RFC 7574
Peer-to-Peer Streaming Peer Protocol (PPSPP)
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 13.12. 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.
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