PPSP                                                           A. Bakker
Internet-Draft                                               R. Petrocco
Intended status: Standards Track                          V. Grishchenko
Expires: July 27, 2013                     Technische Universiteit Delft
                                                        January 23, 2013


              Peer-to-Peer Streaming Peer Protocol (PPSPP)
                    draft-ietf-ppsp-peer-protocol-05

Abstract

   The Peer-to-Peer Streaming Peer Protocol (PPSPP) is a transport
   protocol for disseminating the same content to a group of interested
   parties in a streaming fashion.  PPSPP supports streaming of both
   pre-recorded (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
   LEDBAT for congestion control.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 27, 2013.

Copyright Notice



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   Copyright (c) 2013 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  . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  Joining a Swarm  . . . . . . . . . . . . . . . . . . . . .  8
     2.2.  Exchanging Chunks  . . . . . . . . . . . . . . . . . . . .  9
     2.3.  Leaving a Swarm  . . . . . . . . . . . . . . . . . . . . .  9
   3.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  HANDSHAKE  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  HAVE . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.3.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.4.  ACK  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.5.  INTEGRITY  . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.6.  SIGNED_INTEGRITY . . . . . . . . . . . . . . . . . . . . . 11
     3.7.  REQUEST  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.8.  CANCEL . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.9.  CHOKE and UNCHOKE  . . . . . . . . . . . . . . . . . . . . 12
     3.10. Peer Address Exchange and NAT Hole Punching  . . . . . . . 13
       3.10.1.  PEX_REQ and PEX_RES Messages  . . . . . . . . . . . . 13
       3.10.2.  Hole Punching via PPSPP Messages  . . . . . . . . . . 13
     3.11. Keep Alive Signalling  . . . . . . . . . . . . . . . . . . 13
     3.12. Storage Independence . . . . . . . . . . . . . . . . . . . 14
   4.  Chunk Addressing Schemes . . . . . . . . . . . . . . . . . . . 14
     4.1.  Start-End Ranges . . . . . . . . . . . . . . . . . . . . . 14
       4.1.1.   Chunk Ranges  . . . . . . . . . . . . . . . . . . . . 14
       4.1.2.   Byte Ranges . . . . . . . . . . . . . . . . . . . . . 14
     4.2.  Bin Numbers  . . . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  In Messages  . . . . . . . . . . . . . . . . . . . . . . . 16
       4.3.1.   In HAVE Messages  . . . . . . . . . . . . . . . . . . 16
       4.3.2.   In ACK Messages . . . . . . . . . . . . . . . . . . . 16
     4.4.  Compatibility  . . . . . . . . . . . . . . . . . . . . . . 16



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   5.  Content Integrity Protection . . . . . . . . . . . . . . . . . 17
     5.1.  Merkle Hash Tree Scheme  . . . . . . . . . . . . . . . . . 18
     5.2.  Content Integrity Verification . . . . . . . . . . . . . . 19
     5.3.  The Atomic Datagram Principle  . . . . . . . . . . . . . . 20
     5.4.  INTEGRITY Messages . . . . . . . . . . . . . . . . . . . . 21
     5.5.  Discussion and Overhead  . . . . . . . . . . . . . . . . . 21
     5.6.  Automatic Detection of Content Size  . . . . . . . . . . . 22
       5.6.1.   Peak Hashes . . . . . . . . . . . . . . . . . . . . . 22
       5.6.2.   Procedure . . . . . . . . . . . . . . . . . . . . . . 24
   6.  Live Streaming . . . . . . . . . . . . . . . . . . . . . . . . 25
     6.1.  Content Authentication . . . . . . . . . . . . . . . . . . 25
       6.1.1.   Sign All  . . . . . . . . . . . . . . . . . . . . . . 25
       6.1.2.   Unified Merkle Tree . . . . . . . . . . . . . . . . . 26
     6.2.  Forgetting Chunks  . . . . . . . . . . . . . . . . . . . . 29
   7.  Protocol Options . . . . . . . . . . . . . . . . . . . . . . . 29
     7.1.  End Option . . . . . . . . . . . . . . . . . . . . . . . . 30
     7.2.  Version  . . . . . . . . . . . . . . . . . . . . . . . . . 30
     7.3.  Minimum Version  . . . . . . . . . . . . . . . . . . . . . 30
     7.4.  Swarm Identifier . . . . . . . . . . . . . . . . . . . . . 30
     7.5.  Content Integrity Protection Method  . . . . . . . . . . . 31
     7.6.  Merkle Tree Hash Function  . . . . . . . . . . . . . . . . 31
     7.7.  Live Signature Algorithm . . . . . . . . . . . . . . . . . 32
     7.8.  Chunk Addressing Method  . . . . . . . . . . . . . . . . . 32
     7.9.  Live Discard Window  . . . . . . . . . . . . . . . . . . . 32
     7.10. Supported Messages . . . . . . . . . . . . . . . . . . . . 33
   8.  UDP Encapsulation  . . . . . . . . . . . . . . . . . . . . . . 33
     8.1.  Chunk Size . . . . . . . . . . . . . . . . . . . . . . . . 34
     8.2.  Datagrams and Messages . . . . . . . . . . . . . . . . . . 34
     8.3.  Channels . . . . . . . . . . . . . . . . . . . . . . . . . 35
     8.4.  HANDSHAKE  . . . . . . . . . . . . . . . . . . . . . . . . 36
     8.5.  HAVE . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     8.6.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     8.7.  ACK  . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     8.8.  INTEGRITY  . . . . . . . . . . . . . . . . . . . . . . . . 38
     8.9.  SIGNED_INTEGRITY . . . . . . . . . . . . . . . . . . . . . 38
     8.10. REQUEST  . . . . . . . . . . . . . . . . . . . . . . . . . 38
     8.11. CANCEL . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     8.12. CHOKE and UNCHOKE  . . . . . . . . . . . . . . . . . . . . 38
     8.13. PEX_REQ, PEX_RESv4, PEX_RESv6 and PEX_REScert  . . . . . . 39
     8.14. KEEPALIVE  . . . . . . . . . . . . . . . . . . . . . . . . 39
     8.15. Flow and Congestion Control  . . . . . . . . . . . . . . . 40
   9.  Extensibility  . . . . . . . . . . . . . . . . . . . . . . . . 40
     9.1.  Chunk Picking Algorithms . . . . . . . . . . . . . . . . . 40
     9.2.  Reciprocity Algorithms . . . . . . . . . . . . . . . . . . 40
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 41
     12.1. Security of the Handshake Procedure  . . . . . . . . . . . 41



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       12.1.1.  Protection against attack 1 . . . . . . . . . . . . . 43
       12.1.2.  Protection against attack 2 . . . . . . . . . . . . . 43
       12.1.3.  Protection against attack 3 . . . . . . . . . . . . . 43
     12.2. Secure Peer Address Exchange . . . . . . . . . . . . . . . 44
       12.2.1.  Protection against the Amplification Attack . . . . . 44
       12.2.2.  Example: Tracker as Certification Authority . . . . . 45
       12.2.3.  Protection Against Eclipse Attacks  . . . . . . . . . 46
     12.3. Support for Closed Swarms (PPSP.SEC.REQ-1) . . . . . . . . 46
     12.4. Confidentiality of Streamed Content (PPSP.SEC.REQ-2+3) . . 46
     12.5. Limit Potential Damage and Resource Exhaustion by Bad
           or Broken Peers (PPSP.SEC.REQ-4+6) . . . . . . . . . . . . 47
       12.5.1.  HANDSHAKE . . . . . . . . . . . . . . . . . . . . . . 47
       12.5.2.  HAVE  . . . . . . . . . . . . . . . . . . . . . . . . 47
       12.5.3.  DATA  . . . . . . . . . . . . . . . . . . . . . . . . 47
       12.5.4.  ACK . . . . . . . . . . . . . . . . . . . . . . . . . 48
       12.5.5.  INTEGRITY and SIGNED_INTEGRITY  . . . . . . . . . . . 48
       12.5.6.  REQUEST . . . . . . . . . . . . . . . . . . . . . . . 48
       12.5.7.  CANCEL  . . . . . . . . . . . . . . . . . . . . . . . 48
       12.5.8.  CHOKE . . . . . . . . . . . . . . . . . . . . . . . . 49
       12.5.9.  UNCHOKE . . . . . . . . . . . . . . . . . . . . . . . 49
       12.5.10. PEX_RES . . . . . . . . . . . . . . . . . . . . . . . 49
       12.5.11. Unsolicited Messages in General . . . . . . . . . . . 49
     12.6. Exclude Bad or Broken Peers (PPSP.SEC.REQ-5) . . . . . . . 49
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 50
     13.2. Informative References . . . . . . . . . . . . . . . . . . 51
   Appendix A.  Revision History  . . . . . . . . . . . . . . . . . . 54
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 58























