PPSP Rui S. Cruz
INTERNET-DRAFT Mario S. Nunes
Intended Status: Standards Track IST/INESC-ID/INOV
Expires: December 31, 2012 Yingjie Gu
Jinwei Xia
Huawei
Joao P. Taveira
IST/INOV
June 29, 2012
PPSP Tracker Protocol--Base Protocol (PPSP-TP/1.0)
draft-cruz-ppsp-base-tracker-protocol-00
Abstract
This document specifies the base Peer-to-Peer Streaming Protocol-
Tracker Protocol (PPSP-TP/1.0), an application-layer control
(signaling) protocol for the exchange of meta information between
trackers and peers. The specification outlines the architecture of
the protocol and its functionality, and describes message flows,
message processing instructions, message formats, formal syntax and
semantics. The PPSP Tracker Protocol enables cooperating peers to
form content streaming overlay networks to support near real-time
Structured Media content (audio, video, associated timed text and
metadata) delivery, such as adaptive multi-rate, layered (scalable)
and multi-view (3D), in live, time-shifted and on-demand modes.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html
Copyright and License Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Use Scenarios and Streaming Modes . . . . . . . . . . . . . 4
1.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Enrollment and Bootstrap . . . . . . . . . . . . . . . 6
1.2.2. NAT Traversal . . . . . . . . . . . . . . . . . . . . . 7
1.2.3. Content Information Metadata . . . . . . . . . . . . . 8
1.2.4. Authentication, Confidentiality, Integrity . . . . . . 8
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Architectural and Functional View . . . . . . . . . . . . . . . 11
4. Messaging Model . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Request/Response model . . . . . . . . . . . . . . . . . . . . 13
6. State Machines and Flows of the Protocol . . . . . . . . . . . 14
6.1. Normal Operation . . . . . . . . . . . . . . . . . . . . . 16
6.2. Error Conditions . . . . . . . . . . . . . . . . . . . . . 16
7. Protocol Specification . . . . . . . . . . . . . . . . . . . . 17
7.1. Messages Syntax . . . . . . . . . . . . . . . . . . . . . . 17
7.1.1. Header Fields . . . . . . . . . . . . . . . . . . . . . 18
7.1.2. Methods . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1.3. Message Bodies . . . . . . . . . . . . . . . . . . . . 19
7.1.4. Message Response Codes . . . . . . . . . . . . . . . . 19
7.2. Request/Response Syntax and Format . . . . . . . . . . . . 21
7.2.1. Semantics of PPSPTrackerProtocol elements . . . . . . . 23
7.2.2. Request element in request Messages . . . . . . . . . . 25
7.2.3. Response element in response Messages . . . . . . . . . 26
8. Request/Response Processing . . . . . . . . . . . . . . . . . . 27
8.1. CONNECT Request . . . . . . . . . . . . . . . . . . . . . . 27
8.2. STAT_REPORT Request . . . . . . . . . . . . . . . . . . . . 31
8.3. Error and Recovery conditions . . . . . . . . . . . . . . . 32
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 33
9.1. Authentication between Tracker and Peers . . . . . . . . . 34
9.2. Content Integrity protection against polluting
peers/trackers . . . . . . . . . . . . . . . . . . . . . . 34
9.3. Residual attacks and mitigation . . . . . . . . . . . . . . 34
9.4. Pro-incentive parameter trustfulness . . . . . . . . . . . 35
10. Guidelines for Extending PPSP-TP . . . . . . . . . . . . . . . 35
10.1. Forms of PPSP-TP Extension . . . . . . . . . . . . . . . . 36
10.2. Issues to Be Addressed in PPSP-TP Extensions . . . . . . . 38
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
13.1. Normative References . . . . . . . . . . . . . . . . . . . 39
13.2. Informative References . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
The Peer-to-Peer Streaming Protocol (PPSP) is composed of two
protocols: the PPSP Tracker Protocol and the PPSP Peer Protocol
[I-D.ietf-ppsp-problem-statement] specifies that the Tracker Protocol
should standardize format/encoding of information and messages
between PPSP peers and PPSP trackers and also defines the
requirements.
The PPSP Tracker Protocol provides communication between trackers and
peers, by which peers send meta information to trackers, report
streaming status and obtain peer lists from trackers.
The PPSP architecture requires PPSP peers able to communicate with a
tracker in order to participate in a particular streaming content
swarm. This centralized tracker service is used by PPSP peers for
content registration and location.
The signaling and the media data transfer between PPSP peers is not
in the scope of this specification.
This document describes the base PPSP Tracker protocol and how it
satisfies the requirements for the IETF Peer-to-Peer Streaming
Protocol, in order to derive the implications for the standardization
of the PPSP streaming protocols and to identify open issues and
promote further discussion.
1.1. Use Scenarios and Streaming Modes
This section is tutorial in nature and does not contain any normative
statements.
This section describes some aspects of the use of PPSP-TP. The
examples were chosen to illustrate the basic operation, but not to
limit what PPSP-TP may be used for.
The functional entities related to PPSP protocols are the Client
Media Player, the service Portal, the tracker and the peers. The
complete description of these entities is not discussed here, as not
in the scope the specification.
The Client Media Player is a logical entity providing direct
interface to the end user at the client device, and includes the
functions to select, request, decode and render contents. The Client
Media Player may interface with the local peer application using
request and response standard formats for HTTP Request and Response
messages [RFC2616].
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The service Portal is a logical entity typically used for client
enrollment and content information publishing, searching and
retrieval.
The tracker is a logical entity that maintains the lists of PPSP
active peers storing and exchanging a specific media content. The
tracker also stores the status of active peers in swarms, to help in
the selection of appropriate peers for a requesting peer. The
tracker can be realized by geographically distributed tracker nodes
or multiple server nodes in a data center, increasing the content
availability, the service robustness and the network scalability or
reliability. The management and locating of content index
information are totally internal behaviors of the tracker system,
which is invisible to the PPSP Peer.
The peer is also a logical entity embedding the P2P core engine, with
a client serving side interface to respond to Client Media Player
requests and a network side interface to exchange data and PPSP
signaling with trackers and other peers.
The streaming technique is chunk-based, i.e., client peers obtain
media chunks from serving peers and handle the buffering that is
necessary for the playback processes during the download of the media
chunks.
In Live streaming, all end users are interested in a specific media
coming from an ongoing program, which means that all respective peers
share nearly the same streaming content at a given point of time.
Peers may store the live media for further distribution (known as
time-shift TV), where the stored media is distributed in a VoD-like
manner.
In VoD, different end users watch different parts of the recorded
media content during a past event. In this case, each respective
peer obtains from other peers the information on media chunks they
store and then gets the required media from a selected set of those
peers. While watching VoD, an end user can also switch to any place
of the content, e.g., skip the credits part, or skip the part that it
is not interested in. In this case the respective participating peer
may not store all the content segments. From the whole swarm point
of view, the participating peers typically store different parts of
content.
1.2. Assumptions
This section is tutorial in nature and does not contain any normative
statements.
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The process used for media streaming distribution assumes a segment
(chunk) transfer scheme whereby the original content (that can be
encoded using adaptive or scalable techniques) is chopped into small
segments having the following representations:
1. Adaptive - alternate representations with different qualities and
bitrates; a single representation is non-adaptive;
2. Scalable description levels - multiple additive descriptions
(i.e., addition of descriptions refine the quality of the video);
3. Scalable layered levels - nested dependent layers corresponding to
several hierarchical levels of quality, i.e., higher enhancement
layers refine the quality of the video of lower layers.
4. Scalable multiple views - views correspond to mono (2D) and
stereoscopic (3D) videos, with several hierarchical levels of
quality.
These streaming distribution techniques support dynamic variations in
video streaming quality while ensuring support for a plethora of end
user devices and network connections.
