Internet Engineering Task Force (IETF) R. Cruz Request for Comments: 7846 M. Nunes Category: Standards Track IST/INESC-ID/INOV ISSN: 2070-1721 J. Xia R. Huang, Ed. Huawei J. Taveira IST/INOV D. Lingli China Mobile May 2016
Peer-to-Peer Streaming Tracker Protocol (PPSTP)
This document specifies the base Peer-to-Peer Streaming Tracker Protocol (PPSTP) version 1, 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; it also describes message flows, message processing instructions, message formats, formal syntax, and semantics. The PPSTP enables cooperating peers to form content- streaming overlay networks to support near real-time delivery of structured media content (audio, video, and 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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4 1.1. Terminology ................................................4 1.2. Design Overview ............................................6 1.2.1. Typical PPSP Session ................................7 1.2.2. Example of a PPSP Session ...........................7 2. Protocol Architecture and Functional View ......................10 2.1. Messaging Model ...........................................10 2.2. Request/Response Model ....................................10 2.3. State Machines and Flows of the Protocol ..................12 2.3.1. Normal Operation ...................................14 2.3.2. Error Conditions ...................................15 3. Protocol Specification .........................................16 3.1. Presentation Language .....................................16 3.2. Resource Element Types ....................................16 3.2.1. Version ............................................16 3.2.2. Peer Number Element ................................17 3.2.3. Swarm Action Element ...............................18 3.2.4. Peer Information Elements ..........................18 3.2.5. Statistics and Status Information Element ..........20 3.3. Requests and Responses ....................................21 3.3.1. Request Types ......................................21 3.3.2. Response Types .....................................21 3.3.3. Request Element ....................................22 3.3.4. Response Element ...................................23 3.4. PPSTP Message Element .....................................24 4. Protocol Specification: Encoding and Operation .................24 4.1. Requests and Responses ....................................25 4.1.1. CONNECT Request ....................................25 126.96.36.199. Example ...................................28 4.1.2. FIND Request .......................................32 188.8.131.52. Example ...................................33
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4.1.3. STAT_REPORT Request ................................34 184.108.40.206. Example ...................................35 4.2. Response Element in Response Messages .....................36 4.3. Error and Recovery Conditions .............................37 4.4. Parsing of Unknown Fields in message-body .................38 5. Operations and Manageability ...................................38 5.1. Operational Considerations ................................38 5.1.1. Installation and Initial Setup .....................38 5.1.2. Migration Path .....................................39 5.1.3. Requirements on Other Protocols and Functional Components ..............................39 5.1.4. Impact on Network Operation ........................39 5.1.5. Verifying Correct Operation ........................40 5.2. Management Considerations .................................40 5.2.1. Interoperability ...................................40 5.2.2. Management Information .............................40 5.2.3. Fault Management ...................................41 5.2.4. Configuration Management ...........................41 5.2.5. Accounting Management ..............................41 5.2.6. Performance Management .............................41 5.2.7. Security Management ................................41 6. Security Considerations ........................................42 6.1. Authentication between Tracker and Peers ..................42 6.2. Content Integrity Protection against Polluting Peers/Trackers ............................................43 6.3. Residual Attacks and Mitigation ...........................43 6.4. Pro-incentive Parameter Trustfulness ......................44 6.5. Privacy for Peers .........................................44 7. Guidelines for Extending PPSTP .................................45 7.1. Forms of PPSTP Extension ..................................45 7.2. Issues to Be Addressed in PPSTP Extensions ................47 8. IANA Considerations ............................................48 8.1. MIME Type Registry ........................................48 8.2. PPSTP Version Number Registry .............................49 8.3. PPSTP Request Type Registry ...............................49 8.4. PPSTP Error Code Registry .................................50 9. References .....................................................51 9.1. Normative References ......................................51 9.2. Informative References ....................................53 Acknowledgments ...................................................54 Authors' Addresses ................................................55
The Peer-to-Peer Streaming Protocol (PPSP) is composed of two protocols: the Tracker Protocol (defined in this document) and the Peer Protocol (defined in [RFC7574]). [RFC6972] specifies that the Tracker Protocol should standardize the messages between PPSP peers and PPSP trackers and also defines the requirements.
