| Rfc | 7574 |
|---|
| Title | Peer-to-Peer Streaming Peer Protocol (PPSPP) |
|---|
| Author | A. Bakker, R.
Petrocco, V. Grishchenko |
|---|
| Date | July 2015 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) A. Bakker
Request for Comments: 7574 Vrije Universiteit Amsterdam
Category: Standards Track R. Petrocco
ISSN: 2070-1721 V. Grishchenko
Technische Universiteit Delft
July 2015
Peer-to-Peer Streaming Peer Protocol (PPSPP)
Abstract
The Peer-to-Peer Streaming Peer Protocol (PPSPP) is a protocol for
disseminating the same content to a group of interested parties in a
streaming fashion. PPSPP supports streaming of both prerecorded (on-
demand) and live audio/video content. It is based on the peer-to-
peer paradigm, where clients consuming the content are put on equal
footing with the servers initially providing the content, to create a
system where everyone can potentially provide upload bandwidth. It
has been designed to provide short time-till-playback for the end
user and to prevent disruption of the streams by malicious peers.
PPSPP has also been designed to be flexible and extensible. It can
use different mechanisms to optimize peer uploading, prevent
freeriding, and work with different peer discovery schemes
(centralized trackers or Distributed Hash Tables). It supports
multiple methods for content integrity protection and chunk
addressing. Designed as a generic protocol that can run on top of
various transport protocols, it currently runs on top of UDP using
Low Extra Delay Background Transport (LEDBAT) for congestion control.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7574.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................5
1.1. Purpose ....................................................5
1.2. Requirements Language ......................................6
1.3. Terminology ................................................6
2. Overall Operation ...............................................9
2.1. Example: Joining a Swarm ...................................9
2.2. Example: Exchanging Chunks ................................10
2.3. Example: Leaving a Swarm ..................................10
3. Messages .......................................................11
3.1. HANDSHAKE .................................................11
3.1.1. Handshake Procedure ................................12
3.2. HAVE ......................................................14
3.3. DATA ......................................................15
3.4. ACK .......................................................15
3.5. INTEGRITY .................................................15
3.6. SIGNED_INTEGRITY ..........................................16
3.7. REQUEST ...................................................16
3.8. CANCEL ....................................................16
3.9. CHOKE and UNCHOKE .........................................17
3.10. Peer Address Exchange ....................................17
3.10.1. PEX_REQ and PEX_RES Messages ......................17
3.11. Channels .................................................19
3.12. Keep Alive Signaling .....................................20
4. Chunk Addressing Schemes .......................................21
4.1. Start-End Ranges ..........................................21
4.1.1. Chunk Ranges .......................................21
4.1.2. Byte Ranges ........................................21
4.2. Bin Numbers ...............................................22
4.3. In Messages ...............................................23
4.3.1. In HAVE Messages ...................................23
4.3.2. In ACK Messages ....................................24
5. Content Integrity Protection ...................................24
5.1. Merkle Hash Tree Scheme ...................................25
5.2. Content Integrity Verification ............................26
5.3. The Atomic Datagram Principle .............................27
5.4. INTEGRITY Messages ........................................28
5.5. Discussion and Overhead ...................................28
5.6. Automatic Detection of Content Size .......................29
5.6.1. Peak Hashes ........................................29
5.6.2. Procedure ..........................................31
6. Live Streaming .................................................32
6.1. Content Authentication ....................................32
6.1.1. Sign All ...........................................33
6.1.2. Unified Merkle Tree ................................33
6.1.2.1. Signed Munro Hashes .......................34
6.1.2.2. Munro Signature Calculation ...............36
6.1.2.3. Procedure .................................37
6.1.2.4. Secure Tune In ............................37
6.2. Forgetting Chunks .........................................38
7. Protocol Options ...............................................38
7.1. End Option ................................................39
7.2. Version ...................................................39
7.3. Minimum Version ...........................................40
7.4. Swarm Identifier ..........................................40
7.5. Content Integrity Protection Method .......................41
7.6. Merkle Tree Hash Function .................................41
7.7. Live Signature Algorithm ..................................42
7.8. Chunk Addressing Method ...................................42
7.9. Live Discard Window .......................................43
7.10. Supported Messages .......................................44
7.11. Chunk Size ...............................................44
8. UDP Encapsulation ..............................................45
8.1. Chunk Size ................................................45
8.2. Datagrams and Messages ....................................46
8.3. Channels ..................................................47
8.4. HANDSHAKE .................................................47
8.5. HAVE ......................................................48
8.6. DATA ......................................................48
8.7. ACK .......................................................49
8.8. INTEGRITY .................................................50
8.9. SIGNED_INTEGRITY ..........................................51
8.10. REQUEST ..................................................52
8.11. CANCEL ...................................................52
8.12. CHOKE and UNCHOKE ........................................53
8.13. PEX_REQ, PEX_RESv4, PEX_RESv6, and PEX_REScert ...........53
8.14. KEEPALIVE ................................................55
8.15. Flow and Congestion Control ..............................56
8.16. Example of Operation .....................................57
9. Extensibility ..................................................61
9.1. Chunk Picking Algorithms ..................................61
9.2. Reciprocity Algorithms ....................................62
10. IANA Considerations ...........................................62
10.1. PPSPP Message Type Registry ..............................62
10.2. PPSPP Option Registry ....................................62
10.3. PPSPP Version Number Registry ............................62
10.4. PPSPP Content Integrity Protection Method Registry .......62
10.5. PPSPP Merkle Hash Tree Function Registry .................63
10.6. PPSPP Chunk Addressing Method Registry ...................63
11. Manageability Considerations ..................................63
11.1. Operations ...............................................63
11.1.1. Installation and Initial Setup ....................63
11.1.2. Migration Path ....................................64
11.1.3. Requirements on Other Protocols and
Functional Components .............................64
11.1.4. Impact on Network Operation .......................64
11.1.5. Verifying Correct Operation .......................65
11.1.6. Configuration .....................................65
11.2. Management Considerations ................................66
11.2.1. Management Interoperability and Information .......67
11.2.2. Fault Management ..................................67
11.2.3. Configuration Management ..........................67
11.2.4. Accounting Management .............................68
11.2.5. Performance Management ............................68
11.2.6. Security Management ...............................68
12. Security Considerations .......................................68
12.1. Security of the Handshake Procedure ......................68
12.1.1. Protection against Attack 1 .......................69
12.1.2. Protection against Attack 2 .......................70
12.1.3. Protection against Attack 3 .......................70
12.2. Secure Peer Address Exchange .............................71
12.2.1. Protection against the Amplification Attack .......71
12.2.2. Example: Tracker as Certification Authority .......72
12.2.3. Protection against Eclipse Attacks ................73
12.3. Support for Closed Swarms ................................73
12.4. Confidentiality of Streamed Content ......................74
12.5. Strength of the Hash Function for Merkle Hash Trees ......74
12.6. Limit Potential Damage and Resource Exhaustion by
Bad or Broken Peers ......................................74
12.6.1. HANDSHAKE .........................................75
12.6.2. HAVE ..............................................75
12.6.3. DATA ..............................................75
12.6.4. ACK ...............................................75
12.6.5. INTEGRITY and SIGNED_INTEGRITY ....................76
12.6.6. REQUEST ...........................................76
12.6.7. CANCEL ............................................76
12.6.8. CHOKE .............................................77
12.6.9. UNCHOKE ...........................................77
12.6.10. PEX_RES ..........................................77
12.6.11. Unsolicited Messages in General ..................77
12.7. Exclude Bad or Broken Peers ..............................77
13. References ....................................................78
13.1. Normative References .....................................78
13.2. Informative References ...................................79
Acknowledgements ..................................................84
Authors' Addresses ................................................85
1. Introduction
1.1. Purpose
This document describes the Peer-to-Peer Streaming Peer Protocol
(PPSPP), designed for disseminating the same content to a group of
interested parties in a streaming fashion. PPSPP supports streaming
of both prerecorded (on-demand) and live audio/video content. It is
based on the peer-to-peer paradigm where clients consuming the
content are put on equal footing with the servers initially providing
the content, to create a system where everyone can potentially
provide upload bandwidth.
PPSPP has been designed to provide short time-till-playback for the
end user and to prevent disruption of the streams by malicious peers.
Central in this design is a simple method of identifying content
based on self-certification. In particular, content in PPSPP is
identified by a single cryptographic hash that is the root hash in a
Merkle hash tree calculated recursively from the content [MERKLE]
[ABMRKL]. This self-certifying hash tree allows every peer to
directly detect when a malicious peer tries to distribute fake
content. The tree can be used for both static and live content.
Moreover, it ensures only a small amount of information is needed to
start a download and to verify incoming chunks of content, thus
ensuring short start-up times.
PPSPP has also been designed to be extensible for different
transports and use cases. Hence, PPSPP is a generic protocol that
can run directly on top of UDP, TCP, or other protocols. As such,
PPSPP defines a common set of messages that make up the protocol,
which can have different representations on the wire depending on the
lower-level protocol used. When the lower-level transport allows,
PPSPP can also use different congestion control algorithms.
At present, PPSPP is set to run on top of UDP using LEDBAT for
congestion control [RFC6817]. Using LEDBAT enables PPSPP to serve
the content after playback (seeding) without disrupting the user who
may have moved to different tasks that use its network connection.
PPSPP is also flexible and extensible in the mechanisms it uses to
promote client contribution and prevent freeriding, that is, how to
deal with peers that only download content but never upload to
others. It also allows different schemes for chunk addressing and
content integrity protection, if the defaults are not fit for a
particular use case. In addition, it can work with different peer
discovery schemes, such as centralized trackers or fast Distributed
Hash Tables [JIM11]. Finally, in this default setup, PPSPP maintains
only a small amount of state per peer. A reference implementation of
PPSPP over UDP is available [SWIFTIMPL].
The protocol defined in this document assumes that a peer has already
discovered a list of (initial) peers using, for example, a
centralized tracker [PPSP-TP]. Once a peer has this list of peers,
PPSPP allows the peer to connect to other peers, request chunks of
content, and discover other peers disseminating the same content.
The design of PPSPP is based on our research into making BitTorrent
[BITTORRENT] suitable for streaming content [P2PWIKI]. Most PPSPP
messages have corresponding BitTorrent messages and vice versa.
However, PPSPP is specifically targeted towards streaming audio/video
content and optimizes time-till-playback. It was also designed to be
more flexible and extensible.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.3. Terminology
message
The basic unit of PPSPP communication. A message will have
different representations on the wire depending on the transport
protocol used. Messages are typically multiplexed into a
datagram for transmission.
datagram
A sequence of messages that is offered as a unit to the
underlying transport protocol (UDP, etc.). The datagram is
PPSPP's Protocol Data Unit (PDU).
content
Either a live transmission or a prerecorded multimedia file.
chunk
The basic unit in which the content is divided. For example, a
block of N kilobytes. A chunk may be of variable size.
chunk ID
Unique identifier for a chunk of content (e.g., an integer). Its
type depends on the chunk addressing scheme used.
chunk specification
An expression that denotes one or more chunk IDs.
chunk addressing scheme
Scheme for identifying chunks and expressing the chunk
availability map of a peer in a compact fashion.
chunk availability map
The set of chunks a peer has successfully downloaded and checked
the integrity of.
bin
A number denoting a specific binary interval of the content
(i.e., one or more consecutive chunks) in the bin numbers chunk
addressing scheme (see Section 4).
content integrity protection scheme
Scheme for protecting the integrity of the content while it is
being distributed via the peer-to-peer network. That is, methods
for receiving peers to detect whether a requested chunk has been
modified, either maliciously by the sending peer or accidentally
in transit.
hash
The result of applying a cryptographic hash function, more
specifically a Modification Detection Code (MDC) [HAC01], such as
SHA-256 [FIPS180-4], to a piece of data.
Merkle hash tree
A tree of hashes whose base is formed by the hashes of the chunks
of content, and its higher nodes are calculated by recursively
computing the hash of the concatenation of the two child hashes
(see Section 5.1).
root hash
The root in a Merkle hash tree calculated recursively from the
content (see Section 5.1).
munro hash
The hash of a subtree that is the unit of signing in the Unified
Merkle Tree content authentication scheme for live streaming (see
Section 6.1.2.1).
swarm
A group of peers participating in the distribution of the same
content.
swarm ID
Unique identifier for a swarm of peers, in PPSPP a sequence of
bytes. For video on demand with content integrity protection
enabled, the identifier is the so-called root hash of a Merkle
hash tree over the content. For live streaming, the swarm ID is
a public key.
tracker
An entity that records the addresses of peers participating in a
swarm, usually for a set of swarms, and makes this membership
information available to other peers on request.
choking
When Peer A is choking Peer B, it means that A is currently not
willing to accept requests for content from B.
seeding
Peer A is said to be seeding when A has downloaded a static
content file completely and is now offering it for others to
download.
leeching
Peer A is said to be leeching when A has not completely
downloaded a static content file yet or is not offering to upload
it to others.
channel
A logical connection between two peers. The channel concept
allows peers to use the same transport address for communicating
with different peers.
channel ID
Unique, randomly chosen identifier for a channel, local to each
peer. So the two peers logically connected by a channel each
have a different channel ID for that channel.
heavy payload
A datagram has a heavy payload when it contains DATA messages,
SIGNED_INTEGRITY messages, or a large number of smaller messages.
In this document the prefixes kilo-, mega-, etc., denote base 1024.
2. Overall Operation
The basic unit of communication in PPSPP is the message. Multiple
messages are multiplexed into a single datagram for transmission. A
datagram (and hence the messages it contains) will have different
representations on the wire depending on the transport protocol used
(see Section 8).
The overall operation of PPSPP is illustrated in the following
examples. The examples assume that the content distributed is
static, UDP is used for transport, the Merkle Hash Tree scheme is
used for content integrity protection, and that a specific policy is
used for selecting which chunks to download.
2.1. Example: Joining a Swarm
Consider a user who wants to watch a video. To play the video, the
user clicks on the play button of a HTML5 <video> element shown in
his PPSPP-enabled browser. Imagine this element has a PPSPP URL (to
be defined elsewhere) identifying the video as its source. The
browser passes this URL to its peer-to-peer streaming protocol
handler. Let's call this protocol handler Peer A. Peer A parses the
URL to retrieve the transport address of a peer-to-peer streaming
protocol tracker and swarm metadata of the content. The tracker
address may be optional in the presence of a decentralized tracking
mechanism. The mechanisms for tracking peers are outside of the
scope of this document.
Peer A now registers with the tracker following the peer-to-peer
streaming protocol tracker specification [PPSP-TP] and receives the
IP address and port of peers already in the swarm, say, Peers B, C,
and D. At this point, the PPSPP starts operating. Peer A now sends
a datagram containing a PPSPP HANDSHAKE message to Peers B, C, and D.
This message conveys protocol options. In particular, Peer A
includes the ID of the swarm (part of the swarm metadata) as a
protocol option because the destination peers can listen for multiple
swarms on the same transport address.
Peers B and C respond with datagrams containing a PPSPP HANDSHAKE
message and one or more HAVE messages. A HAVE message conveys (part
of) the chunk availability of a peer; thus, it contains a chunk
specification that denotes what chunks of the content Peers B and C
have, respectively. Peer D sends a datagram with a HANDSHAKE and
HAVE messages, but also with a CHOKE message. The latter indicates
that Peer D is not willing to upload chunks to Peer A at present.
2.2. Example: Exchanging Chunks
In response to Peers B and C, Peer A sends new datagrams to Peers B
and C containing REQUEST messages. A REQUEST message indicates the
chunks that a peer wants to download; thus, it contains a chunk
specification. The REQUEST messages to Peers B and C refer to
disjoint sets of chunks. Peers B and C respond with datagrams
containing HAVE, DATA, and, in this example, INTEGRITY messages. In
the Merkle hash tree content protection scheme (see Section 5.1), the
INTEGRITY messages contain all cryptographic hashes that Peer A needs
to verify the integrity of the content chunk sent in the DATA
message. Using these hashes, Peer A verifies that the chunks
received from Peers B and C are correct against the trusted swarm ID.
Peer A also updates the chunk availability of Peers B and C using the
information in the received HAVE messages. In addition, it passes
the chunks of video to the user's browser for rendering.
After processing, Peer A sends a datagram containing HAVE messages
for the chunks it just received to all its peers. In the datagram to
Peers B and C, it includes an ACK message acknowledging the receipt
of the chunks and adds REQUEST messages for new chunks. ACK messages
are not used when a reliable transport protocol is used. When, for
example, Peer C finds that Peer A obtained a chunk (from Peer B) that
Peer C did not yet have, Peer C's next datagram includes a REQUEST
for that chunk.
Peer D also sends HAVE messages to Peer A when it downloads chunks
from other peers. When Peer D is willing to accept REQUESTs from
Peer A, Peer D sends a datagram with an UNCHOKE message to inform
Peer A. If Peer B or C decides to choke Peer A, they send a CHOKE
message and Peer A should then re-request from other peers. Peers B
and C may continue to send HAVE, REQUEST, or periodic keep-alive
messages such that Peer A keeps sending them HAVE messages.
