PPSP Rui S. Cruz
INTERNET-DRAFT Mario S. Nunes
Intended Status: Standards Track IST/INESC-ID/INOV
Expires: July 4, 2014 Yingjie Gu
Jinwei Xia
Huawei
Joao P. Taveira
IST/INOV
Deng Lingli
China Mobile
December 31, 2013
PPSP Tracker Protocol-Base Protocol (PPSP-TP/1.0)
draft-ietf-ppsp-base-tracker-protocol-03
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."
The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Copyright and License Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Operation and Protocol Architecture Overview . . . . . . . . . 8
2.1 Operation . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Enrollment and Bootstrap . . . . . . . . . . . . . . . . . 9
2.3 Architectural and Functional View . . . . . . . . . . . . . 11
2.3.1 Messaging Model . . . . . . . . . . . . . . . . . . . . 11
2.3.2 Request/Response model . . . . . . . . . . . . . . . . 11
2.4 State Machines and Flows of the Protocol . . . . . . . . . 13
2.4.1 Normal Operation . . . . . . . . . . . . . . . . . . . 15
2.4.2 Error Conditions . . . . . . . . . . . . . . . . . . . 15
3 Protocol Specification . . . . . . . . . . . . . . . . . . . . 17
3.1 Request/Response Syntax and Format . . . . . . . . . . . . 17
3.2 Request element in request Messages . . . . . . . . . . . . 22
3.3
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Operation and Protocol Architecture Overview . . . . . . . . . 9
2.1 Operation . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Enrollment and Bootstrap . . . . . . . . . . . . . . . . . 10
2.3 Architectural and Functional View . . . . . . . . . . . . . 12
2.3.1 Messaging Model . . . . . . . . . . . . . . . . . . . . 12
2.3.2 Request/Response model . . . . . . . . . . . . . . . . 13
2.4 State Machines and Flows of the Protocol . . . . . . . . . 14
2.4.1 Normal Operation . . . . . . . . . . . . . . . . . . . 16
2.4.2 Error Conditions . . . . . . . . . . . . . . . . . . . 17
3 Protocol Specification . . . . . . . . . . . . . . . . . . . . 18
3.1 Request/Response Syntax and Semantics . . . . . . . . . . . 18
3.2 Request element in request Messages . . . . . . . . . . . . 22
3.3 Response element in response Messages . . . . . . . . . . . 23
4 Request/Response Processing . . . . . . . . . . . . . . . . . . 23
4.1 CONNECT Request . . . . . . . . . . . . . . . . . . . . . . 24
4.2 FIND Request . . . . . . . . . . . . . . . . . . . . . . . 26
4.3 STAT_REPORT Request . . . . . . . . . . . . . . . . . . . . 27
4.4 Error and Recovery conditions . . . . . . . . . . . . . . . 27
5 Operations and Manageability . . . . . . . . . . . . . . . . . 29
5.1 Operational Considerations . . . . . . . . . . . . . . . . 29
5.1.1 Installation and Initial Setup . . . . . . . . . . . . 29
5.1.2 Migration Path . . . . . . . . . . . . . . . . . . . . 30
5.1.3 Requirements on Other Protocols and Functional
Components . . . . . . . . . . . . . . . . . . . . . . 30
5.1.4 Impact on Network Operation . . . . . . . . . . . . . . 30
5.1.5 Verifying Correct Operation . . . . . . . . . . . . . . 30
5.2 Management Considerations . . . . . . . . . . . . . . . . . 30
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5.2.1 Interoperability . . . . . . . . . . . . . . . . . . . 30
5.2.2 Management Information . . . . . . . . . . . . . . . . 31
5.2.3 Fault Management . . . . . . . . . . . . . . . . . . . 31
5.2.4 Configuration Management . . . . . . . . . . . . . . . 31
5.2.5 Accounting Management . . . . . . . . . . . . . . . . . 31
5.2.6 Performance Management . . . . . . . . . . . . . . . . 32
5.2.7 Security Management . . . . . . . . . . . . . . . . . . 32
6 Security Considerations . . . . . . . . . . . . . . . . . . . . 33
6.1 Authentication between Tracker and Peers . . . . . . . . . 33
6.2 Content Integrity protection against polluting
peers/trackers . . . . . . . . . . . . . . . . . . . . . . 33
6.3 Residual attacks and mitigation . . . . . . . . . . . . . . 34
6.4 Pro-incentive parameter trustfulness . . . . . . . . . . . 34
7 Guidelines for Extending PPSP-TP . . . . . . . . . . . . . . . 35
7.1 Forms of PPSP-TP Extension . . . . . . . . . . . . . . . . 35
7.2 Issues to Be Addressed in PPSP-TP Extensions . . . . . . . 36
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 37
9 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 37
10 References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1 Normative References . . . . . . . . . . . . . . . . . . . 38
10.2 Informative References . . . . . . . . . . . . . . . . . . 39
Appendix A. Revision History . . . . . . . . . . . . . . . . . . 42
Appendix B. PPSP-TP Message Syntax for HTTP/1.1 . . . . . . . . . 43
B.1 Header Fields . . . . . . . . . . . . . . . . . . . . . . . 44
B.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 44
B.3 Message Bodies . . . . . . . . . . . . . . . . . . . . . . 45
B.4 Message Response Codes . . . . . . . . . . . . . . . . . . 45
Appendix C. Use Scenarios . . . . . . . . . . . . . . . . . . . . 47
C.1 Additional Terminology . . . . . . . . . . . . . . . . . . 47
C.2 Functional Entities . . . . . . . . . . . . . . . . . . . . 48
C.3 Streaming Capabilities . . . . . . . . . . . . . . . . . . 49
C.4 NAT Traversal . . . . . . . . . . . . . . . . . . . . . . . 49
C.5 Content Information Metadata . . . . . . . . . . . . . . . 49
C.6 Authentication, Confidentiality, Integrity . . . . . . . . 50
