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Versions: (draft-cruz-ppsp-base-tracker-protocol) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7846

PPSP                                                         Rui S. Cruz
INTERNET-DRAFT                                            Mario S. Nunes
Intended Status: Standards Track                       IST/INESC-ID/INOV
Expires: July 3, 2016                                         Yingjie Gu
                                                              Jinwei Xia
                                                            Rachel Huang
                                                                  Huawei
                                                         Joao P. Taveira
                                                                IST/INOV
                                                             Deng Lingli
                                                            China Mobile
                                                       December 31, 2015


           PPSP Tracker Protocol-Base Protocol (PPSP-TP/1.0)
                draft-ietf-ppsp-base-tracker-protocol-12


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 delivery (audio, video, associated timed
   text and metadata), such as adaptive multi-rate, layered (scalable)
   and multi-view (3D) videos, 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."




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Copyright and License Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   described in the Simplified BSD License.



Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2  Design Overview . . . . . . . . . . . . . . . . . . . . . .  7
       1.2.1  Typical Use Cases . . . . . . . . . . . . . . . . . . .  8
       1.2.2  Enrollment and Bootstrap  . . . . . . . . . . . . . . .  9
   2  Protocol Architecture and Functional View . . . . . . . . . . . 11
     2.1  Messaging Model . . . . . . . . . . . . . . . . . . . . . . 12
     2.2  Request/Response model  . . . . . . . . . . . . . . . . . . 12
     2.3  State Machines and Flows of the Protocol  . . . . . . . . . 13
       2.3.1  Normal Operation  . . . . . . . . . . . . . . . . . . . 15
       2.3.2  Error Conditions  . . . . . . . . . . . . . . . . . . . 16
   3  Protocol Specification  . . . . . . . . . . . . . . . . . . . . 17
     3.1  Presentation Language . . . . . . . . . . . . . . . . . . . 17
     3.2  Resource Element Types  . . . . . . . . . . . . . . . . . . 17
       3.2.1  Version . . . . . . . . . . . . . . . . . . . . . . . . 17
       3.2.2  Peer Number Element . . . . . . . . . . . . . . . . . . 17
       3.2.3  Swarm Action Element  . . . . . . . . . . . . . . . . . 18
       3.2.4  Peer Information Elements . . . . . . . . . . . . . . . 19
       3.2.5  Statistics and Status Information Element . . . . . . . 20
     3.3  Requests and Responses  . . . . . . . . . . . . . . . . . . 21
       3.3.1  Request Types . . . . . . . . . . . . . . . . . . . . . 21
       3.3.2  Response Types  . . . . . . . . . . . . . . . . . . . . 22



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       3.3.3  Request Element . . . . . . . . . . . . . . . . . . . . 22
       3.3.4  Response Element  . . . . . . . . . . . . . . . . . . . 23
     3.4  PPSP-TP Message Element . . . . . . . . . . . . . . . . . . 24
   4  Protocol Specification: Encoding and Operation  . . . . . . . . 24
     4.1 Requests and Responses . . . . . . . . . . . . . . . . . . . 25
       4.1.1  CONNECT Request . . . . . . . . . . . . . . . . . . . . 25
         4.1.1.1  Example . . . . . . . . . . . . . . . . . . . . . . 27
       4.1.2  FIND Request  . . . . . . . . . . . . . . . . . . . . . 32
         4.1.2.1  Example . . . . . . . . . . . . . . . . . . . . . . 33
       4.1.3  STAT_REPORT Request . . . . . . . . . . . . . . . . . . 35
         4.1.3.1  Example . . . . . . . . . . . . . . . . . . . . . . 36
     4.2  Response element in response Messages . . . . . . . . . . . 37
     4.3  Error and Recovery conditions . . . . . . . . . . . . . . . 37
     4.4  Parsing of Unknown Fields in Message-body . . . . . . . . . 38
   5  Operations and Manageability  . . . . . . . . . . . . . . . . . 39
     5.1  Operational Considerations  . . . . . . . . . . . . . . . . 39
       5.1.1  Installation and Initial Setup  . . . . . . . . . . . . 39
       5.1.2  Migration Path  . . . . . . . . . . . . . . . . . . . . 40
       5.1.3  Requirements on Other Protocols and Functional
              Components  . . . . . . . . . . . . . . . . . . . . . . 40
       5.1.4  Impact on Network Operation . . . . . . . . . . . . . . 40
       5.1.5  Verifying Correct Operation . . . . . . . . . . . . . . 40
     5.2  Management Considerations . . . . . . . . . . . . . . . . . 40
       5.2.1  Interoperability  . . . . . . . . . . . . . . . . . . . 40
       5.2.2  Management Information  . . . . . . . . . . . . . . . . 41
       5.2.3  Fault Management  . . . . . . . . . . . . . . . . . . . 41
       5.2.4  Configuration Management  . . . . . . . . . . . . . . . 41
       5.2.5  Accounting Management . . . . . . . . . . . . . . . . . 42
       5.2.6  Performance Management  . . . . . . . . . . . . . . . . 42
       5.2.7  Security Management . . . . . . . . . . . . . . . . . . 42
   6  Security Considerations . . . . . . . . . . . . . . . . . . . . 42
     6.1  Authentication between Tracker and Peers  . . . . . . . . . 42
     6.2  Content Integrity protection against polluting
          peers/trackers  . . . . . . . . . . . . . . . . . . . . . . 43
     6.3  Residual attacks and mitigation . . . . . . . . . . . . . . 43
     6.4  Pro-incentive parameter trustfulness  . . . . . . . . . . . 43
   7  Guidelines for Extending PPSP-TP  . . . . . . . . . . . . . . . 44
     7.1  Forms of PPSP-TP Extension  . . . . . . . . . . . . . . . . 45
     7.2  Issues to Be Addressed in PPSP-TP Extensions  . . . . . . . 46
   8  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 47
     8.1 MIME Type Registry . . . . . . . . . . . . . . . . . . . . . 47
     8.2 PPSP Tracker Protocol Version Number Registry  . . . . . . . 48
   9  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 48
   10  References . . . . . . . . . . . . . . . . . . . . . . . . . . 49
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 49
     10.2  Informative References . . . . . . . . . . . . . . . . . . 49
   Appendix A.  Revision History  . . . . . . . . . . . . . . . . . . 51
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52



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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.
   [RFC6972] specifies that the Tracker Protocol should standardize the
   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
   acquisition of peer lists.

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

   This document introduces a base PPSP Tracker Protocol which satisfies
   the requirements from [RFC6972].

1.1  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 [RFC2119].

   absolute time:  Absolute time is expressed as ISO 8601 timestamps,
   using zero UTC offset.  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.

   chunk:  An uniformly atomic subset of the resource that constitutes
   the basic unit of data organized in P2P streaming for storage,
   scheduling, advertisement and exchange among peers.

   chunk ID:  A unique resource identifier for a chunk.  The identifier
   type depends on the addressing scheme used, i.e., an integer, an
   HTTP-URL and possibly a byte-range, and is described in the MPD.

   LEECH:  A LEECH refers to the peers in a swarm that download content
   from other peers as well as contribute its downloaded content with
   others. A LEECH should join the swarm with uncompleted media content.

   MPD (Media Presentation Description):  Formalized description for a



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   media presentation, i.e., describes the structure of the media,
   namely, the Representations, the codecs used, the chunks, and the
   corresponding addressing scheme.

   peer: A participant in a P2P streaming system that not only receives
   streaming content, but also caches and streams streaming content to
   other participants.

   peer ID:  The identifier of a peer such that other peers, or the
   Tracker, can refer to the peer by using its ID.  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, or a 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 list: A list of peers that are in the same swarm maintained by
   the tracker. A peer can fetch the peer list of a swarm from the
   tracker.

   PPSP: The abbreviation of Peer-to-Peer Streaming Protocol.

   PPSP-TP: The abbreviation of Peer-to-Peer Streaming Protocols -
   Tracker Protocol.

   SEEDER:  A SEEDER refers to the peers in a swarm that only contribute
   the content they have to others. A SEEDER should join the swarm with
   the complete media content.

   service portal: A logical entity typically used for client enrollment
   and content information publishing, searching and retrieval. It is
   usually located in a server of content provider.

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

   swarm ID: The identifier of a swarm containing a group of peers
   sharing a common streaming content. 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 directory service that maintains a
   list of peers participating in a specific audio/video channel or in
   the distribution of a streaming file. It is a logical component that
   can be deployed in a centralized or distributed way.




