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Versions: 00 01 draft-ietf-ppsp-base-tracker-protocol

PPSP                                                         Rui S. Cruz
INTERNET-DRAFT                                            Mario S. Nunes
Intended Status: Standards Track                       IST/INESC-ID/INOV
Expires: April 24, 2013                                       Yingjie Gu
                                                              Jinwei Xia
                                                                  Huawei
                                                         Joao P. Taveira
                                                                IST/INOV
                                                             Deng Lingli
                                                            China Mobile
                                                        October 21, 2012


          PPSP Tracker Protocol--Base Protocol (PPSP-TP/1.0)
                draft-cruz-ppsp-base-tracker-protocol-01


Abstract

   This document specifies the base Peer-to-Peer Streaming Protocol-
   Tracker Protocol (PPSP-TP/1.0), an application-layer control
   (signaling) protocol for the exchange of meta information between
   trackers and peers.  The specification outlines the architecture of
   the protocol and its functionality, and describes message flows,
   message processing instructions, message formats, formal syntax and
   semantics.  The PPSP Tracker Protocol enables cooperating peers to
   form content streaming overlay networks to support near real-time
   Structured Media content (audio, video, associated timed text and
   metadata) delivery, such as adaptive multi-rate, layered (scalable)
   and multi-view (3D), in live, time-shifted and on-demand modes.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html



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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html


Copyright and License Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document. 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.

































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Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1. Use Scenarios and Streaming Modes . . . . . . . . . . . . .  4
     1.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2.1. Enrollment and Bootstrap  . . . . . . . . . . . . . . .  6
       1.2.2. NAT Traversal . . . . . . . . . . . . . . . . . . . . .  7
       1.2.3. Content Information Metadata  . . . . . . . . . . . . .  8
       1.2.4. Authentication, Confidentiality, Integrity  . . . . . .  8
   2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3. Architectural and Functional View . . . . . . . . . . . . . . . 11
   4. Messaging Model . . . . . . . . . . . . . . . . . . . . . . . . 13
   5. Request/Response model  . . . . . . . . . . . . . . . . . . . . 13
   6. State Machines and Flows of the Protocol  . . . . . . . . . . . 14
     6.1. Normal Operation  . . . . . . . . . . . . . . . . . . . . . 16
     6.2. Error Conditions  . . . . . . . . . . . . . . . . . . . . . 17
   7. Protocol Specification  . . . . . . . . . . . . . . . . . . . . 17
     7.1. Request/Response Syntax and Format  . . . . . . . . . . . . 17
     7.2. Semantics of PPSPTrackerProtocol elements . . . . . . . . . 20
     7.3. Request element in request Messages . . . . . . . . . . . . 22
     7.4. Response element in response Messages . . . . . . . . . . . 23
   8. Request/Response Processing . . . . . . . . . . . . . . . . . . 24
     8.1. CONNECT Request . . . . . . . . . . . . . . . . . . . . . . 24
     8.2. STAT_REPORT Request . . . . . . . . . . . . . . . . . . . . 28
     8.3. Error and Recovery conditions . . . . . . . . . . . . . . . 29
   9. Security Considerations . . . . . . . . . . . . . . . . . . . . 30
     9.1. Authentication between Tracker and Peers  . . . . . . . . . 30
     9.2. Content Integrity protection against polluting
          peers/trackers  . . . . . . . . . . . . . . . . . . . . . . 31
     9.3. Residual attacks and mitigation . . . . . . . . . . . . . . 31
     9.4. Pro-incentive parameter trustfulness  . . . . . . . . . . . 31
   10. Guidelines for Extending PPSP-TP . . . . . . . . . . . . . . . 32
     10.1. Forms of PPSP-TP Extension . . . . . . . . . . . . . . . . 33
     10.2. Issues to Be Addressed in PPSP-TP Extensions . . . . . . . 34
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 35
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     13.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Appendix A. PPSP-TP Message Syntax for HTTT/1.1  . . . . . . . . . 39
     A.1. Header Fields . . . . . . . . . . . . . . . . . . . . . . . 40
     A.2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     A.3. Message Bodies  . . . . . . . . . . . . . . . . . . . . . . 40
     A.4. Message Response Codes  . . . . . . . . . . . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43






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1. Introduction

   The Peer-to-Peer Streaming Protocol (PPSP) is composed of two
   protocols: the PPSP Tracker Protocol and the PPSP Peer Protocol
   [I-D.ietf-ppsp-problem-statement] specifies that the Tracker Protocol
   should standardize format/encoding of information and messages
   between PPSP peers and PPSP trackers and also defines the
   requirements.

   The PPSP Tracker Protocol provides communication between trackers and
   peers, by which peers send meta information to trackers, report
   streaming status and obtain peer lists from trackers.

   The PPSP architecture requires PPSP peers able to communicate with a
   tracker in order to participate in a particular streaming content
   swarm.  This centralized tracker service is used by PPSP peers for
   content registration and location.

   The signaling and the media data transfer between PPSP peers is not
   in the scope of this specification.

   This document describes the base PPSP Tracker protocol and how it
   satisfies the requirements for the IETF Peer-to-Peer Streaming
   Protocol, in order to derive the implications for the standardization
   of the PPSP streaming protocols and to identify open issues and
   promote further discussion.

1.1. Use Scenarios and Streaming Modes

   This section is tutorial in nature and does not contain any normative
   statements.

   This section describes some aspects of the use of PPSP-TP. The
   examples were chosen to illustrate the basic operation, but not to
   limit what PPSP-TP may be used for.

   The functional entities related to PPSP protocols are the Client
   Media Player, the service Portal, the tracker and the peers.  The
   complete description of these entities is not discussed here, as not
   in the scope the specification.

   The Client Media Player is a logical entity providing direct
   interface to the end user at the client device, and includes the
   functions to select, request, decode and render contents.  The Client
   Media Player may interface with the local peer application using
   request and response standard formats for HTTP Request and Response
   messages [RFC2616].




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   The service Portal is a logical entity typically used for client
   enrollment and content information publishing, searching and
   retrieval.

   The tracker is a logical entity that maintains the lists of PPSP
   active peers storing and exchanging a specific media content.  The
   tracker also stores the status of active peers in swarms, to help in
   the selection of appropriate peers for a requesting peer.  The
   tracker can be realized by geographically distributed tracker nodes
   or multiple server nodes in a data center, increasing the content
   availability, the service robustness and the network scalability or
   reliability.  The management and locating of content index
   information are totally internal behaviors of the tracker system,
   which is invisible to the PPSP Peer.

   The peer is also a logical entity in the client device embedding the
   P2P core engine, with a client serving side interface to respond to
   Client Media Player requests and a network side interface to exchange
   data and PPSP signaling with trackers and other peers.

   The streaming technique is chunk-based, i.e., client peers obtain
   media chunks from serving peers and handle the buffering that is
   necessary for the playback processes during the download of the media
   chunks.

   In Live streaming, all end users are interested in a specific media
   coming from an ongoing program, which means that all respective peers
   share nearly the same streaming content at a given point of time.
   Peers may store the live media for further distribution (known as
   time-shift TV), where the stored media is distributed in a VoD-like
   manner.

   In VoD, different end users watch different parts of the recorded
   media content during a past event.  In this case, each respective
   peer obtains from other peers the information on media chunks they
   store and then get the required media from a selected set of those
   peers.  While watching VoD, an end user can also switch to any place
   of the content, e.g., skip the credits part, or skip the part that it
   is not interested in.  In this case the respective participating peer
   may not store all the content segments.  From the whole swarm point
   of view, the participating peers typically store different parts of
   content.

1.2. Assumptions

   This section is tutorial in nature and does not contain any normative
   statements.




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   The process used for media streaming distribution assumes a segment
   (chunk) transfer scheme whereby the original content (that can be
   encoded using adaptive or scalable techniques) is chopped into small
   segments having the following representations:

   1. Adaptive - alternate representations with different qualities and
      bitrates; a single representation is non-adaptive;
   2. Scalable description levels - multiple additive descriptions
      (i.e., addition of descriptions refine the quality of the video);
   3. Scalable layered levels - nested dependent layers corresponding to
      several hierarchical levels of quality, i.e., higher enhancement
      layers refine the quality of the video of lower layers.
   4. Scalable multiple views - views correspond to mono (2D) and
      stereoscopic (3D) videos, with several hierarchical levels of
      quality.

   These streaming distribution techniques support dynamic variations in
   video streaming quality while ensuring support for a plethora of end
   user devices and network connections.

1.2.1. Enrollment and Bootstrap

   In order to join an existing P2P streaming service and to participate
   in content sharing, any peer must first locate a tracker, using for
   example, the following methods (illustrated in Figures 1, 2):

   +--------+      +--------+     +--------+    +---------+  +--------+
   | Player |      | Peer 1 |     | Portal |    | Tracker |  | Peer 2 |
   +--------+      +--------+     +--------+    +---------+  +--------+
       |                |               |              |           |
   (a) |--Page request----------------->|              |           |
       |<--------------Page with links--|              |           |
       |--Select stream (MPD Request)-->|              |           |
       |<--------------------OK+MPD(x)--|              |           |
   (b) |--Start/Resume->|--CONNECT(join x)------------>|           |
       |<-----------OK--|<----------------OK+Peerlist--|           |
       |                |                              |           |
       |--Get(Chunk)--->|<---------- (Peer protocol) ------------->|
       |<--------Chunk--|<---------------------------------Chunks--|
       :                :               :              :           :
       |                |--STAT_REPORT---------------->|           |
       |                |<-------------------------Ok--|           |
       :                :               :              :           :
       |--Get(Chunk)--->|<---------- (Peer protocol) ------------->|
       |<--------Chunk--|<---------------------------------Chunks--|
       :                :               :              :           :

   Figure 1: A typical PPSP session for watching a streaming content.