Appendix D. Implementation Options . . . . . . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 51
Response element in response Messages . . . . . . . . . . . 22 4
Request/Response Processing . . . . . . . . . . . . . . . . . . 24
4.1 CONNECT Request . . . . . . . . . . . . . . . . . . . . . . 24
4.2 FIND Request . . . . . . . . . . . . . . . . . . . . . . . 28
4.3 STAT_REPORT Request . . . . . . . . . . . . . . . . . . . . 30
4.4 Error and Recovery conditions . . . . . . . . . . . . . . . 31 5
Operations and Manageability . . . . . . . . . . . . . . . . . 33
5.1 Operational Considerations . . . . . . . . . . . . . . . . 33
5.1.1 Installation and Initial Setup . . . . . . . . . . . . 33
5.1.2 Migration Path . . . . . . . . . . . . . . . . . . . . 34
5.1.3 Requirements on Other Protocols and Functional
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Components . . . . . . . . . . . . . . . . . . . . . . 34 5.1.4
Impact on Network Operation . . . . . . . . . . . . . . 34 5.1.5
Verifying Correct Operation . . . . . . . . . . . . . . 34 5.2
Management Considerations . . . . . . . . . . . . . . . . . 34
5.2.1 Interoperability . . . . . . . . . . . . . . . . . . . 34
5.2.2 Management Information . . . . . . . . . . . . . . . . 35
5.2.3 Fault Management . . . . . . . . . . . . . . . . . . . 35
5.2.4 Configuration Management . . . . . . . . . . . . . . . 35
5.2.5 Accounting Management . . . . . . . . . . . . . . . . . 35
5.2.6 Performance Management . . . . . . . . . . . . . . . . 36
5.2.7 Security Management . . . . . . . . . . . . . . . . . . 36 6
Security Considerations . . . . . . . . . . . . . . . . . . . . 37
6.1 Authentication between Tracker and Peers . . . . . . . . . 37
6.2 Content Integrity protection against polluting
peers/trackers . . . . . . . . . . . . . . . . . . . . . . 37 6.3
Residual attacks and mitigation . . . . . . . . . . . . . . 38 6.4
Pro-incentive parameter trustfulness . . . . . . . . . . . 38 7
Guidelines for Extending PPSP-TP . . . . . . . . . . . . . . . 39
7.1 Forms of PPSP-TP Extension . . . . . . . . . . . . . . . . 39
7.2 Issues to Be Addressed in PPSP-TP Extensions . . . . . . . 41
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 43
9 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 43
10 References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
10.1 Normative References . . . . . . . . . . . . . . . . . . . 44
10.2 Informative References . . . . . . . . . . . . . . . . . . 44
Appendix A. Revision History . . . . . . . . . . . . . . . . . . 47
Appendix B. PPSP-TP Message Syntax for HTTP/1.1 . . . . . . . . . 48
B.1 Header Fields . . . . . . . . . . . . . . . . . . . . . . . 49
B.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 49
B.3 Message Bodies . . . . . . . . . . . . . . . . . . . . . . 50
B.4 Message Response Codes . . . . . . . . . . . . . . . . . . 50
Appendix C. Use Scenarios . . . . . . . . . . . . . . . . . . . . 52
C.1 Additional Terminology . . . . . . . . . . . . . . . . . . 52
C.2 Functional Entities . . . . . . . . . . . . . . . . . . . . 53
C.3 Streaming Capabilities . . . . . . . . . . . . . . . . . . 54
C.4 NAT Traversal . . . . . . . . . . . . . . . . . . . . . . . 54
C.5 Content Information Metadata . . . . . . . . . . . . . . . 54
C.6 Authentication, Confidentiality, Integrity . . . . . . . . 55
Appendix D. Implementation Options . . . . . . . . . . . . . . . 55
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 56
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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
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 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].
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.
CHUNK: A Chunk is a basic unit of data organized in P2P streaming
for storage, scheduling, advertisement and exchange among peers.
CHUNK ID: A unique resource identifier for a SEGMENT 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.
CONNECTION TRACKER: The node running the tracker service to which
the PPSP peer will connect when it wants to get registered and join
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the PPSP system.
LEECHER: A Peer that has not yet completed the transfer of all
Chunks of the media content.
LIVE STREAMING: It refers to a scenario where all the audiences
receive streaming content for the same ongoing event. It is desired
that the lags between the play points of the audiences and streaming
source be small.
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 joined. 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 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 which are in a same SWARM maintained by
the TRACKER. A PEER can fetch the PEER LIST of a SWARM from the
TRACKER or from other PEERs in order to know which PEERs have the
required streaming content.
PPSP: The abbreviation of Peer-to-Peer Streaming Protocols. PPSP
refer to the primary signaling protocols among various P2P streaming
system components, including the TRACKER and the PEER.
PPSP-TP: The abbreviation of Peer-to-Peer Streaming Protocols -
Tracker Protocol.
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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.
SEEDER: A Peer that holds and shares the complete media content.
SEGMENT: 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.
