Internet Engineering Task Force (IETF) J. Chroboczek
Request for Comments: 8966 IRIF, University of Paris-Diderot
Obsoletes: 6126, 7557 D. Schinazi
Category: Standards Track Google LLC
ISSN: 2070-1721 January 2021
The Babel Routing Protocol
Abstract
Babel is a loop-avoiding, distance-vector routing protocol that is
robust and efficient both in ordinary wired networks and in wireless
mesh networks. This document describes the Babel routing protocol
and obsoletes RFC 6126 and RFC 7557.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8966.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction
1.1. Features
1.2. Limitations
1.3. Specification of Requirements
2. Conceptual Description of the Protocol
2.1. Costs, Metrics, and Neighbourship
2.2. The Bellman-Ford Algorithm
2.3. Transient Loops in Bellman-Ford
2.4. Feasibility Conditions
2.5. Solving Starvation: Sequencing Routes
2.6. Requests
2.7. Multiple Routers
2.8. Overlapping Prefixes
3. Protocol Operation
3.1. Message Transmission and Reception
3.2. Data Structures
3.3. Acknowledgments and Acknowledgment Requests
3.4. Neighbour Acquisition
3.5. Routing Table Maintenance
3.6. Route Selection
3.7. Sending Updates
3.8. Explicit Requests
4. Protocol Encoding
4.1. Data Types
4.2. Packet Format
4.3. TLV Format
4.4. Sub-TLV Format
4.5. Parser State and Encoding of Updates
4.6. Details of Specific TLVs
4.7. Details of specific sub-TLVs
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Cost and Metric Computation
A.1. Maintaining Hello History
A.2. Cost Computation
A.3. Route Selection and Hysteresis
Appendix B. Protocol Parameters
Appendix C. Route Filtering
Appendix D. Considerations for Protocol Extensions
Appendix E. Stub Implementations
Appendix F. Compatibility with Previous Versions
Acknowledgments
Authors' Addresses
1. Introduction
Babel is a loop-avoiding distance-vector routing protocol that is
designed to be robust and efficient both in networks using prefix-
based routing and in networks using flat routing ("mesh networks"),
and both in relatively stable wired networks and in highly dynamic
wireless networks. This document describes the Babel routing
protocol and obsoletes [RFC6126] and [RFC7557].
1.1. Features
The main property that makes Babel suitable for unstable networks is
that, unlike naive distance-vector routing protocols [RIP], it
strongly limits the frequency and duration of routing pathologies
such as routing loops and black-holes during reconvergence. Even
after a mobility event is detected, a Babel network usually remains
loop-free. Babel then quickly reconverges to a configuration that
preserves the loop-freedom and connectedness of the network, but is
not necessarily optimal; in many cases, this operation requires no
packet exchanges at all. Babel then slowly converges, in a time on
the scale of minutes, to an optimal configuration. This is achieved
by using sequenced routes, a technique pioneered by Destination-
Sequenced Distance-Vector routing [DSDV].
More precisely, Babel has the following properties:
* when every prefix is originated by at most one router, Babel never
suffers from routing loops;
* when a single prefix is originated by multiple routers, Babel may
occasionally create a transient routing loop for this particular
prefix; this loop disappears in time proportional to the loop's
diameter, and never again (up to an arbitrary garbage-collection
(GC) time) will the routers involved participate in a routing loop
for the same prefix;
* assuming bounded packet loss rates, any routing black-holes that
may appear after a mobility event are corrected in a time at most
proportional to the network's diameter.
Babel has provisions for link quality estimation and for fairly
arbitrary metrics. When configured suitably, Babel can implement
shortest-path routing, or it may use a metric based, for example, on
measured packet loss.
Babel nodes will successfully establish an association even when they
are configured with different parameters. For example, a mobile node
that is low on battery may choose to use larger time constants (hello
and update intervals, etc.) than a node that has access to wall
power. Conversely, a node that detects high levels of mobility may
choose to use smaller time constants. The ability to build such
heterogeneous networks makes Babel particularly adapted to the
unmanaged or wireless environment.
Finally, Babel is a hybrid routing protocol, in the sense that it can
carry routes for multiple network-layer protocols (IPv4 and IPv6),
regardless of which protocol the Babel packets are themselves being
carried over.
1.2. Limitations
Babel has two limitations that make it unsuitable for use in some
environments. First, Babel relies on periodic routing table updates
rather than using a reliable transport; hence, in large, stable
networks it generates more traffic than protocols that only send
updates when the network topology changes. In such networks,
protocols such as OSPF [OSPF], IS-IS [IS-IS], or the Enhanced
Interior Gateway Routing Protocol (EIGRP) [EIGRP] might be more
suitable.
Second, unless the second algorithm described in Section 3.5.4 is
implemented, Babel does impose a hold time when a prefix is
retracted. While this hold time does not apply to the exact prefix
being retracted, and hence does not prevent fast reconvergence should
it become available again, it does apply to any shorter prefix that
covers it. This may make those implementations of Babel that do not
implement the optional algorithm described in Section 3.5.4
unsuitable for use in networks that implement automatic prefix
aggregation.
1.3. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Conceptual Description of the Protocol
Babel is a loop-avoiding distance-vector protocol: it is based on the
Bellman-Ford algorithm, just like the venerable RIP [RIP], but
includes a number of refinements that either prevent loop formation
altogether, or ensure that a loop disappears in a timely manner and
doesn't form again.
Conceptually, Bellman-Ford is executed in parallel for every source
of routing information (destination of data traffic). In the
following discussion, we fix a source S; the reader will recall that
the same algorithm is executed for all sources.
2.1. Costs, Metrics, and Neighbourship
For every pair of neighbouring nodes A and B, Babel computes an
abstract value known as the cost of the link from A to B, written
C(A, B). Given a route between any two (not necessarily
neighbouring) nodes, the metric of the route is the sum of the costs
of all the links along the route. The goal of the routing algorithm
is to compute, for every source S, the tree of routes of lowest
metric to S.
Costs and metrics need not be integers. In general, they can be
values in any algebra that satisfies two fairly general conditions
(Section 3.5.2).
A Babel node periodically sends Hello messages to all of its
neighbours; it also periodically sends an IHU ("I Heard You") message
to every neighbour from which it has recently heard a Hello. From
the information derived from Hello and IHU messages received from its
neighbour B, a node A computes the cost C(A, B) of the link from A to
B.
2.2. The Bellman-Ford Algorithm
Every node A maintains two pieces of data: its estimated distance to
S, written D(A), and its next-hop router to S, written NH(A).
Initially, D(S) = 0, D(A) is infinite, and NH(A) is undefined.
Periodically, every node B sends to all of its neighbours a route
update, a message containing D(B). When a neighbour A of B receives
the route update, it checks whether B is its selected next hop; if
that is the case, then NH(A) is set to B, and D(A) is set to C(A, B)
+ D(B). If that is not the case, then A compares C(A, B) + D(B) to
its current value of D(A). If that value is smaller, meaning that
the received update advertises a route that is better than the
currently selected route, then NH(A) is set to B, and D(A) is set to
C(A, B) + D(B).
A number of refinements to this algorithm are possible, and are used
by Babel. In particular, convergence speed may be increased by
sending unscheduled "triggered updates" whenever a major change in
the topology is detected, in addition to the regular, scheduled
updates. Additionally, a node may maintain a number of alternate
routes, which are being advertised by neighbours other than its
selected neighbour, and which can be used immediately if the selected
route were to fail.
2.3. Transient Loops in Bellman-Ford
It is well known that a naive application of Bellman-Ford to
distributed routing can cause transient loops after a topology
change. Consider for example the following topology:
B
1 /|
1 / |
S --- A |1
\ |
1 \|
C
After convergence, D(B) = D(C) = 2, with NH(B) = NH(C) = A.
Suppose now that the link between S and A fails:
B
1 /|
/ |
S A |1
\ |
1 \|
C
When it detects the failure of the link, A switches its next hop to B
(which is still advertising a route to S with metric 2), and
advertises a metric equal to 3, and then advertises a new route with
metric 3. This process of nodes changing selected neighbours and
increasing their metric continues until the advertised metric reaches
"infinity", a value larger than all the metrics that the routing
protocol is able to carry.
2.4. Feasibility Conditions
Bellman-Ford is a very robust algorithm: its convergence properties
are preserved when routers delay route acquisition or when they
discard some updates. Babel routers discard received route
announcements unless they can prove that accepting them cannot
possibly cause a routing loop.
More formally, we define a condition over route announcements, known
as the "feasibility condition", that guarantees the absence of
routing loops whenever all routers ignore route updates that do not
satisfy the feasibility condition. In effect, this makes Bellman-
Ford into a family of routing algorithms, parameterised by the
feasibility condition.
Many different feasibility conditions are possible. For example, BGP
can be modelled as being a distance-vector protocol with a (rather
drastic) feasibility condition: a routing update is only accepted
when the receiving node's AS number is not included in the update's
AS_PATH attribute (note that BGP's feasibility condition does not
ensure the absence of transient "micro-loops" during reconvergence).
Another simple feasibility condition, used in the Destination-
Sequenced Distance-Vector (DSDV) routing protocol [DSDV] and in the
Ad hoc On-Demand Distance Vector (AODV) protocol [RFC3561], stems
from the following observation: a routing loop can only arise after a
router has switched to a route with a larger metric than the route
that it had previously selected. Hence, one may define that a route
is feasible when its metric at the local node would be no larger than
the metric of the currently selected route, i.e., an announcement
carrying a metric D(B) is accepted by A when C(A, B) + D(B) <= D(A).
If all routers obey this constraint, then the metric at every router
is nonincreasing, and the following invariant is always preserved: if
A has selected B as its next hop, then D(B) < D(A), which implies
that the forwarding graph is loop-free.
Babel uses a slightly more refined feasibility condition, derived
from EIGRP [DUAL]. Given a router A, define the feasibility distance
of A, written FD(A), as the smallest metric that A has ever
advertised for S to any of its neighbours. An update sent by a
neighbour B of A is feasible when the metric D(B) advertised by B is
strictly smaller than A's feasibility distance, i.e., when D(B) <
FD(A).
It is easy to see that this latter condition is no more restrictive
than DSDV-feasibility. Suppose that node A obeys DSDV-feasibility;
then D(A) is nonincreasing, hence at all times D(A) <= FD(A).
Suppose now that A receives a DSDV-feasible update that advertises a
metric D(B). Since the update is DSDV-feasible, C(A, B) + D(B) <=
D(A), hence D(B) < D(A), and since D(A) <= FD(A), D(B) < FD(A).
To see that it is strictly less restrictive, consider the following
diagram, where A has selected the route through B, and D(A) = FD(A) =
2. Since D(C) = 1 < FD(A), the alternate route through C is feasible
for A, although its metric C(A, C) + D(C) = 5 is larger than that of
the currently selected route:
B
1 / \ 1
/ \
S A
\ /
1 \ / 4
C
To show that this feasibility condition still guarantees loop-
freedom, recall that at the time when A accepts an update from B, the
metric D(B) announced by B is no smaller than FD(B); since it is
smaller than FD(A), at that point in time FD(B) < FD(A). Since this
property is preserved when A sends updates and also when it picks a
different next hop, it remains true at all times, which ensures that
the forwarding graph has no loops.
2.5. Solving Starvation: Sequencing Routes
Obviously, the feasibility conditions defined above cause starvation
when a router runs out of feasible routes. Consider the following
diagram, where both A and B have selected the direct route to S:
A
1 /| D(A) = 1
/ | FD(A) = 1
S |1
\ | D(B) = 2
2 \| FD(B) = 2
B
Suppose now that the link between A and S breaks:
A
|
| FD(A) = 1
S |1
\ | D(B) = 2
2 \| FD(B) = 2
B
The only route available from A to S, the one that goes through B, is
not feasible: A suffers from spurious starvation. At that point, the
whole subtree suffering from starvation must be reset, which is
essentially what EIGRP does when it performs a global synchronisation
of all the routers in the starving subtree (the "active" phase of
EIGRP).
Babel reacts to starvation in a less drastic manner, by using
sequenced routes, a technique introduced by DSDV and adopted by AODV.
In addition to a metric, every route carries a sequence number, a
nondecreasing integer that is propagated unchanged through the
network and is only ever incremented by the source; a pair (s, m),
where s is a sequence number and m a metric, is called a distance.
A received update is feasible when either it is more recent than the
feasibility distance maintained by the receiving node, or it is
equally recent and the metric is strictly smaller. More formally, if
FD(A) = (s, m), then an update carrying the distance (s', m') is
feasible when either s' > s, or s = s' and m' < m.
Assuming the sequence number of S is 137, the diagram above becomes:
A
|
| FD(A) = (137, 1)
S |1
\ | D(B) = (137, 2)
2 \| FD(B) = (137, 2)
B
After S increases its sequence number, and the new sequence number is
propagated to B, we have:
A
|
| FD(A) = (137, 1)
S |1
\ | D(B) = (138, 2)
2 \| FD(B) = (138, 2)
B
at which point the route through B becomes feasible again.