1.2.1. Enrollment and Bootstrap
In order to join an existing P2P streaming service and to participate
in content sharing, any peer must first locate a tracker, using for
example, the following methods (illustrated in Figures 1, 2):
+--------+ +--------+ +--------+ +---------+ +--------+
| Player | | Peer 1 | | Portal | | Tracker | | Peer 2 |
+--------+ +--------+ +--------+ +---------+ +--------+
| | | | |
(a) |--Page request----------------->| | |
|<--------------Page with links--| | |
|--Select stream (MPD Request)-->| | |
|<--------------------OK+MPD(x)--| | |
(b) |--Start/Resume->|--CONNECT(join x)------------>| |
|<-----------OK--|<----------------OK+Peerlist--| |
| | | |
|--Get(Chunk)--->|<---------- (Peer protocol) ------------->|
|<--------Chunk--|<---------------------------------Chunks--|
: : : : :
| |--STAT_REPORT---------------->| |
| |<-------------------------Ok--| |
: : : : :
|--Get(Chunk)--->|<---------- (Peer protocol) ------------->|
|<--------Chunk--|<---------------------------------Chunks--|
: : : : :
Figure 1: A typical PPSP session for watching a streaming content.
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+-------------+ +---------+
| Peer Seeder | | Tracker |
+-------------+ +---------+
| |
Start->|--CONNECT (join x,y,z)-------->|
|<--------------------------OK--|
: :
| |
|--STAT_REPORT----------------->|
|<--------------------------Ok--|
: :
| |
|--STAT_REPORT----------------->|
|<--------------------------Ok--|
: :
Figure 2: A typical PPSP session for a streaming Seeder
1. From a content service provider provisioning mechanism: this is
the typical case used for the provider Seeders (edge caches and/or
Media Servers), where some "Start" activation signal triggers the
process for contents x, y, z (Figure 2).
2. From a web page: a Publishing and Searching Portal may provide
tracker location information to end users.
3. From the Media Presentation Description (MPD) file of a content:
this meta-info file must contain information about the address of
one or more trackers (that can be grouped by tiers of priority)
which are controlling the swarm for that media content, as
illustrated in Figure 1 for a content x.
As illustrated in Figure 1, a peer may initiate a stream using the
above method 3 starting at point (a), or resume a previously
initiated stream, using also method 3 but starting at point (b).
In order to be able to bootstrap, a peer must first obtain a Peer-ID
(identity that can be associated with end user authentication
credentials) and any required security certificates or authorization
tokens from an enrollment service (end user registration).
The specification of the mechanisms used by the Client Media Player
and the peer (to signal start/resume streams or request media
chunks), obtain a Peer-ID, security certificates or tokens are not in
the scope of this document.
1.2.2. NAT Traversal
It is assumed that all trackers must be in the public Internet and
have been placed there deliberately. This document will not describe
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NAT Traversal mechanisms but the protocol facilitates flexible NAT
Traversal techniques, such as those based on ICE [RFC5245],
considering that the tracker node may provide NAT traversal services,
as a STUN-like tracker. Being a STUN-like tracker, it can discover
the reflexive candidate addresses of a peer and make them available
in responses to other requesting peers.
1.2.3. Content Information Metadata
Multimedia contents may consist of several media components (for
example, audio, video, and timed text), each of which might have
different characteristics.
The representations of a media content correspond to encoded
alternatives of the same media component, varying from other
representations by bitrate, resolution, number of channels, or other
characteristics. Each representation consists of one or multiple
segments. Segments are the media stream transport chunks in temporal
sequence.
These characteristics may be described in a Media Presentation
Description (MPD) file. It is envisioned that the content information
metadata used in PPSP may align with MPD formats, such as ISO/IEC
23009-1 [ISO.IEC.23009-1] and [I-D.pantos-http-live-streaming].
1.2.4. Authentication, Confidentiality, Integrity
Channel-oriented security can be used in the communication between
peers and tracker, such as the Transport Layer Security (TLS) to
provide privacy and data integrity. HTTP/1.1 over TLS (HTTPS)
[RFC2818] is the preferred approach for preventing disclosure of peer
critical information via the communication channel.
Due to the transactional nature of the communication between peers
and tracker a method for adding authentication and data security
services via replaceable mechanisms may be employed. One such method
is the OAuth 2.0 Authorization [I-D.ietf-oauth-v2] with bearer token,
providing the peer with the information required to successfully
utilize the access token to make protected requests to the tracker
[I-D.ietf-oauth-v2-bearer].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This draft uses terms defined in [I-D.ietf-ppsp-problem-statement].
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Absolute Time: Absolute time is expressed as ISO 8601
[ISO.8601.2004] timestamps, using zero UTC offset (GMT). Fractions
of a second may be indicated. Example for December 25, 2010 at 14h56
and 20.25 seconds: basic format 20101225T145620.25Z or extended
format 2010-12-25T14:56:20.25Z.
Adaptive Streaming: Multiple alternate representations (different
qualities and bitrates) of the same media content co-exist for the
same streaming session; each alternate representation corresponds to
a different media quality level; peers can choose among the
alternate representations for decode and playback.
Base Layer: The playable representation level in Scalable Video
Coding (SVC) required by all upper level Enhancements Layers for
proper decoding of the video.
Chunk: A chunk is a basic unit of data block organized in P2P
streaming for storage, scheduling, advertisement and exchange among
peers. A chunk therefore refers to a segment of partitioned
streaming media.
Complementary Representation: Representation in a set of content
representations which have inter-representation dependencies and
which, when combined, result in a single representation for decoding
and presentation.
Connection Tracker: The tracker node to which the PPSP peer will
connect when it wants to get registered and join the PPSP system.
Continuous media: Media with an inherent notion of time, for
example, speech, audio, video, timed text or timed metadata.
Enhancement Layer: Enhancement differential quality level
(complementary representation) in Scalable Video Coding (SVC) used to
produce a higher quality, higher definition video in terms of space
(i.e., image resolution), time (i.e., frame rate) or Signal-to-Noise
Ratio (i.e., fidelity) when combined with the playable Base Layer
[ITU-T.H.264].
Join Time: Join time is the absolute time when a peer registers on a
tracker. This value is recorded by the tracker and is used to
calculate Online Time.
Leecher: A Peer that has not yet completed the transfer of all
chunks of the media content.
Live streaming: The scenario where all clients receive streaming
content for the same ongoing event. The lags between the play points
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of the clients and that of the streaming source are small.
Media Component: An encoded version of one individual media type
such as audio, video or timed text with specific attributes, e.g.,
bandwidth, language, or resolution.
Media Presentation Description (MPD): Formalized description for a
media presentation, i.e., describes the structure of the media,
namely, the representations, the codecs used, the segments (chunks),
and the corresponding addressing scheme.
Method: The method is the primary function that a request from a
peer is meant to invoke on a tracker. The method is carried in the
request message itself.
Online Time: Online Time shows how long the peer has been in the P2P
streaming system since it joins. This value indicates the stability
of a peer, and can be calculated by tracker when necessary.
Peer: A peer refers to a participant in a P2P streaming system that
not only receives streaming content, but also stores and uploads
streaming content to other participants.
Peer-ID: Unique identifier for the peer. The Peer-ID and any
required security certificates are obtained from an offline
enrollment server.
Peer-Peer Messages (i.e., Peer Protocol): The Peer Protocol messages
enable each peer to exchange content availability with other peers
and request other peers for content.
PPSP: The abbreviation of Peer-to-Peer Streaming Protocols. PPSP
protocols refer to the key signaling protocols among various P2P
streaming system components, including the tracker and peers.
Representation: Structured collection of one or more media
components.
Request: A message sent from a peer to a tracker, for the purpose of
invoking a particular operation.
Response: A message sent from a tracker to a peer, for indicating
the status of a request sent from the peer to the tracker.
Scalable Streaming: With Multiple Description Coding (MDC), multiple
additive descriptions (that can be independently played-out) to
refine the quality of the video when combined together. With
Scalable Video Coding (SVC), nested dependent enhancement layers
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(hierarchical levels of quality), refine the quality of lower layers,
from the lowest level (the playable Base Layer). With Multiple View
Coding (MVC), multiple views allow the video to be played in 3D when
the views are combined together.
Seeder: A Peer that holds and shares the complete media content.
Segment: A segment is a resource that can be identified, by an ID or
an HTTP-URL and possibly a byte-range, and is included in the MPD.
The segment is a basic unit of partitioned streaming media, which is
used by a peer for the purpose of storage, advertisement and exchange
among peers.
Swarm: A swarm refers to a group of peers sharing the same content
(e.g., video/audio program, digital file, etc.) at a given time.