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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 pre-recorded (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 which
   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.
   LEDBAT may be replaced with a different algorithm when the work of
   the IETF working group on RTP Media Congestion Avoidance Techniques
   (RMCAT) [RMCATCHART] matures.

   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



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   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].

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, a pre-recorded multimedia asset, or a
       file.

   chunk
       The basic unit in which the content is divided.  E.g. a block of
       N kilobyte.

   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.





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   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.  I.e. methods for
       receiving peers to detect whether a requested chunk has been
       maliciously modified by the sending peer.

   hash
       The result of applying a cryptographic hash function, more
       specifically a modification detection code (MDC) [HAC01], such as
       SHA-1 [FIPS180-3], 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).

   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.  When Merkle hash trees are used for content integrity
       protection, the identifier is the so-called root hash of the
       content (video-on-demand).  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 a peer A is choking peer B it means that A is currently not
       willing to accept requests for content from B.






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   seeding/leeching
       When a peer A is seeding it means that A has downloaded a static
       content asset completely and is now offering it for others to
       download.  If peer A does not yet have all content or is not
       offering it for download, A is said to be leeching.


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 UDP is used for transport, the
   recommended method for content integrity protection (Merkle hash
   trees) is used, and that a specific policy is used for selecting
   which chunks to download.

2.1.  Joining a Swarm

   Consider a peer A that wants to download a certain content asset.  To
   commence a PPSPP download, peer A must have the swarm ID of the
   content and a list of one or more tracker contact points (e.g. host+
   port).  The list of trackers is optional in the presence of a
   decentralized tracking mechanism.

   Peer A now registers with the tracker following e.g. the PPSP tracker
   protocol [I-D.ietf-ppsp-reqs] and receives the IP address and port of
   peers already in the swarm, say B, C, and D. Peer A now sends a
   datagram containing a HANDSHAKE message to B, C, and D. This message
   conveys protocol options, in particular, peer A includes the ID of
   the swarm as the destination peers can listen for multiple swarms on
   the same network address.

   Peer B and C respond with datagrams containing a HANDSHAKE message
   and one or more HAVE messages.  A HAVE message conveys (part of) the
   chunk availability of a peer and thus contains a chunk specification
   that denotes what chunks of the content peer B, resp. C have.  Peer D
   sends a datagram with a HANDSHAKE and HAVE messages, but also with a
   CHOKE message.  The latter indicates that D is not willing to upload
   chunks to A at present.







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2.2.  Exchanging Chunks

   In response to B and C, A sends new datagrams to B and C containing
   REQUEST messages.  A REQUEST message indicates the chunks that a peer
   wants to download, and thus contains a chunk specification.  The
   REQUEST messages to B and C refer to disjunct sets of chunks.  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 B and C are correct.  It also
   updates the chunk availability of B and C using the information in
   the received HAVE messages.

   After processing, A sends a datagram containing HAVE messages for the
   chunks it just received to all its peers.  In the datagram to 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 e.g.  C finds that
   A obtained a chunk (from B) that C did not yet have, C's next
   datagram includes a REQUEST for that chunk.

   Peer D also sends HAVE messages to A when it downloads chunks from
   other peers.  When D is willing to accept REQUESTs from A, D sends a
   datagram with an UNCHOKE message to inform A. If B or C decide to
   choke A they sending a CHOKE message and A should then re-request
   from other peers.  B and C may continue to send HAVE, REQUEST, or
   periodic KEEPALIVE messages such that 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 MAY also contact the tracker or another
   source again to obtain more peer addresses.

2.3.  Leaving a Swarm

   To leave a swarm in a graceful way, peer A sends a "close-channel"
   datagram to all its peers and deregisters from the tracker following
   the (PPSP) tracker protocol.  Peers receiving the datagram should
   remove A from their current peer list.  If A crashes ungracefully,
   peers should remove A from their peer list when they detect it no
   longer sends messages.







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3.  Messages

   In general, 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 (i.e., responding with chunks) or bad and only use the good
   ones.  This behavior allows a peer to deal with slow, crashed and
   (silent) malicious peers.

   For the sake of simplicity, one swarm of peers deals with one content
   asset (e.g. file) only.  Retrieval of a collections 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, as described in Section 3.12.

3.1.  HANDSHAKE

   The initiating peer and the addressed peer MUST send a HANDSHAKE
   message as the first message in the first datagrams they exchange.
   The payload of the HANDSHAKE message is a sequence of protocol
   options.  Example options are the content integrity protection scheme
   used and an option to specify the swarm identifier.  The initiating
   peer's HANDSHAKE SHOULD contain the latter option as peers SHOULD
   listen for messages for multiple swarms on the same network address
   in the chosen UDP encapsulation.  Protocol options are specified in
   Section 7.

   After the handshakes are exchanged, the initiator knows that the peer
   really responds.  Hence, the second datagram the initiator sends MAY
   already contain some heavy payload, e.g.  DATA messages.  To minimize
   the number of initialization round-trips, the first two datagrams
   exchanged MAY also contain some minor payload, e.g.  HAVE messages to
   indicate the current progress of a peer or a REQUEST (see
   Section 3.7).

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 has successfully checked the
   integrity of a chunk or interval of chunks it MUST send a HAVE
   message to all peers it wants to interact with in the near future.



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   The latter confinement allows the HAVE message to be used as a method
   of choking.  The HAVE message MUST contain the chunk specification of
   the received chunks.  A receiving peer MUST NOT send a HAVE message
   to peers for which the handshake procedure is still incomplete, see
   Section 12.1.

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

   When PPSPP is run over an unreliable transport protocol, an
   implementation MAY choose to use ACK messages to acknowledge received
   data.  When used, a receiving peer that has successfully checked the
   integrity of a chunk or interval of chunks C it 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.

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 recommended method of
   Merkle hash trees is used, the datagram carrying the DATA message
   MUST include the cryptographic hashes that are necessary for a
   receiver to check the integrity of the chunk in the form of INTEGRITY
   messages.  What are the necessary hashes is explained in Section 5.3.

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 and a digital
   signature.  Its exact payload depends on the live content integrity
   protection scheme used, see Section 6.1.






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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
   request-less push model to save round trips.  PPSPP supports both
   models of operation.

   A peer MAY send a REQUEST message that MUST contain the specification
   of the chunks it wants to download.  A peer receiving a REQUEST
   message MAY send out the requested chunks.  When peer Q receives
   multiple REQUESTs from the same peer P peer Q SHOULD process the
   REQUESTs sequentially.  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 MAY
   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
   MAY 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 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 P. In other words, the HAVE message acts as an
   implicit CANCEL.

