| Rfc | 6693 |
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| Title | Probabilistic Routing Protocol for Intermittently Connected
Networks |
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| Author | A. Lindgren, A. Doria, E. Davies, S. Grasic |
|---|
| Date | August 2012 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | EXPERIMENTAL |
|---|
|
Internet Research Task Force (IRTF) A. Lindgren
Request for Comments: 6693 SICS
Category: Experimental A. Doria
ISSN: 2070-1721 Technicalities
E. Davies
Folly Consulting
S. Grasic
Lulea University of Technology
August 2012
Probabilistic Routing Protocol for Intermittently Connected Networks
Abstract
This document is a product of the Delay Tolerant Networking Research
Group and has been reviewed by that group. No objections to its
publication as an RFC were raised.
This document defines PRoPHET, a Probabilistic Routing Protocol using
History of Encounters and Transitivity. PRoPHET is a variant of the
epidemic routing protocol for intermittently connected networks that
operates by pruning the epidemic distribution tree to minimize
resource usage while still attempting to achieve the best-case
routing capabilities of epidemic routing. It is intended for use in
sparse mesh networks where there is no guarantee that a fully
connected path between the source and destination exists at any time,
rendering traditional routing protocols unable to deliver messages
between hosts. These networks are examples of networks where there
is a disparity between the latency requirements of applications and
the capabilities of the underlying network (networks often referred
to as delay and disruption tolerant). The document presents an
architectural overview followed by the protocol specification.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Research Task
Force (IRTF). The IRTF publishes the results of Internet-related
research and development activities. These results might not be
suitable for deployment. This RFC represents the consensus of the
Delay Tolerant Networking Research Group of the Internet Research
Task Force (IRTF). Documents approved for publication by the IRSG
are not a candidate for any level of Internet Standard; see Section 2
of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6693.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Relation to the Delay-Tolerant Networking Architecture . 7
1.2. Applicability of the Protocol . . . . . . . . . . . . . . 8
1.3. PRoPHET as Compared to Regular Routing Protocols . . . . 10
1.4. Requirements Notation . . . . . . . . . . . . . . . . . . 11
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1. PRoPHET . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1. Characteristic Time Interval . . . . . . . . . . . . 12
2.1.2. Delivery Predictability Calculation . . . . . . . . . 12
2.1.3. Optional Delivery Predictability Optimizations . . . 17
2.1.4. Forwarding Strategies and Queueing Policies . . . . . 18
2.2. Bundle Protocol Agent to Routing Agent Interface . . . . 19
2.3. PRoPHET Zone Gateways . . . . . . . . . . . . . . . . . . 20
2.4. Lower-Layer Requirements and Interface . . . . . . . . . 21
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 22
3.1. Neighbor Awareness . . . . . . . . . . . . . . . . . . . 22
3.2. Information Exchange Phase . . . . . . . . . . . . . . . 23
3.2.1. Routing Information Base Dictionary . . . . . . . . . 25
3.2.2. Handling Multiple Simultaneous Contacts . . . . . . . 26
3.3. Routing Algorithm . . . . . . . . . . . . . . . . . . . . 28
3.4. Bundle Passing . . . . . . . . . . . . . . . . . . . . . 32
3.4.1. Custody . . . . . . . . . . . . . . . . . . . . . . . 33
3.5. When a Bundle Reaches Its Destination . . . . . . . . . . 33
3.6. Forwarding Strategies . . . . . . . . . . . . . . . . . . 34
3.7. Queueing Policies . . . . . . . . . . . . . . . . . . . . 36
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 38
4.1. Header . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2. TLV Structure . . . . . . . . . . . . . . . . . . . . . . 44
4.3. TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3.1. Hello TLV . . . . . . . . . . . . . . . . . . . . . . 45
4.3.2. Error TLV . . . . . . . . . . . . . . . . . . . . . . 47
4.3.3. Routing Information Base Dictionary TLV . . . . . . . 48
4.3.4. Routing Information Base TLV . . . . . . . . . . . . 50
4.3.5. Bundle Offer and Response TLVs (Version 2) . . . . . 51
5. Detailed Operation . . . . . . . . . . . . . . . . . . . . . 55
5.1. High-Level State Tables . . . . . . . . . . . . . . . . . 56
5.2. Hello Procedure . . . . . . . . . . . . . . . . . . . . . 59
5.2.1. Hello Procedure State Tables . . . . . . . . . . . . 61
5.3. Information Exchange Phase . . . . . . . . . . . . . . . 62
5.3.1. State Definitions for the Initiator Role . . . . . . 66
5.3.2. State Definitions for the Listener Role . . . . . . . 71
5.3.3. Recommendations for Information Exchange Timer
Periods . . . . . . . . . . . . . . . . . . . . . . . 77
5.3.4. State Tables for Information Exchange . . . . . . . . 78
5.4. Interaction with Nodes Using Version 1 of PRoPHET . . . . 92
6. Security Considerations . . . . . . . . . . . . . . . . . . . 93
6.1. Attacks on the Operation of the Protocol . . . . . . . . 94
6.1.1. Black-Hole Attack . . . . . . . . . . . . . . . . . . 94
6.1.2. Limited Black-Hole Attack / Identity Spoofing . . . . 95
6.1.3. Fake PRoPHET ACKs . . . . . . . . . . . . . . . . . . 95
6.1.4. Bundle Store Overflow . . . . . . . . . . . . . . . . 96
6.1.5. Bundle Store Overflow with Delivery Predictability
Manipulation . . . . . . . . . . . . . . . . . . . . 96
6.2. Interactions with External Routing Domains . . . . . . . 97
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 97
7.1. DTN Routing Protocol Number . . . . . . . . . . . . . . . 98
7.2. PRoPHET Protocol Version . . . . . . . . . . . . . . . . 98
7.3. PRoPHET Header Flags . . . . . . . . . . . . . . . . . . 99
7.4. PRoPHET Result Field . . . . . . . . . . . . . . . . . . 99
7.5. PRoPHET Codes for Success and Codes for Failure . . . . . 99
7.6. PRoPHET TLV Type . . . . . . . . . . . . . . . . . . . . 100
7.7. Hello TLV Flags . . . . . . . . . . . . . . . . . . . . . 101
7.8. Error TLV Flags . . . . . . . . . . . . . . . . . . . . . 101
7.9. RIB Dictionary TLV Flags . . . . . . . . . . . . . . . . 102
7.10. RIB TLV Flags . . . . . . . . . . . . . . . . . . . . . . 102
7.11. RIB Flags . . . . . . . . . . . . . . . . . . . . . . . . 103
7.12. Bundle Offer and Response TLV Flags . . . . . . . . . . . 103
7.13. Bundle Offer and Response B Flags . . . . . . . . . . . . 104
8. Implementation Experience . . . . . . . . . . . . . . . . . . 104
9. Deployment Experience . . . . . . . . . . . . . . . . . . . . 105
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 105
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 105
11.1. Normative References . . . . . . . . . . . . . . . . . . 105
11.2. Informative References . . . . . . . . . . . . . . . . . 106
Appendix A. PRoPHET Example . . . . . . . . . . . . . . . . . . 108
Appendix B. Neighbor Discovery Example . . . . . . . . . . . . . 110
Appendix C. PRoPHET Parameter Calculation Example . . . . . . . 110
1. Introduction
The Probabilistic Routing Protocol using History of Encounters and
Transitivity (PRoPHET) algorithm enables communication between
participating nodes wishing to communicate in an intermittently
connected network where at least some of the nodes are mobile.
One of the most basic requirements for "traditional" (IP) networking
is that there must exist a fully connected path between communication
endpoints for the duration of a communication session in order for
communication to be possible. There are, however, a number of
scenarios where connectivity is intermittent so that this is not the
case (thus rendering the end-to-end use of traditional networking
protocols impossible), but where it still is desirable to allow
communication between nodes.
Consider a network of mobile nodes using wireless communication with
a limited range that is less than the typical excursion distances
over which the nodes travel. Communication between a pair of nodes
at a particular instant is only possible when the distance between
the nodes is less than the range of the wireless communication. This
means that, even if messages are forwarded through other nodes acting
as intermediate routes, there is no guarantee of finding a viable
continuous path when it is needed to transmit a message.
One way to enable communication in such scenarios is by allowing
messages to be buffered at intermediate nodes for a longer time than
normally occurs in the queues of conventional routers (cf. Delay-
Tolerant Networking [RFC4838]). It would then be possible to exploit
the mobility of a subset of the nodes to bring messages closer to
their destination by transferring them to other nodes as they meet.
Figure 1 shows how the mobility of nodes in such a scenario can be
used to eventually deliver a message to its destination. In this
figure, the four sub-figures (a) - (d) represent the physical
positions of four nodes (A, B, C, and D) at four time instants,
increasing from (a) to (d). The outline around each letter
represents the range of the radio communication used for
communication by the nodes: communication is only possible when the
ranges overlap. At the start time, node A has a message -- indicated
by an asterisk (*) next to that node -- to be delivered to node D,
but there does not exist a path between nodes A and D because of the
limited range of available wireless connections. As shown in sub-
figures (a) - (d), the mobility of the nodes allows the message to
first be transferred to node B, then to node C, and when finally node
C moves within range of node D, it can deliver the message to its
final destination. This technique is known as "transitive
networking".
Mobility and contact patterns in real application scenarios are
likely to be non-random, but rather be predictable, based on the
underlying activities of the higher-level application (this could,
for example, stem from human mobility having regular traffic patterns
based on repeating behavioral patterns (e.g., going to work or the
market and returning home) and social interactions, or from any
number of other node mobility situations where a proportion of nodes
are mobile and move in ways that are not completely random over time
but have a degree of predictability over time). This means that if a
node has visited a location or been in contact with a certain node
several times before, it is likely that it will visit that location
or meet that node again.
PRoPHET can also be used in some networks where such mobility as
described above does not take place. Predictable patterns in node
contacts can also occur among static nodes where varying radio
conditions or power-saving sleeping schedules cause connection
between nodes to be intermittent.
In previously discussed mechanisms to enable communication in
intermittently connected networks, such as Epidemic Routing
[vahdat_00], very general approaches have been taken to the problem
at hand. In an environment where buffer space and bandwidth are
infinite, epidemic routing will give an optimal solution to the
problem of routing in an intermittently connected network with regard
to message delivery ratio and latency. However, in most cases,
neither bandwidth nor buffer space is infinite, but instead they are
rather scarce resources, especially in the case of sensor networks.
PRoPHET is fundamentally an epidemic protocol with strict pruning.
An epidemic protocol works by transferring its data to each and every
node it meets. As data is passed from node to node, it is eventually
passed to all nodes, including the target node. One of the
advantages of an epidemic protocol is that by trying every path, it
is guaranteed to try the best path. One of the disadvantages of an
epidemic protocol is the extensive use of resources with every node
needing to carry every packet and the associated transmission costs.
PRoPHET's goal is to gain the advantages of an epidemic protocol
without paying the price in storage and communication resources
incurred by the basic epidemic protocol. That is, PRoPHET offers an
alternative to basic epidemic routing, with lower demands on buffer
space and bandwidth, with equal or better performance in cases where
those resources are limited, and without loss of generality in
scenarios where it is suitable to use PRoPHET.
In a situation where PRoPHET is applicable, the patterns are expected
to have a characteristic time (such as the expected time between
encounters between mobile stations) that is in turn related to the
expected time that traffic will take to reach its destination in the
part of the network that is using PRoPHET. This characteristic time
provides guidance for configuration of the PRoPHET protocol in a
network. When appropriately configured, the PRoPHET protocol
effectively builds a local model of the expected patterns in the
network that can be used to optimize the usage of resources by
reducing the amount of traffic sent to nodes that are unlikely to
lead to eventual delivery of the traffic to its destination.
+----------------------------+ +----------------------------+
| ___ | | ___ |
| ___ / \ | | / \ |
| / \ ( D ) | | ( D ) |
| ( B ) \___/ | | ___ \___/ |
| \___/ ___ | | /___\ ___ |
|___ / \ | | (/ B*\) / \ |
| \ ( C ) | | (\_A_/) ( C ) |
| A* ) \___/ | | \___/ \___/ |
|___/ | | |
+----------------------------+ +----------------------------+
(a) Time t (b) Time (t + dt)
+----------------------------+ +----------------------------+
| _____ ___ | | ___ ___ |
| / / \ \ / \ | | / \ /___\ |
| ( (B C* ) ( D ) | | ( B ) (/ D*\) |
| \_\_/_/ \___/ | | \___/ (\_C_/) |
| ___ | | ___ \___/ |
| / \ | | / \ |
| ( A ) | | ( A ) |
| \___/ | | \___/ |
| | | |
+----------------------------+ +----------------------------+
(c) Time (t + 2*dt) (d) Time (t + 3*dt)
Figure 1: Example of transitive communication
This document presents a framework for probabilistic routing in
intermittently connected networks, using an assumption of non-random
mobility of nodes to improve the delivery rate of messages while
keeping buffer usage and communication overhead at a low level.
First, a probabilistic metric called delivery predictability is
defined. The document then goes on to define a probabilistic routing
protocol using this metric.
1.1. Relation to the Delay-Tolerant Networking Architecture
The Delay-Tolerant Networking (DTN) architecture [RFC4838] defines an
architecture for communication in environments where traditional
communication protocols cannot be used due to excessive delays, link
outages, and other extreme conditions. The intermittently connected
networks considered here are a subset of those covered by the DTN
architecture. The DTN architecture defines routes to be computed
based on a collection of "contacts" indicating the start time,
duration, endpoints, forwarding capacity, and latency of a link in
the topology graph. These contacts may be deterministic or may be
derived from estimates. The architecture defines some different
types of intermittent contacts. The ones called "opportunistic" and
"predicted" are the ones addressed by this protocol.
Opportunistic contacts are those that are not scheduled, but rather
present themselves unexpectedly and frequently arise due to node
mobility. Predicted contacts are like opportunistic contacts, but,
based on some information, it might be possible to draw some
statistical conclusion as to whether or not a contact will be present
soon.
The DTN architecture also introduces the bundle protocol [RFC5050],
which provides a way for applications to "bundle" an entire session,
including both data and metadata, into a single message, or bundle,
that can be sent as a unit. The bundle protocol also provides end-
to-end addressing and acknowledgments. PRoPHET is specifically
intended to provide routing services in a network environment that
uses bundles as its data transfer mechanism but could be also be used
in other intermittent environments.
1.2. Applicability of the Protocol
The PRoPHET routing protocol is mainly targeted at situations where
at least some of the nodes are mobile in a way that creates
connectivity patterns that are not completely random over time but
have a degree of predictability. Such connectivity patterns can also
occur in networks where nodes switch off radios to preserve power.
Human mobility patterns (often containing daily or weekly periodic
activities) provide one such example where PRoPHET is expected to be
applicable, but the applicability is not limited to scenarios
including humans.
In order for PRoPHET to benefit from such predictability in the
contact patterns between nodes, it is expected that the network exist
under similar circumstances over a longer timescale (in terms of node
encounters) so that the predictability can be accurately estimated.
The PRoPHET protocol expects nodes to be able to establish a local
TCP link in order to exchange the information needed by the PRoPHET
protocol. Protocol signaling is done out-of-band over this TCP link,
without involving the bundle protocol agent [RFC5050]. However, the
PRoPHET protocol is expected to interact with the bundle protocol
agent to retrieve information about available bundles as well as to
request that a bundle be sent to another node (it is expected that
the associated bundle protocol agents are then able to establish a
link (probably over the TCP convergence layer [CLAYER]) to perform
this bundle transfer).
TCP provides a reliable bidirectional channel between two peers and
guarantees in-order delivery of transmitted data. When using TCP,
the guarantee of reliable, in-order delivery allows information
exchanges of each category of information to be distributed across
several messages without requiring the PRoPHET protocol layer to be
concerned that all messages have been received before starting the
exchange of the next category of information. At most, the last
message of the category needs to be marked as such. This allows the
receiver to process earlier messages while waiting for additional
information and allows implementations to limit the size of messages
so that IP fragmentation will be avoided and memory usage can be
optimized if necessary. However, implementations MAY choose to build
a single message for each category of information that is as large as
necessary and rely on TCP to segment the message.
While PRoPHET is currently defined to run over TCP, in future
versions the information exchange may take place over other transport
protocols, and these may not provide message segmentation or
reliable, in-order delivery. The simple message division used with
TCP MUST NOT be used when the underlying transport does not offer
reliable, in-order delivery, as it would be impossible to verify that
all the messages had arrived. Hence, the capability is provided to
segment protocol messages into submessages directly in the PRoPHET
layer. Submessages are provided with sequence numbers, and this,
together with a capability for positive acknowledgements, would allow
PRoPHET to operate over an unreliable protocol such as UDP or
potentially directly over IP.
Since TCP offers reliable delivery, it is RECOMMENDED that the
positive acknowledgment capability is not used when PRoPHET is run
over a TCP transport or similar protocol. When running over TCP,
implementations MAY safely ignore positive acknowledgments.
Whatever transport protocol is used, PRoPHET expects to use a
bidirectional link for the information exchange; this allows for the
information exchange to take place in both directions over the same
link avoiding the need to establish a second link for information
exchange in the reverse direction.
In a large Delay- and Disruption-Tolerant Network (DTN), network
conditions may vary widely, and in different parts of the network,
different routing protocols may be appropriate. In this
specification, we consider routing within a single "PRoPHET zone",
which is a set of nodes among which messages are routed using
PRoPHET. In many cases, a PRoPHET zone will not span the entire DTN,
but there will be other parts of the network with other
characteristics that run other routing protocols. To handle this,
there may be nodes within the zone that act as gateways to other
nodes that are the destinations for bundles generated within the zone
or that insert bundles into the zone. Thus, PRoPHET is not
necessarily used end-to-end, but only within regions of the network
where its use is appropriate.
1.3. PRoPHET as Compared to Regular Routing Protocols
While PRoPHET uses a mechanism for pruning the epidemic forwarding
tree that is similar to the mechanism used in metric-based vector
routing protocols (where the metric might be distance or cost), it
should not be confused with a metric vector protocol.
In a traditional metric-based vector routing protocol, the
information passed from node to node is used to create a single non-
looping path from source to destination that is optimal given the
metric used. The path consists of a set of directed edges selected
from the complete graph of communications links between the network
nodes.
In PRoPHET, that information is used to prune the epidemic tree of
paths by removing paths that look less likely to provide an effective
route for delivery of data to its intended destination. One of the
effects of this difference is that the regular notions of split
horizon, as described in [RFC1058], do not apply to PRoPHET. The
purpose of split horizon is to prevent a distance vector protocol
from ever passing a packet back to the node that sent it the packet
because it is well known that the source does not lie in that
direction as determined when the directed path was computed.
In an epidemic protocol, where that previous system already has the
data, the notion of passing the data back to the node is redundant:
the protocol can readily determine that such a transfer is not
required. Further, given the mobility and constant churn of
encounters possible in a DTN that is dominated by opportunistic
encounters, it is quite possible that, on a future encounter, the
node might have become a better option for reaching the destination.
Such a later encounter may require a re-transfer of the data if
resource constraints have resulted in the data being deleted from the
original carrier between the encounters.
The logic of metric routing protocols does not map directly onto the
family of epidemic protocols. In particular, it is inappropriate to
try to assess such protocols against the criteria used to assess
conventional routing protocols such as the metric vector protocols;
this is not to say that the family of epidemic protocols do not have
weaknesses but they have to be considered independently of
traditional protocols.
1.4. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Architecture
2.1. PRoPHET
This section presents an overview of the main architecture of
PRoPHET, a Probabilistic Routing Protocol using History of Encounters
and Transitivity. The protocol leverages the observations made on
the non-randomness of mobility patterns present in many application
scenarios to improve routing performance. Instead of doing blind
epidemic replication of bundles through the network as previous
protocols have done, it applies "probabilistic routing".
To accomplish this, a metric called "delivery predictability",
0 <= P_(A,B) <= 1, is established at every node A for each known
destination B. This metric is calculated so that a node with a
higher value for a certain destination is estimated to be a better
candidate for delivering a bundle to that destination (i.e., if
P_(A,B)>P_(C,B), bundles for destination B are preferable to forward
to A rather than C). It is later used when making forwarding
decisions. As routes in a DTN are likely to be asymmetric, the
calculation of the delivery predictability reflects this, and P_(A,B)
may be different from P_(B,A).
The delivery predictability values in each node evolve over time both
as a result of decay of the metrics between encounters between nodes
and due to changes resulting from encounters when metric information
for the encountered node is updated to reflect the encounter and
metric information about other nodes is exchanged.
When two PRoPHET nodes have a communication opportunity, they
initially enter a two-part Information Exchange Phase (IEP). In the
first part of the exchange, the delivery predictabilities for all
destinations known by each node are shared with the encountered node.
The exchanged information is used by each node to update the internal
delivery predictability vector as described below. After that, the
nodes exchange information (including destination and size) about the
bundles each node carries, and the information is used in conjunction
with the updated delivery predictabilities to decide which bundles to
request to be forwarded from the other node based on the forwarding
strategy used (as discussed in Section 2.1.4). The forwarding of
bundles is carried out in the latter part of the Information Exchange
Phase.
2.1.1. Characteristic Time Interval
When an application scenario makes PRoPHET applicable, the mobility
pattern will exhibit a characteristic time interval that reflects the
distribution of time intervals between encounters between nodes. The
evolution of the delivery predictabilities, which reflects this
mobility pattern, should reflect this same characteristic time
interval. Accordingly, the parameters used in the equations that
specify the evolution of delivery predictability (see Section 2.1.2)
need to be configured appropriately so that the evolution reflects a
model of the mobility pattern.
2.1.2. Delivery Predictability Calculation
As stated above, PRoPHET relies on calculating a metric based on the
probability of encountering a certain node, and using that to support
the decision of whether or not to forward a bundle to a certain node.
This section describes the operations performed on the metrics stored
in a node when it encounters another node and a communications
opportunity arises. In the operations described by the equations
that follow, the updates are being performed by node A, P_(A,B) is
the delivery predictability value that node A will have stored for
the destination B after the encounter, and P_(A,B)_old is the
corresponding value that was stored before the encounter. If no
delivery predictability value is stored for a particular destination
B, P_(A,B) is considered to be zero.
As a special case, the metric value for a node itself is always
defined to be 1 (i.e., P_(A,A)=1).
The equations use a number of parameters that can be selected to
match the characteristics of the mobility pattern in the PRoPHET zone
where the node is located (see Section 2.1.1). Recommended settings
for the various parameters are given in Section 3.3. The impact on
the evolution of delivery predictabilities if encountering nodes have
different parameter setting is discussed in Section 2.1.2.1.