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              +-------------+                   +---------+
              | Peer Seeder |                   | Tracker |
              +-------------+                   +---------+
                     |                               |
              Start->|--CONNECT (join x,y,z)-------->|
                     |<--------------------------OK--|
                     :                               :
                     |                               |
                     |--STAT_REPORT----------------->|
                     |<--------------------------Ok--|
                     :                               :
                     |                               |
                     |--STAT_REPORT----------------->|
                     |<--------------------------Ok--|
                     :                               :

        Figure 2: A typical PPSP session for a streaming Seeder

   1. From a content service provider provisioning mechanism: this is
      the typical case used for the provider Seeders (edge caches and/or
      Media Servers), where some "Start" activation signal triggers the
      process for contents x, y, z (Figure 2).
   2. From a web page: a Publishing and Searching Portal may provide
      tracker location information to end users.
   3. From the Media Presentation Description (MPD) file of a content:
      this meta-info file must contain information about the address of
      one or more trackers (that can be grouped by tiers of priority)
      which are controlling the swarm for that media content, as
      illustrated in Figure 1 for a content x.

   As illustrated in Figure 1, a peer may initiate a stream using the
   above method 3 starting at point (a), or resume a previously
   initiated stream, using also method 3 but starting at point (b).

   In order to be able to bootstrap, a peer must first obtain a Peer-ID
   (identifier of the peer) and any required security certificates or
   authorization tokens from an enrollment service (end user
   registration).  The specification of the format of the Peer-ID is not
   in the scope of this document.

   The specification of the mechanisms used by the Client Media Player
   and the peer (to signal start/resume streams or request media
   chunks), obtain a Peer-ID, security certificates or tokens are not in
   the scope of this document.

1.2.2. NAT Traversal

   It is assumed that all trackers must be in the public Internet and



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   have been placed there deliberately.  This document will not describe
   NAT Traversal mechanisms but the protocol facilitates flexible NAT
   Traversal techniques, such as those based on ICE [RFC5245],
   considering that the tracker node may provide NAT traversal services,
   as a STUN-like tracker. Being a STUN-like tracker, it can discover
   the reflexive candidate addresses of a peer and make them available
   in responses to other requesting peers.

1.2.3. Content Information Metadata

   Multimedia contents may consist of several media components (for
   example, audio, video, and timed text), each of which might have
   different characteristics.

   The representations of a media content correspond to encoded
   alternatives of the same media component, varying from other
   representations by bitrate, resolution, number of channels, or other
   characteristics. Each representation consists of one or multiple
   segments.  Segments are the media stream transport chunks in temporal
   sequence.

   These characteristics may be described in a Media Presentation
   Description (MPD) file. It is envisioned that the content information
   metadata used in PPSP may align with MPD formats, such as ISO/IEC
   23009-1 [ISO.IEC.23009-1] and [I-D.pantos-http-live-streaming].

1.2.4. Authentication, Confidentiality, Integrity

   Channel-oriented security can be used in the communication between
   peers and tracker, such as the Transport Layer Security (TLS) to
   provide privacy and data integrity.  HTTP/1.1 over TLS (HTTPS)
   [RFC2818] is the preferred approach for preventing disclosure of peer
   critical information via the communication channel.

   Due to the transactional nature of the communication between peers
   and tracker a method for adding authentication and data security
   services via replaceable mechanisms may be employed.  One such method
   is the OAuth 2.0 Authorization [RFC6749] with bearer token, providing
   the peer with the information required to successfully utilize the
   access token to make protected requests to the tracker [RFC6750].

2. Terminology

   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].

   This draft uses terms defined in [I-D.ietf-ppsp-problem-statement].



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   Absolute Time:  Absolute time is expressed as ISO 8601
   [ISO.8601.2004] timestamps, using zero UTC offset (GMT).  Fractions
   of a second may be indicated. Example for December 25, 2010 at 14h56
   and 20.25 seconds: basic format 20101225T145620.25Z or extended
   format 2010-12-25T14:56:20.25Z.

   Adaptive Streaming:  Multiple alternate representations (different
   qualities and bitrates) of the same media content co-exist for the
   same streaming session;  each alternate representation corresponds to
   a different media quality level;  peers can choose among the
   alternate representations for decode and playback.

   Base Layer:  The playable representation level in Scalable Video
   Coding (SVC) required by all upper level Enhancements Layers for
   proper decoding of the video.

   Chunk:  A chunk is a basic unit of data block organized in P2P
   streaming for storage, scheduling, advertisement and exchange among
   peers.  A chunk therefore refers to a segment of partitioned
   streaming media.

   Complementary Representation:  Representation in a set of content
   representations which have inter-representation dependencies and
   which, when combined, result in a single representation for decoding
   and presentation.

   Connection Tracker:  The tracker node to which the PPSP peer will
   connect when it wants to get registered and join the PPSP system.

   Continuous media:  Media with an inherent notion of time, for
   example, speech, audio, video, timed text or timed metadata.

   Enhancement Layer:  Enhancement differential quality level
   (complementary representation) in Scalable Video Coding (SVC) used to
   produce a higher quality, higher definition video in terms of space
   (i.e., image resolution), time (i.e., frame rate) or Signal-to-Noise
   Ratio (i.e., fidelity) when combined with the playable Base Layer
   [ITU-T.H.264].

   Join Time:  Join time is the absolute time when a peer registers on a
   tracker.  This value is recorded by the tracker and is used to
   calculate Online Time.

   Leecher:  A Peer that has not yet completed the transfer of all
   chunks of the media content.

   Live streaming:  The scenario where all clients receive streaming
   content for the same ongoing event.  The lags between the play points



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   of the clients and that of the streaming source are small.

   Media Component:  An encoded version of one individual media type
   such as audio, video or timed text with specific attributes, e.g.,
   bandwidth, language, or resolution.

   Media Presentation Description (MPD):  Formalized description for a
   media presentation, i.e., describes the structure of the media,
   namely, the representations, the codecs used, the segments (chunks),
   and the corresponding addressing scheme.

   Method:  The method is the primary function that a request from a
   peer is meant to invoke on a tracker.  The method is carried in the
   request message itself.

   Online Time:  Online Time shows how long the peer has been in the P2P
   streaming system since it joins.  This value indicates the stability
   of a peer, and can be calculated by tracker when necessary.

   Peer:  A peer refers to a participant in a P2P streaming system that
   not only receives streaming content, but also stores and uploads
   streaming content to other participants.

   Peer-ID:  Unique identifier for the peer.  The Peer-ID is mandatory,
   can take the form of a universal unique identifier (UUID), defined in
   [RFC4122], and can be bound to a network address of the peer, i.e.,
   an IP address uniform resource identifier/locator (URI/URL) that
   uniquely identifies the corresponding peer in the network.  The Peer-
   ID and any required security certificates are obtained from an
   offline enrollment server.

   Peer-Peer Messages (i.e., Peer Protocol): The Peer Protocol messages
   enable each peer to exchange content availability with other peers
   and request other peers for content.

   PPSP:  The abbreviation of Peer-to-Peer Streaming Protocols. PPSP
   protocols refer to the key signaling protocols among various P2P
   streaming system components, including the tracker and peers.

   Representation: Structured collection of one or more media
   components.

   Request:  A message sent from a peer to a tracker, for the purpose of
   invoking a particular operation.

   Response:  A message sent from a tracker to a peer, for indicating
   the status of a request sent from the peer to the tracker.




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   Scalable Streaming:  With Multiple Description Coding (MDC), multiple
   additive descriptions (that can be independently played-out) to
   refine the quality of the video when combined together.  With
   Scalable Video Coding (SVC), nested dependent enhancement layers
   (hierarchical levels of quality), refine the quality of lower layers,
   from the lowest level (the playable Base Layer).  With Multiple View
   Coding (MVC), multiple views allow the video to be played in 3D when
   the views are combined together.

   Seeder:  A Peer that holds and shares the complete media content.

   Segment:  A segment is a resource that can be identified, by an ID or
   an HTTP-URL and possibly a byte-range, and is included in the MPD.
   The segment is a basic unit of partitioned streaming media, which is
   used by a peer for the purpose of storage, advertisement and exchange
   among peers.

   Swarm:  A swarm refers to a group of peers sharing the same content
   (e.g., video/audio program, digital file, etc.) at a given time.

   Swarm-ID:  Unique identifier for a swarm. The Swarm-ID may use a
   universal unique identifier (UUID), e.g., a 64 or 128 bit datum to
   refer to the content resource being shared among peers.