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   transaction ID:  The identifier of a request from the peer to the
   tracker. It is used to disambiguate responses that may arrive in a
   different order of the corresponding requests.


1.2  Design Overview

   The functional entities related to PPSP protocols are the Client
   Media Player, the service portal, the tracker and the peers.  The
   complete description of Client Media Player and service portal is not
   discussed here, as not in the scope of the specification.  The
   functional entities directly involved in the PPSP Tracker Protocol
   are trackers and peers (which may support different capabilities).

   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 [RFC7230].

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

   A peer corresponds to a logical entity (typically in a user device)
   that actually participates 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.

   A tracker is a logical entity that maintains the lists of peers
   storing chunks 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 node,
   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. Communications between these physical nodes to present them
   as a single tracker to peers is not considered in PPSP-TP which is a
   protocol between a tracker and a peer.

   PPSP-TP 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. And PPSP-TP is not designed for applications
   where in-sync reception is needed.






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1.2.1  Typical PPSP Session

   When a peer wants to receive streaming of a selected content (LEECH
   mode):

   1. Peer connects to a tracker and joins a swarm.
   2. Peer acquires a list of other peers in the swarm from the
      tracker.
   3. Peer exchanges its content availability with the
      peers on the obtained peer list.
   4. Peer identifies the peers with desired content.

   5. Peer requests content from the identified peers.

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

   1. Peer connects to a tracker.
   2. Peer sends information to the tracker about the swarms
      it belongs to (joined swarms).
   3. Peer waits other peers as LEECH to connect with it (see previous
   step 3 - 5).

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

1.2.2  An Example of PPSP Session

   In order to be able to bootstrap in the P2P network, a peer must
   first obtain a peer ID and any required security certificates or
   authorization tokens from an enrollment service (end-user
   registration).  The peer ID MUST be unique (see the terminology of
   peer ID in Section 1.1), however, the representation of the peer ID
   is not considered in this document.

   +--------+      +--------+     +--------+    +---------+  +--------+
   | 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) ------------->|



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       |<--------chunk--|<---------------------------------chunks--|
       :                :               :              :           :
       |                |--STAT_REPORT---------------->|           |
       |                |<-------------------------OK--|           |
       :                :               :              :           :
       |                |--FIND----------------------->|           |
       |                |<----------------OK+Peerlist--|           |
       :                :               :              :           :
       |--Get(chunk)--->|<---------- (Peer protocol) ------------->|
       |<--------chunk--|<---------------------------------chunks--|
       :                :               :              :           :
       Figure 1:  A typical PPSP session for streaming a content.

   To join an existing P2P streaming service and to participate in
   content sharing, any peer must first locate a tracker.


   As illustrated in Figure 1, a P2P streaming session may be initiated
   starting at point (a), with the Client Media Player browsing for the
   desired content in order to request it (to the local Peer_1 in the
   figure), or resume a previously initiated stream, but starting at
   point (b). For this example, the Peer_1 is in mode LEECH.

   At point (a) in Figure 1, the Client Media Player accesses the Portal
   and selects the content of interest. The Portal returns the Media
   Presentation Description (MPD) file that includes information about
   the address of one or more trackers (that can be grouped by tiers of
   priority) which are controlling the swarm x for that media content
   (e.g., content x).

   With the information from the MPD the Client Media Player is able to
   trigger the start of the streaming session, requesting to the local
   Peer_1 the chunks of interest.

   The PPSP streaming session is then started (or resumed) at Peer_1 by
   sending a PPSP-TP CONNECT message to the tracker in order to join
   swarm x. The tracker will then return the OK response message
   containing a peer list, if the CONNECT message is successfully
   accepted. From that point, every chunk request is addressed by Peer_1
   to its neighbors (Peer_2 in Figure 1) using peer protocol, e.g.,
   [RFC7574], returning the received chunks to the Client Media Player.

   Once connected, Peer_1 needs to periodically report its status and
   statistics data to the tracker using a STAT_REPORT message.

   If Peer_1 needs to refresh its neighborhood (for example, due to
   churn) it will send a PPSP-TP FIND message (with the desired scope)
   to the tracker.



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   Peers that are only SEEDERs (i.e., serving contents to other peers),
   as are the typical cases of service provider P2P edge caches and/or
   Media Servers, trigger their P2P streaming sessions for contents x,
   y, z... (Figure 2), not from Media Player signals, but from some
   "Start" activation signal received from the service provider
   provisioning mechanism.  In this particular case the peer starts or
   resumes all its streaming sessions just by sending a PPSP-TP CONNECT
   message to the tracker (Figure 2), in order to "join" all the
   requested swarms.


   Periodically, the peer also report its status and statistics data to
   the tracker using a PPSP-TP STAT_REPORT message.

                +---------+                     +---------+
                |  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.

   The specification of the mechanisms used by the Client Media Player
   (or provisioning process) 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.

2  Protocol Architecture and Functional View

   PPSP-TP is designed with a layered approach i.e., a PPSP-TP
   Request/Response layer, a Message layer and a Transport layer.  The
   PPSP-TP Request/Response layer deals with the interactions between
   tracker and peers using request and response messages (see Figure 3).

                    +----------------------+
                    |      Application     |
                    +----------------------+
                    |  Request/Response    |  PPSP-TP



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                    |----------------------|
                    |   (HTTP) Message     |
                    +----------------------+
                    |       TRANSPORT      |
                    +----------------------+

                Figure 3:  Abstract layering of PPSP-TP.

   The Message layer deals with the framing format, for encoding and
   transmitting the data through the underlying transport protocol, as
   well as 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 TCP, or TLS [RFC5246] over it.

2.1  Messaging Model

   The messaging model of PPSP-TP aligns with HTTP protocol and the
   semantics of its messages, currently in version 1.1 [RFC7230], but
   intended to support future versions of HTTP.

2.2  Request/Response model

   PPSP-TP uses a REST-Like (Representational State Transfer) design
   with the goal of leveraging current HTTP implementations and
   infrastructure, as well as familiarity with existing REST-like
   services in popular use.  PPSP-TP messages use the UTF-8 character
   set [RFC3629] and are either requests from peers to a tracker
   service, or responses from a tracker service to peers.  The 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.

   PPSP-TP uses the HTTP POST method to send parameters in requests.
   PPSP-TP messages use JavaScript Object Notation (JSON) [RFC7159] to
   encode message bodies.

   Peers send requests to tracker. Trackers send a single response for
   each request though both requests and responses can be subject to
   fragmentation of messages in transport.

   The request messages of the base protocol are listed in Table 1:

                    +------------------------------+
                    | PPSP-TP/1.0 Request Messages |



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                    +------------------------------+
                    | CONNECT                      |
                    | FIND                         |
                    | STAT_REPORT                  |
                    +------------------------------+

                       Table 1:  Request Messages

   CONNECT:  This request message is used when a peer registers in the
      tracker (or if already registered) to notify 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 associated location information), link status and peer mode
      for the named swarm(s).  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 requesting peer ID. On
      receiving a CONNECT message, the tracker first checks the peer
      mode type (SEEDER/LEECH) for the specified swarm(s) and then
      decides the next steps (more details are referred in section 4.1)

   FIND:  This request message is used by peers to request to the
      tracker, whenever needed, a list of peers active in the named
      swarm.  On receiving a FIND message, the tracker finds the peers,
      listed in content status of the specified swarm that can satisfy
      the requesting peer's requirements, returning the list to the
      requesting peer. To create the peer list, the tracker may take
      peer status, capabilities and peers priority into consideration.
      Peer priority may be determined by network topology preference,
      operator policy preference, etc.

   STAT_REPORT:  This request message is used to allow an active peer to
      send status (and optionally statistic 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.

2.3  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 |<-/
                +------------+    +=========+    +======+
                 (Transient)                         \- (start tracker)



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                   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, 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.

   Once in STARTED state, each peer is instantiated (per peer ID) in the
   tracker state machine with a dedicated transaction state machine
   (Figure 5), which is deleted when the peer ID is removed.

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



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   snd OK         \     +==============+     /  --------------- (3)
   rst track timer  ----|   TRACKING   |----    snd OK response
   snd error (B)        +==============+        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
   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 instantiated
   "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, FIND 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.

2.3.1  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 with
      a tracker by sending a CONNECT message asking for the swarm(s) it
      wants to join.  This request from a new peer ID triggers the
      instantiation in the tracker of a "per-Peer-ID" State Machine. In
      the START state of the new "per-Peer-ID" SM, the tracker registers
      the peer ID and associated information (IP addresses), starts the
      "init timer" and moves to PEER REGISTERED state.