The Peer-to-Peer Streaming Tracker Protocol (PPSTP) 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 to be able to communicate with a tracker in order to participate in a particular streaming content swarm. This centralized tracker service is used by PPSP peers for acquisition of peer lists.
The signaling and the media data transfer between PPSP peers is not in the scope of this specification.
This document introduces a base Peer-to-Peer Streaming Tracker Protocol (PPSTP) that satisfies the requirements in [RFC6972].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
absolute time: Expressed as ISO 8601 timestamps, using zero UTC offset. Fractions of a second may be indicated, for example, December 25, 2010 at 14h56 and 20.25 seconds in basic format is 20101225T145620.25Z and in extended format is 2010-12-25T14:56:20.25Z.
chunk: An uniformly atomic subset of the resource that constitutes the basic unit of data organized in P2P streaming for storage, scheduling, advertisement, and exchange among peers.
chunk ID: A unique resource identifier for a chunk. The identifier type depends on the addressing scheme used, i.e., an integer, an HTTP-URL, and possibly a byte-range. The identifier type is described in the Media Presentation Description (MPD).
LEECH: The peers in a swarm that download content from other peers as well as contribute downloaded content with others. A LEECH should join the swarm with uncompleted media content.
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MPD (Media Presentation Description): Formalized description for a media presentation, i.e., describes the structure of the media, namely, the representations, the codecs used, the chunks, and the corresponding addressing scheme.
peer: A participant in a P2P streaming system that not only receives streaming content, but also caches and streams streaming content to other participants.
peer ID: The identifier of a peer such that other peers, or the Tracker, can refer to the peer using its ID. The peer ID is mandatory, can take the form of a universally unique identifier (UUID), defined in [RFC4122], and can be bound to a network address of the peer, i.e., an IP address or a uniform resource identifier/locator (URI/URL) that uniquely identifies the corresponding peer in the network. The peer ID and any required security certificates are obtained from an offline enrollment server.
peer list: A list of peers that are in the same swarm maintained by the tracker. A peer can fetch the peer list of a swarm from the tracker.
PPSP: The abbreviation of Peer-to-Peer Streaming Protocol.
PPSTP: The abbreviation of Peer-to-Peer Streaming Tracker Protocol.
SEEDER: The peers in a swarm that only contribute the content they have to others. A SEEDER should join the swarm with complete media content.
service portal: A logical entity typically used for client enrollment and for publishing, searching, and retrieving content information. It is usually located in a server of a content provider.
swarm: A group of peers that exchange data to distribute chunks of the same content (e.g., video/audio program, digital file, etc.) at a given time.
swarm ID: The identifier of a swarm containing a group of peers sharing common streaming content. The swarm ID may use a universally unique identifier (UUID), e.g., a 64- or 128-bit datum to refer to the content resource being shared among peers.
tracker: A directory service that maintains a list of peers participating in a specific audio/video channel or in the distribution of a streaming file. It is a logical component that can be deployed in a centralized or distributed way.
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transaction ID: The identifier of a request from the peer to the tracker. It is used to disambiguate responses that may arrive in a different order than the corresponding requests.
The functional entities related to peer-to-peer streaming 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 they are not in the scope of the specification. The functional entities directly involved in PPSTP 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 content. The Client Media Player may interface with the local peer application using the standard format for HTTP request and response messages [RFC7230].
The service portal is a logical entity typically used for client enrollment and for publishing, searching, and retrieving content information.
A peer corresponds to a logical entity (typically in a user device) that actually participates in sharing media content. Peers are organized in various swarms; each swarm corresponds to the group of peers streaming certain content at any given time.
A tracker is a logical entity that maintains the lists of peers storing chunks for a specific live media channel or on-demand media streaming content, answers queries from peers, and collects information on the activity of peers. While a tracker may have an underlying implementation consisting of more than one physical node, logically, the tracker can most simply be thought of as a single element; in this document, it will be treated as a single logical entity. Communication between these physical nodes to present them as a single tracker to peers is not considered in PPSTP, which is a protocol between a tracker and a peer.
PPSTP 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. PPSTP is not designed for applications for which in-sync reception is needed.