   Tracker:  A tracker refers to a centralized logical directory service
   used to communicate with PPSP Peers for content registration and
   location, which maintains the lists of PPSP peers storing segments
   for a specific live content channel or streaming media and answers
   queries from PPSP peers.

   Tracker-Peer Messages (i.e., Tracker Protocol):  The Tracker Protocol
   messages provide communication between peers and trackers, by which
   peers can provide content availability, report streaming status and
   request peer lists from trackers.

   Video-on-demand (VoD):  A kind of application that allows users to
   select and watch video content on demand.

3. Architectural and Functional View

   The PPSP Tracker Protocol architecture is intended to be compatible
   with the web infrastructure. PPSP-TP is designed with a layered
   approach i.e., a PPSP-TP Request/Response layer, a Messaging layer
   and a Transport layer.

   The PPSP-TP Request/Response layer deals with the interactions
   between tracker and peers using Request and Response codes (see
   Figure 3).



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   The messaging layer deals with the framing format, for encoding and
   transmitting the data through the underlying transport protocol, and
   the asynchronous nature of the interactions between tracker and
   peers.

   The transport layer is responsible for the actual transmission of
   requests and responses over network transports, including the
   determination of the connection to use for a request or response
   message when using a connection-oriented transport like TCP, or TLS
   [RFC5246] over it.

                    +----------------------+
                    |      Application     |
                    +----------------------+
                    |  Request/Response    |  PPSP-TP
                    |----------------------|
                    |   (HTTP)Messages     |
                    +----------------------+
                    |       TRANSPORT      |
                    +----------------------+

               Figure 3: Abstract layering of PPSP-TP

   The functional entities involved in the PPSP Tracker Protocol are
   trackers and peers (which may support different capabilities).

   A Peer corresponds to a logical entity (typically in a user device)
   that actually participate in sharing a media content.  Peers are
   organized in (various) swarms corresponding each swarm to the group
   of peers streaming a certain content at any given time.

   The tracker is a logical entity that maintains the lists of peers
   storing segments for a specific Live media channel or on-demand media
   streaming content, answers queries from peers and collects
   information on the activity of peers.  While a tracker may have an
   underlying implementation consisting of more than one physical nodes,
   logically the tracker can most simply be thought of as a single
   element, and in this document it will be treated as a single logical
   entity.

   The Tracker Protocol is not used to exchange actual content data
   (either on-demand or Live streaming) with peers, but information
   about which peers can provide the content.

   When a peer wants to receive streaming of a selected content:

   1. Peer connects to a local connection tracker and joins a swarm.
   2. Peer acquires a list of peers from the connection tracker.



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   3. Peer exchanges its content availability with the peers on the
      obtained peer list (via peer protocol).
   4. Peer identifies the peers with desired content.
   5. Peer requests content from the identified peers (peer protocol).

   When a peer wants to share streaming contents (Seeder mode) with
   other peers:

   1. Peer connects to the connection tracker.
   2. Peer sends information to the connection tracker about the swarms
      it belongs to (joins).

   After having been disconnected due to some termination condition, a
   peer can resume previous activity by connecting and re-joining the
   corresponding swarm(s).

4. Messaging Model

   The messaging model of PPSP-TP aligns with HTTP protocol and the
   semantics of its messages, currently in version 1.1 [RFC2616], but
   intended to support future versions of HTTP and framing formats used
   for encoding and transmitting the data over the wire, e.g., the
   format proposed in [I-D.montenegro-httpbis-speed-mobility].  The
   exchange of messages of PPSP-TP is envisioned to be performed over a
   stream-oriented reliable transport protocol, like TCP.

   Appendix A describes the message syntax when using HTTT/1.1 messages.

5. Request/Response model

   PPSP-TP is a text-based protocol, uses the UTF-8 character set
   [RFC3629] and the protocol messages are either requests from client
   peers to a tracker server, or responses from a tracker server to
   client peers.

   PPSP-TP Request and Response semantics are carried as entities
   (header and body) in messages which correspond to either HTTP request
   methods or HTTP response codes, respectively.

   Requests are sent, and responses returned to these requests.  A
   single request generates a single response (neglecting fragmentation
   of messages in transport).

   The response codes are consistent with HTTP response codes, however,
   not all HTTP response codes are used for the PPSP-TP (section 7).

   The Request Messages of the base protocol, are listed in Table 1:




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                         +---------------+
                         | PPSP-TP/1.0   |
                         | Req. Messages |
                         +---------------+
                         | CONNECT       |
                         | STAT_REPORT   |
                         +---------------+
                    Table 1: Request Messages

   CONNECT: This Request message is used when a peer registers (or if
   already registered) in the tracker notifies it about the
   participation in named swarm(s).  The tracker records the Peer-ID,
   connect-time (referenced to the absolute time), peer IP addresses and
   link status.  The tracker also changes the content availability of
   the valid named swarm(s), i.e., changes the peers lists of the
   corresponding swarm(s) for the requester Peer-ID.  On receiving a
   CONNECT message, the tracker first checks the peer mode type
   (Seed/Leech) for the specified swarm(s) and then decides the next
   steps (more details are referred in section 8.1)

   STAT_REPORT: This Request message allows an active peer to send
   status data to the tracker to signal continuing activity. This
   request message MUST be sent periodically to the tracker while the
   peer is active in the system.

6. State Machines and Flows of the Protocol

   The state machine for the tracker is very simple, as shown in
   Figure 4.

   Peer-ID registrations represent a dynamic piece of state maintained
   by the network.

               --------------------------------------------
              /                                            \
             |  +------------+    +---------+    +------+   |
              \-| TERMINATED |<---| STARTED |<---| INIT |<-/
                +------------+    +---------+    +------+
                                                    \- (start tracker)
                    Figure 4: Tracker State Machine

   When there are no peers connected in the tracker, the state machine
   is in the INIT state.

   When the "first" peer connects for registration with its Peer-ID, the
   state machine moves from INIT to STARTED.

   As long as there is at least one active registration of a Peer-ID,



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   the state machine remains in the STARTED state.  When the "last"
   Peer-ID is removed, the state machine transitions to TERMINATED.
   From there, it immediately transitions back to the INIT state.
   Because of that, the TERMINATED state here is transient.

   In addition to the tracker state machine, each Peer-ID is modeled
   with its own transaction state machine (Figure 5), instantiated per
   Peer-ID in the tracker, and deleted when it is removed.

               --------------------------------------------
              /                                            \
             |  +------------+    +---------+    +------+   |
              \-| TERMINATED |<---| STARTED |<---| INIT |<-/
                +------------+    +---------+    +------+
                                       | (1)        \- (start tracker)
                                       V
                   +-----------+   +-------+  rcv CONNECT
                   | TERMINATE |   | START |  --------------- (1)
                   +-----------+   +-------+  strt init timer
                         ^             |
                         |             |
   on registration error |             v
   on action error       |      +------------+
   ---------------- (A)  +<-----| PEER       | (Transient)
   stop init timer       |      | REGISTERED |
   snd error             |      +------------+
                         |            |
                         |            |   process swarm actions
                         |            |   --------------------- (2)
   on CONNECT Error (B)  |            |   snd OK (PeerList)
   on timeout       (5)  |           /    stop init timer
   ----------------      |          /     strt track timer
   stop track timer      |         /
   clean peer info       |        |
   del registration      |        |
   snd error (B)          \       |     ----
                    ----   \      |   /      \
                  /      \  \     |  |        |
   rcv CONNECT   |  (4)   |  |    |  |        |
   -----------   |        v  |    v  v        | rcv STAT_REPORT
   snd OK         \     +--------------+     /  --------------- (3)
   rst track timer  ----|   TRACKING   |----    snd OK response
                        +--------------+        rst track timer

     Figure 5: Per-Peer-ID Transaction State Machine and Flow Diagram

   Unlike the tracker state machine, which exists even when no Peer-IDs
   are registered, the "per-Peer-ID" transaction state machine is



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   instantiated only when the Peer-ID starts registration in the
   tracker, and is deleted when the Peer-ID is de-registered/removed.
   This allows for an implementation optimization whereby the tracker
   can destroy the objects associated with the "per-Peer-ID" transaction
   state machine once it enters the TERMINATE state (Figure 5).

   When a new Peer-ID is added, the corresponding "per-Peer-ID" state
   machine is instantiated, and it moves into the PEER REGISTERED state.
   Because of that, the START state here is transient.

   When the Peer-ID is no longer bound to a registration, the "per-Peer-
   ID" state machine moves to the TERMINATE state, and the state machine
   is destroyed.

   During the lifetime of streaming activity of a peer, the "per-Peer-
   ID" transaction state machine progresses from one state to another in
   response to various events.  The events that may potentially advance
   the state include:

      o  Reception of CONNECT and STAT_REPORT messages, or
      o  Timeout events.

   The state diagram in Figure 5 illustrates state changes, together
   with the causing events and resulting actions.  Specific error
   conditions are not shown in the state diagram.

6.1. Normal Operation

   On normal operation the process consists of the following steps:

   1) When a Peer wants to access the system it needs to register on a
      tracker by sending a CONNECT message asking for the swarm(s) it
      wants to join. This CONNECT request triggers a new "per-Peer-ID"
      State machine.  In the START state the tracker initiates the
      registration of the Peer-ID and associated information (IP
      addresses), starts the "init timer" and moves to PEER REGISTERED
      state.