SEGMENT CHUNK: For Structured Media contents, a segment may
correspond to a set of nested and dependent CHUNKs, a set of
correlated additive CHUNKs or a set of alternate REPRESENTATION
CHUNKs.
SWARM: A SWARM refers to a group of PEERs who 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.
SUPER-NODE: A SUPER-NODE is a special kind of PEER deployed by ISPs.
This kind of PEER is more stable with higher computing, storage and
bandwidth capabilities than normal 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. Also, the TRACKER answers PEER
LIST queries received from PEERs. The TRACKER is a logical component
which can be centralized or distributed.
TRANSACTION ID: The identifier of a REQUEST from the PEER to the
TRACKER. Used to disambiguate RESPONSES that may arrive in a
different order of the corresponding REQUESTs.
VIDEO-ON-DEMAND (VoD): It refers to a scenario where different
audiences may watch different parts of the same recorded streaming
with downloaded content.
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2 Operation and Protocol Architecture Overview
2.1 Operation
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 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 [RFC2616].
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.
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 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.
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 (Leech
mode):
1. Peer connects to a connection tracker and joins a swarm.
2. Peer acquires a list of other peers in the swarm from the
connection tracker.
3. Peer exchanges its content availability with the peers on the
obtained peer list (the 3~5 steps are in the peer protocol).
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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 the connection tracker.
2. Peer sends information to the connection tracker about the swarms
it belongs to (joined swarms).
After having been disconnected due to some termination condition, a
peer can resume previous activity by connecting and re-joining the
corresponding swarm(s).
2.2 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 the
following (not exhaustive) typical methods (illustrated in Figures 1
and 2):
+--------+ +--------+ +--------+ +---------+ +--------+
| Leecher| | 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--| |
: : : : :
| |--FIND----------------------->| |
| |<----------------OK+Peerlist--| |
: : : : :
|--Get(Chunk)--->|<---------- (Peer protocol) ------------->|
|<--------Chunk--|<---------------------------------Chunks--|
: : : : :
Figure 1: A typical PPSP session for a streaming Leecher.
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A leecher logins on the portal to select the swarm it are interested,
then the portal return the Media Presentation Description (MPD) file
of the selected swarm to the leecher. The MPD includes the
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(e.g.,content x).
Then the leecher starts to initiate a PPSP-TP CONNECT message to the
tracker, or resume a previously initiated swarm at point (b). Tracker
will return OK code with the peer list to the leecher if the CONNECT
message is successfully accepted.
Once the CONNECT is successfully accepted, the leecher needs to
report its status and statisc data to the tracker periodically.
If the leecher plan to switch to another straw, it will initiate a
FIND message to the tracker to ask the information of new swarm.
+---------+ +---------+
| 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.
For the seeder usually located at edge caches and/or Media Servers,
the swarms it can provide are reported to tracker through CONNECT
message.
In order to be able to bootstrap, either leecher or seeder must first
obtain a Peer ID (identifier of the peer) and any required security
certificates or authorization tokens from an enrollment service (end
user registration). How the peer obtain above information is not in
the scope of this document.
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Peer may support different encoding type (e.g., text or binary), in
which case, the portal needs to select an appropriate tracker
supporting the same encoding type as the peer. How the portal learn
the encoding type supported by the peer is not in the scope of this
document.
The specification of the mechanisms used by the Client Media Player
and the peer (to signal start/resume streams or request media
chunks), obtain a Peer ID, security certificates or tokens are not in
the scope of this document.
2.3 Architectural and Functional View
The PPSP Tracker Protocol architecture is intended to be compatible
with the web infrastructure. PPSP-TP is designed with a layered
approach i.e., a PPSP-TP Request/Response layer, a Message layer and
a Transport layer. The PPSP-TP Request/Response layer deals with the
interactions between tracker and peers using Request and Response
codes (see Figure 3).
+----------------------+
| Application |
+----------------------+
| Request/Response | PPSP-TP
|----------------------|
| (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 a connection-oriented transport like TCP
[RFC0793], or TLS [RFC5246] over it.
2.3.1 Messaging Model
The messaging model of PPSP-TP aligns with HTTP protocol and the
semantics of its messages, currently in version 1.1 [RFC2616], but
intended to support future versions of HTTP. The exchange of
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messages of PPSP-TP is envisioned to be performed over a stream-
oriented reliable transport protocol, like TCP [RFC0793].
Appendix B describes the message syntax when using HTTP/1.1.
2.3.2 Request/Response model
PPSP-TP messages 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.
Requests are sent, and responses returned to these requests. A
single request generates a single response (neglecting fragmentation
of messages in transport).
The response codes are consistent with HTTP response codes, however,
not all HTTP response codes are used for the PPSP-TP (section 3 and
Appendix B.4).
The Request Messages of the base protocol are listed in Table 1:
+---------------+
| PPSP-TP/1.0 |
| Req. Messages |
+---------------+
| CONNECT |
| FIND |
| STAT_REPORT |
+---------------+
Table 1: Request Messages
CONNECT: This Request message is an "action signal" 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 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 4.1)
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FIND: This Request message is an "action signal" 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 an "information signal" that
allows 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.
2.4 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)
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 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
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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.
--------------------------------------------
/ \
| +------------+ +=========+ +======+ |
\-| 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 | +------------+
| |
| | process swarm actions
| | --------------------- (2)
on CONNECT Error (B) | | snd OK (PeerList)
on timeout (C) | / stop init timer
---------------- | / strt track timer
stop track timer | /
clean peer info | |
del registration | | rcv FIND
snd error (B) \ | ---- --------------- (3)
---- \ | / \ snd OK (PeerList)
/ \ \ | | | rst track timer
rcv CONNECT | (4) | | | | |
----------- | v | v v | rcv STAT_REPORT
snd OK \ +==============+ / --------------- (3)
rst track timer ----| TRACKING |---- snd OK response
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+==============+ rst track timer
Figure 5: Per-Peer-ID Transaction State Machine and Flow Diagram
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, 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.