Swarm-ID: Unique identifier for a swarm. It is used to describe a
specific resource shared among peers.
Tracker: A tracker refers to a centralized logical directory service
used to communicate with PPSP Peers for content registration and
location, which maintains the lists of PPSP peers storing segments
for a specific live content channel or streaming media and answers
queries from PPSP peers.
Tracker-Peer Messages (i.e., Tracker Protocol): The Tracker Protocol
messages provide communication between peers and trackers, by which
peers can provide content availability, report streaming status and
request peer lists from trackers.
Video-on-demand (VoD): A kind of application that allows users to
select and watch video content on demand.
3. Architectural and Functional View
The PPSP Tracker Protocol architecture uses a two-layer approach
i.e., a messaging layer and a PPSP-TP Request/Response layer.
The messaging layer deals with the underlying transport protocol and
the asynchronous nature of the interactions between tracker and
peers.
The PPSP-TP Request/Response layer deals with the interactions
between tracker and peers using Method and Response codes (see
Figure 3).
The transport layer is responsible for the actual transmission of
requests and responses over network transports, including the
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determination of the connection to use for a request or response
message when using a connection-oriented transport like TCP, or TLS
[RFC5246] over it.
+----------------------+
| Application |
+----------------------+
| Request/Response |
|----------------------| PPSP-TP
| (HTTP)Messages |
+----------------------+
| TRANSPORT |
+----------------------+
Figure 3: Abstract layering of PPSP-TP
The functional entities involved in the PPSP Tracker Protocol are
trackers and peers (which may support different capabilities).
A Peer corresponds to a logical entity (typically in a user device)
that actually participate in sharing a media content. Peers are
organized in (various) swarms corresponding each swarm to the group
of peers streaming a certain content at any given time.
The tracker is a logical entity that maintains the lists of peers
storing segments for a specific Live media channel or on-demand media
streaming content, answers queries from peers and collects
information on the activity of peers. While a tracker may have an
underlying implementation consisting of more than one physical nodes,
logically the tracker can most simply be thought of as a single
element, and in this document it will be treated as a single logical
entity.
The Tracker Protocol is not used to exchange actual content data
(either on-demand or Live streaming) with peers, but information
about which peers can provide the content.
When a peer wants to receive streaming of a selected content:
1. Peer connects to a local connection tracker and joins a swarm.
2. Peer acquires a list of peers from the connection tracker.
3. Peer exchanges its content availability with the peers on the
obtained peer list (via peer protocol).
4. Peer identifies the peers with desired content.
5. Peer requests content from the identified peers (peer protocol).
When a peer wants to share streaming contents (Seeder mode) with
other peers:
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1. Peer connects to the connection tracker.
2. Peer sends information to the connection tracker about the swarms
it belongs to (joins).
After having been disconnected due to some termination condition, a
peer can resume previous activity by connecting and re-joining the
corresponding swarm(s).
4. Messaging Model
The messaging model of PPSP-TP is based on the exchange of messages
that follow the syntax and semantics of the current HTTP/1.1
specification [RFC2616]. The exchange of messages is envisioned to
be performed over a stream-oriented reliable transport protocol, like
TCP.
PPSP-TP is a text-based protocol, uses the UTF-8 character set
[RFC3629] and the protocol messages are either requests from client
peers to a tracker server, or responses from a tracker server to
client peers.
5. Request/Response model
PPSP-TP Request and Response semantics are carried as entities
(header and body) in messages which correspond to either HTTP/1.1
request methods or HTTP/1.1 response codes, respectively.
Requests are sent, and responses returned to these requests. A
single request generates a single response (neglecting fragmentation
of messages in transport).
The response codes are consistent with HTTP/1.1 response codes,
however, not all HTTP/1.1 response codes are used for the PPSP-TP
(section 7).
The Request Messages of the base protocol, are listed in Table 1:
+---------------+
| PPSP-TP/1.0 |
| Req. Messages |
+---------------+
| CONNECT |
| STAT_REPORT |
+---------------+
Table 1: Request Messages
CONNECT: This Request message is used when a peer registers (or if
already registered) in the tracker notifies it about the
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participation in named swarm(s). The tracker records the Peer-ID,
connect-time (referenced to the absolute time), peer IP addresses and
link status. The tracker also changes the content availability of
the valid named swarm(s), i.e., changes the peers lists of the
corresponding swarm(s) for the requester Peer-ID. On receiving a
CONNECT message, the tracker first checks the peer mode type
(Seed/Leech) for the specified swarm(s) and then decides the next
steps (more details are referred in section 8.1)
STAT_REPORT: This Request message allows an active peer to send
status data to the tracker to signal continuing activity. This
request message MUST be sent periodically to the tracker while the
peer is active in the system.
6. State Machines and Flows of the Protocol
The state machine for the tracker is very simple, as shown in
Figure 4.
Peer-ID registrations represent a dynamic piece of state maintained
by the network.
+------+ +---------+ +------------+
| INIT |------------>| STARTED |----------->| TERMINATED |
+------+ +---------+ +------------+
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.
In addition to the tracker state machine, each Peer-ID is modeled
with its own transaction state machine (Figure 5), instantiated per
Peer-ID in the tracker, and deleted when it is removed. 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.
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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).
+-----------+ +-------+ rcv CONNECT
| TERMINATE | | START | --------------- (1)
+-----------+ +-------+ strt init timer
^ |
| |
on registration error | v
on action error | +------------+
---------------- (A) +<-----| PEER | (Transient)
stop init timer | | REGISTERED |
snd error | +------------+
| |
| | process swarm actions
| | --------------------- (2)
on CONNECT Error (B) | | snd OK (PeerList)
on timeout (5) | / stop init timer
---------------- | / strt track timer
stop track timer | /
clean peer info | |
del registration | |
snd error (B) \ | ----
---- \ | / \
/ \ \ | | |
rcv CONNECT | (4) | | | | |
----------- | v | v v | rcv STAT_REPORT
snd OK \ +--------------+ / --------------- (3)
rst track timer ----| TRACKING |---- snd OK response
+--------------+ rst track timer
Figure 5: Per-Peer-ID Transaction State Machine and Flow Diagram
When a new Peer-ID is added, the corresponding "per-Peer-ID" state
machine is instantiated, and it moves into the PEER REGISTERED state.
Because of that, the START state here is transient.
When the Peer-ID is no longer bound to a registration, the "per-Peer-
ID" state machine moves to the TERMINATE state, and the state machine
is destroyed.
During the lifetime of streaming activity of a peer, the "per-Peer-
ID" transaction state machine progresses from one state to another in
response to various events. The events that may potentially advance
the state include:
o Reception of CONNECT and STAT_REPORT messages, or
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o Timeout events.
The state diagram in Figure 5 illustrates state changes, together
with the causing events and resulting actions. Specific error
conditions are not shown in the state diagram.
6.1. Normal Operation
On normal operation the process consists of the following steps:
1) When a Peer wants to access the system it needs to register on a
tracker by sending a CONNECT message asking for the swarm(s) it
wants to join. This CONNECT request triggers a new "per-Peer-ID"
State machine. In the START state the tracker initiates the
registration of the Peer-ID and associated information (IP
addresses), starts the "init timer" and moves to PEER REGISTERED
state.
2) In PEER REGISTERED state, if Peer-ID is valid, the tracker
processes the requested action(s) for the valid swarm information.
In case of success the tracker stops the "init timer", starts the
"track timer" and sends the response to the peer. The response
MAY contain the appropriate list of peers for the joining
swarm(s), depending on peer mode (section 8.1). At the PEER
REGISTERED state, if Peer-ID is considered invalid, the tracker
responds with either error codes 401 Unauthorized or 403 Forbidden
(described in section 7), and transitions to TERMINATE state for
that Peer-ID.
3) In TRACKING state, a STAT_REPORT message received from the peer
resets the "track timer" and is responded with a successful
condition.
4) While TRACKING, a CONNECT message received with valid swarm
actions information (section 8.1) from the peer resets the "track
timer" and is responded with a successful condition.
5) In TRACKING state, without receiving messages from the peer, on
timeout (track timer) the tracker cleans all the information
associated with the Peer-ID in all swarms it was joined, deletes
the registration, and transitions to TERMINATE state for that
Peer-ID.