The calculation of the updates to the delivery predictabilities
during an encounter has three parts.
When two nodes meet, the first thing they do is to update the
delivery predictability for each other, so that nodes that are often
encountered have a high delivery predictability. If node B has not
met node A for a long time or has never met node B, such that
P_(A,B) < P_first_threshold, then P_(A,B) should be set to
P_encounter_first. Because PRoPHET generally has no prior knowledge
about whether this is an encounter that will be repeated relatively
frequently or one that will be a rare event, P_encounter_first SHOULD
be set to 0.5 unless the node has extra information obtained other
than through the PRoPHET protocol about the likelihood of future
encounters. Otherwise, P_(A,B) should be calculated as shown in
Equation 1, where 0 <= P_encounter <= 1 is a scaling factor setting
the rate at which the predictability increases on encounters after
the first, and delta is a small positive number that effectively sets
an upper bound for P_(A,B). The limit is set so that
predictabilities between different nodes stay strictly less than 1.
The value of delta should normally be very small (e.g., 0.01) so as
not to significantly restrict the range of available
predictabilities, but it can be chosen to make calculations efficient
where this is important.
P_(A,B) =
P_(A,B)_old + ( 1 - delta - P_(A,B)_old ) * P_encounter (Eq. 1)
There are practical circumstances where an encounter that is
logically a single encounter in terms of the proximity of the node
hardware and/or from the point of view of the human users of the
nodes results in several communication opportunities closely spaced
in time. For example, mobile nodes communicating with each other
using Wi-Fi ad hoc mode may produce apparent multiple encounters with
a short interval between them but these are frequently due to
artifacts of the underlying physical network when using wireless
connections, where transmission problems or small changes in location
may result in repeated reconnections. In this case, it would be
inappropriate to increase the delivery predictability by the same
amount for each opportunity as it would be increased when encounters
occur at longer intervals in the normal mobility pattern.
In order to reduce the distortion of the delivery predictability in
these circumstances, P_encounter is a function of the interval since
the last encounter resulted in an update of the delivery
predictabilities. The form of the function is as shown in Figure 2.
P_encounter
^
|
P_encounter_max + - - .-------------------------------------
| /
| / .
| /
| / .
| /
| / .
|/
+-------+-------------------------------------> I
I_typ
Figure 2: P_encounter as function of time interval, I,
between updates
The form of the function is chosen so that both the increase of
P_(A,B) resulting from Equation 1 and the decrease that results from
Equation 2 are related to the interval between updates for short
intervals. For intervals longer than the "typical" time (I_typ)
between encounters, P_encounter is set to a fixed value
P_encounter_max. The break point reflects the transition between the
"normal" communication opportunity regime (where opportunities result
from the overall mobility pattern) and the closely spaced
opportunities that result from what are effectively local artifacts
of the wireless technology used to deliver those opportunities.
P_encounter_max is chosen so that the increment in P_(A,B) provided
by Equation 1 significantly exceeds the decay of the delivery
predictability over the typical interval between encounters resulting
from Equation 2.
Making P_encounter dependent on the interval time also avoids
inappropriate extra increments of P_(A,B) in situations where node A
is in communication with several other nodes simultaneously. In this
case, updates from each of the communicating nodes have to be
distributed to the other nodes, possibly leading to several updates
being carried out in a short period. This situation is discussed in
more detail in Section 3.2.2.
If a pair of nodes do not encounter each other during an interval,
they are less likely to be good forwarders of bundles to each other,
thus the delivery predictability values must age, being reduced in
the process. The second part of the updates of the metric values is
application of the aging equation shown in Equation 2, where
0 <= gamma <= 1 is the aging constant, and K is the number of time
units that have elapsed since the last time the metric was aged. The
time unit used can differ and should be defined based on the
application and the expected delays in the targeted network.
P_(A,B) = P_(A,B)_old * gamma^K (Eq. 2)
The delivery predictabilities are aged according to Equation 2 before
being passed to an encountered node so that they reflect the time
that has passed since the node had its last encounter with any other
node. The results of the aging process are sent to the encountered
peer for use in the next stage of the process. The aged results
received from node B in node A are referenced as P_(B,x)_recv.
The delivery predictability also has a transitive property that is
based on the observation that if node A frequently encounters node B,
and node B frequently encounters node C, then node C probably is a
good node to which to forward bundles destined for node A.
Equation 3 shows how this transitivity affects the delivery
predictability, where 0 <= beta <= 1 is a scaling constant that
controls how large an impact the transitivity should have on the
delivery predictability.
P_(A,C) = MAX( P_(A,C)_old, P_(A,B) * P_(B,C)_recv * beta ) (Eq. 3)
Node A uses Equation 3 and the metric values received from the
encountered node B (e.g., P_(B,C)_recv) in the third part of updating
the metric values stored in node A.
2.1.2.1. Impact of Encounters between Nodes with Different Parameter
Settings
The various parameters used in the three equations described in
Section 2.1.2 are set independently in each node, and it is therefore
possible that encounters may take place between nodes that have been
configured with different values of the parameters. This section
considers whether this could be problematic for the operation of
PRoPHET in that zone.
It is desirable that all the nodes operating in a PRoPHET zone should
use closely matched values of the parameters and that the parameters
should be set to values that are appropriate for the operating zone.
More details of how to select appropriate values are given in
Section 3.3. Using closely matched values means that delivery
predictabilities will evolve in the same way in each node, leading to
consistent decision making about the bundles that should be exchanged
during encounters.
Before going on to consider the impact of reasonable but different
settings, it should be noted that malicious nodes can use
inappropriate settings of the parameters to disrupt delivery of
bundles in a PRoPHET zone as described in Section 6.
Firstly and importantly, use of different, but legitimate, settings
in encountering nodes will not cause problems in the protocol itself.
Apart from P_encounter_first, the other parameters control the rate
of change of the metric values or limit the range of valid values
that will be stored in a node. None of the calculations in a node
will be invalidated or result in illegal values if the metric values
received from another node were calculated using different
parameters. Furthermore, the protocol is designed so that it is not
possible to carry delivery predictabilities outside the permissible
range of 0 to 1.
A node MAY consider setting received values greater than (1 - delta)
to (1 - delta) if this would simplify operations. However, there are
some special situations where it may be appropriate for the delivery
predictability for another node to be 1. For example, if a DTN using
PRoPHET has multiple gateways to the continuously connected Internet,
the delivery predictability seen from PRoPHET in one gateway for the
other gateway nodes can be taken as 1 since they are permanently
connected through the Internet. This would allow traffic to be
forwarded into the DTN through the most advantageous gateway even if
it initially arrives at another gateway.
Simulation work indicates that the update calculations are quite
stable in the face of changes to the rate parameters, so that minor
discrepancies will not have a major impact on the performance of the
protocol. The protocol is explicitly designed to deal with
situations where there are random factors in the opportunistic nature
of node encounters, and this randomness dominates over the
discrepancies in the parameters.
More major discrepancies may lead to suboptimal behavior of the
protocol, as certain paths might be more preferred or more deprecated
inappropriately. However, since the protocol overall is epidemic in
nature, this would not generally lead to non-delivery of bundles, as
they would also be passed to other nodes and would still be
delivered, though possibly not on the optimal path.
2.1.3. Optional Delivery Predictability Optimizations
2.1.3.1. Smoothing
To give the delivery predictability a smoother rate of change, a node
MAY apply one of the following methods:
1. Keep a list of NUM_P values for each destination instead of only
a single value. (The recommended value is 4, which has been
shown in simulations to give a good trade-off between smoothness
and rate of response to changes.) The list is held in order of
acquisition. When a delivery predictability is updated, the
value at the "newest" position in the list is used as input to
the equations in Section 2.1.2. The oldest value in the list is
then discarded and the new value is written in the "newest"
position of the list. When a delivery predictability value is
needed (either for sending to a peering PRoPHET node, or for
making a forwarding decision), the average of the values in the
list is calculated, and that value is then used. If less than
NUM_P values have been entered into the list, only the positions
that have been filled should be used for the averaging.
2. In addition to keeping the delivery predictability as described
in Section 2.1.2, a node MAY also keep an exponential weighted
moving average (EWMA) of the delivery predictability. The EWMA
is then used to make forwarding decisions and to report to
peering nodes, but the value calculated according to
Section 2.1.2 is still used as input to the calculations of new
delivery predictabilities. The EWMA is calculated according to
Equation 4, where 0 <= alpha <= 1 is the weight of the most
current value.
P_ewma = P_ewma_old * (1 - alpha) + P * alpha (Eq. 4)
The appropriate choice of alpha may vary depending on application
scenario circumstances. Unless prior knowledge of the scenario is
available, it is suggested that alpha is set to 0.5.
2.1.3.2. Removal of Low Delivery Predictabilities
To reduce the data to be transferred between two nodes, a node MAY
treat delivery predictabilities smaller than P_first_threshold, where
P_first_threshold is a small number, as if they were zero, and thus
they do not need to be stored or included in the list sent during the
Information Exchange Phase. If this optimization is used, care must
be taken to select P_first_threshold to be smaller than delivery
predictability values normally present in the network for
destinations for which this node is a forwarder. It is possible that
P_first_threshold could be calculated based on delivery
predictability ranges and the amount they change historically, but
this has not been investigated yet.
2.1.4. Forwarding Strategies and Queueing Policies
In traditional routing protocols, choosing where to forward a message
is usually a simple task; the message is sent to the neighbor that
has the path to the destination with the lowest cost (often the
shortest path). Normally, the message is also sent to only a single
node since the reliability of paths is relatively high. However, in
the settings we envision here, things are radically different. The
first possibility that must be considered when a bundle arrives at a
node is that there might not be a path to the destination available,
so the node has to buffer the bundle, and upon each encounter with
another node, the decision must be made whether or not to transfer a
particular bundle. Furthermore, having duplicates of messages (on
different nodes, as the bundle offer/request mechanism described in
Section 4.3.5 ensures that a node does not receive a bundle it
already carries) may also be sensible, as forwarding a bundle to
multiple nodes can increase the delivery probability of that bundle.
Unfortunately, these decisions are not trivial to make. In some
cases, it might be sensible to select a fixed threshold and only give
a bundle to nodes that have a delivery predictability over that
threshold for the destination of the bundle. On the other hand, when
encountering a node with a low delivery predictability, it is not
certain that a node with a higher metric will be encountered within a
reasonable time. Thus, there can also be situations where we might
want to be less strict in deciding who to give bundles to.
Furthermore, there is the problem of deciding how many nodes to give
a certain bundle to. Distributing a bundle to a large number of
nodes will of course increase the probability of delivering that
particular bundle to its destination, but this comes at the cost of
consuming more system resources for bundle storage and possibly
reducing the probability of other bundles being delivered. On the
other hand, giving a bundle to only a few nodes (maybe even just a
single node) will use less system resources, but the probability of
delivering a bundle is lower, and the delay incurred is high.
When resources are constrained, nodes may suffer from storage
shortage, and may have to drop bundles before they have been
delivered to their destinations. They may also wish to consider the
length of bundles being offered by an encountered node before
accepting transfer of the bundle in order to avoid the need to drop
the new bundle immediately or to ensure that there is adequate space
to hold the bundle offered, which might require other bundles to be
dropped. As with the decision as to whether or not to forward a
bundle, deciding which bundles to accept and/or drop to still
maintain good performance might require different policies in
different scenarios.
Nodes MAY define their own forwarding strategies and queueing
policies that take into account the special conditions applicable to
the nodes, and local resource constraints. Some default strategies
and policies that should be suitable for most normal operations are
defined in Section 3.6 and Section 3.7.
2.2. Bundle Protocol Agent to Routing Agent Interface
The bundle protocol [RFC5050] introduces the concept of a "bundle
protocol agent" that manages the interface between applications and
the "convergence layers" that provide the transport of bundles
between nodes during communication opportunities. This specification
extends the bundle protocol agent with a routing agent that controls
the actions of the bundle protocol agent during an (opportunistic)
communications opportunity.
This specification defines the details of the PRoPHET routing agent,
but the interface defines a more general interface that is also
applicable to alternative routing protocols.
To enable the PRoPHET routing agent to operate properly, it must be
aware of the bundles stored at the node, and it must also be able to
tell the bundle protocol agent of that node to send a bundle to a
peering node. Therefore, the bundle protocol agent needs to provide
the following interface/functionality to the routing agent:
Get Bundle List
Returns a list of the stored bundles and their attributes to the
routing agent.
Send Bundle
Makes the bundle protocol agent send a specified bundle.
Accept Bundle
Gives the bundle protocol agent a new bundle to store.
Bundle Delivered
Tells the bundle protocol agent that a bundle was delivered to
its destination.
Drop Bundle Advice
Advises the bundle protocol agent that a specified bundle should
not be offered for forwarding in future and may be dropped by
the bundle protocol agent if appropriate.
Route Import
Can be used by a gateway node in a PRoPHET zone to import
reachability information about endpoint IDs (EIDs) that are
external to the PRoPHET zone. Translation functions dependent
on the external routing protocol will be used to set the
appropriate delivery predictabilities for imported destinations
as described in Section 2.3.
Route Export
Can be used by a gateway node in a PRoPHET zone to export
reachability information (destination EIDs and corresponding
delivery predictabilities) for use by routing protocols in other
parts of the DTN.
Implementation Note: Depending on the distribution of functions in
a complete bundle protocol agent supporting PRoPHET, reception and
delivery of bundles may not be carried out directly by the PRoPHET
module. In this case, PRoPHET can inform the bundle protocol
agent about bundles that have been requested from communicating
nodes. Then, the Accept Bundle and Bundle Delivered functions can
be implemented as notifications of the PRoPHET module when the
relevant bundles arrive at the node or are delivered to local
applications.
2.3. PRoPHET Zone Gateways
PRoPHET is designed to handle routing primarily within a "PRoPHET
zone", i.e., a set of nodes that all implement the PRoPHET routing
scheme. However, since we recognize that a PRoPHET routing zone is
unlikely to encompass an entire DTN, there may be nodes within the
zone that act as gateways to other nodes that are the destinations
for bundles generated within the zone or that insert bundles into the
zone.
PRoPHET MAY elect to export and import routes across a bundle
protocol agent interface. The delivery predictability to use for
routes that are imported depends on the routing protocol used to
manage those routes. If a translation function between the external
routing protocol and PRoPHET exists, it SHOULD be used to set the
delivery predictability. If no such translation function exists, the
delivery predictability SHOULD be set to 1. For those routes that
are exported, the current delivery predictability will be exported
with the route.
2.4. Lower-Layer Requirements and Interface
PRoPHET can be run on a large number of underlying networking
technologies. To accommodate its operation on all kinds of lower
layers, it requires the lower layers to provide the following
functionality and interfaces.
Neighbor discovery and maintenance
A PRoPHET node needs to know the identity of its neighbors and
when new neighbors appear and old neighbors disappear. Some
wireless networking technologies might already contain
mechanisms for detecting neighbors and maintaining this state.
To avoid redundancies and inefficiencies, neighbor discovery is
thus not included as a part of PRoPHET, but PRoPHET relies on
such a mechanism in lower layers. The lower layers MUST provide
the two functions listed below. If the underlying networking
technology does not support such services, a simple neighbor
discovery scheme using local broadcasts of beacon messages could
be run in between PRoPHET and the underlying layer. An example
of a simple neighbor discovery mechanism that could be used is
in Appendix B.
New Neighbor
Signals to the PRoPHET agent that a new node has become a
neighbor. A neighbor is defined here as another node that
is currently within communication range of the wireless
networking technology in use. The PRoPHET agent should now
start the Hello procedure as described in Section 5.2.
Neighbor Gone
Signals to the PRoPHET agent that one of its neighbors has
left.
Local Address
An address used by the underlying communication layer (e.g., an
IP or Media Access Control (MAC) address) that identifies the
sender address of the current message. This address must be
unique among the nodes that can currently communicate and is
only used in conjunction with an Instance Number to identify a
communicating pair of nodes as described in Section 4.1. This
address and its format is dependent on the communication layer
that is being used by the PRoPHET layer.
3. Protocol Overview
The PRoPHET protocol involves two principal phases:
o becoming aware of new neighbors that implement the protocol and
establishing a point-to-point connection between each pair of
encountering nodes, and
o using the connection for information exchange needed to establish
PRoPHET routing and to exchange bundles.
3.1. Neighbor Awareness
Since the operation of the protocol is dependent on the encounters of
nodes running PRoPHET, the nodes must be able to detect when a new
neighbor is present. The protocol may be run on several different
networking technologies, and as some of them might already have
methods available for detecting neighbors, PRoPHET does not include a
mechanism for neighbor discovery. Instead, it requires the
underlying layer to provide a mechanism to notify the protocol of
when neighbors appear and disappear as described in Section 2.4.
When a new neighbor has been detected, the protocol starts to set up
a link with that node through the Hello message exchange as described
in Section 5.2. The Hello message exchange allows for negotiation of
capabilities between neighbors. At present, the only capability is a
request that the offering node should or should not include bundle
payload lengths with all offered bundles rather than just for
fragments. Once the link has been set up, the protocol may continue
to the Information Exchange Phase (see Section 3.2). Once this has
been completed, the nodes will normally recalculate the delivery
predictabilities using the equations and mechanisms described in
Sections 2.1.2 and 2.1.3.
As described in Section 2.1.2, there are some circumstances in which
a single logical encounter may result in several actual communication
opportunities. To avoid the delivery predictability of the
encountered node being increased excessively under these
circumstances, the value of P_encounter is made dependent on the
interval time between delivery predictability updates when the
interval is less than the typical interval between encounters, but it
is a constant for longer intervals.
In order to make use of this time dependence, PRoPHET maintains a
list of recently encountered nodes identified by the Endpoint
Identifier (EID) that the node uses to identify the communication
session and containing the start time of the last communication
session with that node. The size of this list is controlled because
nodes that are not in contact and that started their last connection
more than a time I_typ before the present can be dropped from the
list. It also maintains a record of the time at which the decay
function (Equation 2) was last applied to the delivery
predictabilities in the node.
3.2. Information Exchange Phase
The Information Exchange Phase involves two parts:
o establishing the Router Information Base (RIB Exchange Sub-Phase),
and
o exchanging bundles using this information (Bundle Passing Sub-
Phase).
Four types of information are exchanged during this process:
o Routing Information Base Dictionary (RIB Dictionary or RIBD),
o Routing Information Base (RIB),
o Bundle Offers, and
o Bundle Responses.
During a communication opportunity, several sets of each type of
information may be transferred in each direction as explained in the
rest of this section. Each set can be transferred in one or more
messages. When (and only when) using a connection-oriented reliable
transport protocol such as TCP as envisaged in this document, a set
can be partitioned across messages by the software layer above the
PRoPHET protocol engine.
In this case, the last message in a set is flagged in the protocol.
This allows the higher-level software to minimize the buffer memory
requirements by avoiding the need to build very large messages in one
go and allows the message size to be controlled outside of PRoPHET.
However, this scheme is only usable if the transport protocol
provides reliable, in-order delivery of messages, as the messages are
not explicitly sequence numbered and the overall size of the set is
not passed explicitly.
The specification of PRoPHET also provides a submessage mechanism and
retransmission that allows large messages specified by the higher
level to be transmitted in smaller chunks. This mechanism was
originally provided to allow PRoPHET to operate over unreliable
transport protocols such as UDP, but can also be used with reliable
transports if the higher-level software does not want to handle
message fragmentation. However, the sequencing and length adds
overhead that is redundant if the transport protocol already provides
reliable, in-order delivery.
The first step in the Information Exchange Phase is for the protocol
to send one or more messages containing a RIB Dictionary TLV (Type-
Length-Value message component) to the node with which it is peering.
This set of messages contain a dictionary of the Endpoint Identifiers
(EIDs) of the nodes that will be listed in the Routing Information
Base (RIB); see Section 3.2.1 for more information about this
dictionary. After this, one or more messages containing a Routing
Information Base TLV are sent. This TLV contains a list of the EIDs
that the node has knowledge of, and the corresponding delivery
predictabilities for those nodes, together with flags describing the
capabilities of the sending node. Upon reception of a complete set
of these messages, the peer node updates its delivery predictability
table according to the equations in Section 2.1.2. The peer node
then applies its forwarding strategy (see Section 2.1.4) to determine
which of its stored bundles it wishes to offer the node that sent the
RIB; that node will then be the receiver for any bundles to be
transferred.
After making this decision, one or more Bundle Offer TLVs are
prepared, listing the bundle identifiers and their destinations for
all bundles the peer node wishes to offer to the receiver node that
sent the RIB. As described in [RFC5050], a bundle identifier
consists of up to five component parts. For a complete bundle, the
identifier consists of
o source EID,
o creation timestamp - time of creation, and
o creation timestamp - sequence number.
Additionally, for a bundle fragment, the identifier also contains
o offset within the payload at which the fragment payload data
starts, and
o length of the fragment payload data.
If any of the Bundle Offer TLVs lists a bundle for which the source
or destination EID was not included in the previous set of RIBD
information sent, one or more new RIBD TLVs are sent next with an
incremental update of the dictionary. When the receiver node has a
dictionary with all necessary EIDs, the Bundle Offer TLVs are sent to
it. The Bundle Offer TLVs also contain a list of PRoPHET ACKs (see
Section 3.5). If requested by the receiver node during the Hello
phase, the Bundle Offer TLV will also specify the payload length for
all bundles rather than for just fragments. This information can be
used by the receiving node to assist with the selection of bundles to
be accepted from the offered list, especially if the available bundle
storage capacity is limited.
The receiving node then examines the list of offered bundles and
selects bundles that it will accept according to its own policies,
considering the bundles already present in the node and the current
availability of resources in the node. The list is sorted according
to the priority that the policies apply to the selected bundles, with
the highest priority bundle first in the list. The offering node
will forward the selected bundles in this order. The prioritized
list is sent to the offering node in one or more Bundle Response TLVs
using the same EID dictionary as was used for the Bundle Offer TLV.
When a new bundle arrives at a node, the node MAY inspect its list of
available neighbors, and if one of them is a candidate to forward the
bundle, a new Bundle Offer TLV MAY be sent to that node. If two
nodes remain connected over a longer period of time, the Information
Exchange Phase will be periodically re-initiated to allow new
delivery predictability information to be spread through the network
and new bundle exchanges to take place.
The Information Exchange Phase of the protocol is described in more
detail in Section 5.3.
3.2.1. Routing Information Base Dictionary
To reduce the overhead of the protocol, the Routing Information Base
and Bundle Offer/Response TLVs utilize an EID dictionary. This
dictionary maps variable-length EIDs (as defined in [RFC4838]), which
may potentially be quite long, to shorter numerical identifiers,
coded as Self-Delimiting Numeric Values (SDNVs -- see Section 4.1. of
RFC 5050 [RFC5050]), which are used in place of the EIDs in
subsequent TLVs.