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   2) In PEER REGISTERED state, if peer ID is valid, the tracker either
      a) processes the requested action(s) for the valid swarm
      information contained in the CONNECT request and 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), as
      detailed in section 4.1, or b) moves the valid FIND request to
      TRACKING state.

   3) In TRACKING state, STAT_REPORT or FIND messages received from that
      peer ID will reset the "track timer" and the tracker responds to
      the respectively requests with a) a successful condition, b) a
      successful condition containing the appropriate list of peers for
      the named swarm (section 4.2).

   4) While TRACKING, a CONNECT message received from that peer ID with
      valid swarm actions information (section 4.1.1) resets the "track
      timer" and the tracker responds to the request with a successful
      condition.

2.3.2  Error Conditions

      Peers are required not to 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 6.1.1), the tracker responds with
      PPSP-TP error code specified in Section 4.3, deletes the
      registration, transition to TERMINATE state for that peer ID and
      the SM is destroyed.

      At the PEER REGISTERED state, if the peer ID is considered invalid
      (in the case of a CONNECT request or in the case of FIND or
      STAT_REPORT requests received from an unregistered peer ID), the
      tracker responds with either error codes authentication required
      or Forbidden (described in section 4.3), transitions to TERMINATE
      state for that peer ID and the SM is destroyed.

   B) At the TRACKING state (while the "track timer" has not expired)
      receiving a CONNECT message from that peer ID with invalid swarm
      actions (section 5.1), or receiving a FIND/STAT_REPORT message
      from that peer ID with invalid swarm ID is considered an error
      condition. The tracker responds with corresponding error code
      (described in section 4.3).



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   C) 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, transitions to TERMINATE state for that peer ID
      and the SM is destroyed.

   NOTE:  These situations may correspond to malfunctions at the peer or
   to malicious conditions.  As preventive measure, the tracker proceeds
   to TERMINATE state for that peer ID.


3  Protocol Specification

3.1  Presentation Language

   PPSP-TP uses a REST-Like design, encoding the requests and responses
   using JSON [RFC7159]. For a generalization of the definition of
   protocol elements and fields, their types and structures, this
   document uses a C-style notation, similar to the presentation
   language used to define TLS [RFC5246].

   A JSON object consists of name/value pairs with the grammar specified
   in [RFC7159]. In this document, comments begin with "//", and the
   "ppsp_tp_string_t" and "ppsp_tp_integer_t" types are used to indicate
   the JSON string and number, respectively. Optional fields are
   enclosed in "[ ]" brackets.  An array is indicated by two numbers in
   angle brackets, <min..max>, where "min" indicates the minimal number
   of values and "max" the maximum.  An "*" is used to denote a no upper
   bound value for "max".

3.2  Resource Element Types

   This section details the format of PPSP-TP resource element types.

3.2.1  Version

   For both requests and responses, the version of PPSP-TP being used
   MUST be indicated by the attribute version, defined as follows:

      ppsp_tp_integer_t ppsp_tp_version_t = 1

   The defined value for ppsp_tp_version_t is listed in Table 2

     +----------------------------------------------------------+
     | ppsp_tp_version_t |  Description                         |
     +----------------------------------------------------------+
     | 0                 |  Reserved                            |
     | 1                 |  Protocol specified in this document |



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     | 2-255             |  Unassigned                          |
     +----------------------------------------------------------+

            Table 2:  PPSP Tracker Protocol Version Numbers

3.2.2  Peer Number Element

   The peer number element is a scope selector optionally present in
   CONNECT and FIND requests.

   This element contains the attribute peer_count to indicate the
   maximum number of peers in the returned peer list.  Peer_count should
   be less than 30 in this specification.  The other 4 attributes, i.e.,
   ability_nat, concurrent_links, online_time and upload_bandwidth may
   be also contained in this element to inform the tracker the status of
   the peer so that the tracker could return some eligible peers based
   on the implementing rules set by the service providers:

   o ability_nat is used to indicate the preferred NAT traversal
     situation of the requesting peer.

   o concurrent_links means the number of P2P links the peer currently
     has.

   o online_time represents online duration time of the peer. The unit
     is second.

   o upload_bandwidth is the maximum upload bandwidth capability of the
     peer. The unit is kbps.

   The definition of the scope selector element and attributes is
   defined as follows:

      Object {
              ppsp_tp_integer_t   peer_count;
              [ppsp_tp_string_t   ability_nat = "NO_NAT"
                                              | "STUN"
                                              | "TURN";]
              [ppsp_tp_integer_t   concurrent_links;]
              [ppsp_tp_integer_t   online_time;]
              [ppsp_tp_integer_t   upload_bandwidth;]
      } ppsp_tp_peer_num_t;


3.2.3  Swarm Action Element

   The swarm action element identifies the action(s) to be taken in the
   named swarm(s) as well as the corresponding peer mode (if the peer is



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   LEECH or SEEDER in that swarm).

      Object {
              ppsp_tp_string_t  swarm_id;   //swarm ID
              ppsp_tp_string_t  action = "JOIN"
                                        |"LEAVE"; // Action type of
                                                  // the CONNECT
                                                  // message

              ppsp_tp_string_t  peer_mode = "SEEDER"
                                          | "LEECH"; // Mode of the peer
                                                     // participating
                                                     // in this swarm
      } ppsp_tp_swarm_action_t;


3.2.4  Peer Information Elements

   The peer information elements provides network identification
   information of peers.  A peer information consists of peer identifier
   and the IP related addressing information.

      Object {
              ppsp_tp_string_t    peer_id;
              ppsp_tp_peer_addr_t peer_addr;
      }ppsp_tp_peer_info_t;


   The ppsp_tp_peer_addr_t element includes the IP address and port,
   with a few optional attributes related with connection type and
   network location (in terms of ASN) as well as, optionally, the
   identifier of the Peer Protocol being used.

      Object {
              ppsp_tp_ip_address       ip_address;
              ppsp_tp_integer_t        port;
              ppsp_tp_integer_t        priority;
              ppsp_tp_string_t         type = "HOST"
                                            | "REFLEXIVE"
                                            | "PROXY";
             [ppsp_tp_string_t         connection = "wireless"
                                                  | "wired";]
             [ppsp_tp_string_t         asn;]
             [ppsp_tp_string_t         peer_protocol;]
      } ppsp_tp_peer_addr_t;

   The semantics of ppsp_tp_peer_addr_t attributes are listed in
   Table 3:



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      +----------------------+----------------------------------+
      | Element or Attribute | Description                      |
      +----------------------+----------------------------------+
      |      ip_address      | IP Address information           |
      |      port            | IP service port value            |
      |      priority        | The priority of this interface.It|
      |                      | may be determined by network     |

      |                      | topology preference, operator    |
      |                      | policy preference, etc. How to   |
      |                      | create a priority is outside of  |
      |                      | the scope. The larger the value, |
      |                      | the higher the priority.         |
      |      type            | Describes the address for NAT    |
      |                      | traversal, which can be HOST     |
      |                      | REFLEXIVE or PROXY               |
      |      connection      | Access type (wireless or wired.) |
      |      asn             | Autonomous System Number         |
      |      peer_protocol   | PPSP Peer Protocol supported     |
      +----------------------+----------------------------------+

              Table 3:  Semantics of ppsp_tp_peer_addr_t.

   In this document, IP address is specified as ppsp_tp_addr_value.  The
   exact characters and format depend on address_type:

   o The IPv4 address is encoded as specified by the IPv4address rule in
     Section 3.2.2 of [RFC3986].

   o The IPv6 address is encoded as specified in section 4 of [RFC5952].