2.4.1 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 from a new Peer ID triggers
the instantiation of a "per-Peer-ID" State Machine. In the START
state of the new "per-Peer-ID" SM, 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 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 are respectively
responded 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) resets the "track
timer" and is responded with a successful condition.
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2.4.2 Error Conditions
Peers MUST NOT generate protocol elements that are invalid. However,
several situations of a peer may lead to abnormal conditions in the
interaction with the tracker. The situations may be related with
peer malfunction or communications errors. The tracker reacts to the
abnormal situations depending on its current state related to a Peer
ID, as follows:
A) At PEER REGISTERED state, when a CONNECT Request only contains
invalid swarm actions (section 4.1), the tracker responds with
error code 403 Forbidden, 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 401 Unauthorized or 403
Forbidden (described in section 4.4), 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 4.1) is considered an error condition. The
tracker responds with error code 403 Forbidden (described in
section 3), stops the "track timer", deletes the registration,
transitions to TERMINATE state for that Peer ID and the SM is
destroyed.
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 and the SM is destroyed.
NOTE: These situations may correspond to a malfunction at the peer
or to malicious conditions. Therefore, as preventive measure, the
tracker proceeds to TERMINATE state for the Peer ID by de-registering
the peer and cleaning all peer information.
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3 Protocol Specification
3.1 Request/Response Syntax and Semantics
PPSP-TP is a message-oriented request/response protocol. The messages
could be encoded using binary type or text type, which can be
indicated in the Content-Type field in HTTP/1.1 [RFC2616], but is not
in the scope of this draft. In this draft, only the syntax and format
of the PPSP-TP messages are defined, at here, a C-like syntax based
on the presentation language used to defined TLS [RFC5246] is
recommended to present the message structures.
The generic format of a Request is the following:
struct {
uint8 request_type;
uint64 transaction_id;
PeerInfo peer_information;
select (request_type){
case CONNECT :
unit16 swarm_length;
SwarmInfo swarm_information[swarm_length];
uint16 peer_number;
PeerAttr peer_attribute;
case FIND :
unit16 swarm_length;
SwarmInfo swarm_information[swarm_length];
uint16 peer_number;
PeerAttr peer_attribute;
case STAT_REPORT :
uint16 swarm_length;
StatisticsGroup statistics_list[swarm_length];
}
} PPSP_TP_Req;
struct {
uint64 peer_id_low;
uint64 peer_id_high;
IPv4AddrPort v4addr_port;
IPv6AddrPort v6addr_port;
} PeerInfo;
struct {
uint32 addr;
uint16 port;
uint8 priority
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uint8 type; /* REFLEXIVE or PROXY */
uint8 connection; /* 3G, ADSL etc */
uint32 asn;
} IPv4AddrPort;
struct {
uint128 addr;
uint16 port;
uint8 priority
uint8 type; /* REFLEXIVE or PROXY */
uint8 connection; /* 3G, ADSL etc */
uint32 asn;
} IPv6AddrPort;
struct {
uint64 swarm_id_low;
uint64 swarm_id_high;
uint8 action_type; /* JOIN or LEAVE */
uint8 peer_mode; /* LEECH or SEED */
} SwarmInfo;
struct {
uint8 abilityNAT;
uint8 concurrentLinks;
uint8 onlineTime;
uint8 uploadBWlevel;
} PeerAttr;
struct {
uint64 swarm_id_low;
uint64 swarm_id_high;
uint64 uploadedBytes;
uint64 downloadedBytes;
uint64 availBandwidth;
} StatisticsGroup;
The Request element MUST be present in requests and corresponds to
the request method type for the message. The request type includes
CONNECT, FIND and STAT_REPORT.
The element Transaction_ID MUST be present in requests to uniquely
identify the transaction. Responses to completed transactions use
the same TransactionID as the request they correspond to.
All Request messages MUST contain a Peer_Info element to uniquely
identify the requesting peer in the network. It includes the Peer_ID,
IP addresses and ports, with a few of optional attributes.
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The Peer_ID information may be present on the following levels:
- The identifier of the requesting peer on PPSP-TP level.
- The identifier of the candidate peer from tracker on PeerGroup
level.
The peer also MAY include some attributes:
- Priority: the preference of IP address on which the requesting peer
get the swarm.
- Type: Describes the address for NAT traversal, which can be HOST
REFLEXIVE or PROXY.
- Connection: Access type (3G, ADSL, etc.).
- ASN: Autonomous System Number.
If STUN-like functions are enabled in the tracker and a PPSP-ICE
method [RFC5245] 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.
The Swarm_Info element includes the Swarm_ID and the actions on the
specific swarm.
The element Swarm_ID MUST be present in requests to identify the
actions to be taken to the specified swarms. It MUST be present in
CONNECT and FIND Requests and SHOULD be present in STAT_REPORT
Requests on the following levels:
- The identifier of the swarm requested by peer On PPSP-TP level.
- The identifier of the swarm whose statistics are be reported to
tracker on StatisticsGroup level.
The swarm also include a couple of attributes:
- Action: This behavior MUST be set to JOIN or LEAVE.