This dictionary is a shared resource between the two peering nodes.
Each can add to the dictionary by sending a RIB Dictionary TLV to its
peer. To allow either node to add to the dictionary at any time, the
identifiers used by each node are taken from disjoint sets:
identifiers originated by the node that started the Hello procedure
have the least significant bit set to 0 (i.e., are even numbers)
whereas those originated by the other peer have the least significant
bit set to 1 (i.e., are odd numbers). This means that the dictionary
can be expanded by either node at any point in the Information
Exchange Phase and the new identifiers can then be used in subsequent
TLVs until the dictionary is re-initialized.
The dictionary that is established only persists through a single
encounter with a node (i.e., while the same link set up by the Hello
procedure, with the same instance numbers, remains open).
Having more then one identifier for the same EID does not cause any
problems. This means that it is possible for the peers to create
their dictionary entries independently if required by an
implementation, but this may be inefficient as a dictionary entry for
an EID might be sent in both directions between the peers.
Implementers can choose to inspect entries sent by the node that
started the Hello procedure and thereby eliminate any duplicates
before sending the dictionary entries from the other peer. Whether
postponing sending the other peer's entries is more efficient depends
on the nature of the physical link technology and the transport
protocol used. With a genuinely full-duplex link, it may be faster
to accept possible duplication and send dictionary entries
concurrently in both directions. If the link is effectively half-
duplex (e.g., Wi-Fi), then it will generally be more efficient to
wait and eliminate duplicates.
If a node receives a RIB Dictionary TLV containing an identifier that
is already in use, the node MUST confirm that the EID referred to is
identical to the EID in the existing entry. Otherwise, the node must
send an error response to the message with the TLV containing the
error and ignore the TLV containing the error. If a node receives a
RIB, Bundle Offer, or Bundle Response TLV that uses an identifier
that is not in its dictionary, the node MUST send an error response
and ignore the TLV containing the error.
3.2.2. Handling Multiple Simultaneous Contacts
From time to time, a mobile node may, for example, be in wireless
range of more than one other mobile node. The PRoPHET neighbor
awareness protocol will establish multiple simultaneous contacts with
these nodes and commence information exchanges with each of them.
When updating the delivery predictabilities as described in
Section 2.1.2 using the values passed from each of the contacts in
turn, some special considerations apply when multiple contacts are in
progress:
SC1 When aging the delivery predictabilities according to
Equation 2, the value of K to be used in each set of
calculations is always the amount of time since the last aging
was done. For example, if node Z makes contact with node A and
then with node B, the value of K used when the delivery
predictabilities are aged in node Z for the contact with node B
will be the time since the delivery predictabilities were aged
for the contact with node A.
SC2 When a new contact starts, the value of P_encounter used when
applying Equation 1 for the newly contacted node is always
selected according to the time since the last encounter with
that node. Thus, the application of Equation 1 to update
P_(Z,A) when the contact of nodes Z and A starts (in the aging
example just given) and the updating of P_(Z,B) when the contact
of nodes Z and B starts will use the appropriate value of
P_encounter according to how long it is since node Z previously
encountered node A and node B, respectively.
SC3 If, as with the contact between nodes Z and B, there is another
active contact in progress, such as with node A when the contact
with node B starts, Equation 1 should *also* be applied to
P_(z,x) for all the nodes "x" that have ongoing contacts with
node Z (i.e., node A in the example given). However, the value
of P_encounter used will be selected according to the time since
the previous update of the delivery predictabilities as a result
of information received from any other node. In the example
given here, P_(Z,A) would also have Equation 1 applied when the
delivery predictabilities are received from node B, but the
value of P_encounter used would be selected according to the
time since the updates done when the encounter between nodes Z
and A started rather than the time since the previous encounter
between nodes A and Z.
If these simultaneous contacts persist for some time, then, as
described in Section 3.2, the Information Exchange Phase will be
periodically rerun for each contact according to the configured timer
interval. When the delivery predictability values are recalculated
during each rerun, Equation 1 will be applied as in special
consideration SC3 above, but it will be applied to the delivery
predictability for each active contact using the P_encounter value
selected according to the time since the last set of updates were
performed on the delivery predictabilities, irrespective of which
nodes triggered either the previous or current updates. This means
that, in the example discussed here, P_(Z,A) and P_(Z,B) will be
updated using the same value of P_encounter whether node A or node B
initiated the update while the three nodes remain connected.
The interval between reruns of the information exchange will
generally be set to a small fraction of the expected time between
independent encounters of pairs of nodes. This ensures that, for
example, the delivery predictability information obtained by node Z
from node A will be passed on to node B whether or not nodes A and B
can communicate directly during this encounter. This avoids problems
that may arise from peculiarities of radio propagation during this
sort of encounter, but the scaling of the P_encounter factor
according to the time between updates of the delivery
predictabilities means that the predictabilities for the nodes that
are in contact are not increased excessively as would be the case if
each information exchange were treated as a separate encounter with
the value of P_encounter_max used each time. When several nodes are
in mutual contact, the delivery predictabilities in each node
stabilize after a few exchanges due to the scaling of P_encounter as
well as the form of Equation 3 where a "max" function is used. This
has been demonstrated by simulation.
The effect of the updates of the delivery predictabilities when there
are multiple simultaneous contacts is that the information about good
routes on which to forward bundles is correctly passed between sets
of nodes that are simultaneously in contact through the transitive
update of Equation 3 during each information exchange, but the
delivery predictabilities for the direct contacts are not
exaggerated.
3.3. Routing Algorithm
The basic routing algorithm of the protocol is described in
Section 2.1. The algorithm uses some parameter values in the
calculation of the delivery predictability metric. These parameters
are configurable depending on the usage scenario, but Figure 3
provides some recommended default values. A brief explanation of the
parameters and some advice on setting appropriate values is given
below.
I_typ
I_typ provides a fundamental timescale for the mobility pattern
in the PRoPHET scenario where the protocol is being applied. It
represents the typical or mean time interval between encounters
between a given pair of nodes in the normal course of mobility.
The interval should reflect the "logical" time between
encounters and should not give significant weight to multiple
connection events as explained in Section 2.1.2. This time
interval informs the settings of many of the other parameters
but is not necessarily directly used as a parameter.
Consideration needs to be given to the higher statistical
moments (e.g., standard deviation) as well as the mean (first
moment) of the distribution of intervals between encounters and
the nature of that distribution (e.g., how close to a normal
distribution it is). There is further discussion of this point
later in this section and in Appendix C.
P_encounter_max
P_encounter_max is used as the upper limit of a scaling factor
that increases the delivery predictability for a destination
when the destination node is encountered. A larger value of
P_encounter_max will increase the delivery predictability
faster, and fewer encounters will be required for the delivery
predictability to reach a certain level. Given that relative
rather than absolute delivery predictability values are what is
interesting for the forwarding mechanisms defined, the protocol
is very robust to different values of P_encounter as long as the
same value is chosen for all nodes. The value should be chosen
so that the increase in the delivery predictability resulting
from using P_encounter_max in Equation 1 more than compensates
for the decay of the delivery predictability resulting from
Equation 3 with a time interval of I_typ.
P_encounter(intvl)
As explained in Section 2.1.2, the parameter P_encounter used in
Equation 1 is a function of the time interval "intvl". The
function should be an approximation to
P_encounter(intvl) =
P_encounter_max * (intvl / I_typ) for 0<= intvl <= I_typ
P_encounter_max for intvl > I_typ
The function can be quantized and adapted to suit the mobility
pattern and to make implementation easier. The overall effect
should be that be that if Equation 1 is applied a number of
times during a long-lived communication opportunity lasting
I_typ, the overall increase in the delivery predictability
should be approximately the same as if there had been two
distinct encounters spaced I_typ apart. This second case would
result in one application of Equation 1 using P_encounter_max.
P_first_threshold
As described in Section 2.1.2, the delivery predictability for a
destination is gradually reduced over time unless increased as a
result of direct encounters or through the transitive property.
If the delivery predictability falls below the value
P_first_threshold, then the node MAY discard the delivery
predictability information for the destination and treat
subsequent encounters as if they had never encountered the node
previously. This allows the node to reduce the storage needed
for delivery predictabilities and decreases the amount of
information that has to be exchanged between nodes; otherwise,
the reduction algorithm would result in very small but non-zero
predictabilities being maintained for nodes that were last
encountered a long time ago.
P_encounter_first
As described in Section 2.1.2, PRoPHET does not, by default,
make any assumptions about the likelihood that an encountered
node will be encountered repeatedly in the future or,
alternatively, that this is a one-off chance encounter that is
unlikely to be repeated. During an encounter where the
encountering node has no delivery predictability information for
the encountered destination node, either because this is really
the first encounter between the nodes or because the previous
encounter was so long ago that the predictability had fallen
below P_first_threshold and therefore had been discarded, the
encountering node sets the delivery predictability for the
destination node to P_encounter_first. The suggested value for
P_encounter_first is 0.5: this value is RECOMMENDED as
appropriate in the usual case where PRoPHET has no extra (e.g.,
out-of-band) information about whether future encounters with
this node will be regular or otherwise.
alpha
The alpha parameter is used in the optional smoothing of the
delivery predictabilities described in Section 2.1.3.1. It is
used to determine the weight of the most current P-value in the
calculation of an EWMA.
beta
The beta parameter adjusts the weight of the transitive property
of PRoPHET, that is, how much consideration should be given to
information about destinations that is received from encountered
nodes. If beta is set to zero, the transitive property of
PRoPHET will not be active, and only direct encounters will be
used in the calculation of the delivery predictability. The
higher the value of beta, the more rapidly encounters will
increase predictabilities through the transitive rule.
gamma
The gamma parameter determines how quickly delivery
predictabilities age. A lower value of gamma will cause the
delivery predictability to age faster. The value of gamma
should be chosen according to the scenario and environment in
which the protocol will be used. If encounters are expected to
be very frequent, a lower value should be chosen for gamma than
if encounters are expected to be rare.
delta
The delta parameter sets the maximum value of the delivery
predictability for a destination other than for the node itself
(i.e., P_(A,B) for all cases except P_(A,A)) as (1 - delta).
Delta should be set to a small value to allow the maximum
possible range for predictabilities but can be configured to
make the calculation efficient if needed.
To set an appropriate gamma value, one should consider the "average
expected delivery" time I_aed in the PRoPHET zone where the protocol
is to be used, and the time unit used (the resolution with which the
delivery predictability is being updated). The I_aed time interval
can be estimated according to the average number of hops that bundles
have to pass and the average interval between encounters I_typ.
Clearly, if bundles have a Time To Live (TTL), i.e., the time left
until the expiry time stored in the bundle occurs, that is less than
I_aed, they are unlikely to survive in the network to be delivered to
a node in this PRoPHET zone. However, the TTL for bundles created in
nodes in this zone should not be chosen solely on this basis because
they may pass through other networks.
After estimating I_aed and selecting how much we want the delivery
predictability to age in one I_aed time period (call this A), we can
calculate K, the number of time units in one I_aed, using
K = (I_aed / time unit). This can then be used to calculate gamma as
gamma = K'th-root( A ).
I_typ, I_aed, K, and gamma can then be used to inform the settings of
P_encounter_first, P_encounter_max, P_first_threshold, delta, and the
detailed form of the function P_encounter(intvl).
First, considering the evolution of the delivery predictability
P_(A,B) after a single encounter between nodes A and B, P_(A,B) is
initially set to P_encounter_first and will then steadily decay until
it reaches P_first_threshold. The ratio between P_encounter_first
and P_first_threshold should be set so that P_first_threshold is
reached after a small multiple (e.g., 3 to 5) of I_aed has elapsed,
making it likely that any subsequent encounter between the nodes
would have occurred before P_(A,B) decays below P_first_threshold.
If the statistics of the distribution of times between encounters is
known, then a small multiple of the standard deviation of the
distribution would be a possible period instead of using a multiple
of I_aed.
Second, if a second encounter between A and B occurs, the setting of
P_encounter_max should be sufficiently high to reverse the decay that
would have occurred during I_typ and to increase P_(A,B) above the
value of P_encounter_first. After several further encounters,
P_(A,B) will reach (1 - delta), its upper limit. As with setting up
P_first_threshold, P_encounter_max should be set so that the upper
limit is reached after a small number of encounters spaced apart by
I_typ have occurred, but this should generally be more than 2 or 3.
Finally, beta can be chosen to give some smoothing of the influence
of transitivity.
These instructions on how to set the parameters are only given as a
possible method for selecting appropriate values, but network
operators are free to set parameters as they choose. Appendix C goes
into some more detail on linking the parameters defined here and the
more conventional ways of expressing the mobility model in terms of
distributions of times between events of various types.
Recommended starting parameter values when specific network
measurements have not been done are below. Note: There are no "one
size fits all" default values, and the ideal values vary based on
network characteristics. It is not inherently necessary for the
parameter values to be identical at all nodes, but it is recommended
that similar values are used at all nodes within a PRoPHET zone as
discussed in Section 2.1.2.1.
+========================================+
| Parameter | Recommended value |
+========================================+
| P_encounter_max | 0.7 |
+----------------------------------------+
| P_encounter_first | 0.5 |
+----------------------------------------+
| P_first_threshold | 0.1 |
+----------------------------------------+
| alpha | 0.5 |
+----------------------------------------+
| beta | 0.9 |
+----------------------------------------+
| gamma | 0.999 |
+----------------------------------------+
| delta | 0.01 |
+========================================+
Figure 3: Default parameter settings
3.4. Bundle Passing
Upon reception of the Bundle Offer TLV, the node inspects the list of
bundles and decides which bundles it is willing to store for future
forwarding or that it is able to deliver to their destinations. This
decision has to be made using local policies and considering
parameters such as available buffer space and, if the node requested
bundle lengths, the lengths of the offered bundles. For each such
acceptable bundle, the node sends a Bundle Response TLV to its
peering node, which responds by sending the requested bundle. If a
node has some bundles it would prefer to receive ahead of others
offered (e.g., bundles that it can deliver to their final
destination), it MAY request the bundles in that priority order.
This is often desirable as there is no guarantee that the nodes will
remain in contact with each other for long enough to transfer all the
acceptable bundles. Otherwise, the node SHOULD assume that the
bundles are listed in a priority order determined by the peering
node's forwarding strategy and request bundles in that order.
3.4.1. Custody
To free up local resources, a node may give custody of a bundle to
another node that offers custody. This is done to move the
retransmission requirement further toward the destination. The
concept of custody transfer, and more details on the motivation for
its use can be found in [RFC4838]. PRoPHET takes no responsibilities
for making custody decisions. Such decisions should be made by a
higher layer.
3.5. When a Bundle Reaches Its Destination
A PRoPHET ACK is only a confirmation that a bundle has been delivered
to its destination in the PRoPHET zone (within the part of the
network where PRoPHET is used for routing, bundles might traverse
several different types of networks using different routing
protocols; thus, this might not be the final destination of the
bundle). When nodes exchange Bundle Offer TLVs, bundles that have
been ACKed are also listed, having the "PRoPHET ACK" flag set. The
node that receives this list updates its own list of ACKed bundles to
be the union of its previous list and the received list. To prevent
the list of ACKed bundles growing indefinitely, each PRoPHET ACK
should have a timeout that MUST NOT be longer than the timeout of the
bundle to which the ACK corresponds.
When a node receives a PRoPHET ACK for a bundle it is carrying, it
MAY delete that bundle from its storage, unless the node holds
custody of that bundle. The PRoPHET ACK only indicates that a bundle
has been delivered to its destination within the PRoPHET zone, so the
reception of a PRoPHET ACK is not a guarantee that the bundle has
been delivered to its final destination.
Nodes MAY track to which nodes they have sent PRoPHET ACKs for
certain bundles, and MAY in that case refrain from sending multiple
PRoPHET ACKs for the same bundle to the same node.
If necessary in order to preserve system resources, nodes MAY drop
PRoPHET ACKs prematurely but SHOULD refrain from doing so if
possible.
It is important to keep in mind that PRoPHET ACKs and bundle ACKs
[RFC5050] are different things. PRoPHET ACKs are only valid within
the PRoPHET part of the network, while bundle ACKs are end-to-end
acknowledgments that may go outside of the PRoPHET zone.
3.6. Forwarding Strategies
During the Information Exchange Phase, nodes need to decide on which
bundles they wish to exchange with the peering node. Because of the
large number of scenarios and environments that PRoPHET can be used
in, and because of the wide range of devices that may be used, it is
not certain that this decision will be based on the same strategy in
every case. Therefore, each node MUST operate a _forwarding
strategy_ to make this decision. Nodes may define their own
strategies, but this section defines a few basic forwarding
strategies that nodes can use. Note: If the node being encountered
is the destination of any of the bundles being carried, those bundles
SHOULD be offered to the destination, even if that would violate the
forwarding strategy. Some of the forwarding strategies listed here
have been evaluated (together with a number of queueing policies)
through simulations, and more information about that and
recommendations on which strategies to use in different situations
can be found in [lindgren_06]. If not chosen differently due to the
characteristics of the deployment scenario, nodes SHOULD choose GRTR
as the default forwarding strategy.
The short names applied to the forwarding strategies should be read
as mnemonic handles rather than as specific acronyms for any set of
words in the specification.
We use the following notation in our descriptions below. A and B are
the nodes that encounter each other, and the strategies are described
as they would be applied by node A. The destination node is D.
P_(X,Y) denotes the delivery predictability stored at node X for
destination Y, and NF is the number of times node A has given the
bundle to some other node.
GRTR
Forward the bundle only if P_(B,D) > P_(A,D).
When two nodes meet, a bundle is sent to the other node if the
delivery predictability of the destination of the bundle is
higher at the other node. The first node does not delete the
bundle after sending it as long as there is sufficient buffer
space available (since it might encounter a better node, or even
the final destination of the bundle in the future).
GTMX
Forward the bundle only if P_(B,D) > P_(A,D) && NF < NF_max.
This strategy is like the previous one, but each bundle is given
to at most NF_max other nodes in addition to the destination.
GTHR
Forward the bundle only if
P_(B,D) > P_(A,D) OR P_(B,D) > FORW_thres,
where FORW_thres is a threshold value above which a bundle
should always be given to the node unless it is already present
at the other node.
This strategy is similar to GRTR, but among nodes with very high
delivery predictability, bundles for that particular destination
are spread epidemically.
GRTR+
Forward the bundle only if Equation 5 holds, where P_max is the
largest delivery predictability reported by a node to which the
bundle has been sent so far.
P_(B,D) > P_(A,D) && P_(B,D) > P_max (Eq. 5)
This strategy is like GRTR, but each node forwarding a bundle
keeps track of the largest delivery predictability of any node
it has forwarded this bundle to, and only forwards the bundle
again if the currently encountered node has a greater delivery
predictability than the maximum previously encountered.
GTMX+
Forward the bundle only if Equation 6 holds.
P_(B,D) > P_(A,D) && P_(B,D) > P_max && NF < NF_max (Eq. 6)
This strategy is like GTMX, but nodes keep track of P_max as in
GRTR+.
GRTRSort
Select bundles in descending order of the value of
P_(B,D) - P_(A,D).
Forward the bundle only if P_(B,D) > P_(A,D).
This strategy is like GRTR, but instead of just going through
the bundle queue linearly, this strategy looks at the difference
in delivery predictabilities for each bundle between the two
nodes and forwards the bundles with the largest difference
first. As bandwidth limitations or disrupted connections may
result in not all bundles that would be desirable being
exchanged, it could be desirable to first send bundles that get
a large improvement in delivery predictability.
GRTRMax
Select bundles in descending order of P_(B,D).
Forward the bundle only if P_(B,D) > P_(A,D).
This strategy begins by considering the bundles for which the
encountered node has the highest delivery predictability. The
motivation for doing this is the same as in GRTRSort, but based
on the idea that it is better to give bundles to nodes with high
absolute delivery predictabilities, instead of trying to
maximize the improvement.
3.7. Queueing Policies
Because of limited buffer resources, nodes may need to drop some
bundles. As is the case with the forwarding strategies, which bundle
to drop is also dependent on the scenario. Therefore, each node MUST
also operate a queueing policy that determines how its bundle queue
is handled. This section defines a few basic queueing policies, but
nodes MAY use other policies if desired. Some of the queueing
policies listed here have been evaluated (together with a number of
forwarding strategies) through simulations. More information about
that and recommendations on which policies to use in different
situations can be found in [lindgren_06]. If not chosen differently
due to the characteristics of the deployment scenario, nodes SHOULD
choose FIFO as the default queueing policy.
The short names applied to the queueing policies should be read as
mnemonic handles rather than as specific acronyms for any set of
words in the specification.
FIFO - First In First Out.
The bundle that was first entered into the queue is the first
bundle to be dropped.
MOFO - Evict most forwarded first.
In an attempt to maximize the delivery rate of bundles, this
policy requires that the routing agent keep track of the number
of times each bundle has been forwarded to some other node. The
bundle that has been forwarded the largest number of times is
the first to be dropped.
MOPR - Evict most favorably forwarded first.
Keep a variable FAV for each bundle in the queue, initialized to
zero. Each time the bundle is forwarded, update FAV according
to Equation 7, where P is the predictability metric that the
node the bundle is forwarded to has for its destination.
FAV_new = FAV_old + ( 1 - FAV_old ) * P (Eq. 7)
The bundle with the highest FAV value is the first to be
dropped.
Linear MOPR - Evict most favorably forwarded first; linear increase.
Keep a variable FAV for each bundle in the queue, initialized to
zero. Each time the bundle is forwarded, update FAV according
to Equation 8, where P is the predictability metric that the
node the bundle is forwarded to has for its destination.
FAV_new = FAV_old + P (Eq. 8)
The bundle with the highest FAV value is the first to be
dropped.
SHLI - Evict shortest life time first.
As described in [RFC5050], each bundle has a timeout value
specifying when it no longer is meaningful to its application
and should be deleted. Since bundles with short remaining Time
To Live will soon be dropped anyway, this policy decides to drop
the bundle with the shortest remaining lifetime first. To
successfully use a policy like this, there needs to be some form
of time synchronization between nodes so that it is possible to
know the exact lifetimes of bundles. However, this is not
specific to this routing protocol, but a more general DTN
problem.
LEPR - Evict least probable first.
Since the node is least likely to deliver a bundle for which it
has a low delivery predictability, drop the bundle for which the
node has the lowest delivery predictability, and that has been
forwarded at least MF times, where MF is a minimum number of
forwards that a bundle must have been forwarded before being
dropped (if such a bundle exists).