Once Peer A has received all content (video-on-demand use case), it
stops sending messages to all other peers that have all content
(a.k.a. seeders). Peer A can also contact the tracker or another
source again to obtain more peer addresses.
2.3. Example: Leaving a Swarm
To leave a swarm in a graceful way, Peer A sends a specific HANDSHAKE
message to all its peers (see Section 8.4) and deregisters from the
tracker following the tracker specification [PPSP-TP]. Peers
receiving the datagram should remove Peer A from their current peer
list. If Peer A crashes ungracefully, peers should remove Peer A
from their peer list when they detect it no longer sends messages
(see Section 3.12).
3. Messages
No error codes or responses are used in the protocol; absence of any
response indicates an error. Invalid messages are discarded, and
further communication with the peer SHOULD be stopped. The rationale
is that it is sufficient to classify peers as either good or bad and
only use the good ones. A good peer is a peer that responds with
chunks; a peer that does not respond, or does not respond in time is
classified as bad. The idea is that, in PPSPP, the content is
available from multiple sources (unlike HTTP), so a peer should not
invest too much effort in trying to obtain it from a particular
source. This classification in good or bad allows a peer to deal
with slow, crashed, and (silent) malicious peers.
Multiple messages MUST be multiplexed into a single datagram for
transmission. Messages in a single datagram MUST be processed in the
strict order in which they appear in the datagram. If an invalid
message is found in a datagram, the remaining messages MUST be
discarded.
For the sake of simplicity, one swarm of peers deals with one content
file or stream only. There is a single division of the content into
chunks that all peers in the swarm adhere to, determined by the
content publisher. Distribution of a collection of files can be done
either by using multiple swarms or by using an external storage
mapping from the linear byte space of a single swarm to different
files, transparent to the protocol. In other words, the audio/video
container format used is outside the scope of this document.
3.1. HANDSHAKE
For Peer P to establish communication with Peer Q in Swarm S, the
peers must first exchange HANDSHAKE messages by means of a handshake
procedure. The initiating Peer P needs to know the metadata of Swarm
S, which consists of:
(a) the swarm ID of the content (see Sections 5.1 and 6),
(b) the chunk size used,
(c) the chunk addressing method used,
(d) the content integrity protection method used, and
(e) the Merkle hash tree function used (if applicable).
(f) If automatic content size detection (see Section 5.6) is not
used, the content length is also part of the metadata (for
static content.)
This document assumes the swarm metadata is obtained from a trusted
source. In addition, Peer P needs to know a transport address for
Peer Q, obtained from a peer discovery/tracking protocol.
The payload of the HANDSHAKE message contains a sequence of protocol
options. The protocol options encode the swarm metadata just
described to enable an end-to-end check to see whether the peers are
in the right swarm. Additionally, the options encode a number of
per-peer configuration parameters. The complete set of protocol
options are specified in Section 7. The HANDSHAKE message also
contains a channel ID for multiplexing communication and security
(see Sections 3.11 and 12.1). A HANDSHAKE message MUST always be the
first message in a datagram.
3.1.1. Handshake Procedure
The handshake procedure for a peer, Peer P, to start communication
with another peer, Peer Q, in Swarm S is now as follows.
1. The first datagram the initiating Peer P sends to Peer Q MUST
start with a HANDSHAKE message. This HANDSHAKE message MUST
contain:
* A channel ID, chanP, randomly chosen as specified in
Section 12.1.
* The metadata of Swarm S, encoded as protocol options, as
specified in Section 7. In particular, the initiating Peer P
MUST include the swarm ID.
* The capabilities of Peer P, in particular, its supported
protocol versions, "Live Discard Window" (in case of a live
swarm) and "Supported Messages", encoded as protocol options.
This first datagram MUST be prefixed with the (destination)
channel ID 0; see Section 3.11. Hence, the datagram contains two
channel IDs: the destination channel ID prefixed to the datagram
and the channel ID chanP included in the HANDSHAKE message inside
the datagram. This datagram MAY also contain some minor
additional payload, e.g., HAVE messages to indicate Peer P's
current progress, but it MUST NOT include any heavy payload
(defined in Section 1.3), such as a DATA message. Allowing minor
payload minimizes the number of initialization round trips, thus
improving time-till-playback. Forbidding heavy payload prevents
an amplification attack (see Section 12.1).
2. The receiving Peer Q checks the HANDSHAKE message from Peer P.
If any check by Peer Q fails, or if Peers P and Q are not in the
same swarm, Peer Q MUST NOT send a HANDSHAKE (or any other)
message back, as the message from Peer P may have been spoofed
(see Section 12.1). Otherwise, if Peer Q is interested in
communicating with Peer P, Peer Q MUST send a datagram to Peer P
that starts with a HANDSHAKE message. This reply HANDSHAKE MUST
contain:
* A channel ID, chanQ, randomly chosen as specified in
Section 12.1.
* The metadata of Swarm S, encoded as protocol options, as
specified in Section 7. In particular, the responding Peer Q
MAY include the swarm ID.
* The capabilities of Peer Q, in particular, its supported
protocol versions, its "Live Discard Window" (in case of a
live swarm) and "Supported Messages", encoded as protocol
options.
This reply datagram MUST be prefixed with the channel ID chanP
sent by Peer P in the first HANDSHAKE message (see Section 3.11).
This reply datagram MAY also contain some minor additional
payload, e.g., HAVE messages to indicate Peer Q's current
progress, or REQUEST messages (see Section 3.7), but it MUST NOT
include any heavy payload.
3. The initiating Peer P checks the reply datagram from Peer Q. If
the reply datagram is not prefixed with (destination) channel ID
chanP, Peer P MUST discard the datagram. Peer P SHOULD continue
to process datagrams from Peer Q that do meet this requirement.
This check prevents interference by spoofing, see Section 12.1.
If Peer P's channel ID is echoed correctly, the initiator Peer P
knows that the addressed Peer Q really responds.
4. Next, Peer P checks the HANDSHAKE message in the datagram from
Peer Q. If any check by Peer P fails, or Peer P is no longer
interested in communicating with Peer Q, Peer P MAY send a
HANDSHAKE message to inform Peer Q it will cease communication.
This closing HANDSHAKE message MUST contain an all zeros channel
ID and a list of protocol options. The list MUST either be empty
or contain the maximum version number Peer P supports, following
the min/max versioning scheme defined in [RFC6709], Section 4.1.
The datagram containing this closing HANDSHAKE message MUST be
prefixed with the (destination) channel ID chanQ. Peer P MAY
also simply cease communication.
5. If the addressed peer, Peer Q, does not respond to initiating
Peer P's first datagram, Peer P MAY resend that datagram until
Peer Q is considered dead, according to the rules specified in
Section 3.12.
6. If the reply datagram by Peer Q does pass the checks by Peer P,
and Peer P wants to continue interacting with Peer Q, Peer P can
now send REQUEST, PEX_REQ, and other messages to Peer Q.
Datagrams carrying these messages MUST be prefixed with the
channel ID chanQ sent by Peer Q. More specifically, because Peer
P knows that Peer Q really responds, Peer P MAY start sending
Peer Q messages with heavy payload. That means that Peer P MAY
start responding to any REQUEST messages that Peer Q may have
sent in this first reply datagram with DATA messages. Hence,
transfer of chunks can start soon in PPSPP.
7. If Peer Q receives any datagram (apparently) from Peer P that
does not contain channel ID chanQ, Peer Q MUST discard the
datagram but SHOULD continue to process datagrams from Peer P
that do meet this requirement. Once Peer Q receives a datagram
from Peer P that does contain the channel ID chanQ, Peer Q knows
that Peer P really received its reply datagram, and the three-way
handshake and channel establishment is complete. Peer Q MAY now
also start sending messages with heavy payload to Peer P.
8. If Peer P decides it no longer wants to communicate with Peer Q,
or vice versa, the peer SHOULD send a closing HANDSHAKE message
to the other, as described above.
3.2. HAVE
The HAVE message is used to convey which chunks a peer has available
for download. The set of chunks it has available may be expressed
using different chunk addressing and availability map compression
schemes, described in Section 4. HAVE messages can be used both for
sending a complete overview of a peer's chunk availability as well as
for updates to that set.
In particular, whenever a receiving Peer P has successfully checked
the integrity of a chunk, or interval of chunks, it MUST send a HAVE
message to all peers Q1..Qn it wants to allow to download those
chunks. A policy in Peer P determines when the HAVE is sent. Peer P
may send it directly, or Peer P may wait either until it has other
data to send to Peer Qi or until it has received and checked multiple
chunks. The policy will depend on how urgent it is to distribute
this information to the other peers. This urgency is generally
determined in turn by the chunk picking policy (see Section 9.1). In
general, the HAVE messages can be piggybacked onto other messages.
Peers that do not receive HAVE messages are effectively prevented
from downloading the newly available chunks; hence, the HAVE message
can be used as a method of choking.
The HAVE message MUST contain the chunk specification of the received
and verified chunks. A receiving peer MUST NOT send a HAVE message
to peers for which the handshake procedure is still incomplete, see
Section 12.1. A peer SHOULD NOT send a HAVE message to peers that
have the complete content already (e.g., in video-on-demand
scenarios).
3.3. DATA
The DATA message is used to transfer chunks of content. The DATA
message MUST contain the chunk ID of the chunk and chunk itself. A
peer MAY send the DATA messages for multiple chunks in the same
datagram. The DATA message MAY contain additional information if
needed by the specific congestion control mechanism used. At
present, PPSPP uses LEDBAT [RFC6817] for congestion control, which
requires the current system time to be sent along with the DATA
message, so the current system time MUST be included.
3.4. ACK
ACK messages MUST be sent to acknowledge received chunks if PPSPP is
run over an unreliable transport protocol. ACK messages MAY be sent
if a reliable transport protocol is used. In the former case, a
receiving peer that has successfully checked the integrity of a
chunk, or interval of chunks C, MUST send an ACK message containing a
chunk specification for C. As LEDBAT is used, an ACK message MUST
contain the one-way delay, computed from the peer's current system
time received in the DATA message. A peer MAY delay sending ACK
messages as defined in the LEDBAT specification [RFC6817].
3.5. INTEGRITY
The INTEGRITY message carries information required by the receiver to
verify the integrity of a chunk. Its payload depends on the content
integrity protection scheme used. When the Merkle Hash Tree scheme
is used, an INTEGRITY message MUST contain a cryptographic hash of a
subtree of the Merkle hash tree and the chunk specification that
identifies the subtree.
As a typical example, when a peer wants to send a chunk and Merkle
hash trees are used, it creates a datagram that consists of several
INTEGRITY messages containing the hashes the receiver needs to verify
the chunk and the actual chunk itself encoded in a DATA message.
What are the necessary hashes and the exact rules for encoding them
into datagrams is specified in Sections 5.3, and 5.4, respectively.
3.6. SIGNED_INTEGRITY
The SIGNED_INTEGRITY message carries digitally signed information
required by the receiver to verify the integrity of a chunk in live
streaming. It logically contains a chunk specification, a timestamp,
and a digital signature. Its exact payload depends on the live
content integrity protection scheme used, see Section 6.1.
3.7. REQUEST
While bulk download protocols normally do explicit requests for
certain ranges of data (i.e., use a pull model, for example,
BitTorrent [BITTORRENT]), live streaming protocols quite often use a
push model without requests to save round trips. PPSPP supports both
models of operation.
The REQUEST message is used to request one or more chunks from
another peer. A REQUEST message MUST contain the specification of
the chunks the requester wants to download. A peer receiving a
REQUEST message MAY send out the requested chunks (by means of DATA
messages). When Peer Q receives multiple REQUESTs from the same Peer
P, Peer Q SHOULD process the REQUESTs in the order received.
Multiple REQUEST messages MAY be sent in one datagram, for example,
when a peer wants to request several rare chunks at once.
When live streaming via a push model, a peer receiving REQUESTs also
MAY send some other chunks in case it runs out of requests or for
some other reason. In that case, the only purpose of REQUEST
messages is to provide hints and coordinate peers to avoid
unnecessary data retransmission.
3.8. CANCEL
When downloading on-demand or live streaming content, a peer can
request urgent data from multiple peers to increase the probability
of it being delivered on time. In particular, when the specific
chunk picking algorithm (see Section 9.1), detects that a request for
urgent data might not be served on time, a request for the same data
can be sent to a different peer. When a Peer P decides to request
urgent data from a Peer Q, Peer P SHOULD send a CANCEL message to all
the peers to which the data has been previously requested. The
CANCEL message contains the specification of the chunks Peer P no
longer wants to request. In addition, when Peer Q receives a HAVE
message for the urgent data from Peer P, Peer Q MUST also cancel the
previous REQUEST(s) from Peer P. In other words, the HAVE message
acts as an implicit CANCEL.
3.9. CHOKE and UNCHOKE
Peer A can send a CHOKE message to Peer B to signal it will no longer
be responding to REQUEST messages from Peer B, for example, because
Peer A's upload capacity is exhausted. Peer A MAY send a subsequent
UNCHOKE message to signal that it will respond to new REQUESTs from
Peer B again (Peer A SHOULD discard old requests). When Peer B
receives a CHOKE message from Peer A, it MUST NOT send new REQUEST
messages and it cannot expect answers to any outstanding ones, as the
transfer of chunks is choked. When Peer B is choked but receives a
HAVE message from Peer A, it is not automatically unchoked and MUST
NOT send any new REQUEST messages. The CHOKE and UNCHOKE messages
are informational as responding to REQUESTs is OPTIONAL, see
Section 3.7.
3.10. Peer Address Exchange
3.10.1. PEX_REQ and PEX_RES Messages
Peer Exchange (PEX) messages are common in many peer-to-peer
protocols. They allow peers to exchange the transport addresses of
the peers they are currently interacting with, thereby reducing the
need to contact a central tracker (or Distributed Hash Table) to
discovery new peers. The strength of this mechanism is therefore
that it enables decentralized peer discovery: after an initial
bootstrap, a central tracker is no longer needed. Its weakness is
that it enables a number of attacks, so it should not be used on the
Internet unless extra security measures are in place.
PPSPP supports peer-address exchange on the Internet and in benign
private networks as an OPTIONAL feature (not mandatory to implement)
under certain conditions. The general mechanism works as follows.
To obtain some peer addresses, a Peer A MAY send a PEX_REQ message to
Peer B. Peer B MAY respond with one or more PEX_REScert messages.
Logically, a PEX_REScert reply message contains the address of a
single peer Ci. Peer B MUST have exchanged messages with Peer Ci in
the last 60 seconds to guarantee liveliness. Upon receipt, Peer A
may contact any or none of the returned peers Ci. Alternatively,
peers MAY ignore PEX_REQ and PEX_REScert messages if uninterested in
obtaining new peers or because of security considerations (rate
limiting) or any other reason. The PEX messages can be used to
construct a dedicated tracker peer.
To use PEX in PPSPP on the Internet, two conditions must be met:
1. Peer transport addresses must be relatively stable.
2. A peer must not obtain all its peer addresses through PEX.
The full security analysis for PEX messages can be found in
Section 12.2. Physically, a PEX_REScert message carries a swarm-
membership certificate rather than an IP address and port. A
membership certificate for Peer C states that Peer C at address
(ipC,portC) is part of Swarm S at Time T and is cryptographically
signed by an issuer. The receiver Peer A can check the certificate
for a valid signature by a trusted issuer, the right swarm, and
liveliness and only then consider contacting C. These swarm-
membership certificates correspond to signed node descriptors in
secure decentralized peer sampling services [SPS].
Several designs are possible for the security environment for these
membership certificates. That is, there are different designs
possible for who signs the membership certificates and how public
keys are distributed. Section 12.2.2 describes an example where a
central tracker acts as the Certification Authority.
In a hostile environment, such as the Internet, peers must also
ensure that they do not end up interacting only with malicious peers
when using the peer-address exchange feature. To this extent, peers
MUST ensure that part of their connections are to peers whose
addresses came from a trusted and secured tracker (see
Section 12.2.3).
In addition to the PEX_REScert, there are two other PEX reply
messages. The PEX_RESv4 message contains a single IPv4 address and
port. The PEX_RESv6 message contains a single IPv6 address and port.
They MUST only be used in a benign environment, such as a private
network, as they provide no guarantees that the host addressed
actually participates in a PPSPP swarm.
Once a PPSPP implementation has obtained a list of peers (either via
PEX, from a central tracker, or via a Distributed Hash Table (DHT)),
it has to determine which peers to actually contact. In this
process, a PPSPP implementation can benefit from information by
network or content providers to help improve network usage and boost
PPSPP performance. How a peer-to-peer (P2P) system like PPSPP can
perform these optimizations using the Application-Layer Traffic
Optimization (ALTO) protocol is described in detail in [RFC7285],
Section 7.
3.11. Channels
It is increasingly complex for peers to enable communication between
each other due to NATs and firewalls. Therefore, PPSPP uses a
multiplexing scheme, called channels, to allow multiple swarms to use
the same transport address. Channels loosely correspond to TCP
connections and each channel belongs to a single swarm, as
illustrated in Figure 1. As with TCP connections, a channel is
identified by a unique identifier local to the peer at each end of
the connection (cf. TCP port), which MUST be randomly chosen. In
other words, the two peers connected by a channel use different IDs
to denote the same channel. The IDs are different and random for
security reasons, see Section 12.1.