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- Peer_Mode: Mode of Peer participation in the swarm MUST be set to
LEECH or SEED.
The Peer_Number element MAY be present in CONNECT and FIND requests
and MAY contain the Peer_Attribute to inform the tracker the numbers
of the candidate peers the requesting peer hope to get, followed by
the optional attributes of the candidate peers:
- AbilityNAT: Type of NAT traversal peers, as No-NAT, STUN, TURN or
PROXY.
- ConcurrentLinks: Concurrent connectivity level of peers, HIGH, LOW
or NORMAL.
- OnlineTime: Availability or online duration of peers, HIGH or
NORMAL.
- UploadBWlevel: Upload bandwidth capability of peers, HIGH or
NORMAL.
The StatisticsGroup element MAY be present in STAT_REPORT requests.
Several properties relevant to P2P network are presented:
- Swarm_ID: To identify the specified swarms whose properties need to
be reported.
- UploadedBytes: Bytes sent to swarm.
- DownloadedBytes: Bytes received from swarm.
- AvailBandwidth: Upstream Bandwidth available.
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
sibling elements and new attributes (guidelines in section 7).
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The generic format of a Response is the following:
struct {
uint8 response_code;
uint64 transaction_id;
select (request_type){
case CONNECT :
PeerInfo peer_information;
PeerGroup peer_group;
case FIND :
PeerInfo peer_information;
PeerGroup peer_group;
} PPSP_TP_Res;
struct {
unit16 swarm_length;
SwarmGroup swarm_group[swarm_length];
} PeerGroup;
struct {
uint64 swarm_id_low;
uint64 swarm_id_high;
uint16 peer_list_length;
PeerInfo peer_list[peer_list_length];
} SwarmGroup;
The Response element MUST be present in responses and corresponds to
the response method type of the message.
The element Transaction_ID 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 PeerGroup element MAY be present in Response to CONNECT and FIND
requests, and MAY contain the peer list for the requested swarm.
Request and Response processing is provided in section 4 for each
message.
3.2 Request element in request Messages
Table 6 defines the valid string representations for the requests.
These values MUST be treated as case-sensitive.
+----------------------+
| Request Types |
+----------------------+
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| CONNECT 0 |
| FIND 1 |
| STAT_REPORT 2 |
+----------------------+
Table 6: Request Type of PPSP-TP requests.
3.3 Response element in response Messages
Table 7 defines the valid code for Response messages that require
message-body.
Response messages not requiring message-body only use the standard
HTTP Status-Code and Reason-Phrase (appended, if appropriate, with
detail phrase, as described in section 4.4).
+-------------------------+---------------------+
| Response Code | HTTP Status-Code |
| | and Reason-Phrase |
+-------------------------+---------------------+
| SUCCESSFUL 0 | 200 OK |
| Error_AUTHENTICATION 1 | 401 Unauthorized |
+-------------------------+---------------------+
Table 7: Valid Codes 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.
To CONNECT Request: returns information about the successful
registration of the peer and/or of each swarm to which Action
element requested. MAY additionally return the list of peers
corresponding to the JOIN attribute requested.
To FIND Request: returns the list of peers corresponding to the
requested swarm.
To STAT_REPORT Request: confirms the success of the requested
operation.
Error_AUTHENTICATION: request is denied by the tracker.
4 Request/Response Processing
Upon reception, a message is examined to ensure that it is properly
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formed. The receiver MUST check that the HTTP message itself is
properly formed, and if not, appropriate standard HTTP errors MUST be
generated.
4.1 CONNECT Request
This method is used when a peer registers to the system and/or
requests swarm actions.
The peer MUST properly set the Request type to CONNECT, generate and
set the TransactionIDs, set the PeerInfo and MAY include the swarm
the peer is interested in, followed by the corresponding action_type
and peer_mode.
The following example of the message-body of a CONNECT Request
corresponds to a peer that wants to start (or re-start) sharing its
previously streamed contents (peerMode is of SEED). Note for this
case that the peer also requests from the Tracker an appropriate list
of peers (PeerNum element) already active in the swarm, i.e., a list
of 15 peers having STUN capabilities in terms of NAT. In the case of
a Super-Node peer of an ISP, the CONNECT request would be similar
but, optionally not including the PeerNum element:
When a peer agrees to share its previous swarm to others, it should
set action_type to JOIN to the swarm, as well set the peer_mode to
SEED during its start (or re-start)period.
When a peer requests to join a swarm, it should set action_type to
JOIN to the swarm , as well set the peer_mode to LEECH during its
start (or re-start)period.
In above two cases, the peers could 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 to the swarm regardless of the peer_mode.
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 valid sets of SwarmID whose action_type is combined with
peer_mode for the CONNECT Request logic are enumerated in Table 8
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(referring to the tracker "per-Peer-ID" state machine in section
2.4).
+-----------+-----------+---------+----------+-----------+----------+
| 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 8: Validity of action _type and peer_mode combinations
The element PeerInfo, if present, 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 the peer wants to
advertise.
The element peer_number 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.
When a peer's request to join a swarm as leecher was accepted by the
tracker, the Response message tracker will search out the list of
peers for the swarm and select an appropriate peer list satisfying
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the conditions set by the requesting peer besides of the SUCCESSFUL
response_code. The peer list returned MUST contain 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.
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].
When a peer's request to join a swarm as seeder was accepted by the
tracker, the tracker responds with a SUCCESSFUL response_code and
enters the peer information into the corresponding swarm activity.