Note that while sequence numbers are used for determining
feasibility, they are not used in route selection: a node ignores the
sequence number when selecting the best route to a given destination
(Section 3.6). Doing otherwise would cause route oscillation while a
sequence number propagates through the network, and might even cause
persistent black-holes with some exotic metrics.
2.6. Requests
In DSDV, the sequence number of a source is increased periodically.
A route becomes feasible again after the source increases its
sequence number, and the new sequence number is propagated through
the network, which may, in general, require a significant amount of
time.
Babel takes a different approach. When a node detects that it is
suffering from a potentially spurious starvation, it sends an
explicit request to the source for a new sequence number. This
request is forwarded hop by hop to the source, with no regard to the
feasibility condition. Upon receiving the request, the source
increases its sequence number and broadcasts an update, which is
forwarded to the requesting node.
Note that after a change in network topology not all such requests
will, in general, reach the source, as some will be sent over links
that are now broken. However, if the network is still connected,
then at least one among the nodes suffering from spurious starvation
has an (unfeasible) route to the source; hence, in the absence of
packet loss, at least one such request will reach the source.
(Resending requests a small number of times compensates for packet
loss.)
Since requests are forwarded with no regard to the feasibility
condition, they may, in general, be caught in a forwarding loop; this
is avoided by having nodes perform duplicate detection for the
requests that they forward.
2.7. Multiple Routers
The above discussion assumes that each prefix is originated by a
single router. In real networks, however, it is often necessary to
have a single prefix originated by multiple routers: for example, the
default route will be originated by all of the edge routers of a
routing domain.
Since synchronising sequence numbers between distinct routers is
problematic, Babel treats routes for the same prefix as distinct
entities when they are originated by different routers: every route
announcement carries the router-id of its originating router, and
feasibility distances are not maintained per prefix, but per source,
where a source is a pair of a router-id and a prefix. In effect,
Babel guarantees loop-freedom for the forwarding graph to every
source; since the union of multiple acyclic graphs is not in general
acyclic, Babel does not in general guarantee loop-freedom when a
prefix is originated by multiple routers, but any loops will be
broken in a time at most proportional to the diameter of the loop --
as soon as an update has "gone around" the routing loop.
Consider for example the following topology, where A has selected the
default route through S, and B has selected the one through S':
1 1 1
::/0 -- S --- A --- B --- S' -- ::/0
Suppose that both default routes fail at the same time; then nothing
prevents A from switching to B, and B simultaneously switching to A.
However, as soon as A has successfully advertised the new route to B,
the route through A will become unfeasible for B. Conversely, as
soon as B will have advertised the route through A, the route through
B will become unfeasible for A.
In effect, the routing loop disappears at the latest when routing
information has gone around the loop. Since this process can be
delayed by lost packets, Babel makes certain efforts to ensure that
updates are sent reliably after a router-id change (Section 3.7.2).
Additionally, after the routers have advertised the two routes, both
sources will be in their source tables, which will prevent them from
ever again participating in a routing loop involving routes from S
and S' (up to the source GC time, which, available memory permitting,
can be set to arbitrarily large values).
2.8. Overlapping Prefixes
In the above discussion, we have assumed that all prefixes are
disjoint, as is the case in flat ("mesh") routing. In practice,
however, prefixes may overlap: for example, the default route
overlaps with all of the routes present in the network.
After a route fails, it is not correct in general to switch to a
route that subsumes the failed route. Consider for example the
following configuration:
1 1
::/0 -- A --- B --- C
Suppose that node C fails. If B forwards packets destined to C by
following the default route, a routing loop will form, and persist
until A learns of B's retraction of the direct route to C. B avoids
this pitfall by installing an "unreachable" route after a route is
retracted; this route is maintained until it can be guaranteed that
the former route has been retracted by all of B's neighbours
(Section 3.5.4).
3. Protocol Operation
Every Babel speaker is assigned a router-id, which is an arbitrary
string of 8 octets that is assumed unique across the routing domain.
For example, router-ids could be assigned randomly, or they could be
derived from a link-layer address. (The protocol encoding is
slightly more compact when router-ids are assigned in the same manner
as the IPv6 layer assigns host IDs; see the definition of the "R"
flag in Section 4.6.9.)
3.1. Message Transmission and Reception
Babel protocol packets are sent in the body of a UDP datagram (as
described in Section 4). Each Babel packet consists of zero or more
TLVs. Most TLVs may contain sub-TLVs.
Babel's control traffic can be carried indifferently over IPv6 or
over IPv4, and prefixes of either address family can be announced
over either protocol. Thus, there are at least two natural
deployment models: using IPv6 exclusively for all control traffic, or
running two distinct protocol instances, one for each address family.
The exclusive use of IPv6 for all control traffic is RECOMMENDED,
since using both protocols at the same time doubles the amount of
traffic devoted to neighbour discovery and link quality estimation.
The source address of a Babel packet is always a unicast address,
link-local in the case of IPv6. Babel packets may be sent to a well-
known (link-local) multicast address or to a (link-local) unicast
address. In normal operation, a Babel speaker sends both multicast
and unicast packets to its neighbours.
With the exception of acknowledgments, all Babel TLVs can be sent to
either unicast or multicast addresses, and their semantics does not
depend on whether the destination is a unicast or a multicast
address. Hence, a Babel speaker does not need to determine the
destination address of a packet that it receives in order to
interpret it.
A moderate amount of jitter may be applied to packets sent by a Babel
speaker: outgoing TLVs are buffered and SHOULD be sent with a random
delay. This is done for two purposes: it avoids synchronisation of
multiple Babel speakers across a network [JITTER], and it allows for
the aggregation of multiple TLVs into a single packet.
The maximum amount of delay a TLV can be subjected to depends on the
TLV. When the protocol description specifies that a TLV is urgent
(as in Section 3.7.2 and Section 3.8.1), then the TLV MUST be sent
within a short time known as the urgent timeout (see Appendix B for
recommended values). When the TLV is governed by a timeout
explicitly included in a previous TLV, such as in the case of
Acknowledgments (Section 4.6.4), Updates (Section 3.7), and IHUs
(Section 3.4.2), then the TLV MUST be sent early enough to meet the
explicit deadline (with a small margin to allow for propagation
delays). In all other cases, the TLV SHOULD be sent out within one-
half of the Multicast Hello interval.
In order to avoid packet drops (either at the sender or at the
receiver), a delay SHOULD be introduced between successive packets
sent out on the same interface, within the constraints of the
previous paragraph. Note, however, that such packet pacing might
impair the ability of some link layers (e.g., IEEE 802.11
[IEEE802.11]) to perform packet aggregation.
3.2. Data Structures
In this section, we describe the data structures that every Babel
speaker maintains. This description is conceptual: a Babel speaker
may use different data structures as long as the resulting protocol
is the same as the one described in this document. For example,
rather than maintaining a single table containing both selected and
unselected (fallback) routes, as described in Section 3.2.6, an
actual implementation would probably use two tables, one with
selected routes and one with fallback routes.
3.2.1. Sequence Number Arithmetic
Sequence numbers (seqnos) appear in a number of Babel data
structures, and they are interpreted as integers modulo 2^(16). For
the purposes of this document, arithmetic on sequence numbers is
defined as follows.
Given a seqno s and a non-negative integer n, the sum of s and n is
defined by the following:
s + n (modulo 2^(16)) = (s + n) MOD 2^(16)
or, equivalently,
s + n (modulo 2^(16)) = (s + n) AND 65535
where MOD is the modulo operation yielding a non-negative integer,
and AND is the bitwise conjunction operation.
Given two sequence numbers s and s', the relation s is less than s'
(s < s') is defined by the following:
s < s' (modulo 2^(16)) when 0 < ((s' - s) MOD 2^(16)) < 32768
or, equivalently,
s < s' (modulo 2^(16)) when s /= s' and ((s' - s) AND 32768) = 0.
3.2.2. Node Sequence Number
A node's sequence number is a 16-bit integer that is included in
route updates sent for routes originated by this node.
A node increments its sequence number (modulo 2^(16)) whenever it
receives a request for a new sequence number (Section 3.8.1.2). A
node SHOULD NOT increment its sequence number (seqno) spontaneously,
since increasing seqnos makes it less likely that other nodes will
have feasible alternate routes when their selected routes fail.
3.2.3. The Interface Table
The interface table contains the list of interfaces on which the node
speaks the Babel protocol. Every interface table entry contains the
interface's outgoing Multicast Hello seqno, a 16-bit integer that is
sent with each Multicast Hello TLV on this interface and is
incremented (modulo 2^(16)) whenever a Multicast Hello is sent.
(Note that an interface's Multicast Hello seqno is unrelated to the
node's seqno.)
There are two timers associated with each interface table entry. The
periodic multicast hello timer governs the sending of scheduled
Multicast Hello and IHU packets (Section 3.4). The periodic Update
timer governs the sending of periodic route updates (Section 3.7.1).
See Appendix B for suggested time constants.
3.2.4. The Neighbour Table
The neighbour table contains the list of all neighbouring interfaces
from which a Babel packet has been recently received. The neighbour
table is indexed by pairs of the form (interface, address), and every
neighbour table entry contains the following data:
* the local node's interface over which this neighbour is reachable;
* the address of the neighbouring interface;
* a history of recently received Multicast Hello packets from this
neighbour; this can, for example, be a sequence of n bits, for
some small value n, indicating which of the n hellos most recently
sent by this neighbour have been received by the local node;
* a history of recently received Unicast Hello packets from this
neighbour;
* the "transmission cost" value from the last IHU packet received
from this neighbour, or FFFF hexadecimal (infinity) if the IHU
hold timer for this neighbour has expired;
* the expected incoming Multicast Hello sequence number for this
neighbour, an integer modulo 2^(16).
* the expected incoming Unicast Hello sequence number for this
neighbour, an integer modulo 2^(16).
* the outgoing Unicast Hello sequence number for this neighbour, an
integer modulo 2^(16) that is sent with each Unicast Hello TLV to
this neighbour and is incremented (modulo 2^(16)) whenever a
Unicast Hello is sent. (Note that the outgoing Unicast Hello
seqno for a neighbour is distinct from the interface's outgoing
Multicast Hello seqno.)
There are three timers associated with each neighbour entry -- the
multicast hello timer, which is set to the interval value carried by
scheduled Multicast Hello TLVs sent by this neighbour, the unicast
hello timer, which is set to the interval value carried by scheduled
Unicast Hello TLVs, and the IHU timer, which is set to a small
multiple of the interval carried in IHU TLVs (see "IHU Hold time" in
Appendix B for suggested values).
Note that the neighbour table is indexed by IP addresses, not by
router-ids: neighbourship is a relationship between interfaces, not
between nodes. Therefore, two nodes with multiple interfaces can
participate in multiple neighbourship relationships, a situation that
can notably arise when wireless nodes with multiple radios are
involved.
3.2.5. The Source Table
The source table is used to record feasibility distances. It is
indexed by triples of the form (prefix, plen, router-id), and every
source table entry contains the following data:
* the prefix (prefix, plen), where plen is the prefix length in
bits, that this entry applies to;
* the router-id of a router originating this prefix;
* a pair (seqno, metric), this source's feasibility distance.
There is one timer associated with each entry in the source table --
the source garbage-collection timer. It is initialised to a time on
the order of minutes and reset as specified in Section 3.7.3.
3.2.6. The Route Table
The route table contains the routes known to this node. It is
indexed by triples of the form (prefix, plen, neighbour), and every
route table entry contains the following data:
* the source (prefix, plen, router-id) for which this route is
advertised;
* the neighbour (an entry in the neighbour table) that advertised
this route;
* the metric with which this route was advertised by the neighbour,
or FFFF hexadecimal (infinity) for a recently retracted route;
* the sequence number with which this route was advertised;
* the next-hop address of this route;
* a boolean flag indicating whether this route is selected, i.e.,
whether it is currently being used for forwarding and is being
advertised.
There is one timer associated with each route table entry -- the
route expiry timer. It is initialised and reset as specified in
Section 3.5.3.
Note that there are two distinct (seqno, metric) pairs associated
with each route: the route's distance, which is stored in the route
table, and the feasibility distance, which is stored in the source
table and shared between all routes with the same source.
3.2.7. The Table of Pending Seqno Requests
The table of pending seqno requests contains a list of seqno requests
that the local node has sent (either because they have been
originated locally, or because they were forwarded) and to which no
reply has been received yet. This table is indexed by triples of the
form (prefix, plen, router-id), and every entry in this table
contains the following data:
* the prefix, plen, router-id, and seqno being requested;
* the neighbour, if any, on behalf of which we are forwarding this
request;
* a small integer indicating the number of times that this request
will be resent if it remains unsatisfied.
There is one timer associated with each pending seqno request; it
governs both the resending of requests and their expiry.
3.3. Acknowledgments and Acknowledgment Requests
A Babel speaker may request that a neighbour receiving a given packet
reply with an explicit acknowledgment within a given time. While the
use of acknowledgment requests is optional, every Babel speaker MUST
be able to reply to such a request.