In the PPSP-over-UDP encapsulation (Section 8.3), when a Channel C
has been established between Peer A and Peer B, the datagrams
containing messages from Peer A to Peer B are prefixed with the four-
byte channel ID allocated by Peer B, and vice versa for datagrams
from Peer B to A. The channel IDs used are exchanged as part of the
handshake procedure, see Section 8.4. In that procedure, the channel
ID with value 0 is used for the datagram that initiates the
handshake. PPSPP can be used in combination with Session Traversal
Utilities for NAT (STUN) [RFC5389].
_________ _________ _________
| | | | | |
| Swarm | | Swarm | | Swarm |
| Mgr | | A | | B |
|_______| |_______| |_______|
| | / \
| | / \
____|____ ____|____ ______/__ _\_______
| | | | | | | |
| Chan | | Chan | | Chan | | Chan |
| 0 | | 481 | | 836 | | 372 |
|_______| |_______| |_______| |_______|
| | | |
| | | |
____|____________|____________|____________|____
| |
| UDP |
| port 6778 |
|______________________________________________|
Network stack of a PPSPP peer that is reachable on UDP port 6778 and
is connected via channel 481 to one peer in Swarm A and two peers in
Swarm B via channels 836 and 372, respectively. Channel ID 0 is
special and is used for handshaking.
Figure 1
3.12. Keep Alive Signaling
A peer SHOULD send a "keep alive" message periodically to each peer
it is interested in, but has no other messages to send to them at
present. The goal of the keep alives is to keep a signaling channel
open to peers that are of interest. Which peers those are is
determined by a policy that decides which peers are of interest now
and in the near future. This document does not prescribe a policy,
but examples of interesting peers are (a) peers that have chunks on
offer that this client needs or (b) peers that currently do not have
interesting chunks on offer (because they are still downloading
themselves, or in live streaming) but gave good performance in the
past. When these peers have new chunks to offer, the peer that kept
a signaling channel open can use them again. Periodically sending
"keep alive" messages prevents other peers declaring the peer dead.
A guideline for declaring a peer dead when using UDP consists of a
three minute delay since that last packet has been received from that
peer and at least three datagrams having been sent to that peer
during the same period. When a peer is declared dead, the channel to
it is closed, no more messages will be sent to that peer and the
local administration about the peer is discarded. Busy servers can
force idle clients to disconnect by not sending keep alives. PPSPP
does not define an explicit message type for "keep alive" messages.
In the PPSP-over-UDP encapsulation they are implemented as simple
datagrams consisting of a four-byte channel ID only, see Sections 8.3
and 8.4.
4. Chunk Addressing Schemes
PPSPP can use different methods of chunk addressing, that is, support
different ways of identifying chunks and different ways of expressing
the chunk availability map of a peer in a compact fashion.
All peers in a swarm MUST use the same chunk addressing method.
4.1. Start-End Ranges
A chunk specification consists of a single (start specification,end
specification) pair that identifies a range of chunks (end
inclusive). The start and end specifications can use one of multiple
addressing schemes. Two schemes are currently defined: chunk ranges
and byte ranges.
4.1.1. Chunk Ranges
The start and end specification are both chunk identifiers. Chunk
identifiers are 32-bit or 64-bit unsigned integers. A PPSPP peer
MUST support this scheme.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Start chunk (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ End chunk (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.2. Byte Ranges
The start and end specification are 64-bit byte offsets in the
content. The support for this scheme is OPTIONAL.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start byte offset (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| End byte offset (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2. Bin Numbers
PPSPP introduces a novel method of addressing chunks of content
called "bin numbers" (or "bins" for short). Bin numbers allow the
addressing of a binary interval of data using a single integer. This
reduces the amount of state that needs to be recorded per peer and
the space needed to denote intervals on the wire, making the protocol
lightweight. In general, this numbering system allows PPSPP to work
with simpler data structures, e.g., to use arrays instead of binary
trees, thus reducing complexity. The support for this scheme is
OPTIONAL.
In bin addressing, the smallest binary interval is a single chunk
(e.g., a block of bytes that may be of variable size), the largest
interval is a complete range of 2**63 chunks. In a novel addition to
the classical scheme, these intervals are numbered in a way that lays
them out into a vector nicely, which is called bin numbering, as
follows. Consider a chunk interval of width W. To derive the bin
numbers of the complete interval and the subintervals, a minimal
balanced binary tree is built that is at least W chunks wide at the
base. The leaves from left-to-right correspond to the chunks 0..W-1
in the interval, and have bin number I*2 where I is the index of the
chunk (counting beyond W-1 to balance the tree). The bin number of
higher-level node P in the tree is calculated as follows:
binP = (binL + binR) / 2
where binL is the bin of node P's left-hand child and binR is the bin
of node P's right-hand child. Given that each node in the tree
represents a subinterval of the original interval, each such
subinterval now is addressable by a bin number, a single integer.
The bin number tree of an interval of width W=8 looks like this:
7
/ \
/ \
/ \
/ \
3 11
/ \ / \
/ \ / \
/ \ / \
1 5 9 13
/ \ / \ / \ / \
0 2 4 6 8 10 12 14
C0 C1 C2 C3 C4 C5 C6 C7
The bin number tree of an interval of width W=8
Figure 2
So bin 7 represents the complete interval, bin 3 represents the
interval of chunk C0..C3, bin 1 represents the interval of chunks C0
and C1, and bin 2 represents chunk C1. The special numbers
0xFFFFFFFF (32-bit) or 0xFFFFFFFFFFFFFFFF (64-bit) stands for an
empty interval, and 0x7FFF...FFF stands for "everything".
When bin numbering is used, the ID of a chunk is its corresponding
(leaf) bin number in the tree, and the chunk specification in HAVE
and ACK messages is equal to a single bin number (32-bit or 64-bit),
as follows.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Bin number (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.3. In Messages
4.3.1. In HAVE Messages
When a receiving peer has successfully checked the integrity of a
chunk or interval of chunks, it MUST send a HAVE message to all peers
it wants to allow download of those chunk(s) from. The ability to
withhold HAVE messages allows them to be used as a method of choking.
The HAVE message MUST contain the chunk specification of the biggest
complete interval of all chunks the receiver has received and checked
so far that fully includes the interval of chunks just received. So
the chunk specification MUST denote at least the interval received,
but the receiver is supposed to aggregate and acknowledge bigger
intervals, when possible.
As a result, every single chunk is acknowledged a logarithmic number
of times. That provides some necessary redundancy of
acknowledgements and sufficiently compensates for unreliable
transport protocols.
Implementation note:
To record which chunks a peer has in the state that an
implementation keeps for each peer, an implementation MAY use the
efficient "binmap" data structure, which is a hybrid of a bitmap
and a binary tree, discussed in detail in [BINMAP].
4.3.2. In ACK Messages
PPSPP peers MUST use ACK messages to acknowledge received chunks if
an unreliable transport protocol is used. When a receiving peer has
successfully checked the integrity of a chunk or interval of chunks
C, it MUST send an ACK message containing the chunk specification of
its biggest, complete interval covering C to the sending peer (see
HAVE).
5. Content Integrity Protection
PPSPP can use different methods for protecting the integrity of the
content while it is being distributed via the peer-to-peer network.
More specifically, PPSPP can use different methods for receiving
peers to detect whether a requested chunk has been maliciously
modified by the sending peer. In benign environments, content
integrity protection can be disabled.
For static content, PPSPP currently defines one method for protecting
integrity, called the Merkle Hash Tree scheme. If PPSPP operates
over the Internet, this scheme MUST be used. If PPSPP operates in a
benign environment, this scheme MAY be used. So the scheme is
mandatory to implement, to satisfy the requirement of strong security
for an IETF protocol [RFC3365]. An extended version of the scheme is
used to efficiently protect dynamically generated content (live
streams), as explained below and in Section 6.1.
The Merkle Hash Tree scheme can work with different chunk addressing
schemes. All it requires is the ability to address a range of
chunks. In the following description abstract node IDs are used to
identify nodes in the tree. On the wire, these are translated to the
corresponding range of chunks in the chosen chunk addressing scheme.
5.1. Merkle Hash Tree Scheme
PPSPP uses a method of naming content based on self-certification.
In particular, content in PPSPP is identified by a single
cryptographic hash that is the root hash in a Merkle hash tree
calculated recursively from the content [ABMRKL]. This self-
certifying hash tree allows every peer to directly detect when a
malicious peer tries to distribute fake content. It also ensures
only a small the amount of information is needed to start a download
(the root hash and some peer addresses). For live streaming, a
dynamic tree and a public key are used, see below.
The Merkle hash tree of a content file that is divided into N chunks
is constructed as follows. Note the construction does not assume
chunks of content to be of a fixed size. Given a cryptographic hash
function, more specifically an MDC [HAC01], such as SHA-256, the
hashes of all the chunks of the content are calculated. Next, a
binary tree of sufficient height is created. Sufficient height means
that the lowest level in the tree has enough nodes to hold all chunk
hashes in the set, as with bin numbering. The figure below shows the
tree for a content file consisting of 7 chunks. As with the content
addressing scheme, the leaves of the tree correspond to a chunk and,
in this case, are assigned the hash of that chunk, starting at the
leftmost leaf. As the base of the tree may be wider than the number
of chunks, any remaining leaves in the tree are assigned an empty
hash value of all zeros. Finally, the hash values of the higher
levels in the tree are calculated, by concatenating the hash values
of the two children (again left to right) and computing the hash of
that aggregate. If the two children are empty hashes, the parent is
an empty all-zeros hash as well (to save computation). This process
ends in a hash value for the root node, which is called the "root
hash". Note the root hash only depends on the content and any
modification of the content will result in a different root hash.
7 = root hash
/ \
/ \
/ \
/ \
3* 11
/ \ / \
/ \ / \
/ \ / \
1 5 9 13* = uncle hash
/ \ / \ / \ / \
0 2 4 6 8 10* 12 14
C0 C1 C2 C3 C4 C5 C6 E
=chunk index ^^ = empty hash
Merkle hash tree of a content file with N=7 chunks
Figure 3
5.2. Content Integrity Verification
Assuming a peer receives the root hash of the content it wants to
download from a trusted source, it can check the integrity of any
chunk of that content it receives as follows. It first calculates
the hash of the chunk it received, for example, chunk C4 in the
previous figure. Along with this chunk, it MUST receive the hashes
required to check the integrity of that chunk. In principle, these
are the hash of the chunk's sibling (C5) and that of its "uncles". A
chunk's uncles are the sibling Y of its parent X, and the uncle of
that Y, recursively until the root is reached. For chunk C4, the
uncles are nodes 13 and 3 and the sibling is 10; all marked with a *
in the figure. Using this information, the peer recalculates the
root hash of the tree and compares it to the root hash it received
from the trusted source. If they match, the chunk of content has
been positively verified to be the requested part of the content.
Otherwise, the sending peer sent either the wrong content or the
wrong sibling or uncle hashes. For simplicity, the set of sibling
and uncle hashes is collectively referred to as the "uncle hashes".
In the case of live streaming, the tree of chunks grows dynamically
and the root hash is undefined or, more precisely, transient, as long
as new data is generated by the live source. Section 6.1.2 defines a
method for content integrity verification for live streams that works
with such a dynamic tree. Although the tree is dynamic, content
verification works the same for both live and predefined content,
resulting in a unified method for both types of streaming.
5.3. The Atomic Datagram Principle
As explained above, a datagram consists of a sequence of messages.
Ideally, every datagram sent must be independent of other datagrams:
each datagram SHOULD be processed separately, and a loss of one
datagram must not disrupt the flow of datagrams between two peers.
Thus, as a datagram carries zero or more messages, both messages and
message interdependencies SHOULD NOT span over multiple datagrams.
This principle implies that as any chunk is verified using its uncle
hashes, the necessary hashes SHOULD be put into the same datagram as
the chunk's data. If this is not possible because of a limitation on
datagram size, the necessary hashes MUST be sent first in one or more
datagrams. As a general rule, if some additional data is still
missing to process a message within a datagram, the message SHOULD be
dropped.
The hashes necessary to verify a chunk are, in principle, its
sibling's hash and all its uncle hashes, but the set of hashes to
send can be optimized. Before sending a packet of data to the
receiver, the sender inspects the receiver's previous
acknowledgements (HAVE or ACK) to derive which hashes the receiver
already has for sure. Suppose the receiver had acknowledged chunks
C0 and C1 (the first two chunks of the file), then it must already
have uncle hashes 5, 11, and so on. That is because those hashes are
necessary to check C0 and C1 against the root hash. Then, hashes 3,
7, and so on must also be known as they are calculated in the process
of checking the uncle hash chain. Hence, to send chunk C7, the
sender needs to include just the hashes for nodes 14 and 9, which let
the data be checked against hash 11, which is already known to the
receiver.
The sender MAY optimistically skip hashes that were sent out in
previous, still-unacknowledged datagrams. It is an optimization
trade-off between redundant hash transmission and the possibility of
collateral data loss in the case in which some necessary hashes were
lost in the network so some delivered data cannot be verified and
thus had to be dropped. In either case, the receiver builds the
Merkle hash tree on-demand, incrementally, starting from the root
hash, and uses it for data validation.
In short, the sender MUST put into the datagram the hashes he
believes are necessary for the receiver to verify the chunk. The
receiver MUST remember all the hashes it needs to verify missing
chunks that it still wants to download. Note that the latter implies
that a hardware-limited receiver MAY forget some hashes if it does
not plan to announce possession of these chunks to others (i.e., does
not plan to send HAVE messages.)
5.4. INTEGRITY Messages
Concretely, a peer that wants to send a chunk of content creates a
datagram that MUST consist of a list of INTEGRITY messages followed
by a DATA message. If the INTEGRITY messages and DATA message cannot
be put into a single datagram because of a limitation on datagram
size, the INTEGRITY messages MUST be sent first in one or more
datagrams. The list of INTEGRITY messages sent MUST contain an
INTEGRITY message for each hash the receiver misses for integrity
checking. An INTEGRITY message for a hash MUST contain the chunk
specification corresponding to the node ID of the hash and the hash
data itself. The chunk specification corresponding to a node ID is
defined as the range of chunks formed by the leaves of the subtree
rooted at the node. For example, node 3 in Figure 3 denotes chunks
0, 2, 4, and 6, so the chunk specification should denote that
interval. The list of INTEGRITY messages MUST be sorted in order of
the tree height of the nodes, descending (the leaves are at height
0). The DATA message MUST contain the chunk specification of the
chunk and the chunk itself. A peer MAY send the required messages
for multiple chunks in the same datagram, depending on the
encapsulation.
5.5. Discussion and Overhead
The current method for protecting content integrity in BitTorrent
[BITTORRENT] is not suited for streaming. It involves providing
clients with the hashes of the content's chunks before the download
commences by means of metadata files (called .torrent files in
BitTorrent.) However, when chunks are small, as in the current UDP
encapsulation of PPSPP, this implies having to download a large
number of hashes before content download can begin. This, in turn,
increases time-till-playback for end users, making this method
unsuited for streaming.
The overhead of using Merkle hash trees is limited. The size of the
hash tree expressed as the total number of nodes depends on the
number of chunks the content is divided (and hence the size of
chunks) following this formula:
nnodes = math.pow(2,math.log(nchunks,2)+1)
In principle, the hash values of all these nodes will have to be sent
to a peer once for it to verify all of the chunks. Hence, the
maximum on-the-wire overhead is hashsize * nnodes. However, the
actual number of hashes transmitted can be optimized as described in
Section 5.3.
To see a peer can verify all chunks whilst receiving not all hashes,
consider the example tree in Section 5.1. In the case of a simple
progressive download, of chunks 0, 2, 4, 6, etc., the sending peer
will send the following hashes:
+-------+---------------------------------------------+
| Chunk | Node IDs of hashes sent |
+-------+---------------------------------------------+
| 0 | 2,5,11 |
| 2 | - (receiver already knows all) |
| 4 | 6 |
| 6 | - |
| 8 | 10,13 (hash 3 can be calculated from 0,2,5) |
| 10 | - |
| 12 | 14 |
| 14 | - |
| Total | # hashes 7 |
+-------+---------------------------------------------+
Table 1: Overhead for the Example Tree
So the number of hashes sent in total (7) is less than the total
number of hashes in the tree (16), as a peer does not need to send
hashes that are calculated and verified as part of earlier chunks.
5.6. Automatic Detection of Content Size
In PPSPP, the size of a static content file, such as a video file,
can be reliably and automatically derived from information received
from the network when fixed-size chunks are used. As a result, it is
not necessary to include the size of the content file as the metadata
of the content for such files. Implementations of PPSPP MAY use this
automatic detection feature. Note this feature is the only feature
of PPSPP that requires that a fixed-size chunk is used. This feature
builds on the Merkle hash tree and the trusted root hash as swarm ID
as follows.