In order to be able to bootstrap in the P2P network, a peer must first obtain a peer ID and any required security certificates or authorization tokens from an enrollment service (end-user registration). The peer ID MUST be unique (see the definition of "peer ID" in Section 1.1); however, the representation of the peer ID is not considered in this document.
Figure 1: A Typical PPSP Session for Streaming Content
To join an existing P2P streaming service and to participate in content sharing, a peer must first locate a tracker.
As illustrated in Figure 1, a P2P streaming session may be initiated starting at point (a), with the Client Media Player browsing for the desired content in order to request it (to the local Peer_1 in the figure), or resume a previously initiated stream, but starting at point (b). For this example, the Peer_1 is in mode LEECH.
At point (a) in Figure 1, the Client Media Player accesses the portal and selects the content of interest. The portal returns the Media Presentation Description (MPD) file that includes information about the address of one or more trackers (which can be grouped by tiers of priority) that control the swarm x for that media content (e.g., content x).
With the information from the MPD, the Client Media Player is able to trigger the start of the streaming session, requesting to the local Peer_1 the chunks of interest.
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The PPSP streaming session is then started (or resumed) at Peer_1 by sending a PPSTP CONNECT message to the tracker in order to join swarm x. The tracker will then return the OK response message containing a peer list, if the CONNECT message is successfully accepted. From that point, every chunk request is addressed by Peer_1 to its neighbors (Peer_2 in Figure 1) using a peer protocol, e.g., [RFC7574], returning the received chunks to the Client Media Player.
Once connected, Peer_1 needs to periodically report its status and statistics data to the tracker using a STAT_REPORT message.
If Peer_1 needs to refresh its neighborhood (for example, due to churn), it will send a PPSTP FIND message (with the desired scope) to the tracker.
Peers that are only SEEDERs (i.e., serving content to other peers), as are the typical cases of service provider P2P edge caches and/or media servers, trigger their P2P streaming sessions for content x, y, z... (Figure 2), not from Media Player signals, but from some "Start" activation signal received from the service provider provisioning mechanism. In this particular case, the peer starts or resumes all its streaming sessions just by sending a PPSTP CONNECT message to the tracker (Figure 2), in order to "join" all the requested swarms.
Periodically, the peer also reports its status and statistics data to the tracker using a PPSTP STAT_REPORT message.
Figure 2: A Typical PPSP Session for a Streaming SEEDER
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The specification of the mechanisms used by the Client Media Player (or provisioning process) and the peer to signal start/resume of streams, request media chunks, and obtain a peer ID, security certificates, or tokens is not in the scope of this document.
2. Protocol Architecture and Functional View
PPSTP is designed with a layered approach i.e., a PPSTP Request/Response layer, a Message layer, and a Transport layer (see Figure 3).
The PPSTP Request/Response layer deals with the interactions between tracker and peers using request and response messages.
The Message layer deals with the framing format for encoding and transmitting data through the underlying transport protocol, as well as the asynchronous nature of the interactions between tracker and peers.
The Transport layer is responsible for the actual transmission of requests and responses over network transports, including the determination of the connection to use for a request or response message when using TCP or Transport Layer Security (TLS) [RFC5246] over it.
PPSTP uses a design like REST (Representational State Transfer) with the goal of leveraging current HTTP implementations and infrastructure, as well as familiarity with existing REST-like
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services in popular use. PPSTP messages use the UTF-8 character set [RFC3629] and are either requests from peers to a tracker service or responses from a tracker service to peers. The request and response semantics are carried as entities (header and body) in messages that correspond to either HTTP request methods or HTTP response codes, respectively.
Peers send requests to trackers. Trackers send a single response for each request though both requests and responses can be subject to fragmentation of messages in transport.
The request messages of the base protocol are listed in Table 1:
CONNECT: This request message is used when a peer registers in the tracker to notify it about participation in the named swarm(s). If the peer is already registered in the tracker, this request message simply notifies the tracker about participation in the 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 peer's lists of the corresponding swarm(s) for the requesting peer ID. On receiving a CONNECT message, the tracker first checks the peer mode type (SEEDER/LEECH) for the specified swarm(s) and then decides the next steps (see Section 4.1 for more details).