An acknowledgment MUST be sent to a unicast destination. On the
other hand, acknowledgment requests may be sent to either unicast or
multicast destinations, in which case they request an acknowledgment
from all of the receiving nodes.
When to request acknowledgments is a matter of local policy; the
simplest strategy is to never request acknowledgments and to rely on
periodic updates to ensure that any reachable routes are eventually
propagated throughout the routing domain. In order to improve
convergence speed and to reduce the amount of control traffic,
acknowledgment requests MAY be used in order to reliably send urgent
updates (Section 3.7.2) and retractions (Section 3.5.4), especially
when the number of neighbours on a given interface is small. Since
Babel is designed to deal gracefully with packet loss on unreliable
media, sending all packets with acknowledgment requests is not
necessary and NOT RECOMMENDED, as the acknowledgments cause
additional traffic and may force additional Address Resolution
Protocol (ARP) or Neighbour Discovery (ND) exchanges.
3.4. Neighbour Acquisition
Neighbour acquisition is the process by which a Babel node discovers
the set of neighbours heard over each of its interfaces and
ascertains bidirectional reachability. On unreliable media,
neighbour acquisition additionally provides some statistics that may
be useful for link quality computation.
Before it can exchange routing information with a neighbour, a Babel
node MUST create an entry for that neighbour in the neighbour table.
When to do that is implementation-specific; suitable strategies
include creating an entry when any Babel packet is received, or
creating an entry when a Hello TLV is parsed. Similarly, in order to
conserve system resources, an implementation SHOULD discard an entry
when it has been unused for long enough; suitable strategies include
dropping the neighbour after a timeout, and dropping a neighbour when
the associated Hello histories become empty (see Appendix A.2).
3.4.1. Reverse Reachability Detection
Every Babel node sends Hello TLVs to its neighbours, at regular or
irregular intervals, to indicate that it is alive. Each Hello TLV
carries an increasing (modulo 2^(16)) sequence number and an upper
bound on the time interval until the next Hello of the same type (see
below). If the time interval is set to 0, then the Hello TLV does
not establish a new promise: the deadline carried by the previous
Hello of the same type still applies to the next Hello (if the most
recent scheduled Hello of the right kind was received at time t0 and
carried interval i, then the previous promise of sending another
Hello before time t0 + i still holds). We say that a Hello is
"scheduled" if it carries a nonzero interval, and "unscheduled"
otherwise.
There are two kinds of Hellos: Multicast Hellos, which use a per-
interface Hello counter (the Multicast Hello seqno), and Unicast
Hellos, which use a per-neighbour counter (the Unicast Hello seqno).
A Multicast Hello with a given seqno MUST be sent to all neighbours
on a given interface, either by sending it to a multicast address or
by sending it to one unicast address per neighbour (hence, the term
"Multicast Hello" is a slight misnomer). A Unicast Hello carrying a
given seqno should normally be sent to just one neighbour (over
unicast), since the sequence numbers of different neighbours are not
in general synchronised.
Multicast Hellos sent over multicast can be used for neighbour
discovery; hence, a node SHOULD send periodic (scheduled) Multicast
Hellos unless neighbour discovery is performed by means outside of
the Babel protocol. A node MAY send Unicast Hellos or unscheduled
Hellos of either kind for any reason, such as reducing the amount of
multicast traffic or improving reliability on link technologies with
poor support for link-layer multicast.
A node MAY send a scheduled Hello ahead of time. A node MAY change
its scheduled Hello interval. The Hello interval MAY be decreased at
any time; it MAY be increased immediately before sending a Hello TLV,
but SHOULD NOT be increased at other times. (Equivalently, a node
SHOULD send a scheduled Hello immediately after increasing its Hello
interval.)
How to deal with received Hello TLVs and what statistics to maintain
are considered local implementation matters; typically, a node will
maintain some sort of history of recently received Hellos. An
example of a suitable algorithm is described in Appendix A.1.
After receiving a Hello, or determining that it has missed one, the
node recomputes the association's cost (Section 3.4.3) and runs the
route selection procedure (Section 3.6).
3.4.2. Bidirectional Reachability Detection
In order to establish bidirectional reachability, every node sends
periodic IHU ("I Heard You") TLVs to each of its neighbours. Since
IHUs carry an explicit interval value, they MAY be sent less often
than Hellos in order to reduce the amount of routing traffic in dense
networks; in particular, they SHOULD be sent less often than Hellos
over links with little packet loss. While IHUs are conceptually
unicast, they MAY be sent to a multicast address in order to avoid an
ARP or Neighbour Discovery exchange and to aggregate multiple IHUs
into a single packet.
In addition to the periodic IHUs, a node MAY, at any time, send an
unscheduled IHU packet. It MAY also, at any time, decrease its IHU
interval, and it MAY increase its IHU interval immediately before
sending an IHU, but SHOULD NOT increase it at any other time.
(Equivalently, a node SHOULD send an extra IHU immediately after
increasing its Hello interval.)
Every IHU TLV contains two pieces of data: the link's rxcost
(reception cost) from the sender's perspective, used by the neighbour
for computing link costs (Section 3.4.3), and the interval between
periodic IHU packets. A node receiving an IHU sets the value of the
txcost (transmission cost) maintained in the neighbour table to the
value contained in the IHU, and resets the IHU timer associated to
this neighbour to a small multiple of the interval value received in
the IHU (see "IHU Hold time" in Appendix B for suggested values).
When a neighbour's IHU timer expires, the neighbour's txcost is set
to infinity.
After updating a neighbour's txcost, the receiving node recomputes
the neighbour's cost (Section 3.4.3) and runs the route selection
procedure (Section 3.6).
3.4.3. Cost Computation
A neighbourship association's link cost is computed from the values
maintained in the neighbour table: the statistics kept in the
neighbour table about the reception of Hellos, and the txcost
computed from received IHU packets.
For every neighbour, a Babel node computes a value known as this
neighbour's rxcost. This value is usually derived from the Hello
history, which may be combined with other data, such as statistics
maintained by the link layer. The rxcost is sent to a neighbour in
each IHU.
Since nodes do not necessarily send periodic Unicast Hellos but do
usually send periodic Multicast Hellos (Section 3.4.1), a node SHOULD
use an algorithm that yields a finite rxcost when only Multicast
Hellos are received, unless interoperability with nodes that only
send Multicast Hellos is not required.
How the txcost and rxcost are combined in order to compute a link's
cost is a matter of local policy; as far as Babel's correctness is
concerned, only the following conditions MUST be satisfied:
* the cost is strictly positive;
* if no Hello TLVs of either kind were received recently, then the
cost is infinite;
* if the txcost is infinite, then the cost is infinite.
See Appendix A.2 for RECOMMENDED strategies for computing a link's
cost.
3.5. Routing Table Maintenance
Conceptually, a Babel update is a quintuple (prefix, plen, router-id,
seqno, metric), where (prefix, plen) is the prefix for which a route
is being advertised, router-id is the router-id of the router
originating this update, seqno is a nondecreasing (modulo 2^(16))
integer that carries the originating router seqno, and metric is the
announced metric.
Before being accepted, an update is checked against the feasibility
condition (Section 3.5.1), which ensures that the route does not
create a routing loop. If the feasibility condition is not
satisfied, the update is either ignored or prevents the route from
being selected, as described in Section 3.5.3. If the feasibility
condition is satisfied, then the update cannot possibly cause a
routing loop.
3.5.1. The Feasibility Condition
The feasibility condition is applied to all received updates. The
feasibility condition compares the metric in the received update with
the metrics of the updates previously sent by the receiving node;
updates that fail the feasibility condition, and therefore have
metrics large enough to cause a routing loop, are either ignored or
prevent the resulting route from being selected.
A feasibility distance is a pair (seqno, metric), where seqno is an
integer modulo 2^(16) and metric is a positive integer. Feasibility
distances are compared lexicographically, with the first component
inverted: we say that a distance (seqno, metric) is strictly better
than a distance (seqno', metric'), written
(seqno, metric) < (seqno', metric')
when
seqno > seqno' or (seqno = seqno' and metric < metric')
where sequence numbers are compared modulo 2^(16).
Given a source (prefix, plen, router-id), a node's feasibility
distance for this source is the minimum, according to the ordering
defined above, of the distances of all the finite updates ever sent
by this particular node for the prefix (prefix, plen) and the given
router-id. Feasibility distances are maintained in the source table,
the exact procedure is given in Section 3.7.3.
A received update is feasible when either it is a retraction (its
metric is FFFF hexadecimal), or the advertised distance is strictly
better, in the sense defined above, than the feasibility distance for
the corresponding source. More precisely, a route advertisement
carrying the quintuple (prefix, plen, router-id, seqno, metric) is
feasible if one of the following conditions holds:
* metric is infinite; or
* no entry exists in the source table indexed by (prefix, plen,
router-id); or
* an entry (prefix, plen, router-id, seqno', metric') exists in the
source table, and either
- seqno' < seqno or
- seqno = seqno' and metric < metric'.
Note that the feasibility condition considers the metric advertised
by the neighbour, not the route's metric; hence, a fluctuation in a
neighbour's cost cannot render a selected route unfeasible. Note
further that retractions (updates with infinite metric) are always
feasible, since they cannot possibly cause a routing loop.
3.5.2. Metric Computation
A route's metric is computed from the metric advertised by the
neighbour and the neighbour's link cost. Just like cost computation,
metric computation is considered a local policy matter; as far as
Babel is concerned, the function M(c, m) used for computing a metric
from a locally computed link cost c and the metric m advertised by a
neighbour MUST only satisfy the following conditions:
* if c is infinite, then M(c, m) is infinite;
* M is strictly monotonic: M(c, m) > m.
Additionally, the metric SHOULD satisfy the following condition:
* M is left-distributive: if m <= m', then M(c, m) <= M(c, m').
While strict monotonicity is essential to the integrity of the
network (persistent routing loops may arise if it is not satisfied),
left-distributivity is not: if it is not satisfied, Babel will still
converge to a loop-free configuration, but might not reach a global
optimum (in fact, a global optimum may not even exist).
The conditions above are easily satisfied by using the additive
metric, i.e., by defining M(c, m) = c + m. Since the additive metric
is useful with a large range of cost computation strategies, it is
the RECOMMENDED default. See also Appendix C, which describes a
technique that makes it possible to tweak the values of individual
metrics without running the risk of creating routing loops.
3.5.3. Route Acquisition
When a Babel node receives an update (prefix, plen, router-id, seqno,
metric) from a neighbour neigh, it checks whether it already has a
route table entry indexed by (prefix, plen, neigh).
If no such entry exists:
* if the update is unfeasible, it MAY be ignored;
* if the metric is infinite (the update is a retraction of a route
we do not know about), the update is ignored;
* otherwise, a new entry is created in the route table, indexed by
(prefix, plen, neigh), with source equal to (prefix, plen, router-
id), seqno equal to seqno, and an advertised metric equal to the
metric carried by the update.
If such an entry exists:
* if the entry is currently selected, the update is unfeasible, and
the router-id of the update is equal to the router-id of the
entry, then the update MAY be ignored;
* otherwise, the entry's sequence number, advertised metric, metric,
and router-id are updated, and if the advertised metric is not
infinite, the route's expiry timer is reset to a small multiple of
the interval value included in the update (see "Route Expiry time"
in Appendix B for suggested values). If the update is unfeasible,
then the (now unfeasible) entry MUST be immediately unselected.
If the update caused the router-id of the entry to change, an
update (possibly a retraction) MUST be sent in a timely manner as
described in Section 3.7.2.
Note that the route table may contain unfeasible routes, either
because they were created by an unfeasible update or due to a metric
fluctuation. Such routes are never selected, since they are not
known to be loop-free. Should all the feasible routes become
unusable, however, the unfeasible routes can be made feasible and
therefore possible to select by sending requests along them (see
Section 3.8.2).
When a route's expiry timer triggers, the behaviour depends on
whether the route's metric is finite. If the metric is finite, it is
set to infinity and the expiry timer is reset. If the metric is
already infinite, the route is flushed from the route table.
After the route table is updated, the route selection procedure
(Section 3.6) is run.
3.5.4. Hold Time
When a prefix P is retracted (because all routes are unfeasible or
have an infinite metric, whether due to the expiry timer or for other
reasons), and a shorter prefix P' that covers P is reachable, P'
cannot in general be used for routing packets destined to P without
running the risk of creating a routing loop (Section 2.8).
To avoid this issue, whenever a prefix P is retracted, a route table
entry with infinite metric is maintained as described in
Section 3.5.3. As long as this entry is maintained, packets destined
to an address within P MUST NOT be forwarded by following a route for
a shorter prefix. This entry is removed as soon as a finite-metric
update for prefix P is received and the resulting route selected. If
no such update is forthcoming, the infinite metric entry SHOULD be
maintained at least until it is guaranteed that no neighbour has
selected the current node as next hop for prefix P. This can be
achieved by either:
* waiting until the route's expiry timer has expired
(Section 3.5.3); or
* sending a retraction with an acknowledgment request (Section 3.3)
to every reachable neighbour that has not explicitly retracted
prefix P, and waiting for all acknowledgments.