      Object {
              ppsp_tp_string_t   address_type;
              ppsp_tp_addr_value address;
      } ppsp_tp_ip_address;


   The peer Information in responses is grouped in a
   ppsp_tp_peer_group_t element:

      Object {
              ppsp_tp_peer_info_t peer_info<1..*>;
      } ppsp_tp_peer_group_t;


3.2.5  Statistics and Status Information Element

   The statistics element (stat) is used to describe several properties



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   relevant to the P2P network.  These properties can be related to
   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.  This specification
   only defines the property type "STREAM_STATS".













































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   The definition of the statistic element and attributes is as follows:

      Object {
             ppsp_tp_string_t  swarm_id;
             ppsp_tp_integer_t uploaded_bytes;
             ppsp_tp_integer_t downloaded_bytes;
             ppsp_tp_integer_t available_bandwidth;
             ppsp_tp_integer_t concurrent_links;
      } stream_stats;


   The semantics of stream_stats attributes are listed in Table 4:

      +----------------------+----------------------------------+
      | Element or Attribute | Description                      |
      +----------------------+----------------------------------+
      | swarm_id             | Swarm ID                         |
      | uploaded_bytes       | Bytes sent to swarm              |
      | downloaded_bytes     | Bytes received from swarm        |
      | available_bandwidth  | available instantaneous upload   |
      |                      | bandwidth                        |
      | concurrent_links     | The number of concurrent links   |
      +----------------------+----------------------------------+

                  Table 4:  Semantics of stream_stats.

   The Stat Information is grouped in the ppsp_tp_stat_group_t element:

      Object {
         ppsp_tp_string_t     type = "STREAM_STATS"; // property type
         stream_stats         stat<1..*>;
      } ppsp_tp_stat_group_t


   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 specific information for new properties, elements or
   attributes (guidelines in section 7).

3.3  Requests and Responses

   This section defines the structure of PPSP-TP requests and responses.

3.3.1  Request Types




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   The request type includes CONNECT, FIND and STAT_REPORT, defined as
   follows:


      ppsp_tp_string_t ppsp_tp_request_type_t = "CONNECT"
                                              | "FIND"
                                              | "STAT_REPORT";


3.3.2  Response Types

   Response type corresponds to the response method type of the message,
   defined as follows:

      JSONValue ppsp_tp_response_type_t = 0x00    // SUCCESSFUL
                                        | 0x01;   // FAILED

3.3.3  Request Element

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

   The generic definition of a request element is the following:

      Object {
              [ppsp_tp_peer_num_t      peer_num;]
              [ppsp_tp_peer_addr_t     peer_addr<1..*>;]
              ppsp_tp_swarm_action_t   swarm_action<1..*>;
      } ppsp_tp_request_connect;

      Object {
              ppsp_tp_string_t         swarm_id;
             [ppsp_tp_peer_num_t       peer_num;]
      } ppsp_tp_request_find;

      Object {
              ppsp_tp_version_t        version;
              ppsp_tp_request_type_t   request_type;
              ppsp_tp_string_t         transaction_id;
              ppsp_tp_string_t         peer_id;
              JSONValue request_data = ppsp_tp_req_connect connect
                                     | ppsp_tp_req_find     find
                                     | ppsp_tp_stat_group_t stat_report;
      } ppsp_tp_request;

   A request element consists the version of PPSP-TP, the request type,
   a transaction ID and the requesting peer ID, as well as the
   requesting body, i.e., request_data. The request_data MUST be



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   correctly set to the corresponding element based on the request type
   (see Table 5).

          +----------------------+----------------------+
          | request_type         | request_data         |
          +----------------------+----------------------+
          | "CONNECT"            | "connect"            |
          | "FIND"               | "find"               |
          | "STAT_REPORT"        | "stat_report"        |
          +----------------------+----------------------+

   Table 5:  The relationship between request_type and request_data.

3.3.4  Response Element

   The generic definition of a response element is the following:

      Object {
              ppsp_tp_version_t             version;
              ppsp_tp_response_type_t       response_type;
              ppsp_tp_integer_t             error_code;
              ppsp_tp_string_t              transaction_id;
             [ppsp_tp_peer_addr_t           peer_addr;]
             [ppsp_tp_swarm_action_result_t swarm_result<1..*>;]
      } ppsp_tp_response;


   A response element consists the version of PPSP-TP, the response
   type, the error code, a transaction ID, and optionally the public
   address of the requesting peer and one or multiple swarm action
   result elements. Normally, swarm action result elements SHOULD be
   present and error_code MUST be set to 0 when response_type is 0x00.
   Swarm action result elements SHOULD NOT be set when error_code is
   0x01. Detailed selection of error_code is introduced in Section 4.3;

      Object {
          ppsp_tp_string_t           swarm_id;
          ppsp_tp_response_type_t    result;
          [ppsp_tp_peer_group_t      peer_group;]
      }ppsp_tp_swarm_action_result_t;

   A swarm action result element represents the result of an action
   requested by the peer. It contains a swarm identifier which globally
   indicates the swarm, the result for the peer of this action which it
   could be CONNECT ("JOIN" or "LEAVE"), FIND or STAT_REQPORT, and
   optionally one peer group element. The attribute result indicates the
   operation result of the corresponding request. When the response
   element is corresponding to the STAT_REPORT request, or the result



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   attribute is set to 0x01, the peer group element SHOULD NOT be set.

3.4  PPSP-TP Message Element

   PPSP-TP messages (requests or responses) are designed to have a
   similar structure with a root field named "PPSPTrackerProtocol"
   containing meta information and data pertaining to a request or a
   response.

   The base type of PPSP-TP message is defined as follows:

      Object {
              JSONValue PPSPTrackerProtocol = ppsp_tp_request  Request
                                            | ppsp_tp_response Response;
      } ppsp_tp_message_root;


4  Protocol Specification: Encoding and Operation

   PPSP-TP is a message-oriented request/response protocol.  PPSP-TP
   messages use a text type encoding in JSON [RFC7159], which MUST be
   indicated in the Content-Type field in HTTP/1.1 [RFC7231], specifying
   the application/ppsp-tracker+json media type for all PPSP-TP request
   parameters and responses.

   Implementations MUST support the "https" URI scheme [RFC2818] and
   Transport Layer Security (TLS) [RFC5246].

   For deployment scenarios where peer (Client) authentication is
   desired at the tracker, HTTP Digest Authentication [RFC7616] MUST be
   supported, with TLS Client Authentication as the preferred mechanism,
   if available.

   PPSP-TP uses the HTTP POST method to send parameters in requests to
   provide information resources that are the function of one or more of
   those input parameters.  Input parameters are encoded in JSON in the
   HTTP entity body of the request.

   The section describes the operation of the three types of requests of
   PPSP-TP and provides some examples of usage.











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4.1 Requests and Responses

4.1.1  CONNECT Request

   This method is used when a peer registers to the system and/or
   requests some swarm actions (join/leave).  The peer MUST properly set
   the request type to CONNECT, generate and set the transaction_ids,
   set the peer_id and MUST include swarms the peer is interested in,
   followed by the corresponding action type and peer mode.

   o When a peer already possesses a content and agrees to share it to
     others, it should set the action type to the value JOIN, as well as
     set the peer mode to SEEDER during its start (or re-start) period.

   o When a peer makes a request to join a swarm to consume content, it
     should set the action type to the value JOIN, as well as set the
     peer mode to LEECH during its start (or re-start) period.

   In the above cases, the peer can provide optional information on the
   addresses of its network interface(s), for example, the priority,
   type, connection and ASN.

   When a peer plans to leave a previously joined swarm, it should set
   action type to LEAVE, regardless of the peer mode.

   When receiving a well-formed CONNECT request message, the tracker
   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 valid sets of number of swarms whose action type is combined with
   peer mode for the CONNECT request logic are enumerated in Table 6
   (referring to the tracker "per-Peer-ID" state machine in
   Section 2.3).














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   +-----------+-----------+---------+----------+-----------+----------+
   | Swarm     | peer_mode |  action | Initial  | Final     | Request  |
   | Number    |  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     |  SEEDER   |  JOIN   |  START   | TRACKING  |  Valid   |
   +-----------+-----------+---------+----------+-----------+----------+
   |     N     |  SEEDER   |  JOIN   | TRACKING | TERMINATE | Invalid  |
   +-----------+-----------+---------+----------+-----------+----------+
   |     N     |  SEEDER   |  LEAVE  | TRACKING | TERMINATE |  Valid   |
   +-----------+-----------+---------+----------+-----------+----------+

     Table 6:  Validity of action combinations in CONNECT request.

   In the CONNECT request message, multiple swarm action elements
   ppsp_tp_swarm_action_t could be contained.  Each of them contains the
   request action and the peer_mode of the peer.  The peer_mode
   attribute MUST be set to the type of participation of the peer in the
   swarm (SEEDER or LEECH).

   The CONNECT message may contain multiple peer_addr elements with
   attributes ip_address, port, priority and type (if ICE [RFC5245] NAT
   traversal techniques are used), and optionally connection, asn and
   peer_protocol corresponding to each of the network interfaces the
   peer wants to advertise.