FIND: This request message is used by peers to request a list of peers active in the named swarm from the tracker whenever needed. On receiving a FIND message, the tracker finds the peers listed in the content status of the specified swarm that can satisfy the requesting peer's requirements and returns the list to the
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requesting peer. To create the peer list, the tracker may take peer status, capabilities, and peer priority into consideration. Peer priority may be determined by network topology preference, operator policy preference, etc.
STAT_REPORT: This request message is used to allow an active peer to send status (and optionally statistic data) to the tracker to signal continuing activity. This request message MUST be sent periodically to the tracker while the peer is active in the system.
When there are no peers connected in the tracker, the state machine is in INIT state.
When the first peer connects to register 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 STARTED state. When the last peer ID is removed, the state machine transitions to TERMINATED. From there, it immediately transitions back to INIT state. Because of this, TERMINATED state is transient.
Once in STARTED state, each peer is instantiated (per peer ID) in the tracker state machine with a dedicated transaction state machine (Figure 5), which is deleted when the peer ID is removed.
Figure 5: "Per-Peer-ID" State Machine and Flow Diagram
Unlike the tracker state machine, which exists even when no peer IDs are registered, the "per-Peer-ID" 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" 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.
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When the peer ID is no longer bound to a registration, the "per-Peer- ID" State Machine moves to the TERMINATE state, and the state machine is destroyed.
During the lifetime of streaming activity of a peer, the instantiated "per-Peer-ID" 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
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.
For normal operation, the process consists of the following steps:
1) When a peer wants to access the system, it needs to register with a tracker by sending a CONNECT message asking for the swarm(s) it wants to join. This request from a new peer ID triggers the instantiation in the tracker of a "per-Peer-ID" State Machine. In the START state of the new "per-Peer-ID" State Machine, the tracker registers the peer ID and associated information (IP addresses), starts the "init timer", and moves to PEER REGISTERED state.
2) In PEER REGISTERED state, if the peer ID is valid, the tracker either:
a) processes the requested action(s) for the valid swarm information contained in the CONNECT requests, and if successful, the tracker stops the "init timer", starts the "track timer", and sends the response to the peer (the response may contain the appropriate list of peers for the joining swarm(s), as detailed in Section 4.1), or
b) moves the valid FIND request to TRACKING state.
3) In TRACKING state, STAT_REPORT or FIND messages received from that peer ID will reset the "track timer", and the tracker responds to the requests with the following, respectively:
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a) a successful condition, or
b) a successful condition containing the appropriate list of peers for the named swarm (Section 4.2).
4) While in TRACKING state, a CONNECT message received from that peer ID with valid swarm action information (Section 4.1.1) resets the "track timer", and the tracker responds to the request with a successful condition.
Peers are required not to generate protocol elements that are invalid. However, several situations may lead to abnormal conditions in the interaction with the tracker. These situations may be related to peer malfunction or communication errors. The tracker reacts to these abnormal situations depending on its current state related to a peer ID, as follows:
A) In PEER REGISTERED state, when a CONNECT request only contains invalid swarm actions (Section 4.1.1), the tracker responds with a PPSTP error code as specified in Section 4.3, deletes the registration, and transitions to TERMINATE state for that peer ID. The state machine is destroyed.
B) In 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 a 06 (Authentication Required) error_code or a 03 (Forbidden Action) error_code as described in Section 4.3 and transitions to TERMINATE state for that peer ID. The state machine is destroyed.
C) In TRACKING state (while the "track timer" has not expired), receiving a CONNECT message from a peer ID with invalid swarm actions (Section 4.1.1) or receiving a FIND/STAT_REPORT message from a peer ID with an invalid swarm ID is considered an error condition. The tracker responds with the corresponding error code (described in Section 4.3).
D) In TRACKING state, without receiving messages from the peer on timeout (the "track timer" has expired), the tracker cleans all the information associated with the peer ID in all swarms it was joined, deletes the registration, and transitions to TERMINATE state for that peer ID. The state machine is destroyed.