The former option is simpler and ensures that, at that point, any
routes for prefix P pointing at the current node have expired.
However, since the expiry time can be as high as a few minutes, doing
that prevents automatic aggregation by creating spurious black-holes
for aggregated routes. The latter option is RECOMMENDED as it
dramatically reduces the time for which a prefix is unreachable in
the presence of aggregated routes.
3.6. Route Selection
Route selection is the process by which a single route for a given
prefix is selected to be used for forwarding packets and to be re-
advertised to a node's neighbours.
Babel is designed to allow flexible route selection policies. As far
as the algorithm's correctness is concerned, the route selection
policy MUST only satisfy the following properties:
* a route with infinite metric (a retracted route) is never
selected;
* an unfeasible route is never selected.
Babel nodes using different route selection strategies will
interoperate and will not create routing loops as long as these two
properties hold.
Route selection MUST NOT take seqnos into account: a route MUST NOT
be preferred just because it carries a higher (more recent) seqno.
Doing otherwise would cause route oscillation while a new seqno
propagates across the network, and might create persistent black-
holes if the metric being used is not left-distributive
(Section 3.5.2).
The obvious route selection strategy is to pick, for every
destination, the feasible route with minimal metric. When all
metrics are stable, this approach ensures convergence to a tree of
shortest paths (assuming that the metric is left-distributive, see
Section 3.5.2). There are two reasons, however, why this strategy
may lead to instability in the presence of continuously varying
metrics. First, if two parallel routes oscillate around a common
value, then the smallest metric strategy will keep switching between
the two. Second, the selection of a route increases congestion along
it, which might increase packet loss, which in turn could cause its
metric to increase; this kind of feedback loop is prone to causing
persistent oscillations.
In order to limit these kinds of instabilities, a route selection
strategy SHOULD include some form of hysteresis, i.e., be sensitive
to a route's history: the strategy should only switch from the
currently selected route to a different route if the latter has been
consistently good for some period of time. A RECOMMENDED hysteresis
algorithm is given in Appendix A.3.
After the route selection procedure is run, triggered updates
(Section 3.7.2) and requests (Section 3.8.2) are sent.
3.7. Sending Updates
A Babel speaker advertises to its neighbours its set of selected
routes. Normally, this is done by sending one or more multicast
packets containing Update TLVs on all of its connected interfaces;
however, on link technologies where multicast is significantly more
expensive than unicast, a node MAY choose to send multiple copies of
updates in unicast packets, especially when the number of neighbours
is small.
Additionally, in order to ensure that any black-holes are reliably
cleared in a timely manner, a Babel node may send retractions
(updates with an infinite metric) for any recently retracted
prefixes.
If an update is for a route injected into the Babel domain by the
local node (e.g., it carries the address of a local interface, the
prefix of a directly attached network, or a prefix redistributed from
a different routing protocol), the router-id is set to the local
node's router-id, the metric is set to some arbitrary finite value
(typically 0), and the seqno is set to the local router's sequence
number.
If an update is for a route learnt from another Babel speaker, the
router-id and sequence number are copied from the route table entry,
and the metric is computed as specified in Section 3.5.2.
3.7.1. Periodic Updates
Every Babel speaker MUST advertise each of its selected routes on
every interface, at least once every Update interval. Since Babel
doesn't suffer from routing loops (there is no "counting to
infinity") and relies heavily on triggered updates (Section 3.7.2),
this full dump only needs to happen infrequently (see Appendix B for
suggested intervals).
3.7.2. Triggered Updates
In addition to periodic routing updates, a Babel speaker sends
unscheduled, or triggered, updates in order to inform its neighbours
of a significant change in the network topology.
A change of router-id for the selected route to a given prefix may be
indicative of a routing loop in formation; hence, whenever it changes
the selected router-id for a given destination, a node MUST send an
update as an urgent TLV (as defined in Section 3.1). Additionally,
it SHOULD make a reasonable attempt at ensuring that all reachable
neighbours receive this update.
There are two strategies for ensuring that. If the number of
neighbours is small, then it is reasonable to send the update
together with an acknowledgment request; the update is resent until
all neighbours have acknowledged the packet, up to some number of
times. If the number of neighbours is large, however, requesting
acknowledgments from all of them might cause a non-negligible amount
of network traffic; in that case, it may be preferable to simply
repeat the update some reasonable number of times (say, 3 for
wireless and 2 for wired links). The number of copies MUST NOT
exceed 5, and the copies SHOULD be separated by a small delay, as
described in Section 3.1.
A route retraction is somewhat less worrying: if the route retraction
doesn't reach all neighbours, a black-hole might be created, which,
unlike a routing loop, does not endanger the integrity of the
network. When a route is retracted, a node SHOULD send a triggered
update and SHOULD make a reasonable attempt at ensuring that all
neighbours receive this retraction.
Finally, a node MAY send a triggered update when the metric for a
given prefix changes in a significant manner, due to a received
update, because a link's cost has changed or because a different next
hop has been selected. A node SHOULD NOT send triggered updates for
other reasons, such as when there is a minor fluctuation in a route's
metric, when the selected next hop changes without inducing a
significant change to the route's metric, or to propagate a new
sequence number (except to satisfy a request, as specified in
Section 3.8).
3.7.3. Maintaining Feasibility Distances
Before sending an update (prefix, plen, router-id, seqno, metric)
with finite metric (i.e., not a route retraction), a Babel node
updates the feasibility distance maintained in the source table.
This is done as follows.
If no entry indexed by (prefix, plen, router-id) exists in the source
table, then one is created with value (prefix, plen, router-id,
seqno, metric).
If an entry (prefix, plen, router-id, seqno', metric') exists, then
it is updated as follows:
* if seqno > seqno', then seqno' := seqno, metric' := metric;
* if seqno = seqno' and metric' > metric, then metric' := metric;
* otherwise, nothing needs to be done.
The garbage-collection timer for the entry is then reset. Note that
the feasibility distance is not updated and the garbage-collection
timer is not reset when a retraction (an update with infinite metric)
is sent.
When the garbage-collection timer expires, the entry is removed from
the source table.
3.7.4. Split Horizon
When running over a transitive, symmetric link technology, e.g., a
point-to-point link or a wired LAN technology such as Ethernet, a
Babel node SHOULD use an optimisation known as split horizon. When
split horizon is used on a given interface, a routing update for
prefix P is not sent on the particular interface over which the
selected route towards prefix P was learnt.
Split horizon SHOULD NOT be applied to an interface unless the
interface is known to be symmetric and transitive; in particular,
split horizon is not applicable to decentralised wireless link
technologies (e.g., IEEE 802.11 in ad hoc mode) when routing updates
are sent over multicast.
3.8. Explicit Requests
In normal operation, a node's route table is populated by the regular
and triggered updates sent by its neighbours. Under some
circumstances, however, a node sends explicit requests in order to
cause a resynchronisation with the source after a mobility event or
to prevent a route from spuriously expiring.
The Babel protocol provides two kinds of explicit requests: route
requests, which simply request an update for a given prefix, and
seqno requests, which request an update for a given prefix with a
specific sequence number. The former are never forwarded; the latter
are forwarded if they cannot be satisfied by the receiver.
3.8.1. Handling Requests
Upon receiving a request, a node either forwards the request or sends
an update in reply to the request, as described in the following
sections. If this causes an update to be sent, the update is either
sent to a multicast address on the interface on which the request was
received, or to the unicast address of the neighbour that sent the
request.
The exact behaviour is different for route requests and seqno
requests.
3.8.1.1. Route Requests
When a node receives a route request for a given prefix, it checks
its route table for a selected route to this exact prefix. If such a
route exists, it MUST send an update (over unicast or over
multicast); if such a route does not exist, it MUST send a retraction
for that prefix.
When a node receives a wildcard route request, it SHOULD send a full
route table dump. Full route dumps SHOULD be rate-limited,
especially if they are sent over multicast.
3.8.1.2. Seqno Requests
When a node receives a seqno request for a given router-id and
sequence number, it checks whether its route table contains a
selected entry for that prefix. If a selected route for the given
prefix exists and has finite metric, and either the router-ids are
different or the router-ids are equal and the entry's sequence number
is no smaller (modulo 2^(16)) than the requested sequence number, the
node MUST send an update for the given prefix. If the router-ids
match, but the requested seqno is larger (modulo 2^(16)) than the
route entry's, the node compares the router-id against its own
router-id. If the router-id is its own, then it increases its
sequence number by 1 (modulo 2^(16)) and sends an update. A node
MUST NOT increase its sequence number by more than 1 in reaction to a
single seqno request.
Otherwise, if the requested router-id is not its own, the received
node consults the Hop Count field of the request. If the hop count
is 2 or more, and the node is advertising the prefix to its
neighbours, the node selects a neighbour to forward the request to as
follows:
* if the node has one or more feasible routes towards the requested
prefix with a next hop that is not the requesting node, then the
node MUST forward the request to the next hop of one such route;
* otherwise, if the node has one or more (not feasible) routes to
the requested prefix with a next hop that is not the requesting
node, then the node SHOULD forward the request to the next hop of
one such route.
In order to actually forward the request, the node decrements the hop
count and sends the request in a unicast packet destined to the
selected neighbour. The forwarded request SHOULD be sent as an
urgent TLV (as defined in Section 3.1).
A node SHOULD maintain a list of recently forwarded seqno requests
and forward the reply (an update with a seqno sufficiently large to
satisfy the request) as an urgent TLV (as defined in Section 3.1). A
node SHOULD compare every incoming seqno request against its list of
recently forwarded seqno requests and avoid forwarding the request if
it is redundant (i.e., if the node has recently sent a request with
the same prefix, router-id, and a seqno that is not smaller modulo
2^(16)).
Since the request-forwarding mechanism does not necessarily obey the
feasibility condition, it may get caught in routing loops; hence,
requests carry a hop count to limit the time during which they remain
in the network. However, since requests are only ever forwarded as
unicast packets, the initial hop count need not be kept particularly
low, and performing an expanding horizon search is not necessary. A
single request MUST NOT be duplicated: it MUST NOT be forwarded to a
multicast address, and it MUST NOT be forwarded to multiple
neighbours. However, if a seqno request is resent by its originator,
the subsequent copies may be forwarded to a different neighbour than
the initial one.
3.8.2. Sending Requests
A Babel node MAY send a route or seqno request at any time to a
multicast or a unicast address; there is only one case when
originating requests is required (Section 3.8.2.1).
3.8.2.1. Avoiding Starvation
When a route is retracted or expires, a Babel node usually switches
to another feasible route for the same prefix. It may be the case,
however, that no such routes are available.
A node that has lost all feasible routes to a given destination but
still has unexpired unfeasible routes to that destination MUST send a
seqno request; if it doesn't have any such routes, it MAY still send
a seqno request. The router-id of the request is set to the router-
id of the route that it has just lost, and the requested seqno is the
value contained in the source table plus 1. The request carries a
hop count, which is used as a last-resort mechanism to ensure that it
eventually vanishes from the network; it MAY be set to any value that
is larger than the diameter of the network (64 is a suitable default
value).
If the node has any (unfeasible) routes to the requested destination,
then it MUST send the request to at least one of the next-hop
neighbours that advertised these routes, and SHOULD send it to all of
them; in any case, it MAY send the request to any other neighbours,
whether they advertise a route to the requested destination or not.
A simple implementation strategy is therefore to unconditionally
multicast the request over all interfaces.
Similar requests will be sent by other nodes that are affected by the
route's loss. If the network is still connected, and assuming no
packet loss, then at least one of these requests will be forwarded to
the source, resulting in a route being advertised with a new sequence
number. (Due to duplicate suppression, only a small number of such
requests are expected to actually reach the source.)
In order to compensate for packet loss, a node SHOULD repeat such a
request a small number of times if no route becomes feasible within a
short time (see "Request timeout" in Appendix B for suggested
values). In the presence of heavy packet loss, however, all such
requests might be lost; in that case, the mechanism in the next
section will eventually ensure that a new seqno is received.
3.8.2.2. Dealing with Unfeasible Updates
When a route's metric increases, a node might receive an unfeasible
update for a route that it has currently selected. As specified in
Section 3.5.1, the receiving node will either ignore the update or
unselect the route.
In order to keep routes from spuriously expiring because they have
become unfeasible, a node SHOULD send a unicast seqno request when it
receives an unfeasible update for a route that is currently selected.
The requested sequence number is computed from the source table as in
Section 3.8.2.1.
Additionally, since metric computation does not necessarily coincide
with the delay in propagating updates, a node might receive an
unfeasible update from a currently unselected neighbour that is
preferable to the currently selected route (e.g., because it has a
much smaller metric); in that case, the node SHOULD send a unicast
seqno request to the neighbour that advertised the preferable update.
3.8.2.3. Preventing Routes from Expiring
In normal operation, a route's expiry timer never triggers: since a
route's hold time is computed from an explicit interval included in
Update TLVs, a new update (possibly a retraction) should arrive in
time to prevent a route from expiring.