More than one queueing policy MAY be combined in an ordered set,
where the first policy is used primarily, the second only being used
if there is a need to tie-break between bundles given the same
eviction priority by the primary policy, and so on. As an example,
one could select the queueing policy to be {MOFO; SHLI; FIFO}, which
would start by dropping the bundle that has been forwarded the
largest number of times. If more than one bundle has been forwarded
the same number of times, the one with the shortest remaining
lifetime will be dropped, and if that also is the same, the FIFO
policy will be used to drop the bundle first received.
It is worth noting that a node MUST NOT drop bundles for which it has
custody unless the bundle's lifetime expires.
4. Message Formats
This section defines the message formats of the PRoPHET routing
protocol. In order to allow for variable-length fields, many numeric
fields are encoded as Self-Delimiting Numeric Values (SDNVs). The
format of SDNVs is defined in [RFC5050]. Since many of the fields
are coded as SDNVs, the size and alignment of fields indicated in
many of the specification diagrams below are indicative rather than
prescriptive. Where SDNVs and/or text strings are used, the octets
of the fields will be packed as closely as possible with no
intervening padding between fields.
Explicit-length fields are specified for all variable-length string
fields. Accordingly, strings are not null terminated and just
contain the exact set of octets in the string.
The basic message format shown in Figure 4 consists of a header (see
Section 4.1) followed by a sequence of one or more Type-Length-Value
components (TLVs) taken from the specifications in Section 4.2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Header ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TLV 1 ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
~ . ~
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TLV n ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Basic PRoPHET Message Format
4.1. Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol Number|Version| Flags | Result | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Instance | Sender Instance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transaction Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| SubMessage Number | Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Message Body ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: PRoPHET Message Header
Protocol Number
The DTN Routing Protocol Number encoded as 8-bit unsigned
integer in network bit order. The value of this field is 0.
The PRoPHET header is organized in this way so that in principle
PRoPHET messages could be sent as the Protocol Data Unit of an
IP packet if an IP protocol number was allocated for PRoPHET.
At present, PRoPHET is only specified to use a TCP transport for
carriage of PRoPHET packets, so that the protocol number serves
only to identify the PRoPHET protocol within DTN. Transmitting
PRoPHET packets directly as an IP protocol on a public IP
network such as the Internet would generally not work well
because middleboxes (such as firewalls and NAT boxes) would be
unlikely to allow the protocol to pass through, and the protocol
does not provide any congestion control. However, it could be
so used on private networks for experimentation or in situations
where all communications are between isolated pairs of nodes.
Also, in the future, other protocols that require transmission
of metadata between DTN nodes could potentially use the same
format and protocol state machinery but with a different
Protocol Number.
Version
The version of the PRoPHET Protocol. Encoded as a 4-bit
unsigned integer in network bit order. This document defines
version 2.
Flags
Reserved field of 4 bits.
Result
Field that is used to indicate whether a response is required to
the request message if the outcome is successful. A value of
"NoSuccessAck" indicates that the request message does not
expect a response if the outcome is successful, and a value of
"AckAll" indicates that a response is expected if the outcome is
successful. In both cases, a failure response MUST be generated
if the request fails. If running over a TCP transport or
similar protocol that offers reliable in order delivery,
deployments MAY choose not to send "Success" responses when an
outcome is successful. To achieve this, the Result field is set
to the "NoSuccessAck" value in all request messages.
In a response message, the result field can have two values:
"Success" and "Failure". The "Success" result indicates a
success response. All messages that belong to the same success
response will have the same Transaction Identifier. The
"Success" result indicates a success response that may be
contained in a single message or the final message of a success
response spanning multiple messages.
ReturnReceipt is a value of the result field used to indicate
that an acknowledgement is required for the message. The
default for messages is that the controller will not acknowledge
responses. In the case where an acknowledgement is required, it
will set the Result Field to ReturnReceipt in the header of the
Message.
The result field is encoded as an 8-bit unsigned integer in
network bit order. The following values are currently defined:
NoSuccessAck: Result = 1
AckAll: Result = 2
Success: Result = 3
Failure: Result = 4
ReturnReceipt Result = 5
Code
This field gives further information concerning the result in a
response message. It is mostly used to pass an error code in a
failure response but can also be used to give further
information in a success response message or an event message.
In a request message, the code field is not used and is set to
zero.
If the Code field indicates that the Error TLV is included in
the message, further information on the error will be found in
the Error TLV, which MUST be the first TLV after the header.
The Code field is encoded as an 8-bit unsigned integer in
network bit order. Separate number code spaces are used for
success and failure response messages. In each case, a range of
values is reserved for use in specifications and another range
for private and experimental use. For success messages, the
following values are defined:
Generic Success 0x00
Submessage Received 0x01
Unassigned 0x02 - 0x7F
Private/Experimental Use 0x80 - 0xFF
The Submessage Received code is used to acknowledge reception of
a message segment. The Generic Success code is used to
acknowledge receipt of a complete message and successful
processing of the contents.
For failure messages, the following values are defined:
Reserved 0x00 - 0x01
Unspecified Failure 0x02
Unassigned 0x03 - 0x7F
Private/Experimental Use 0x80 - 0xFE
Error TLV in message 0xFF
The Unspecified Failure code can be used to report a failure for
which there is no more specific code or Error TLV value defined.
Sender Instance
For messages during the Hello phase with the Hello SYN, Hello
SYNACK, and Hello ACK functions (which are explained in
Section 5.2), it is the sender's instance number for the link.
It is used to detect when the link comes back up after going
down or when the identity of the entity at the other end of the
link changes. The instance number is a 16-bit number that is
guaranteed to be unique within the recent past and to change
when the link or node comes back up after going down. Zero is
not a valid instance number. For the RSTACK function (also
explained in detail in Section 5.2), the Sender Instance field
is set to the value of the Receiver Instance field from the
incoming message that caused the RSTACK function to be
generated. Messages sent after the Hello phase is completed
should use the sender's instance number for the link. The
Sender Instance is encoded as a 16-bit unsigned integer in
network bit order.
Receiver Instance
For messages during the Hello phase with the Hello SYN, Hello
SYNACK, and Hello ACK functions, it is what the sender believes
is the current instance number for the link, allocated by the
entity at the far end of the link. If the sender of the message
does not know the current instance number at the far end of the
link, this field MUST be set to zero. For the RSTACK message,
the Receiver Instance field is set to the value of the Sender
Instance field from the incoming message that caused the RSTACK
message to be generated. Messages sent after the Hello phase is
completed should use what the sender believes is the current
instance number for the link, allocated by the entity at the far
end of the link. The Sender Instance is encoded as a 16-bit
unsigned integer in network bit order.
Transaction Identifier
Used to associate a message with its response message. This
should be set in request messages to a value that is unique for
the sending host within the recent past. Reply messages contain
the Transaction Identifier of the request to which they are
responding. The Transaction Identifier is a bit pattern of 32
bits.
S-flag
If S is set (value 1), then the SubMessage Number field
indicates the total number of SubMessage segments that compose
the entire message. If it is not set (value 0), then the
SubMessage Number field indicates the sequence number of this
SubMessage segment within the whole message. The S field will
only be set in the first submessage of a sequence.
SubMessage Number
When a message is segmented because it exceeds the MTU of the
link layer or otherwise, each segment will include a SubMessage
Number to indicate its position. Alternatively, if it is the
first submessage in a sequence of submessages, the S-flag will
be set, and this field will contain the total count of
SubMessage segments. The SubMessage Number is encoded as a
15-bit unsigned integer in network bit order. The SubMessage
number is zero-based, i.e., for a message divided into n
submessages, they are numbered from 0 to (n - 1). For a message
that is not divided into submessages, the single message has the
S-flag cleared (value 0), and the SubMessage Number is set to 0
(zero).
Length
Length in octets of this message including headers and message
body. If the message is fragmented, this field contains the
length of this SubMessage. The Length is encoded as an SDNV.
Message Body
As specified in Section 4, the Message Body consists of a
sequence of one or more of the TLVs specified in Section 4.2.
The protocol also requires extra information about the link that the
underlying communication layer MUST provide. This information is
used in the Hello procedure described in more detail in Section 5.2.
Since this information is available from the underlying layer, there
is no need to carry it in PRoPHET messages. The following values are
defined to be provided by the underlying layer:
Sender Local Address
An address that is used by the underlying communication layer as
described in Section 2.4 and identifies the sender address of
the current message. This address must be unique among the
nodes that can currently communicate, and it is only used in
conjunction with the Receiver Local Address, Receiver Instance,
and Sender Instance to identify a communicating pair of nodes.
Receiver Local Address
An address that is used by the underlying communication layer as
described in Section 2.4 and identifies the receiver address of
the current message. This address must be unique among the
nodes that can currently communicate, and is only used in
conjunction with the Sender Local Address, Receiver Instance,
and Sender Instance to identify a communicating pair of nodes.
When PRoPHET is run over TCP, the IP addresses of the communicating
nodes are used as Sender and Receiver Local Addresses.
4.2. TLV Structure
All TLVs have the following format, and can be nested.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | TLV Flags | TLV Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TLV Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: TLV Format
TLV Type
Specific TLVs are defined in Section 4.3. The TLV Type is
encoded as an 8-bit unsigned integer in network bit order. Each
TLV will have fields defined that are specific to the function
of that TLV.
TLV Flags
These are defined per TLV type. Flag n corresponds to bit 15-n
in the TLV. Any flags that are specified as reserved in
specific TLVs SHOULD be transmitted as 0 and ignored on receipt.
TLV Length
Length of the TLV in octets, including the TLV header and any
nested TLVs. Encoded as an SDNV. Note that TLVs are not padded
to any specific alignment unless explicitly required in the
description of the TLV. No TLVs in this document specify any
padding.
4.3. TLVs
This section describes the various TLVs that can be used in PRoPHET
messages.
4.3.1. Hello TLV
The Hello TLV is used to set up and maintain a link between two
PRoPHET nodes. Hello messages with the SYN function are transmitted
periodically as beacons or keep-alives. The Hello TLV is the first
TLV exchanged between two PRoPHET nodes when they encounter each
other. No other TLVs can be exchanged until the first Hello sequence
is completed.
Once a communication link is established between two PRoPHET nodes,
the Hello TLV will be sent once for each interval as defined in the
interval timer. If a node experiences the lapse of HELLO_DEAD Hello
intervals without receiving a Hello TLV on a connection in the
INFO_EXCH state (as defined in the state machine in Section 5.1), the
connection SHOULD be assumed broken.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type=0x01 |L| Resv | HF | TLV Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timer (SDNV) |EID Length,SDNV| Sender EID (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Hello TLV Format
TLV Flags
The TLV Flags field contains two 1-bit flags (S and L) and a
3-bit Hello Function (HF) number that specifies one of four
functions for the Hello TLV. The remaining 3 bits (Resv) are
unused and reserved:
HF
TLV Flags bits 0, 1, and 2 are treated as an unsigned 3-bit
integer coded in network bit order. The value of the
integer specifies the Hello Function (HF) of the Hello TLV.
Four functions are specified for the Hello TLV.
The encoding of the Hello Function is:
SYN: HF = 1
SYNACK: HF = 2
ACK: HF = 3
RSTACK: HF = 4
The remaining values (0, 5, 6 and 7) are unused and reserved. If a
Hello TLV with any of these values is received, the link should be
reset.
Resv
TLV Flags bits 3, 4, 5, and 6 are reserved. They SHOULD be
set to 0 on transmission and ignored on reception.
L
The L bit flag (TLV Flags bit 7) is set (value 1) to
request that the Bundle Offer TLV sent during the
Information Exchange Phase contains bundle payload lengths
for all bundles, rather than only for bundle fragments as
when the L flag is cleared (value 0), when carried in a
Hello TLV with Hello Function SYN or SYNACK. The flag is
ignored for other Hello Function values.
TLV Data
Timer
The Timer field is used to inform the receiver of the timer
value used in the Hello processing of the sender. The
timer specifies the nominal time between periodic Hello
messages. It is a constant for the duration of a session.
The timer field is specified in units of 100 ms and is
encoded as an SDNV.
EID Length
The EID Length field is used to specify the length of the
Sender EID field in octets. If the Endpoint Identifier
(EID) has already been sent at least once in a message with
the current Sender Instance, a node MAY choose to set this
field to zero, omitting the Sender EID from the Hello TLV.
The EID Length is encoded as an SDNV, and the field is thus
of variable length.
Sender EID
The Sender EID field specifies the DTN endpoint identifier
(EID) of the sender that is to be used in updating routing
information and making forwarding decisions. If a node has
multiple EIDs, one should be chosen for PRoPHET routing.
This field is of variable length.
4.3.2. Error TLV
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV type=0x02 | TLV Flags | TLV Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TLV Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Error TLV Format
TLV Flags
For Error TLVs, the TLV Flags field carries an identifier for
the Error TLV type as an 8-bit unsigned integer encoded in
network bit order. A range of values is available for private
and experimental use in addition to the values defined here.
The following Error TLV types are defined:
Dictionary Conflict 0x00
Bad String ID 0x01
Reserved 0x02 - 0x7F
Private/Experimental Use 0x80 - 0xFF
TLV Data
The contents and interpretation of the TLV Data field are
specific to the type of Error TLV. For the Error TLVs defined
in this document, the TLV Data is defined as follows:
Dictionary Conflict
The TLV Data consists of the String ID that is causing the
conflict encoded as an SDNV followed by the EID string that
conflicts with the previously installed value. The
Endpoint Identifier is NOT null terminated. The length of
the EID can be determined by subtracting the length of the
TLV Header and the length of the SDNV containing the String
ID from the TLV Length.
Bad String ID
The TLV Data consists of the String ID that is not found in
the dictionary encoded as an SDNV.
4.3.3. Routing Information Base Dictionary TLV
The Routing Information Base Dictionary includes the list of endpoint
identifiers used in making routing decisions. The referents remain
constant for the duration of a session over a link where the instance
numbers remain the same and can be used by both the Routing
Information Base messages and the bundle offer/response messages.
The dictionary is a shared resource (see Section 3.2.1) built in each
of the paired peers from the contents of one or more incoming TLVs of
this type and from the information used to create outgoing TLVs of
this type.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV type=0xA0 | TLV Flags | TLV Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIBD Entry Count (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Variable-Length Routing Address Strings ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Routing Address String 1 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| String ID 1 (SDNV) | Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Endpoint Identifier 1 (variable length) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
~ Routing Address String n . ~
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| String ID n (SDNV) | Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Endpoint Identifier n (variable length) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Routing Information Base Dictionary TLV Format
TLV Flags
The encoding of the Header flag field relates to the
capabilities of the source node sending the RIB Dictionary:
Flag 0: Sent by Listener 0b1
Flag 1: Reserved 0b1
Flag 2: Reserved 0b1
Flag 3: Unassigned 0b1
Flag 4: Unassigned 0b1
Flag 5: Unassigned 0b1
Flag 6: Unassigned 0b1
Flag 7: Unassigned 0b1
The "Sent by Listener" flag is set to 0 if this TLV was sent by
a node in the Initiator role and set to 1 if this TLV was sent
by a node in the Listener role (see Section 3.2 for explanations
of these roles).
TLV Data
RIBD Entry Count
Number of entries in the database. Encoded as SDNV.
String ID
SDNV identifier that is constant for the duration of a
session. String ID zero is predefined as the node that
initiates the session through sending the Hello SYN
message, and String ID one is predefined as the node that
responds with the Hello SYNACK message. These entries do
not need to be sent explicitly as the EIDs are exchanged
during the Hello procedure.
In order to ensure that the String IDs originated by the
two peers do not conflict, the String IDs generated in the
node that sent the Hello SYN message MUST have their least
significant bit set to 0 (i.e., are even numbers), and the
String IDs generated in the node that responded with the
Hello SYNACK message MUST have their least significant bit
set to 1 (i.e., they are odd numbers).
Length
Length of Endpoint Identifier in this entry. Encoded as
SDNV.
Endpoint Identifier
Text string representing the Endpoint Identifier. Note
that it is NOT null terminated as the entry contains the
length of the identifier.
4.3.4. Routing Information Base TLV
The Routing Information Base lists the destinations (endpoints) a
node knows of and the delivery predictabilities it has associated
with them. This information is needed by the PRoPHET algorithm to
make decisions on routing and forwarding.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type=0xA1 | TLV Flags | TLV Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIB String Count (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIBD String ID 1 (SDNV) | P-value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIB Flags 1 | . ~
+-+-+-+-+-+-+-+-+ . ~
~ . ~
~ . ~
~ . ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIBD String ID n (SDNV) | P-value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIB Flags n |
+-+-+-+-+-+-+-+-+
Figure 10: Routing Information Base TLV Format
TLV Flags
The encoding of the Header flag field relates to the
capabilities of the Source node sending the RIB:
Flag 0: More RIB TLVs 0b1
Flag 1: Reserved 0b1
Flag 2: Reserved 0b1
Flag 3: Unassigned 0b1
Flag 4: Unassigned 0b1
Flag 5: Unassigned 0b1
Flag 6: Unassigned 0b1
Flag 7: Unassigned 0b1
The "More RIB TLVs" flag is set to 1 if the RIB requires more
TLVs to be sent in order to be fully transferred. This flag is
set to 0 if this is the final TLV of this RIB.
TLV Data
RIB String Count
Number of routing entries in the TLV. Encoded as an SDNV.
RIBD String ID
String ID of the endpoint identifier of the destination for
which this entry specifies the delivery predictability as
predefined in a dictionary TLV. Encoded as an SDNV.
P-value
Delivery predictability for the destination of this entry
as calculated from previous encounters according to the
equations in Section 2.1.2, encoded as a 16-bit unsigned
integer. The encoding of this field is a linear mapping
from [0,1] to [0, 0xFFFF] (e.g., for a P-value of 0.75, the
mapping would be 0.75*65535=49151=0xBFFF; thus, the P-value
would be encoded as 0xBFFF).
RIB Flag
The encoding of the 8-bit RIB Flag field is:
Flag 0: Unassigned 0b1
Flag 1: Unassigned 0b1
Flag 2: Unassigned 0b1
Flag 3: Unassigned 0b1
Flag 4: Unassigned 0b1
Flag 5: Unassigned 0b1
Flag 6: Unassigned 0b1
Flag 7: Unassigned 0b1
4.3.5. Bundle Offer and Response TLVs (Version 2)
After the routing information has been passed, the node will ask the
other node to review available bundles and determine which bundles it
will accept for relay. The source relay will determine which bundles
to offer based on relative delivery predictabilities as explained in
Section 3.6.
Note: The original versions of these TLVs (TLV Types 0xA2 and
0xA3) used in version 1 of the PRoPHET protocol have been
deprecated, as they did not contain the complete information
needed to uniquely identify bundles and could not handle bundle
fragments.
Depending on the bundles stored in the offering node, the Bundle
Offer TLV might contain descriptions of both complete bundles and
bundle fragments. In order to correctly identify bundle fragments, a
bundle fragment descriptor MUST contain the offset of the payload
fragment in the bundle payload and the length of the payload
fragment. If requested by the receiving node by setting the L flag
in the SYN or SYNACK message during the neighbor awareness phase, the
offering node MUST include the length of the payload in the
descriptor for complete bundles. The appropriate flags MUST be set
in the B_flags for the descriptor to indicate if the descriptor
contains the payload length field (set for fragments in all cases and
for complete bundles if the L flag was set) and if the descriptor
contains a payload offset field (fragments only).
The Bundle Offer TLV also lists the bundles for which a PRoPHET
acknowledgement has been issued. Those bundles have the PRoPHET ACK
flag set in their entry in the list. When a node receives a PRoPHET
ACK for a bundle, it SHOULD, if possible, signal to the bundle
protocol agent that this bundle is no longer required for
transmission by PRoPHET. Despite no longer transmitting the bundle,
it SHOULD keep an entry for the acknowledged bundle to be able to
further propagate the PRoPHET ACK.
The Response TLV format is identical to the Offer TLV with the
exception of the TLV Type field. Bundles that are being accepted
from the corresponding Offer are explicitly marked with a B_flag.
Specifications for bundles that are not being accepted MAY either be
omitted or left in but not marked as accepted. The payload length
field MAY be omitted for complete bundles in the Response message
even if it was included in the Offer message. The B_flags payload
length flag MUST be set correctly to indicate if the length field is
included or not. The Response message MUST include both payload
offset and payload length fields for bundle fragments, and the
B_flags MUST be set to indicate that both are present.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | TLV Flags | TLV Length (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle Offer Count (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| B_flags | Bundle Source | Bundle Destination |
| | String ID 1 (SDNV) | String ID 1 (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle 1 Creation Timestamp Time |
| (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle 1 Creation Timestamp Sequence Number |
| (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle 1 Payload Offset - only present if bundle is a fragment|
| (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle 1 Payload Length - only present if bundle is a fragment|
| or transmission of length requested (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ . ~
~ . ~
~ . ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| B_flags | Bundle Source | Bundle Destination |
| | String ID n (SDNV) | String ID n (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle n Creation Timestamp Time |
| (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle n Creation Timestamp Sequence Number |
| (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle n Payload Offset - only present if bundle is a fragment|
| (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle n Payload Length - only present if bundle is a fragment|
| or transmission of length requested (SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Bundle Offer and Response TLV Format
TLV Type
The TLV Type for a Bundle Offer is 0xA4. The TLV Type for a
Bundle Response is 0xA5.
TLV Flags
The encoding of the Header flag field relates to the
capabilities of the source node sending the RIB:
Flag 0: More Offer/Response
TLVs Following 0b1
Flag 1: Unassigned 0b1
Flag 2: Unassigned 0b1
Flag 3: Unassigned 0b1
Flag 4: Unassigned 0b1
Flag 5: Unassigned 0b1
Flag 6: Unassigned 0b1
Flag 7: Unassigned 0b1
If the Bundle Offers or Bundle Responses are divided between
several TLVs, the "More Offer/Response TLVs Following" flag MUST
be set to 1 in all but the last TLV in the sequence where it
MUST be set to 0.
TLV Data
Bundle Offer Count
Number of bundle offer/response entries. Encoded as an
SDNV. Note that 0 is an acceptable value. In particular,
a Bundle Response TLV with 0 entries is used to signal that
a cycle of information exchange and bundle passing is
completed.
B Flags
The encoding of the B Flags is:
Flag 0: Bundle Accepted 0b1
Flag 1: Bundle is a Fragment 0b1
Flag 2: Bundle Payload Length
included in TLV 0b1
Flag 3: Unassigned 0b1
Flag 4: Unassigned 0b1
Flag 5: Unassigned 0b1
Flag 6: Unassigned 0b1
Flag 7: PRoPHET ACK 0b1
Bundle Source String ID
String ID of the source EID of the bundle as predefined in
a dictionary TLV. Encoded as an SDNV.