   The element peer_num indicates the maximum number of peers to be
   returned in a list from the tracker. The returned peer list can be
   optionally filtered by some indicated properties, such as ability_nat
   for NAT traversal, and concurrent_links, online_time and
   upload_bandwidth for the preferred capabilities.

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

   The response may include peer_addr data of the requesting peer public



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   IP address. Peers can use Session Traversal Utilities for NAT (STUN)
   [RFC5389] and Traversal Using Relays around NAT (TURN) [RFC5766] to
   gather their candidates, in which case peer_addr SHOULD NOT present
   in the response. If no STUN is used and the tracker is able to work
   as a "STUN-like" server which can inspect the public address of a
   peer, the tracker can return the address back with a " REFLEXIVE "
   attribute type. The swarm_result may also include peer_addr 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 of the referenced peer.

   In case the peer_mode is SEEDER, the tracker responds with a
   SUCCESSFUL response and enters the peer information into the
   corresponding swarm activity.  In case the peer_mode is LEECH (or if
   a SEEDER includes a peer_num element in the request), 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
   [RFC7285].

   IMPLEMENTATION NOTE: If no peer_num attributes are present in the
   request the tracker may return a random sample from the peer
   population.

4.1.1.1  Example

   The following example of a CONNECT request corresponds to a peer that
   wants to start (or re-start) sharing its previously streamed contents
   (peer_mode is SEEDER).
















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      POST https://tracker.example.com/video_1 HTTP/1.1
      Host: tracker.example.com
      Content-Length: 494
      Content-Type: application/ppsp-tracker+json
      Accept: application/ppsp-tracker+json

      {
        "PPSPTrackerProtocol": {
          "version":              1,
          "request_type":         "CONNECT",
          "transaction_id":       "12345",
          "peer_id":              "656164657220",
          "connect":{
              "peer_addr": {
                     "ip_address": {
                          "address_type":     "ipv4",
                          "address":          "192.0.2.2"
                     },
                     "port":         80,
                     "priority":     1,
                     "type":         "HOST",
                     "connection":   "wired",
                     "asn":          "45645"
              },
              "swarm_action": [{
                  "swarm_id":       "1111",
                  "action":         "JOIN",
                  "peer_mode":      "SEEDER"
              },
              {
                  "swarm_id":       "2222",
                  "action":         "JOIN",
                  "peer_mode":      "SEEDER"
              }]
          }
        }
      }

   Another example of the message-body of a CONNECT request corresponds
   to a peer (peer_mode is LEECH, meaning that the peer is not in
   possession of the content) requesting join to a swarm, in order to
   start receiving the stream, and providing optional information on the
   addresses of its network interface(s):

      {
        "PPSPTrackerProtocol": {
          "version":               1,
          "request_type":          "CONNECT",



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          "transaction_id":        "12345.0",
          "peer_id":               "656164657221",
          "connect":{
              "peer_num": {
                  "peer_count":        5,
                  "ability_nat":       "STUN",
                  "concurrent_links":  "5",
                  "online_time":       "200",
                  "upload_bandwidth":  "600"
               },
               "peer_addr": [{
                     "ip_address": {
                          "address_type":     "ipv4",
                          "address":          "192.0.2.2"
                     },
                     "port":         80,
                     "priority":     1,
                     "type":         "HOST",
                     "connection":   "wired",
                     "asn":          "3256546"
               },
               {
                     "ip_address":{
                         "address_type":     "ipv6",
                         "address":          "2001:db8::2"
                     },
                     "port":         80,
                     "priority":     2,
                     "type":         "HOST",
                     "connection":   "wireless",
                     "asn":          "34563456",
                     "peer_protocol": "PPSP-PP"
               }],
               "swarm_action": {
                  "swarm_id":       "1111",
                  "action":         "JOIN",
                  "peer_mode":      "LEECH"
               }
          }
        }
      }


   The next example of a CONNECT request corresponds to a peer "leaving"
   a previously joined swarm and requesting join to a new swarm. This is
   the typical example of a user watching a live channel but then
   deciding to switch to a different one:




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      {
        "PPSPTrackerProtocol": {
          "version":              1,
          "request_type":         "CONNECT",
          "transaction_id":       "12345",
          "peer_id":              "656164657221",
          "connect":{
              "peer_num": {
                  "peer_count":        5,
                  "ability_nat":       "STUN",
                  "concurrent_links":  "5",
                  "online_time":       "200",
                  "upload_bandwidth":  "600"
              },
              "swarm_action": [{
                  "swarm_id":          "1111",
                  "action":            "LEAVE",
                  "peer_mode":         "LEECH"
              },
              {
                  "swarm_id":          "2222",
                  "action":            "JOIN",
                  "peer_mode":         "LEECH"
              }]
          }
        }
      }

   The next example illustrates the response for the previous example of
   CONNECT request where the peer requested two swarm actions and not
   more than 5 other peers, receiving from the tracker a peer list with
   only 2 two other peers in the swarm "2222":

      HTTP/1.1 200 OK
      Content-Length: 1342
      Content-Type: application/ppsp-tracker+json

      {
        "PPSPTrackerProtocol": {
          "version":               1,
          "response_type":         0,
          "error_code":            0,
          "transaction_id":        "12345",
          "peer_addr": {
              "ip_address": {
                  "address_type":     "ipv4",
                  "address":          "198.51.100.1"
              },



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              "port":          80,
              "priority":      1,
              "asn":           "64496"
         },
         "swarm_result": {
              "swarm_id":        "2222",
              "result":          0,
              "peer_group": {
                  "peer_info": [{
                      "peer_id":    "956264622298",
                      "peer_addr": {
                          "ip_address": {
                              "address_type":     "ipv4",
                              "address":          "198.51.100.22"
                          },
                          "port":          80,
                          "priority":      2,
                          "type":          "REFLEXIVE",
                          "connection":    "wired",
                          "asn":           "64496",
                          "peer_protocol": "PPSP-PP"
                      }
                  },
                  {
                      "peer_id":    "3332001256741",
                      "peer_addr": {
                          "ip_address": {
                              "address_type":     "ipv4",
                              "address":          "198.51.100.201"
                          },
                          "port":          80,
                          "priority":      2,
                          "type":          "REFLEXIVE",

                          "connection":    "wired",
                          "asn":           "64496",
                          "peer_protocol": "PPSP-PP"
                      }
                  }]
                }
             }
         }
      }

4.1.2  FIND Request

   This method allows peers to request to the tracker, whenever needed,
   a new peer list for the swarm.



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   The FIND request may include a peer_number element to indicate to the
   tracker the maximum number of peers to be returned in a list
   corresponding to the indicated conditions set by the requesting peer,
   being ability_nat for NAT traversal (considering that PPSP-ICE NAT
   traversal techniques may be used), and optionally concurrent_links,
   online_time and upload_bandwidth for the preferred capabilities.

   When receiving a well-formed FIND request the tracker processes the
   information to check if it is valid.  In case of success a response
   message with a response value of SUCCESSFUL will be generated and the
   tracker will search out the list of peers for the swarm and select an
   appropriate peer list satisfying the conditions set by the requesting
   peer. The peer list returned MUST contain the peer IDs and the
   corresponding IP Addresses.

   The tracker may take the ability of peers and popularity of the
   requested content into consideration.  For example, the tracker could
   select peers with higher ability than the current peers that provide
   the content if the content is relatively popular (see Section 5.1.1);
   and the tracker could also select peers with lower ability than the
   current peers that provide the content when the content is relatively
   uncommon. The tracker may take network location information into
   consideration as well, to express network topology preferences or
   operators' policy preferences. It can implement other IETF efforts
   like ALTO[RFC7285], which is out of the scope of this document.

   The response MUST include peer_group element which contains the peer
   IDs and the corresponding IP Addresses, 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 of the referenced peer.

   The response may also include peer_addr element that includes the
   requesting peer public IP address. If no STUN is used and the tracker
   is able to work as a "STUN-like" server which can inspect the public
   address of a peer, the tracker can return the address back with a "
   REFLEXIVE " attribute type.

   IMPLEMENTATION NOTE: If no peer_num attributes are present in the
   request the tracker may return a random sample from the peer
   population.