3.9.  CHOKE and UNCHOKE

   Peer A MAY send a CHOKE message to peer B to signal it will no longer
   be responding to REQUEST messages from B, for example, because A's
   upload capacity is exhausted.  Peer A MAY send a subsequent UNCHOKE
   message to signal that it will respond to new REQUESTs from B again
   (A SHOULD discard old requests).  When peer B receives a CHOKE
   message from A it MUST NOT send new REQUEST messages and SHOULD NOT
   expect answers to any outstanding ones.  The CHOKE and UNCHOKE
   messages are informational as a peer is not required to respond to



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   REQUESTs, see Section 3.7.

3.10.  Peer Address Exchange and NAT Hole Punching

3.10.1.  PEX_REQ and PEX_RES Messages

   Peer address exchange messages (or PEX messages for short) are common
   in many peer-to-peer protocols.  By exchanging peer addresses in
   gossip fashion, peers relieve central coordinating entities (the
   trackers) from unnecessary work.  PPSPP optionally features two types
   of PEX messages: PEX_REQ and PEX_RES.  A peer that wants to retrieve
   some peer addresses MUST send a PEX_REQ message.  The receiving peer
   MAY respond with a PEX_RES message containing the (potentially
   signed) addresses of several peers.  The addresses MUST be of peers
   it has exchanged messages with in the last 60 seconds to guarantee
   liveliness.  Alternatively, the receiving peer MAY ignore PEX
   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.

   As peer-address exchange enables a number of attacks it should not be
   used outside a benign environment unless extra security measures are
   in place.  These security measures, which involve exchanging
   addresses in cryptographically signed swarm-membership certificates,
   are described in Section 12.2.

3.10.2.  Hole Punching via PPSPP Messages

   PPSPP can be used in combination with STUN [RFC5389].  In addition,
   the native PEX messages can be used to do simple NAT hole punching
   [SNP], as follows.  When peer A introduces peer B to peer C by
   sending a PEX_RES message to C, it SHOULD also send a PEX_RES message
   to B introducing C. These messages SHOULD be within 2 seconds from
   each other, but MAY not be simultaneous, instead leaving a gap of
   twice the "typical" RTT, i.e. 300-600 ms.  As a result, the peers are
   supposed to initiate handshakes to each other thus forming a simple
   NAT hole punching pattern where the introducing peer effectively acts
   as a STUN server.  Note that the PEX_RES message is sent without a
   prior PEX_REQ in this case.  Also note the PEX_RES from A to B is
   likely to arrive because recent communication between A and B is a
   prerequisite for A introducing B to C, see previous section.

3.11.  Keep Alive Signalling

   A peer MUST send a "keep alive" message periodically to each peer it
   wants to interact with in the future, but has no other messages to
   send them at present.  PPSPP does not define an explicit message type
   for "keep alive" messages.  In the PPSP-over-UDP encapsulation they



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   are implemented as simple datagrams consisting of a 4-byte channel
   number only, see Section 8.3 and Section 8.4.

3.12.  Storage Independence

   Note PPSPP does not prescribe how chunks are stored.  This also
   allows users of PPSPP to map different files into a single swarm as
   in BitTorrent's multi-file torrents [BITTORRENT], and more innovative
   storage solutions when variable-sized chunks are used.


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.

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.  A PPSPP
   peer MUST support this scheme.

4.1.2.  Byte Ranges

   The start and end specification are byte offsets in the content.  A
   PPSPP peer MAY support this scheme.

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
   light-weight.  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.  A PPSPP peer MAY support this
   scheme.

   In bin addressing, the smallest binary interval is a single chunk



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   (e.g. a block of bytes which 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 which
   lays them out into a vector nicely, which is called bin numbering, as
   follows.  Consider an 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 nodes 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 1

   So bin 7 represents the complete interval, bin 3 represents the
   interval of chunk 0..3, bin 1 represents the interval of chunks 0 and
   1, 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



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   ACK messages is equal to a single bin number, as follows.

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 interact with.  The latter allows the HAVE message 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 acknowledgments
   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

   When PPSPP is run over an unreliable transport protocol, an
   implementation MAY choose to use ACK messages to acknowledge received
   data.  When a receiving peer has successfully checked the integrity
   of a chunk or interval of chunks C it MUST send a ACK message
   containing the chunk specification of its biggest, complete, interval
   covering C to the sending peer (see HAVE).

4.4.  Compatibility

   In principle, peers using range addressing and peers using bin
   numbering can interact, with some limitations.  Alternatively, a peer
   A MAY refuse to interact with a peer B using a different addressing
   scheme.  In that case, A MUST respond to B'S HANDSHAKE message by
   sending an explicit close (see Section 8.4).  PPSPP presently
   supports only interaction between willing peers when fixed sized
   chunks are used, as follows:

   When a bin peer sends a message containing a chunk specification to a
   byte-range peer it MUST translate its internal bin numbers to byte



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   ranges.  When a byte range peer sends a message with a chunk
   specification message to a bin peer, it MUST round its internal byte
   ranges to 1 or more bins.  For the latter translation, the byte-range
   peer MUST know the fixed chunk size used (which it should receive
   along with the swarm identifier).  When a range translates to
   multiple bins, the byte-range peer should send multiple e.g.  HAVE
   messages.  Note that the bin peer may not be able to request all
   content the byte-range peer has if it does not have an integral
   number of chunks.

   Aside: Translation from bytes to bins is possible for variable sized
   chunks only when the byte-range peer has extra information.  In
   particular, it will need to know the individual sizes of the chunks
   from the start of the content till the byte range it wants to convey
   to the bin peer.

   A similar translation MUST be done for translating between bins and
   chunk ranges.  Chunk ranges are directly translatable to bins.
   Assuming ranges are intervals of a list of chunks numbered 0...N, for
   a given bin number "bin" and bitwise operations AND and OR:

       startrange = (bin AND (bin + 1))/2

       endrange = ((bin OR (bin + 1)) - 1)/2

   The reverse translation may require a chunk range to be rounded to
   the largest binary interval it covers, or for a range be translated
   to a series of bin numbers that should be sent using multiple (e.g.
   HAVE) messages.

   Finally, byte-range peers can interact with chunk-range peers, by
   using the direct translation from chunks into bytes and by rounding
   byte ranges into chunk ranges.  The latter requires the byte-range
   peer to know the fixed chunk size.


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.

   This section describes the recommended method for bad content
   detection, the Merkle Hash Tree scheme, which SHOULD be implemented
   for protecting static content.  It can also be efficiently used in
   protecting live streams, as explained below and in Section 6.1.



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   The Merkle hash tree scheme can use 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.
   When bin numbering is used, node IDs correspond directly to bin
   numbers in the INTEGRITY message, see below.

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 asset that is divided into N chunks
   is constructed as follows.  Note the construction does not assume
   chunks of content to be fixed size.  Given a cryptographic hash
   function, more specifically a modification detection code (MDC)
   [HAC01] , such as SHA1, 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 asset consisting of 7
   chunks.  As before 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 left-most 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.









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                               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


             The Merkle hash tree of an interval of width W=8

                                 Figure 2

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 its
   uncles are nodes 13 and 3, marked with * 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
   either sent the wrong content or the wrong sibling or uncle hashes.
   For simplicity, the set of sibling and uncles 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.



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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,
   so each datagram SHOULD be processed separately and a loss of one
   datagram MUST NOT disrupt the flow.  Thus, as a datagram carries zero
   or more messages, neither messages nor message interdependencies
   SHOULD 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 acknowledgments (HAVE or ACK)
   to derive which hashes the receiver already has for sure.  Suppose,
   the receiver had acknowledged chunks C0 and C1 (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 be also 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 which were sent out in
   previous, still unacknowledged datagrams.  It is an optimization
   trade-off between redundant hash transmission and possibility of
   collateral data loss in the case some necessary hashes were lost in
   the network so some delivered data cannot be verified and thus has to
   be dropped.  In either case, the receiver builds the Merkle 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 missing hashes
   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.)