5.6.1. Peak Hashes
The ability for a newcomer peer to detect the size of the content
depends heavily on the concept of peak hashes. The concept of peak
hashes depends on the concepts of filled and incomplete nodes.
Recall that when constructing the binary trees for content
verification and addressing the base of the tree may have more leaves
than the number of chunks in the content. In the Merkle hash tree,
these leaves were assigned empty all-zero hashes to be able to
calculate the higher-level hashes. A filled node is now defined as a
node that corresponds to an interval of leaves that consists only of
hashes of content chunks, not empty hashes. Reversely, an incomplete
(not filled) node corresponds to an interval that also contains empty
hashes, typically, an interval that extends past the end of the file.
In the following figure, nodes 7, 11, 13, and 14 are incomplete: the
rest is filled.
Formally, a peak hash is the hash of a filled node in the Merkle hash
tree, whose sibling is an incomplete node. Practically, suppose a
file is 7162 bytes long and a chunk is 1 kilobyte. That file fits
into 7 chunks, the tail chunk being 1018 bytes long. The Merkle hash
tree for that file is shown in Figure 4. Following the definition,
the peak hashes of this file are in nodes 3, 9, and 12, denoted with
an *. E denotes an empty hash.
7
/ \
/ \
/ \
/ \
3* 11
/ \ / \
/ \ / \
/ \ / \
1 5 9* 13
/ \ / \ / \ / \
0 2 4 6 8 10 12* 14
C0 C1 C2 C3 C4 C5 C6 E
= 1018 bytes
Peak hashes in a Merkle hash tree
Figure 4
Peak hashes can be explained by the binary representation of the
number of chunks the file occupies. The binary representation for 7
is 111. Every "1" in binary representation of the file's packet
length corresponds to a peak hash. For this particular file, there
are indeed three peaks: nodes 3, 9, and 12. Therefore, the number of
peak hashes for a file is also, at most, logarithmic with its size.
A peer knowing which nodes contain the peak hashes for the file can
therefore calculate the number of chunks it consists of; thus, it
gets an estimate of the file size (given all chunks but the last are
of a fixed size). Which nodes are the peaks can be securely
communicated from one (untrusted) peer, Peer A, to another peer, Peer
B, by letting Peer A send the peak hashes and their node IDs to Peer
B. It can be shown that the root hash that Peer B obtained from a
trusted source is sufficient to verify that these are indeed the
right peak hashes, as follows.
Lemma: Peak hashes can be checked against the root hash.
Proof: (a) Any peak hash is always the left sibling. Otherwise, if
it is the right sibling, its left neighbor/sibling must also be a
filled node, because of the way chunks are laid out in the leaves,
which contradicts the definition of a peak hash. (b) For the
rightmost peak hash, its right sibling is zero. (c) For any peak
hash, the right sibling might be calculated using peak hashes to the
left and zeros for empty nodes. (d) Once the right sibling of the
leftmost peak hash is calculated, its parent might be calculated. (e)
Once that parent is calculated, we might trivially get to the root
hash by concatenating the hash with zeros and hashing it repeatedly.
Informally, the Lemma might be expressed as follows: peak hashes
cover all data, so the remaining hashes are either trivial (zeros) or
might be calculated from peak hashes and zero hashes.
Finally, once Peer B has obtained the number of chunks in the
content, it can determine the exact file size as follows. Given that
all chunks except the last are of a fixed size, Peer B just needs to
know the size of the last chunk. Knowing the number of chunks, Peer
B can calculate the node ID of the last chunk and download it. As
always, Peer B verifies the integrity of this chunk against the
trusted root hash. As there is only one chunk of data that leads to
a successful verification, the size of this chunk must be correct.
Peer B can then determine the exact file size as:
(number of chunks -1) * fixed chunk size + size of last chunk
5.6.2. Procedure
A PPSPP implementation that wants to use automatic size detection
MUST operate as follows. When Peer A sends a DATA message for the
first time to Peer B, Peer A MUST first send all the peak hashes for
the content, in INTEGRITY messages, unless Peer B has already
signaled that it knows the peak hashes by having acknowledged any
chunk. If they are needed, the peak hashes MUST be sent as an extra
list of uncle hashes for the chunk, before the list of actual uncle
hashes of the chunk as described in Section 5.3. The receiver, Peer
B, MUST check the peak hashes against the root hash to determine the
approximate content size. To obtain the definite content size, Peer
B MUST download the last chunk of the content from any peer that
offers it.
As an example, let's consider a 7162-byte file, which fits in 7
chunks of 1 kilobyte, distributed by Peer A. Figure 4 shows the
relevant Merkle hash tree. Peer B, which only knows the root hash of
the file after successfully connecting to Peer A, requests the first
chunk of data, C0 in Figure 4. Peer A replies to Peer B by including
in the datagram the following messages in this specific order: first,
the three peak hashes of this particular file, the hashes of nodes 3,
9, and 12; second, the uncle hashes of C0, followed by the DATA
message containing the actual content of C0. Upon receiving the peak
hashes, Peer B checks them against the root hash determining that the
file is 7 chunks long. To establish the exact size of the file, Peer
B needs to request and retrieve the last chunk containing data, C6 in
Figure 4. Once the last chunk has been retrieved and verified, Peer
B concludes that it is 1018 bytes long, hence determining that the
file is exactly 7162 bytes long.
6. Live Streaming
The set of messages defined above can be used for live streaming as
well. In a pull-based model, a live streaming injector can announce
the chunks it generates via HAVE messages, and peers can retrieve
them via REQUEST messages. Areas that need special attention are
content authentication and chunk addressing (to achieve an infinite
stream of chunks).
6.1. Content Authentication
For live streaming, PPSPP supports two methods for a peer to
authenticate the content it receives from another peer, called "Sign
All" and "Unified Merkle Tree".
In the "Sign All" method, the live injector signs each chunk of
content using a private key. Upon receiving the chunk, peers check
the signature using the corresponding public key obtained from a
trusted source. Support for this method is OPTIONAL.
In the "Unified Merkle Tree" method, PPSPP combines the Merkle Hash
Tree scheme for static content with signatures to unify the video-on-
demand and live streaming scenarios. The use of Merkle hash trees
reduces the number of signing and verification operations, hence
providing a similar signature amortization to the approach described
in [SIGMCAST]. If PPSPP operates over the Internet, the "Unified
Merkle Tree" method MUST be used. If the protocol operates in a
benign environment, the "Unified Merkle Tree" method MAY be used. So
this method is mandatory to implement.
In both methods, the swarm ID consists of a public key encoded as in
a DNSSEC DNSKEY resource record without Base64 encoding [RFC4034].
In particular, the swarm ID consists of a 1-byte Algorithm field that
identifies the public key's cryptographic algorithm and determines
the format of the Public Key field that follows. The value of this
Algorithm field is one of the values in the "Domain Name System
Security (DNSSEC) Algorithm Numbers" registry [IANADNSSECALGNUM].
The RSASHA1 [RFC4034], RSASHA256 [RFC5702], ECDSAP256SHA256 and
ECDSAP384SHA384 [RFC6605] algorithms are mandatory to implement.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algo Number(8)| ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ DNSSEC Public Key (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.1.1. Sign All
In the "Sign All" method, the live injector signs each chunk of
content using a private key and peers, upon receiving the chunk,
check the signature using the corresponding public key obtained from
a trusted source. In particular, in PPSPP, the swarm ID of the live
stream is that public key.
A peer that wants to send a chunk of content creates a datagram that
MUST contain a SIGNED_INTEGRITY message with the chunk's signature,
followed by a DATA message with the actual chunk. If the
SIGNED_INTEGRITY message and DATA message cannot be contained into a
single datagram, because of a limitation on datagram size, the
SIGNED_INTEGRITY message MUST be sent first in a separate datagram.
The SIGNED_INTEGRITY message consists of the chunk specification, the
timestamp, and the digital signature.
The digital signature algorithm that is used, is determined by the
Live Signature Algorithm protocol option, see Section 7.7. The
signature is computed over a concatenation of the on-the-wire
representation of the chunk specification, a 64-bit timestamp in NTP
Timestamp format [RFC5905], and the chunk, in that order. The
timestamp is the time signature that was made at the injector in UTC.
6.1.2. Unified Merkle Tree
In this method, the chunks of content are used as the basis for a
Merkle hash tree as for static content. However, because chunks are
continuously generated, this tree is not static, but dynamic. As a
result, the tree does not have a root hash, or, more precisely, it
has a transient root hash. Therefore, a public key serves as swarm
ID of the content. It is used to digitally sign updates to the tree
allowing peers to expand it based on trusted information using the
following process.
6.1.2.1. Signed Munro Hashes
The live injector generates a number of chunks, denoted
NCHUNKS_PER_SIG, corresponding to fixed power of 2
(NCHUNKS_PER_SIG>=2), which are added as new leaves to the existing
hash tree. As a result of this expansion, the hash tree contains a
new subtree that is NCHUNKS_PER_SIG chunks wide at the base. The
root of this new subtree is referred to as the munro of that subtree,
and its hash as the munro hash of the subtree, illustrated in
Figure 5. In this figure, node 5 is the new munro, labeled with a $
sign.
3
/ \
/ \
/ \
1 5$
/ \ / \
0 2 4 6
Expanded live tree. With NCHUNKS_PER_SIG=2, node 5 is the munro for
the new subtree spanning 4 and 6. Node 1 is the munro for the
subtree spanning chunks 0 and 2, created in the previous iteration.
Figure 5
Informally, the process now proceeds as follows. The injector signs
only the munro hash of the new subtree using its private key. Next,
the injector announces the existence of the new subtree to its peers
using HAVE messages. When a peer, in response to the HAVE messages,
requests a chunk from the new subtree, the injector first sends the
signed munro hash corresponding to the requested chunk. Afterwards,
similar to static content, the injector sends the uncle hashes
necessary to verify that chunk, as in Section 5.1. In particular,
the injector sends the uncle hashes necessary to verify the requested
chunk against the munro hash. This differs from static content,
where the verification takes places against the root hash. Finally,
the injector sends the actual chunk.
The receiving peer verifies the signature on the signed munro using
the swarm ID (a public key) and updates its hash tree. As the peer
now knows the munro hash is trusted, it can verify all chunks in the
subtree against this munro hash, using the accompanying uncle hashes
as in Section 5.1.
To illustrate this procedure, lets consider the next iteration in the
process. The injector has generated the current tree shown in
Figure 5, and it is connected to several peers that currently have
the same tree and all posses chunks 0, 2, 4, and 6. When the
injector generates two new chunks, NCHUNKS_PER_SIG=2, the hash tree
expands as shown in Figure 6. The two new chunks, 8 and 10, extend
the tree on the right side, and to accommodate them, a new root is
created: node 7. As this tree is wider at the base than the actual
number of chunks, there are currently two empty leaves. The munro
node for the new subtree is 9, labeled with a $ sign.
7
/ \
/ \
/ \
/ \
3 11
/ \ / \
/ \ / \
/ \ / \
1 5 9$ 13
/ \ / \ / \ / \
0 2 4 6 8 10 E E
Expanded live tree. With NCHUNKS_PER_SIG=2, node 9 is the munro of
the newly added subtree spanning chunks 8 and 10.
Figure 6
The injector now needs to inform its peers of the updated tree,
communicating the addition of the new munro hash 9. Hence, it sends
a HAVE message with a chunk specification for nodes 8 + 10 to its
peers. As a response, Peer P requests the newly created chunk, e.g.,
chunk 8, from the injector by sending a REQUEST message. In reply,
the injector sends the signed munro hash of node 9 as an INTEGRITY
message with the hash of node 9, and a SIGNED_INTEGRITY message with
the signature of the hash of node 9. These messages are followed by
an INTEGRITY message with the hash of node 10 and a DATA message with
chunk 8.
Upon receipt, Peer P verifies the signature of the munro and expands
its view of the tree. Next, the peer computes the hash of chunk 8
and combines it with the received hash of node 10, computing the
expected hash of node 9. He can then verify the content of chunk 8
by comparing the computed hash of node 9 with the munro hash of the
same node he just received; hence, Peer P has successfully verified
the integrity of chunk 8.
This procedure requires just one signing operation for every
NCHUNKS_PER_SIG chunks created, and one verification operation for
every NCHUNKS_PER_SIG received, making it much cheaper than "Sign
All". A receiving peer does additionally need to check one or more
hashes per chunk via the Merkle Hash Tree scheme, but this has less
hardware requirements than a signature verification for every chunk.
This approach is similar to signature amortization via Merkle Tree
Chaining [SIGMCAST]. The downside of this scheme is in an increased
latency. A peer cannot download the new chunks until the injector
has computed the signature and announced the subtree. A peer MUST
check the signature before forwarding the chunks to other peers
[POLLIVE].
The number of chunks per signature NCHUNKS_PER_SIG MUST be a fixed
power of 2 for simplicity. NCHUNKS_PER_SIG MUST be larger than 1 for
performance reasons. There are two related factors to consider when
choosing a value for NCHUNKS_PER_SIG. First, the allowed CPU load on
clients due to signature verifications, given the expected bitrate of
the stream. To achieve a low CPU load in a high bitrate stream,
NCHUNKS_PER_SIG should be high. Second, the effect on latency, which
increases when NCHUNKS_PER_SIG gets higher, as just discussed. Note
how the procedure does not preclude the use of variable-size chunks.
This method of integrity verification provides an additional benefit.
If the system includes some peers that saved the complete broadcast,
as soon as the broadcast ends, the content is available as a video-
on-demand download using the now stabilized tree and the final root
hash as swarm identifier. Peers that saved all the chunks, can now
announce the root hash to the tracking infrastructure and instantly
seed the content.
6.1.2.2. Munro Signature Calculation
The digital signature algorithm used is determined by the Live
Signature Algorithm protocol option, see Section 7.7. The signature
is computed over a concatenation of the on-the-wire representation of
the chunk specification of the munro node (see Section 6.1.2.1), a
timestamp in 64-bit NTP Timestamp format [RFC5905], and the hash
associated with the munro node, in that order. The timestamp is the
time signature that was made at the injector in UTC.
6.1.2.3. Procedure
Formally, the injector MUST NOT send a HAVE message for chunks in the
new subtree until it has computed the signed munro hash for that
subtree.
When Peer B requests a chunk C from Peer A (either the injector or
another peer), and Peer A decides to reply, it must do so as follows.
First, Peer A MUST send an INTEGRITY message with the chunk
specification for the munro of chunk C and the munro's hash, followed
by a SIGNED_INTEGRITY message with the chunk specification for the
munro, timestamp, and its signature in a single datagram, unless Peer
B indicated earlier in the exchange that it already possess a chunk
with the same corresponding munro (by means of HAVE or ACK messages).
Following these two messages (if any), Peer A MUST send the necessary
missing uncles hashes needed for verifying the chunk against its
munro hash, and the chunk itself, as described in Section 5.4,
sharing datagrams if possible.
6.1.2.4. Secure Tune In
When a peer tunes in to a live stream, it has to determine what is
the last chunk the injector has generated. To facilitate this
process in the Unified Merkle Tree scheme, each peer shares its
knowledge about the injector's chunks with the others by exchanging
their latest signed munro hashes, as follows.
Recall that, in PPSPP, when Peer A initiates a channel with Peer B,
Peer A sends a first datagram with a HANDSHAKE message, and Peer B
responds with a second datagram also containing a HANDSHAKE message
(see Section 3.1). When Peer A sends a third datagram to Peer B, and
it is received by Peer B, both peers know that the other is listening
on its stated transport address. Peer B is then allowed to send
heavy payload like DATA messages in the fourth datagram. Peer A can
already safely do that in the third datagram.
In the Unified Merkle Tree scheme, Peer A MUST send its rightmost
signed munro hash to Peer B in the third datagram, and in any
subsequent datagrams to Peer B, until Peer B indicates that it
possess a chunk with the same corresponding munro or a more recent
munro (by means of a HAVE or ACK message). Peer B may already have
indicated this fact by means of HAVE messages in the second datagram.
Conversely, when Peer B sends the fourth datagram or any subsequent
datagram to Peer A, Peer B MUST send its rightmost signed munro hash,
unless Peer A indicated knowledge of it or more recent munros. The
rightmost signed munro hash of a peer is defined as the munro hash
signed by the injector of the rightmost subtree of width
NCHUNKS_PER_SIG chunks in the peer's Merkle hash tree. Peer A MUST
NOT send the signed munro hash in the first datagram of the HANDSHAKE
procedure and Peer B MUST NOT send it in the second datagram as it is
considered heavy payload.
When a peer receives a SIGNED_INTEGRITY message with a signed munro
hash but the timestamp is too old, the peer MUST discard the message.
Otherwise, it SHOULD use the signed munro to update its hash tree and
pick a tune-in in the live stream. A peer may use the information
from multiple peers to pick the tune-in point.
6.2. Forgetting Chunks
As a live broadcast progresses, a peer may want to discard the chunks
that it already played out. Ideally, other peers should be aware of
this fact so that they will not try to request these chunks from this
peer. This could happen in scenarios where live streams may be
paused by viewers, or viewers are allowed to start late in a live
broadcast (e.g., start watching a broadcast at 20:35 when it actually
began at 20:30).