In the presence of packet loss, however, it may be the case that no
update is successfully received for an extended period of time,
causing a route to expire. In order to avoid such spurious expiry,
shortly before a selected route expires, a Babel node SHOULD send a
unicast route request to the neighbour that advertised this route;
since nodes always send either updates or retractions in response to
non-wildcard route requests (Section 3.8.1.1), this will usually
result in the route being either refreshed or retracted.
4. Protocol Encoding
A Babel packet MUST be sent as the body of a UDP datagram, with
network-layer hop count set to 1, destined to a well-known multicast
address or to a unicast address, over IPv4 or IPv6; in the case of
IPv6, these addresses are link-local. Both the source and
destination UDP port are set to a well-known port number. A Babel
packet MUST be silently ignored unless its source address is either a
link-local IPv6 address or an IPv4 address belonging to the local
network, and its source port is the well-known Babel port. It MAY be
silently ignored if its destination address is a global IPv6 address.
In order to minimise the number of packets being sent while avoiding
lower-layer fragmentation, a Babel node SHOULD maximise the size of
the packets it sends, up to the outgoing interface's MTU adjusted for
lower-layer headers (28 octets for UDP over IPv4, 48 octets for UDP
over IPv6). It MUST NOT send packets larger than the attached
interface's MTU adjusted for lower-layer headers or 512 octets,
whichever is larger, but not exceeding 2^(16) - 1 adjusted for lower-
layer headers. Every Babel speaker MUST be able to receive packets
that are as large as any attached interface's MTU adjusted for lower-
layer headers or 512 octets, whichever is larger. Babel packets MUST
NOT be sent in IPv6 jumbograms [RFC2675].
4.1. Data Types
4.1.1. Representation of Integers
All multi-octet fields that represent integers are encoded with the
most significant octet first (in Big-Endian format [IEN137], also
called network order). The base protocol only carries unsigned
values; if an extension needs to carry signed values, it will need to
specify their encoding (e.g., two's complement).
4.1.2. Interval
Relative times are carried as 16-bit values specifying a number of
centiseconds (hundredths of a second). This allows times up to
roughly 11 minutes with a granularity of 10 ms, which should cover
all reasonable applications of Babel (see also Appendix B).
4.1.3. Router-Id
A router-id is an arbitrary 8-octet value. A router-id MUST NOT
consist of either all binary zeroes (0000000000000000 hexadecimal) or
all binary ones (FFFFFFFFFFFFFFFF hexadecimal).
4.1.4. Address
Since the bulk of the protocol is taken by addresses, multiple ways
of encoding addresses are defined. Additionally, within Update TLVs
a common subnet prefix may be omitted when multiple addresses are
sent in a single packet -- this is known as address compression
(Section 4.6.9).
Address encodings (AEs):
AE 0: Wildcard address. The value is 0 octets long.
AE 1: IPv4 address. Compression is allowed. 4 octets or less.
AE 2: IPv6 address. Compression is allowed. 16 octets or less.
AE 3: Link-local IPv6 address. Compression is not allowed. The
value is 8 octets long, a prefix of fe80::/64 is implied.
The address family associated with an address encoding is either IPv4
or IPv6: it is undefined for AE 0, IPv4 for AE 1, and IPv6 for AEs 2
and 3.
4.1.5. Prefixes
A network prefix is encoded just like a network address, but it is
stored in the smallest number of octets that are enough to hold the
significant bits (up to the prefix length).
4.2. Packet Format
A Babel packet consists of a 4-octet header, followed by a sequence
of TLVs (the packet body), optionally followed by a second sequence
of TLVs (the packet trailer). The format is designed to be
extensible; see Appendix D for extensibility considerations.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic | Version | Body length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Body...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Packet Trailer...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Magic The arbitrary but carefully chosen value 42 (decimal);
packets with a first octet different from 42 MUST be
silently ignored.
Version This document specifies version 2 of the Babel protocol.
Packets with a second octet different from 2 MUST be
silently ignored.
Body length The length in octets of the body following the packet
header (excluding the Magic, Version, and Body length
fields, and excluding the packet trailer).
Packet Body The packet body; a sequence of TLVs.
Packet Trailer The packet trailer; another sequence of TLVs.
The packet body and trailer are both sequences of TLVs. The packet
body is the normal place to store TLVs; the packet trailer only
contains specialised TLVs that do not need to be protected by
cryptographic security mechanisms. When parsing the trailer, the
receiver MUST ignore any TLV unless its definition explicitly states
that it is allowed to appear there. Among the TLVs defined in this
document, only Pad1 and PadN are allowed in the trailer; since these
TLVs are ignored in any case, an implementation MAY silently ignore
the packet trailer without even parsing it, unless it implements at
least one protocol extension that defines TLVs that are allowed to
appear in the trailer.
4.3. TLV Format
With the exception of Pad1, all TLVs have the following structure:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Payload...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type The type of the TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
Payload The TLV payload, which consists of a body and, for selected
TLV types, an optional list of sub-TLVs.
TLVs with an unknown type value MUST be silently ignored.
4.4. Sub-TLV Format
Every TLV carries an explicit length in its header; however, most
TLVs are self-terminating, in the sense that it is possible to
determine the length of the body without reference to the explicit
Length field. If a TLV has a self-terminating format, then the space
between the natural size of the TLV and the size announced in the
Length field may be used to store a sequence of sub-TLVs.
Sub-TLVs have the same structure as TLVs. With the exception of
Pad1, all TLVs have the following structure:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Body...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type The type of the sub-TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
Body The sub-TLV body, the interpretation of which depends on
both the type of the sub-TLV and the type of the TLV within
which it is embedded.
The most significant bit of the sub-TLV type (the bit with value 80
hexadecimal), is called the mandatory bit; in other words, sub-TLV
types 128 through 255 have the mandatory bit set. This bit indicates
how to handle unknown sub-TLVs. If the mandatory bit is not set,
then an unknown sub-TLV MUST be silently ignored, and the rest of the
TLV is processed normally. If the mandatory bit is set, then the
whole enclosing TLV MUST be silently ignored (except for updating the
parser state by a Router-Id, Next Hop, or Update TLV, as described in
the next section).
4.5. Parser State and Encoding of Updates
In a large network, the bulk of Babel traffic consists of route
updates; hence, some care has been given to encoding them
efficiently. The data conceptually contained in an update
(Section 3.5) is split into three pieces:
* the prefix, seqno, and metric are contained in the Update TLV
itself (Section 4.6.9);
* the router-id is taken from the Router-Id TLV that precedes the
Update TLV and may be shared among multiple Update TLVs
(Section 4.6.7);
* the next hop is taken either from the source address of the
network-layer packet that contains the Babel packet or from an
explicit Next Hop TLV (Section 4.6.8).
In addition to the above, an Update TLV can omit a prefix of the
prefix being announced, which is then extracted from the preceding
Update TLV in the same address family (IPv4 or IPv6). Finally, as a
special optimisation for the case when a router-id coincides with the
interface-id part of an IPv6 address, the Router-Id TLV itself may be
omitted, and the router-id is derived from the low-order bits of the
advertised prefix (Section 4.6.9).
In order to implement these compression techniques, Babel uses a
stateful parser: a TLV may refer to data from a previous TLV. The
parser state consists of the following pieces of data:
* for each address encoding that allows compression, the current
default prefix: this is undefined at the start of the packet and
is updated by each Update TLV with the Prefix flag set
(Section 4.6.9);
* for each address family (IPv4 or IPv6), the current next hop: this
is the source address of the enclosing packet for the matching
address family at the start of a packet, and it is updated by each
Next Hop TLV (Section 4.6.8);
* the current router-id: this is undefined at the start of the
packet, and it is updated by each Router-Id TLV (Section 4.6.7)
and by each Update TLV with Router-Id flag set.
Since the parser state must be identical across implementations, it
is updated before checking for mandatory sub-TLVs: parsing a TLV MUST
update the parser state even if the TLV is otherwise ignored due to
an unknown mandatory sub-TLV or for any other reason.
None of the TLVs that modify the parser state are allowed in the
packet trailer; hence, an implementation may choose to use a
dedicated stateless parser to parse the packet trailer.
4.6. Details of Specific TLVs
4.6.1. Pad1
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
Fields:
Type Set to 0 to indicate a Pad1 TLV.
This TLV is silently ignored on reception. It is allowed in the
packet trailer.
4.6.2. PadN
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length | MBZ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type Set to 1 to indicate a PadN TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
MBZ Must be zero, set to 0 on transmission.
This TLV is silently ignored on reception. It is allowed in the
packet trailer.
4.6.3. Acknowledgment Request
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This TLV requests that the receiver send an Acknowledgment TLV within
the number of centiseconds specified by the Interval field.
Fields:
Type Set to 2 to indicate an Acknowledgment Request TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
Reserved Sent as 0 and MUST be ignored on reception.
Opaque An arbitrary value that will be echoed in the receiver's
Acknowledgment TLV.
Interval A time interval in centiseconds after which the sender will
assume that this packet has been lost. This MUST NOT be 0.
The receiver MUST send an Acknowledgment TLV before this
time has elapsed (with a margin allowing for propagation
time).
This TLV is self-terminating and allows sub-TLVs.
4.6.4. Acknowledgment
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length | Opaque |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This TLV is sent by a node upon receiving an Acknowledgment Request
TLV.
Fields:
Type Set to 3 to indicate an Acknowledgment TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
Opaque Set to the Opaque value of the Acknowledgment Request that
prompted this Acknowledgment.
Since Opaque values are not globally unique, this TLV MUST be sent to
a unicast address.
This TLV is self-terminating and allows sub-TLVs.
4.6.5. Hello
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seqno | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This TLV is used for neighbour discovery and for determining a
neighbour's reception cost.
Fields:
Type Set to 4 to indicate a Hello TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
Flags The individual bits of this field specify special handling
of this TLV (see below).
Seqno If the Unicast flag is set, this is the value of the
sending node's outgoing Unicast Hello seqno for this
neighbour. Otherwise, it is the sending node's outgoing
Multicast Hello seqno for this interface.
Interval If nonzero, this is an upper bound, expressed in
centiseconds, on the time after which the sending node will
send a new scheduled Hello TLV with the same setting of the
Unicast flag. If this is 0, then this Hello represents an
unscheduled Hello and doesn't carry any new information
about times at which Hellos are sent.
The Flags field is interpreted as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|X|X|X|X|X|X|X|X|X|X|X|X|X|X|X|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
U (Unicast) flag (8000 hexadecimal): if set, then this Hello
represents a Unicast Hello, otherwise it represents a
Multicast Hello;
X: all other bits MUST be sent as 0 and silently ignored on
reception.
Every time a Hello is sent, the corresponding seqno counter MUST be
incremented. Since there is a single seqno counter for all the
Multicast Hellos sent by a given node over a given interface, if the
Unicast flag is not set, this TLV MUST be sent to all neighbours on
this link, which can be achieved by sending to a multicast
destination or by sending multiple packets to the unicast addresses
of all reachable neighbours. Conversely, if the Unicast flag is set,
this TLV MUST be sent to a single neighbour, which can achieved by
sending to a unicast destination. In order to avoid large
discontinuities in link quality, multiple Hello TLVs SHOULD NOT be
sent in the same packet.
This TLV is self-terminating and allows sub-TLVs.
4.6.6. IHU
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 | Length | AE | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rxcost | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address...
+-+-+-+-+-+-+-+-+-+-+-+-
An IHU ("I Heard You") TLV is used for confirming bidirectional
reachability and carrying a link's transmission cost.
Fields:
Type Set to 5 to indicate an IHU TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
AE The encoding of the Address field. This should be 1 or 3
in most cases. As an optimisation, it MAY be 0 if the TLV
is sent to a unicast address, if the association is over a
point-to-point link, or when bidirectional reachability is
ascertained by means outside of the Babel protocol.
Reserved Sent as 0 and MUST be ignored on reception.
Rxcost The rxcost according to the sending node of the interface
whose address is specified in the Address field. The value
FFFF hexadecimal (infinity) indicates that this interface
is unreachable.
Interval An upper bound, expressed in centiseconds, on the time
after which the sending node will send a new IHU; this MUST
NOT be 0. The receiving node will use this value in order
to compute a hold time for this symmetric association.
Address The address of the destination node, in the format
specified by the AE field. Address compression is not
allowed.
Conceptually, an IHU is destined to a single neighbour. However, IHU
TLVs contain an explicit destination address, and MAY be sent to a
multicast address, as this allows aggregation of IHUs destined to
distinct neighbours into a single packet and avoids the need for an
ARP or Neighbour Discovery exchange when a neighbour is not being
used for data traffic.
IHU TLVs with an unknown value in the AE field MUST be silently
ignored.
This TLV is self-terminating and allows sub-TLVs.
4.6.7. Router-Id
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Router-Id +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A Router-Id TLV establishes a router-id that is implied by subsequent
Update TLVs, as described in Section 4.5. This TLV sets the router-
id even if it is otherwise ignored due to an unknown mandatory sub-
TLV.