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NOTE: These situations may correspond to malfunctions at the peer or to malicious conditions. As a preventive measure, the tracker proceeds to TERMINATE state for that peer ID.
PPSTP uses a REST-like design, encoding the requests and responses using JSON [RFC7159]. For a generalization of the definition of protocol elements and fields, as well as their types and structures, this document uses a C-style notation, similar to the presentation language used to define TLS [RFC5246].
A JSON object consists of name/value pairs with the grammar specified in [RFC7159]. In this document, comments begin with "//", and the "ppsp_tp_string_t" and "ppsp_tp_integer_t" types are used to indicate the JSON string and number, respectively. Optional fields are enclosed in "[ ]" brackets. An array is indicated by two numbers in angle brackets, <min..max>, where "min" indicates the minimal number of values and "max" the maximum. An "*" is used to denote a no upper-bound value for "max".
The peer number element is a scope selector optionally present in CONNECT and FIND requests.
This element contains the attribute peer_count to indicate the maximum number of peers in the returned peer list. peer_count should be less than 30 in this specification. The other 4 attributes, i.e., ability_nat, concurrent_links, online_time, and upload_bandwidth may also be contained in this element to inform the tracker the status of the peer so that the tracker could return some eligible peers based on the implementing rules set by the service providers:
o ability_nat is used to indicate the preferred NAT traversal situation of the requesting peer.
o concurrent_links means the number of P2P links the peer currently has.
o online_time represents online duration time of the peer. The unit is second.
o upload_bandwidth is the maximum upload bandwidth capability of the peer. The unit is Kbps.
The scope selector element and its attributes are defined as follows:
The ppsp_tp_peer_addr_t element includes the IP address and port, with a few optional attributes related to connection type and network location (in terms of ASN) as well as, optionally, the identifier of the peer protocol being used.
The semantics of ppsp_tp_peer_addr_t attributes are listed in Table 3:
+----------------------+----------------------------------+ | Element or Attribute | Description | +----------------------+----------------------------------+ | ip_address | IP address information | | port | IP service port value | | priority | The priority of this interface. | | | It may be determined by network | | | topology preference, operator | | | policy preference, etc. How to | | | create a priority is outside of | | | the scope. The larger the value,| | | the higher the priority. | | type | Describes the address for NAT | | | traversal, which can be HOST | | | REFLEXIVE or PROXY | | connection | Access type (wireless or wired) | | asn | Autonomous System Number | | peer_protocol | Peer-to-Peer Streaming Peer | | | Protocol (PPSPP) supported | +----------------------+----------------------------------+
Table 3: Semantics of ppsp_tp_peer_addr_t
In this document, IP address is specified as ppsp_tp_addr_value. The exact characters and format depend on address_type:
The statistics element (stat) is used to describe several properties relevant to the P2P network. These properties can be related to stream statistics and peer status information. Each stat element will correspond to a property type, and several stat blocks can be reported in a single STAT_REPORT message, corresponding to some or all the swarms the peer is actively involved. This specification only defines the property type "STREAM_STATS".
The definition of the statistic element and attributes is as follows:
A request element consists of the version of PPSTP, the request type, a transaction ID, the requesting peer ID, and requesting body (i.e., request_data). The request_data MUST be correctly set to the corresponding element based on the request type (see Table 5).
A response element consists of the version of PPSTP, the response type, the error code, a transaction ID, and optionally the public address of the requesting peer and one or multiple swarm action result elements. Normally, swarm action result elements SHOULD be present and error_code MUST be set to 00 (No Error) when response_type is 0x00. Swarm action result elements SHOULD NOT be set when error_code is 01 (Bad Request). Detailed selection of error_code is introduced in Section 4.3.
A swarm action result element represents the result of an action requested by the peer. It contains a swarm identifier that globally indicates the swarm, the result for the peer of this action (which could be CONNECT ("JOIN" or "LEAVE"), FIND, or STAT_REPORT), and optionally one peer group element. The attribute result indicates the operation result of the corresponding request. When the response element corresponds to the STAT_REPORT request or the result attribute is set to 0x01, the peer group element SHOULD NOT be set.
PPSTP messages (requests or responses) are designed to have a similar structure with a root field named "PPSPTrackerProtocol" containing meta information and data pertaining to a request or a response.