PPSPP provides a simple solution for peers to stay up to date with
the chunk availability of a discarding peer. A discarding peer in a
live stream MUST enable the Live Discard Window protocol option,
specifying how many chunks/bytes it caches before the last chunk/byte
it advertised as being available (see Section 7.9). Its peers SHOULD
apply this number as a sliding window filter over the peer's chunk
availability as conveyed via its HAVE messages.
Three factors are important when deciding for an appropriate value
for this option: the desired amount of playback buffer for peers, the
bitrate of the stream, and the available resources of the peer.
Consider the case of a fresh peer joining the stream. The size of
the discard window of the peers it connects to influences how much
data it can directly download to establish its prebuffer. If the
window is smaller than the desired buffer, the fresh peer has to wait
until the peers downloaded more of the stream before it can start
playback. As media buffers are generally specified in terms of a
number of seconds, the size of the discard window is also related to
the (average) bitrate of the stream. Finally, if a peer has few
resources to store chunks and metadata, it should choose a small
discard window.
7. Protocol Options
The HANDSHAKE message in PPSPP can contain the following protocol
options. Unless stated otherwise, a protocol option consists of an
8-bit code followed by an 8-bit value. Larger values are all encoded
big-endian. Each protocol option is explained in the following
subsections. The list of protocol options MUST be sorted on code
value (ascending) in a HANDSHAKE message.
+--------+-------------------------------------+
| Code | Description |
+--------+-------------------------------------+
| 0 | Version |
| 1 | Minimum Version |
| 2 | Swarm Identifier |
| 3 | Content Integrity Protection Method |
| 4 | Merkle Hash Tree Function |
| 5 | Live Signature Algorithm |
| 6 | Chunk Addressing Method |
| 7 | Live Discard Window |
| 8 | Supported Messages |
| 9 | Chunk Size |
| 10-254 | Unassigned |
| 255 | End Option |
+--------+-------------------------------------+
Table 2: PPSPP Options
7.1. End Option
A peer MUST conclude the list of protocol options with the end
option. Subsequent octets should be considered protocol messages.
The code for the end option is 255, and unlike others, it has no
value octet, so the option's length is 1 octet.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1|
+-+-+-+-+-+-+-+-+
7.2. Version
A peer MUST include the maximum version of the PPSPP it supports as
the first protocol option in the list. The code for this option is
0. Defined values are listed in Table 3.
+---------+----------------------------------------+
| Version | Description |
+---------+----------------------------------------+
| 0 | Reserved |
| 1 | Protocol as described in this document |
| 2-255 | Unassigned |
+---------+----------------------------------------+
Table 3: PPSPP Version Numbers
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Version (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.3. Minimum Version
When a peer initiates the handshake, it MUST include the minimum
version of the PPSPP it supports in the list of protocol options,
following the min/max versioning scheme defined in [RFC6709],
Section 4.1, strategy 5. The code for this option is 1. Defined
values are listed in Table 3.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Min. Ver. (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.4. Swarm Identifier
When a peer initiates the handshake, it MUST include a single swarm
identifier option. If the peer is not the initiator, it MAY include
a swarm identifier option, as an end-to-end check. This option has
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 0| Swarm ID Length (16) | ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Swarm Identifier (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Swarm ID Length field contains the length of the single Swarm
Identifier that follows in bytes. The Length field is 16 bits wide
to allow for large public keys as identifiers in live streaming.
Each PPSPP peer knows the IDs of the swarms it joins, so this
information can be immediately verified upon receipt.
7.5. Content Integrity Protection Method
A peer MUST include the content integrity method used by a swarm.
The code for this option is 3. Defined values are listed in Table 4.
+--------+-------------------------+
| Method | Description |
+--------+-------------------------+
| 0 | No integrity protection |
| 1 | Merkle Hash Tree |
| 2 | Sign All |
| 3 | Unified Merkle Tree |
| 4-255 | Unassigned |
+--------+-------------------------+
Table 4: PPSPP Content Integrity Protection Methods
The "Merkle Hash Tree" method is the default for static content, see
Section 5.1. "Sign All", and "Unified Merkle Tree" are for live
content, see Section 6.1, with "Unified Merkle Tree" being the
default.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 1| CIPM (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.6. Merkle Tree Hash Function
When the content integrity protection method is "Merkle Hash Tree",
this option defining which hash function is used for the tree MUST be
included. The code for this option is 4. Defined values are listed
in Table 5 (see [FIPS180-4] for the function semantics).
+----------+-------------+
| Function | Description |
+----------+-------------+
| 0 | SHA-1 |
| 1 | SHA-224 |
| 2 | SHA-256 |
| 3 | SHA-384 |
| 4 | SHA-512 |
| 5-255 | Unassigned |
+----------+-------------+
Table 5: PPSPP Merkle Hash Functions
Implementations MUST support SHA-1 (see Section 12.5) and SHA-256.
SHA-256 is the default.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 0| MHF (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.7. Live Signature Algorithm
When the content integrity protection method is "Sign All" or
"Unified Merkle Tree", this option MUST be defined. The code for
this option is 5. The 8-bit value of this option is one of the
values listed in the "Domain Name System Security (DNSSEC) Algorithm
Numbers" registry [IANADNSSECALGNUM]. The RSASHA1 [RFC4034],
RSASHA256 [RFC5702], ECDSAP256SHA256 and ECDSAP384SHA384 [RFC6605]
algorithms are mandatory to implement. Default is ECDSAP256SHA256.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 1| LSA (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.8. Chunk Addressing Method
A peer MUST include the chunk addressing method it uses. The code
for this option is 6. Defined values are listed in Table 6.
+--------+---------------------+
| Method | Description |
+--------+---------------------+
| 0 | 32-bit bins |
| 1 | 64-bit byte ranges |
| 2 | 32-bit chunk ranges |
| 3 | 64-bit bins |
| 4 | 64-bit chunk ranges |
| 5-255 | Unassigned |
+--------+---------------------+
Table 6: PPSPP Chunk Addressing Methods
Implementations MUST support "32-bit chunk ranges" and "64-bit chunk
ranges". Default is "32-bit chunk ranges".
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 0| CAM (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.9. Live Discard Window
A peer in a live swarm MUST include the discard window it uses. The
code for this option is 7. The unit of the discard window depends on
the chunk addressing method used, see Table 6. For bins and chunk
ranges, it is a number of chunks; for byte ranges, it is a number of
bytes. Its data type is the same as for a bin, or one value in a
range specification. In other words, its value is a 32-bit or 64-bit
integer in big-endian format. If this option is used, the Chunk
Addressing Method MUST appear before it in the list. This option has
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 1| Live Discard Window (32 or 64) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A peer that does not, under normal circumstances, discard chunks MUST
set this option to the special value 0xFFFFFFFF (32-bit) or
0xFFFFFFFFFFFFFFFF (64-bit). For example, peers that record a
complete broadcast to offer it directly as a static file after the
broadcast ends use these values (see Section 6.1.2). Section 6.2
explains how to determine a value for this option.
7.10. Supported Messages
Peers may support just a subset of the PPSPP messages. For example,
peers running over TCP may not accept ACK messages or peers used with
a centralized tracking infrastructure may not accept PEX messages.
For these reasons, peers who support only a proper subset of the
PPSPP messages MUST signal which subset they support by means of this
protocol option. The code for this option is 8. The value of this
option is a length octet (SupMsgLen) indicating the length, in bytes,
of the compressed bitmap that follows.
The set of messages supported can be derived from the compressed
bitmap by padding it with bytes of value 0 until it is 256 bits in
length. Then, a 1 bit in the resulting bitmap at position X
(numbering left to right) corresponds to support for message type X,
see Table 7. In other words, to construct the compressed bitmap,
create a bitmap with a 1 for each message type supported and a 0 for
a message type that is not, store it as an array of bytes, and
truncate it to the last non-zero byte. An example of the first 16
bits of the compressed bitmap for a peer supporting every message
except ACKs and PEXs is 11011001 11110000.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 0| SupMsgLen (8) | ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Supported Messages Bitmap (variable, max 256) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.11. Chunk Size
A peer in a swarm MUST include the chunk size the swarm uses. The
code for this option is 9. Its value is a 32-bit integer denoting
the size of the chunks in bytes in big-endian format. When variable
chunk sizes are used, this option MUST be set to the special value
0xFFFFFFFF. Section 8.1 explains how content publishers can
determine a value for this option.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 1| Chunk Size (32) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ |
+-+-+-+-+-+-+-+-+
8. UDP Encapsulation
PPSPP implementations MUST use UDP as transport protocol and MUST use
LEDBAT for congestion control [RFC6817]. Using LEDBAT enables PPSPP
to serve the content after playback (seeding) without disrupting the
user who may have moved to different tasks that use its network
connection. Future PPSPP versions can also run over other transport
protocols or use different congestion control algorithms.
8.1. Chunk Size
In general, a UDP datagram containing PPSPP messages SHOULD fit
inside a single IP packet, so its maximum size depends on the MTU of
the network. If the UDP datagram does not fit, its chance of getting
lost in the network increases as the loss of a single fragment of the
datagram causes the loss of the complete datagram.
The largest message in a PPSPP datagram is the DATA message carrying
a chunk of content. So the (maximum) size of a chunk to choose for a
particular swarm depends primarily on the expected MTU. The chunk
size should be chosen such that a chunk and its required INTEGRITY
messages can generally be carried inside a single datagram, following
the Atomic Datagram Principle (Section 5.3). Other considerations
are the hardware capabilities of the peers. Having large chunks and
therefore less chunks per megabyte of content reduces processing
costs. The chunk addressing schemes can all work with different
chunk sizes, see Section 4.
The RECOMMENDED approach is to use fixed-size chunks of 1024 bytes,
as this size has a high likelihood of traveling end-to-end across the
Internet without any fragmentation. In particular, with this size, a
UDP datagram with a DATA message can be transmitted as a single IP
packet over an Ethernet network with 1500-byte frames.
A PPSPP implementation MAY use a variant of the Packetization Layer
Path MTU Discovery (PLPMTUD), described in [RFC4821], for discovering
the optimal MTU between sender and destination. As in PLPMTUD,
progressively larger probing packets are used to detect the optimal
MTU for a given path. However, in PPSPP, probe packets SHOULD
contain actual messages, in particular, multiple DATA messages. By
using actual DATA messages as probe packets, the returning ACK
messages will confirm the probe delivery, effectively updating the
MTU estimate on both ends of the link. To be able to scale up probe
packets with sensible increments, a minimum chunk size of 512 bytes
SHOULD be used. Smaller chunk sizes lead to an inefficient protocol.
An implication is that PPSPP supports datagrams over IPv4 of 576
bytes or more only. This variant is not mandatory to implement.
The chunk size used for a particular swarm, or the fact that it is
variable, MUST be part of the swarm's metadata (which then minimally
consists of the swarm ID and the chunk nature and size).
8.2. Datagrams and Messages
When using UDP, the abstract datagram described above corresponds
directly to a UDP datagram. Most messages within a datagram have a
fixed length, which generally depends on the type of the message.
The first byte of a message denotes its type. The currently defined
types are:
+----------+------------------+
| Msg Type | Description |
+----------+------------------+
| 0 | HANDSHAKE |
| 1 | DATA |
| 2 | ACK |
| 3 | HAVE |
| 4 | INTEGRITY |
| 5 | PEX_RESv4 |
| 6 | PEX_REQ |
| 7 | SIGNED_INTEGRITY |
| 8 | REQUEST |
| 9 | CANCEL |
| 10 | CHOKE |
| 11 | UNCHOKE |
| 12 | PEX_RESv6 |
| 13 | PEX_REScert |
| 14-254 | Unassigned |
| 255 | Reserved |
+----------+------------------+
Table 7: PPSPP Message Types
Furthermore, integers are serialized in network (big-endian) byte
order. So, consider the example of a HAVE message (Section 3.2)
using bin chunk addressing. It has a message type of 0x03 and a
payload of a bin number, a 4-byte integer (say, 1); hence, its on-
the-wire representation for UDP can be written in hex as
"0300000001".
All messages are idempotent or recognizable as duplicates.
Idempotent means that processing a message more than once does not
lead to a different state from if it was processed just once. In
particular, a peer MAY resend DATA, ACK, HAVE, INTEGRITY, PEX_*,
SIGNED_INTEGRITY, REQUEST, CANCEL, CHOKE, and UNCHOKE messages
without problems when loss is suspected. When a peer resends a
HANDSHAKE message, it can be recognized as duplicate by the receiver,
because it already recorded the first connection attempt, and be
dealt with.
8.3. Channels
As described in Section 3.11, PPSPP uses a multiplexing scheme,
called channels, to allow multiple swarms to use the same UDP port.
In the UDP encapsulation, each datagram from Peer A to Peer B is
prefixed with the channel ID allocated by Peer B. The peers learn
about each other's channel ID during the handshake as explained in
Section 3.1.1. A channel ID consists of 4 bytes and MUST be
generated following the requirements in [RFC4960] (Section 5.1.3).
8.4. HANDSHAKE
A channel is established with a handshake. To start a handshake, the
initiating peer needs to know the swarm metadata, defined in
Section 3.1 and the IP address and UDP port of a peer. A datagram
containing a HANDSHAKE message then looks as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Channel ID (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Source Channel ID (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Protocol Options ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Destination Channel ID:
If the datagram is sent by the initiating peer, then it MUST be
an all-zeros channel ID.
If the datagram is sent by the responding peer, then it MUST
consist of the Source Channel ID from the sender's HANDSHAKE
message.
The octet 0x00: The HANDSHAKE message type
Source Channel ID: A locally unused channel ID
Protocol Options: A list of protocol options encoding the swarm's
metadata, as defined in Section 7.
A peer SHOULD explicitly close a channel by sending a HANDSHAKE
message that MUST contain an all zeros Source Channel ID and a list
of protocol options. The list MUST either be empty or contain the
maximum version number the sender supports, following the min/max
versioning scheme defined in [RFC6709], Section 4.1.
8.5. HAVE
A HAVE message (type 0x03) consists of a single chunk specification
that states that the sending peer has those chunks and successfully
checked their integrity. The single chunk specification represents a
consecutive range of verified chunks. A bin consists of a single
integer, and a chunk or byte range of two integers, of the width
specified by the Chunk Addressing protocol options, encoded big-
endian.
A HAVE message using 32-bit chunk ranges as Chunk Addressing method:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
where the first octet is the HAVE message (0x03) followed by the
start chunk and the end chunk describing the chunk range. Note this
diagram shows a message and not a datagram, so it is not prefixed by
the destination Channel ID. This holds for all subsequent message
diagrams.
8.6. DATA
A DATA message (type 0x01) consists of a chunk specification, a
timestamp, and the actual chunk. In case a datagram contains one
DATA message, a sender MUST always put the DATA message in the tail
of the datagram. A datagram MAY contain multiple DATA messages when
the chunk size is fixed and when none of the DATA messages carry the
last chunk, if that is smaller than the chunk size. As LEDBAT
congestion control is used, a sender MUST include a timestamp, in
particular, a 64-bit integer representing the current system time
with microsecond accuracy. The timestamp MUST be included between
chunk specification and the actual chunk.
A DATA message using 32-bit chunk ranges as Chunk Addressing method:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the first octet is the DATA message (0x01) followed by the
start chunk and the end chunk describing the single chunk, the
timestamp, and the actual data.
8.7. ACK
An ACK message (type 0x02) acknowledges data that was received from
its addressee; to comply with the LEDBAT delay-based congestion
control, an ACK message consists of a chunk specification and a
timestamp representing a one-way delay sample. The one-way delay
sample is a 64-bit integer with microsecond accuracy, and it is
computed from the timestamp received from the previous DATA message
containing the chunk being acknowledged following the LEDBAT
specification.
An ACK message using 32-bit chunk ranges as Chunk Addressing method:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 0| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| One-way delay sample (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
where the first octet is the ACK message (0x02) followed by the start
chunk and the end chunk describing the chunk range and the one-way
delay sample.
8.8. INTEGRITY
An INTEGRITY message (type 0x04) consists of a chunk specification
and the cryptographic hash for the specified chunk or node. The type
and format of the hash depends on the protocol options.
An INTEGRITY message using 32-bit chunk ranges as Chunk Addressing
method and a SHA-256 hash:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 0| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Hash (256) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
where the first octet is the INTEGRITY message (0x04) followed by the
start chunk and the end chunk describing the chunk range and the
hash.
8.9. SIGNED_INTEGRITY
A SIGNED_INTEGRITY message (type 0x07) consists of a chunk
specification, a 64-bit timestamp in NTP Timestamp format [RFC5905]
and a digital signature encoded as a Signature field would be in an
RRSIG record in DNSSEC without the Base64 encoding [RFC4034]. The
signature algorithm is defined by the Live Signature Algorithm
protocol option, see Section 7.7. The plaintext over which the
signature is taken depends on the content integrity protection method
used, see Section 6.1.