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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 a
   INTEGRITY message for each hash the receiver misses for integrity
   checking.  A 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 2 denotes chunks
   0,2,4,6, so the chunk specification should denote that interval.  The
   list of INTEGRITY messages MUST be sorted in order of tree height of
   the nodes, descending.  The DATA message MUST contain the chunk
   specification of the chunk and 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 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 case of a simple progressive download, of chunks 0,2,4,6, etc. the



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   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 root hash of a static content asset, such as a video
   file, along with some peer addresses is sufficient to start a
   download.  In addition, PPSPP can reliably and automatically derive
   the size of such content from information received from the network
   when fixed sized chunks are used.  As a result, it is not necessary
   to include the size of the content asset as the metadata of the
   content, in addition to the root hash.  Implementations of PPSPP MAY
   use this automatic detection feature.  Note this feature is the only
   feature of PPSPP that requires that a fixed-sized chunk is used.

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.  Peak hashes, in
   general, enable two cornerstone features of PPSPP: reliable file size
   detection and download/live streaming unification (see Section 6).
   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



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   incomplete (not filled) node corresponds to an interval that contains
   also 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
   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 tree
   for that file is shown in Figure 3.  Following the definition the
   peak hashes of this file are in nodes 3, 9 and 12, denoted with a *.
   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 3

   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, 12.  The number of peak hashes
   for a file is therefore 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, and thus get
   an estimate of the file size (given all chunks but the last are fixed
   size).  Which nodes are the peaks can be securely communicated from
   one (untrusted) peer A to another B by letting A send the peak hashes
   and their node IDs to B. It can be shown that the root hash that B
   obtained from a trusted source is sufficient to verify that these are



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   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, be
   it the right sibling, its left neighbor/sibling must also be a filled
   node, because of the way chunks are laid out in the leaves,
   contradiction. (b) For the rightmost peak hash, its right sibling is
   zero. (c) For any peak hash, its 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 fixed size B just needs to know the size
   of the last chunk.  Knowing the number of chunks B can calculate the
   node ID of the last chunk and download it.  As always 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.  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 a peer B sends a DATA message for the
   first time to a peer A, B MUST first send all the peak hashes for the
   content, unless A has already signalled earlier in the exchange 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 A MUST check the
   peak hashes against the root hash to determine the approximate
   content size.  To obtain the definite content size peer A MUST
   download the last chunk of the content from any peer that offers it.







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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 and peers, upon receiving the chunk,
   check the signature using the corresponding public key obtained from
   a trusted source.  In particular, in PPSP, the swarm ID of the live
   stream is that public key.  The signature is sent along with the DATA
   message containing the relevant chunk using the SIGNED_INTEGRITY
   message.

   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 case.  The use of Merkle hash trees reduces
   the number of signing and verification operations per second, that
   is, provide signature amortization similar to the approach described
   in [SIGMCAST].

   In both methods the swarm ID consists of a public key encoded as in a
   DNSSEC DNSKEY resource record without BASE-64 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.

6.1.1.  Sign All

   Even with "Sign All", the number of cryptographic operations per
   second may be limited.  For example, consider a 25 frame/second video
   transmitted over UDP.  When each frame is transmitted in its own
   chunk, only 25 signature verification operations per second are
   required, for the receiving peer, for bitrates up to ~12.8 megabit/
   second over UDP.  For higher bitrates multiple UDP packets per frame
   are needed.






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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 has a
   transient root hash.  A public key therefore 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 procedure.

   The live injector creates a number of chunks N, a fixed power of 2
   (N>=2), which are added as new leaves to the existing hash tree,
   expanding the tree as required.  As a result of this expansion, the
   tree will have gotten a set of new peak hashes (see Section 5.6.1).
   The injector now signs only the peak hashes in this set that are not
   in the old set of peak hashes.  For N being a power of 2 there will
   just be one new peak hash (see below).  This complementary signed
   peak is distributed to the peers.  Receiving peers will verify the
   signature on the signed peak against the swarm ID, update their tree
   and request the new chunks.

   To illustrate this procedure, consider the injector has generated the
   tree shown in Figure 4 and it is connected to several peers that
   currently have the same tree and all chunks.  In this tree the root
   node 3 is also the peak node for this tree.  Now the injector
   generates N=2 new chunks.  As a result the tree expands as shown in
   Figure 5.  The two new pieces 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 peak nodes in this tree are 3 and 9.


                                     3*
                                    / \
                                   /   \
                                  /     \
                                 1       5
                                / \     / \
                               0   2   4   6


                             Current live tree

                                 Figure 4






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                                     7
                                    / \
                                  /     \
                                /         \
                              /             \
                            3*               11
                           / \              / \
                          /   \            /   \
                         /     \          /     \
                        1       5        9*      13
                       / \     / \      / \      / \
                      0   2   4   6    8   10   E   E


                          Next current live tree

                                 Figure 5

   The injector now needs to inform its peers of the changed tree, in
   particular the addition of the new complementary peak hash 9.  To
   this extent, it sends an INTEGRITY message with the hash of node 9, a
   SIGNED_INTEGRITY message with the signature of the hash of node 9 and
   a HAVE message for node 9.  The receiving peers now expand their view
   of the tree.  Next, the peers will request e.g. chunk 8 from the
   injector by sending a REQUEST message.  The injector responds by
   sending the requester an INTEGRITY message with the hash of node 10,
   and a DATA message with chunk 8.  This allows the peer to verify the
   chunk against peak hash 9 which is signed by the trusted injector.

   The injector MAY send HAVE messages for the chunks it creates
   immediately, and allow peers to retrieve them.  This optimizes the
   use of the injector's bandwidth.  Peers MUST NOT forward these chunks
   to others until they have received and checked the peak hash
   signature and the necessary hashes.

   This procedure generates just 1 new peak hash for every N blocks, so
   it requires just one signature on each iteration, making it N times
   cheaper than "Sign All".  To see why just 1 new peak hash is
   generated each iteration let's return to the definition of a peak
   hash in a tree, from Section 5.6.1.  A peak hash is the hash of a
   filled node in the Merkle tree, whose sibling is an incomplete node.
   Now consider the above procedure where N chunks (with N a power of 2)
   are added to a tree at each iteration.  In the first iteration, the
   tree consists of just N leaves, therefore the only peak is the root
   of the tree.  In the second iteration, the tree consists of 2N chunks
   and the only peak is the root of that bigger tree (depicted in
   Figure 4 for N=2).  In the third iteration, we have 3N chunks as
   leaves and a tree that is 4N wide (to span the 3N chunks) and hence



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   has N empty leaves (depicted in Figure 5 for N=2).  This implies that
   the tree has 2 peaks, notably the peak from the previous iteration
   (node 3 in the figure) and the top of the subtree of the N chunks
   that were added last (node 9 in the figure).  Although this iteration
   has two peaks, there is only one new peak as the expanded tree
   overlaps with the tree from the previous iteration.  In the fourth
   iteration, we have a complete balanced tree again, and just a single
   new peak.  It is now easy to see that this process in which previous
   peaks are either consumed into a single new peak, or peak sets
   overlap with just 1 new addition yields a single new peak per N
   chunks.