4.1.2.1  Example

   An example of the message-body of a FIND request, where the peer
   requests to the tracker an list of not more than 5 peers in the swarm
   "1111" conforming to the characteristics expressed (concurrent links,
   online time, and upload bandwidth level) is the following:



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      {
        "PPSPTrackerProtocol": {
            "version":             1,
            "request_type":        "FIND",
            "transaction_id":      "12345",
            "peer_id":             "656164657221",
            "swarm_id":            "1111",
            "peer_num": {
                "peer_count":        5,
                "ability_nat":       "STUN",
                "concurrent_links":  "5",
                "online_time":       "200",
                "upload_bandwidth":  "600"
            }
        }
      }

   An example of the message-body of a response for the above FIND
   request, including the requesting peer public IP address information,
   is the following:

      {
        "PPSPTrackerProtocol": {
            "version":             1,
            "response_type":       0,
            "error_code":          0,
            "transaction_id":      "12345",
            "swarm_result": {
                "swarm_id":        "1111",
                "result":          0,
                "peer_group": {
                    "peer_info": [{

                        "peer_id":    "656164657221",
                        "peer_addr": {
                            "ip_address": {
                                "address_type":     "ipv4",
                                "address":          "198.51.100.1"
                            },
                            "port":          80,
                            "priority":      1,
                            "type":          "REFLEXIVE",
                            "connection":    "wireless",
                            "asn":           "64496"
                        }
                    },
                    {
                        "peer_id":    "956264622298",



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                        "peer_addr": {
                            "ip_address": {
                                "address_type":     "ipv4",
                                "address":          "198.51.100.22"
                            },
                            "port":          80,
                            "priority":      1,
                            "type":          "REFLEXIVE",
                            "connection":    "wireless",
                            "asn":           "64496"
                        }
                    },
                    {
                        "peer_id":    "3332001256741",
                        "peer_addr": {
                            "ip_address": {
                                "address_type":     "ipv4",
                                "address":          "198.51.100.201"
                            },
                            "port":          80,
                            "priority":      1,
                            "type":          "REFLEXIVE",

                            "connection":    "wireless",
                            "asn":           "64496"
                        }
                    }]
                }
            }
        }
      }

4.1.3  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 set the request_type to "STAT_REPORT", set the peer_id
   with the identifier of the peer, and generate and set the
   transaction_id.

   The report may include multiple statistics elements describing
   several properties relevant to a specific swarm.  These properties
   can be related with stream statistics and peer status information,
   including uploaded_bytes, downloaded_bytes, available_bandwidth,
   concurrent_links and etc.




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   Other properties may be defined (guidelines in Section 7.1) related
   for example, with incentives and reputation mechanisms.  In case no
   Statistics Group is included, the STAT_REPORT is used as a "keep-
   alive" message to prevent the tracker from de-registering the peer
   when "track timer" expires.

   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 transaction_id value as the request.

4.1.3.1  Example

   An example of the message-body of a STAT_REPORT request is:

      {
        "PPSPTrackerProtocol": {
            "version":             1,
            "request_type":        "STAT_REPORT",
            "transaction_id":      "12345",
            "peer_id":             "656164657221",
            "stat_report": {
                "type":  "STREAM_STATS",
                "Stat": {
                      "swarm_id":              "1111",
                      "uploaded_bytes":        512,
                      "downloaded_bytes":      768,
                      "available_bandwidth":   1024000,
                      "concurrent_links":      5
                }
            }
        }
      }

   An example of the message-body of a response for the START_REPORT
   request is:

      {
        "PPSPTrackerProtocol": {
            "version":              1,
            "response_type":        0,
            "error_code":           0,
            "transaction_id":       "12345",
            "swarm_result": {
                "swarm_id":     "1111",
                "result":       0
            }



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        }
      }

4.2  Response Element in Response Messages

   Table 7 indicates the response type and corresponding semantics.

              +--------------------+---------------------+
              | Response Type      | Semantics           |
              |                    |                     |
              +--------------------+---------------------+
              | 0                  |   SUCCESSFUL        |
              | 1                  |   FAILED            |
              +--------------------+---------------------+

          Table 7:  Semantics for the Value of Response Type.

   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
   transaction_id of the corresponding request.

      CONNECT:  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 action attribute
      requested.

      FIND:  returns the list of peers corresponding to the requested
      scope.

      STAT_REPORT:  confirms the success of the requested operation.

   FAILED: indicates that the request has not been processed properly.
      And corresponding error_code SHOULD be set according to the
      conditions described in Section 4.3.

4.3  Error and Recovery conditions

   If the peer receives an invalid response, the same request with
   identical content including the same transaction_id MUST be repeated.

   The transaction_id on a request can be reused if and only if all of
   the content is identical, including 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 MUST be prepared to receive a request with a repeated



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   transaction_id.

   Error situations resulting from the Normal Operation or from abnormal
   conditions (Section 2.3.2) MUST be responded with response_type set
   to 0x01 and with the adequate error_code, as described here:

   o If the message is found to be incorrectly formed, the receiver MUST
     respond with a 01 (bad request) error_code with an empty message-
     body (no peer_addr and swarm_result attributes).

   o If the version number of the protocol is for a version the receiver
     does not supports, the receiver MUST respond with a 02 (Unsupported
     Version Number) error_code with an empty message-body (no peer_addr
     and swarm_result attributes).

   o In the PEER REGISTERED and TRACKING states of the tracker, certain
     requests are not allowed (Section 2.3.2).  The tracker MUST respond
     with a 03 (Forbidden) error_code with an empty message-body (no
     peer_addr and swarm_result attributes).

   o If the tracker is unable to process a request message due to
     unexpected condition, it SHOULD respond with a 04 (Internal Server
     Error) error_code with an empty message-body (no peer_addr and
     swarm_result attributes).

   o If the tracker is unable to process a request message for being in
     an overloaded state, it SHOULD respond with a 05 (Service
     Unavailable) error_code with an empty message-body (no peer_addr
     and swarm_result attributes).

   o If authentication is required for the peer to make the request, the
     tracker SHOULD respond with a 06 (Authentication Required)
     error_code with an empty message-body (no peer_addr and
     swarm_result attributes).

4.4  Parsing of Unknown Fields in Message-body

   This document only details object members used by this specification.
   Extensions may include additional members within JSON objects defined
   in this document.  PPSP-TP implementations MUST ignore unknown
   members when processing PPSP-TP messages.

5  Operations and Manageability

   This section provides the operational and managements aspects that
   are required to be considered in implementations of PPSP-TP. These
   aspects follow the recommendations expressed in [RFC5706].




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5.1  Operational Considerations

   The PPSP-TP provides communication between trackers and peers and is
   conceived as a "client-server" mechanism, allowing the exchange of
   information about the participant peers sharing multimedia streaming
   contents.

   The "serving" component, i.e., the tracker, is a logical entity that
   can be envisioned as a centralized service (implemented in one or
   more physical nodes), or a fully distributed service.

   The "client" component can be implemented at each peer participating
   in the streaming of contents.

5.1.1  Installation and Initial Setup

   Content providers wishing to use PPSP for content distribution should
   setup at least a PPSP tracker and a service portal (public web
   server) to publish links of the content descriptions, for access to
   their on-demand or live original contents sources.  Content/Service
   providers should also create conditions to generate peer IDs and any
   required security certificates, as well as chunk IDs and swarm IDs
   for each streaming content.  The configuration processes for the PPSP
   Tracking facility, the service portal and content sources are not
   standardized, enabling all the flexibility for implementers.

   The swarm IDs of available contents, as well as the addresses of the
   PPSP Tracking facility, can be distributed to end-users in various
   ways, but it is common practice to include both the swarm ID and the
   corresponding PPSP tracker addresses (as URLs) in the MPD of the
   content, which is obtainable (a link) from the service portal.

   The available contents could have different importance attribute
   values to indicate whether the content is popular or not.  However,
   it is a totally implementation design and outside of this
   specification.  For example, the importance attribute values of the
   contents could be set by content providers when distributing them or
   could be determined by the tracker based on the statistics of the
   requests from the peers that request the content.  The tracker could
   set a upper threshold to decide that the content is popular enough
   when the importance attribute value is higher than the upper
   threshold.  And the tracker could also set a lower threshold to
   decide that the content is uncommon enough when the importance
   attribute value is lower than the lower threshold.

   End-users browse and search for the desired contents in the service
   portal, selecting by clicking the links of the corresponding MPDs.
   This action typically requires security certificates or authorization



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   tokens from an enrollment service (end user registration), and then
   launches the Client Media Player (with PPSP awareness) which will
   then, using PPSP-TP, contact the PPSP tracker to join the
   corresponding swarm and obtain the transport addresses of other PPSP
   peers in order to start streaming the content.