6.2. Error Conditions
Peers MUST NOT generate protocol elements that are invalid.
However, several situations of a peer may lead to abnormal
conditions in the interaction with the tracker. The situations
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may be related with peer malfunction or communications errors.
The tracker reacts to the abnormal situations depending on its
current state related to a peer-ID, as follows:
A) At PEER REGISTERED state, when a CONNECT Request only contains
invalid swarm actions (section 8.1), the tracker responds with
error code 403 Forbidden, deletes the registration and transition
to TERMINATE state for that Peer-ID.
B) At the TRACKING state (while the "track timer" has not expired)
receiving a CONNECT message from the peer with invalid swarm
actions (section 8.1) is considered an error condition. The
tracker responds with error code 403 Forbidden (described in
section 7), stops the "track timer", deletes the registration and
transitions to TERMINATE state for that Peer-ID.
NOTE: These situations may correspond to a malfunction at the peer
or to malicious conditions. Therefore, the adequate preventive
measure is to proceed to TERMINATE state for the Peer-ID by de-
registering the peer and cleaning all peer information.
7. Protocol Specification
7.1. Messages Syntax
PPSP-TP messages use the generic message format of RFC 5322 [RFC5322]
for transferring the payload of the message (Requests and
Responses).
PPSP-TP messages consist of a start-line, one or more header fields,
an empty line indicating the end of the header fields, and, when
applicable, a message-body.
The start-line, each message-header line, and the empty line MUST be
terminated by a carriage-return line-feed sequence (CRLF). Note that
the empty line MUST be present even if the message-body is not.
The PPSP-TP message and header field syntax is identical to HTTP/1.1
[RFC2616].
A Request message is a standard HTTP/1.1 message starting with a
Request-Line generated by the HTTP client peer. The Request-Line
contains a method name, a Request-URI, and the protocol version
separated by a single space (SP) character.
Request-Line =
Method SP Request-URI SP HTTP-Version CRLF
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A Request message example is the following:
<Method> /<Resource> HTTP/1.1
Host: <Host>
Content-Lenght: <ContentLenght>
Content-Type: <ContentType>
Authorization: <AuthToken>
[Request_Body]
The HTTP Method token and Request-URI (the Resource) identifies the
resource upon which to apply the operation requested.
The Response message is also a standard HTTP/1.1 message starting
with a Status-Line generated by the tracker. The Status-Line
consists of the protocol version followed by a numeric Status-Code
and its associated Reason-Phrase, with each element separated by a
single SP character.
Status-Line =
HTTP-Version SP Status-Code SP Reason-Phrase CRLF
A Response message example is the following:
HTTP/1.1 <Status-Code> <Reason-Phrase>
Content-Lenght: <ContentLenght>
Content-Type: <ContentType>
Content-Encoding: <ContentCoding>
[Response_Body]
The Status-Code element is a 3-digit integer result code that
indicates the outcome of an attempt to understand and satisfy a
request.
The Reason-Phrase element is intended to give a short textual
description of the Status-Code.
7.1.1. Header Fields
The header fields are identical to HTTP/1.1 header fields in both
syntax and semantics.
Some header fields only make sense in requests or responses. If a
header field appears in a message not matching its category (such as
a request header field in a response), it MUST be ignored.
The Host request-header field in the request message follows the
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standard rules for the HTTP/1.1 Host header.
The Content-Type entity-header field MUST be used in requests and
responses containing message-bodies to define the Internet media type
of the message-body.
The Content-Encoding entity-header field MAY be used in response
messages with "gzip" compression scheme [RFC1952] for faster
transmission times and less network bandwidth usage.
The Content-Length entity-header field MUST be used in messages
containing message-bodies to locate the end of each message in a
stream.
The Authorization header field in the request message allows a peer
to authenticate itself with a tracker, containing authentication
information.
7.1.2. Methods
PPSP-TP uses HTTP/1.1 POST method token for all request messages.
7.1.3. Message Bodies
PPSP-TP requests MUST contain message-bodies.
PPSP-TP responses MAY include a message-body.
If the message-body has undergone any encoding such as compression,
then this MUST be indicated by the Content-Encoding header field;
otherwise, Content-Encoding MUST be omitted.
The character set of the message body is indicated as part of the
Content-Type header-field, and the default value for PPSP-TP messages
is "UTF-8".
7.1.4. Message Response Codes
The response codes in PPSP-TP response messages are consistent with
HTTP/1.1 response status-codes. However, not all HTTP/1.1 response
status-codes are appropriate for PPSP-TP, and only those that are
appropriate are given here. Other HTTP/1.1 response codes SHOULD NOT
be used in PPSP-TP.
The class of the response is defined by the first digit of the
Status-Code. The last two digits do not have any categorization
role. For this reason, any response with a Status-Code between 200
and 299 is referred to as a "2xx response", and similarly to the
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other supported classes:
2xx: Success -- the action was successfully received, understood, and
accepted;
4xx: Peer Error -- the request contains bad syntax or cannot be
fulfilled at this tracker;
5xx: Tracker Error -- the tracker failed to fulfill an apparently
valid request;
The valid response codes are the following (Status-Code Reason-
Phrase):
200 OK -- The request has succeeded. The information returned with
the response describes or contains the result of the action;
400 Bad Request -- The request could not be understood due to
malformed syntax.
401 Unauthorized -- The request requires authentication.
403 Forbidden -- The tracker understood the request, but is refusing
to fulfill it. The request SHOULD NOT be repeated.
404 Not Found -- This status is returned if the tracker did not find
anything matching the Request-URI.
408 Request Timeout -- The peer did not produce a request within the
time that the tracker was prepared to wait.
411 Length Required -- The tracker refuses to accept the request
without a defined Content-Length. The peer MAY repeat the request
if it adds a valid Content-Length header field containing the
length of the message-body in the request message.
414 Request-URI Too Long -- The tracker is refusing to service the
request because the Request-URI is longer than the tracker is
willing to interpret. This rare condition is likely to occur
when the tracker is under attack by a client attempting to
exploit security holes.
500 Internal Server Error -- The tracker encountered an unexpected
condition which prevented it from fulfilling the request.
503 Service Unavailable -- The tracker is currently unable to handle
the request due to a temporary overloading or maintenance
condition.
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7.2. Request/Response Syntax and Format
The message-body for Requests and Responses requiring it, is encoded
in XML [XML].
The XML message-body MUST begin with an XML declaration line
specifying the version of XML being used and indicating the character
encoding, that SHOULD be "UTF-8". The root element MUST begin with
an <PPSPTrackerProtocol> element. This element identifies the start
of an PPSP-TP protocol element, the namespace used within the
protocol, and the location of the protocol schema.
The generic format of a Request is the following:
<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Request></Request>
<TransactionID></TransactionID>
<PeerID></PeerID>
<SwarmID></SwarmID>
<PeerNum></PeerNum>
<PeerGroup></PeerGroup>
<StatisticsGroup></StatisticsGroup>
</PPSPTrackerProtocol>
The generic format of a Response is the following:
<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Response></Response>
<TransactionID></TransactionID>
<PeerGroup></PeerGroup>
</PPSPTrackerProtocol>
The Request element MUST be present in requests and corresponds to
the request method type for the message.
The Response element MUST be present in responses and corresponds to
the response method type of the message.
The element TransactionID MUST be present in requests to uniquely
identify the transaction. Responses to completed transactions use
the same TransactionID as the request they correspond to.
The element SwarmID MUST be present in requests to identify the
actions to be taken in the specified swarms.
The version of PPSP-TP being used is indicated by the attribute
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@version of the root element, specified in an XML Schema
([XMLSchema.1] and [XMLSchema.2]) with XML namespaces [XMLNameSpace]
to identify protocol grammars.
All Request messages MUST contain a PeerID element to uniquely
identify the peer (Peer-ID) in the network.
The PeerID information may be present on the following levels:
- On PPSPTrackerProtocol level in PPSPTrackerProtocol.PeerID element.
For details refer to 7.2.1 Table 2.
- On PeerGroup level in PeerGroup.PeerInfo.PeerID element. For
details refer to 7.2.1 Table 3.