A SIGNED_INTEGRITY message using 32-bit chunk ranges as Chunk
Addressing method:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (64) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Signature ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the first octet is the SIGNED_INTEGRITY message (0x07) followed
by the start chunk and the end chunk describing the chunk range, the
timestamp, and the Signature.
The length of the digital signature can be derived from the Live
Signature Algorithm protocol option and the swarm ID as follows. The
first mandatory algorithms are RSASHA1 and RSASHA256. For those
algorithms, the swarm ID consists of a 1-byte Algorithm field
followed by an RSA public key stored as a tuple (exponent length,
exponent, modulus) [RFC3110]. Given the exponent length and the
length of the public key tuple in the swarm ID, the length of the
modulus in bytes can be calculated. This yields the length of the
signature, as in RSA this is the length of the modulus [HAC01]. The
other mandatory algorithms are ECDSAP256SHA256 and ECDSAP384SHA384
[RFC6605]. For these algorithms, the length of the digital signature
is 64 and 96 bytes, respectively.
8.10. REQUEST
A REQUEST message (type 0x08) consists of a chunk specification for
the chunks the requester wants to download.
A REQUEST message using 32-bit chunk ranges as Chunk Addressing
method:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 0| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
where the first octet is the REQUEST message (0x08) followed by the
start chunk and the end chunk describing the chunk range.
8.11. CANCEL
A CANCEL message (type 0x09) consists of a chunk specification for
the chunks the requester no longer is interested in.
A CANCEL message using 32-bit chunk ranges as Chunk Addressing
method:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 1| Start chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | End chunk (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
where the first octet is the CANCEL message (0x09) followed by the
start chunk and the end chunk describing the chunk range.
8.12. CHOKE and UNCHOKE
Both CHOKE and UNCHOKE messages (types 0x0a and 0x0b, respectively)
carry no payload.
A CHOKE message:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 1 0|
+-+-+-+-+-+-+-+-+
where the first octet is the CHOKE message (0x0a).
An UNCHOKE message:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 1 1|
+-+-+-+-+-+-+-+-+
where the first octet is the UNCHOKE message (0x0b).
8.13. PEX_REQ, PEX_RESv4, PEX_RESv6, and PEX_REScert
A PEX_REQ (0x06) message has no payload. A PEX_RESv4 (0x05) message
consists of an IPv4 address in big-endian format followed by a UDP
port number in big-endian format. A PEX_RESv6 (0x0c) message
contains a 128-bit IPv6 address instead of an IPv4 one. If a PEX_REQ
message does not originate from a private, unique-local, link-local,
or multicast address [RFC1918] [RFC4193] [RFC4291], then the PEX_RES*
messages sent in reply MUST NOT contain such addresses. This is to
prevent leaking of internal addresses to external peers.
A PEX_REQ message:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 1 0|
+-+-+-+-+-+-+-+-+
where the first octet is the PEX_REQ message (0x06).
A PEX_RESv4 message:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 1 0 1| IPv4 Address (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the first octet is the PEX_RESv4 message (0x05) followed by the
IPv4 address and the port number.
A PEX_RESv6 message:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 1 0 0| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Address (128) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Port (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the first octet is the PEX_RESv6 message (0x0c), followed by
the IPv6 address and the port number.
A PEX_REScert (0x0d) message consists of a 16-bit integer in big-
endian specifying the size of the membership certificate that
follows, see Section 12.2.1. This membership certificate states that
Peer P at Time T is a member of Swarm S and is a X.509v3 certificate
[RFC5280] that is encoded using the ASN.1 distinguished encoding
rules (DER) [CCITT.X690.2002]. The certificate MUST contain a
"Subject Alternative Name" extension, marked as critical, of type
uniformResourceIdentifier.
A PEX_REScert message:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 1 0 1| Size of Memb. Cert. (16) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Membership Certificate ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the first octet is the PEX_REScert message (0x0d) followed by
the size of the membership certificate and the membership
certificate.
The URL contained in the name extension MUST follow the generic
syntax for URLs [RFC3986], where its scheme component is "file", the
host in the authority component is the DNS name or IP address of Peer
P, the port in the authority component is the port of Peer P, and the
path contains the swarm identifier for Swarm S, in hexadecimal form.
In particular, the preferred form of the swarm identifier is
xxyyzz..., where the 'x's, 'y's, and 'z's are 2 hexadecimal digits of
the 8-bit pieces of the identifier. The validity time of the
certificate is set with notBefore UTCTime set to T and notAfter
UTCTime set to T plus some expiry time defined by the issuer. An
example URL:
file://192.0.2.0:6778/e5a12c7ad2d8fab33c699d1e198d66f79fa610c3
8.14. KEEPALIVE
Keep alives do not have a message type on UDP. They are just simple
datagrams consisting of the 4-byte channel ID of the destination
only.
A keep-alive datagram:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel ID (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8.15. Flow and Congestion Control
Explicit flow control is not required for PPSPP over UDP. In the
case of video on demand, the receiver explicitly requests the content
from peers, and is therefore in control of how much data is coming
towards it. In the case of live streaming, where a push model may be
used, the amount of data incoming is limited to the stream bitrate,
which the receiver must be able to process for a continuous playback.
Should, for any reason, the receiver get saturated with data, the
congestion control at the sender side will detect the situation and
adjust the sending rate accordingly.
PPSPP over UDP can support different congestion control algorithms.
At present, it uses the LEDBAT congestion control algorithm
[RFC6817]. LEDBAT is a delay-based congestion control algorithm that
is used every day by millions of users as part of the uTP
transmission protocol of BitTorrent [LBT] [LCOMPL] and is suitable
for P2P streaming [PPSPPERF].
LEDBAT monitors the delay of the packets on the data path. It uses
the one-way delay variations to react early and limit the congestion
that the stream may induce in the network [RFC6817]. Using LEDBAT
enables PPSPP to serve the content to other interested peers after
the playback has finished (seeding), without disrupting the user.
After the playback, the user might move to different tasks that use
its network link, which are prioritized over PPSPP traffic. Hence,
the user does not notice the background PPSPP traffic, which in turn
increases the chances of seeding the content for a longer period of
time.
The property of reacting early is not a problem in a peer-to-peer
system where multiple sources offer the content. Considering the
case of congestion near the sender, LEDBAT's early reaction impacts
the transmission of chunks to the receiver. However, for the
receiver, it is actually beneficial to learn early that the
transmission from a particular source is impacted. The receiver can
then choose to download time-critical chunks from other sources
during its chunk picking phase.
If the bottleneck is near the receiver, the receiver is indeed
unlucky that transmissions from any source that runs through this
bottleneck will back off quite fast due to LEDBAT. However, for the
rest of the network (and the network operator), this is beneficial as
the video-streaming system will back off early enough and not
contribute too much to the congestion.
The power of LEDBAT is that its behavior can be configured. In the
case of live streaming, a PPSPP deployer may want a more aggressive
behavior to ensure quality of service. In that case, LEDBAT can be
configured to be more aggressive. In particular, LEDBAT's queuing
target delay value (TARGET in [RFC6817]) and other parameters can be
adjusted such that it acts as aggressive as TCP (or even more).
Hence, LEDBAT is an algorithm that works for many scenarios in a
peer-to-peer context.
8.16. Example of Operation
We present a small example of communication between a leecher and a
seeder. The example presents the transmission of the file "Hello
World!", which fits within a 1024-byte chunk. For an easy
understanding, we use the message description names, as listed in
Table 7, and the protocol option names as listed in Table 2, rather
than the actual binary value.
To do the handshake, the initiating peer sends a datagram that MUST
start with an all-zeros channel ID (0x00000000); followed by a
HANDSHAKE message, whose payload is a locally unused; a random
channel ID (in this case 0x00000001); and a list of protocol options.
Channel IDs MUST be randomly chosen, as described in Section 12.1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HANDSHAKE |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Version |0 0 0 0 0 0 0 1| Min Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Swarm ID |0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 0 0 1 1 1 1 0 1 0 0 0 0 0 0 0 0 1 0 0 1 1 1 1 1 0 0 1 1 0|
~ ..... ~
|1 0 0 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1 0 0 0 0 0 0 1 1 1 0 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cont. Int. |0 0 0 0 0 0 0 1| Mer.H.Tree F. |0 0 0 0 0 0 1 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Add. |0 0 0 0 0 0 1 0| Chunk Size |0 0 0 0 0 0 0 0~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0| End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The protocol options are:
Version: 1
Minimum supported Version: 1
Swarm Identifier: A 32-byte root hash (47a0...b03b) identifying
the content
Content Integrity Protection Method: Merkle Hash Tree
Merkle Tree Hash Function: SHA-256
Chunk Addressing Method: 32-bit chunk ranges
Chunk Size: 1024
The receiving peer MAY respond, in which case the returned datagram
MUST consist of the channel ID from the sender's HANDSHAKE message
(0x00000001); a HANDSHAKE message, whose payload is a locally unused;
a random channel ID (0x00000008); and a list of protocol options;
followed by any other messages it wants to send.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HANDSHAKE |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 1 0 0 0| Version |0 0 0 0 0 0 0 1| Cont. Int. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1| Mer.H.Tree F. |0 0 0 0 0 0 1 0| Chunk Add. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 1 0| Chunk Size |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0| End | HAVE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
With the protocol options, the receiving peer agrees on speaking
protocol version 1, on using the Merkle Hash Tree as the Content
Integrity Protection Method, SHA-256 hash as the Merkle Tree Hash
Function, 32-bit chunk ranges as the Chunk Addressing Method, and
Chunk Size 1024. Furthermore, it sends a HAVE message within the
same datagram, announcing that it has locally available the first
chunk of content.
At this point, the initiator knows that the peer really responds; for
that purpose, channel IDs MUST be random enough to prevent easy
guessing. So, the third datagram of a handshake MAY already contain
some heavy payload. To minimize the number of initialization round
trips, the first two datagrams MAY also contain some minor payload,
e.g., the HAVE message.
The initiating peer MAY send a request for the chunks of content it
wants to retrieve from the receiving peer, e.g., the first chunk
announced during the handshake. It always precedes the message with
the channel ID of the peer it is communicating with (0x00000008 in
our example), as described in Section 3.11. Furthermore, it MAY add
additional messages such as a PEX_REQ.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| REQUEST |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| PEX_REQ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When receiving the third datagram, both peers have proof that they
really talk to each other; the three-way handshake is complete. The
receiving peer responds to the request by sending a DATA message
containing the requested content.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DATA |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 1 0 1 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 1 1 0 1 1 1 1 1 0 1 1 0 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 0 0 1 0 0|0 1 0 0 1 0 0 0 0 1 1 0 0 1 0 1 0 1 1 0 1 1 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ..... ~
|0 1 1 0 1 1 0 0 0 1 1 0 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 0 1 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The DATA message consists of:
The 32-bit chunk range: 0,0 (the first chunk)
The timestamp value: 0004e94180b7db44
The data: 48656c6c6f20776f726c6421 (the "Hello world!" file)
Note that the above datagram does not include the INTEGRITY message,
as the entire content can fit into a single message; hence, the
initiating peer is able to verify it against the root hash. Also, in
this example, the peer does not respond to the PEX_REQ as it does not
know any third peer participating in the swarm.
Upon receiving the requested data, the initiating peer responds with
an ACK message for the first chunk, containing a one-way delay sample
(100 ms). Furthermore, it also adds a HAVE message for the chunk.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 1 0 0 1 0 0| HAVE |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
At this point, the initiating peer has successfully retrieved the
entire file. Then, it explicitly closes the connection by sending a
HANDSHAKE message that contains an all-zeros Source Channel 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HANDSHAKE |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| End |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9. Extensibility
9.1. Chunk Picking Algorithms
Chunk (or piece) picking entirely depends on the receiving peer. The
sending peer is made aware of preferred chunks by the means of
REQUEST messages. In some (live) scenarios, it may be beneficial to
allow the sender to ignore those hints and send unrequested data.
The chunk picking algorithm is external to the PPSPP and will
generally be a pluggable policy that uses the mechanisms provided by
PPSPP. The algorithm will handle the choices made by the user
consuming the content, such as seeking or switching audio tracks or
subtitles. Example policies for P2P streaming can be found in
[BITOS], and [EPLIVEPERF].
9.2. Reciprocity Algorithms
The role of reciprocity algorithms in peer-to-peer systems is to
promote client contribution and prevent freeriding. A peer is said
to be freeriding if it only downloads content but never uploads to
others. Examples of reciprocity algorithms are tit-for-tat as used
in BitTorrent [TIT4TAT] and Give-to-Get [GIVE2GET]. In PPSPP,
reciprocity enforcement is the sole responsibility of the sending
peer.
10. IANA Considerations
IANA has created a new top-level registry called "Peer-to-Peer
Streaming Peer Protocol (PPSPP)", which hosts the six new sub-
registries defined below for the extensibility of the protocol. For
all registries, assignments consist of a name and its associated
value. Also, for all registries, the "Unassigned" ranges designated
are governed by the policy "IETF Review" as described in [RFC5226].
10.1. PPSPP Message Type Registry
The registry name is "PPSPP Message Type Registry". Values are
integers in the range 0-255, with initial assignments and
reservations given in Table 7.
10.2. PPSPP Option Registry
The registry name is "PPSPP Option Registry". Values are integers in
the range 0-255, with initial assignments and reservations given in
Table 2.
10.3. PPSPP Version Number Registry
The registry name is "PPSPP Version Number Registry". Values are
integers in the range 0-255, with initial assignments and
reservations given in Table 3.
10.4. PPSPP Content Integrity Protection Method Registry
The registry name is "PPSPP Content Integrity Protection Method
Registry". Values are integers in the range 0-255, with initial
assignments and reservations given in Table 4.
10.5. PPSPP Merkle Hash Tree Function Registry
The registry name is "PPSPP Merkle Hash Tree Function Registry".
Values are integers in the range 0-255, with initial assignments and
reservations given in Table 5.
10.6. PPSPP Chunk Addressing Method Registry
The registry name is "PPSPP Chunk Addressing Method Registry".
Values are integers in the range 0-255, with initial assignments and
reservations given in Table 6.
11. Manageability Considerations
This section presents operations and management considerations
following the checklist in [RFC5706], Appendix A.
In this section, "PPSPP client" is defined as a PPSPP peer acting on
behalf of an end user which may not yet have a copy of the content,
and "PPSPP server" as a PPSPP peer that provides the initial copies
of the content to the swarm on behalf of a content provider.
11.1. Operations
11.1.1. Installation and Initial Setup
A content provider wishing to use PPSPP to distribute content should
set up at least one PPSPP server. PPSPP servers need to have access
to either some static content or some live audio/video sources. To
provide flexibility for implementors, this configuration process is
not standardized. The output of this process will be a list of
metadata records, one for each swarm. A metadata record consists of
the swarm ID, the chunk size used, the chunk addressing method used,
the content integrity protection method used, and the Merkle hash
tree function used (if applicable). If automatic content size
detection (see Section 5.6) is not used, the content length is also
part of the metadata record for static content. Note the swarm ID
already contains the Live Signature Algorithm used, in case of a live
stream.
In addition, a content provider should set up a tracking facility for
the content by configuring, for example, a peer-to-peer streaming
protocol tracker [PPSP-TP] or a Distributed Hash Table. The output
of the latter process is a list of transport addresses for the
tracking facility.
The list of metadata records of available content, and transport
address for the tracking facility, can be distributed to users in
various ways. Typically, they will be published on a website as
links. When a user clicks such a link, the PPSPP client is launched,
either as a standalone application or by invoking the browser's
internal PPSPP protocol handler, as exemplified in Section 2. The
clients use the tracking facility to obtain the transport address of
the PPSPP server(s) and other peers from the swarm, executing the
peer protocol to retrieve and redistribute the content. The format
of the PPSPP URLs should be defined in an extension document. The
default protocol options should be exploited to keep the URLs small.
The minimal information a tracking facility must return when queried
for a list of peers for a swarm is as follows. Assuming the
communication between tracking facility and requester is protected,
the facility must at least return for each peer in the list its IP
address, transport protocol identifier (i.e., UDP), and transport
protocol port number.
11.1.2. Migration Path
This document does not detail a migration path since there is no
previous standard protocol providing similar functionality.
11.1.3. Requirements on Other Protocols and Functional Components
When using the peer-to-peer streaming protocol tracker, PPSPP
requires a specific behavior from this protocol for security reasons,
as detailed in Section 12.2.
11.1.4. Impact on Network Operation
PPSPP is a peer-to-peer protocol that takes advantage of the fact
that content is available from multiple sources to improve
robustness, scalability, and performance. At the same time, poor
choices in determining which exact sources to use can lead to bad
experience for the end user and high costs for network operators.
Hence, PPSPP can benefit from the ALTO protocol to steer peer
selection, as described in Section 3.10.1.
11.1.5. Verifying Correct Operation
PPSPP is operating correctly when all peers obtain the desired
content on time. Therefore, the PPSPP client is the ideal location
to verify the protocol's correct operation. However, it is not
feasible to mandate logging the behavior of PPSPP peers in all
implementations and deployments, for example, due to privacy reasons.