5.1.2  Migration Path

   There is no previous standard protocol providing similar
   functionality as PPSP-TP. However, some popular proprietary
   protocols, e.g., BitTorrent, are used in existing systems. There is
   no way for PPSP-TP to migrate to proprietary protocols like
   BitTorrent tracker protocol. And because PPSP-TP is an application
   level protocol, there is no harm for PPSP-TP having no migration
   path. However, proprietary protocols migrating to standard protocols
   like PPSP-TP can solve the problems raised in [RFC6972]. It is also
   possible for systems to use PPSP-TP as the management protocol to
   work with exiting propriety peer protocols like BitTorrent peer
   protocol.

5.1.3  Requirements on Other Protocols and Functional Components

   For security reasons, when using PPSP Peer protocol with PPSP-TP, the
   mechanisms described in Section 6.1 should be observed.

5.1.4  Impact on Network Operation

   As the messaging model of PPSP-TP aligns with HTTP protocol and the
   semantics of its messages, the impact on Network Operation is similar
   to using HTTP.

5.1.5  Verifying Correct Operation

   The correct operation of PPSP-TP can be verified both at the tracker
   and at the peer by logging the behavior of PPSP-TP.  Additionally,
   the PPSP tracker collects the status of the peers including peer's
   activity, and such information can be used to monitor and obtain the
   global view of the operation.

5.2  Management Considerations

   The management considerations for PPSP-TP are similar to other
   solutions using HTTP for large-scale content distribution.  The PPSP
   tracker can be realized by geographically distributed tracker nodes
   or multiple server nodes in a data center.  As these nodes are akin
   to WWW nodes, their configuration procedures, detection of faults,
   measurement of performance, usage accounting and security measures
   can be achieved by standard solutions and facilities.



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5.2.1  Interoperability


   Interoperability refers to allowing information sharing and
   operations between multiple devices and multiple management
   applications.  For PPSP-TP, distinct types of devices host PPSP-TP
   trackers and peers.  Therefore, support for multiple standard schema
   languages, management protocols and information models, suited to
   different purposes, was considered in the PPSP-TP design.
   Specifically, management functionality for PPSP-TP devices can be
   achieved with Simple Network Management Protocol (SNMP) [RFC3410],
   syslog [RFC5424] and NETCONF [RFC6241].

5.2.2  Management Information

   PPSP trackers may implement SNMP management interfaces, namely the
   Application Management MIB [RFC2564] without the need to instrument
   the tracker application itself. The channel, connections and
   transaction objects of the Application Management MIB can be used to
   report the basic behavior of the PPSP tracker service.

   The Application Performance Measurement MIB (APM-MIB) [RFC3729] and
   the Transport Performance Metrics MIB (TPM-MIB) [RFC4150] can be used
   with PPSP-TP, providing adequate metrics for the analysis of
   performance for transaction flows in the network, in direct
   relationship to the transport of PPSP-TP.

   The Host Resources MIB [RFC2790] can be used to supply information on
   the hardware, the operating system, and the installed and running
   software on a PPSP tracker host.

   The TCP-MIB [RFC4022] can additionally be considered for network
   monitoring.

   Logging is an important functionality for PPSP-TP tracker and peer,
   done via syslog [RFC5424].

5.2.3  Fault Management

   As PPSP tracker failures can be mainly attributed to host or network
   conditions, the facilities previously described for verifying the
   correct operation of PPSP-TP and the management of PPSP tracker
   servers, appear sufficient for PPSP-TP fault monitoring.

5.2.4  Configuration Management

   PPSP tracker deployments, when realized by geographically distributed
   tracker nodes or multiple server nodes in a data center,  may benefit



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   from a standard way of replicating atomic configuration updates over
   a set of server nodes.  This functionality can be provided via
   NETCONF [RFC6241].
















































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5.2.5  Accounting Management

   PPSP-TP implementations, namely for content provider environments,
   can benefit from accounting standardization efforts as defined in
   [RFC2975], in terms of resource consumption data, for the purposes of
   capacity and trend analysis, cost allocation, auditing, and billing.

5.2.6  Performance Management

   Being transaction-oriented, PPSP-TP performance, in terms of
   availability and responsiveness, can be measured with the facilities
   of the APM-MIB [RFC3729] and the TPM-MIB [RFC4150].

5.2.7  Security Management

   Standard SNMP notifications for PPSP tracker management [RFC5590] and
   syslog messages [RFC5424] can be used, to alert operators to the
   conditions identified in the security considerations (Section 6).

   The statistics collected about the operation of PPSP-TP can be used
   for detecting attacks, such as the receipt of malformed messages,
   messages out of order, or messages with invalid timestamps. However,
   collecting such endpoint properties may also raise some security
   issues. For example, the statistics collected by the tracker may be
   disclosed to an unauthorized third party which has some malicious
   intention. To address such risk, the provider of the tracker should
   evaluate how much information is revealed and the associated risks.
   And confidentiality mechanism must be provided by HTTP over TLS to
   guarantee the confidentiality of PPSP-TP.

6  Security Considerations

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

   No security system can guarantee 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
   security considerations described in [RFC7230] and [RFC7231] also



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   apply. 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.

6.1  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
   this specification.

   Transport Layer Security (TLS) [RFC5246] MUST be used in the
   communication between peers and tracker to provide privacy and data
   integrity. Software engineers developing and service providers
   deploying the tracker should make themselves familiar with the Best
   Current Practices (BCP) on configuring HTTP over TLS [RFC7525].

   OAuth 2.0 Authorization [RFC6749] SHOULD be also considered when
   digest authentication [RFC7616] and HTTPS client certificates are
   required.

6.2  Content Integrity Protection Against Polluting Peers/Trackers

   Malicious peers may claim ownership of popular content to the tracker
   and try to serve polluted (i.e., decoy content or even virus/trojan
   infected contents) to other peers. Since trackers don't exchange
   content information among peers, they are difficult to detect if a
   peer is polluting the content or not. Usually, this kind of pollution
   can be detected by PPSPP [RFC7574] with requiring the use of Merkle
   Hash Tree scheme for protecting the integrity of the content. More
   details can be seen in Section 5 of [RFC7574].

   Some attackers that disrupt P2P streaming on behalf of content
   providers may provide false or modified content or peer list
   information to achieve certain malicious goals. Peers connecting to
   those portals or trackers provided by the attackers may be redirected
   to some corrupted malicious content. However, there is no standard
   ways for peers to avoid this kind of situations completely. Peers can
   have mechanisms to detect undesirable content or results themselves.
   For example, if a peer finds the portal returns some undesired



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   content information or the tracker returns some malicious peer lists,
   the peer may choose quitting the swarm, or switching to other P2P
   streaming services provided by other content providers.

6.3  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.

6.4  Pro-incentive parameter trustfulness


   Property types for STAT_REPORT messages may consider additional pro-
   incentive parameters (guidelines for extension in Section 7), which
   can enable the tracker to improve the performance of the whole P2P
   streaming system.  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.

   One such solution could be a reputation-incentive mechanism, based on
   the notions of reputation, social awareness and fairness.  The
   mechanism would promote cooperation among participants (via each
   peer's reputation) based on the history of past transactions, such
   as, count of chunk requests (sent, received) in a swarm, contribution
   time of the peer, cumulative uploaded and downloaded content, JOIN
   and LEAVE timestamps, attainable rate, etc.

   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].

6.5  Privacy for Peers




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   The PPSP-TP provides mechanisms in which the peers can send message
   containing IP addresses, ports and other information to the tracker.
   A tracker or a third party who is able to intercept such messages can
   store and process the obtained information in order to analyze peers'
   behaviors and communication patterns. Such analysis can lead to
   privacy risks. For example, an unauthorized party may snoop on the
   data transmission from the peer to a tracker in order to introduce
   some corrupted chunks.

   The PPSP peer protocol [RFC7574] has already introduced some
   mechanisms to protect the streamed content, see section 12.3 and 12.4
   of [RFC7574]. For PPSP-TP, peer implementations as well as tracker
   implementations MUST support the "https" URI scheme [RFC2818] and
   Transport Layer Security (TLS) [RFC5246]. In addition, a peer should
   be cognizant about potential tracker tracking through queries of
   peers, e.g., by using HTTP cookies. The PPSP-TP protocol specified in
   this document does not rely on HTTP cookies. Thus, peers may decide
   not to return cookies received from the tracker, in order to making
   some additional tracking more difficult.

7  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 [RFC6709].