Bundle Destination String ID
String ID of the destination EID of the bundle as
predefined in a dictionary TLV. Encoded as an SDNV.
Bundle Creation Timestamp Time
Time component of the Bundle Creation Timestamp for the
bundle. Encoded as an SDNV.
Bundle Creation Timestamp Sequence Number
Sequence Number component of the Bundle Creation Timestamp
for the bundle. Encoded as an SDNV.
Bundle Payload Offset
Only included if the bundle is a fragment and the fragment
bit is set (value 1) in the bundle B Flags. Offset of the
start of the fragment payload in the complete bundle
payload. Encoded as an SDNV.
Bundle Payload Length
Only included if the bundle length included bit is set
(value 1) in the bundle B Flags. Length of the payload in
the bundle specified. This is either the total payload
length if the bundle is a complete bundle or the bundle
fragment payload length if the bundle is a fragment.
Encoded as an SDNV.
5. Detailed Operation
In this section, some more details on the operation of PRoPHET are
given along with state tables to help in implementing the protocol.
As explained in Section 1.2, it is RECOMMENDED that "Success"
responses should not be requested or sent when operating over a
reliable, in-order transport protocol such as TCP. If in the future
PRoPHET were operated over an unreliable transport protocol, positive
acknowledgements would be necessary to signal successful delivery of
(sub)messages. In this section, the phrase "send a message" should
be read as *successful* sending of a message, signaled by receipt of
the appropriate "Success" response if running over an unreliable
protocol, but guaranteed by TCP or another reliable protocol
otherwise. Hence, the state descriptions below do not explicitly
mention positive acknowledgements, whether they are being sent or
not.
5.1. High-Level State Tables
This section gives high-level state tables for the operation of
PRoPHET. The following sections will describe each part of the
operation in more detail (including state tables for the internal
states of those procedures).
The following main or high-level states are used in the state tables:
WAIT_NB This is the state all nodes start in. Nodes remain in this
state until they are notified that a new neighbor is available.
At that point, the Hello procedure should be started with the
new neighbor, and the node transitions into the HELLO state.
Nodes SHOULD be able to handle multiple neighbors in parallel,
maintaining separate state machines for each neighbor. This
could be handled by creating a new thread or process during the
transition to the HELLO state that then takes care of the
communication with the new neighbor while the parent remains in
state WAIT_NB waiting for additional neighbors to communicate.
In this case, when the neighbor can no longer be communicated
with (described as "Neighbor Gone" in the tables below), the
thread or process created is destroyed and, when a connection-
oriented protocol is being used to communicate with the
neighbor, the connection is closed. The current version of the
protocol is specified to use TCP for neighbor connections so
that these will be closed when the neighbor is no longer
accessible.
HELLO Nodes are in the HELLO state from when a new neighbor is
detected until the Hello procedure is completed and a link is
established (which happens when the Hello procedure enters the
ESTAB state as described in Section 5.2; during this procedure,
the states ESTAB, SYNSENT, and SYNRCVD will be used, but these
are internal to the Hello procedure and are not listed here).
If the node is notified that the neighbor is no longer in range
before a link has been established, it returns to the WAIT_NB
state, and, if appropriate, any additional process or thread
created to handle the neighbor MAY be destroyed.
INFO_EXCH After a link has been set up by the Hello procedure, the
node transitions to the INFO_EXCH state in which the
Information Exchange Phase is done. The node remains in this
state as long as Information Exchange Phase TLVs (Routing RIB,
Routing RIB Dictionary, Bundle Offer, Bundle Response) are
being received. If the node is notified that the neighbor is
no longer in range before all information and bundles have been
exchanged, any associated connection is closed and the node
returns to the WAIT_NB state to await new neighbors. The
Timer(keep_alive) is used to ensure that the connection remains
active.
In the INFO_EXCH state, the nodes at both ends of the
established link are able to update their delivery
predictability information using data from the connected peer
and then make offers of bundles for exchange which may be
accepted or not by the peer. To manage these processes, each
node acts both as an Initiator and a Listener for the
Information Exchange Phase processes, maintaining subsidiary
state machines for the two roles. The Initiator and Listener
terms refer to the sending of the Routing RIB information: it
is perhaps counterintuitive that the Listener becomes the
bundle offeror and the Initiator the bundle acceptor during the
bundling passing part.
The protocol is designed so that the two exchanges MAY be
carried out independently but concurrently, with the messages
multiplexed onto on a single bidirectional link (such as is
provided by the TCP connection). Alternatively, the exchanges
MAY be carried out partially or wholly sequentially if
appropriate for the implementation. The Information Exchange
Phase is explained in more detail in Section 3.2.
When an empty Bundle Response TLV (i.e., no more bundles to
send) is received, the node starts the Timer(next_exchange).
When this timer expires, assuming that the neighbor is still
connected, the Initiator reruns the Information Exchange Phase.
If there is only one neighbor connected at this time, this will
have the effect of further increasing the delivery
predictability for this node in the neighbor, and changing the
delivery predictabilities as a result of the transitive
property (Equation 3). If there is more than one neighbor
connected or other communication opportunities have happened
since the previous information exchange occurred, then the
changes resulting from these other encounters will be passed on
to the connected neighbor. The next_exchange timer is
restarted once the information exchange has completed again.
If one or more new bundles are received by this node while
waiting for the Timer(next_exchange) to expire and the delivery
predictabilities indicate that it would be appropriate to
forward some or all of the bundles to the connected node, the
bundles SHOULD be immediately offered to the connected neighbor
and transferred if accepted.
State: WAIT_NB
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| New Neighbor | Start Hello procedure for neighbor| HELLO |
| | Keep waiting for more neighbors | WAIT_NB |
+==================================================================+
State: HELLO
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| Hello TLV rcvd | | HELLO |
+------------------+-----------------------------------+-----------+
| Enter ESTAB state| Start Information Exchange Phase | INFO_EXCH |
+------------------+-----------------------------------+-----------+
| Neighbor Gone | | WAIT_NB |
+==================================================================+
State: INFO_EXCH
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| On entry | Start Timer(keep-alive) | |
| | Uses Hello Timer interval | INFO_EXCH |
+------------------+-----------------------------------+-----------+
|Info Exch TLV rcvd| (processed by subsidiary state | |
| | machines) | INFO_EXCH |
+------------------+-----------------------------------+-----------+
| No more bundles | Start Timer(next_exchange) | INFO_EXCH |
+------------------+-----------------------------------+-----------+
| Keep-alive expiry| Send Hello SYN message | INFO_EXCH |
+------------------+-----------------------------------+-----------+
| Hello SYN rcvd | Record reception | |
| | Restart Timer(keep-alive) | INFO_EXCH |
+------------------+-----------------------------------+-----------+
| Neighbor Gone | | WAIT_NB |
+==================================================================+
The keep-alive messages (messages with Hello SYN TLV) are processed
by the high-level state machine in the INFO_EXCH state. All other
messages are delegated to the subsidiary state machines of the
Information Exchange Phase described in Section 5.3. The receipt of
keep-alive messages is recorded and may be used by the subsidiary
machines to check if the peer is still functioning. The connection
will be aborted (as described in Section 4.3.1) if several keep-alive
messages are not received.
5.2. Hello Procedure
The Hello procedure is described by the following rules and state
tables. In this section, the messages sent consist of the PRoPHET
header and a single Hello TLV (see Figure 4 and Section 4.3.1) with
the HF (Hello Function) field set to the specified value (SYN,
SYNACK, ACK or RSTACK).
The state of the L flag in the latest SYN or SYNACK message is
recorded in the node that receives the message. If the L flag is set
(value 1), the receiving node MUST send the payload length for each
bundle that it offers to the peer during the Information Exchange
Phase.
The rules and state tables use the following operations:
o The "Update Peer Verifier" operation is defined as storing the
values of the Sender Instance and Sender Local Address fields from
a Hello SYN or Hello SYNACK function message received from the
entity at the far end of the link.
o The procedure "Reset the link" is defined as:
When using TCP or other reliable connection-oriented transport:
Close the connection and terminate any separate thread or
process managing the connection.
Otherwise:
1. Generate a new instance number for the link.
2. Delete the peer verifier (set to zero the values of
Sender Instance and Sender Local Address previously
stored by the Update Peer Verifier operation).
3. Send a SYN message.
4. Transition to the SYNSENT state.
o The state tables use the following Boolean terms and operators:
A The Sender Instance in the incoming message matches the value
stored from a previous message by the "Update Peer Verifier"
operation.
B The Sender Instance and Sender Local Address fields in the
incoming message match the values stored from a previous
message by the "Update Peer Verifier" operation.
C The Receiver Instance and Receiver Local Address fields in
the incoming message match the values of the Sender Instance
and Sender Local Address used in outgoing Hello SYN, Hello
SYNACK, and Hello ACK messages.
SYN A Hello SYN message has been received.
SYNACK A Hello SYNACK message has been received.
ACK A Hello ACK message has been received.
&& Represents the logical AND operation
|| Represents the logical OR operation
! Represents the logical negation (NOT) operation.
o A timer is required for the periodic generation of Hello SYN,
Hello SYNACK, and Hello ACK messages. The value of the timer is
announced in the Timer field. To avoid synchronization effects,
uniformly distributed random jitter of +/-5% of the Timer field
SHOULD be added to the actual interval used for the timer.
There are two independent events: the timer expires, and a packet
arrives. The processing rules for these events are:
Timer Expires: Reset Timer
If state = SYNSENT Send SYN message
If state = SYNRCVD Send SYNACK message
If state = ESTAB Send ACK message
Packet Arrives:
If incoming message is an RSTACK message:
If (A && C && !SYNSENT) Reset the link
Else discard the message.
If incoming message is a SYN, SYNACK, or ACK message:
Response defined by the following State Tables.
If incoming message is any other PRoPHET TLV and
state != ESTAB:
Discard incoming message.
If state = SYNSENT Send SYN message(Note 1)
If state = SYNRCVD Send SYNACK message(Note 1)
Note 1: No more than two SYN or SYNACK messages should be
sent within any time period of length defined by the timer.
o A connection across a link is considered to be achieved when the
protocol reaches the ESTAB state. All TLVs, other than Hello
TLVs, that are received before synchronization is achieved will be
discarded.
5.2.1. Hello Procedure State Tables
State: SYNSENT
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| SYNACK && C | Update Peer Verifier; | ESTAB |
| | Send ACK message | |
+------------------+-----------------------------------+-----------+
| SYNACK && !C | Send RSTACK message | SYNSENT |
+------------------+-----------------------------------+-----------+
| SYN | Update Peer Verifier; | SYNRCVD |
| | Send SYNACK message | |
+------------------+-----------------------------------+-----------+
| ACK | Send RSTACK message | SYNSENT |
+==================================================================+
State: SYNRCVD
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| SYNACK && C | Update Peer Verifier; | ESTAB |
| | Send ACK message | |
+------------------+-----------------------------------+-----------+
| SYNACK && !C | Send RSTACK message | SYNRCVD |
+------------------+-----------------------------------+-----------+
| SYN | Update Peer Verifier; | SYNRCVD |
| | Send SYNACK message | |
+------------------+-----------------------------------+-----------+
| ACK && B && C | Send ACK message | ESTAB |
+------------------+-----------------------------------+-----------+
| ACK && !(B && C) | Send RSTACK message | SYNRCVD |
+==================================================================+
State: ESTAB
+==================================================================+
| Condition | Action | New State |
+=================+====================================+===========+
| SYN || SYNACK | Send ACK message (notes 2 and 3) | ESTAB |
+-----------------+------------------------------------+-----------+
| ACK && B && C | Send ACK message (note 3) | ESTAB |
+-----------------+------------------------------------+-----------+
| ACK && !(B && C)| Send RSTACK message | ESTAB |
+==================================================================+
Note 2: No more than two ACK messages should be sent within any
time period of length defined by the timer. Thus, one ACK message
MUST be sent every time the timer expires. In addition, one
further ACK message may be sent between timer expirations if the
incoming message is a SYN or SYNACK. This additional ACK allows
the Hello functions to reach synchronization more quickly.
Note 3: No more than one ACK message should be sent within any
time period of length defined by the timer.
5.3. Information Exchange Phase
After the Hello messages have been exchanged, and the nodes are in
the ESTAB state, the Information Exchange Phase, consisting of the
RIB Exchange and Bundle Passing Sub-Phases, is initiated. This
section describes the procedure and shows the state transitions
necessary in these sub-phases; the following sections describe in
detail the various TLVs passed in these phases. On reaching the
ESTAB state in the high-level HELLO state, there is an automatic
transition to the INFO_EXCH high-level state.
PRoPHET runs over a bidirectional transport as documented in
Section 1.2 so that when a pair of nodes (A and B) have reached the
ESTAB state, they are able to perform the Information Exchange Phase
processes for both the A-to-B and B-to-A directions over the link
that has just been established. In principle, these two processes
are independent of each other and can be performed concurrently.
However, complete concurrency may not be the most efficient way to
implement the complete process. As explained in Section 3.2.1, the
Routing Information Base Dictionary is a shared resource assembled
from a combination of information generated locally on each node and
information passed from the peer node. Overlaps in this information,
and hence the amount of information that has to be passed between the
nodes, can be minimized by sequential rather than concurrent
operation of the dictionary generation and update processes. It may
also be possible to reduce the number of bundles that need to be
offered by the second offeror by examining the offers received from
the first offeror -- there is no need for the second offeror to offer
a bundle that is already present in the first offeror's offer list,
as it will inevitably be refused.
All implementations MUST be capable of operating in a fully
concurrent manner. Each implementation needs to define a policy,
which SHOULD be configurable, as to whether it will operate in a
concurrent or sequential manner during the Information Exchange
Phase. If it is to operate sequentially, then further choices can be
made as to whether to interleave dictionary, offer, and response
exchange parts, or to complete all parts in one direction before
initiating the other direction.
Sequential operation will generally minimize the amount of data
transferred across the PRoPHET link and is especially appropriate if
the link is half-duplex. However it is probably not desirable to
postpone starting the information exchange in the second direction
until the exchange of bundles has completed. If the contact between
the nodes ends before all possible bundles have been exchanged, it is
possible that postponing the start of bundle exchange in the second
direction can lead to bundle exchange being skewed in favor of one
direction over the other. It may be preferable to share the
available contact time and bandwidth between directions by
overlapping the Information Exchange Phases and running the actual
bundle exchanges concurrently if possible. Also, if encounters
expected in the current PRoPHET zone are expected to be relatively
short, it MAY not be appropriate to use sequential operation.
One possible interleaving strategy is to alternate between sending
from the two nodes. For example, if the Hello SYN node sends its
initial dictionary entries while the Hello SYNACK node waits until
this is complete, the Hello SYNACK node can then prune its proposed
dictionary entries before sending in order to avoid duplication.
This approach can be repeated for the second tranche of dictionary
entries needed for the Bundle Offers and Responses, and also for the
Bundle Offers, where any bundles that are offered by the Hello SYN
node that are already present in the Hello SYNACK node need not be
offered to the Hello SYN node. This approach is well suited to a
transport protocol and physical medium that is effectively half-
duplex.
At present, the decision to operate concurrently or sequentially is
purely a matter of local policy in each node. If nodes have
inconsistent policies, the behavior at each encounter will depend on
which node takes the SYN role; this is a matter of chance depending
on random timing of the start of communications during the encounter.
To manage the information transfer, two subsidiary state machines are
created in each node to control the stages of the RIB Exchange Sub-
Phase and Bundle Passing Sub-Phase processes within the INFO_EXCH
high-level state as shown in Figure 12. Each subsidiary state
machine consists of two essentially independent components known as
the "Initiator role" and the "Listener role". One of these
components is instantiated in each node. The Initiator role starts
the Information Exchange Phase in each node and the Listener role
responds to the initial messages, but it is not a passive listener as
it also originates messages. The transition from the ESTAB state is
a "forking" transition in that it starts both subsidiary state
machines. The two subsidiary state machines operate in parallel for
as long as the neighbor remains in range and connected.
+ - - - - - - - - + + - - - - - - - - +
| SYN node | PRoPHET messages with: | SYNACK node |
| +-------------+ | A. Delivery Predictabilities | +-------------+ |
| Subsidiary |--->---->---->---->---->---->---->| Subsidiary |
| | State | | C. Bundle Responses | | State | |
| Machine 1: | | Machine 1: |
| | Initiator | | B. Bundle Offers | | Listener | |
| Role |<----<----<----<----<----<----<---| Role |
| +-------------+ | D. Requested Bundles | +-------------+ |
| +-------------+ | A. Delivery Predictabilities | +-------------+ |
| Subsidiary |<----<----<----<----<----<----<---| Subsidiary |
| | State | | C. Bundle Responses | | State | |
| Machine 2: | | Machine 2: |
| | Listener | | B. Bundle Offers | | Initiator | |
| Role |--->---->---->---->---->---->---->| Role |
| +-------------+ | D. Requested Bundles | +-------------+ |
+ - - - - - - - - + + - - - - - - - - +
The letters (A - D) indicate the sequencing of messages.
Figure 12: Information Exchange Phase Subsidiary State Machines
These subsidiary state machines can be thought of as mirror images:
for each state machine, one node takes on the Initiator role while
the other node takes on the Listener role. TLVs sent by a node from
the Initiator role will be processed by the peer node in the Listener
role and vice versa. As indicated in Figure 12, the Initiator role
handles sending that node's current set of delivery predictabilities
for known destinations to the Listener role node. The Listener role
node uses the supplied values to update its delivery predictabilities
according to the update algorithms described in Section 2.1.2. It
then decides which bundles that it has in store should be offered for
transfer to the Initiator role node as a result of comparing the
local predictabilities and those supplied by the Initiator node.
When these offers are delivered to the Initiator role node, it
decides which ones to accept and supplies the Listener role node with
a prioritized list of bundles that it wishes to accept. The Listener
role node then sends the requested bundles.
These exchanges are repeated periodically for as long as the nodes
remain in contact. Additionally, if new bundles arrive from other
sources, they may be offered, accepted, and sent in between these
exchanges.
The PRoPHET protocol is designed so that in most cases the TLV type
determines the role in which it will be processed on reception. The
only exception to this is that both roles may send RIB Dictionary
TLVs: the Initiator role sends dictionary entries for use in the
subsequent RIB TLV(s), and the Listener role may send additional
dictionary entries for use in subsequent Bundle Offer TLVs. The two
cases are distinguished by a TLV flag to ensure that they are
processed in the right role context on reception. If this flag was
not provided, there are states where both roles could accept the RIB
Dictionary TLV, making it impossible to ensure that the correct role
state machine accepts the RIB Dictionary TLV. Note that the correct
updates would be made to the dictionary whichever role processed the
TLV and that the ambiguity would not arise if the roles are adopted
completely sequentially, i.e., if the RIB Exchange Sub-Phase and
associated Bundle Passing Sub-Phase run to completion in one
direction before the process for the reverse direction is started.
If sequential operation is selected, the node that sent the Hello SYN
function message MUST be the node that sends the first message in the
Information Exchange Phase process. This ensures that there is a
well-defined order of events with the Initiator role in the Hello SYN
node (i.e., the node identified by String ID 0) starting first. The
Hello SYNACK node MAY then postpone sending its first message until
the Listener role state machine in the Hello SYNACK node has reached
any of a number of points in its state progression according to
locally configured policy and the nature of the physical link for the
current encounter between the nodes as described above. If
concurrent operation is selected, the Hello SYNACK node can start
sending messages immediately without waiting to receive messages from
the peer.
The original design of the PRoPHET protocol allowed it to operate
over unreliable datagram-type transports as well as the reliable, in-
order delivery transport of TCP that is currently specified. When
running over TCP, protocol errors and repeated timeouts during the
Information Exchange Phase SHOULD result in the connection being
terminated.
5.3.1. State Definitions for the Initiator Role
The state machine component with the Initiator role in each node
starts the transfer of information from one node to its peer during
the Information Exchange Phase. The process from the Initiator's
point of view does the following:
o The Initiator role determines the set of delivery predictabilities
to be sent to the peer node and sends RIB dictionary entries
necessary to interpret the set of RIB predictability values that
are sent after the dictionary updates. On second and subsequent
executions of this state machine during a single session with the
same peer, there may be no RIB Dictionary entries to send. Either
an empty TLV can be sent or the TLV can be omitted.
o The Initiator then waits to receive any RIB Dictionary updates
followed by bundle offers from the Listener role on the peer node.
o The Initiator determines which of the bundle offers should be
accepted and, if necessary, reorders the offers to suit its own
priorities. The possibly reordered list of accepted bundles is
sent to the peer node using one or more bundle responses.
o The peer then sends the accepted bundles to the Initiator in turn.
o Assuming that the link remains open during the bundle sending
process, the Initiator signals that the Bundle Passing Sub-Phase
is complete by sending a message with an empty Bundle Response TLV
(i.e, with the Bundle Offer Count set to 0 and no bundle offers
following the TLV header).
o When the bundle transfer is complete, the Initiator starts the
Timer(next_exchange). Assuming that the connection to the
neighbor remains open, when the timer expires, the Initiator
restarts the Information Exchange Phase. During this period,
Hello SYN messages are exchanged as keep-alives to check that the
neighbor is still present. The keep-alive mechanism is common to
the Initiator and Listener machines and is handled in the high-
level state machine (see Section 5.1.
A timer is provided that restarts the Initiator role state machine if
Bundle Offers are not received after sending the RIB. If this node
receives a Hello ACK message containing an Error TLV indicating there
has been a protocol problem, then the connection MUST be terminated.
The following states are used:
CREATE_DR
The initial transition to this state from the ESTAB state is
immediate and automatic for the node that sent the Hello SYN
message. For the peer (Hello SYNACK sender) node, it may be
immediate for nodes implementing a fully concurrent process or may
be postponed until the corresponding Listener has reached a
specified state if a sequential process is configured in the node
policy.
The local dictionary is initialized when this state is entered for
the first time from the ESTAB state. The initial state of the
dictionary contains two entries: the EID of the node that sent the
Hello SYN (String ID 0) and the EID of the node that sent the
Hello SYNACK (String ID 1). If the peer reports via a Hello ACK
message containing an Error TLV reporting a Dictionary Conflict or
Bad String ID error, then the connection MUST be terminated.
The CREATE_DR state will be entered in the same way from the
REQUEST state when the Timer(next_exchange) expires, signaling the
start of a new round of information exchange and bundle passing.
When in this state:
* Determine the destination EIDs for which delivery
predictabilities will be sent to the peer in a RIB TLV, if any.