The SwarmID element MUST be be present in CONNECT Requests and SHOULD
be present in STAT_REPORT Requests on the following levels:
- On PPSPTrackerProtocol level in PPSPTrackerProtocol.SwarmID
element. For details refer to 7.2.1 Table 2.
- On StatisticsGroup level in StatisticsGroup.Stat.SwarmID element.
For details refer to 7.2.1 Table 4.
The PeerNum element MAY be present in CONNECT requests and MAY
contain the attribute @abilityNAT to inform the tracker on the
preferred type of peers, in what concerns their NAT traversal
situation, to be returned in a peer list.
The StatisticsGroup element MAY be present in STAT_REPORT requests.
Details of usage in 8.2.
The semantics of the attributes and elements within a
PPSPTrackerProtocol root element is described in subsection 7.2.1.
Request and Response processing is provided in section 8 for each
message.
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7.2.1. Semantics of PPSPTrackerProtocol elements
The semantics of PPSPTrackerProtocol elements and attributes are
described in the following tables.
+----------------------+---------+----------------------------------+
| Element Name or | Use | Description |
| Attribute Name | | |
+----------------------+---------+----------------------------------+
| PPSPTrackerProtocol | 1 | The root element. |
| @version | M | Provides the version of PPSP-TP. |
| Request | 0...1 | Provides the request method |
| | | and MUST be present in Request. |
| Response | 0...1 | Provides the response method |
| | | and MUST be present in Response. |
| TransactionID | M | Root transaction Identification. |
| Result | 0...N | Result of @action MUST be present|
| | | in Responses. |
| @transactionID | CM | Identifier of the @action. |
| PeerID | 0...1 | Peer Identification. |
| | | MUST be present in Request. |
| SwarmID | 0...N | Swarm Identification. |
| | | MUST be present in Requests. |
| @action | CM | Must be set to JOIN or LEAVE. |
| @peerMode | CM | Mode of Peer participation in |
| | | the swarm, "LEECH" or "SEED". |
| @transactionID | CM | Identifier for the @action. |
| PeerNUM | 0...1 | Maximum peers to be received |
| | | with capabilities indicated. |
| @abilityNAT | CM | Type of NAT traversal peers, as |
| | | "No-NAT","STUN","TURN" or "PROXY"|
| @concurrentLinks | CM | Concurrent connectivity level of |
| | | peers, "HIGH", "LOW" or "NORMAL" |
| @onlineTime | CM | Availability or online duration |
| | | of peers, "HIGH" or "NORMAL" |
| @uploadBWlevel | CM | Upload bandwidth capability of |
| | | peers, "HIGH" or "NORMAL" |
| PeerGroup | 0...1 | Information on peers (Table 3) |
| StatisticsGroup | 0...1 | Statistic data of peer (Table 4) |
+----------------------+---------+----------------------------------+
| Legend: |
| Use for attributes: M=Mandatory, OP=Optional, |
| CM=Conditionally Mandatory |
| Use for elements: minOccurs...maxOccurs (N=unbounded) |
| Elements are represented by their name (case-sensitive) |
| Attribute names (case-sensitive) are preceded with an @ |
+-------------------------------------------------------------------+
Table 2: Semantics of the base PPSPTrackerProtocol.
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+----------------------+---------+----------------------------------+
| Element Name or | Use | Description |
| Attribute Name | | |
+----------------------+---------+----------------------------------+
| PeerGroup | 0...1 | Contains description of peers. |
| PeerInfo | 1...N | Provides information on a peer. |
| @swarmID | 0...1 | Swarm Identification. |
| PeerID | 0...1 | Peer Identification. |
| | | MAY be present in responses. |
| PeerAddress | 1...N | IP Address information. |
| @addrType | M | Type of IP address, which can be |
| | | "ipv4" or "ipv6" |
| @priority | CM | The priority of this interface. |
| | | Used for NAT traversal. |
| @type | CM | Describes the address for NAT |
| | | traversal, which can be "HOST" |
| | | "REFLEXIVE" or "PROXY". |
| @connection | OP | Access type ("3G", "ADSL", etc.) |
| @asn | OP | Autonomous System number. |
| @ip | M | IP address value. |
| @port | M | IP service port value. |
+----------------------+---------+----------------------------------+
| Legend: |
| Use for attributes: M=Mandatory, OP=Optional, |
| CM=Conditionally Mandatory |
| Use for elements: minOccurs...maxOccurs (N=unbounded) |
| Elements are represented by their name (case-sensitive) |
| Attribute names (case-sensitive) are preceded with an @ |
+-------------------------------------------------------------------+
Table 3: Semantics of PeerGroup.
If STUN-like functions are enabled in the tracker and a PPSP-ICE
method is used the attributes @type and @priority MUST be returned
with the transport address candidates in responses to CONNECT
requests.
The @asn attribute MAY be used to inform about the network location,
in terms of Autonomous System, for each of the active public network
interfaces of the peer.
The @connection attribute is informative on the type of access
network of the respective interface.
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+----------------------+---------+----------------------------------+
| Element Name or | Use | Description |
| Attribute Name | | |
+----------------------+---------+----------------------------------+
| StatisticsGroup | 0...1 | Provides statistic data on peer |
| | | and content. |
| Stat | 1...N | Groups statistics property data. |
| @property | M | The property to be reported. |
| | | Property values in Table 6. |
| SwarmID | 0...1 | Swarm Identification. |
| UploadedBytes | 0...1 | Bytes sent to swarm. |
| DownloadedBytes | 0...1 | Bytes received from swarm. |
| AvailBandwidth | 0...1 | Upstream Bandwidth available. |
+----------------------+---------+----------------------------------+
| Legend: |
| Use for attributes: M=Mandatory, OP=Optional, |
| CM=Conditionally Mandatory |
| Use for elements: minOccurs...maxOccurs (N=unbounded) |
| Elements are represented by their name (case-sensitive) |
| Attribute names (case-sensitive) are preceded with an @ |
+-------------------------------------------------------------------+
Table 4: Semantics of StatisticsGroup.
The Stat element is used to describe several properties relevant to
the P2P network. These properties can be related with stream
statistics and peer status information. Each Stat element will
correspond to a @property type and several Stat blocks can be
reported in a single STAT_REPORT message, corresponding to some or
all the swarms the peer is actively involved.
Other properties may be defined, related, for example, with
incentives and reputation mechanisms, like peer online time, or
connectivity conditions, like physical link status, etc.
For that purpose, the Stat element may be extended to provide
additional scheme specific information for new @property groups, new
elements and new attributes.
7.2.2. Request element in request Messages
Table 5 defines the valid string representations for the requests.
These values MUST be treated as case-sensitive.
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+----------------------+
| XML Request Methods |
| String Values |
+----------------------+
| CONNECT |
| STAT_REPORT |
+----------------------+
Table 5: Valid Strings for Request element of requests.
7.2.3. Response element in response Messages
Table 6 defines the valid string representations for Response
messages that require message-body. These values MUST be treated as
case-sensitive.
Response messages not requiring message-body only use the standard
HTTP/1.1 Status-Code and Reason-Phrase (appended, if appropriate,
with detail phrase, as described in section 8.6).
+-------------------------+---------------------+
| XML Response Method | HTTP Status-Code |
| String Values | and Reason-Phrase |
+-------------------------+---------------------+
| SUCCESSFUL | 200 OK |
| AUTHENTICATION REQUIRED | 401 Unauthorized |
+------------------------+----------------------+
Table 6: Valid Strings for Response element of responses.
SUCCESSFUL: indicates that the request has been processed properly
and the desired operation has completed. The body of the response
message includes the requested information and MUST include the same
TransactionID of the corresponding request.
In CONNECT Request: returns information about the successful
registration of the peer and/or of each swarm @action requested.
MAY additionally return the list of peers corresponding to the
join @action requested.
In STAT_REPORT Request: confirms the success of the requested
operation.
AUTHENTICATION REQUIRED: Authentication is required for the peer to
make the request.
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8. Request/Response Processing
When a PPSP-TP message is received some basic processing is
performed, regardless of the message type.
Upon reception, a message is examined to ensure that it is properly
formed. The receiver MUST check that the HTTP message itself is
properly formed, and if not, appropriate standard HTTP errors MUST be
generated. The receiver must also verify that the XML body is
properly formed. In case of error due to malformed messages
appropriate responses MUST be returned, as described in 8.6.