Fields:
Type Set to 6 to indicate a Router-Id TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
Reserved Sent as 0 and MUST be ignored on reception.
Router-Id The router-id for routes advertised in subsequent Update
TLVs. This MUST NOT consist of all zeroes or all ones.
This TLV is self-terminating and allows sub-TLVs.
4.6.8. Next Hop
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 | Length | AE | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next hop...
+-+-+-+-+-+-+-+-+-+-+-+-
A Next Hop TLV establishes a next-hop address for a given address
family (IPv4 or IPv6) that is implied in subsequent Update TLVs, as
described in Section 4.5. This TLV sets up the next hop for
subsequent Update TLVs even if it is otherwise ignored due to an
unknown mandatory sub-TLV.
Fields:
Type Set to 7 to indicate a Next Hop TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
AE The encoding of the Address field. This SHOULD be 1 (IPv4)
or 3 (link-local IPv6), and MUST NOT be 0.
Reserved Sent as 0 and MUST be ignored on reception.
Next hop The next-hop address advertised by subsequent Update TLVs
for this address family.
When the address family matches the network-layer protocol over which
this packet is transported, a Next Hop TLV is not needed: in the
absence of a Next Hop TLV in a given address family, the next-hop
address is taken to be the source address of the packet.
Next Hop TLVs with an unknown value for the AE field MUST be silently
ignored.
This TLV is self-terminating, and allows sub-TLVs.
4.6.9. Update
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 8 | Length | AE | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Plen | Omitted | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seqno | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-
An Update TLV advertises or retracts a route. As an optimisation, it
can optionally have the side effect of establishing a new implied
router-id and a new default prefix, as described in Section 4.5.
Fields:
Type Set to 8 to indicate an Update TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
AE The encoding of the Prefix field.
Flags The individual bits of this field specify special handling
of this TLV (see below).
Plen The length in bits of the advertised prefix. If AE is 3
(link-local IPv6), the Omitted field MUST be 0.
Omitted The number of octets that have been omitted at the
beginning of the advertised prefix and that should be taken
from a preceding Update TLV in the same address family with
the Prefix flag set.
Interval An upper bound, expressed in centiseconds, on the time
after which the sending node will send a new update for
this prefix. This MUST NOT be 0. The receiving node will
use this value to compute a hold time for the route table
entry. The value FFFF hexadecimal (infinity) expresses
that this announcement will not be repeated unless a
request is received (Section 3.8.2.3).
Seqno The originator's sequence number for this update.
Metric The sender's metric for this route. The value FFFF
hexadecimal (infinity) means that this is a route
retraction.
Prefix The prefix being advertised. This field's size is
(Plen/8 - Omitted) rounded upwards.
The Flags field is interpreted as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|P|R|X|X|X|X|X|X|
+-+-+-+-+-+-+-+-+
P (Prefix) flag (80 hexadecimal): if set, then this Update TLV
establishes a new default prefix for subsequent Update TLVs
with a matching address encoding within the same packet,
even if this TLV is otherwise ignored due to an unknown
mandatory sub-TLV;
R (Router-Id) flag (40 hexadecimal): if set, then this TLV
establishes a new default router-id for this TLV and
subsequent Update TLVs in the same packet, even if this TLV
is otherwise ignored due to an unknown mandatory sub-TLV.
This router-id is computed from the first address of the
advertised prefix as follows:
* if the length of the address is 8 octets or more, then
the new router-id is taken from the 8 last octets of the
address;
* if the length of the address is smaller than 8 octets,
then the new router-id consists of the required number
of zero octets followed by the address, i.e., the
address is stored on the right of the router-id. For
example, for an IPv4 address, the router-id consists of
4 octets of zeroes followed by the IPv4 address.
X: all other bits MUST be sent as 0 and silently ignored on
reception.
The prefix being advertised by an Update TLV is computed as follows:
* the first Omitted octets of the prefix are taken from the previous
Update TLV with the Prefix flag set and the same address encoding,
even if it was ignored due to an unknown mandatory sub-TLV; if the
Omitted field is not zero and there is no such TLV, then this
Update MUST be ignored;
* the next (Plen/8 - Omitted) rounded upwards octets are taken from
the Prefix field;
* if Plen is not a multiple of 8, then any bits beyond Plen (i.e.,
the low-order (8 - Plen MOD 8) bits of the last octet) are
cleared;
* the remaining octets are set to 0.
If the Metric field is finite, the router-id of the originating node
for this announcement is taken from the prefix advertised by this
Update if the Router-Id flag is set, computed as described above.
Otherwise, it is taken either from the preceding Router-Id TLV, or
the preceding Update TLV with the Router-Id flag set, whichever comes
last, even if that TLV is otherwise ignored due to an unknown
mandatory sub-TLV; if there is no suitable TLV, then this update is
ignored.
The next-hop address for this update is taken from the last preceding
Next Hop TLV with a matching address family (IPv4 or IPv6) in the
same packet even if it was otherwise ignored due to an unknown
mandatory sub-TLV; if no such TLV exists, it is taken from the
network-layer source address of this packet if it belongs to the same
address family as the prefix being announced; otherwise, this Update
MUST be ignored.
If the metric field is FFFF hexadecimal, this TLV specifies a
retraction. In that case, the router-id, next hop, and seqno are not
used. AE MAY then be 0, in which case this Update retracts all of
the routes previously advertised by the sending interface. If the
metric is finite, AE MUST NOT be 0; Update TLVs with finite metric
and AE equal to 0 MUST be ignored. If the metric is infinite and AE
is 0, Plen and Omitted MUST both be 0; Update TLVs that do not
satisfy this requirement MUST be ignored.
Update TLVs with an unknown value in the AE field MUST be silently
ignored.
This TLV is self-terminating and allows sub-TLVs.
4.6.10. Route Request
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Length | AE | Plen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-
A Route Request TLV prompts the receiver to send an update for a
given prefix, or a full route table dump.
Fields:
Type Set to 9 to indicate a Route Request TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
AE The encoding of the Prefix field. The value 0 specifies
that this is a request for a full route table dump (a
wildcard request).
Plen The length in bits of the requested prefix.
Prefix The prefix being requested. This field's size is Plen/8
rounded upwards.
A Request TLV prompts the receiver to send an update message
(possibly a retraction) for the prefix specified by the AE, Plen, and
Prefix fields, or a full dump of its route table if AE is 0 (in which
case Plen must be 0 and Prefix is of length 0). A Request TLV with
AE set to 0 and Plen not set to 0 MUST be ignored.
This TLV is self-terminating and allows sub-TLVs.
4.6.11. Seqno Request
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 10 | Length | AE | Plen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seqno | Hop Count | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Router-Id +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix...
+-+-+-+-+-+-+-+-+-+-+
A Seqno Request TLV prompts the receiver to send an Update for a
given prefix with a given sequence number, or to forward the request
further if it cannot be satisfied locally.
Fields:
Type Set to 10 to indicate a Seqno Request TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
AE The encoding of the Prefix field. This MUST NOT be 0.
Plen The length in bits of the requested prefix.
Seqno The sequence number that is being requested.
Hop Count The maximum number of times that this TLV may be
forwarded, plus 1. This MUST NOT be 0.
Reserved Sent as 0 and MUST be ignored on reception.
Router-Id The Router-Id that is being requested. This MUST NOT
consist of all zeroes or all ones.
Prefix The prefix being requested. This field's size is Plen/8
rounded upwards.
A Seqno Request TLV prompts the receiving node to send a finite-
metric Update for the prefix specified by the AE, Plen, and Prefix
fields, with either a router-id different from what is specified by
the Router-Id field, or a Seqno no less (modulo 2^(16)) than what is
specified by the Seqno field. If this request cannot be satisfied
locally, then it is forwarded according to the rules set out in
Section 3.8.1.2.
While a Seqno Request MAY be sent to a multicast address, it MUST NOT
be forwarded to a multicast address and MUST NOT be forwarded to more
than one neighbour. A request MUST NOT be forwarded if its Hop Count
field is 1.
This TLV is self-terminating and allows sub-TLVs.
4.7. Details of specific sub-TLVs
4.7.1. Pad1
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
Fields:
Type Set to 0 to indicate a Pad1 sub-TLV.
This sub-TLV is silently ignored on reception. It is allowed within
any TLV that allows sub-TLVs.
4.7.2. PadN
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length | MBZ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type Set to 1 to indicate a PadN sub-TLV.
Length The length of the body in octets, exclusive of the Type and
Length fields.
MBZ Must be zero, set to 0 on transmission.
This sub-TLV is silently ignored on reception. It is allowed within
any TLV that allows sub-TLVs.
5. IANA Considerations
IANA has registered the UDP port number 6696, called "babel", for use
by the Babel protocol.
IANA has registered the IPv6 multicast group ff02::1:6 and the IPv4
multicast group 224.0.0.111 for use by the Babel protocol.
IANA has created a registry called "Babel TLV Types". The allocation
policy for this registry is Specification Required [RFC8126] for
Types 0-223 and Experimental Use for Types 224-254. The values in
this registry are as follows:
+=========+==========================================+===========+
| Type | Name | Reference |
+=========+==========================================+===========+
| 0 | Pad1 | RFC 8966 |
+---------+------------------------------------------+-----------+
| 1 | PadN | RFC 8966 |
+---------+------------------------------------------+-----------+
| 2 | Acknowledgment Request | RFC 8966 |
+---------+------------------------------------------+-----------+
| 3 | Acknowledgment | RFC 8966 |
+---------+------------------------------------------+-----------+
| 4 | Hello | RFC 8966 |
+---------+------------------------------------------+-----------+
| 5 | IHU | RFC 8966 |
+---------+------------------------------------------+-----------+
| 6 | Router-Id | RFC 8966 |
+---------+------------------------------------------+-----------+
| 7 | Next Hop | RFC 8966 |
+---------+------------------------------------------+-----------+
| 8 | Update | RFC 8966 |
+---------+------------------------------------------+-----------+
| 9 | Route Request | RFC 8966 |
+---------+------------------------------------------+-----------+
| 10 | Seqno Request | RFC 8966 |
+---------+------------------------------------------+-----------+
| 11 | TS/PC | [RFC7298] |
+---------+------------------------------------------+-----------+
| 12 | HMAC | [RFC7298] |
+---------+------------------------------------------+-----------+
| 13 | Reserved | |
+---------+------------------------------------------+-----------+
| 14 | Reserved | |
+---------+------------------------------------------+-----------+
| 15 | Reserved | |
+---------+------------------------------------------+-----------+
| 224-254 | Reserved for Experimental Use | RFC 8966 |
+---------+------------------------------------------+-----------+
| 255 | Reserved for expansion of the type space | RFC 8966 |
+---------+------------------------------------------+-----------+
Table 1
IANA has created a registry called "Babel Sub-TLV Types". The
allocation policy for this registry is Specification Required for
Types 0-111 and 128-239, and Experimental Use for Types 112-126 and
240-254. The values in this registry are as follows:
+=========+===============================+===================+
| Type | Name | Reference |
+=========+===============================+===================+
| 0 | Pad1 | RFC 8966 |
+---------+-------------------------------+-------------------+
| 1 | PadN | RFC 8966 |
+---------+-------------------------------+-------------------+
| 2 | Diversity | [BABEL-DIVERSITY] |
+---------+-------------------------------+-------------------+
| 3 | Timestamp | [BABEL-RTT] |
+---------+-------------------------------+-------------------+
| 4-111 | Unassigned | |
+---------+-------------------------------+-------------------+
| 112-126 | Reserved for Experimental Use | RFC 8966 |
+---------+-------------------------------+-------------------+
| 127 | Reserved for expansion of the | RFC 8966 |
| | type space | |
+---------+-------------------------------+-------------------+
| 128 | Source Prefix | [BABEL-SS] |
+---------+-------------------------------+-------------------+
| 129-239 | Unassigned | |
+---------+-------------------------------+-------------------+
| 240-254 | Reserved for Experimental Use | RFC 8966 |
+---------+-------------------------------+-------------------+
| 255 | Reserved for expansion of the | RFC 8966 |
| | type space | |
+---------+-------------------------------+-------------------+
Table 2
IANA has created a registry called "Babel Address Encodings". The
allocation policy for this registry is Specification Required for
Address Encodings (AEs) 0-223, and Experimental Use for AEs 224-254.