   2) In PEER REGISTERED state, if Peer-ID is valid, the tracker
      processes the requested action(s) for the valid swarm information.
      In case of success the tracker stops the "init timer", starts the
      "track timer" and sends the response to the peer.  The response
      MAY contain the appropriate list of peers for the joining
      swarm(s), depending on peer mode (section 8.1).  At the PEER
      REGISTERED state, if Peer-ID is considered invalid, the tracker
      responds with either error codes 401 Unauthorized or 403 Forbidden
      (described in section 7), and transitions to TERMINATE state for
      that Peer-ID.



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   3) In TRACKING state, a STAT_REPORT message received from the peer
      resets the "track timer" and is responded with a successful
      condition.

   4) While TRACKING, a CONNECT message received with valid swarm
      actions information (section 8.1) from the peer resets the "track
      timer" and is responded with a successful condition.

   5) In TRACKING state, without receiving messages from the peer, on
      timeout (track timer) the tracker cleans all the information
      associated with the Peer-ID in all swarms it was joined, deletes
      the registration, and transitions to TERMINATE state for that
      Peer-ID.

6.2. Error Conditions

      Peers MUST NOT generate protocol elements that are invalid.
      However, several situations of a peer may lead to abnormal
      conditions in the interaction with the tracker.  The situations
      may be related with peer malfunction or communications errors.
      The tracker reacts to the abnormal situations depending on its
      current state related to a peer-ID, as follows:

   A) At PEER REGISTERED state, when a CONNECT Request only contains
      invalid swarm actions (section 8.1), the tracker responds with
      error code 403 Forbidden, deletes the registration and transition
      to TERMINATE state for that Peer-ID.

   B) At the TRACKING state (while the "track timer" has not expired)
      receiving a CONNECT message from the peer with invalid swarm
      actions (section 8.1) is considered an error condition.  The
      tracker responds with error code 403 Forbidden (described in
      section 7), stops the "track timer", deletes the registration and
      transitions to TERMINATE state for that Peer-ID.

      NOTE: These situations may correspond to a malfunction at the peer
      or to malicious conditions.  Therefore, the adequate preventive
      measure is to proceed to TERMINATE state for the Peer-ID by de-
      registering the peer and cleaning all peer information.

7. Protocol Specification

7.1. Request/Response Syntax and Format

   The message-body for Requests and Responses requiring it, correspond
   to a XML document [XML], optionally represented in a binary compact
   form using for example Efficient XML Interchange (EXI) Format [EXI
   Format].



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   The XML message-body MUST begin with an XML declaration line
   specifying the version of XML being used and indicating the character
   encoding, which SHOULD be "UTF-8".  The root element MUST begin with
   an <PPSPTrackerProtocol> element.  This element identifies the start
   of an PPSP-TP protocol element, the namespace used within the
   protocol, and the location of the protocol schema.

   The generic format of a Request is the following:

   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Request></Request>
     <TransactionID></TransactionID>
     <PeerID></PeerID>
     <SwarmID></SwarmID>
     <PeerNum></PeerNum>
     <PeerGroup></PeerGroup>
     <StatisticsGroup></StatisticsGroup>
   </PPSPTrackerProtocol>

   The generic format of a Response is the following:

   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Response></Response>
     <TransactionID></TransactionID>
     <PeerGroup></PeerGroup>
   </PPSPTrackerProtocol>

   The Request element MUST be present in requests and corresponds to
   the request method type for the message.

   The Response element MUST be present in responses and corresponds to
   the response method type of the message.

   The element TransactionID MUST be present in requests to uniquely
   identify the transaction.  Responses to completed transactions use
   the same TransactionID as the request they correspond to.

   The element SwarmID MUST be present in requests to identify the
   actions to be taken in the specified swarms.

   The version of PPSP-TP being used is indicated by the attribute
   @version of the root element, specified in an XML Schema
   ([XMLSchema.1] and [XMLSchema.2]) with XML namespaces [XMLNameSpace]
   to identify protocol grammars.

   All Request messages MUST contain a PeerID element to uniquely



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   identify the peer (Peer-ID) in the network.

   The PeerID information may be present on the following levels:

   - On PPSPTrackerProtocol level in PPSPTrackerProtocol.PeerID element.
     For details refer to 7.2.1 Table 2.

   - On PeerGroup level in PeerGroup.PeerInfo.PeerID element. For
     details refer to 7.2.1 Table 3.

   The SwarmID element MUST be be present in CONNECT Requests and SHOULD
   be present in STAT_REPORT Requests on the following levels:

   - On PPSPTrackerProtocol level in PPSPTrackerProtocol.SwarmID
     element. For details refer to 7.2.1 Table 2.

   - On StatisticsGroup level in StatisticsGroup.Stat.SwarmID element.
     For details refer to 7.2.1 Table 4.


   The PeerNum element MAY be present in CONNECT requests and MAY
   contain the attribute @abilityNAT to inform the tracker on the
   preferred type of peers, in what concerns their NAT traversal
   situation, to be returned in a peer list.

   The StatisticsGroup element MAY be present in STAT_REPORT requests.
   Details of usage in 8.2.

   The semantics of the attributes and elements within a
   PPSPTrackerProtocol root element is described in subsection 7.2.1.

   Request and Response processing is provided in section 8 for each
   message.


















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7.2. Semantics of PPSPTrackerProtocol elements

   The semantics of PPSPTrackerProtocol elements and attributes are
   described in the following tables.

   +----------------------+---------+----------------------------------+
   | Element Name or      | Use     | Description                      |
   | Attribute Name       |         |                                  |
   +----------------------+---------+----------------------------------+
   | PPSPTrackerProtocol  | 1       | The root element.                |
   |   @version           | M       | Provides the version of PPSP-TP. |
   |   Request            | 0...1   | Provides the request method      |
   |                      |         | and MUST be present in Request.  |
   |   Response           | 0...1   | Provides the response method     |
   |                      |         | and MUST be present in Response. |
   |   TransactionID      | M       | Root transaction Identification. |
   |     Result           | 0...N   | Result of @action MUST be present|
   |                      |         | in Responses.                    |
   |     @transactionID   | CM      | Identifier of the @action.       |
   |   PeerID             | 0...1   | Peer Identification.             |
   |                      |         | MUST be present in Request.      |
   |   SwarmID            | 0...N   | Swarm Identification.            |
   |                      |         | MUST be present in Requests.     |
   |     @action          | CM      | Must be set to JOIN or LEAVE.    |
   |     @peerMode        | CM      | Mode of Peer participation in    |
   |                      |         | the swarm, "LEECH" or "SEED".    |
   |     @transactionID   | CM      | Identifier for the @action.      |
   |   PeerNUM            | 0...1   | Maximum peers to be received     |
   |                      |         | with capabilities indicated.     |
   |     @abilityNAT      | CM      | Type of NAT traversal peers, as  |
   |                      |         | "No-NAT","STUN","TURN" or "PROXY"|
   |     @concurrentLinks | CM      | Concurrent connectivity level of |
   |                      |         | peers, "HIGH", "LOW" or "NORMAL" |
   |     @onlineTime      | CM      | Availability or online duration  |
   |                      |         | of peers, "HIGH" or "NORMAL"     |
   |     @uploadBWlevel   | CM      | Upload bandwidth capability of   |
   |                      |         | peers, "HIGH" or "NORMAL"        |
   |   PeerGroup          | 0...1   | Information on peers (Table 3)   |
   |   StatisticsGroup    | 0...1   | Statistic data of peer (Table 4) |
   +----------------------+---------+----------------------------------+
   | Legend:                                                           |
   | Use for attributes: M=Mandatory, OP=Optional,                     |
   |                     CM=Conditionally Mandatory                    |
   | Use for elements: minOccurs...maxOccurs (N=unbounded)             |
   | Elements are represented by their name (case-sensitive)           |
   | Attribute names (case-sensitive) are preceded with an @           |
   +-------------------------------------------------------------------+
            Table 2: Semantics of the base PPSPTrackerProtocol.



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   +----------------------+---------+----------------------------------+
   | Element Name or      | Use     | Description                      |
   | Attribute Name       |         |                                  |
   +----------------------+---------+----------------------------------+
   | PeerGroup            | 0...1   | Contains description of peers.   |
   |   PeerInfo           | 1...N   | Provides information on a peer.  |
   |     @swarmID         | 0...1   | Swarm Identification.            |
   |     PeerID           | 0...1   | Peer Identification.             |
   |                      |         | MAY be present in responses.     |
   |     PeerAddress      | 0...N   | IP Address information.          |
   |       @addrType      | M       | Type of IP address, which can be |
   |                      |         | "ipv4" or "ipv6"                 |
   |       @priority      | CM      | The priority of this interface.  |
   |                      |         | Used for NAT traversal.          |
   |       @type          | CM      | Describes the address for NAT    |
   |                      |         | traversal, which can be "HOST"   |
   |                      |         | "REFLEXIVE" or "PROXY".          |
   |       @connection    | OP      | Access type ("3G", "ADSL", etc.) |
   |       @asn           | OP      | Autonomous System number.        |
   |       @ip            | M       | IP address value.                |
   |       @port          | M       | IP service port value.           |
   |       @peerProtocol  | OP      | PPSP Peer Protocol supported.    |
   +----------------------+---------+----------------------------------+
   | Legend:                                                           |
   | Use for attributes: M=Mandatory, OP=Optional,                     |
   |                     CM=Conditionally Mandatory                    |
   | Use for elements: minOccurs...maxOccurs (N=unbounded)             |
   | Elements are represented by their name (case-sensitive)           |
   | Attribute names (case-sensitive) are preceded with an @           |
   +-------------------------------------------------------------------+
                  Table 3: Semantics of PeerGroup.