   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 MUST be documented in standards-track RFCs if there are
   requirements for coordination, interoperability, and broad
   distribution.


7.1  Forms of PPSP-TP Extension



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   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 Statistics Group can be
      extended to include additional elements needed to express status
      information about the activity of the peer, such as online time
      for the Stat element.

   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
      it would join a first swarm as SEEDER, later join a second swarm
      as LEECH, 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:




















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                 +--------+                     +---------+
                 |  Peer  |                     | Tracker |
                 +--------+                     +---------+
                     |                               |
                     |--CONNECT--------------------->|
                     |<--------------------------OK--|
                     |--JOIN(swarm_a;SEEDER)---------->|
                     |<--------------------------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.

7.2  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: The new extension MUST be backward
      compatible with the base PPSP-TP specified in this document.


   -  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



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      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:  As security is an important component of any
      protocol, designers of PPSP-TP extensions need to carefully
      consider security requirements, e.g., 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.

8  IANA Considerations

8.1 MIME Type Registry

   This document registers application/ppsp-tracker+json media types.

   Type name:  application

   Subtype name:  ppsp-tracker+json

   Required parameters:  n/a

   Optional parameters:  n/a

   Encoding considerations:  Encoding considerations are identical to
   those specified for the "application/json" media type.  See
   [RFC7159].

   Security considerations: See Section 6.

   Interoperability considerations:  This document specifies format of
   conforming messages and the interpretation thereof.



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   Published specification:  This document.

   Applications that use this media type:  PPSP trackers and peers
   either stand alone or embedded within other applications.

   Additional information:

      Magic number(s):  n/a

      File extension(s):  n/a.

      Macintosh file type code(s):  n/a

      Fragment identifier considerations:  n/a

   Person & email address to contact for further information:  See
   Authors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author:  See Authors' Addresses section.

   Change controller:  IESG (iesg@ietf.org)

8.2 PPSP Tracker Protocol Version Number Registry

   Registry name is "PPSP Tracker Protocol Version Number Registry".
   Values are integers in the range 0-255, with initial assignments and
   reservations given in Table 2. New PPSP-TP version types are assigned
   after IETF Review [RFC5226] to ensure that proper documentation
   regarding the new version types and their usage has been provided.

8.3 PPSP Tracker Protocol Request Type Registry

   Registry name is "PPSP Tracker Protocol Request Type Registry".
   Values are strings listed in Table 8. New PPSP-TP request types are
   assigned after IETF Review [RFC5226] to ensure that proper
   documentation regarding the new request types and their usage has
   been provided.

   +----------------------+-------------------------------------------+
   | request_type         | Description                               |
   +----------------------+-------------------------------------------+
   | "CONNECT"            | CONNECT message specified in this document|
   | "FIND"               | FIND message specified in this document   |
   | "STAT_REPORT"        | STAT_REPORT message specified in this     |



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   |                      | document                                  |
   +----------------------+-------------------------------------------+
       Table 8:  The PPSP Tracker Protocol Request Type Registry.

8.4  PPSP Tracker Protocol Error Code Registry

   Registry name is "PPSP Tracker Protocol Error Code Registry". Values
   are strings listed in Table 9. New PPSP-TP error codes are assigned
   after IETF Review [RFC5226] to ensure that proper documentation
   regarding the new error codes and their usage has been provided.

      +---------------+-------------------------------------------+
      | error_code    | Description                               |
      +---------------+-------------------------------------------+
      | 00            | no error                                  |
      | 01            | bad request                               |
      | 02            | unsupported version number                |
      | 03            | forbidden action                          |
      | 04            | internal server error                     |
      | 05            | service unavailable                       |
      | 06            | authentication required                   |
      +---------------+-------------------------------------------+
        Table 9:  The PPSP Tracker Protocol Error Code Registry.


9  Acknowledgments

   The authors would like to thank many people for for their help and
   comments, particularly: Zhang Yunfei, Liao Hongluan, Roni Even, Dave
   Cottlehuber, 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, Riccardo Petrocco and
   Arno Bakker.

   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 [SARACEN], the European Commission, Huawei or
   China Mobile.

10  References

10.1  Normative References

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




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

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

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

   [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.

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

   [RFC5590]  Harrington, D. and J., Schoenwaelder, "Transport Subsystem
              for the Simple Network Management Protocol (SNMP)", RFC
              5590, June 2009.

   [RFC5766]  Mahy, R., Matthews, P. and J., Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC5766, April 2010.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J. and A.,
              Bierman, "Network Configuration Protocol (NETCONF)", RFC
              6241, June 2011.

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

   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

   [RFC7230]  Fielding, R., and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

   [RFC7231]  Fielding, R., and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.




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   [RFC7285]  Alimi, R., Penno, R., Yang, Y., Kiesel S., Previdi, S.,
              Roome, W., Shalunov, S. and R. Woundy, "Application-Layer
              Traffic Optimization (ALTO) Protocol", RF 7285, September
              2014.

   [RFC7574]  Bakker, A., Petrocco, R. and V., Grishchenko, "Peer-to-
              Peer Streaming Peer Protocol (PPSPP)", RFC 7574, July
              2015.

   [RFC7616]  Shekh-Yusef, R., Ahrens, D., and S. Bremer, "HTTP Digest
              Access Authentication", RFC 7616, September 2015.


10.2  Informative References

   [RFC2564]  Kalbfleisch, C., Krupczak, C., Presuhn, R., and J.
              Saperia, "Application Management MIB", RFC 2564, May 1999.

   [RFC2790]  Waldbusser, S. and P. Grillo, "Host Resources MIB", RFC
              2790, March 2000.

   [RFC2975]  Aboba, B., Arkko, J., and D. Harrington, "Introduction to
              Accounting Management", RFC 2975, October 2000.

   [RFC3410]  Case, J., Mundy, R., Partain, D., and B. Stewart,
              "Introduction and Applicability Statements for Internet-
              Standard Management Framework", RFC 3410, December 2002.

   [RFC3729]  Waldbusser, S., "Application Performance Measurement MIB",
              RFC 3729, March 2004.

   [RFC4022]  Raghunarayan, R., Ed., "Management Information Base for
              the Transmission Control Protocol (TCP)", RFC 4022, March
              2005.

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

   [RFC4150]  Dietz, R. and R. Cole, "Transport Performance Metrics
              MIB", RFC 4150, August 2005.

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

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and



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              Management of New Protocols and Protocol Extensions", RFC
              5706, November 2009.

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

   [RFC6972]  Zhang, Y., and N., Zong, "Problem Statement and
              Requirement of the Peer-to-Peer Streaming Protocol
              (PPSP)", RFC 6972, July 2013.

   [RFC7525]  Sheffer, Y., Holz, R., and P., Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7525, May 2015.

   [SARACEN] "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.

Appendix A.  Revision History

   -00     2013-02-14 Initial version.
   -01     2013-02-14 Minor revision.
   -02     2013-10-21 Minor revision.
   -03     2013-12-31 Major revision
       +   Introduced a generalization of the protocol specification
           using a C-style notation.
       -   removed all examples of protocol message encoding in XML
   -04     2014-07-01 Minor Revision
       -   removed Appendix referencing the use of HTTP
       +   refined the presentation language specification to include
           protocol elements definitions.
   -05     2014-07-04 Minor Revision
   -06     2014-10-27 Minor Revision
   -07     2014-12-12 Major Revision
       +   introduced a text-based (JSON) protocol encoding with
           examples for all the messages
       +   corrections in the specifications of protocol elements
       +   section 5 specification of protocol elements semantics
       +   introduced a IANA MIME Type registry
   -08     2015-01-08  Major Revision
       *   merge sections 5 and 4 with section 3; renumbered all other
       +   refined the protocol elements definitions for consistency
           with the JSON data structures



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       +   revised protocol messages encoding examples
       +   additional IANA registry for protocol version
       *   editorial corrections
   -09     (2015-3-27) Major Revision
       +   Add concurrent_link in the stream_stats specification.
       +   Remove "PROXY" value from "ability_nat" specification.
       +   Dividing attributes by "," in the example
       *   editorial corrections
   -10     (Current version) Major Revision
       +   Update dates
   -11     (2015-12-12) Address the comments from IESG review


Authors' Addresses


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

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

   Rachel Huang (Editor)
   Huawei
   Email: rachel.huang@huawei.com

   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
   Email: denglingli@chinamobile.com

   Gu Yingjie
   Email: guyingjie@gmail.com



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