   From this we can conclude that the injector has to sign less hashes
   than in the "Sign All" method.  A receiving peer therefore also has
   to verify less signatures.  It does additionally need to check one or
   more hashes per chunk via the Merkle Tree scheme, but this requires
   less CPU than a signature verification for every chunk.  This
   approach is similar to signature amortization via Merkle Tree
   Chaining [SIGMCAST].  The downside of this amortization of signature
   costs over several chunks is that latency will increase.  A receiving
   peer now has to wait for the signature before delivering the chunks
   to the higher layers responsible for playback [POLLIVE], unless some
   (optimistic) optimisations are made.  It MUST check the signature
   before forwarding the chunks to other peers.

   The number of chunks per signature N MUST be a fixed power of 2
   (N>=2).  The procedure does not preclude using variable-sized chunks.
   Using a variable number N, however, is not allowed as this breaks the
   property of generating just 1 new peak per iteration.

   Unification of static content checking and live content checking is
   achieved by sending the signed peak hashes on-demand, ahead of the
   actual data.  As before, the sender SHOULD use acknowledgments to
   derive which content range the receiver has peak hashes for, and to
   prepend the data hashes with the necessary (signed) peak hashes.
   Except for the fact that the set of peak hashes changes with time,
   other parts of the protocol work as described in Section 5.1.

   This method of integrity verification has an added benefit if the
   system includes some peers that saved the complete broadcast.  The
   benefit is that 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 have saved
   all chunks can now announce this root hash to the tracking
   infrastructure and instantly seed it.






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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 such 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 whereas it 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.


7.  Protocol Options

   The HANDSHAKE message in PPSPP can contain the following protocol
   options (cf. [RFC2132] (DHCP 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.

              +-------+-------------------------------------+
              | 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                  |
              | 9-254 | Unassigned                          |
              | 255   | End Option                          |
              +-------+-------------------------------------+

                    Table 2: PPSP Peer Protocol Options







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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.

7.2.  Version

   A peer MUST include the maximum version of the PPSPP protocol 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                            |
           +---------+----------------------------------------+
           | 1       | Protocol as described in this document |
           | 2-255   | Unassigned                             |
           +---------+----------------------------------------+

                Table 3: PPSP Peer Protocol Version Numbers

7.3.  Minimum Version

   When a peer initiates the handshake it MUST include the minimum
   version of the PPSPP protocol it supports in the list of protocol
   options, following the Min/max versioning scheme defined in
   [RFC6709], Section 4.1.  The code for this option is 1.  Defined
   values are listed in Table 3.

7.4.  Swarm Identifier

   When a peer initiates the handshake it MUST include a swarm
   identifier option.  In other cases a peer MAY include a swarm
   identifier option, as an end-to-end check.  This option has the
   following structure:

   +------+-------------+------------------+
   | Code |    Length   | Swarm Identifier |
   +------+-------------+------------------+
   |   2  | n (16 bits) |     i1,i2,...    |
   +------+-------------+------------------+

   Each PPSPP peer knows the IDs of the swarms it joins so this
   information can be immediately verified upon receipt.  The length
   field is 2 octets to allow for large public keys as identifiers in
   live streaming.




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7.5.  Content Integrity Protection Method

   A peer MUST include the content integrity method used by a swarm,
   unless it uses the default, in which case it MAY include the method.
   The code for this option is 3.  Defined values are listed in Table 4.

                   +--------+-------------------------+
                   | Method | Description             |
                   +--------+-------------------------+
                   | 0      | No integrity protection |
                   | 1      | Merkle Hash Trees       |
                   | 2      | Sign All                |
                   | 3      | Unified Merkle Tree     |
                   | 4-255  | Unassigned              |
                   +--------+-------------------------+

          Table 4: PPSP Peer Content Integrity Protection Methods

   The "Merkle Hash Trees" method is for static content, see
   Section 5.1.  "Sign All", and "Unified Merkle Tree" are for live
   content, see Section 6.1.

   The veracity of this information will come out when the receiver
   successfully verifies the first chunk from any peer.

7.6.  Merkle Tree Hash Function

   When the content integrity protection method is Merkle Hash Trees
   this option defining which hash function is used for the tree MUST
   also be defined.  The code for this option is 4.  Defined values are
   listed in Table 5 (see [FIPS180-3] for the function semantics).

                        +----------+-------------+
                        | Function | Description |
                        +----------+-------------+
                        | 0        | SHA1        |
                        | 1        | SHA-224     |
                        | 2        | SHA-256     |
                        | 3        | SHA-384     |
                        | 4        | SHA-512     |
                        | 5-255    | Unassigned  |
                        +----------+-------------+

             Table 5: PPSP Peer Protocol Merkle Hash Functions

   The veracity of this information will come out when the receiver
   successfully verifies the first chunk from any peer.




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7.7.  Live Signature Algorithm

   When the content integrity protection method is "Sign All" or
   "Unified Merkle Tree" this option MUST also be defined.  The code for
   this option is 5.  The 8-bit value of this option is one of the
   Domain Name System Security (DNSSEC) Algorithm Numbers
   [IANADNSSECALGNUM].  If necessary, the key size that impacts
   signature length can be derived from the swarm identifier which is
   the signing public key in live streaming.

   The veracity of this information will come out when the receiver
   successfully verifies the first chunk from any peer.

7.8.  Chunk Addressing Method

   A peer MUST include the chunk addressing method it uses, unless it
   uses the default, in which case it MAY include the method.  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: PPSP Peer Chunk Addressing Methods

   The veracity of this information will come out when the receiver
   parses the first message containing a chunk specification from any
   peer.

7.9.  Live Discard Window

   A peer in a live swarm MUST include the discard window it uses.  The
   unit of the discard window depends on the chunk addressing method
   used.  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 list of protocol options MUST therefore contain
   the Chunk Addressing Method option and the method option MUST appear
   before the window option in the list.  This option has the following
   structure:



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   +------+-------------------+
   | Code |       Window      |
   +------+-------------------+
   |   7  | w (32 or 64-bits) |
   +------+-------------------+

   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 asset after the
   broadcast ends use these values (see Section 6.1.2).

   The veracity of this information does not impact a receiving peer
   more than when a sender peer just does not respond to REQUEST
   messages.

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 value of this option is a length octet
   and a 256-bit bitmap where each bit represents a message type.  The
   bitmap may be truncated to the last non-zero byte.

   +------+------------+----------------+
   | Code |   Length   | Message Bitmap |
   +------+------------+----------------+
   |   8  | n (8-bits) |    m1,m2,...   |
   +------+------------+----------------+


8.  UDP Encapsulation

   Currently, PPSPP-over-UDP is the preferred deployment option.
   Effectively, UDP allows the use of IP with minimal overhead and it
   also allows userspace implementations.  LEDBAT is used 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.  LEDBAT may
   be replaced with a different algorithm when the work of the IETF
   working group on RTP Media Congestion Avoidance Techniques (RMCAT)
   [RMCATCHART] matures.






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8.1.  Chunk Size

   In general, an UDP datagram containing PPSPP messages SHOULD fit
   inside a single IP packet, so its maximum size depends on the MTU of
   the network.  The default is to use fixed-sized chunks of 1 kilobyte
   such that a UDP datagram with a DATA message can be transmitted as a
   single IP packet over an Ethernet network with 1500-byte frames.
   PPSPP implementations can use larger chunk sizes.  For example, on
   CPU-limited hardware 8 kilobyte chunks MAY be used, transported as a
   single UDP datagram fragmented over multiple IP packets (with the
   increased chance of that UDP datagram getting lost).  The chunk
   addressing schemes can all work with different chunk sizes, see
   Section 4.

   The chunk size used for a particular swarm MUST be part of the
   swarm's metadata (which then consists of the swarm ID and the chunk
   size), unless it is the 1 KB default.

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:


























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                      +----------+------------------+
                      | 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: PPSP Peer Protocol Message Types

   Furthermore, integers are serialized in the network (big-endian) byte
   order.  So consider the example of a HAVE message (Section 3.2) using
   bin chunk addressing.  It has message type of 0x02 and a payload of a
   bin number, a four-byte integer (say, 1); hence, its on the wire
   representation for UDP can be written in hex as: "0200000001".