8.1. CONNECT Request
This method is used when a peer registers to the system and/or
requests swarm actions.
The peer MUST properly form the XML message-body, set the Request
method to CONNECT, generate and set the TransactionIDs, set the
PeerID with the identifier of the peer and include SwarmID action(s)
to be performed. The peer SHOULD also include the IP addresses of
its network interfaces in the CONNECT message.
An example of the message-body of a CONNECT Request for a peer Seeder
is the following:
<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Request>CONNECT</Request>
<PeerID>656164657220</PeerID>
<SwarmID action="JOIN" peerMode="SEED"
transactionID="12345.1">1111</SwarmID>
<SwarmID action="JOIN" peerMode="SEED"
transactionID="12345.2">2222</SwarmID>
<TransactionID>12345.0</TransactionID>
<PeerGroup>
<PeerInfo>
<PeerAddress addrType="ipv4" ip="192.0.2.1" port="80"
priority="1" />
<PeerAddress addrType="ipv6" ip="2001:db8::1" port="80"
priority="2"
type="HOST"
connection="ADSL" />
</PeerInfo>
</PeerGroup>
</PPSPTrackerProtocol>
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An example of the message-body of a CONNECT Request for a peer
Leecher requesting join to a swarm is the following:
<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Request>CONNECT</Request>
<PeerID>656164657221</PeerID>
<PeerNum abilityNAT="STUN">5</PeerNum>
<SwarmID action="JOIN" peerMode="LEECH"
transactionID="12345.1">1111</SwarmID>
<TransactionID>12345.0</TransactionID>
<PeerGroup>
<PeerInfo>
<PeerAddress addrType="ipv4" ip="192.0.2.2" port="80"
priority="1" />
<PeerAddress addrType="ipv6" ip="2001:db8::2" port="80"
priority="2"
type="HOST"
connection="3G" />
</PeerInfo>
</PeerGroup>
</PPSPTrackerProtocol>
An example of the message-body of a CONNECT Request for a peer
Leecher "leaving" a previously joined swarm and requesting join to a
new swarm is the following:
<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Request>CONNECT</Request>
<PeerID>656164657221</PeerID>
<PeerNum abilityNAT="STUN">5</PeerNum>
<SwarmID action="LEAVE" peerMode="LEECH"
transactionID="12345.1">1111</SwarmID>
<SwarmID action="JOIN" peerMode="LEECH"
transactionID="12345.2">2222</SwarmID>
<TransactionID>12345.0</TransactionID>
<PeerGroup>
<PeerInfo>
<PeerAddress addrType="ipv4" ip="192.0.2.2" port="80"
priority="1" />
<PeerAddress addrType="ipv6" ip="2001:db8::2" port="80"
priority="2"
type="HOST"
connection="3G" />
</PeerInfo>
</PeerGroup>
</PPSPTrackerProtocol>
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When receiving a well-formed CONNECT Request message, the tracker
MAY, when applicable, start by pre-processing the peer authentication
information (provided as Authorization scheme and token in the HTTP
message) to check whether it is valid and that it can connect to the
service, then proceed to register the peer in the service and perform
the swarm actions requested. In case of success a Response message
with a corresponding response value of SUCCESSFUL will be generated.
The element SwarmID MUST be present with cardinality 1 to N, each
containing the request for @action, the @peerMode of the peer and the
child @transactionID for that swarm. The @peerMode element MUST be
set to the type of participation of the peer in the swarm (SEED or
LEECH).
The valid sets of SwarmID @action combinations for the CONNECT
Request logic in PPSP-TP base protocol are summarized in Table 7
(referring to the tracker State Machine).
+-----------+-----------+---------+----------+-----------+----------+
| SwarmID | @peerMode | @action | Initial | Final | Request |
| Elements | value | value | State | State | validity |
+-----------+-----------+---------+----------+-----------+----------|
| 1 | LEECH | JOIN | START | TRACKING | Valid |
+-----------+-----------+---------+----------+-----------+----------+
| 1 | LEECH | LEAVE | START | TERMINATE | Invalid |
+-----------+-----------+---------+----------+-----------+----------+
| 1 | LEECH | LEAVE | TRACKING | TERMINATE | Valid |
+-----------+-----------+---------+----------+-----------+----------+
| 1 | LEECH | JOIN | START | TERMINATE | Invalid |
| 1 | LEECH | LEAVE | | | |
+-----------+-----------+---------+----------+-----------+----------+
| 1 | LEECH | JOIN | TRACKING | TRACKING | Valid |
| 1 | LEECH | LEAVE | | | |
+-----------+-----------+---------+----------+-----------+----------+
| N | SEED | JOIN | START | TRACKING | Valid |
+-----------+-----------+---------+----------+-----------+----------+
| N | SEED | JOIN | TRACKING | TERMINATE | Invalid |
+-----------+-----------+---------+----------+-----------+----------+
| N | SEED | LEAVE | TRACKING | TERMINATE | Valid |
+-----------+-----------+---------+----------+-----------+----------+
Table 7: Validity of SwarmID @action combinations in CONNECT Request.
The element PeerInfo MAY contain multiple PeerAddress child elements
with attributes @addrType, @ip, and @port, and optionally @priority
and @type (if PPSP-ICE NAT traversal techniques are used)
corresponding to each of the network interfaces of the peer.
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The element PeerNum indicates to the tracker the number of peers to
be returned in a list corresponding to the indicated properties,
being @abilityNAT for NAT traversal (considering that PPSP-ICE NAT
traversal techniques may be used), and optionally @concurrentLinks,
@onlineTime and @uploadBWlevel for the preferred capabilities.
If STUN-like function is enabled in the tracker, the response MAY
include the peer reflexive address.
The response MUST have the same TransactionID values as the
corresponding request and actions.
An example of a Response message for the CONNECT Request is the
following:
<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Response>SUCCESSFUL</Response>
<TransactionID>
<Result transactionID="12345.0">200 OK</Result>
<Result transactionID="12345.1">200 OK</Result>
<Result transactionID="12345.2">200 OK</Result>
</TransactionID>
<PeerGroup>
<PeerInfo>
<PeerID>656164657221</PeerID>
<PeerAddress addrType="ipv4" ip="198.51.100.1" port="80"
priority="1"
type="REFLEXIVE"
connection="3G"
asn="64496" />
</PeerInfo>
<PeerInfo swarmID="2222">
<PeerID>956264622298</PeerID>
<PeerAddress addrType="ipv4" ip="198.51.100.22" port="80"
asn="64496"/>
</PeerInfo>
<PeerInfo swarmID="2222">
<PeerID>3332001256741</PeerID>
<PeerAddress addrType="ipv4" ip="198.51.100.201" port="80"
asn="64496"/>
</PeerInfo>
</PeerGroup>
</PPSPTrackerProtocol>
The Response MUST include a PeerGroup with PeerInfo data of the
requesting peer public IP address. If STUN-like function is enabled
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in the tracker, the PeerAddress includes the attribute @type with a
value of REFLEXIVE, corresponding to the transport address
"candidate" of the peer. The PeerGroup MAY also include PeerInfo data
corresponding to the Peer-IDs and public IP addresses of the selected
active peers in the requested swarm.
The tracker MAY also include the attribute @asn with network location
information of the transport address, corresponding to the Autonomous
System Number of the access network provider.
In case the @peerMode is SEED, the tracker responds with a SUCCESSFUL
response and enters the peer information into the corresponding swarm
activity.
In case the @peerMode is LEECH the tracker will search and select an
appropriate list of peers satisfying the conditions set by the
requesting peer. The peer list returned MUST contain the Peer-IDs
and the corresponding IP Addresses. To create the peer list, the
tracker may take peer status and network location information into
consideration, to express network topology preferences or Operators'
policy preferences, with regard to the possibility of connecting with
other IETF efforts such as ALTO [I.D.ietf-alto-protocol].
8.2. STAT_REPORT Request
This method allows peers to send status and statistic data to
trackers. The method is initiated by the peer, periodically while
active.
The peer MUST properly form the XML message-body, set the Request
method to STAT_REPORT, set the PeerID with the identifier of the
peer, and generate and set the TransactionID.