   If STUN-like functions are enabled in the tracker and a PPSP-ICE
   method is used the attributes @type and @priority MUST be returned
   with the transport address candidates in responses to CONNECT
   requests.

   The @asn attribute MAY be used to inform about the network location,
   in terms of Autonomous System, for each of the active public network
   interfaces of the peer.

   The @connection attribute is informative on the type of access
   network of the respective interface.








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   +----------------------+---------+----------------------------------+
   | Element Name or      | Use     | Description                      |
   | Attribute Name       |         |                                  |
   +----------------------+---------+----------------------------------+
   | StatisticsGroup      | 0...1   | Provides statistic data on peer  |
   |                      |         | and content.                     |
   |   Stat               | 1...N   | Groups statistics property data. |
   |     @property        | M       | The property to be reported.     |
   |                      |         | Property values in Table 6.      |
   |     SwarmID          | 0...1   | Swarm Identification.            |
   |     UploadedBytes    | 0...1   | Bytes sent to swarm.             |
   |     DownloadedBytes  | 0...1   | Bytes received from swarm.       |
   |     AvailBandwidth   | 0...1   | Upstream Bandwidth available.    |
   +----------------------+---------+----------------------------------+
   | Legend:                                                           |
   | Use for attributes: M=Mandatory, OP=Optional,                     |
   |                     CM=Conditionally Mandatory                    |
   | Use for elements: minOccurs...maxOccurs (N=unbounded)             |
   | Elements are represented by their name (case-sensitive)           |
   | Attribute names (case-sensitive) are preceded with an @           |
   +-------------------------------------------------------------------+

              Table 4: Semantics of StatisticsGroup.

   The Stat element is used to describe several properties relevant to
   the P2P network.  These properties can be related with stream
   statistics and peer status information.  Each Stat element will
   correspond to a @property type and several Stat blocks can be
   reported in a single STAT_REPORT message, corresponding to some or
   all the swarms the peer is actively involved.

   Other properties may be defined, related, for example, with
   incentives and reputation mechanisms, like peer online time, or
   connectivity conditions, like physical link status, etc.

   For that purpose, the Stat element may be extended to provide
   additional scheme specific information for new @property groups, new
   elements and new attributes.

7.3. Request element in request Messages

   Table 5 defines the valid string representations for the requests.
   These values MUST be treated as case-sensitive.








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                         +----------------------+
                         | XML Request Methods  |
                         | String Values        |
                         +----------------------+
                         | CONNECT              |
                         | STAT_REPORT          |
                         +----------------------+

          Table 5: Valid Strings for Request element of requests.

7.4. Response element in response Messages

   Table 6 defines the valid string representations for Response
   messages that require message-body.  These values MUST be treated as
   case-sensitive.

   Response messages not requiring message-body only use the standard
   HTTP Status-Code and Reason-Phrase (appended, if appropriate, with
   detail phrase, as described in section 8.6).

          +-------------------------+---------------------+
          | XML Response Method     | HTTP Status-Code    |
          | String Values           | and Reason-Phrase   |
          +-------------------------+---------------------+
          | SUCCESSFUL              |   200 OK            |
          | AUTHENTICATION REQUIRED |   401 Unauthorized  |
          +------------------------+----------------------+

     Table 6: Valid Strings for Response element of responses.

   SUCCESSFUL: indicates that the request has been processed properly
   and the desired operation has completed.  The body of the response
   message includes the requested information and MUST include the same
   TransactionID of the corresponding request.

      In CONNECT Request:  returns information about the successful
      registration of the peer and/or of each swarm @action requested.
      MAY additionally return the list of peers corresponding to the
      join @action requested.

      In STAT_REPORT Request:  confirms the success of the requested
      operation.

   AUTHENTICATION REQUIRED: Authentication is required for the peer to
   make the request.






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8. Request/Response Processing

   When a PPSP-TP message is received some basic processing is
   performed, regardless of the message type.

   Upon reception, a message is examined to ensure that it is properly
   formed.  The receiver MUST check that the HTTP message itself is
   properly formed, and if not, appropriate standard HTTP errors MUST be
   generated.  The receiver must also verify that the XML body is
   properly formed.  In case of error due to malformed messages
   appropriate responses MUST be returned, as described in 8.6.

8.1. CONNECT Request

   This method is used when a peer registers to the system and/or
   requests swarm actions.

   The peer MUST properly form the XML message-body, set the Request
   method to CONNECT, generate and set the TransactionIDs, set the
   PeerID with the identifier of the peer and include SwarmID action(s)
   to be performed.  The peer MAY also include the IP addresses of its
   network interfaces in the CONNECT message.

   An example of the message-body of a CONNECT Request for a peer Seeder
   is the following:

   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Request>CONNECT</Request>
     <PeerID>656164657220</PeerID>
     <SwarmID action="JOIN" peerMode="SEED"
              transactionID="12345.1">1111</SwarmID>
     <SwarmID action="JOIN" peerMode="SEED"
              transactionID="12345.2">2222</SwarmID>
     <TransactionID>12345.0</TransactionID>
   </PPSPTrackerProtocol>

   An example of the message-body of a CONNECT Request for a peer
   Leecher requesting join to a swarm and providing optional information
   on the addresses of its interfaces is the following:











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   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Request>CONNECT</Request>
     <PeerID>656164657221</PeerID>
     <PeerNum abilityNAT="STUN">5</PeerNum>
     <SwarmID action="JOIN" peerMode="LEECH"
              transactionID="12345.1">1111</SwarmID>
     <TransactionID>12345.0</TransactionID>
     <PeerGroup>
       <PeerInfo>
         <PeerAddress addrType="ipv4" ip="192.0.2.2" port="80"
                      priority="1" peerProtocol="PPSP-PP" />
         <PeerAddress addrType="ipv6" ip="2001:db8::2" port="80"
                      priority="2" peerProtocol="PPSP-PP" />
       </PeerInfo>
     </PeerGroup>
   </PPSPTrackerProtocol>

   An example of the message-body of a CONNECT Request for a peer
   Leecher "leaving" a previously joined swarm and requesting join to a
   new swarm is the following:

   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Request>CONNECT</Request>
     <PeerID>656164657221</PeerID>
     <PeerNum abilityNAT="STUN">5</PeerNum>
     <SwarmID action="LEAVE" peerMode="LEECH"
              transactionID="12345.1">1111</SwarmID>
     <SwarmID action="JOIN" peerMode="LEECH"
              transactionID="12345.2">2222</SwarmID>
     <TransactionID>12345.0</TransactionID>
   </PPSPTrackerProtocol>

   When receiving a well-formed CONNECT Request message, the tracker
   MAY, when applicable, start by pre-processing the peer authentication
   information (provided as Authorization scheme and token in the HTTP
   message) to check whether it is valid and that it can connect to the
   service, then proceed to register the peer in the service and perform
   the swarm actions requested.  In case of success a Response message
   with a corresponding response value of SUCCESSFUL will be generated.

   The element SwarmID MUST be present with cardinality 1 to N, each
   containing the request for @action, the @peerMode of the peer and the
   child @transactionID for that swarm. The @peerMode element MUST be
   set to the type of participation of the peer in the swarm (SEED or
   LEECH).




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   The valid sets of SwarmID @action combinations for the CONNECT
   Request logic in PPSP-TP base protocol are summarized in Table 7
   (referring to the tracker State Machine).

   +-----------+-----------+---------+----------+-----------+----------+
   | SwarmID   | @peerMode | @action | Initial  | Final     | Request  |
   | Elements  |  value    |  value  |  State   | State     | validity |
   +-----------+-----------+---------+----------+-----------+----------|
   |     1     |  LEECH    |  JOIN   |  START   | TRACKING  |  Valid   |
   +-----------+-----------+---------+----------+-----------+----------+
   |     1     |  LEECH    |  LEAVE  |  START   | TERMINATE | Invalid  |
   +-----------+-----------+---------+----------+-----------+----------+
   |     1     |  LEECH    |  LEAVE  | TRACKING | TERMINATE |  Valid   |
   +-----------+-----------+---------+----------+-----------+----------+
   |     1     |  LEECH    |  JOIN   |  START   | TERMINATE | Invalid  |
   |     1     |  LEECH    |  LEAVE  |          |           |          |
   +-----------+-----------+---------+----------+-----------+----------+
   |     1     |  LEECH    |  JOIN   | TRACKING | TRACKING  |  Valid   |
   |     1     |  LEECH    |  LEAVE  |          |           |          |
   +-----------+-----------+---------+----------+-----------+----------+
   |     N     |  SEED     |  JOIN   |  START   | TRACKING  |  Valid   |
   +-----------+-----------+---------+----------+-----------+----------+
   |     N     |  SEED     |  JOIN   | TRACKING | TERMINATE | Invalid  |
   +-----------+-----------+---------+----------+-----------+----------+
   |     N     |  SEED     |  LEAVE  | TRACKING | TERMINATE |  Valid   |
   +-----------+-----------+---------+----------+-----------+----------+

   Table 7: Validity of SwarmID @action combinations in CONNECT Request.