The values in this registry are as follows:
+=========+========================================+===========+
| AE | Name | Reference |
+=========+========================================+===========+
| 0 | Wildcard address | RFC 8966 |
+---------+----------------------------------------+-----------+
| 1 | IPv4 address | RFC 8966 |
+---------+----------------------------------------+-----------+
| 2 | IPv6 address | RFC 8966 |
+---------+----------------------------------------+-----------+
| 3 | Link-local IPv6 address | RFC 8966 |
+---------+----------------------------------------+-----------+
| 4-223 | Unassigned | |
+---------+----------------------------------------+-----------+
| 224-254 | Reserved for Experimental Use | RFC 8966 |
+---------+----------------------------------------+-----------+
| 255 | Reserved for expansion of the AE space | RFC 8966 |
+---------+----------------------------------------+-----------+
Table 3
IANA has renamed the registry called "Babel Flags Values" to "Babel
Update Flags Values". The allocation policy for this registry is
Specification Required. The values in this registry are as follows:
+=====+===================+===========+
| Bit | Name | Reference |
+=====+===================+===========+
| 0 | Default prefix | RFC 8966 |
+-----+-------------------+-----------+
| 1 | Default router-id | RFC 8966 |
+-----+-------------------+-----------+
| 2-7 | Unassigned | |
+-----+-------------------+-----------+
Table 4
IANA has created a new registry called "Babel Hello Flags Values".
The allocation policy for this registry is Specification Required.
The initial values in this registry are as follows:
+======+============+===========+
| Bit | Name | Reference |
+======+============+===========+
| 0 | Unicast | RFC 8966 |
+------+------------+-----------+
| 1-15 | Unassigned | |
+------+------------+-----------+
Table 5
IANA has replaced all references to RFCs 6126 and 7557 in all of the
registries mentioned above with references to this document.
6. Security Considerations
As defined in this document, Babel is a completely insecure protocol.
Without additional security mechanisms, Babel trusts any information
it receives in plaintext UDP datagrams and acts on it. An attacker
that is present on the local network can impact Babel operation in a
variety of ways; for example they can:
* spoof a Babel packet, and redirect traffic by announcing a route
with a smaller metric, a larger sequence number, or a longer
prefix;
* spoof a malformed packet, which could cause an insufficiently
robust implementation to crash or interfere with the rest of the
network;
* replay a previously captured Babel packet, which could cause
traffic to be redirected, black-holed, or otherwise interfere with
the network.
When carried over IPv6, Babel packets are ignored unless they are
sent from a link-local IPv6 address; since routers don't forward
link-local IPv6 packets, this mitigates the attacks outlined above by
restricting them to on-link attackers. No such natural protection
exists when Babel packets are carried over IPv4, which is one of the
reasons why it is recommended to deploy Babel over IPv6
(Section 3.1).
It is usually difficult to ensure that packets arriving at a Babel
node are trusted, even in the case where the local link is believed
to be secure. For that reason, it is RECOMMENDED that all Babel
traffic be protected by an application-layer cryptographic protocol.
There are currently two suitable mechanisms, which implement
different trade-offs between implementation simplicity and security:
* Babel over DTLS [RFC8968] runs the majority of Babel traffic over
DTLS and leverages DTLS to authenticate nodes and provide
confidentiality and integrity protection;
* MAC authentication [RFC8967] appends a message authentication code
(MAC) to every Babel packet to prove that it originated at a node
that knows a shared secret, and includes sufficient additional
information to prove that the packet is fresh (not replayed).
Both mechanisms enable nodes to ignore packets generated by attackers
without the proper credentials. They also ensure integrity of
messages and prevent message replay. While Babel-DTLS supports
asymmetric keying and ensures confidentiality, Babel-MAC has a much
more limited scope (see Sections 1.1, 1.2, and 7 of [RFC8967]).
Since Babel-MAC is simpler and more lightweight, it is recommended in
preference to Babel-DTLS in deployments where its limitations are
acceptable, i.e., when symmetric keying is sufficient and where the
routing information is not considered confidential.
Every implementation of Babel SHOULD implement BABEL-MAC.
One should be aware that the information that a mobile Babel node
announces to the whole routing domain is sufficient to determine the
mobile node's physical location with reasonable precision, which
might cause privacy concerns even if the control traffic is protected
from unauthenticated attackers by a cryptographic mechanism such as
Babel-DTLS. This issue may be mitigated somewhat by using randomly
chosen router-ids and randomly chosen IP addresses, and changing them
often enough.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8967] Dô, C., Kolodziejak, W., and J. Chroboczek, "MAC
Authentication for the Babel Routing Protocol", RFC 8967,
DOI 10.17487/RFC8967, January 2021,
<https://www.rfc-editor.org/info/rfc8967>.
7.2. Informative References
[BABEL-DIVERSITY]
Chroboczek, J., "Diversity Routing for the Babel Routing
Protocol", Work in Progress, Internet-Draft, draft-
chroboczek-babel-diversity-routing-01, 15 February 2016,
<https://tools.ietf.org/html/draft-chroboczek-babel-
diversity-routing-01>.
[BABEL-RTT]
Jonglez, B. and J. Chroboczek, "Delay-based Metric
Extension for the Babel Routing Protocol", Work in
Progress, Internet-Draft, draft-ietf-babel-rtt-extension-
00, 26 April 2019, <https://tools.ietf.org/html/draft-
ietf-babel-rtt-extension-00>.
[BABEL-SS] Boutier, M. and J. Chroboczek, "Source-Specific Routing in
Babel", Work in Progress, Internet-Draft, draft-ietf-
babel-source-specific-07, 28 October 2020,
<https://tools.ietf.org/html/draft-ietf-babel-source-
specific-07>.
[DSDV] Perkins, C. and P. Bhagwat, "Highly dynamic Destination-
Sequenced Distance-Vector routing (DSDV) for mobile
computers", ACM SIGCOMM '94: Proceedings of the conference
on Communications architectures, protocols and
applications, 234-244, DOI 10.1145/190314.190336, October
1994, <https://doi.org/10.1145/190314.190336>.
[DUAL] Garcia Luna Aceves, J. J., "Loop-free routing using
diffusing computations", IEEE/ACM Transactions on
Networking, 1:1, DOI 10.1109/90.222913, February 1993,
<https://doi.org/10.1109/90.222913>.
[EIGRP] Albrightson, B., Garcia Luna Aceves, J. J., and J. Boyle,
"EIGRP -- a Fast Routing Protocol Based on Distance
Vectors", Proc. Networld/Interop 94, 1994.
[ETX] De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
high-throughput path metric for multi-hop wireless
networks", MobiCom '03: Proceedings of the 9th annual
international conference on Mobile computing and
networking, 134-146, DOI 10.1145/938985.939000, September
2003, <https://doi.org/10.1145/938985.939000>.
[IEEE802.11]
IEEE, "IEEE Standard for Information technology--
Telecommunications and information exchange between
systems Local and metropolitan area networks--Specific
requirements Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications",
IEEE 802.11-2012, DOI 10.1109/ieeestd.2012.6178212, April
2012, <https://doi.org/10.1109/ieeestd.2012.6178212>.
[IEN137] Cohen, D., "On Holy Wars and a Plea for Peace", IEN 137, 1
April 1980.
[IS-IS] International Organization for Standardization,
"Information technology -- Telecommunications and
information exchange between systems -- Intermediate
System to Intermediate System intra-domain routeing
information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode network
service (ISO 8473)", ISO/IEC 10589:2002, 2002.
[JITTER] Floyd, S. and V. Jacobson, "The Synchronization of
Periodic Routing Messages", IEEE/ACM Transactions on
Networking, 2, 2, 122-136, DOI 10.1109/90.298431, April
1994, <https://doi.org/10.1109/90.298431>.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[PACKETBB] Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
"Generalized Mobile Ad Hoc Network (MANET) Packet/Message
Format", RFC 5444, DOI 10.17487/RFC5444, February 2009,
<https://www.rfc-editor.org/info/rfc5444>.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, DOI 10.17487/RFC2675, August 1999,
<https://www.rfc-editor.org/info/rfc2675>.
[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
Demand Distance Vector (AODV) Routing", RFC 3561,
DOI 10.17487/RFC3561, July 2003,
<https://www.rfc-editor.org/info/rfc3561>.
[RFC6126] Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
DOI 10.17487/RFC6126, April 2011,
<https://www.rfc-editor.org/info/rfc6126>.
[RFC7298] Ovsienko, D., "Babel Hashed Message Authentication Code
(HMAC) Cryptographic Authentication", RFC 7298,
DOI 10.17487/RFC7298, July 2014,
<https://www.rfc-editor.org/info/rfc7298>.
[RFC7557] Chroboczek, J., "Extension Mechanism for the Babel Routing
Protocol", RFC 7557, DOI 10.17487/RFC7557, May 2015,
<https://www.rfc-editor.org/info/rfc7557>.
[RFC8968] Décimo, A., Schinazi, D., and J. Chroboczek, "Babel
Routing Protocol over Datagram Transport Layer Security",
RFC 8968, DOI 10.17487/RFC8968, January 2021,
<https://www.rfc-editor.org/info/rfc8968>.
[RIP] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
DOI 10.17487/RFC2453, November 1998,
<https://www.rfc-editor.org/info/rfc2453>.
Appendix A. Cost and Metric Computation
The strategy for computing link costs and route metrics is a local
matter; Babel itself only requires that it comply with the conditions
given in Section 3.4.3 and Section 3.5.2. Different nodes may use
different strategies in a single network and may use different
strategies on different interface types. This section describes a
set of strategies that have been found to work well in actual
networks.
In summary, a node maintains per-neighbour statistics about the last
16 received Hello TLVs of each kind (Appendix A.1), it computes costs
by using the 2-out-of-3 strategy (Appendix A.2.1) on wired links and
Expected Transmission Cost (ETX) (Appendix A.2.2) on wireless links.
It uses an additive algebra for metric computation (Section 3.5.2).
A.1. Maintaining Hello History
For each neighbour, a node maintains two sets of Hello history, one
for each kind of Hello, and an expected sequence number, one for
Multicast and one for Unicast Hellos. Each Hello history is a vector
of 16 bits, where a 1 value represents a received Hello, and a 0
value a missed Hello. For each kind of Hello, the expected sequence
number, written ne, is the sequence number that is expected to be
carried by the next received Hello from this neighbour.
Whenever it receives a Hello packet of a given kind from a neighbour,
a node compares the received sequence number nr for that kind of
Hello with its expected sequence number ne. Depending on the outcome
of this comparison, one of the following actions is taken:
* if the two differ by more than 16 (modulo 2^(16)), then the
sending node has probably rebooted and lost its sequence number;
the whole associated neighbour table entry is flushed and a new
one is created;
* otherwise, if the received nr is smaller (modulo 2^(16)) than the
expected sequence number ne, then the sending node has increased
its Hello interval without our noticing; the receiving node
removes the last (ne - nr) entries from this neighbour's Hello
history (we "undo history");
* otherwise, if nr is larger (modulo 2^(16)) than ne, then the
sending node has decreased its Hello interval, and some Hellos
were lost; the receiving node adds (nr - ne) 0 bits to the Hello
history (we "fast-forward").
The receiving node then appends a 1 bit to the Hello history and sets
ne to (nr + 1). If the Interval field of the received Hello is not
zero, it resets the neighbour's hello timer to 1.5 times the
advertised Interval (the extra margin allows for delay due to
jitter).
Whenever either hello timer associated with a neighbour expires, the
local node adds a 0 bit to the corresponding Hello history, and
increments the expected Hello number. If both Hello histories are
empty (they contain 0 bits only), the neighbour entry is flushed;
otherwise, the relevant hello timer is reset to the value advertised
in the last Hello of that kind received from this neighbour (no extra
margin is necessary in this case, since jitter was already taken into
account when computing the timeout that has just expired).
A.2. Cost Computation
This section describes two algorithms suitable for computing costs
(Section 3.4.3) based on Hello history. Appendix A.2.1 applies to
wired links and Appendix A.2.2 to wireless links. RECOMMENDED
default values of the parameters that appear in these algorithms are
given in Appendix B.
A.2.1. k-out-of-j
K-out-of-j link sensing is suitable for wired links that are either
up, in which case they only occasionally drop a packet, or down, in
which case they drop all packets.
The k-out-of-j strategy is parameterised by two small integers k and
j, such that 0 < k <= j, and the nominal link cost, a constant C >=
1. A node keeps a history of the last j hellos; if k or more of
those have been correctly received, the link is assumed to be up, and
the rxcost is set to C; otherwise, the link is assumed to be down,
and the rxcost is set to infinity.
Since Babel supports two kinds of Hellos, a Babel node performs k-
out-of-j twice for each neighbour, once on the Unicast Hello history
and once on the Multicast Hello history. If either of the instances
of k-out-of-j indicates that the link is up, then the link is assumed
to be up, and the rxcost is set to C; if both instances indicate that
the link is down, then the link is assumed to be down, and the rxcost
is set to infinity. In other words, the resulting rxcost is the
minimum of the rxcosts yielded by the two instances of k-out-of-j
link sensing.
The cost of a link performing k-out-of-j link sensing is defined as
follows:
* cost = FFFF hexadecimal if rxcost = FFFF hexadecimal;
* cost = txcost otherwise.
A.2.2. ETX
Unlike wired links which are bimodal (either up or down), wireless
links exhibit continuous variation of the link quality. Naive
application of hop-count routing in networks that use wireless links
for transit tends to select long, lossy links in preference to
shorter, lossless links, which can dramatically reduce throughput.
For that reason, a routing protocol designed to support wireless
links must perform some form of link quality estimation.