IMPLEMENTATION NOTE: If no PeerNum attributes are present in the
request the tracker MAY return a random sample from the peer
population.
4.2 FIND Request
This method allows peers to request to the tracker, whenever needed,
a new peer list for the swarm.
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 AbilityNAT for NAT traversal (considering that PPSP-ICE NAT
traversal techniques may be used), and optionally ConcurrentLinks,
OnlineTime and UploadBWlevel 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 peer status and network location information
into consideration when selecting the peer list to return, 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].
To provide more choices for the requesting peer, the tracker may
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select a new peer list with lower priority from the list of peers and
return it to the requesting peer later.
The peer list MUST contain 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.
IMPLEMENTATION NOTE: If no PeerNum attributes are present in the
request the tracker MAY return a random sample from the peer
population.
4.3 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 report MAY include a StatisticsGroup containing multiple
statistics elements describing several properties relevant to a
specific swarm. These properties can be related with stream
statistics and peer status information, including ploadedBytes,
DownloadedBytes, AvailBandwidth and etc.
Other properties may be defined (guidelines in section 7.1) related
for example, with incentives and reputation mechanisms. In case no
StatisticsGroup 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 TransactionID value as the request.
4.4 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 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.
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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 2.4.2) MUST be responded with the adequate
response codes, as described here:
If the message is found to be incorrectly formed, the receiver MUST
respond with a 400 (Bad Request) response with an empty message-
body. The Reason-Phrase SHOULD identify the syntax problem in more
detail, for example, "Missing Content-Type header field".
If the version number of the protocol is for a version the receiver
does not supports, the receiver MUST respond with a 400 (Bad
Request) with an empty message-body. Additional information SHOULD
be provided in the Reason-Phrase, for example, "PPSP Version #.#".
If the length of the received 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 2.4.2). 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.
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5 Operations and Manageability
This section provides the operational and managements aspects that
are required to be considered in implementations of the PPSP Tracker
Protocol. These aspects follow the recommendations expressed in
RFC 5706 [RFC5706].
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.
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 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.
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5.1.2 Migration Path
Since there is no previous standard protocol providing similar
functionality, this specification does not details a migration path.
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.
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
servers (Trackers) and clients (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 functionalities for
PPSP-TP devices can be achieved with Simple Network Management
Protocol (SNMP) [RFC3410], syslog [RFC5424] and NETCONF [RFC6241].
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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 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 server (Tracker)
and client (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
from a standard way of replicating atomic configuration updates over
a set of server nodes. This functionality can be provided via
NETCONF [RFC6241].
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.
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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 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.
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6 Security Considerations
P2P streaming systems are subject to attacks by malicious/unfriendly
peers/trackers that may eavesdrop on signaling, forge/deny
information/knowledge about streaming content and/or its
availability, impersonating to be another valid participant, or
launch DoS attacks to a chosen victim.
No security system can guarantees complete security in an open P2P
streaming system where participants may be malicious or
uncooperative. The goal of security considerations described here is
to provide sufficient protection for maintaining some security
properties during the tracker-peer communication even in the face of
a large number of malicious peers and/or eventual distrustful
trackers (under the distributed tracker deployment scenario).
Since the protocol uses HTTP to transfer signaling most of the same
security considerations described in RFC 2616 also apply [RFC2616].
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.
A channel-oriented security mechanism should be used in the
communication between peers and tracker, such as the Transport Layer
Security (TLS) to provide privacy and data integrity.
Due to the transactional nature of the communication between peers
and tracker the method for adding authentication and data security
services can be the OAuth 2.0 Authorization [RFC6749] with bearer
token, which provides the peer with the information required to
successfully utilize an access token to make protected requests to
the tracker [RFC6750].
6.2 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.
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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.
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 (such as those of
property="StreamStatistics" in Table 5 of section 3.2) 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].
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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 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.
7.1 Forms of PPSP-TP Extension
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 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.
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,
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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.
7.2 Issues to Be Addressed in PPSP-TP Extensions
There are several issues that all extensions should take into
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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.
8 IANA Considerations
There are presently no IANA considerations with this document.
9 Acknowledgments
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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.
Rui Cruz, Mario Nunes and Joao Taveira are partially supported by the
SARACEN project [SARACEN], a research project of the European Union
7th Framework Programme (contract no. ICT-248474).
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, the European Commission, Huawei or China Mobile.
This I-D document was prepared using NroffEdit version 2.08
(http://aaa-sec.com/nroffedit/).
10 References
10.1 Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[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.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and F.
Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", W3C xml, November 2008,
.
[XMLSchema.1] Thompson, H., Beech, D., Maloney, M. and N.
Mendelsohn, "XML Schema Part 1: Structures Second
Edition", W3C xmlschema-1, October 2004,
.
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[XMLSchema.2] Biron, P. and A. Malhotra, "XML Schema Part 2:
Datatypes Second Edition", W3C xmlschema-2, October 2004,
.
[XMLNameSpace] Bray, T., Hollander, D., Layman, A., Tobin, R. and H.
Thompson, "Namespaces in XML", W3C REC-xml-names, December
2009, .
[EXI] Schneider, J. and T. Kamiya, "Efficient XML Interchange (EXI)
Format 1.0", W3C exi, March 2011,
.
10.2 Informative References
[RFC1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, May 1996.
[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.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 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.
[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.
[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.
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[RFC4150] Dietz, R. and R. Cole, "Transport Performance Metrics
MIB", RFC 4150, August 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.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, November 2009.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, June 2011.
[RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
September 2012.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, October 2012.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012.