   The element PeerInfo, if present, MAY contain multiple PeerAddress
   child elements with attributes @addrType, @ip, @port and
   @peerProtocol, and optionally @priority and @type (if PPSP-ICE NAT
   traversal techniques are used) corresponding to each of the network
   interfaces of the peer.

   The element PeerNum indicates to the tracker the number of peers to
   be returned in a list corresponding to the indicated properties,
   being @abilityNAT for NAT traversal (considering that PPSP-ICE NAT
   traversal techniques may be used), and optionally @concurrentLinks,
   @onlineTime and @uploadBWlevel for the preferred capabilities.

   If STUN-like function is enabled in the tracker, the response MAY
   include the peer reflexive address.

   The response MUST have the same TransactionID values as the
   corresponding request and actions.

   An example of a Response message for the CONNECT Request is the



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   following:

   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Response>SUCCESSFUL</Response>
     <TransactionID>
       <Result transactionID="12345.0">200 OK</Result>
       <Result transactionID="12345.1">200 OK</Result>
       <Result transactionID="12345.2">200 OK</Result>
     </TransactionID>
     <PeerGroup>
       <PeerInfo>
         <PeerID>656164657221</PeerID>
         <PeerAddress addrType="ipv4" ip="198.51.100.1" port="80"
                      priority="1"
                      type="REFLEXIVE"
                      connection="3G"
                      asn="64496" />
       </PeerInfo>
       <PeerInfo swarmID="2222">
          <PeerID>956264622298</PeerID>
          <PeerAddress addrType="ipv4" ip="198.51.100.22" port="80"
                       asn="64496" peerProtocol="PPSP-PP" />
       </PeerInfo>
       <PeerInfo swarmID="2222">
          <PeerID>3332001256741</PeerID>
          <PeerAddress addrType="ipv4" ip="198.51.100.201" port="80"
                       asn="64496" peerProtocol="PPSP-PP" />
       </PeerInfo>
     </PeerGroup>
   </PPSPTrackerProtocol>


   The Response MUST include a PeerGroup with PeerInfo data of the
   requesting peer public IP address. If STUN-like function is enabled
   in the tracker, the PeerAddress includes the attribute @type with a
   value of REFLEXIVE, corresponding to the transport address
   "candidate" of the peer. The PeerGroup MAY also include PeerInfo data
   corresponding to the Peer-IDs and public IP addresses of the selected
   active peers in the requested swarm.

   The tracker MAY also include the attribute @asn with network location
   information of the transport address, corresponding to the Autonomous
   System Number of the access network provider.

   In case the @peerMode is SEED, the tracker responds with a SUCCESSFUL
   response and enters the peer information into the corresponding swarm
   activity.



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   In case the @peerMode is LEECH the tracker will search and select an
   appropriate list of peers satisfying the conditions set by the
   requesting peer.  The peer list returned MUST contain the Peer-IDs
   and the corresponding IP Addresses. To create the peer list, the
   tracker may take peer status and network location information into
   consideration, to express network topology preferences or Operators'
   policy preferences, with regard to the possibility of connecting with
   other IETF efforts such as ALTO [I.D.ietf-alto-protocol].

8.2. STAT_REPORT Request

   This method allows peers to send status and statistic data to
   trackers. The method is initiated by the peer, periodically while
   active.

   The peer MUST properly form the XML message-body, set the Request
   method to STAT_REPORT, set the PeerID with the identifier of the
   peer, and generate and set the TransactionID.

   The report MAY include a StatisticsGroup containing multiple Stat
   elements describing several properties relevant to the P2P network.
   These properties can be related with stream statistics and peer
   status information.

   Other properties may be defined, related for example, with incentives
   and reputation mechanisms.

   In case no StatisticsGroup is included, the STAT_REPORT may be used
   as a "keep-alive" message, to prevent the Tracker from de-registering
   the peer when timer expired.

   An example of the message-body of a STAT_REPORT Request is the
   following:

   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Request>STAT_REPORT</Request>
     <PeerID>656164657221</PeerID>
     <TransactionID>12345</TransactionID>
     <StatisticsGroup>
       <Stat property="StreamStatistics">
         <SwarmID>1111</SwarmID>
         <UploadedBytes>512</UploadedBytes>
         <DownloadedBytes>768</DownloadedBytes>
         <AvailBandwidth>1024000</AvailBandwidth>
       </Stat>
     </StatisticsGroup>
   </PPSPTrackerProtocol>



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   If the request is valid the tracker processes the received
   information for future use, and generates a response message with a
   Response value of SUCCESSFUL.

   The response MUST have the same TransactionID value as the request.

   An example of a Response message for the START_REPORT Request is the
   following:

   <?xml version="1.0" encoding="UTF-8"?>
   <PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
     <Response>SUCCESSFUL</Response>
     <TransactionID>12345</TransactionID>
   </PPSPTrackerProtocol>

8.3. Error and Recovery conditions

   If the peer fails to read the tracker response, the same Request with
   identical content, including the same TransactionID, SHOULD be
   repeated, if the condition is transient.

   The TransactionID on a Request can be reused if and only if all of
   the content is identical, including eventual Date/Time information.
   Details of the retry process (including time intervals to pause,
   number of retries to attempt, and timeouts for retrying) are
   implementation dependent.

   The tracker SHOULD be prepared to receive a Request with a repeated
   TransactionID.

   Error situations resulting from the Normal Operation or from abnormal
   conditions (section 6.2) MUST be responded with the adequate response
   codes, as described here:

     If the message is found to be incorrectly formed, the receiver MUST
     respond with a 400 (Bad Request) response with an empty message-
     body.  The Reason-Phrase SHOULD identify the syntax problem in more
     detail, for example, "Missing Content-Type header field".

     If the version number of the protocol is for a version the receiver
     does not supports, the receiver MUST respond with a 400 (Bad
     Request) with an empty message-body.  Additional information SHOULD
     be provided in the Reason-Phrase, for example, "PPSP Version #.#".

     If the length of the message does not matches the Content-Length
     specified in the message header, or the message is received without
     a defined Content-Length, the receiver MUST respond with a 411
     (Length Required) response with an empty message-body.



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     If the Request-URI in a Request message is longer than the tracker
     is willing to interpret, the tracker MUST respond with a 414
     (Request-URI Too Long) response with an empty message-body.

   In the PEER REGISTERED and TRACKING states of the tracker, certain
   requests are not allowed (section 6.2).  The tracker MUST respond
   with a 403 (Forbidden) response with an empty message-body.  The
   Reason-Phrase SHOULD identify the error condition in more detail, for
   example, "Action not allowed".

   If the tracker is unable to process a Request message due to
   unexpected condition, it SHOULD respond with a 500 (Internal Server
   Error) response with an empty message-body.

   If the tracker is unable to process a Request message for being in an
   overloaded state, it SHOULD respond with a 503 (Service Unavailable)
   response with an empty message-body.

9. Security Considerations

   P2P streaming systems are subject to attacks by malicious/unfriendly
   peers/trackers that may eavesdrop on signaling, forge/deny
   information/knowledge about streaming content and/or its
   availability, impersonating to be another valid participant, or
   launch DoS attacks to a chosen victim.

   No security system can guarantees complete security in an open P2P
   streaming system where participants may be malicious or
   uncooperative.  The goal of security considerations described here is
   to provide sufficient protection for maintaining some security
   properties during the tracker-peer communication even in the face of
   a large number of malicious peers and/or eventual distrustful
   trackers (under the distributed tracker deployment scenario).

   Since the protocol uses HTTP to transfer signaling most of the same
   security considerations described in RFC 2616 also apply [RFC2616].

9.1. Authentication between Tracker and Peers

   To protect the PPSP-TP signaling from attackers pretending to be
   valid peers (or peers other than themselves) all messages received in
   the tracker SHOULD be received from authorized peers.

   For that purpose a peer SHOULD enroll in the system via a centralized
   enrollment server. The enrollment server is expected to provide a
   proper Peer-ID for the peer and information about the authentication
   mechanisms.  The specification of the enrollment method and the
   provision of identifiers and authentication tokens is out of scope of



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   this specification.

   A Channel-oriented security mechanism should be used in the
   communication between peers and tracker, such as the Transport Layer
   Security (TLS) to provide privacy and data integrity.

   Due to the transactional nature of the communication between peers
   and tracker the method for adding authentication and data security
   services can be the OAuth 2.0 Authorization [RFC6749] with bearer
   token, which provides the peer with the information required to
   successfully utilize an access token to make protected requests to
   the tracker [RFC6750].