Record the prior state of the local dictionary (assuming that
String IDs are numbers allocated sequentially, the state
information needed is just the highest ID used before this
process started) so that the process can be restarted if
necessary. Update the local dictionary if any new EIDS are
required; format one or more RIB Dictionary TLVs and one or
more RIB TLVs and send them to the peer. If there are no
dictionary entries to send, TLVs with zero entries MAY be sent,
or the TLV can be omitted, but an empty RIB TLV MUST be sent if
there is no data to send. The RIB Dictionary TLVs generated
here MUST have the Sent by Listener flag set to 0 to indicate
that they were sent by the Initiator.
* If an Error TLV indicating a Dictionary Conflict or
Bad String ID is received during or after sending the RIB
Dictionary TLVs and/or the RIB TLVs, abort any in-progress
Initiator or Listener process, and terminate the connection to
the peer.
* Start a timer (known as Timer(info)) and transition to the
SEND_DR state.
Note that when (and only when) running over a transport protocol
such as TCP, both the RIB Dictionary and RIB information MAY be
spread across multiple TLVs and messages if required by known
constraints of the transport protocol or to reduce the size of
memory buffers. Alternatively, the information can be formatted
using a single RIB Dictionary TLV and a single RIB TLV. These
TLVs may be quite large, so it may be necessary to segment the
message either using the PRoPHET submessage capability or, if the
transport protocol has appropriate capabilities, using those
inherent capabilities. This discussion of segmentation applies to
the other states and the bundle offer and bundle response messages
and will not be repeated.
If more than one RIB TLV is to be used, all but the last one have
the "More RIB TLVs" flag set to 1 in the TLV flags. It is not
necessary to distinguish the last RIB Dictionary TLV because the
actions taken at the receiver are essentially passive (recording
the contents), and the sequence is ended by the sending of the
first RIB TLV.
SEND_DR
In this state, the Initiator node expects to be receiving Bundle
Offers and sending Bundle Responses. The Initiator node builds a
list of bundles offered by the peer while in this state:
* Clear the set of bundles offered by the peer on entry to the
state.
* If the Timer(info) expires, re-send the RIB Dictionary and RIB
information sent in the previous CREATE_DR state using the
stored state to re-create the information. The RIB dictionary
update process in the peer is idempotent provided that the
mappings between the EID and the String ID in the re-sent RIB
Dictionary TLVs are the same as in the original. This means
that it does not matter if some of the RIB Dictionary TLVs had
already been processed in the peer. Similarly, re-sending RIB
TLVs will not cause a problem.
* If a message with a RIB Dictionary TLV marked as sent by a
Listener is received, update the local dictionary based on the
received TLV. If any of the entries in the RIB Dictionary TLV
conflict with existing entries (i.e., an entry is received that
uses the same String ID as some previously received entry but
the EID in the entry is different), send a Response message
with an Error TLV containing a Dictionary Conflict indicator,
abort any in-progress Initiator or Listener process, and
terminate the connection to the peer. Note that in some
circumstances no dictionary updates are needed, and the first
message received in this state will carry a Bundle Offer TLV.
* If a message with a Bundle Offer TLV is received, restart the
Timer(info) if the "More Offer/Response TLVs Following" flag is
set in the TLV; otherwise, stop the Timer(info). Then process
any PRoPHET ACKs in the TLV by informing the bundle protocol
agent, and add the bundles offered in the TLV to the set of
bundles offered. If the "More Offer/Response TLVs Following"
flag is set in the TLV, wait for further Bundle Offer TLVs. If
a Bundle Offer TLV is received with a String ID that is not in
the dictionary, send a message with an Error TLV containing a
Bad String ID indicator, abort any in-progress Initiator or
Listener process, and terminate the connection to the peer.
* If the "More Offer/Response TLVs Following" flag is clear in
the last Bundle Offer TLV received, inspect the set of bundles
offered to determine the set of bundles that are to be accepted
using the configured queueing policy. Record the set of
bundles accepted so that reception can be checked in the Bundle
Passing Sub-Phase. Format one or more Bundle Response TLVs
flagging the accepted offers and send them to the peer. If
more than one Bundle Response TLV is sent, all but the last one
should have the "More Offer/Response TLVs Following" flag set
to 1. At least one Bundle Response TLV MUST be sent even if
the node does not wish to accept any of the offers. In this
case, the Bundle Response TLV contains an empty set of
acceptances.
* If an Error TLV indicating a Bad String ID is received during
or after sending the Bundle Response TLVs, abort any in-
progress Initiator or Listener process, re-initialize the local
dictionary, and terminate the connection to the peer.
* Restart the Timer(info) timer in case the peer does not start
sending the requested bundles.
* Transition to state REQUEST.
REQUEST
In this state, the Initiator node expects to be receiving the
bundles accepted in the Bundle Response TLV(s):
* Keep track of the bundles received and delete them from the set
of bundles accepted.
* If the Timer(info) expires while waiting for bundles, format
and send one or more Bundle Response TLVs listing the bundles
previously accepted but not yet received. If more than one
Bundle Response TLV is sent, all but the last one should have
the "More Offer/Response TLVs Following" flag set to 1.
* If an Error TLV indicating a Bad String ID is received during
or after sending the Bundle Response TLVs, abort any in-
progress Initiator or Listener process, re-initialize the local
dictionary, and terminate the connection to the peer.
* Restart the Timer(info) timer after each bundle is received in
case the peer does not continue sending the requested bundles.
* When all the requested bundles have been received, format a
Bundle Response TLV with the Bundle Offer Count set to zero and
with the "More Offer/Response TLVs Following" flag cleared to 0
to signal completion to the peer node. Also, signal the
Listener in this node that the Initiator has completed. If the
peer node is using a sequential policy, the Listener may still
be in the initial state, in which case, it needs to start a
timer to ensure that it detects if the peer fails to start the
Initiator state machine. Thereafter, coordinate with the
Listener state machine in the same node: when the Listener has
received the completion notification from the peer node and
this Initiator has sent its completion notification, start
Timer(next_exchange).
* If the Timer(next_exchange) expires, transition to state
CREATE_DR to restart the Information Exchange Phase.
Note that if Timer(info) timeout occurs a number of times
(configurable, typically 3) without any bundles being received,
then this SHOULD generally be interpreted as the problem that the
link to the peer is no longer functional and the session should be
terminated. However, some bundles may be very large and take a
long time to transmit. Before terminating the session, this state
machine needs to check if a large bundle is actually being
received although no new completed bundles have been received
since the last expiry of the timer. In this case the timer should
be restarted without sending the Bundle Response TLV. Also, if
the bundles are being exchanged over a transport protocol that can
detect link failure, then the session MUST be terminated if the
bundle exchange link is shut down because it has failed.
5.3.2. State Definitions for the Listener Role
The state machine component with the Listener role in each node
initially waits to receive a RIB Dictionary update followed by a set
of RIB delivery predictabilities during the Information Exchange
Phase. The process from the point of view of the Listener does the
following:
o Receive RIB Dictionary updates and RIB values from the peer. Note
that in some circumstances no dictionary updates are needed, and
the RIBD TLV will contain no entries or may be omitted completely.
o When all RIB messages have been received, the delivery
predictability update algorithms are run (see Section 2.1.2) using
the values received from the Initiator node and applying any of
the optional optimizations configured for this node (see
Section 2.1.3).
o Using the updated delivery predictabilities and the queueing
policy and forwarding strategy configured for this node (see
Section 2.1.4) examine the set of bundles currently stored in the
Listener node to determine the set of bundles to be offered to the
Initiator and order the list according to the forwarding strategy
in use. The Bundle Offer TLVs are also used to notify the peer of
any PRoPHET ACKs that have been received by the Listener role
node.
o Send the list of bundles in one or more bundle offers, preceded if
necessary by one or more RIB dictionary updates to add any EIDs
required for the source or destination EIDs of the offered
bundles. These updates MUST be marked as being sent by the
Listener role so that they will be processed by the Initiator role
in the peer.
o Wait for the Initiator to send bundle responses indicating which
bundles should be sent and possibly a modified order for the
sending. Send the accepted bundles in the specified order. The
bundle sending will normally be carried out over a separate
connection using a suitable DTN convergence layer.
o On completion of the sending, wait for a message with an empty
Bundle Response TLV indicating correct completion of the process.
o The Listener process will be notified if any new bundles or
PRoPHET ACKs are received by the node after the completion of the
bundle sending that results from this information exchange. The
forwarding policy and the current delivery predictabilities will
then be applied to determine if this information should be sent to
the peer. If it is determined that one or more bundles and/or
ACKs ought to be forwarded, a new set of bundle offers are sent to
the peer. If the peer accepts them by sending bundle responses,
the bundles and/or ACKS are transferred as previously.
o Periodically, the Initiator in the peer will restart the complete
information exchange by sending a RIB TLV that may be, optionally,
preceded by RIB Dictionary entries if they are required for the
updated RIB.
Timers are used to ensure that the Listener does not lock up if
messages are not received from the Initiator in a timely fashion.
The Listener is restarted if the RIB is not received, and a Hello ACK
message is sent to force the Initiator to restart. If bundle
response messages are not received in a timely fashion, the Listener
re-sends the bundle offers and associated dictionary updates. The
following states are used:
WAIT_DICT
The Listener subsidiary state machine transitions to this state
automatically and immediately from the state ESTAB in both peers.
This state will be entered in the same way if the
Timer(next_exchange) expires in the peer, signaling the start of a
new round of information exchange and bundle passing. This will
result in one or more RIB TLVs being sent to the Listener by the
peer node's Initiator.
* When a RIB Dictionary TLV is received, use the TLV to update
the local dictionary, start or (if it is running) restart the
Timer(peer) and transition to state WAIT_RIB. If any of the
entries in the RIB Dictionary TLV conflict with existing
entries (i.e., an entry is received that uses the same String
ID as some previously received entry, but the EID in the entry
is different), send a Response message with an Error TLV
containing a Dictionary Conflict indicator, abort any in-
progress Initiator or Listener process, and terminate the
connection to the peer.
* If a Hello ACK message is received from the peer node,
transition to state WAIT_DICT and restart the process.
If multiple timeouts occur (configurable, typically 3), assume
that the link is broken and terminate the session. Note that the
RIB Dictionary and RIB TLVs may be combined into a single message.
The RIB TLV should be passed on to be processed in the WAIT_RIB
state.
WAIT_RIB
In this state, the Listener expects to be receiving one or more
RIB TLVs and possibly additional RIB Dictionary TLVs.
* On entry to this state, clear the set of received delivery
predictabilities.
* Whenever a new message is received, restart the Timer(peer)
timer.
* If a RIB dictionary TLV is received, use it to update the local
dictionary and remain in this state. If any of the entries in
the RIB Dictionary TLV conflict with existing entries (i.e., an
entry is received that uses the same String ID as some
previously received entry, but the EID in the entry is
different), send a message with an Error TLV containing a
Dictionary Conflict indicator, abort any in-progress Initiator
or Listener process, and terminate the connection to the peer.
* If a RIB TLV is received, record the received delivery
predictabilities for use in recalculating the local delivery
predictabilities. If a delivery predictability value is
received for an EID that is already in the set of received
delivery predictabilities, overwrite the previously received
value with the latest value. If a delivery predictability
value is received with a String ID that is not in the
dictionary, send a message with an Error TLV containing a
Bad String ID indicator, abort any in-progress Initiator or
Listener process, and terminate the connection to the peer.
* When a RIB TLV is received with the "More RIB TLVs" flag
cleared, initiate the recalculation of delivery
predictabilities and stop the Timer(peer). Use the revised
delivery predictabilities and the configured queueing and
forwarding strategies to create a list of bundles to be offered
to the peer node.
* Record the state of the local dictionary in case the offer
procedure has to be restarted. Determine if any new dictionary
entries are required for use in the Bundle Offer TLV(s). If
so, record them in the local dictionary, then format and send
RIB Dictionary entries in zero or more RIB Dictionary TLV
messages to update the dictionary in the peer if necessary.
* Format and send Bundle Offer TLV(s) carrying the identifiers of
the bundles to be offered together with any PRoPHET ACKs
received or generated by this node. If more than one Bundle
Offer TLV is sent, all but the last Bundle Offer TLV sent MUST
have the "More Offer/Response TLVs Following" flag set to 1.
* When all Bundle Offer TLVs have been sent, start the
Timer(info) and transition to state OFFER.
* If the Timer(peer) expires, send a Hello ACK TLV to the peer,
restart the timer, and transition to state WAIT_DICT.
* If an Error TLV indicating a Dictionary Conflict or
Bad String ID is received during or after sending the RIB
Dictionary TLVs and/or the Bundle Offer TLVs, abort any in-
progress Initiator or Listener process, and terminate the
connection to the peer.
* If a Hello ACK message is received from the peer node,
transition to state WAIT_DICT and restart the process.
OFFER
In this state, the Listener expects to be receiving one or more
Bundle Response TLVs detailing the bundles accepted by the
Initiator node. The ordered list of accepted bundles is
communicated to the bundle protocol agent, which controls sending
them to the peer node over a separate connection.
* When a Bundle Response TLV is received with a non-zero count of
Bundle Offers, extract the list of accepted bundles and send
the list to the bundle protocol agent so that it can start
transmission to the peer node. Ensure that the order of offers
from the TLV is maintained. Restart the Timer(info) unless the
last Bundle Response TLV received has the "More Offer/
Response TLVs Following" flag set to 0. If a Bundle Response
TLV is received with a String ID that is not in the dictionary,
send a message with an Error TLV containing a Bad String ID
indicator, abort any in-progress Initiator or Listener process,
and terminate the connection to the peer.
* After receiving a Bundle Response TLV with the "More Offer/
Response TLVs Following" flag set to 0 stop the Timer(info) and
transition to state SND_BUNDLE.
* If the Timer(info) expires, send a Hello ACK TLV to the peer,
restart the timer and transition to state WAIT_DICT.
* If a Hello ACK message is received from the peer node,
transition to state WAIT_DICT and restart the process.
SND_BUNDLE
In this state the Listener monitors the sending of bundles to the
Initiator peer node. In the event of disruption in transmission,
the Initiator node will, if possible, re-send the list of bundles
that were accepted but have not yet been received. The bundle
protocol agent has to be informed of any updates to the list of
bundles to send (this is likely to involve re-sending one or more
bundles). Otherwise, the Listener is quiescent in this state.
* When a Bundle Response TLV is received with a non-zero count of
Bundle Offers, extract the list of accepted bundles and update
the list previously passed to the bundle protocol agent so that
it can (re)start transmission to the peer node. Ensure that
the order of offers from the TLV is maintained so far as is
possible. Restart the Timer(info) unless the last Bundle
Response TLV received has the "More Offer/Response TLVs
Following" flag set to 0. If a Bundle Response TLV is received
with a String ID that is not in the dictionary, send a message
with an Error TLV containing a Bad String ID indicator, abort
any in-progress Initiator or Listener process, re-initialize
the local dictionary, and restart the Information Exchange
Phase as if the ESTAB state had just been reached.
* After receiving a Bundle Response TLV with the "More Offer/
Response TLVs Following" flag set to 0, stop the Timer(info)
and wait for completion of bundle sending.
* If the Timer(info) expires, send a Hello ACK TLV to the peer,
restart the timer, and transition to state WAIT_DICT.
* If a Hello ACK message is received from the peer node,
transition to state WAIT_DICT and restart the process.
* When a Bundle Response TLV is received with a zero count of
Bundle Offers, the Bundle Passing Sub-Phase is complete.
Notify the Initiator that the Listener process is complete and
transition to state WAIT_MORE.
As explained in the Initiator state REQUEST description, depending
on the transport protocol (convergence layer) used to send the
bundles to the peer node, it may be necessary during the bundle
sending process to monitor the liveness of the connection to the
peer node in the Initiator process using a timer.
WAIT_MORE
In this state, the Listener monitors the reception of new bundles
that might be received from a number of sources, including
* local applications on the node,
* other mobile nodes that connect to the node while this
connection is open, and
* permanent connections such as might occur at an Internet
gateway.
When the Listener is notified of received bundles, it determines
if they should be offered to the peer. The peer may also re-
initiate the Information Exchange Phase periodically.
* When the bundle protocol agent notifies the Listener that new
bundles and/or new PRoPHET ACKs have been received, the
Listener applies the selected forwarding policy and the current
delivery predictabilities to determine if any of the items
ought to be offered to the connected peer. If so, it carries
out the same operations as are described in the WAIT_RIB state
to build and send any necessary RIB Dictionary TLVs and RIB
TLVs to the Initiator in the peer.
* When all Bundle Offer TLVs have been sent, start the
Timer(info) and transition to state OFFER.
* If a RIB dictionary TLV is received, use it to update the local
dictionary and transition to state WAIT_RIB. If any of the
entries in the RIB Dictionary TLV conflict with existing
entries (i.e., an entry is received that uses the same String
ID as some previously received entry, but the EID in the entry
is different), send a message with an Error TLV containing a
Dictionary Conflict indicator, abort any in-progress Initiator
or Listener process, and terminate the connection to the peer.
Note that the RIB Dictionary and RIB TLVs may be combined into a
single message. The RIB TLV should be passed on to be processed
in the WAIT_RIB state.
5.3.3. Recommendations for Information Exchange Timer Periods
The Information Exchange Phase (IEP) state definitions include a
number of timers. This section provides advice and recommendations
for the periods that are appropriate for these timers.
Both Timer(info) and Timer(peer) are used to ensure that the state
machines do not become locked into inappropriate states if the peer
node does not apparently respond to messages sent in a timely fashion
either because of message loss in the network or unresponsiveness
from the peer. The appropriate values are to some extent dependent
on the speed of the network connection between the nodes and the
capabilities of the nodes executing the PRoPHET implementations.
Values in the range 1 to 10 seconds SHOULD be used, with a value of 5
seconds RECOMMENDED as default. The period should not be set to too
low a value, as this might lead to inappropriate restarts if the
hardware is relatively slow or there are large numbers of pieces of
information to process before responding. When using a reliable
transport protocol such as TCP, these timers effectively provide a
keep-alive mechanism and ensure that a failed connection is detected
as rapidly as possible so that remedial action can be taken (if
possible) or the connection shut down tidily if the peer node has
moved out of range.
Timer(next_exchange) is used to determine the maximum frequency of
(i.e., minimum period between) successive re-executions of the
information exchange state machines during a single session between a
pair of nodes. Selection of the timer period SHOULD reflect the
trade-off between load on the node processor and desire for timely
forwarding of bundles received from other nodes. It is RECOMMENDED
that the timer periods used should be randomized over a range from
50% to 150% of the base value in order to avoid synchronization
between multiple nodes. Consideration SHOULD be given to the
expected length of typical encounters and the likelihood of
encounters between groups of nodes when setting this period. Base
values in the range of 20 to 60 seconds are RECOMMENDED.
5.3.4. State Tables for Information Exchange
This section shows the state transitions that nodes go through during
the Information Exchange Phase. State tables are given for the
Initiator role and for the Listener role of the subsidiary state
machines. Both nodes will be running machines in each role during
the Information Exchange Phase, and this can be done either
concurrently or sequentially, depending on the implementation, as
explained in Section 5.3. The state tables in this section should be
read in conjunction with the state descriptions in Sections 5.3.1 and
5.3.2.
5.3.4.1. Common Notation, Operations and Events
The following notation is used:
nS Node that sent the Hello SYN message.
nA Node that sent the Hello SYNACK message.
The following events are common to the Initiator and Listener state
tables:
ErrDC Dictionary Conflict Error TLV received.
ErrBadSI Bad String ID Error TLV received.
HelloAck Hello ACK TLV received. This message is delivered to
both Initiator and Listener roles in order to cause a
restart of the Information Exchange Phase in the event
of message loss or protocol problems.
InitStart Sent by Listener role to Initiator role to signal the
Initiator role to commence sending messages to peer.
If the Listener instance is running in the node that
sent the Hello SYN (nS), then InitStart is signaled
immediately when the state is entered. For the node
that sent the Hello SYNACK (nA), InitStart may be
signaled immediately if the operational policy requires
concurrent operation of the Initiator and Listener
roles or postponed until the Listener role state
machine has reached a state defined by the configured
policy.
RIBnotlast RIB TLV received with "More RIB TLVs" flag set to 1.
RIBlast RIB TLV received with "More RIB TLVs" flag set to 0.
REQnotlast Bundle Response TLV received with More Offer/Response
TLVs Following flag set to 1.
REQlast Bundle Response TLV received with More Offer/Response
TLVs Following flag set to 0.
RIBDi RIBD TLV received with Sent by Listener flag set to 0
(i.e., it was sent by Initiator role).
RIBDl RIBD TLV received with Sent by Listener flag set to 1
(i.e., it was sent by Listener role).
Timeout(info) The Timer(info) has expired.
Timeout(peer) The Timer(peer) has expired.
Both the Initiator and Listener state tables use the following common
operations:
o The "Initialize Dictionary" operation is defined as emptying any
existing local dictionary and inserting the two initial entries:
the EID of the node that sent the Hello SYN (String ID 0) and the
EID of the node that sent the Hello SYNACK (String ID 1).
o The "Send RIB Dictionary Updates" operation is defined as:
1. Determining what dictionary updates will be needed for any
extra EIDs in the previously selected RIB entries set that are
not already in the dictionary and updating the local
dictionary with these EIDs. The set of dictionary updates may
be empty if no extra EIDs are needed. The set may be empty
even on the first execution if sequential operation has been
selected, this is the second node to start and the necessary
EIDs were in the set previously sent by the first node to
start.
2. Formatting zero or more RIBD TLVs for the set of dictionary
updates identified in the "Build RIB Entries" operation and
sends them to the peer. The RIBD TLVs MUST have the "Sent by
Listener" flag set to 0 if the updates are sent by the
Initiator role and to 1 if sent by the Listener role. In the
case of the Initiator role, an empty RIBD TLV MUST be sent
even if the set of updates is empty in order to trigger the
Listener state machine.
o The "Update Dictionary" operation uses received RIBD TLV entries
to update the local dictionary. The received entries are checked
against the existing dictionary. If the String ID in the entry is
already in use, the entry is accepted if the EID in the received
entry is identical to that stored in the dictionary previously.
If it is identical, the entry is unchanged, but if it is not a
Response message with an Error TLV indicating Dictionary Conflict
is sent to the peer in an Error Response message, the whole
received RIBD TLV is ignored, and the Initiator and Listener
processes are restarted as if the ESTAB state has just been
reached.
o The "Abort Exchange" operation is defined as aborting any in-
progress information exchange state machines and terminating the
connection to the peer.
o The "Start TI" operation is defined as (re)starting the
Timer(info) timer.
o The "Start TP" operation is defined as (re)starting the
Timer(peer) timer.
o The "Cancel TI" operation is defined as canceling the Timer(info)
timer.
o The "Cancel TP" operation is defined as canceling the Timer(info)
timer.