   All messages are idempotent or recognizable as duplicates.  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 and be
   dealt with.

8.3.  Channels

   As it is increasingly complex for peers to enable UDP communication
   between each other due to NATs and firewalls, PPSPP-over-UDP uses a
   multiplexing scheme, called "channels", to allow multiple swarms to
   use the same UDP port.  Channels loosely correspond to TCP
   connections and each channel belongs to a single swarm.  To support
   channels, each datagram starts with four bytes corresponding to the
   receiving channel number.






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8.4.  HANDSHAKE

   A channel is established with a handshake.  To start a handshake, the
   initiating peer needs to know:

   1.  the IP address of a peer

   2.  peer's UDP port and

   3.  the swarm ID of the content (see Section 5.1 and Section 6).

   4.  the chunk size used, unless the 1 KB default

   To do the handshake the initiating peer sends a datagram that MUST
   start with an all 0-zeros channel number, followed by a HANDSHAKE
   message, whose payload is a locally unused channel number and a list
   of protocol options (see Section 7 for which options are required and
   recommended.)

   On the wire the datagram will look something like this:

       (CHANNEL) 00000000 HANDSHAKE 00000011 v=01 si=123...1234 ca=0 end

   (to unknown channel, handshake from channel 0x11 speaking protocol
   version 0x01, initiating a transfer of a file with a root hash
   123...1234 using bins for chunk addressing)

   The receiving peer MAY respond in which case the returned datagram
   MUST consist of the channel number from the sender's HANDSHAKE
   message, a HANDSHAKE message, whose payload is a locally unused
   channel number and a list of protocol options, followed by any other
   messages it wants to send.

   Peer's response datagram on the wire:

       (CHANNEL) 00000011 HANDSHAKE 00000022 v=01 protocol options end

   (peer to the initiator: use channel number 0x22 for this transfer and
   proto version 0x01.)

   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
   roundtrips, the first two datagrams MAY also contain some minor
   payload, e.g. a couple of HAVE messages roughly indicating the
   current progress of a peer or a REQUEST (see Section 3.7).  When
   receiving the third datagram, both peers have the proof they really



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   talk to each other; the three-way handshake is complete.

   A peer MAY explicit close a channel by sending a HANDSHAKE message
   that MUST contain an all 0-zeros channel number and a list of
   protocol options.  The list MUST be either empty or contain the
   maximum version number the sender supports, following the Min/max
   versioning scheme defined in [RFC6709], Section 4.1.

   On the wire:

       (CHANNEL) 00000022 HANDSHAKE 00000000 end

8.5.  HAVE

   A HAVE message (type 0x03) consists of a chunk specification that
   states that the sending peer has those chunks and successfully
   checked their integrity.  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 for bin 3 on the wire:

       HAVE 00000003

   (received and checked first four kilobytes of a file/stream)

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 unless one
   of the chunks is the last chunk and smaller than the chunk size.  As
   the 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 for bin 0, with timestamp 12345678, and some data on
   the wire:

       DATA 00000000 12345678 48656c6c6f20776f726c6421

   (This message accommodates an entire file: "Hello world!")







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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 an one-way delay sample.  The one-way delay
   sample is a 64-bit integer with microsecond accuracy, and is computed
   from the timestamp received from the previous DATA message containing
   the chunk being acknowledged following the LEDBAT specification.

   An ACK message for bin 2 with one-way delay 12345678 on the wire:

       ACK 00000002 12345678

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 for bin 0 with a SHA1 hash on the wire:

       INTEGRITY 00000000 1234123412341234123412341234123412341234

8.9.  SIGNED_INTEGRITY

   A SIGNED_INTEGRITY message (type 0x07) consists of a chunk
   specification and a digital signature encoded as in DNSSEC without
   the BASE-64 encoding [RFC4034].  The signature algorithm is defined
   by the Live Signature Algorithm protocol option, see Section 7.7.

8.10.  REQUEST

   A REQUEST message (type 0x08) consists of a chunk specification for
   the chunks the requester wants to download.

8.11.  CANCEL

   A CANCEL message (type 0x09) consists of a chunk specification for
   the chunks the requester no longer is interested in.

8.12.  CHOKE and UNCHOKE

   Both CHOKE and UNCHOKE messages (types 0x0a and 0x0b, respectively)
   carry no payload.






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8.13.  PEX_REQ, PEX_RESv4, PEX_RESv6 and PEX_REScert

   A PEX_REQ (0x06) message has no payload.  A PEX_RES (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 sender of
   the PEX_REQ message does not have a private or link-local address,
   then the PEX_RES* messages MUST NOT contain such addresses
   [RFC1918][RFC4291].

   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.X208.1988].  The certificate MUST contain a
   "Subject Alternative Name" extension, marked as critical, of type
   uniformResourceIdentifier.

   The URL the name extension contains MUST follow the generic syntax
   for URLs [RFC3986], where its scheme component is "ppsp", 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:

       ppsp://192.168.0.1:6778/e5a12c7ad2d8fab33c699d1e198d66f79fa610c3

8.14.  KEEPALIVE

   Keepalives do not have a message type on UDP.  They are just simple
   datagrams consisting of a 4-byte channel number only.

   On the wire:

       (CHANNEL) 00000022

   A guideline for declaring a peer dead consist of a 3 minute delay
   since that last packet has been received from that peer, and at least
   3 datagrams were sent to that peer during the same period.







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8.15.  Flow and Congestion Control

   Explicit flow control is not necessary in PPSPP-over-UDP.  In the
   case of video-on-demand the receiver will request data explicitly
   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 bitrate, which
   the receiver must be able to process otherwise it cannot play the
   stream.  Should, for any reason, the receiver get saturated with data
   that situation is perfectly detected by the congestion control.
   PPSPP-over-UDP can support different congestion control algorithms.
   At present, it uses the LEDBAT congestion control algorithm
   [RFC6817].


9.  Extensibility

9.1.  Chunk Picking Algorithms

   Chunk (or piece) picking entirely depends on the receiving peer.  The
   sender 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 protocol 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, 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 sender
   peer.


10.  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



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   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.

   The PPSPP protocol was designed by Victor Grishchenko at Technische
   Universiteit Delft.  The authors would like to thank the following
   people for their contributions to this draft: the chairs and members
   of the IETF PPSP working group, and Mihai Capota, Raul Jimenez,
   Flutra Osmani, Johan Pouwelse, and Raynor Vliegendhart.


11.  IANA Considerations

   The new registries defined below are requested for the extensibility
   of the protocol.  The "Unassigned" ranges designated are governed by
   the policy 'RFC Required' as described in [RFC5226].

   o  PPSP Peer Protocol Message Type Registry, see Table 7.

   o  PPSP Peer Protocol Option Registry, see Table 2.

   o  PPSP Peer Protocol Version Number Registry, see Table 3.

   o  PPSP Peer Protocol Content Integrity Protection Method Registry,
      see Table 4.

   o  PPSP Peer Protocol Merkle Hash Tree Function Registry, see
      Table 5.

   o  PPSP Peer Protocol Chunk Addressing Method Registry, see Table 6.


12.  Security Considerations

   As any other network protocol, the 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 PPSP peer protocol may
   be attacked with 3 types of denial-of-service attacks:




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   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 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] (Sec. 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 0s.  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 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 A sends may already contain heavy payload,
   i.e., a chunk.  This next datagram to B will be prefixed with the
   chanB channel ID.  When 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] (Sec. 5.1)).

   A->B: chanB + HAVE + DATA + ...