9.2. Content Integrity protection against polluting peers/trackers

   Malicious peers may declaim ownership of popular content to the
   tracker but try to serve polluted (i.e., decoy content or even
   virus/trojan infected contents) to other peers.

   This kind of pollution can be detected by incorporating integrity
   verification schemes for published shared contents.  As content
   chunks are transferred independently and concurrently, a
   correspondent chunk-level integrity verification MUST be used,
   checked with signed fingerprints received from authentic origin.

9.3. Residual attacks and mitigation

   To mitigate the impact of Sybil attackers, impersonating a large
   number of valid participants by repeatedly acquiring different peer
   identities, the enrollment server SHOULD carefully regulate the rate
   of peer/tracker admission.

   There is no guarantee that peers honestly report their status to the
   tracker, or serve authentic content to other peers as they claim to
   the tracker.  It is expected that a global trust mechanism, where the
   credit of each peer is accumulated from evaluations for previous
   transactions, may be taken into account by other peers when selecting
   partners for future transactions, helping to mitigate the impact of
   such malicious behaviors.  A globally trusted tracker MAY also take
   part of the trust mechanism by collecting evaluations, computing
   credit values and providing them to joining peers.

9.4. Pro-incentive parameter trustfulness

   Property types for STAT_REPORT messages may consider pro-incentive
   parameters, which can enable the tracker to improve the performance
   of the whole P2P streaming system.




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   Trustworthiness of these pro-incentive parameters is critical to the
   effectiveness of the incentive mechanisms.

   Furthermore, both the amount of uploaded and downloaded data should
   be reported to the tracker to allow checking if there is any
   inconsistency between the upload and download report, and establish
   an appropriate credit/trust system.  Alternatively, exchange of
   cryptographic receipts signed by receiving peers can be used to
   attest to the upload contribution of a peer to the swarm, as
   suggested in [Contracts].

10. Guidelines for Extending PPSP-TP

   Extension mechanisms allow designers to add new features or to
   customize existing features of a protocol for different operating
   environments.

   Since the beginning that extensibility has been a design goal of
   PPSP-TP, as the protocol is represented in the Extensible Markup
   Language [XML].

   A powerful feature of XML is the extensibility, but this feature also
   provides multiple opportunities to make poor design decisions.

   Extending a protocol implies either the addition of features without
   changing the protocol itself or the addition of new elements creating
   new versions of an existing schema and therefore new versions of the
   protocol.

   In PPSP-TP it means that an extension MUST NOT alter an existing
   protocol schema as the changes would result in a new version of an
   existing schema, not an extension of an existing schema, typically
   non-backwards-compatible.

   Additionally, a designer MUST remember that extensions themselves MAY
   also be extensible.

   Extensions MUST adhere to the principles described in this section in
   order to be considered valid.

   Extensions MAY be documented as Internet-Draft and RFC documents if
   there are requirements for coordination, interoperability, and broad
   distribution.

   Extensions need not be published as Internet-Draft or RFC documents
   if they are intended for operation in a closed environment or are
   otherwise intended for a limited audience.




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10.1. Forms of PPSP-TP Extension

   PPSP-TP extensions MUST be specified in XML.  This ensures that
   parsers capable of processing PPSP-TP structures will also be capable
   of processing PPSP-TP extensions.  Guidelines for the use of XML in
   IETF protocols can be found in RFC 3470 [RFC3470].

   In PPSP-TP two extension mechanisms can be used: a Request-Response
   Extension or a Protocol-level Extension.

   o  Request-Response Extension: Adding elements or attributes to an
      existing element mapping in the schema is the simplest form of
      extension. This form should be explored before any other. This
      task can be accomplished by extending an existing element mapping.

      For example, an element mapping for the StatisticsGroup (Section
      7.2.1, Table 4) can be extended to include additional elements
      needed to express status information about the activity of the
      peer, such as OnlineTime for the Stat element:

            <Stat property="StreamStatistics">
              <OnlineTime>3600</OnlineTime>
            </Stat>

      Another example also for the StatisticsGroup is for additional
      @property attributes and elements, such as those necessary to
      report content chunk availability information:

            <Stat property="ContentMap">
              <SegmentInfo chunkmapSize="25">
                  A/8D/wP/A/8D/wP/A/8D/wP/A/8D/wP/....
              </SegmentInfo>
            </Stat>

   o  Protocol-level Extension: If there is no existing element mapping
      that can be extended to meet the requirements and the existing
      PPSP-TP Request and Response message structures are insufficient,
      then extending the protocol should be considered in order to
      define new operational Requests and Responses.

      For example, to enhance the level of control and the granularity
      of the operations, a new version of the protocol with new messages
      (JOIN, DISCONNECT), a retro-compatible change in semantics of an
      existing CONNECT Request/Response and an extension in STAT_REPORT
      could be considered.

      As illustrated in Figure 6, the peer would use an enhanced CONNECT
      Request to perform the initial registration in the system.  Then



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      it would JOIN a first swarm as Seeder, later JOIN a second swarm
      as Leecher, and then DISCONNECT from the latter swarm but keeping
      as Seeder for the first one.  When deciding to leave the system,
      the peer DISCONNECTs gracefully from it:

                 +--------+                     +---------+
                 |  Peer  |                     | Tracker |
                 +--------+                     +---------+
                     |                               |
                     |--CONNECT--------------------->|
                     |<--------------------------OK--|
                     |--JOIN(swarm_a;SEED)---------->|
                     |<--------------------------OK--|
                     :                               :
                     |--STAT_REPORT(activity)------->|
                     |<--------------------------Ok--|
                     :                               :
                     |--JOIN(swarm_b;LEECH)--------->|
                     |<-----------------OK+PeerList--|
                     :                               :
                     |--STAT_REPORT(ChunkMap_b)----->|
                     |<--------------------------Ok--|
                     :                               :
                     |--DISCONNECT(swarm_b)--------->|
                     |<--------------------------Ok--|
                     :                               :
                     |--STAT_REPORT(activity)------->|
                     |<--------------------------Ok--|
                     :                               :
                     |--DISCONNECT------------------>|
                     |<---------------------Ok(BYE)--|

      Figure 6: Example of a session for a PPSP-TP extended version.

10.2. Issues to Be Addressed in PPSP-TP Extensions

   There are several issues that all extensions should take into
   consideration.

   -  Overview of the Extension:  It is RECOMMENDED that extensions to
      PPSP-TP have a protocol overview section that discusses the basic
      operation of the extension. The most important processing rules
      for the elements in the message flows SHOULD also be mentioned.

   -  Backward Compatibility:  One of the most important issues to
      consider is whether the new extension is backward compatible with
      the base PPST-TP.




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   -  Syntactic Issues:  Extensions that define new Request/Response
      methods SHOULD use all capitals for the method name, keeping with
      a long-standing convention in many protocols, such as HTTP. Method
      names are case sensitive in PPSP-TP.  Method names SHOULD be
      shorter than 16 characters and SHOULD attempt to convey the
      general meaning of the Request or Response.

   -  Semantic Issues:  PPSP-TP extensions MUST clearly define the
      semantics of the extensions.  Specifically, the extension MUST
      specify the behaviors expected from both the Peer and the Tracker
      in processing the extension, with the processing rules in temporal
      order of the common messaging scenario.

      Processing rules generally specify actions to be taken on receipt
      of messages and expiration of timers.

      The extension SHOULD specify procedures to be taken in exceptional
      conditions that are recoverable.  Handling of unrecoverable errors
      does not require specification.

   -  Security Issues:  Being security an important component of any
      protocol, designers of PPSP-TP extensions need to carefully
      consider security requirements, namely authorization requirements
      and requirements for end-to-end integrity.

   -  Examples of Usage:  The specification of the extension SHOULD give
      examples of message flows and message formatting and include
      examples of messages containing new syntax.  Examples of message
      flows should be given to cover common cases and at least one
      failure or unusual case.

11. IANA Considerations

   There are presently no IANA considerations with this document.

12. Acknowledgments

   The authors would like to thank many people for for their help and
   comments, particularly: Zhang Yunfei, Liao Hongluan, Roni Even,
   Bhumip Khasnabish, Wu Yichuan, Peng Jin, Chi Jing, Zong Ning, Song
   Haibin, Chen Wei, Zhijia Chen, Christian Schmidt, Lars Eggert, David
   Harrington, Henning Schulzrinne, Kangheng Wu, Martin Stiemerling,
   Jianyin Zhang, Johan Pouwelse and Arno Bakker.

   The authors would also like to thank the people participating in the
   EU FP7 project SARACEN (contract no. ICT-248474)
   [refs.saracenwebpage] for contributions and feedback to this
   document.



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   The views and conclusions contained herein are those of the authors
   and should not be interpreted as necessarily representing the
   official policies or endorsements, either expressed or implied, of
   the SARACEN project or the European Commission.

13. References

13.1. Normative References

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [ISO.8601.2004]  International Organization for Standardization,
              "Data elements and interchange formats - Information
              interchange - Representation of dates and times", ISO
              Standard 8601, December 2004.

   [XML]  Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and F.
              Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", W3C xml, November 2008,
              <http://www.w3.org/TR/xml/>.