The base type of a PPSTP message is defined as follows:
PPSTP is a message-oriented request/response protocol. PPSTP messages use a text type encoding in JSON [RFC7159], which MUST be indicated in the Content-Type field in HTTP/1.1 [RFC7231], specifying the "application/ppsp-tracker+json" media type for all PPSTP request parameters and responses.
Implementations MUST support the "https" URI scheme [RFC2818] and Transport Layer Security (TLS) [RFC5246].
For deployment scenarios where peer (client) authentication is desired at the tracker, HTTP Digest Access Authentication [RFC7616] MUST be supported, with TLS Client Authentication as the preferred mechanism, if available.
PPSTP uses the HTTP POST method to send parameters in requests to provide information resources that are the function of one or more of those input parameters. Input parameters are encoded in JSON in the HTTP entity body of the request.
The section describes the operation of the three types of requests of PPSTP and provides some examples of usage.
This method is used when a peer registers to the system and/or requests some swarm actions (join/leave). The peer MUST properly set the request type to CONNECT, generate and set the transaction_ids, set the peer_id, and include swarms the peer is interested in, followed by the corresponding action type and peer mode.
o When a peer already possesses content and agrees to share it with others, it should set the action type to the value JOIN, as well as set the peer mode to SEEDER during its start (or re-start) period.
o When a peer makes a request to join a swarm to consume content, it should set the action type to the value JOIN, as well as set the peer mode to LEECH during its start (or re-start) period.
In the above cases, the peer can provide optional information on the addresses of its network interface(s), for example, the priority, type, connection, and ASN.
When a peer plans to leave a previously joined swarm, it should set action type to LEAVE, regardless of the peer mode.
When receiving a well-formed CONNECT request message, the tracker starts 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. If successful, a response message with a corresponding response value of SUCCESSFUL will be generated.
The valid sets of the number of swarms whose action type is combined with peer mode for the CONNECT request logic are enumerated in Table 6 (referring to the "per-Peer-ID" State Machine in Section 2.3).
Table 6: Validity of Action Combinations in CONNECT Requests
In the CONNECT request message, multiple swarm action elements ppsp_tp_swarm_action_t could be contained. Each of them contains the request action and the peer_mode of the peer. The peer_mode attribute MUST be set to the type of participation of the peer in the swarm (SEEDER or LEECH).
The CONNECT message may contain multiple peer_addr elements with attributes ip_address, port, priority, and type (if Interactive Connectivity Establishment (ICE) [RFC5245] NAT traversal techniques are used), and optionally connection, asn, and peer_protocol corresponding to each of the network interfaces the peer wants to advertise.
The element peer_num indicates the maximum number of peers to be returned in a list from the tracker. The returned peer list can be optionally filtered by some indicated properties, such as ability_nat for NAT traversal, and concurrent_links, online_time and upload_bandwidth for the preferred capabilities.
The element transaction_id MUST be present in requests to uniquely identify the transaction. Responses to completed transactions use the same transaction_id as the request they correspond to.
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The response may include peer_addr data of the requesting peer public IP address. Peers can use Session Traversal Utilities for NAT (STUN) [RFC5389] and Traversal Using Relays around NAT (TURN) [RFC5766] to gather their candidates, in which case peer_addr SHOULD NOT present in the response. If no STUN is used and the tracker is able to work as a "STUN-like" server that can inspect the public address of a peer, the tracker can return the address back with a "REFLEXIVE" attribute type. The swarm_result may also include peer_addr data corresponding to the peer IDs and public IP addresses of the selected active peers in the requested swarm. The tracker may also include the attribute asn with network location information of the transport address, corresponding to the Autonomous System Number of the access network provider of the referenced peer.
If the peer_mode is SEEDER, the tracker responds with a SUCCESSFUL response and enters the peer information into the corresponding swarm activity. If the peer_mode is LEECH (or if a SEEDER includes a peer_num element in the request), the tracker will search and select an appropriate list of peers satisfying the conditions set by the requesting peer. The peer list returned MUST contain the peer IDs and the corresponding IP addresses. To create the peer list, the tracker may take peer status and network location information into consideration to express network topology preferences or operators' policy preferences with regard to the possibility of connecting with other IETF efforts such as Application-Layer Traffic Optimization (ALTO) [RFC7285].