[RFC6972] Zhang, Y. and N. Zong, "Problem Statement and Requirements
of the Peer-to-Peer Streaming Protocol (PPSP)", RFC 6972,
July 2013.
[I-D.ietf-httpbis-http2] Belshe, M., Twist, Peon, R., Thomson, M.,
and A. Melnikov, "Hypertext Transfer Protocol version
2.0", draft-ietf-httpbis-http2-06 (work in progress),
August 2013.
[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.
[I.D.ietf-alto-protocol] Alimi, R., Penno, R. and Y. Yang, "ALTO
Protocol", draft-ietf-alto-protocol-20, (work in
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progress), October 2013.
[I-D.pantos-http-live-streaming] Pantos, R. and W. May, "HTTP Live
Streaming", draft-pantos-http-live-streaming-12 (work in
progress), October 2013.
[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.
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Appendix A. Revision History
-00 2013-02-14 Initial version.
-01 2013-02-14 Minor revision.
* Terminology definitions related with streaming media contents
moved to Appendix C. - Use Scenarios and Assumptions.
* Terminology updated to follow reviewed PPSP Problem Statement
[RFC6972] definitions.
- Definition of JOIN TIME, PEER-PEER MESSAGES and TRACKER-PEER
MESSAGES removed.
+ Definitions of PEER LIST, CHUNK ID, PPSP-TP, SEGMENT CHUNK,
TRANSACTION ID added.
+ Definitions of SEGMENT updated.
+ Descriptions of Client Media Player and service Portal,
related to PPSP protocols, were added to section 2.1.
+ In section 2.2, "not exhaustive" and "typical" was added to
first paragraph.
+ In section 2.3.2, the description of CONNECT was updated to
clarify the information recorded and maintained by the
tracker related with Peer addressing, location and activity
mode in the swarms.
+ In section 3.1 and Table 2, PeerNum element justified to be
limited to 30 entries.
+ Section 6.4 enhanced with complementary information on
incentive mechanisms.
+ Section 5 completed.
-02 2013-10-21 Minor revision.
+ In section 2.2, Figure 1 was complemented to include the FIND
request.
- In section 2.3.1, references to HTTP/2.0 and framing formats
were removed.
+ In section 2.3.2, Table 1 was updated with FIND request
message and the corresponding description added in text.
+ In section 2.4, Figure 5 and the text were updated to reflect
the FIND request message.
+ In sections 2.4.1 and 2.4.2, the PEER REGISTERED and TRACKING
state descriptions were modified to include the FIND request
message.
- In sections 3.1, references to EXI were removed and text was
modified to reflect the FIND request message.
+ In sections 3.3, Table 1 was updated with FIND request
message.
+ In sections 3.4, the text was updated to include the response
to FIND requests.
+ Section 4.2 renamed to 4.3, and new section 4.2 describes in
detail the processing of FIND requests. All references to
previous section 4.2 updated to 4.3.
+ Appendix D added.
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Appendix B. PPSP-TP Message Syntax for HTTP/1.1
PPSP-TP messages use the generic message format of RFC 5322 [RFC5322]
for transferring the payload of the message (Requests and Responses).
PPSP-TP messages consist of a start-line, one or more header fields,
an empty line indicating the end of the header fields, and, when
applicable, a message-body.
The start-line, each message-header line, and the empty line MUST be
terminated by a carriage-return line-feed sequence (CRLF). Note that
the empty line MUST be present even if the message-body is not.
The PPSP-TP message and header field syntax is identical to HTTP/1.1
[RFC2616].
A Request message is a standard HTTP/1.1 message starting with a
Request-Line generated by the HTTP client peer. The Request-Line
contains a method name, a Request-URI, and the protocol version
separated by a single space (SP) character.
Request-Line =
Method SP Request-URI SP HTTP-Version CRLF
A Request message example is the following:
/ HTTP/1.1
Host:
Content-Lenght:
Content-Type:
Authorization:
[Request_Body]
The HTTP Method token and Request-URI (the Resource) identifies the
resource upon which to apply the operation requested.
The Response message is also a standard HTTP/1.1 message starting
with a Status-Line generated by the tracker. The Status-Line
consists of the protocol version followed by a numeric Status-Code
and its associated Reason-Phrase, with each element separated by a
single SP character.
Status-Line =
HTTP-Version SP Status-Code SP Reason-Phrase CRLF
A Response message example is the following:
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HTTP/1.1
Content-Lenght:
Content-Type:
Content-Encoding:
[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.
B.1 Header Fields
The header fields are identical to HTTP/1.1 header fields in both
syntax and semantics.
Some header fields only make sense in requests or responses. If a
header field appears in a message not matching its category (such as
a request header field in a response), it MUST be ignored.
The Host request-header field in the request message follows the
standard rules for the HTTP/1.1 Host header.
The Content-Type entity-header field MUST be used in requests and
responses containing message-bodies to define the Internet media type
of the message-body.
The Content-Encoding entity-header field MAY be used in response
messages with "gzip" compression scheme [RFC1952] for faster
transmission times and less network bandwidth usage.
The Content-Length entity-header field MUST be used in messages
containing message-bodies to locate the end of each message in a
stream.
The Authorization header field in the request message allows a peer
to authenticate itself with a tracker, containing authentication
information.
B.2 Methods
PPSP-TP uses HTTP/1.1 POST method token for all request messages.
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B.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".