5.3.4.2. State Tables for the Initiator Role
The rules and state tables for the Initiator role use the following
operations:
o The "Build RIB Entries" operation is defined as:
1. Recording the state of the local dictionary.
2. Determining the set of EIDs for which RIB entries should be
sent during this execution of the Initiator role state machine
component. If this is a second or subsequent run of the state
machine in this node during the current session with the
connected peer, then the set of EIDs may be empty if no
changes have occurred since the previous run of the state
machine.
3. Determining and extracting the current delivery predictability
information for the set of EIDs selected.
o The "Send RIB Entries" operation formats one or more RIB TLVs with
the set of RIB entries identified in the "Build RIB Entries"
operation and sends them to the peer. If the set is empty, a
single RIB TLV with zero entries is sent. If more than one RIB
TLV is sent, all but the last one MUST have the "More RIB TLVs"
flag set to 1; the last or only one MUST have the flag set to 0.
o The "Clear Bundle Lists" operation is defined as emptying the
lists of bundles offered by the peer and bundles requested from
the peer.
o The "Notify ACKs" operation is defined as informing the bundle
protocol agent that PRoPHET ACKs has been received for one or more
bundles in a Bundle Offer TLV using the Bundle Delivered interface
(see Section 2.2).
o The "Record Offers" operation is defined as recording all the
bundles offered in a Bundle Offer TLV in the list of bundles
offers.
o The "Select for Request" operation prunes and sorts the list of
offered bundles held into the list of requested bundles according
to policy and the available resources ready for sending to the
offering node.
o The "Send Requests" operation is defined as formatting one or more
non-empty Bundle Response TLVs and sending them to the offering
node. If more than one Bundle Offer TLV is sent, all but the last
one MUST have the "More Offer/Response TLVs Following" flag set to
1; the last or only one MUST have the flag set to 0.
o The "Record Bundle Received" operation deletes a successfully
received bundle from the list of requests.
o The "All Requests Done" operation is defined as formatting and
sending an empty Bundle Offer TLV, with the "More Offer/Response
TLVs Following" flag set to 0, to the offering node.
o The "Check Receiving" operation is defined as checking with the
node bundle protocol agent if bundle reception from the peer node
is currently in progress. This is needed in case a timeout occurs
while waiting for bundle reception and a very large bundle is
being processed.
o The "Start NE" operation is defined as (re)starting the
Timer(next_exchange).
The following events are specific to the Initiator role state
machine:
LastBndlRcvd Bundle received from peer that is the only remaining
bundle in Bundle Requests List.
NotLastBndlRcvd Bundle received from peer that is not the only
remaining bundle in Bundle Requests List.
OFRnotlast Bundle Offer TLV received with "More Offer/Response
TLVs Following" flag set to 1.
OFRlast Bundle Offer TLV received with "More Offer/Response
TLVs Following" flag set to 0
Timeout(next_exch) The Timer(next_exchange) has expired
State: CREATE_DR
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| On Entry | If previous state was ESTAB: | |
| | Initialize Dictionary | |
| | Always: | |
| | Build RIB Entries | |
| | Wait for Init Start | CREATE_DR |
+------------------+-----------------------------------+-----------+
| InitStart | Send RIB Dictionary Updates | |
| | Send RIB Entries | |
| | Start TI | SEND_DR |
+------------------+-----------------------------------+-----------+
| ErrDC | Abort Exchange |(finished) |
+------------------+-----------------------------------+-----------+
| ErrBadSI | Abort Exchange |(finished) |
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | CREATE_DR |
+==================================================================+
State: SEND_DR
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| On Entry | Clear Bundle Lists | SEND_DR |
+------------------+-----------------------------------+-----------+
| Timeout(info) | Send RIB Dictionary Updates | |
| | Send RIB Entries (note 1) | SEND_DR |
+------------------+-----------------------------------+-----------+
| RIBDl received | Update Dictionary (note 2) | |
| | If Dictionary Conflict found: | |
| | Abort Exchange | CREATE_DR |
| | Else: | |
| | Start TI | SEND_DR |
+------------------+-----------------------------------+-----------+
| OFRnotlast | Notify ACKs | |
| | Record Offers | |
| | Start TI | SEND_DR |
+------------------+-----------------------------------+-----------+
| OFRlast | Cancel TI | |
| | Notify ACKs | |
| | Record Offers | |
| | Select for Request | |
| | Send Requests | |
| | Start TI | REQUEST |
+------------------+-----------------------------------+-----------+
| ErrDC | Abort Exchange |(finished) |
+------------------+-----------------------------------+-----------+
| ErrBadSI | Abort Exchange |(finished) |
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | CREATE_DR |
+==================================================================+
State: REQUEST
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| Timeout(info) | Check Receiving | |
| | If bundle reception in progress: | |
| | Start TI | REQUEST |
| | Otherwise: | |
| | Send Requests | |
| | Start TI (note 3) | REQUEST |
+------------------+-----------------------------------+-----------+
| NotLastBndlRcvd | Record Bundle Received | |
| | Start TI | REQUEST |
+------------------+-----------------------------------+-----------+
| LastBndlRcvd | Cancel TI | |
| | All Requests Done | |
| | Start NE | REQUEST |
+------------------+-----------------------------------+-----------+
|Timeout(next_exch)| | CREATE_DR |
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | CREATE_DR |
+==================================================================+
Note 1:
No response to the RIB has been received before the timer expired,
so we re-send the dictionary and RIB TLVs. If the timeout occurs
repeatedly, it is likely that communication has failed and the
connection MUST be terminated.
Note 2:
If a Dictionary Conflict error has to be sent, the state machine
will be aborted. If this event occurs repeatedly, it is likely
that there is either a serious software problem or a security
issue. The connection MUST be terminated.
Note 3:
Remaining requested bundles have not arrived before the timer
expired, so we re-send the list of outstanding requests. If the
timeout occurs repeatedly, it is likely that communication has
failed and the connection MUST be terminated.
5.3.4.3. State Tables for the Listener Role
The rules and state tables for the Listener role use the following
operations:
o The "Clear Supplied RIBs" operation is defined as setting up an
empty container to hold the set of RIBs supplied by the peer node.
o The "Record RIBs Supplied" operation is defined as:
1. Taking the RIB entries from a received RIB TLV.
2. Verifying that the String ID used in each entry is present in
the dictionary. If not, an Error TLV containing the offending
String ID is sent to the peer, and the Initiator and Listener
processes are aborted and restarted as if the ESTAB state had
just been reached.
3. If all the String IDs are present in the dictionary, record
the delivery predictabilities for each EID in the entries.
o The "Recalc Dlvy Predictabilities" operation uses the algorithms
defined in Section 2.1.2 to update the local set of delivery
predictabilities using the using the set of delivery
predictabilities supplied by the peer in RIB TLVs.
o The "Determine Offers" operation determines the set of bundles to
be offered to the peer. The local delivery predictabilities and
the delivery predictabilities supplied by the peer are compared,
and a prioritized choice of the bundles stored in this node to be
offered to the peer is made according to the configured queueing
policy and forwarding strategy.
o The "Determine ACKs" operation is defined as obtaining the set of
PRoPHET ACKs recorded by the bundle protocol agent that need to be
forwarded to the peer. The list of PRoPHET ACKs is maintained
internally by the PRoPHET protocol implementation rather than the
main bundle protocol agent (see Section 3.5).
o The "Determine Offer Dict Updates" operation is defined as
determining any extra EIDs that are not already in the dictionary,
recording the previous state of the local dictionary, and then
adding the required extra entries to the dictionary.
o The "Send Offers" operation is defined as formatting one or more
non-empty Bundle Offer TLVs, incorporating the sets of Offers and
PRoPHET ACKs previously determined, and sending them to the peer
node. If more than one Bundle Offer TLV is sent, all but the last
one MUST have the "More Offer/Response TLVs Following" flag set to
1; the last or only one MUST have the flag set to 0.
o The "Record Requests" operation is defined as recording all the
bundles offered in a Bundle Offer TLV in the list of bundles
offers. Duplicates MUST be ignored. The order of requests in the
TLVs MUST be maintained so far as is possible (it is possible that
a bundle has to be re-sent, and this may result in out-of-order
delivery).
o The "Send Bundles" operation is defined as sending, in the order
requested, the bundles in the requested list. This requires the
list to be communicated to the bundle protocol agent (see
Section 2.2).
o The "Check Initiator Start Point" operation is defined as checking
the configured sequential operation policy to determine if the
Listener role has reached the point where the Initiator role
should be started. If so, the InitStart notification is sent to
the Initiator role in the same node.
The following events are specific to the Listener role state machine:
RIBnotlast RIB TLV received with "More RIB TLVs" flag set to 1.
RIBlast RIB TLV received with "More RIB TLVs" flag set to 0 and
a non-zero count of RIB Entries.
REQnotlast Bundle Response TLV received with More Offer/Response
TLVs Following flag set to 1.
REQlast Bundle Response TLV received with More Offer/Response
TLVs Following flag set to 0 and a non-zero count of
bundle offers.
REQempty Bundle Response TLV received with More Offer/Response
TLVs Following flag set to 0 and a zero count of bundle
offers.
State: WAIT_DICT
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| On Entry | Check Initiator Start Point | WAIT_DICT |
+------------------+-----------------------------------+-----------+
| RIBDi | Update Dictionary (note 1) | |
| | If Dictionary Conflict found: | |
| | Abort Exchange |(finished) |
| | Else: | |
| | Start TP | WAIT_RIB |
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | WAIT_DICT |
+==================================================================+
State: WAIT_RIB
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| On Entry | Clear Supplied RIBS | WAIT_RIB |
+------------------+-----------------------------------+-----------+
| RIBDi | Update Dictionary (note 1) | |
| | If Dictionary Conflict found: | |
| | Abort Exchange |(finished) |
| | Else: | |
| | Start TP | WAIT_RIB |
+------------------+-----------------------------------+-----------+
| RIBnotlast | Record RIBS Supplied (note 2) | |
| | If EID missing in dictionary: | |
| | Abort Exchange |(finished) |
| | Else: | |
| | Start TP | WAIT_RIB |
+------------------+-----------------------------------+-----------
| RIBlast | Check Initiator Start Point | |
| | Record RIBS Supplied (note 2) | |
| | If EID missing in dictionary: | |
| | Abort Exchange |(finished) |
| | Otherwise | |
| | Recalc Dlvy | |
| | Predictabilities | |
| | Cancel TP | |
| | Determine Offers | |
| | Determine ACKs | |
| | Determine Offer | |
| | Dict Updates | |
| | Send RIB Dictionary | |
| | Updates | |
| | Send Offers | |
| | Start TI | OFFER |
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | WAIT_DICT |
+------------------+-----------------------------------+-----------+
|Any Other TLV rcvd| Abort Exchange |(finished) |
+------------------+-----------------------------------+-----------+
| Timeout(peer) | Send RIB Dictionary Updates | |
| | Send Offers | |
| | Start TI (note 3) | OFFER |
+==================================================================+
State: OFFER
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| REQnotlast | Send Bundles | |
| | Start TI | OFFER |
+------------------+-----------------------------------+-----------+
| REQlast | Cancel TI | |
| | Check Initiator Start Point | |
| | Send Bundles | SND_BUNDLE|
+------------------+-----------------------------------+-----------+
| REQempty | Cancel TI | |
| | Check Initiator Start Point | WAIT_MORE|
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | WAIT_DICT |
+------------------+-----------------------------------+-----------+
| Timeout(info) | Send RIB Dictionary Updates | |
| | Send Offers | |
| | Start TI (note 3) | OFFER |
+==================================================================+
State: SND_BUNDLE
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| REQnotlast | Send Bundles | |
| | Start TI | SND_BUNDLE|
+------------------+-----------------------------------+-----------+
| REQlast | Cancel TI | |
| | Send Bundles | SND_BUNDLE|
+------------------+-----------------------------------+-----------+
| REQempty | Cancel TI | |
| | Check Initiator Start Point | WAIT_MORE|
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | WAIT_DICT |
+------------------+-----------------------------------+-----------+
| Timeout(info) | Send RIB Dictionary Updates | |
| | Send Offers | |
| | Start TI (note 3) | OFFER |
+==================================================================+
State: WAIT_MORE
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| More Bundles | Determine Offers | |
| | Determine ACKs | |
| | Determine Offer | |
| | Dict Updates | |
| | Send RIB Dictionary | |
| | Updates | |
| | Send Offers | |
| | Start TI | OFFER |
+------------------+-----------------------------------+-----------+
| RIBDi | Update Dictionary (note 1) | |
| | If Dictionary Conflict found: | |
| | Abort Exchange |(finished) |
| | Else: | |
| | Start TP | WAIT_RIB |
+------------------+-----------------------------------+-----------+
| REQnotlast | Send Bundles | |
| | Start TI | SND_BUNDLE|
+------------------+-----------------------------------+-----------+
| REQlast | Cancel TI | |
| | Send Bundles | SND_BUNDLE|
+------------------+-----------------------------------+-----------+
| REQempty | Cancel TI | |
| | Check Initiator Start Point | SND_BUNDLE|
+------------------+-----------------------------------+-----------+
| HelloAck | Abort Exchange | WAIT_DICT |
+------------------+-----------------------------------+-----------+
| Timeout(info) | Send RIB Dictionary Updates | |
| | Send Offers | |
| | Start TI (note 3) | OFFER |
+==================================================================+
Note 1:
Both the dictionary and the RIB TLVs may come in the same PRoPHET
message. In that case, the state will change to WAIT_RIB, and the
RIB will then immediately be processed.
Note 2:
Send an ACK if the timer for the peering node expires. Either the
link has been broken, and then the link setup will restart, or it
will trigger the Information Exchange Phase to restart.
Note 3:
When the RIB is received, it is possible for the PRoPHET agent to
update its delivery predictabilities according to Section 2.1.2.
The delivery predictabilities and the RIB is then used together
with the forwarding strategy in use to create a bundle offer TLV.
This is sent to the peering node.
Note 4:
No more bundles are requested by the other node; transfer is
complete.
Note 5:
No response to the bundle offer has been received before the timer
expired, so we re-send the bundle offer.
5.4. Interaction with Nodes Using Version 1 of PRoPHET
There are existing implementations of PRoPHET based on draft versions
of this specification that use version 1 of the protocol. There are
a number of significant areas of difference between version 1 and
version 2 as described in this document:
o In version 1, the delivery predictability update equations were
significantly different, and in the case of the transitivity
equation (Equation 3) could lead to degraded performance or non-
delivery of bundles in some circumstances.
o In the current version , constraints were placed on the String IDs
generated by each node to ensure that it was not possible for
there to be a conflict if the IDs were generated concurrently and
independently in the two nodes.
o In the current version, a flag has been added to the Routing
Information Base Dictionary TLV to distinguish dictionary updates
sent by the Initiator role and by the Listener role.
o In the current version, the Bundle Offer and Response TLVs have
been significantly revised. The version 2 TLVs have been
allocated new TLV Type numbers, and the version 1 TLVs (types 0xA2
and 0xA3) are now deprecated. For each bundle specifier, the
source EID is transmitted in addition to the creation timestamp by
version 2 to ensure that the bundle is uniquely identified.
Version 2 also transmits the fragment payload offset and length
when the offered bundle is a bundle fragment. The payload length
can optionally be transmitted for each bundle (whether or not it
is a fragment) to give the receiver additional information that
can be useful when determining which bundle offers to accept.
o The behavior of the system after the first Information Exchange
Phase has been better defined. The state machine has been altered
to better describe how the ongoing operations work. This has
involved the removal of the high-level state WAIT_INFO and the
addition of two states in the Listener role subsidiary state
machine (SND_BUNDLE and WAIT_MORE). The protocol on the wire has
not been altered by this change to the description of the state
machine. However, the specification of the later stages of
operation was slightly vague and might have been interpreted
differently by various implementers.
A node implementing version 2 of the PRoPHET protocol as defined in
this document MAY ignore a communication opportunity with a node that
sends a HELLO message indicating that it uses version 1, or it MAY
partially downgrade and respond to messages as if it were a version 1
node. This means that the version field in all message headers MUST
contain 1.
It is RECOMMENDED that the version 2 node use the metric update
equations defined in this document even when communicating with a
version 1 node as this will partially inhibit the problems with the
transitivity equation in version 1, and that the version 2 node
modify any received metrics that are greater than (1 - delta) to be
(1 - delta) to avoid becoming a "sink" for bundles that are not
destined for this node. Also version 1 nodes cannot be explicitly
offered bundle fragments, and an exchange with a node supporting
version 1 MUST use the, now deprecated, previous versions of the
Bundle Offer and Response TLVs.
Generally, nodes using version 1 should be upgraded if at all
possible because of problems that have been identified.
6. Security Considerations
Currently, PRoPHET does not specify any special security measures.
As a routing protocol for intermittently connected networks, PRoPHET
is a target for various attacks. The various known possible
vulnerabilities are discussed in this section.
The attacks described here are not problematic if all nodes in the
network can be trusted and are working towards a common goal. If
there exist such a set of nodes, but there also exist malicious
nodes, these security problems can be solved by introducing an
authentication mechanism when two nodes meet, for example, using a
public key system. Thus, only nodes that are known to be members of
the trusted group of nodes are allowed to participate in the routing.
This of course introduces the additional problem of key distribution,
but that is not addressed here.
Where suitable, the mechanisms (such as key management and bundle
authentication or integrity checks) and terminology specified by the
Bundle Security Protocol [RFC6257] are to be used.
6.1. Attacks on the Operation of the Protocol
There are a number of kinds of attacks on the operation of the
protocol that it would be possible to stage on a PRoPHET network.
The attacks and possible remedies are listed here.
6.1.1. Black-Hole Attack
A malicious node sets its delivery predictabilities for all
destinations to a value close to or exactly equal to 1 and/or
requests all bundles from nodes it meets, and does not forward any
bundles. This has two effects, both causing messages to be drawn
towards the black hole instead of to their correct destinations.
1. A node encountering a malicious node will try to forward all its
bundles to the malicious node, creating the belief that the
bundle has been very favorably forwarded. Depending on the
forwarding strategy and queueing policy in use, this might hamper
future forwarding of the bundle and/or lead to premature dropping
of the bundle.
2. Due to the transitivity, the delivery predictabilities reported
by the malicious node will affect the delivery predictabilities
of other nodes. This will create a gradient for all destinations
with the black hole as the "center of gravity" towards which all
bundles traverse. This should be particularly severe in
connected parts of the network.
6.1.1.1. Attack Detection
A node receiving a set of delivery predictabilities that are all at
or close to 1 should be suspicious. Similarly, a node that accepts
all bundles and offers none might be considered suspicious. However,
these conditions are not impossible in normal operation.
6.1.1.2. Attack Prevention/Solution
To prevent this attack, authentication between nodes that meet needs
to be present. Nodes can also inspect the received metrics and
bundle acceptances/offers for suspicious patterns and terminate
communications with nodes that appear suspicious. The natural
evolution of delivery predictabilities should mean that a genuine
node would not be permanently ostracized even if the values lead to
termination of a communication opportunity on one occasion. The
epidemic nature of PRoPHET would mean that such a termination rarely
leads to non-delivery of bundles.
6.1.2. Limited Black-Hole Attack / Identity Spoofing
A malicious node misrepresents itself by claiming to be someone else.
The effects of this attack are:
1. The effects of the black-hole attack listed above hold for this
attack as well, with the exception that only the delivery
predictabilities and bundles for one particular destination are
affected. This could be used to "steal" the data that should be
going to a particular node.
2. In addition to the above problems, PRoPHET ACKs will be issued
for the bundles that are delivered to the malicious node. This
will cause these bundles to be removed from the network, reducing
the chance that they will reach their real destination.
6.1.2.1. Attack Detection
The destination can detect that this kind of attack has occurred (but
it cannot prevent the attack) when it receives a PRoPHET ACK for a
bundle destined to itself but for which it did not receive the
corresponding bundle.
6.1.2.2. Attack Prevention/Solution
To prevent this attack, authentication between nodes that meet needs
to be present.
6.1.3. Fake PRoPHET ACKs
A malicious node may issue fake PRoPHET ACKs for all bundles (or only
bundles for a certain destination if the attack is targeted at a
single node) carried by nodes it met. The affected bundles will be
deleted from the network, greatly reducing their probability of being
delivered to the destination.
6.1.3.1. Attack Prevention/Solution
If a public key cryptography system is in place, this attack can be
prevented by mandating that all PRoPHET ACKs be signed by the
destination. Similarly to other solutions using public key
cryptography, this introduces the problem of key distribution.
6.1.4. Bundle Store Overflow
After encountering and receiving the delivery predictability
information from the victim, a malicious node may generate a large
number of fake bundles for the destination for which the victim has
the highest delivery predictability. This will cause the victim to
most likely accept these bundles, filling up its bundle storage,
possibly at the expense of other, legitimate, bundles. This problem
is transient as the messages will be removed when the victim meets
the destination and delivers the messages.
6.1.4.1. Attack Detection
If it is possible for the destination to figure out that the bundles
it is receiving are fake, it could report that malicious actions are
underway.
6.1.4.2. Attack Prevention/Solution
This attack could be prevented by requiring sending nodes to sign all
bundles they send. By doing this, intermediate nodes could verify
the integrity of the messages before accepting them for forwarding.
6.1.5. Bundle Store Overflow with Delivery Predictability Manipulation
A more sophisticated version of the attack in the previous section
can be attempted. The effect of the previous attack was lessened
since the destination node of the fake bundles existed. This caused
fake bundles to be purged from the network when the destination was
encountered. The malicious node may now use the transitive property
of the protocol to boost the victim's delivery predictabilities for a
non-existent destination. After this, it creates a large number of
fake bundles for this non-existent destination and offers them to the
victim. As before, these bundles will fill up the bundle storage of
the victim. The impact of this attack will be greater as there is no
probability of the destination being encountered and the bundles
being acknowledged. Thus, they will remain in the bundle storage
until they time out (the malicious node may set the timeout to a
large value) or until they are evicted by the queueing policy.
The delivery predictability for the fake destination may spread in
the network due to the transitivity, but this is not a problem, as it
will eventually age and fade away.
The impact of this attack could be increased if multiple malicious
nodes collude, as network resources can be consumed at a greater
speed and at many different places in the network simultaneously.