   [XMLSchema.1]  Thompson, H., Beech, D., Maloney, M. and N.
              Mendelsohn, "XML Schema Part 1: Structures Second
              Edition", W3C xmlschema-1, October 2004,
              <http://www.w3.org/TR/xmlschema-1/>.

   [XMLSchema.2]  Biron, P. and A. Malhotra, "XML Schema Part 2:
              Datatypes Second Edition", W3C xmlschema-2, October 2004,
              <http://www.w3.org/TR/xmlschema-2/>.



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   [XMLNameSpace]  Bray, T., Hollander, D., Layman, A., Tobin, R. and H.
              Thompson, "Namespaces in XML", W3C REC-xml-names, December
              2009, <http://www.w3.org/TR/REC-xml-names/>.

   [EXI Format] Schneider, J. and T. Kamiya, "Efficient XML Interchange
              (EXI) Format 1.0",  W3C exi, March 2011,
              <http://www.w3.org/TR/exi/>.

13.2. Informative References

   [RFC1952]  Deutsch, P., "GZIP file format specification version 4.3",
              RFC 1952, May 1996.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3470]  Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines for
              the Use of Extensible Markup Language (XML) within IETF
              Protocols", BCP 70, RFC 3470, January 2003.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122, July
              2005.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, October 2012.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, October 2012.

   [I-D.ietf-ppsp-problem-statement] Zhang, Y., Zong, N., Camarillo, G.,
              Seng, J. and Y. Yang, "Problem Statement of P2P Streaming
              Protocol (PPSP)", draft-ietf-ppsp-problem-statement-11
              (work in progress), October 2012.

   [I-D.pantos-http-live-streaming] Pantos. R. and W. May, "HTTP Live
              Streaming", draft-pantos-http-live-streaming-10 (work in
              progress), October 2012.

   [I.D.ietf-alto-protocol] Alimi, R., Penno, R. and Y. Yang, "ALTO
              Protocol", draft-ietf-alto-protocol-13, (work in
              progress), September 2012.

   [I-D.montenegro-httpbis-speed-mobility] Trace, R., Foresti, A.,
              Singhal, S., Mazahir, O., Nielsen, H., Raymor, B., Rao, R.
              and G. Montenegro, "HTTP Speed+Mobility," draft-
              montenegro-httpbis-speed-mobility-02 (work in progress),
              June 2012.




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   [ISO.IEC.23009-1] ISO/IEC, "Information technology -- Dynamic
              adaptive streaming over HTTP (DASH) -- Part 1: Media
              presentation description and segment formats", ISO/IEC DIS
              23009-1, Aug. 2011.

   [refs.saracenwebpage] "SARACEN Project Website",
              http://www.saracen-p2p.eu/.

   [Contracts] Piatek, M., Venkataramani, A., Yang, R., Zhang, D. and A.
              Jaffe, "Contracts: Practical Contribution Incentives for
              P2P Live Streaming", in NSDI '10: USENIX Symposium on
              Networked Systems Design and Implementation, April 2010.







































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Appendix A. PPSP-TP Message Syntax for HTTT/1.1

   PPSP-TP messages use the generic message format of RFC 5322 [RFC5322]
   for transferring the payload of the message (Requests and
   Responses).

   PPSP-TP messages consist of a start-line, one or more header fields,
   an empty line indicating the end of the header fields, and, when
   applicable, a message-body.

   The start-line, each message-header line, and the empty line MUST be
   terminated by a carriage-return line-feed sequence (CRLF).  Note that
   the empty line MUST be present even if the message-body is not.

   The PPSP-TP message and header field syntax is identical to HTTP/1.1
   [RFC2616].

   A Request message is a standard HTTP/1.1 message starting with a
   Request-Line generated by the HTTP client peer.  The Request-Line
   contains a method name, a Request-URI, and the protocol version
   separated by a single space (SP) character.

   Request-Line =
       Method SP Request-URI SP HTTP-Version CRLF

   A Request message example is the following:

      <Method> /<Resource> HTTP/1.1
      Host: <Host>
      Content-Lenght: <ContentLenght>
      Content-Type: <ContentType>
      Authorization: <AuthToken>

      [Request_Body]

   The HTTP Method token and Request-URI (the Resource) identifies the
   resource upon which to apply the operation requested.

   The Response message is also a standard HTTP/1.1 message starting
   with a Status-Line generated by the tracker.  The Status-Line
   consists of the protocol version followed by a numeric Status-Code
   and its associated Reason-Phrase, with each element separated by a
   single SP character.

   Status-Line =
       HTTP-Version SP Status-Code SP Reason-Phrase CRLF

   A Response message example is the following:



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      HTTP/1.1 <Status-Code> <Reason-Phrase>
      Content-Lenght: <ContentLenght>
      Content-Type: <ContentType>
      Content-Encoding: <ContentCoding>

      [Response_Body]

   The Status-Code element is a 3-digit integer result code that
   indicates the outcome of an attempt to understand and satisfy a
   request.

   The Reason-Phrase element is intended to give a short textual
   description of the Status-Code.

A.1. Header Fields

   The header fields are identical to HTTP/1.1 header fields in both
   syntax and semantics.

   Some header fields only make sense in requests or responses.  If a
   header field appears in a message not matching its category (such as
   a request header field in a response), it MUST be ignored.

   The Host request-header field in the request message follows the
   standard rules for the HTTP/1.1 Host header.

   The Content-Type entity-header field MUST be used in requests and
   responses containing message-bodies to define the Internet media type
   of the message-body.

   The Content-Encoding entity-header field MAY be used in response
   messages with "gzip" compression scheme [RFC1952] for faster
   transmission times and less network bandwidth usage.

   The Content-Length entity-header field MUST be used in messages
   containing message-bodies to locate the end of each message in a
   stream.

   The Authorization header field in the request message allows a peer
   to authenticate itself with a tracker, containing authentication
   information.

A.2. Methods

   PPSP-TP uses HTTP/1.1 POST method token for all request messages.

A.3. Message Bodies




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   PPSP-TP requests MUST contain message-bodies.

   PPSP-TP responses MAY include a message-body.

   If the message-body has undergone any encoding such as compression,
   then this MUST be indicated by the Content-Encoding header field;
   otherwise, Content-Encoding MUST be omitted.

   The character set of the message body is indicated as part of the
   Content-Type header-field, and the default value for PPSP-TP messages
   is "UTF-8".

A.4. Message Response Codes

   The response codes in PPSP-TP response messages are consistent with
   HTTP/1.1 response status-codes.  However, not all HTTP/1.1 response
   status-codes are appropriate for PPSP-TP, and only those that are
   appropriate are given here.  Other HTTP/1.1 response codes SHOULD NOT
   be used in PPSP-TP.

   The class of the response is defined by the first digit of the
   Status-Code.  The last two digits do not have any categorization
   role.  For this reason, any response with a Status-Code between 200
   and 299 is referred to as a "2xx response", and similarly to the
   other supported classes:

   2xx: Success -- the action was successfully received, understood, and
        accepted;
   4xx: Peer Error -- the request contains bad syntax or cannot be
        fulfilled at this tracker;
   5xx: Tracker Error -- the tracker failed to fulfill an apparently
        valid request;

   The valid response codes are the following (Status-Code Reason-
   Phrase):

   200 OK -- The request has succeeded.  The information returned with
       the response describes or contains the result of the action;

   400 Bad Request -- The request could not be understood due to
       malformed syntax.

   401 Unauthorized -- The request requires authentication.

   403 Forbidden -- The tracker understood the request, but is refusing
       to fulfill it.  The request SHOULD NOT be repeated.

   404 Not Found -- This status is returned if the tracker did not find



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       anything matching the Request-URI.

   408 Request Timeout -- The peer did not produce a request within the
       time that the tracker was prepared to wait.

   411 Length Required -- The tracker refuses to accept the request
       without a defined Content-Length. The peer MAY repeat the request
       if it adds a valid Content-Length header field containing the
       length of the message-body in the request message.

   414 Request-URI Too Long -- The tracker is refusing to service the
       request because the Request-URI is longer than the tracker is
       willing to interpret.  This rare condition is likely to occur
       when the tracker is under attack by a client attempting to
       exploit security holes.

   500 Internal Server Error -- The tracker encountered an unexpected
       condition which prevented it from fulfilling the request.

   503 Service Unavailable -- The tracker is currently unable to handle
       the request due to a temporary overloading or maintenance
       condition.





























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Authors' Addresses

       Rui Santos Cruz
       IST/INESC-ID/INOV
       Phone: +351.939060939
       Email: rui.cruz@ieee.org

       Gu Yingjie
       Huawei
       Phone: +86-25-56624760
       Fax:   +86-25-56624702
       Email: guyingjie@huawei.com

       Mario Serafim Nunes
       IST/INESC-ID/INOV
       Rua Alves Redol, n.9
       1000-029 LISBOA, Portugal
       Phone: +351.213100256
       Email: mario.nunes@inov.pt

       Jinwei Xia
       Huawei
       Nanjing, Baixia District  210001, China
       Phone: +86-025-86622310
       Email: xiajinwei@huawei.com

       Joao P. Taveira
       IST/INOV
       Email: joao.silva@inov.pt

       Deng Lingli
       China Mobile



















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