There are two alternative options:
o Monitoring the PPSPP servers initially providing the content,
using standard metrics such as bandwidth usage, peer connections,
and activity, can help identify trouble, see next section and
[RFC2564].
o The tracker protocol [PPSP-TP] may be used to gather information
about all peers in a swarm, to obtain a global view of operation,
according to PPSP.OAM.REQ-3 in [RFC6972].
Basic operation of the protocol can be easily verified when a tracker
and swarm metadata are known by starting a PPSPP download. Deep
packet inspection for DATA and ACK messages help to establish that
actual content transfer is happening and that the chunk availability
signaling and integrity checking are working.
11.1.6. Configuration
Table 8 shows the PPSPP parameters, their defaults, and where the
parameter is defined. For parameters that have no default, the table
row contains the word "var" and refers to the section discussing the
considerations to make when choosing a value.
+-------------------------+-----------------------+-----------------+
| Name | Default | Definition |
+-------------------------+-----------------------+-----------------+
| Chunk Size | var, 1024 bytes | Section 8.1 |
| | recommended | |
| | | |
| Static Content | 1 (Merkle Hash Tree) | Section 7.5 |
| Integrity Protection | | |
| Method | | |
| | | |
| Live Content Integrity | 3 (Unified Merkle | Section 7.5 |
| Protection Method | Tree) | |
| | | |
| Merkle Hash Tree | 2 (SHA-256) | Section 7.6 |
| Function | | |
| | | |
| Live Signature | 13 (ECDSAP256SHA256) | Section 7.7 |
| Algorithm | | |
| | | |
| Chunk Addressing Method | 2 (32-bit chunk | Section 7.8 |
| | ranges) | |
| | | |
| Live Discard Window | var | Section 6.2, |
| | | Section 7.9 |
| | | |
| NCHUNKS_PER_SIG | var | Section 6.1.2.1 |
| | | |
| Dead peer detection | No reply in 3 minutes | Section 3.12 |
| | + 3 datagrams | |
+-------------------------+-----------------------+-----------------+
Table 8: PPSPP Defaults
11.2. Management Considerations
The management considerations for PPSPP are very similar to other
protocols that are used for large-scale content distribution, in
particular HTTP. How does one manage large numbers of servers? How
does one push new content out to a server farm and allows staged
releases? How are faults detected and how are servers and end-user
performance measured? As standard solutions to these challenges are
still being developed, this section cannot provide a definitive
recommendation on how PPSPP should be managed. Hence, it describes
the standard solutions available at this time and assumes a future
extension document will provide more complete guidelines.
11.2.1. Management Interoperability and Information
As just stated, PPSPP servers providing initial copies of the content
are akin to WWW and FTP servers. They can also be deployed in large
numbers and thus can benefit from standard management facilities.
Therefore, PPSPP servers may implement an SNMP management interface
based on the APPLICATION-MIB [RFC2564], where the file object can be
used to report on swarms.
What is missing is the ability to remove or rate limit specific PPSPP
swarms on a server. This corresponds to removing or limiting
specific virtual servers on a web server. In other words, as
multiple pieces of content (swarms, virtual WWW servers) are
multiplexed onto a single server process, more fine-grained
management of that process is required. This functionality is
currently missing.
Logging is an important functionality for PPSPP servers and,
depending on the deployment, PPSPP clients. Logging should be done
via syslog [RFC5424].
11.2.2. Fault Management
The facilities for verifying correct operation and server management
(just discussed) appear sufficient for PPSPP fault monitoring. This
can be supplemented with host resource [RFC2790] and UDP/IP network
monitoring [RFC4113], as PPSPP server failures can generally be
attributed directly to conditions on the host or network.
Since PPSPP has been designed to work in a hostile environment, many
benign faults will be handled by the mechanisms used for managing
attacks. For example, when a malfunctioning peer starts sending the
wrong chunks, this is detected by the content integrity protection
mechanism and another source is sought.
11.2.3. Configuration Management
Large-scale deployments may benefit from a standard way of
replicating a new piece of content on a set of initial PPSPP servers.
This functionality may need to include controlled releasing, such
that content becomes available only at a specific point in time
(e.g., the release of a movie trailer). This functionality could be
provided via NETCONF [RFC6241], to enable atomic configuration
updates over a set of servers. Uploading the new content could be
one configuration change, making the content available for download
by the public another.
11.2.4. Accounting Management
Content providers may offer PPSPP hosting for different customers and
will want to bill these customers, for example, based on bandwidth
usage. This situation is a common accounting scenario, similar to
billing per virtual server for web servers. PPSPP can therefore
benefit from general standardization efforts in this area [RFC2975]
when they come to fruition.
11.2.5. Performance Management
Depending on the deployment scenarios, the application performance
measurement facilities of [RFC3729] and associated [RFC4150] can be
used with PPSPP.
In addition, when the PPSPP tracker protocol is used, it provides a
built-in, application-level, performance measurement infrastructure
for different metrics. See PPSP.OAM.REQ-3 in [RFC6972].
11.2.6. Security Management
Malicious peers should ideally be locked out long term. This is
primarily for performance reasons, as the protocol is robust against
attacks (see next section). Section 12.7 describes a procedure for
long-term exclusion.
12. Security Considerations
As any other network protocol, PPSPP faces a common set of security
challenges. An implementation must consider the possibility of
buffer overruns, DoS attacks and manipulation (i.e., reflection
attacks). Any guarantee of privacy seems unlikely, as the user is
exposing its IP address to the peers. A probable exception is the
case of the user being hidden behind a public NAT or proxy. This
section discusses the protocol's security considerations in detail.
12.1. Security of the Handshake Procedure
Borrowing from the analysis in [RFC5971], the PPSPP may be attacked
with three types of denial-of-service attacks:
1. DoS amplification attack: attackers try to use a PPSPP peer to
generate more traffic to a victim.
2. DoS flood attack: attackers try to deny service to other peers by
allocating lots of state at a PPSPP peer.
3. Disrupt service to an individual peer: attackers send bogus,
e.g., REQUEST and HAVE messages appearing to come from victim
Peer A to the Peers B1..Bn serving that peer. This causes Peer A
to receive chunks it did not request or to not receive the chunks
it requested.
The basic scheme to protect against these attacks is the use of a
secure handshake procedure. In the UDP encapsulation, the handshake
procedure is secured by the use of randomly chosen channel IDs as
follows. The channel IDs must be generated following the
requirements in [RFC4960] (Section 5.1.3).
When UDP is used, all datagrams carrying PPSPP messages are prefixed
with a 4-byte channel ID. These channel IDs are random numbers,
established during the handshake phase as follows. Peer A initiates
an exchange with Peer B by sending a datagram containing a HANDSHAKE
message prefixed with the channel ID consisting of all zeros. Peer
A's HANDSHAKE contains a randomly chosen channel ID, chanA:
A->B: chan0 + HANDSHAKE(chanA) + ...
When Peer B receives this datagram, it creates some state for Peer A,
that at least contains the channel ID chanA. Next, Peer B sends a
response to Peer A, consisting of a datagram containing a HANDSHAKE
message prefixed with the chanA channel ID. Peer B's HANDSHAKE
contains a randomly chosen channel ID, chanB.
B->A: chanA + HANDSHAKE(chanB) + ...
Peer A now knows that Peer B really responds, as it echoed chanA. So
the next datagram that Peer A sends may already contain heavy
payload, i.e., a chunk. This next datagram to Peer B will be
prefixed with the chanB channel ID. When Peer B receives this
datagram, both peers have the proof they are really talking to each
other, the three-way handshake is complete. In other words, the
randomly chosen channel IDs act as tags (cf. [RFC4960]
(Section 5.1)).
A->B: chanB + HAVE + DATA + ...
12.1.1. Protection against Attack 1
In short, PPSPP does a so-called return routability check before
heavy payload is sent. This means that attack 1 is fended off: PPSPP
does not send back much more data than it received, unless it knows
it is talking to a live peer. Attackers sending a spoofed HANDSHAKE
to Peer B pretending to be Peer A now need to intercept the message
from Peer B to Peer A to get Peer B to send heavy payload, and ensure
that that heavy payload goes to the victim, something assumed too
hard to be a practical attack.
Note the rule is that no heavy payload may be sent until the third
datagram. This has implications for PPSPP implementations that use
chunk addressing schemes that are verbose. If a PPSPP implementation
uses large bitmaps to convey chunk availability, these may not be
sent by Peer B in the second datagram.
12.1.2. Protection against Attack 2
On receiving the first datagram Peer B will record some state about
Peer A. At present, this state consists of the chanA channel ID, and
the results of processing the other messages in the first datagram.
In particular, if Peer A included some HAVE messages, Peer B may add
a chunk availability map to Peer A's state. In addition, Peer B may
request some chunks from Peer A in the second datagram, and Peer B
will maintain state about these outgoing requests.
So presently, PPSPP is somewhat vulnerable to attack 2. An attacker
could send many datagrams with HANDSHAKEs and HAVEs and thus allocate
state at the PPSPP peer. Therefore, Peer A MUST respond immediately
to the second datagram, if it is still interested in Peer B.
The reason for using this slightly vulnerable three-way handshake
instead of the safer handshake procedure of Stream Control
Transmission Protocol (SCTP) [RFC4960] (Section 5.1) is quicker
response time for the user. In the SCTP procedure, Peers A and B
cannot request chunks until datagrams 3 and 4 respectively, as
opposed to 2 and 1 in the proposed procedure. This means that the
user has to wait less time in PPSPP between starting the video stream
and seeing the first images.
12.1.3. Protection against Attack 3
In general, channel IDs serve to authenticate a peer. Hence, to
attack, a malicious Peer T would need to be able to eavesdrop on
conversations between victim A and a benign Peer B to obtain the
channel ID Peer B assigned to Peer A, chanB. Furthermore, attacker
Peer T would need to be able to spoof, e.g., REQUEST and HAVE
messages from Peer A to cause Peer B to send heavy DATA messages to
Peer A, or prevent Peer B from sending them, respectively.
The capability to eavesdrop is not common, so the protection afforded
by channel IDs will be sufficient in most cases. If not, point-to-
point encryption of traffic should be used, see below.
12.2. Secure Peer Address Exchange
As described in Section 3.10, Peer A can send Peer-Exchange messages
PEX_RES to Peer B, which contain the IP address and port of other
peers that are supposedly also in the current swarm. The strength of
this mechanism is that it allows decentralized tracking: after an
initial bootstrap, no central tracker is needed. The vulnerability
of this mechanism (and DHTs) is that malicious peers can use it for
an Amplification attack.
In particular, a malicious Peer T could send PEX_RES messages to
well-behaved Peer A with addresses of Peers B1..Bn; on receipt, Peer
A could send a HANDSHAKE to all these peers. So, in the worst case,
a single datagram results in N datagrams. The actual damage depends
on Peer A's behavior. For example, when Peer A already has
sufficient connections, it may not connect to the offered ones at
all; but if it is a fresh peer, it may connect to all directly.
In addition, PEX can be used in Eclipse attacks [ECLIPSE] where
malicious peers try to isolate a particular peer such that it only
interacts with malicious peers. Let us distinguish two specific
attacks:
E1. Malicious peers try to eclipse the single injector in live
streaming.
E2. Malicious peers try to eclipse a specific consumer peer.
Attack E1 has the most impact on the system as it would disrupt all
peers.
12.2.1. Protection against the Amplification Attack
If peer addresses are relatively stable, strong protection against
the attack can be provided by using public key cryptography and
certification. In particular, a PEX_REScert message will carry
swarm-membership certificates rather than IP address and port. A
membership certificate for Peer B states that Peer B at address
(ipB,portB) is part of Swarm S at Time T and is cryptographically
signed. The receiver Peer A can check the certificate for a valid
signature, the right swarm and liveliness, and only then consider
contacting Peer B. These swarm-membership certificates correspond to
signed node descriptors in secure decentralized peer sampling
services [SPS].
Several designs are possible for the security environment for these
membership certificates. That is, there are different designs
possible for who signs the membership certificates and how public
keys are distributed. As an example, we describe a design where the
peer-to-peer streaming protocol tracker acts as certification
authority.
12.2.2. Example: Tracker as Certification Authority
Peer A wanting to join Swarm S sends a certificate request message to
a Tracker X for that swarm. Upon receipt, the tracker creates a
membership certificate from the request with Swarm ID S, a Timestamp
T, and the external IP and port it received the message from, signed
with the tracker's private key. This certificate is returned to Peer
A.
Peer A then includes this certificate when it sends a PEX_REScert to
Peer B. Receiver Peer B verifies it against the tracker public key.
This tracker public key should be part of the swarm's metadata, which
Peer B received from a trusted source. Subsequently, Peer B can send
the member certificate of Peer A to other peers in PEX_REScert
messages.
Peer A can send the certification request when it first contacts the
tracker or at a later time. Furthermore, the responses the tracker
sends could contain membership certificates instead of plain
addresses, such that they can be gossiped securely as well.
We assume the tracker is protected against attacks and does a return
routability check. The latter ensures that malicious peers cannot
obtain a certificate for a random host, just for hosts where they can
eavesdrop on incoming traffic.
The load generated on the tracker depends on churn and the lifetime
of a certificate. Certificates can be fairly long lived, given that
the main goal of the membership certificates is to prevent that
malicious Peer T can cause good Peer A to contact *random* hosts.
The freshness of the timestamp just adds extra protection in addition
to achieving that goal. It protects against malicious hosts causing
a good Peer A to contact hosts that previously participated in the
swarm.
The membership certificate mechanism itself can be used for a kind of
amplification attack against good peers. Malicious Peer T can cause
Peer A to spend some CPU to verify the signatures on the membership
certificates that Peer T sends. To counter this, Peer A SHOULD check
a few of the certificates sent and discard the rest if they are
defective.
The same membership certificates described above can be registered in
a Distributed Hash Table that has been secured against the well-known
DHT specific attacks [SECDHTS].
Note that this scheme does not work for peers behind a symmetric
Network Address Translator, but neither does normal tracker
registration.
12.2.3. Protection against Eclipse Attacks
Before we can discuss Eclipse attacks, we first need to establish the
security properties of the central tracker. A tracker is vulnerable
to Amplification attacks, too. A malicious Peer T could register a
victim Peer B with the tracker, and many peers joining the swarm will
contact Peer B. Trackers can also be used in Eclipse attacks. If
many malicious peers register themselves at the tracker, the
percentage of bad peers in the returned address list may become high.
Leaving the protection of the tracker to the peer-to-peer streaming
protocol tracker specification [PPSP-TP], we assume for the following
discussion that it returns a true random sample of the actual swarm
membership (achieved via Sybil attack protection). This means that
if 50% of the peers are bad, you'll still get 50% good addresses from
the tracker.
Attack E1 on PEX can be fended off by letting live injectors disable
PEX -- or at least, letting live injectors ensure that part of their
connections are to peers whose addresses came from the trusted
tracker.
The same measures defend against attack E2 on PEX. They can also be
employed dynamically. When the current set of Peers B that Peer A is
connected to doesn't provide good quality of service, Peer A can
contact the tracker to find new candidates.
12.3. Support for Closed Swarms
Regarding PPSP.SEC.REQ-1 in [RFC6972], the Closed Swarms [CLOSED] and
Enhanced Closed Swarms [ECS] mechanisms provide swarm-level access
control. The basic idea is that a peer cannot download from another
peer unless it shows a Proof-of-Access. Enhanced Closed Swarms
improve on the original Closed Swarms by adding on-the-wire
encryption against man-in-the-middle attacks and more flexible access
control rules.
The exact mapping of ECS to PPSPP is defined in [ECS-protocol].
12.4. Confidentiality of Streamed Content
Regarding PPSP.SEC.REQ-1 in [RFC6972], no extra mechanism is needed
to support confidentiality in PPSPP. A content publisher wishing
confidentiality should just distribute content in ciphertext and/or
in a format to which Digital Rights Management (DRM) techniques have
been applied. In that case, it is assumed a higher layer handles key
management out-of-band. Alternatively, pure point-to-point
encryption of content and traffic can be provided by the proposed
Closed Swarms access control mechanism, by DTLS [RFC6347], or by
IPsec [RFC4301].
When transmitting over DTLS, PPSPP can obtain the PMTU estimate
maintained by the IP layer to determine how much payload can be put
in a single datagram without fragmentation ([RFC6347],
Section 4.1.1.1). If PMTU changes and the chunk size becomes too
large to fit into a single datagram, PPSPP can choose to allow
fragmentation by clearing the Don't Fragment (DF) bit.
Alternatively, the content publisher can decide to use smaller chunks
and transmit multiple in the same datagram when the MTU allows.
12.5. Strength of the Hash Function for Merkle Hash Trees
Implementations MUST support SHA-1 as the hash function for content
integrity protection via Merkle hash trees. SHA-1 may be preferred
over stronger hash functions by content providers because it reduces
on-the-wire overhead. As such, it presents a trade-off between
performance and security. The security considerations for SHA-1 are
discussed in [RFC6194].
In general, note that the hash function is used in a hash tree, which
makes it more complex to create collisions. In particular, if
attackers manage to find a collision for a hash, it can replace just
one chunk, so the impact is limited. If fixed-size chunks are used,
the collision even has to be of the same size as the original chunk.