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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 B pretending to be A now need to intercept the message from B to A
   to get 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 A included some HAVE messages, B may add a chunk
   availability map to A's state.  In addition, B may request some
   chunks from A in the second datagram, and 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 SCTP [RFC4960] (Sec. 5.1)
   is quicker response time for the user.  In the SCTP procedure, peer 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 shorter 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 B assigned to A, chanB.  Furthermore, attacker T would
   need to be able to spoof e.g.  REQUEST and HAVE messages from A to
   cause B to send heavy DATA messages to A, or prevent B from sending



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   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, a peer A can send a Peer-Exchange
   message PEX_RES to a peer B, which contains 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 anymore.  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 a PEX_RES to well-
   behaved peer A containing a list of address B1,B2,...,BN and 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 A's behaviour.  E.g. when 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_RES 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 A can check the cert for a valid signature, the
   right swarm and liveliness and only then consider contacting B. These
   swarm-membership certificates correspond to signed node descriptors



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   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
   PPSP tracker acts as certification authority.

12.2.2.  Example: Tracker as Certification Authority

   A 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 A.

   Peer A then includes this certificate when it sends a PEX_RES to peer
   B. Receiver B verifies it against the tracker public key.  This
   tracker public key should be part of the swarm's metadata, which B
   received from a trusted source.  Subsequently, peer B can send the
   member certificate of A to other peers in PEX_RES 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 T sends.  To counter this, 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



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   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 B with the tracker, and many peers joining the swarm will
   contact 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 PPSP tracker protocol specification,
   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 is 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, let 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, A can contact
   the tracker to find new candidates.

12.3.  Support for Closed Swarms (PPSP.SEC.REQ-1)

   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
   [I-D.gabrijelcic-ppsp-ecs].

12.4.  Confidentiality of Streamed Content (PPSP.SEC.REQ-2+3)

   No extra mechanism is needed to support confidentiality in PPSPP.  A
   content publisher wishing confidentiality should just distribute



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   content in cyphertext / DRM-ed format.  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, or by DTLS
   [RFC6347] or IPsec [RFC4301].

12.5.  Limit Potential Damage and Resource Exhaustion by Bad or Broken
       Peers (PPSP.SEC.REQ-4+6)

   In this section an analysis is given of the potential damage a
   malicious peer can do with each message in the protocol, and how it
   is prevented by the protocol (implementation).

12.5.1.  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 A by initiating the connection to A.

      Solution: Peer A must not let other peers fill all its available
      connection slots, i.e., A must initiate connections itself too, to
      prevent isolation.

12.5.2.  HAVE

   o  Threat HAVE.1: Malicious peer T can claim to have content which it
      hasn't.  Subsequently T won't respond to requests.

      Solution: peer A will consider 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.

12.5.3.  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.




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      To protect against this threat we need network-level DoS
      prevention.

12.5.4.  ACK

   o  Threat ACK.1: peer T acknowledges wrong chunks.

      Solution: peer A will detect inconsistencies with the data it sent
      to 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
      there is no congestion when in fact there is, causing 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.

12.5.5.  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 A will stop
      asking T because of the atomic datagram principle and the content
      integrity protection.  Subsequent unsolicited traffic from T will
      be ignored.

12.5.6.  REQUEST

   o  Threat REQUEST.1: peer T could request lots from A, leaving 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.

12.5.7.  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



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      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.

12.5.8.  CHOKE

   o  Threat CHOKE.1: peer T sends REQUEST messages after peer A sent B
      a CHOKE message.

      Solution: peer A will just discard the unwanted REQUESTs and
      resend the CHOKE, assuming it got lost.

12.5.9.  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 T to be a slow peer and not ask it
      again.

12.5.10.  PEX_RES

   o  Secured against amplification and Eclipse attacks as described in
      Section 12.2.

12.5.11.  Unsolicited Messages in General

   o  Threat: peer T could send a spoofed PEX_REQ or REQUEST from peer B
      to peer A, causing A to send a PEX_RES/DATA to B.

      Solution: the message from peer T won't be accepted unless T does
      a handshake first, in which case the reply goes to T, not victim
      B.

12.6.  Exclude Bad or Broken Peers (PPSP.SEC.REQ-5)

   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



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   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.X208.1988]
              International International Telephone and Telegraph
              Consultative Committee, "Specification of Abstract Syntax
              Notation One (ASN.1)", CCITT Recommendation X.208,
              November 1988.

   [FIPS180-3]
              Information Technology Laboratory,  National Institute of
              Standards and Technology, "Federal Information Processing
              Standards: Secure Hash Standard (SHS)", Publication 180-3,
              Oct 2008.

   [IANADNSSECALGNUM]
              IANA, "Domain Name System Security (DNSSEC) Algorithm
              Numbers",
              <http://www.iana.org/assignments/dns-sec-alg-numbers>.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.




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   [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, May 2008.

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              December 2012.

13.2.  Informative References

   [ABMRKL]   Bakker, A., "Merkle hash torrent extension", BitTorrent
              Enhancement Proposal 30, Mar 2009,
              <http://bittorrent.org/beps/bep_0030.html>.

   [BINMAP]   Grishchenko, V. and J. Pouwelse, "Binmaps: hybridizing
              bitmaps and binary trees", Technical Report PDS-2011-005,
              Parallel and  Distributed Systems Group, Fac. of
              Electrical Engineering,  Mathematics, and Computer
              Science, Delft University of Technology,  The Netherlands,
              Apr 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, Apr 2006.

   [BITTORRENT]
              Cohen, B., "The BitTorrent Protocol Specification",
              BitTorrent Enhancement Proposal 3, Feb 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, Oct 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, Nov 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.



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   [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,
              Aug 2011.

   [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, Jun 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, California, USA, Jan 2008.

   [HAC01]    Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
              of Applied Cryptography", CRC Press, (Fifth Printing,
              August 2001), Oct 1996.

   [I-D.gabrijelcic-ppsp-ecs]
              Gabrijelcic, D., "Enhanced Closed Swarm protocol",
              draft-ppsp-gabrijelcic-ecs (work in progress),
              November 2012.

   [I-D.ietf-ppsp-reqs]
              Williams, C., Xiao, L., Zong, N., Pascual, V., and Y.
              Zhang, "P2P Streaming Protocol (PPSP) Requirements",
              draft-ietf-ppsp-reqs-05 (work in progress), October 2011.

   [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, Aug 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.

   [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.



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   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5971]  Schulzrinne, H. and R. Hancock, "GIST: General Internet
              Signalling Transport", RFC 5971, October 2010.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6709]  Carpenter, B., Aboba, B., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              September 2012.

   [RMCATCHART]
              Eggert, L. and others, "RTP Media Congestion Avoidance
              Techniques (rmcat) Description of Working Group", 2012,
              <http://datatracker.ietf.org/wg/rmcat/charter/>.

   [SECDHTS]  Urdaneta, G., Pierre, G., and M. van Steen, "A Survey of
              DHT Security Techniques", ACM Computing Surveys vol.
              43(2), Jun 2011.

   [SIGMCAST]
              Wong, C. and S. Lam, "Digital Signatures for Flows and
              Multicasts", IEEE/ACM Transactions on Networking 7(4), pp.
              502-513, 1999.

   [SNP]      Ford, B., Srisuresh, P., and D. Kegel, "Peer-to-Peer
              Communication Across Network Address Translators",
              Feb 2005, <http://www.brynosaurus.com/pub/net/p2pnat/>.

   [SPS]      Jesi, G., Montresor, A., and M. van Steen, "Secure Peer
              Sampling", Computer Networks vol. 54(12), pp. 2086-2098,
              Elsevier, Aug 2010.

   [SWIFTIMPL]
              Grishchenko, V., Paananen, J., Pronchenkov, A., Bakker,
              A., and R. Petrocco, "Swift reference implementation",



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              2012, <https://github.com/triblerteam/libswift/>.