6.2. Interactions with External Routing Domains
Users may opt to connect two regions of sparsely connected nodes
through a connected network such as the Internet where another
routing protocol is running. To this network, PRoPHET traffic would
look like any other application-layer data. Extra care must be taken
in setting up these gateway nodes and their interconnections to make
sure that malicious nodes cannot use them to launch attacks on the
infrastructure of the connected network. In particular, the traffic
generated should not be significantly more than what a single regular
user end host could create on the network.
7. IANA Considerations
Following the policies outlined in "Guidelines for Writing an IANA
Considerations Section in RFCs" (RFC 5226 [RFC5226]), the following
name spaces are defined in PRoPHET.
o For fields in the PRoPHET message header (Section 4.1):
* DTN Routing Protocol Number
* PRoPHET Protocol Version
* PRoPHET Header Flags
* PRoPHET Result Field
* PRoPHET Codes for Success and Codes for Failure
o Identifiers for TLVs carried in PRoPHET messages:
* PRoPHET TLV Type (Section 4.2)
o Definitions of TLV Flags and other flag fields in TLVs:
* Hello TLV Flags (Section 4.3.1)
* Error TLV Flags (Section 4.3.2)
* Routing Information Base (RIB) Dictionary TLV Flags
(Section 4.3.3)
* Routing Information Base (RIB) TLV Flags (Section 4.3.4)
* Routing Information Base (RIB) Flags per entry (Section 4.3.4)
* Bundle Offer and Response TLV Flags (Section 4.3.5)
* Bundle Offer and Response B Flags per offer or response
(Section 4.3.5)
The following subsections list the registries that have been created.
Initial values for the registries are given below; future assignments
for unassigned values are to be made through the Specification
Required policy. Where specific values are defined in the IANA
registries according to the specifications in the subsections below,
the registry refers to this document as defining the allocation.
7.1. DTN Routing Protocol Number
The encoding of the Protocol Number field in the PRoPHET header
(Section 4.1) is:
+--------------------------+-----------+---------------+
| Protocol | Value | Reference |
+--------------------------+-----------+---------------+
| PRoPHET Protocol | 0x00 | This document |
| Unassigned | 0x01-0xEF | |
| Private/Experimental Use | 0xF0-0xFF | This document |
+--------------------------+-----------+---------------+
7.2. PRoPHET Protocol Version
The encoding of the PRoPHET Version field in the PRoPHET header
(Section 4.1) is:
+----------------------------+-----------+---------------+
| Version | Value | Reference |
+----------------------------+-----------+---------------+
| Reserved (do not allocate) | 0x00 | This document |
| PRoPHET v1 | 0x01 | This document |
| PRoPHET v2 | 0x02 | This document |
| Unassigned | 0x03-0xEF | |
| Private/Experimental Use | 0xF0-0xFE | This document |
| Reserved | 0xFF | |
+----------------------------+-----------+---------------+
7.3. PRoPHET Header Flags
The following Flags are defined for the PRoPHET Header (Section 4.1):
+------------+--------------+-----------+
| Meaning | Bit Position | Reference |
+------------+--------------+-----------+
| Unassigned | Bit 0 | |
| Unassigned | Bit 1 | |
| Unassigned | Bit 2 | |
| Unassigned | Bit 3 | |
+------------+--------------+-----------+
7.4. PRoPHET Result Field
The encoding of the Result field in the PRoPHET header (Section 4.1)
is:
+--------------------------+-------------+---------------+
| Result Value | Value | Reference |
+--------------------------+-------------+---------------+
| Reserved | 0x00 | This document |
| NoSuccessAck | 0x01 | This document |
| AckAll | 0x02 | This document |
| Success | 0x03 | This document |
| Failure | 0x04 | This document |
| ReturnReceipt | 0x05 | This document |
| Unassigned | 0x06 - 0x7F | |
| Private/Experimental Use | 0x80 - 0xFF | This document |
+--------------------------+-------------+---------------+
7.5. PRoPHET Codes for Success and Codes for Failure
The encoding for Code field in the PRoPHET header (Section 4.1) for
"Success" messages is:
+--------------------------+-------------+---------------+
| Code Name | Values | Reference |
+--------------------------+-------------+---------------+
| Generic Success | 0x00 | This document |
| Submessage Received | 0x01 | This document |
| Unassigned | 0x02 - 0x7F | |
| Private/Experimental Use | 0x80 - 0xFF | This document |
+--------------------------+-------------+---------------+
The encoding for Code in the PRoPHET header (Section 4.1) for
"Failure" messages is:
+----------------------------+-------------+---------------+
| Code Name | Values | Reference |
+----------------------------+-------------+---------------+
| Reserved (do not allocate) | 0x00 - 0x01 | This document |
| Unspecified Failure | 0x02 | This document |
| Unassigned | 0x03 - 0x7F | |
| Private/Experimental Use | 0x80 - 0xFE | This document |
| Error TLV in Message | 0xFF | This document |
+----------------------------+-------------+---------------+
7.6. PRoPHET TLV Type
The TLV Types defined for PRoPHET (Section 4.2) are:
+------------------------------+-------------+---------------+
| Type | Value | Reference |
+------------------------------+-------------+---------------+
| Reserved (do not allocate) | 0x00 | This document |
| Hello TLV | 0x01 | This document |
| Error TLV | 0x02 | This document |
| Unsassigned | 0x03 - 0x9F | |
| RIB dictionary TLV | 0xA0 | This document |
| RIB TLV | 0xA1 | This document |
| Bundle Offer (deprecated) | 0xA2 | This document |
| Bundle Response (deprecated) | 0xA3 | This document |
| Bundle Offer (v2) | 0xA4 | This document |
| Bundle Response (v2) | 0xA5 | This document |
| Unassigned | 0xA6 - 0xCF | |
| Private/Experimental Use | 0xD0 - 0xFF | This document |
+------------------------------+-------------+---------------+
7.7. Hello TLV Flags
The following TLV Flags are defined for the Hello TLV
(Section 4.3.1). Flag numbers 0, 1, and 2 are treated as a 3-bit
unsigned integer with 5 of the 8 possible values allocated, and the
other 3 reserved. The remaining bits are treated individually:
+----------------------------+---------------------+---------------+
| Meaning | Value | Reference |
+----------------------------+---------------------+---------------+
| | (Flags 0, 1, and 2) | |
| Reserved (do not allocate) | 0b000 | This document |
| SYN | 0b001 | This document |
| SYNACK | 0b010 | This document |
| ACK | 0b011 | This document |
| RSTACK | 0b100 | This document |
| Unassigned | 0b101 - 0b111 | |
| | (Flags 3 - 7) | |
| Unassigned | Flag 3 | |
| Unassigned | Flag 4 | |
| Unassigned | Flag 5 | |
| Unassigned | Flag 6 | |
| L Flag | Flag 7 | This document |
+----------------------------+---------------------+---------------+
7.8. Error TLV Flags
The TLV Flags field in the Error TLV (Section 4.3.2) is treated as an
unsigned 8-bit integer encoding the Error TLV number. The following
values are defined:
+--------------------------+------------------+---------------+
| Error TLV Name | Error TLV Number | Reference |
+--------------------------+------------------+---------------+
| Dictionary Conflict | 0x00 | This document |
| Bad String ID | 0x01 | This document |
| Unassigned | 0x02 - 0x7F | |
| Private/Experimental Use | 0x80 - 0xFF | This document |
+--------------------------+------------------+---------------+
7.9. RIB Dictionary TLV Flags
The following TLV Flags are defined for the RIB Base Dictionary TLV
(Section 4.3.3):
+----------------------------+--------------+---------------+
| Meaning | Bit Position | Reference |
+----------------------------+--------------+---------------+
| Sent by Listener | Flag 0 | This document |
| Reserved (do not allocate) | Flag 1 | This document |
| Reserved (do not allocate) | Flag 2 | This document |
| Unassigned | Flag 3 | |
| Unassigned | Flag 4 | |
| Unassigned | Flag 5 | |
| Unassigned | Flag 6 | |
| Unassigned | Flag 7 | |
+----------------------------+--------------+---------------+
7.10. RIB TLV Flags
The following TLV Flags are defined for the RIB TLV (Section 4.3.4):
+----------------------------+--------------+---------------+
| Meaning | Bit Position | Reference |
+----------------------------+--------------+---------------+
| More RIB TLVs | Flag 0 | This document |
| Reserved (do not allocate) | Flag 1 | This document |
| Reserved (do not allocate) | Flag 2 | This document |
| Unassigned | Flag 3 | |
| Unassigned | Flag 4 | |
| Unassigned | Flag 5 | |
| Unassigned | Flag 6 | |
| Unassigned | Flag 7 | |
+----------------------------+--------------+---------------+
7.11. RIB Flags
The following RIB Flags are defined for the individual entries in the
RIB TLV (Section 4.3.4):
+------------+--------------+-----------+
| Meaning | Bit Position | Reference |
+------------+--------------+-----------+
| Unassigned | Flag 0 | |
| Unassigned | Flag 1 | |
| Unassigned | Flag 2 | |
| Unassigned | Flag 3 | |
| Unassigned | Flag 4 | |
| Unassigned | Flag 5 | |
| Unassigned | Flag 6 | |
| Unassigned | Flag 7 | |
+------------+--------------+-----------+
7.12. Bundle Offer and Response TLV Flags
The following TLV Flags are defined for the Bundle Offer and Response
TLV (Section 4.3.5):
+------------------------------------+--------------+---------------+
| Meaning | Bit Position | Reference |
+------------------------------------+--------------+---------------+
| More Offer/Response TLVs Following | Flag 0 | This document |
| Unassigned | Flag 1 | |
| Unassigned | Flag 2 | |
| Unassigned | Flag 3 | |
| Unassigned | Flag 4 | |
| Unassigned | Flag 5 | |
| Unassigned | Flag 6 | |
| Unassigned | Flag 7 | |
+------------------------------------+--------------+---------------+
7.13. Bundle Offer and Response B Flags
The following B Flags are defined for each Bundle Offer in the Bundle
Offer and Response TLV (Section 4.3.5):
+------------------------------------+--------------+---------------+
| Meaning | Bit Position | Reference |
+------------------------------------+--------------+---------------+
| Bundle Accepted | Flag 0 | This document |
| Bundle is a Fragment | Flag 1 | This document |
| Bundle Payload Length Included in | Flag 2 | This document |
| TLV | | |
| Unassigned | Flag 3 | |
| Unassigned | Flag 4 | |
| Unassigned | Flag 5 | |
| Unassigned | Flag 6 | |
| PRoPHET ACK | Flag 7 | This document |
+------------------------------------+--------------+---------------+
8. Implementation Experience
Multiple independent implementations of the PRoPHET protocol exist.
The first implementation is written in Java, and has been optimized
to run on the Lego MindStorms platform that has very limited
resources. Due to the resource constraints, some parts of the
protocol have been simplified or omitted, but the implementation
contains all the important mechanisms to ensure proper protocol
operation. The implementation is also highly modular and can be run
on another system with only minor modifications (it has currently
been shown to run on the Lego MindStorms platform and on regular
laptops).
Another implementation is written in C++ and runs in the OmNet++
simulator to enable testing and evaluation of the protocol and new
features. Experience and feedback from the implementers on early
versions of the protocol have been incorporated into the current
version.
An implementation compliant to an Internet-Draft (which was posted in
2006 and eventually evolved into this RFC) has been written at Baylor
University. This implementation has been integrated into the DTN2
reference implementation.
An implementation of the protocol in C++ was developed by one of the
authors (Samo Grasic) at Lulea University of Technology (LTU) as part
of the Saami Networking Connectivity project (see Section 9) and
continues to track the development of the protocol. This work is now
part of the Networking for Communications Challenged Communities
(N4C) project and is used in N4C testbeds.
9. Deployment Experience
During a week in August 2006, a proof-of-concept deployment of a DTN
system, using the LTU PRoPHET implementation for routing was made in
the Swedish mountains -- the target area for the Saami Network
Connectivity project [ccnc07] [doria_02]. Four fixed camps with
application gateways, one Internet gateway, and seven mobile relays
were deployed. The deployment showed PRoPHET to be able to route
bundles generated by different applications such as email and web
caching.
Within the realms of the SNC and N4C projects, multiple other
deployments, both during summer and winter conditions, have been done
at various scales during 2007-2010 [winsdr08].
An implementation has been made for Android-based mobile telephones
in the Bytewalla project [bytewalla].
10. Acknowledgements
The authors would like to thank Olov Schelen and Kaustubh S. Phanse
for contributing valuable feedback regarding various aspects of the
protocol. We would also like to thank all other reviewers and the
DTNRG chairs for the feedback in the process of developing the
protocol. The Hello TLV mechanism is loosely based on the Adjacency
message developed for RFC 3292. Luka Birsa and Jeff Wilson have
provided us with feedback from doing implementations of the protocol
based on various preliminary versions of the document. Their
feedback has helped us make the document easier to read for an
implementer and has improved the protocol.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, November 2007.
11.2. Informative References
[CLAYER] Demmer, M., Ott, J., and S. Perreault, "Delay Tolerant
Networking TCP Convergence Layer Protocol", Work
in Progress, August 2012.
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058,
June 1988.
[RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L.,
Durst, R., Scott, K., Fall, K., and H. Weiss, "Delay-
Tolerant Networking Architecture", RFC 4838,
April 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
[RFC6257] Symington, S., Farrell, S., Weiss, H., and P. Lovell,
"Bundle Security Protocol Specification", RFC 6257,
May 2011.
[bytewalla] Prasad, M., "Bytewalla 3: Network architecture and
PRoPHET implementation", Bytewalla Project, KTH Royal
Institute of Technology, Stockholm, Sweden, October
2010,
<http://www.bytewalla.org/sites/bytewalla.org/files/
Bytewalla3_Network_architecture_and_PRoPHET_v1.0.pdf>.
[ccnc07] Lindgren, A. and A. Doria, "Experiences from Deploying
a Real-life DTN System", Proceedings of the 4th Annual
IEEE Consumer Communications and Networking Conference
(CCNC 2007), Las Vegas, Nevada, USA, January 2007.
[doria_02] Doria, A., Uden, M., and D. Pandey, "Providing
connectivity to the Saami nomadic community",
Proceedings of the 2nd International Conference on
Open Collaborative Design for Sustainable Innovation
(dyd 02), Bangalore, India, December 2002.
[lindgren_06] Lindgren, A. and K. Phanse, "Evaluation of Queueing
Policies and Forwarding Strategies for Routing in
Intermittently Connected Networks", Proceedings of
COMSWARE 2006, January 2006.
[vahdat_00] Vahdat, A. and D. Becker, "Epidemic Routing for
Partially Connected Ad Hoc Networks", Duke University
Technical Report CS-200006, April 2000.
[winsdr08] Lindgren, A., Doria, A., Lindblom, J., and M. Ek,
"Networking in the Land of Northern Lights - Two Years
of Experiences from DTN System Deployments",
Proceedings of the ACM Wireless Networks and Systems
for Developing Regions Workshop (WiNS-DR), San
Francisco, California, USA, September 2008.
Appendix A. PRoPHET Example
To help grasp the concepts of PRoPHET, an example is provided to give
an understanding of the transitive property of the delivery
predictability and the basic operation of PRoPHET. In Figure 13, we
revisit the scenario where node A has a message it wants to send to
node D. In the bottom right corner of subfigures a-c, the delivery
predictability tables for the nodes are shown. Assume that nodes C
and D encounter each other frequently (Figure 13a), making the
delivery predictability values they have for each other high. Now
assume that node C also frequently encounters node B (Figure 13b).
Nodes B and C will get high delivery predictability values for each
other, and the transitive property will also increase the value B has
for D to a medium level. Finally, node B meets node A (Figure 13c),
which has a message for node D. Figure 13d shows the message
exchange between node A and node B. Summary vectors and delivery
predictability information is exchanged, delivery predictabilities
are updated, and node A then realizes that P_(b,d) > P_(a,d), and
thus forwards the message for node D to node B.
+----------------------------+ +----------------------------+
| | | |
| C | | D |
| D | | |
| B | | B C |
| | | |
| | | |
| | | |
| | | |
| A* | | A* |
+-------------+--------------+ +-------------+--------------+
| A | B | C | D | | A | B | C | D |
|B:low |A:low |A:low |A:low | |B:low |A:low |A:low |A:low |
|C:low |C:low |B:low |B:low | |C:low |C:high|B:high |B:low |
|D:low |D:low |D:high |C:high| |D:low |D:med |D:high |C:high|
+-------------+--------------+ +-------------+--------------+
(a) (b)
+----------------------------+ A B
| | | |
| D | |Summary vector&delivery pred|
| | |--------------------------->|
| C | |Summary vector&delivery pred|
| | |<---------------------------|
| | | |
| B* | Update delivery predictabilities
| A | | |
| | Packet for D not in SV |
+-------------+--------------+ P(b,d)>P(a,d) |
| A | B | C | D | Thus, send |
|B:low |A:low |A:low |A:low | | |
|C:med |C:high|B:high |B:low | | Packet for D |
|D:low+|D:med |D:high |C:high| |--------------------------->|
+-------------+--------------+ | |
(c) (d)
Figure 13: PRoPHET example
Appendix B. Neighbor Discovery Example
This section outlines an example of a simple neighbor discovery
protocol that can be run in-between PRoPHET and the underlying layer
in case lower layers do not provide methods for neighbor discovery.
It assumes that the underlying layer supports broadcast messages as
would be the case if a wireless infrastructure was involved.
Each node needs to maintain a list of its active neighbors. The
operation of the protocol is as follows:
1. Every BEACON_INTERVAL milliseconds, the node does a local
broadcast of a beacon that contains its identity and address, as
well as the BEACON_INTERVAL value used by the node.
2. Upon reception of a beacon, the following can happen:
A. The sending node is already in the list of active neighbors.
Update its entry in the list with the current time, and
update the node's BEACON_INTERVAL if it has changed.
B. The sending node is not in the list of active neighbors. Add
the node to the list of active neighbors and record the
current time and the node's BEACON_INTERVAL. Notify the
PRoPHET agent that a new neighbor is available ("New
Neighbor", as described in Section 2.4).
3. If a beacon has not been received from a node in the list of
active neighbors within a time period of NUM_ACCEPTED_LOSSES *
BEACON_INTERVAL (for the BEACON_INTERVAL used by that node), it
should be assumed that this node is no longer a neighbor. The
entry for this node should be removed from the list of active
neighbors, and the PRoPHET agent should be notified that a
neighbor has left ("Neighbor Gone", as described in Section 2.4).
Appendix C. PRoPHET Parameter Calculation Example
The evolution of the delivery predictabilities in a PRoPHET node is
controlled by three main equations defined in Section 2.1.2. These
equations use a number of parameters that need to be appropriately
configured to ensure that the delivery predictabilities evolve in a
way that mirrors the mobility model that applies in the PRoPHET zone
where the node is operating.
When trying to describe the mobility model, it is more likely that
the model will be couched in terms of statistical distribution of
times between encounters and times to deliver a bundle in the zone.
In this section, one possible way of deriving the PRoPHET parameters
from a more usual description of the model is presented. It should
be remembered that this may not be the only solution, and its
appropriateness will depend both on the overall mobility model and
the distribution of the times involved. There is an implicit
assumption in this work that these distributions can be characterized
by a normal-type distribution with a well-defined first moment
(mean). The exact form of the distribution is not considered here,
but more detailed models may wish to use more specific knowledge
about the distributions to refine the derivation of the parameters.
To characterize the model, we consider the following parameters:
P1 The time resolution of the model.
P2 The average time between encounters between nodes, I_typ, where
the identity of the nodes is not taken into account.
P3 The average number of encounters that a node has between meeting
a particular node and meeting the same node again.
P4 The average number of encounters needed to deliver a bundle in
this zone.
P5 The multiple of the average number of encounters needed to
deliver a bundle (P4) after which it can be assumed that a node
is not going to encounter a particular node again in the
foreseeable future so that the delivery predictability ought to
be decayed below P_first_threshold.
P6 The number of encounters between a particular pair of nodes that
should result in the delivery predictability of the encountered
node getting close to the maximum possible delivery
predictability (1 - delta).
We can use these parameters to derive appropriate values for gamma
and P_encounter_max, which are the key parameters in the evolution of
the delivery predictabilities. The values of the other parameters
P_encounter_first (0.5), P_first_threshold (0.1), and delta (0.01),
with the default values suggested in Figure 3, generally are not
specific to the mobility model, although in special cases
P_encounter_first may be different if extra information is available.
To select a value for gamma:
After a single, unrepeated encounter, the delivery predictability of
the encountered node should decay from P_encounter_first to
P_first_threshold in the expected time for P4 * P5 encounters. Thus:
P_first_threshold = P_encounter_first * gamma ^ ((P2 * P4 * P5)/P1)
which can be rearranged as
gamma =
exp(ln(P_first_threshold/P_encounter_first) * P1 / (P2* P4 * P5)).
Typical values of gamma will be less than 1, but very close to 1
(usually greater than 0.99). The value has to be stored to several
decimal places of accuracy, but implementations can create a table of
values for specific intervals to reduce the amount of on-the-fly
calculation required.
Selecting a value for P_encounter_max:
Once gamma has been determined, the decay factor for the average time
between encounters between a specific pair of nodes can be
calculated:
Decay_typ = gamma ^ ((P2 * P3)/P1)
Starting with P_encounter_first, using Decay_typ and applying
Equation 1 from Section 2.1.2 (P6 - 1) times, we can calculate the
typical delivery predictability for the encountered node after P6
encounters. The nature of Equation 1 is such that it is not easy to
produce a closed form that generates a value of P_encounter_max from
the parameter values, but using a spreadsheet to apply the equation
repeatedly and tabulate the results will allow a suitable value of
P_encounter_max to be chosen very simply. The evolution is not very
sensitive to the value of P_encounter_max, and values in the range
0.4 to 0.8 will generally be appropriate. A value of 0.7 is
recommended as a default.
Once a PRoPHET zone has been in operation for some time, the logs of
the actual encounters can and should be used to check that the
selected parameters were appropriate and to tune them as necessary.
In the longer term, it may prove possible to install a learning mode
in nodes so that the parameters can be adjusted dynamically to
maintain best congruence with the mobility model that may itself
change over time.
Authors' Addresses
Anders F. Lindgren
Swedish Institute of Computer Science
Box 1263
Kista SE-164 29
SE
Phone: +46707177269
EMail: andersl@sics.se
URI: http://www.sics.se/~andersl
Avri Doria
Technicalities
Providence RI
US
EMail: avri@acm.org
URI: http://psg.com/~avri
Elwyn Davies
Folly Consulting
Soham
UK
EMail: elwynd@folly.org.uk
Samo Grasic
Lulea University of Technology
Lulea SE-971 87
SE
EMail: samo.grasic@ltu.se