The report MAY include a StatisticsGroup containing multiple Stat
elements describing several properties relevant to the P2P network.
These properties can be related with stream statistics and peer
status information.
Other properties may be defined, related for example, with incentives
and reputation mechanisms.
In case no StatisticsGroup is included, the STAT_REPORT may be used
as a "keep-alive" message, to prevent the Tracker from de-registering
the peer when timer expired.
An example of the message-body of a STAT_REPORT Request is the
following:
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<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Request>STAT_REPORT</Request>
<PeerID>656164657221</PeerID>
<TransactionID>12345</TransactionID>
<StatisticsGroup>
<Stat property="StreamStatistics">
<SwarmID>1111</SwarmID>
<UploadedBytes>512</UploadedBytes>
<DownloadedBytes>768</DownloadedBytes>
<AvailBandwidth>1024000</AvailBandwidth>
</Stat>
</StatisticsGroup>
</PPSPTrackerProtocol>
If the request is valid the tracker processes the received
information for future use, and generates a response message with a
Response value of SUCCESSFUL.
The response MUST have the same TransactionID value as the request.
An example of a Response message for the START_REPORT Request is the
following:
<?xml version="1.0" encoding="UTF-8"?>
<PPSPTrackerProtocol xmlns="TBD" schemaLocation="TBD" version="1.0">
<Response>SUCCESSFUL</Response>
<TransactionID>12345</TransactionID>
</PPSPTrackerProtocol>
8.3. Error and Recovery conditions
If the peer fails to read the tracker response, the same Request with
identical content, including the same TransactionID, SHOULD be
repeated, if the condition is transient.
The TransactionID on a Request can be reused if and only if all of
the content is identical, including eventual Date/Time information.
Details of the retry process (including time intervals to pause,
number of retries to attempt, and timeouts for retrying) are
implementation dependent.
The tracker SHOULD be prepared to receive a Request with a repeated
TransactionID.
Error situations resulting from the Normal Operation or from abnormal
conditions (section 6.2) MUST be responded with the adequate response
codes, as described here:
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If the message is found to be incorrectly formed, the receiver MUST
respond with a 400 (Bad Request) response with an empty message-
body. The Reason-Phrase SHOULD identify the syntax problem in more
detail, for example, "Missing Content-Type header field".
If the version number of the protocol is for a version the receiver
does not supports, the receiver MUST respond with a 400 (Bad
Request) with an empty message-body. Additional information SHOULD
be provided in the Reason-Phrase, for example, "PPSP Version #.#".
If the length of the message does not matches the Content-Length
specified in the message header, or the message is received without
a defined Content-Length, the receiver MUST respond with a 411
(Length Required) response with an empty message-body.
If the Request-URI in a Request message is longer than the tracker
is willing to interpret, the tracker MUST respond with a 414
(Request-URI Too Long) response with an empty message-body.
In the PEER REGISTERED and TRACKING states of the tracker, certain
requests are not allowed (section 6.2). The tracker MUST respond
with a 403 (Forbidden) response with an empty message-body. The
Reason-Phrase SHOULD identify the error condition in more detail, for
example, "Action not allowed".
If the tracker is unable to process a Request message due to
unexpected condition, it SHOULD respond with a 500 (Internal Server
Error) response with an empty message-body.
If the tracker is unable to process a Request message for being in an
overloaded state, it SHOULD respond with a 503 (Service Unavailable)
response with an empty message-body.
9. Security Considerations
P2P streaming systems are subject to attacks by malicious/unfriendly
peers/trackers that may eavesdrop on signaling, forge/deny
information/knowledge about streaming content and/or its
availability, impersonating to be another valid participant, or
launch DoS attacks to a chosen victim.
No security system can guarantees complete security in an open P2P
streaming system where participants may be malicious or
uncooperative. The goal of security considerations described here is
to provide sufficient protection for maintaining some security
properties during the tracker-peer communication even in the face of
a large number of malicious peers and/or eventual distrustful
trackers (under the distributed tracker deployment scenario).
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Since the protocol uses HTTP to transfer signaling most of the same
security considerations described in RFC 2616 also apply [RFC2616].
9.1. Authentication between Tracker and Peers
To protect the PPSP-TP signaling from attackers pretending to be
valid peers (or peers other than themselves) all messages received in
the tracker SHOULD be received from authorized peers.
For that purpose a peer SHOULD enroll in the system via a centralized
enrollment server. The enrollment server is expected to provide a
proper Peer-ID for the peer and information about the authentication
mechanisms. The specification of the enrollment method and the
provision of identifiers and authentication tokens is out of scope of
this specification.
A Channel-oriented security mechanism should be used in the
communication between peers and tracker, such as the Transport Layer
Security (TLS) to provide privacy and data integrity.
Due to the transactional nature of the communication between peers
and tracker the method for adding authentication and data security
services can be the OAuth 2.0 Authorization [I-D.ietf-oauth-v2] with
bearer token, which provides the peer with the information required
to successfully utilize an access token to make protected requests to
the tracker [I-D.ietf-oauth-v2-bearer].
9.2. Content Integrity protection against polluting peers/trackers
Malicious peers may declaim ownership of popular content to the
tracker but try to serve polluted (i.e., decoy content or even
virus/trojan infected contents) to other peers.
This kind of pollution can be detected by incorporating integrity
verification schemes for published shared contents. As content
chunks are transferred independently and concurrently, a
correspondent chunk-level integrity verification MUST be used,
checked with signed fingerprints received from authentic origin.
9.3. Residual attacks and mitigation
To mitigate the impact of Sybil attackers, impersonating a large
number of valid participants by repeatedly acquiring different peer
identities, the enrollment server SHOULD carefully regulate the rate
of peer/tracker admission.
There is no guarantee that peers honestly report their status to the
tracker, or serve authentic content to other peers as they claim to
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the tracker. It is expected that a global trust mechanism, where the
credit of each peer is accumulated from evaluations for previous
transactions, may be taken into account by other peers when selecting
partners for future transactions, helping to mitigate the impact of
such malicious behaviors. A globally trusted tracker MAY also take
part of the trust mechanism by collecting evaluations, computing
credit values and providing them to joining peers.
9.4. Pro-incentive parameter trustfulness
Property types for STAT_REPORT messages may consider pro-incentive
parameters, which can enable the tracker to improve the performance
of the whole P2P streaming system.
Trustworthiness of these pro-incentive parameters is critical to the
effectiveness of the incentive mechanisms.
Furthermore, both the amount of uploaded and downloaded data should
be reported to the tracker to allow checking if there is any
inconsistency between the upload and download report, and establish
an appropriate credit/trust system. Alternatively, exchange of
cryptographic receipts signed by receiving peers can be used to
attest to the upload contribution of a peer to the swarm, as
suggested in [Contracts].
10. Guidelines for Extending PPSP-TP
Extension mechanisms allow designers to add new features or to
customize existing features of a protocol for different operating
environments.
Since the beginning that extensibility has been a design goal of
PPSP-TP, as the protocol is represented in the Extensible Markup
Language [XML].
A powerful feature of XML is the extensibility, but this feature also
provides multiple opportunities to make poor design decisions.
Extending a protocol implies either the addition of features without
changing the protocol itself or the addition of new elements creating
new versions of an existing schema and therefore new versions of the
protocol.
In PPSP-TP it means that an extension MUST NOT alter an existing
protocol schema as the changes would result in a new version of an
existing schema, not an extension of an existing schema, typically
non-backwards-compatible.
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Additionally, a designer MUST remember that extensions themselves MAY
also be extensible.
Extensions MUST adhere to the principles described in this section in
order to be considered valid.
Extensions MAY be documented as Internet-Draft and RFC documents if
there are requirements for coordination, interoperability, and broad
distribution.
Extensions need not be published as Internet-Draft or RFC documents
if they are intended for operation in a closed environment or are
otherwise intended for a limited audience.
10.1. Forms of PPSP-TP Extension
PPSP-TP extensions MUST be specified in XML. This ensures that
parsers capable of processing PPSP-TP structures will also be capable
of processing PPSP-TP extensions. Guidelines for the use of XML in
IETF protocols can be found in RFC 3470 [RFC3470].