B.4 Message Response Codes
The response codes in PPSP-TP response messages are consistent with
HTTP/1.1 response status-codes. However, not all HTTP/1.1 response
status-codes are appropriate for PPSP-TP, and only those that are
appropriate are given here. Other HTTP/1.1 response codes SHOULD NOT
be used in PPSP-TP.
The class of the response is defined by the first digit of the
Status-Code. The last two digits do not have any categorization
role. For this reason, any response with a Status-Code between 200
and 299 is referred to as a "2xx response", and similarly to the
other supported classes:
2xx: Success -- the action was successfully received, understood, and
accepted;
4xx: Peer Error -- the request contains bad syntax or cannot be
fulfilled at this tracker;
5xx: Tracker Error -- the tracker failed to fulfill an apparently
valid request;
The valid response codes are the following (Status-Code Reason-
Phrase):
200 OK -- The request has succeeded. The information returned with
the response describes or contains the result of the action;
400 Bad Request -- The request could not be understood due to
malformed syntax.
401 Unauthorized -- The request requires authentication.
403 Forbidden -- The tracker understood the request, but is refusing
to fulfill it. The request SHOULD NOT be repeated.
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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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Appendix C. Use Scenarios
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.
C.1 Additional Terminology
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.
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.
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.
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.
SCALABLE STREAMING: With Multiple Description Coding (MDC), multiple
additive descriptions (that can be independently played-out) to
refine the quality of the video when combined together. With
Scalable Video Coding (SVC), nested dependent enhancement layers
(hierarchical levels of quality), refine the quality of lower layers,
from the lowest level (the playable Base Layer). With Multiple View
Coding (MVC), multiple views nested in dependent enhancement layers
(levels of quality) allow the video to be played in compatible 3D
rendering devices when the views are combined together.
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C.2 Functional Entities
The functional entities related to PPSP protocols are the Client
Media Player, the service Portal, the tracker and the peers.
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].
The service Portal is a logical entity typically used for client
enrollment and content information publishing, searching and
retrieval.
The tracker is a logical entity that maintains the lists of PPSP
active peers storing and exchanging a specific media content. The
tracker also stores the status of active peers in swarms, to help in
the selection of appropriate peers for a requesting peer. The
tracker can be realized by geographically distributed tracker nodes
or multiple server nodes in a data center, increasing the content
availability, the service robustness and the network scalability or
reliability. The management and locating of content index
information are totally internal behaviors of the tracker system,
which is invisible to the PPSP Peer.
The peer is also a logical entity in the client device embedding the
P2P core engine, with a client serving side interface to respond to
Client Media Player requests and a network side interface to exchange
data and PPSP signaling with trackers and other peers.
The streaming technique is chunk-based, i.e., client peers obtain
media chunks from serving peers and handle the buffering that is
necessary for the playback processes during the download of the media
chunks.
In Live streaming, all end users are interested in a specific media
coming from an ongoing program, which means that all respective peers
share nearly the same streaming content at a given point of time.
Peers may store the live media for further distribution (known as
time-shift TV), where the stored media is distributed in a VoD-like
manner.
In VoD, different end users watch different parts of the recorded
media content during a past event. In this case, each respective
peer obtains from other peers the information on media chunks they
store and then get the required media from a selected set of those
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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.
C.3 Streaming Capabilities
This section is tutorial in nature and does not contain any normative
statements.
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.
C.4 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
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.
C.5 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.
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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].
C.6 Authentication, Confidentiality, Integrity
Channel-oriented security can be used in the communication between
peers and tracker, such as the Transport Layer Security (TLS) to
provide privacy and data integrity. HTTP/1.1 over TLS (HTTPS)
[RFC2818] is the preferred approach for preventing disclosure of peer
critical information via the communication channel.
Due to the transactional nature of the communication between peers
and tracker a method for adding authentication and data security
services via replaceable mechanisms may be employed. One such method
is the OAuth 2.0 Authorization [RFC6749] with bearer token, providing
the peer with the information required to successfully utilize the
access token to make protected requests to the tracker [RFC6750].
Appendix D. Implementation Options
With newer versions of HTTP, the framing formats used for encoding
and transmitting the data over the wire, e.g., the encapsulation
proposed for HTTP/2.0 [I-D.ietf-httpbis-http2] may be used in PPSP-
TP.
Similarly, for compact on the wire representation, PPSP-TP Requests
and Responses can be represented in a binary form using, for example,
Efficient XML Interchange (EXI) Format [EXI].
When using the above binary format for both the transport protocol
and the PPSP messages the returned Peer List from a CONNECT or a FIND
Request should be restricted to a number of entries that makes the
total message size smaller than 1 KiB, allowing it to fit inside a
single IP packet. The RECOMMENDED approach is to limit the list to 30
entries (less than 1KiB in size), as this size has a high likelihood
of traveling across the Internet without fragmentation, or be
transmitted as a single IP packet over an Ethernet network (1500 byte
frame size).
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Authors' Addresses
Rui Santos Cruz
IST/INESC-ID/INOV
Phone: +351.939060939
Email: rui.cruz@ieee.org
Gu Yingjie
Email: guyingjie@gmail.com
Mario Serafim Nunes
IST/INESC-ID/INOV
Rua Alves Redol, n.9
1000-029 LISBOA, Portugal
Phone: +351.213100256
Email: mario.nunes@inov.pt
Jinwei Xia
Huawei
Nanjing, Baixia District 210001, China
Phone: +86-025-86622310
Email: xiajinwei@huawei.com
Joao P. Taveira
IST/INOV
Email: joao.silva@inov.pt
Deng Lingli
China Mobile
Email: denglingli@chinamobile.com
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