   [TIT4TAT]  Cohen, B., "Incentives Build Robustness in BitTorrent",
              1st Workshop on Economics of Peer-to-Peer
              Systems, Berkeley, CA, USA, Jun 2003.


Appendix A.  Revision History

   -00        2011-12-19 Initial version.

   -01        2012-01-30 Minor text revision:

       *   Changed heading to "A. Bakker"

       *   Changed title to *Peer* Protocol, and abbreviation PPSPP.

       *   Replaced swift with PPSPP.

       *   Removed Sec. 6.4.  "HTTP (as PPSP)".

       *   Renamed Sec. 8.4. to "Chunk Picking Algorithms".

       *   Resolved Ticket #3: Removed sentence about random set of
           peers.

       *   Resolved Ticket #6: Added clarification to "Chunk Picking
           Algorithms" section.

       *   Resolved Ticket #11: Added Sec. 3.12 on Storage Independence

       *   Resolved Ticket #14: Added clarification to "Automatic Size
           Detection" section.

       *   Resolved Ticket #15: Operation section now states it shows
           example behaviour for a specific set of policies and schemes.

       *   Resolved Ticket #30: Explained why multiple REQUESTs in one
           datagram.

       *   Resolved Ticket #31: Renamed PEX_ADD message to PEX_RES.

       *   Resolved Ticket #32: Renamed Sec 3.8. to "Keep Alive
           Signaling", and updated explanation.

       *   Resolved Ticket #33: Explained NAT hole punching via only
           PPSPP messages.




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       *   Resolved Ticket #34: Added section about limited overhead of
           the Merkle hash tree scheme.

   -02        2012-04-17 Major revision

       *   Allow different chunk addressing and content integrity
           protection schemes (ticket #13):

       *   Added chunk ID, chunk specification, chunk addressing scheme,
           etc. to terminology.

       *   Created new Sections 4 and 5 discussing chunk addressing and
           content integrity protection schemes, respectively and moved
           relevant sections on bin numbering and Merkle hash trees
           there.

       *   Renamed Section 4 to "Merkle Hash Trees and The Automatic
           Detection of Content Size".

       *   Reformulated automatic size detection in terms of nodes, not
           bins.

       *   Extended HANDSHAKE message to carry protocol options and
           created Section 8 on Protocol options.  VERSION and
           MSGTYPE_RCVD messages replaced with protocol options.

       *   Renamed HASH message to INTEGRITY.

       *   Renamed HINT to REQUEST.

       *   Added description of chunk addressing via (start,end) ranges.

       *   Resolved Ticket #26: Extended "Security Considerations" with
           section on the handshake procedure.

       *   Resolved Ticket #17: Defined recently as "in last 60 seconds"
           in PEX.

       *   Resolved Ticket #20: Extended "Security Considerations" with
           design to make Peer Address Exchange more secure.

       *   Resolved Ticket #38+39 / PPSP.SEC.REQ-2+3: Extended "Security
           Considerations" with a section on confidentiality of content.

       *   Resolved Ticket #40+42 / PPSP.SEC.REQ-4+6: Extended "Security
           Considerations" with a per-message analysis of threats and
           how PPSPP is protected from them.




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       *   Progressed Ticket #41 / PPSP.SEC.REQ-5: Extended "Security
           Considerations" with a section on possible ways of excluding
           bad or broken peers from the system.

       *   Moved Rationale to Appendix.

       *   Resolved Ticket #43: Updated Live Streaming section to
           include "Sign All" content authentication, and reference to
           [SIGMCAST] following discussion with Fabio Picconi.

       *   Resolved Ticket #12: Added a CANCEL message to cancel
           REQUESTs for the same data that were sent to multiple peers
           at the same time in time-critical situations.

   -03        2012-10-22 Major revision

       *   Updated Abstract and Introduction, removing download case.

       *   Resolved Ticket #4: Added explicit CHOKE/UNCHOKE messages.

       *   Removed directory lists unused in streaming.

       *   Resolved Ticket #22, #23, #28: Failure behaviour, error codes
           and dealing with peer crashes.

       *   Resolved Ticket #13: Chunk ranges are the default chunk
           addressing scheme that all peers MUST support.

       *   Added a section on compatibility between chunk addressing
           schemes.

       *   Expanded the explanation of Unified Merkle Trees as a method
           for content integrity protection for live streams.

       *   Added a section on forgetting chunks in live streaming.

       *   Added "End" option to protocol options and corrected bugs in
           UDP encapsulation, following Karl Knutsson's comments.

       *   Added SHA-2 support for Merkle Hash functions.

       *   Added content integrity protection methods for live streaming
           to the relevant protocol option.

       *   Added a Live Signature Algorithm protocol option.

       *   Resolved Ticket #24+27: The choice for UDP + LEDBAT as
           transport has now been reflected in the draft.  TCP and RTP



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           encapsulations have been removed.

       *   Superfluous parts of Section 10 on extensibility have been
           removed.

       *   Removed appendix with Rationale.

       *   Resolved Ticket #21+25: PPSPP currently uses LEDBAT and the
           DATA and ACK messages now contain the time fields it
           requires.  Should other congestion control algorithms be
           supported in the future, a protocol option will be added.

   -04        2012-11-07 Minor revision

       *   Corrected typos.

       *   Added empty protocol option list when HANDSHAKE is used for
           explicitly closing a channel in the UDP encapsulation.

       *   Corrected definition of a range chunk specification to be a
           single (start,end) pair.  To send multiple disjunct ranges
           multiple messages should be used.

       *   Clarified that in a range chunk specification the end is
           inclusive.  I.e., [start,end] not [start,end)

       *   Added PEX_REScert message to carry a membership certificate.
           Renamed PEX_RES to PEX_RESv4.

       *   Added a guideline about private and link-local addresses in
           PEX_RES messages.

       *   Defined the format of the public key that is used as swarm ID
           in live streaming.

       *   Clarified that a HANDSHAKE message must be the first message
           in a datagram.

       *   Clarified sending INTEGRITY messages ahead in a separate
           datagram if not all necessary hashes that still need to be
           sent and the chunk fit into a single datagram.  Defined an
           order for the INTEGRITY messages.

       *   Clarified rare case of sending multiple DATA messages in one
           datagram.

       *   Clarified UDP datagrams carrying PPSPP should adhere to the
           network's MTU to avoid IP fragmentation.



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       *   Defined value for version protocol option.

       *   Added small clarifications and corrected typos.

       *   Extended versioning scheme to Min/max versioning scheme
           defined in [RFC6709], Section 4.1, following Riccardo
           Bernardini's suggestion.

       *   Processed comments on unclear phrasing from Riccardo
           Bernardini.

       *   Added a guideline on when to declare a peer dead.

       *   Made sure all essential references are listed as Normative
           references following RFC3967.

   -05        2012-11-07 Minor revision

       *   Corrected category to Standards Track.

       *   Clarified that swarm identifier is a required protocol option
           in an initiating HANDSHAKE in the UDP encapsulation.

       *   Added IANA considerations and tablised name spaces for
           registry definition.


Authors' Addresses

   Arno Bakker
   Technische Universiteit Delft
   Mekelweg 4
   Delft,   2628CD
   The Netherlands

   Phone:
   Email: arno@cs.vu.nl


   Riccardo Petrocco
   Technische Universiteit Delft
   Mekelweg 4
   Delft,   2628CD
   The Netherlands

   Phone:
   Email: r.petrocco@gmail.com




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   Victor Grishchenko
   Technische Universiteit Delft
   Mekelweg 4
   Delft,   2628CD
   The Netherlands

   Phone:
   Email: victor.grishchenko@gmail.com











































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