In PPSP-TP two extension mechanisms can be used: a Request-Response
Extension or a Protocol-level Extension.
o Request-Response Extension: Adding elements or attributes to an
existing element mapping in the schema is the simplest form of
extension. This form should be explored before any other. This
task can be accomplished by extending an existing element mapping.
For example, an element mapping for the StatisticsGroup (Section
7.2.1, Table 4) can be extended to include additional elements
needed to express status information about the activity of the
peer, such as OnlineTime for the Stat element:
<Stat property="StreamStatistics">
<OnlineTime>3600</OnlineTime>
</Stat>
Another example also for the StatisticsGroup is for additional
@property attributes and elements, such as those necessary to
report content chunk availability information:
<Stat property="ContentMap">
<SegmentInfo chunkmapSize="25">
A/8D/wP/A/8D/wP/A/8D/wP/A/8D/wP/....
</SegmentInfo>
</Stat>
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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 Leecher, and then DISCONNECT from the latter swarm but keeping
as Seeder for the first one. When deciding to leave the system,
the peer DISCONNECTs gracefully from it:
+--------+ +---------+
| Peer | | Tracker |
+--------+ +---------+
| |
|--CONNECT--------------------->|
|<--------------------------OK--|
|--JOIN(swarm_a;SEED)---------->|
|<--------------------------OK--|
: :
|--STAT_REPORT(activity)------->|
|<--------------------------Ok--|
: :
|--JOIN(swarm_b;LEECH)--------->|
|<-----------------OK+PeerList--|
: :
|--STAT_REPORT(ChunkMap_b)----->|
|<--------------------------Ok--|
: :
|--DISCONNECT(swarm_b)--------->|
|<--------------------------Ok--|
: :
|--STAT_REPORT(activity)------->|
|<--------------------------Ok--|
: :
|--DISCONNECT------------------>|
|<---------------------Ok(BYE)--|
Figure 6: Example of a session for a PPSP-TP extended version.
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10.2. Issues to Be Addressed in PPSP-TP Extensions
There are several issues that all extensions should take into
consideration.
- Overview of the Extension: It is RECOMMENDED that extensions to
PPSP-TP have a protocol overview section that discusses the basic
operation of the extension. The most important processing rules
for the elements in the message flows SHOULD also be mentioned.
- Backward Compatibility: One of the most important issues to
consider is whether the new extension is backward compatible with
the base PPST-TP.
- Syntactic Issues: Extensions that define new Request/Response
methods SHOULD use all capitals for the method name, keeping with
a long-standing convention in many protocols, such as HTTP. Method
names are case sensitive in PPSP-TP. Method names SHOULD be
shorter than 16 characters and SHOULD attempt to convey the
general meaning of the Request or Response.
- Semantic Issues: PPSP-TP extensions MUST clearly define the
semantics of the extensions. Specifically, the extension MUST
specify the behaviors expected from both the Peer and the Tracker
in processing the extension, with the processing rules in temporal
order of the common messaging scenario.
Processing rules generally specify actions to be taken on receipt
of messages and expiration of timers.
The extension SHOULD specify procedures to be taken in exceptional
conditions that are recoverable. Handling of unrecoverable errors
does not require specification.
- Security Issues: Being security an important component of any
protocol, designers of PPSP-TP extensions need to carefully
consider security requirements, namely authorization requirements
and requirements for end-to-end integrity.
- Examples of Usage: The specification of the extension SHOULD give
examples of message flows and message formatting and include
examples of messages containing new syntax. Examples of message
flows should be given to cover common cases and at least one
failure or unusual case.
11. IANA Considerations
There are presently no IANA considerations with this document.
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12. Acknowledgments
The authors would like to thank many people for for their help and
comments, particularly: Zhang Yunfei, Liao Hongluan, Roni Even,
Bhumip Khasnabish, Wu Yichuan, Peng Jin, Chi Jing, Zong Ning, Song
Haibin, Chen Wei, Zhijia Chen, Christian Schmidt, Lars Eggert, David
Harrington, Henning Schulzrinne, Kangheng Wu, Martin Stiemerling,
Jianyin Zhang, Johan Pouwelse and Arno Bakker.
The authors would also like to thank the people participating in the
EU FP7 project SARACEN (contract no. ICT-248474)
[refs.saracenwebpage] for contributions and feedback to this
document.
The views and conclusions contained herein are those of the authors
and should not be interpreted as necessarily representing the
official policies or endorsements, either expressed or implied, of
the SARACEN project or the European Commission.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[ISO.8601.2004] International Organization for Standardization,
"Data elements and interchange formats - Information
interchange - Representation of dates and times", ISO
Standard 8601, December 2004.
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[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and F.
Yergeau, "Extensible Markup Language (XML) 1.0 (5th ed)",
W3C REC-xml, November 2008, <http://www.w3.org/TR/REC-
xml>.
[XMLSchema.1] Thompson, H., Beech, D., Maloney, M. and N.
Mendelsohn, "XML Schema Part 1: Structures", W3C REC-
xmlschema-1, October 2004,
<http://www.w3.org/TR/xmlschema-1/>.
[XMLSchema.2] Biron, P. and A. Malhotra, "XML Schema Part 2:
Datatypes", W3C REC-xmlschema-2, October 2004,
<http://www.w3.org/TR/xmlschema-2/>.
[XMLNameSpace] Bray, T., Hollander, D., Layman, A., Tobin, R. and H.
Thompson, "Namespaces in XML", W3C REC-xml-names, December
2009, <http://www.w3.org/TR/REC-xml-names/>.
13.2. Informative References
[RFC1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, May 1996.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3470] Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines for
the Use of Extensible Markup Language (XML) within IETF
Protocols", BCP 70, RFC 3470, January 2003.
[I-D.ietf-ppsp-problem-statement] Zhang, Y., Zong, N., Camarillo, G.,
Seng, J. and Y. Yang, "Problem Statement of P2P Streaming
Protocol (PPSP)", draft-ietf-ppsp-problem-statement-08
(work in progress), February 2012.
[I-D.pantos-http-live-streaming] Pantos. R. and W. May, "HTTP Live
Streaming", draft-pantos-http-live-streaming-08 (work in
progress), March 2012.
[I.D.ietf-alto-protocol] Alimi, R., Penno, R. and Y. Yang, "ALTO
Protocol", draft-ietf-alto-protocol-11, (work in
progress), March 2012.
[I-D.ietf-oauth-v2] Hammer-Lahav, E., Recordon, D. and D. Hardt, "The
OAuth 2.0 Authorization Protocol," draft-ietf-oauth-v2-28
(work in progress), June 2012.
[I-D.ietf-oauth-v2-bearer] Jones, M., Hardt, D. and D. Recordon, "The
OAuth 2.0 Authorization Protocol: Bearer Tokens," draft-
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ietf-oauth-v2-bearer-21 (work in progress), June 2012.
[ISO.IEC.23009-1] ISO/IEC, "Information technology -- Dynamic
adaptive streaming over HTTP (DASH) -- Part 1: Media
presentation description and segment formats", ISO/IEC DIS
23009-1, Aug. 2011.
[refs.saracenwebpage] "SARACEN Project Website",
http://www.saracen-p2p.eu/.
[Contracts] Piatek, M., Venkataramani, A., Yang, R., Zhang, D. and A.
Jaffe, "Contracts: Practical Contribution Incentives for
P2P Live Streaming", in NSDI '10: USENIX Symposium on
Networked Systems Design and Implementation, April 2010.
Authors' Addresses
Rui Santos Cruz
IST/INESC-ID/INOV
Phone: +351.939060939
Email: rui.cruz@ieee.org
Gu Yingjie
Huawei
Phone: +86-25-56624760
Fax: +86-25-56624702
Email: guyingjie@huawei.com
Mario Serafim Nunes
IST/INESC-ID/INOV
Rua Alves Redol, n.9
1000-029 LISBOA, Portugal
Phone: +351.213100256
Email: mario.nunes@inov.pt
Jinwei Xia
Huawei
Nanjing, Baixia District 210001, China
Phone: +86-025-86622310
Email: xiajinwei@huawei.com
Joao P. Taveira
IST/INOV
Email: joao.silva@inov.pt
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