For hashes higher up in the hash tree, a collision must be a
concatenation of two hashes. In sum, finding collisions that fit
with the hash tree are generally harder to find than regular
collisions.
12.6. Limit Potential Damage and Resource Exhaustion by Bad or Broken
Peers
Regarding PPSP.SEC.REQ-2 in [RFC6972], this section provides an
analysis of the potential damage a malicious peer can do with each
message in the protocol, and how it is prevented by the protocol
(implementation).
12.6.1. HANDSHAKE
o Secured against DoS Amplification attacks as described in
Section 12.1.
o Threat HS.1: An Eclipse attack where Peers T1..Tn fill all
connection slots of Peer A by initiating the connection to Peer A.
Solution: Peer A must not let other peers fill all its available
connection slots, i.e., Peer A must initiate connections itself
too, to prevent isolation.
12.6.2. HAVE
o Threat HAVE.1: Malicious Peer T can claim to have content that it
does not. Subsequently, Peer T won't respond to requests.
Solution: Peer A will consider Peer T to be a slow peer and not
ask it again.
o Threat HAVE.2: Malicious Peer T can claim not to have content.
Hence, it won't contribute.
Solution: Peer and chunk selection algorithms external to the
protocol will implement fairness and provide sharing incentives.
12.6.3. DATA
o Threat DATA.1: Peer T sending bogus chunks.
Solution: The content integrity protection schemes defend against
this.
o Threat DATA.2: Peer T sends Peer A unrequested chunks.
To protect against this threat we need network-level DoS
prevention.
12.6.4. ACK
o Threat ACK.1: Peer T acknowledges wrong chunks.
Solution: Peer A will detect inconsistencies with the data it sent
to Peer T.
o Threat ACK.2: Peer T modifies timestamp in ACK to Peer A used for
time-based congestion control.
Solution: In theory, by decreasing the timestamp, Peer T could
fake that there is no congestion when in fact there is, causing
Peer A to send more data than it should. [RFC6817] does not list
this as a security consideration. Possibly, this attack can be
detected by the large resulting asymmetry between round-trip time
and measured one-way delay.
12.6.5. INTEGRITY and SIGNED_INTEGRITY
o Threat INTEGRITY.1: An amplification attack where Peer T sends
bogus INTEGRITY or SIGNED_INTEGRITY messages, causing Peer A to
checks hashes or signatures, thus spending CPU unnecessarily.
Solution: If the hashes/signatures don't check out, Peer A will
stop asking Peer T because of the atomic datagram principle and
the content integrity protection. Subsequent unsolicited traffic
from Peer T will be ignored.
o Threat INTEGRITY.2: An attack where Peer T sends old
SIGNED_INTEGRITY messages in the Unified Merkle Tree scheme,
trying to make Peer A tune in at a past point in the live stream.
Solution: The timestamp in the SIGNED_INTEGRITY message protects
against such replays. Subsequent traffic from Peer T will be
ignored.
12.6.6. REQUEST
o Threat REQUEST.1: Peer T could request lots from Peer A, leaving
Peer A without resources for others.
Solution: A limit is imposed on the upload capacity a single peer
can consume, for example, by using an upload bandwidth scheduler
that takes into account the need of multiple peers. A natural
upper limit of this upload quotum is the bitrate of the content,
taking into account that this may be variable.
12.6.7. CANCEL
o Threat CANCEL.1: Peer T sends CANCEL messages for content it never
requested to Peer A.
Solution: Peer A will detect the inconsistency of the messages and
ignore them. Note that CANCEL messages may be received
unexpectedly when a transport is used where REQUEST messages may
be lost or reordered with respect to the subsequent CANCELs.
12.6.8. CHOKE
o Threat CHOKE.1: Peer T sends REQUEST messages after Peer A sent
Peer B a CHOKE message.
Solution: Peer A will just discard the unwanted REQUESTs and
resend the CHOKE, assuming it got lost.
12.6.9. UNCHOKE
o Threat UNCHOKE.1: Peer T sends an UNCHOKE message to Peer A
without having sent a CHOKE message before.
Solution: Peer A can easily detect this violation of protocol
state, and ignore it. Note this can also happen due to loss of a
CHOKE message sent by a benign peer.
o Threat UNCHOKE.2: Peer T sends an UNCHOKE message to Peer A, but
subsequently does not respond to its REQUESTs.
Solution: Peer A will consider Peer T to be a slow peer and not
ask it again.
12.6.10. PEX_RES
o Secured against amplification and Eclipse attacks as described in
Section 12.2.
12.6.11. Unsolicited Messages in General
o Threat: Peer T could send a spoofed PEX_REQ or REQUEST from Peer B
to Peer A, causing Peer A to send a PEX_RES/DATA to Peer B.
Solution: the message from Peer T won't be accepted unless Peer T
does a handshake first, in which case the reply goes to Peer T,
not victim Peer B.
12.7. Exclude Bad or Broken Peers
This section is regarding PPSP.SEC.REQ-2 in [RFC6972]. A receiving
peer can detect malicious or faulty senders as just described, which
it can then subsequently ignore. However, excluding such a bad peer
from the system completely is complex. Random monitoring by trusted
peers that would blacklist bad peers as described in [DETMAL] is one
option. This mechanism does require extra capacity to run such
trusted peers, which must be indistinguishable from regular peers,
and requires a solution for the timely distribution of this blacklist
to peers in a scalable manner.
13. References
13.1. Normative References
[CCITT.X690.2002]
International Telephone and Telegraph Consultative
Committee, "ASN.1 encoding rules: Specification of basic
encoding Rules (BER), Canonical encoding rules (CER) and
Distinguished encoding rules (DER)", CCITT Recommendation
X.690, July 2002.
[FIPS180-4]
National Institute of Standards and Technology,
Information Technology Laboratory, "Federal Information
Processing Standards: Secure Hash Standard (SHS)", FIPS
PUB 180-4, March 2012.
[IANADNSSECALGNUM]
IANA, "Domain Name System Security (DNSSEC) Algorithm
Numbers", March 2014,
<http://www.iana.org/assignments/dns-sec-alg-numbers>.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., J. de Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <http://www.rfc-editor.org/info/rfc1918>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3110] Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
Domain Name System (DNS)", RFC 3110, DOI 10.17487/RFC3110,
May 2001, <http://www.rfc-editor.org/info/rfc3110>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<http://www.rfc-editor.org/info/rfc4034>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5702] Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
and RRSIG Resource Records for DNSSEC", RFC 5702,
DOI 10.17487/RFC5702, October 2009,
<http://www.rfc-editor.org/info/rfc5702>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
[RFC6605] Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
Signature Algorithm (DSA) for DNSSEC", RFC 6605,
DOI 10.17487/RFC6605, April 2012,
<http://www.rfc-editor.org/info/rfc6605>.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<http://www.rfc-editor.org/info/rfc6817>.
13.2. Informative References
[ABMRKL] Bakker, A., "Merkle hash torrent extension", BitTorrent
Enhancement Proposal 30, March 2009,
<http://bittorrent.org/beps/bep_0030.html>.
[BINMAP] Grishchenko, V. and J. Pouwelse, "Binmaps: Hybridizing
Bitmaps and Binary Trees", Delft University of Technology
Parallel and Distributed Systems Report Series, Report
number PDS-2011-005, ISSN 1387-2109, April 2009.
[BITOS] Vlavianos, A., Iliofotou, M., Mathieu, F., and M.
Faloutsos, "BiToS: Enhancing BitTorrent for Supporting
Streaming Applications", IEEE INFOCOM Global Internet
Symposium, Barcelona, Spain, April 2006.
[BITTORRENT]
Cohen, B., "The BitTorrent Protocol Specification",
BitTorrent Enhancement Proposal 3, February 2008,
<http://bittorrent.org/beps/bep_0003.html>.
[CLOSED] Borch, N., Mitchell, K., Arntzen, I., and D. Gabrijelcic,
"Access Control to BitTorrent Swarms Using Closed Swarms",
ACM workshop on Advanced Video Streaming Techniques for
Peer-to-Peer Networks and Social Networking (AVSTP2P '10),
Florence, Italy, October 2010,
<http://doi.acm.org/10.1145/1877891.1877898>.
[DETMAL] Shetty, S., Galdames, P., Tavanapong, W., and Ying. Cai,
"Detecting Malicious Peers in Overlay Multicast
Streaming", IEEE Conference on Local Computer Networks,
(LCN'06), Tampa, FL, USA, November 2006.
[ECLIPSE] Sit, E. and R. Morris, "Security Considerations for Peer-
to-Peer Distributed Hash Tables", IPTPS '01: Revised
Papers from the First International Workshop on Peer-to-
Peer Systems, pp. 261-269, Springer-Verlag, 2002.
[ECS] Jovanovikj, V., Gabrijelcic, D., and T. Klobucar, "Access
Control in BitTorrent P2P Networks Using the Enhanced
Closed Swarms Protocol", International Conference on
Emerging Security Information, Systems and Technologies
(SECURWARE 2011), pp. 97-102, Nice, France, August 2011.
[ECS-protocol]
Gabrijelcic, D., "Enhanced Closed Swarm protocol", Work in
Progress, draft-ppsp-gabrijelcic-ecs-01, June 2013.
[EPLIVEPERF]
Bonald, T., Massoulie, L., Mathieu, F., Perino, D., and A.
Twigg, "Epidemic live streaming: optimal performance
trade-offs", Proceedings of the 2008 ACM SIGMETRICS
International Conference on Measurement and Modeling of
Computer Systems, Annapolis, MD, USA, June 2008.
[GIVE2GET] Mol, J., Pouwelse, J., Meulpolder, M., Epema, D., and H.
Sips, "Give-to-Get: Free-riding-resilient Video-on-Demand
in P2P Systems", Proceedings Multimedia Computing and
Networking conference (Proceedings of SPIE, Vol. 6818),
San Jose, CA, USA, January 2008.
[HAC01] Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
of Applied Cryptography", CRC Press, (Fifth Printing,
August 2001), October 1996.
[JIM11] Jimenez, R., Osmani, F., and B. Knutsson, "Sub-Second
Lookups on a Large-Scale Kademlia-Based Overlay", IEEE
International Conference on Peer-to-Peer Computing
(P2P'11), Kyoto, Japan, August 2011.
[LBT] Rossi, D., Testa, C., Valenti, S., and L. Muscariello,
"LEDBAT: the new BitTorrent congestion control protocol",
Computer Communications and Networks (ICCCN), Zurich,
Switzerland, August 2010.
[LCOMPL] Testa, C. and D. Rossi, "On the impact of uTP on
BitTorrent completion time", IEEE International Conference
on Peer-to-Peer Computing (P2P'11), Kyoto, Japan, August
2011.
[MERKLE] Merkle, R., "Secrecy, Authentication, and Public Key
Systems", Ph.D. thesis, Dept. of Electrical Engineering,
Stanford University, CA, USA, pp 40-45, 1979.
[P2PWIKI] Bakker, A., Petrocco, R., Dale, M., Gerber, J.,
Grishchenko, V., Rabaioli, D., and J. Pouwelse, "Online
video using BitTorrent and HTML5 applied to Wikipedia",
IEEE International Conference on Peer-to-Peer Computing
(P2P'10), Delft, The Netherlands, August 2010.
[POLLIVE] Dhungel, P., Hei, Xiaojun., Ross, K., and N. Saxena,
"Pollution in P2P Live Video Streaming", International
Journal of Computer Networks & Communications (IJCNC) Vol.
1, No. 2, Jul 2009.
[PPSP-TP] Cruz, R., Nunes, M., Yingjie, G., Xia, J., Huang, R.,
Taveira, J., and D. Lingli, "PPSP Tracker Protocol-Base
Protocol (PPSP-TP/1.0)", Work in Progress,
draft-ietf-ppsp-base-tracker-protocol-09, March 2015.
[PPSPPERF] Petrocco, R., Pouwelse, J., and D. Epema, "Performance
Analysis of the Libswift P2P Streaming Protocol", IEEE
International Conference on Peer-to-Peer Computing
(P2P'12), Tarragona, Spain, September 2012.
[RFC2564] Kalbfleisch, C., Krupczak, C., Presuhn, R., and J.
Saperia, "Application Management MIB", RFC 2564,
DOI 10.17487/RFC2564, May 1999,
<http://www.rfc-editor.org/info/rfc2564>.
[RFC2790] Waldbusser, S. and P. Grillo, "Host Resources MIB", RFC
2790, DOI 10.17487/RFC2790, March 2000,
<http://www.rfc-editor.org/info/rfc2790>.
[RFC2975] Aboba, B., Arkko, J., and D. Harrington, "Introduction to
Accounting Management", RFC 2975, DOI 10.17487/RFC2975,
October 2000, <http://www.rfc-editor.org/info/rfc2975>.
[RFC3365] Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61, RFC
3365, DOI 10.17487/RFC3365, August 2002,
<http://www.rfc-editor.org/info/rfc3365>.
[RFC3729] Waldbusser, S., "Application Performance Measurement MIB",
RFC 3729, DOI 10.17487/RFC3729, March 2004,
<http://www.rfc-editor.org/info/rfc3729>.
[RFC4113] Fenner, B. and J. Flick, "Management Information Base for
the User Datagram Protocol (UDP)", RFC 4113,
DOI 10.17487/RFC4113, June 2005,
<http://www.rfc-editor.org/info/rfc4113>.
[RFC4150] Dietz, R. and R. Cole, "Transport Performance Metrics
MIB", RFC 4150, DOI 10.17487/RFC4150, August 2005,
<http://www.rfc-editor.org/info/rfc4150>.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<http://www.rfc-editor.org/info/rfc4193>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424,
DOI 10.17487/RFC5424, March 2009,
<http://www.rfc-editor.org/info/rfc5424>.
[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions", RFC
5706, DOI 10.17487/RFC5706, November 2009,
<http://www.rfc-editor.org/info/rfc5706>.
[RFC5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, DOI 10.17487/RFC5971,
October 2010, <http://www.rfc-editor.org/info/rfc5971>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<http://www.rfc-editor.org/info/rfc6194>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<http://www.rfc-editor.org/info/rfc6241>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
DOI 10.17487/RFC6709, September 2012,
<http://www.rfc-editor.org/info/rfc6709>.
[RFC6972] Zhang, Y. and N. Zong, "Problem Statement and Requirements
of the Peer-to-Peer Streaming Protocol (PPSP)", RFC 6972,
DOI 10.17487/RFC6972, July 2013,
<http://www.rfc-editor.org/info/rfc6972>.
[RFC7285] Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
"Application-Layer Traffic Optimization (ALTO) Protocol",
RFC 7285, DOI 10.17487/RFC7285, September 2014,
<http://www.rfc-editor.org/info/rfc7285>.
[SECDHTS] Urdaneta, G., Pierre, G., and M. van Steen, "A Survey of
DHT Security Techniques", ACM Computing Surveys,
vol. 43(2), January 2011.
[SIGMCAST]
Wong, C. and S. Lam, "Digital Signatures for Flows and
Multicasts", IEEE/ACM Transactions on Networking 7(4),
pp. 502-513, August 1999.
[SPS] Jesi, G., Montresor, A., and M. van Steen, "Secure Peer
Sampling", Computer Networks vol. 54(12), pp. 2086-2098,
Elsevier, August 2010.
[SWIFTIMPL]
Grishchenko, V., Paananen, J., Pronchenkov, A., Bakker,
A., and R. Petrocco, "Swift reference implementation",
2015, <https://github.com/libswift/libswift>.
[TIT4TAT] Cohen, B., "Incentives Build Robustness in BitTorrent",
1st Workshop on Economics of Peer-to-Peer Systems,
Berkeley, CA, USA, May 2003.
Acknowledgements
Arno Bakker, Riccardo Petrocco, and Victor Grishchenko are partially
supported by the P2P-Next project <http://www.p2p-next.org/>, a
research project supported by the European Community under its 7th
Framework Programme (grant agreement no. 216217). The views and
conclusions contained herein are those of the authors and should not
be interpreted as necessarily representing the official policies or
endorsements, either expressed or implied, of the P2P-Next project or
the European Commission.
PPSPP was designed by Victor Grishchenko at Technische Universiteit
Delft under supervision of Johan Pouwelse. The authors would like to
thank the following people for their contributions to this document:
the chairs (Martin Stiemerling, Yunfei Zhang, Stefano Previdi, and
Ning Zong) and members of the IETF PPSP working group, and Mihai
Capota, Raul Jimenez, Flutra Osmani, and Raynor Vliegendhart.
Authors' Addresses
Arno Bakker
Vrije Universiteit Amsterdam
De Boelelaan 1081
Amsterdam 1081HV
The Netherlands
Email: arno@cs.vu.nl
Riccardo Petrocco
Technische Universiteit Delft
Mekelweg 4
Delft 2628CD
The Netherlands
Email: r.petrocco@gmail.com
Victor Grishchenko
Technische Universiteit Delft
Mekelweg 4
Delft 2628CD
The Netherlands
Email: victor.grishchenko@gmail.com