The Expected Transmission Cost algorithm, or ETX [ETX], is a simple
link quality estimation algorithm that is designed to work well with
the IEEE 802.11 MAC [IEEE802.11]. By default, the IEEE 802.11 MAC
performs Automatic Repeat Query (ARQ) and rate adaptation on unicast
frames, but not on multicast frames, which are sent at a fixed rate
with no ARQ; therefore, measuring the loss rate of multicast frames
yields a useful estimate of a link's quality.
A node performing ETX link quality estimation uses a neighbour's
Multicast Hello history to compute an estimate, written beta, of the
probability that a Hello TLV is successfully received. Beta can be
computed as the fraction of 1 bits within a small number (say, 6) of
the most recent entries in the Multicast Hello history, or it can be
an exponential average, or some combination of both approaches. Let
rxcost be 256/beta.
Let alpha be MIN(1, 256/txcost), an estimate of the probability of
successfully sending a Hello TLV. The cost is then computed by
cost = 256/(alpha * beta)
or, equivalently,
cost = (MAX(txcost, 256) * rxcost) / 256.
Since the IEEE 802.11 MAC performs ARQ on unicast frames, unicast
frames do not provide a useful measure of link quality, and therefore
ETX ignores the Unicast Hello history. Thus, a node performing ETX
link quality estimation will not route through neighbouring nodes
unless they send periodic Multicast Hellos (possibly in addition to
Unicast Hellos).
A.3. Route Selection and Hysteresis
Route selection (Section 3.6) is the process by which a node selects
a single route among the routes that it has available towards a given
destination. With Babel, any route selection procedure that only
ever chooses feasible routes with a finite metric will yield a set of
loop-free routes; however, in the presence of continuously variable
metrics such as ETX (Appendix A.2.2), a naive route selection
procedure might lead to persistent oscillations. Such oscillations
can be limited or avoided altogether by implementing hysteresis
within the route selection algorithm, i.e., by making the route
selection algorithm sensitive to a route's history. Any reasonable
hysteresis algorithm should yield good results; the following
algorithm is simple to implement and has been successfully deployed
in a variety of environments.
For every route R, in addition to the route's metric m(R), maintain a
smoothed version of m(R) written ms(R) (we RECOMMEND computing ms(R)
as an exponentially smoothed average (see Section 3.7 of [RFC793]) of
m(R) with a time constant equal to the Hello interval multiplied by a
small number such as 3). If no route to a given destination is
selected, then select the route with the smallest metric, ignoring
the smoothed metric. If a route R is selected, then switch to a
route R' only when both m(R') < m(R) and ms(R') < ms(R).
Intuitively, the smoothed metric is a long-term estimate of the
quality of a route. The algorithm above works by only switching
routes when both the instantaneous and the long-term estimates of the
route's quality indicate that switching is profitable.
Appendix B. Protocol Parameters
The choice of time constants is a trade-off between fast detection of
mobility events and protocol overhead. Two instances of Babel
running with different time constants will interoperate, although the
resulting worst-case convergence time will be dictated by the slower
of the two.
The Hello interval is the most important time constant: an outage or
a mobility event is detected within 1.5 to 3.5 Hello intervals. Due
to Babel's use of a redundant route table, and due to its reliance on
triggered updates and explicit requests, the Update interval has
little influence on the time needed to reconverge after an outage: in
practice, it only has a significant effect on the time needed to
acquire new routes after a mobility event. While the protocol allows
intervals as low as 10 ms, such low values would cause significant
amounts of protocol traffic for little practical benefit.
The following values have been found to work well in a variety of
environments and are therefore RECOMMENDED default values:
Multicast Hello interval: 4 seconds.
Unicast Hello interval: infinite (no Unicast Hellos are sent).
Link cost: estimated using ETX on wireless links; 2-out-of-3 with
C=96 on wired links.
IHU interval: the advertised IHU interval is always 3 times the
Multicast Hello interval. IHUs are actually sent with each
Hello on lossy links (as determined from the Hello
history), but only with every third Multicast Hello on
lossless links.
Update interval: 4 times the Multicast Hello interval.
IHU Hold time: 3.5 times the advertised IHU interval.
Route Expiry time: 3.5 times the advertised update interval.
Request timeout: initially 2 seconds, doubled every time a request
is resent, up to a maximum of three times.
Urgent timeout: 0.2 seconds.
Source GC time: 3 minutes.
Appendix C. Route Filtering
Route filtering is a procedure where an instance of a routing
protocol either discards some of the routes announced by its
neighbours or learns them with a metric that is higher than what
would be expected. Like all distance-vector protocols, Babel has the
ability to apply arbitrary filtering to the routes it learns, and
implementations of Babel that apply different sets of filtering rules
will interoperate without causing routing loops. The protocol's
ability to perform route filtering is a consequence of the latitude
given in Section 3.5.2: Babel can use any metric that is strictly
monotonic, including one that assigns an infinite metric to a
selected subset of routes. (See also Section 3.8.1, where requests
for nonexistent routes are treated in the same way as requests for
routes with infinite metric.)
It is not in general correct to learn a route with a metric smaller
than the one it was announced with, or to replace a route's
destination prefix with a more specific (longer) one. Doing either
of these may cause persistent routing loops.
Route filtering is a useful tool, since it allows fine-grained tuning
of the routing decisions made by the routing protocol. Accordingly,
some implementations of Babel implement a rich configuration language
that allows applying filtering to sets of routes defined, for
example, by incoming interface and destination prefix.
In order to limit the consequences of misconfiguration, Babel
implementations provide a reasonable set of default filtering rules
even when they don't allow configuration of filtering by the user.
At a minimum, they discard routes with a destination prefix in
fe80::/64, ff00::/8, 127.0.0.1/32, 0.0.0.0/32, and 224.0.0.0/8.
Appendix D. Considerations for Protocol Extensions
Babel is an extensible protocol, and this document defines a number
of mechanisms that can be used to extend the protocol in a backwards
compatible manner:
* increasing the version number in the packet header;
* defining new TLVs;
* defining new sub-TLVs (with or without the mandatory bit set);
* defining new AEs;
* using the packet trailer.
This appendix is intended to guide designers of protocol extensions
in choosing a particular encoding.
The version number in the Babel header should only be increased if
the new version is not backwards compatible with the original
protocol.
In many cases, an extension could be implemented either by defining a
new TLV or by adding a new sub-TLV to an existing TLV. For example,
an extension whose purpose is to attach additional data to route
updates can be implemented either by creating a new "enriched" Update
TLV, by adding a nonmandatory sub-TLV to the Update TLV, or by adding
a mandatory sub-TLV.
The various encodings are treated differently by implementations that
do not understand the extension. In the case of a new TLV or of a
sub-TLV with the mandatory bit set, the whole TLV is ignored by
implementations that do not implement the extension, while in the
case of a nonmandatory sub-TLV, the TLV is parsed and acted upon, and
only the unknown sub-TLV is silently ignored. Therefore, a
nonmandatory sub-TLV should be used by extensions that extend the
Update in a compatible manner (the extension data may be silently
ignored), while a mandatory sub-TLV or a new TLV must be used by
extensions that make incompatible extensions to the meaning of the
TLV (the whole TLV must be thrown away if the extension data is not
understood).
Experience shows that the need for additional data tends to crop up
in the most unexpected places. Hence, it is recommended that
extensions that define new TLVs should make them self-terminating and
allow attaching sub-TLVs to them.
Adding a new AE is essentially equivalent to adding a new TLV: Update
TLVs with an unknown AE are ignored, just like unknown TLVs.
However, adding a new AE is more involved than adding a new TLV,
since it creates a new set of compression state. Additionally, since
the Next Hop TLV creates state specific to a given address family, as
opposed to a given AE, a new AE for a previously defined address
family must not be used in the Next Hop TLV if backwards
compatibility is required. A similar issue arises with Update TLVs
with unknown AEs establishing a new router-id (due to the Router-Id
flag being set). Therefore, defining new AEs must be done with care
if compatibility with unextended implementations is required.
The packet trailer is intended to carry cryptographic signatures that
only cover the packet body; storing the cryptographic signatures in
the packet trailer avoids clearing the signature before computing a
hash of the packet body, and makes it possible to check a
cryptographic signature before running the full, stateful TLV parser.
Hence, only TLVs that don't need to be protected by cryptographic
security protocols should be allowed in the packet trailer. Any such
TLVs should be easy to parse and, in particular, should not require
stateful parsing.
Appendix E. Stub Implementations
Babel is a fairly economic protocol. Updates take between 12 and 40
octets per destination, depending on the address family and how
successful compression is; in a dual-stack flat network, an average
of less than 24 octets per update is typical. The route table
occupies about 35 octets per IPv6 entry. To put these values into
perspective, a single full-size Ethernet frame can carry some 65
route updates, and a megabyte of memory can contain a 20,000-entry
route table and the associated source table.
Babel is also a reasonably simple protocol. One complete
implementation consists of less than 12,000 lines of C code, and it
compiles to less than 120 KB of text on a 32-bit CISC architecture;
about half of this figure is due to protocol extensions and user-
interface code.
Nonetheless, in some very constrained environments, such as PDAs,
microwave ovens, or abacuses, it may be desirable to have subset
implementations of the protocol.
There are many different definitions of a stub router, but for the
needs of this section, a stub implementation of Babel is one that
announces one or more directly attached prefixes into a Babel network
but doesn't re-announce any routes that it has learnt from its
neighbours, and always prefers the direct route to a directly
attached prefix to a route learnt over the Babel protocol, even when
the prefixes are the same. It may either maintain a full routing
table or simply select a default gateway through any one of its
neighbours that announces a default route. Since a stub
implementation never forwards packets except from or to a directly
attached link, it cannot possibly participate in a routing loop, and
hence it need not evaluate the feasibility condition or maintain a
source table.
No matter how primitive, a stub implementation must parse sub-TLVs
attached to any TLVs that it understands and check the mandatory bit.
It must answer acknowledgment requests and must participate in the
Hello/IHU protocol. It must also be able to reply to seqno requests
for routes that it announces, and it should be able to reply to route
requests.
Experience shows that an IPv6-only stub implementation of Babel can
be written in less than 1,000 lines of C code and compile to 13 KB of
text on 32-bit CISC architecture.
Appendix F. Compatibility with Previous Versions
The protocol defined in this document is a successor to the protocol
defined in [RFC6126] and [RFC7557]. While the two protocols are not
entirely compatible, the new protocol has been designed so that it
can be deployed in existing RFC 6126 networks without requiring a
flag day.
There are three optional features that make this protocol
incompatible with its predecessor. First of all, RFC 6126 did not
define Unicast Hellos (Section 3.4.1), and an implementation of RFC
6126 will misinterpret a Unicast Hello for a Multicast one; since the
sequence number space of Unicast Hellos is distinct from the sequence
number space of Multicast Hellos, sending a Unicast Hello to an
implementation of RFC 6126 will confuse its link quality estimator.
Second, RFC 6126 did not define unscheduled Hellos, and an
implementation of RFC 6126 will mis-parse Hellos with an interval
equal to 0. Finally, RFC 7557 did not define mandatory sub-TLVs
(Section 4.4), and thus an implementation of RFCs 6126 and 7557 will
not correctly ignore a TLV that carries an unknown mandatory sub-TLV;
depending on the sub-TLV, this might cause routing pathologies.
An implementation of this specification that never sends Unicast or
unscheduled Hellos and doesn't implement any extensions that use
mandatory sub-TLVs is safe to deploy in a network in which some nodes
implement the protocol described in RFCs 6126 and 7557.
Two changes need to be made to an implementation of RFCs 6126 and
7557 so that it can safely interoperate in all cases with
implementations of this protocol. First, it needs to be modified
either to ignore or to process Unicast and unscheduled Hellos.
Second, it needs to be modified to parse sub-TLVs of all the TLVs
that it understands and that allow sub-TLVs, and to ignore the TLV if
an unknown mandatory sub-TLV is found. It is not necessary to parse
unknown TLVs, as these are ignored in any case.
There are other changes, but these are not of a nature to prevent
interoperability:
* the conditions on route acquisition (Section 3.5.3) have been
relaxed;
* route selection should no longer use the route's sequence number
(Section 3.6);
* the format of the packet trailer has been defined (Section 4.2);
* router-ids with a value of all-zeros or all-ones have been
forbidden (Section 4.1.3);
* the compression state is now specific to an address family rather
than an address encoding (Section 4.5);
* packet pacing is now recommended (Section 3.1).
Acknowledgments
A number of people have contributed text and ideas to this
specification. The authors are particularly indebted to Matthieu
Boutier, Gwendoline Chouasne, Margaret Cullen, Donald Eastlake, Toke
Høiland-Jørgensen, Benjamin Kaduk, Joao Sobrinho, and Martin
Vigoureux. The previous version of this specification [RFC6126]
greatly benefited from the input of Joel Halpern. The address
compression technique was inspired by [PACKETBB].
Authors' Addresses
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris CEDEX 13
France
Email: jch@irif.fr
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 94043
United States of America