IMPLEMENTATION NOTE: If no peer_num attributes are present in the request, the tracker may return a random sample from the peer population.
The next example of a CONNECT request corresponds to a peer leaving a previously joined swarm and requesting to join a new swarm. This is the typical example of a user watching a live channel but then deciding to switch to a different one:
The next example illustrates the response for the previous example of a CONNECT request where the peer requested two swarm actions and not more than 5 other peers, receiving from the tracker a peer list with only two other peers in the swarm "2222":
HTTP/1.1 200 OK Content-Length: 1342 Content-Type: application/ppsp-tracker+json
This method allows peers to request a new peer list for the swarm from the tracker whenever needed.
The FIND request may include a peer_number element to indicate to the tracker the maximum number of peers to be returned in a list corresponding to the indicated conditions set by the requesting peer, being ability_nat for NAT traversal (considering that PPSP-ICE NAT traversal techniques may be used), and optionally concurrent_links, online_time, and upload_bandwidth for the preferred capabilities.
When receiving a well-formed FIND request, the tracker processes the information to check if it is valid. If successful, a response message with a response value of SUCCESSFUL will be generated, and the tracker will search out the list of peers for the swarm and select an appropriate peer list satisfying the conditions set by the requesting peer. The peer list returned MUST contain the peer IDs and the corresponding IP addresses.
The tracker may take the ability of peers and popularity of the requested content into consideration. For example, the tracker could select peers with higher ability than the current peers that provide the content if the content is relatively popular (see Section 5.1.1); the tracker could also select peers with lower ability than the current peers that provide the content when the content is relatively uncommon. The tracker may take network location information into consideration as well, to express network topology preferences or operators' policy preferences. It can implement other IETF efforts like ALTO [RFC7285], which is out of the scope of this document.
The response MUST include a peer_group element that contains the peer IDs and the corresponding IP addresses; it may also include the attribute asn with network location information of the transport address, corresponding to the Autonomous System Number of the access network provider of the referenced peer.
The response may also include a peer_addr element that includes the requesting peer public IP address. If no STUN is used and the tracker is able to work as a "STUN-like" server that can inspect the public address of a peer, the tracker can return the address back with a "REFLEXIVE" attribute type.
IMPLEMENTATION NOTE: If no peer_num attributes are present in the request, the tracker may return a random sample from the peer population.
An example of the message-body of a FIND request, where the peer requests from the tracker a list of not more than 5 peers in the swarm "1111" conforming to the characteristics expressed (concurrent links, online time, and upload bandwidth level) is as follows:
This method allows peers to send status and statistic data to trackers. The method is periodically initiated by the peer while it is active.
The peer MUST set the request_type to "STAT_REPORT", set the peer_id with the identifier of the peer, and generate and set the transaction_id.
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The report may include multiple statistics elements describing several properties relevant to a specific swarm. These properties can be related with stream statistics and peer status information, including uploaded_bytes, downloaded_bytes, available_bandwidth, concurrent_links, etc.
Other properties may be defined (see the guidelines in Section 7.1), for example, those related to incentives and reputation mechanisms. If no Statistics Group is included, the STAT_REPORT is used as a "keep-alive" message to prevent the tracker from de-registering the peer when the "track timer" expires.
If the request is valid, the tracker processes the received information for future use and generates a response message with a response value of SUCCESSFUL.
The response MUST have the same transaction_id value as the request.
SUCCESSFUL: Indicates that the request has been processed properly and the desired operation has completed. The body of the response message includes the requested information and MUST include the same transaction_id as the corresponding request.
CONNECT: Returns information about the successful registration of the peer and/or of each swarm action requested. May additionally return the list of peers corresponding to the action attribute requested.
FIND: Returns the list of peers corresponding to the requested scope.
STAT_REPORT: Confirms the success of the requested operation.
FAILED: Indicates that the request has not been processed properly. A corresponding error_code SHOULD be set according to the conditions described in Section 4.3.
If the peer receives an invalid response, the same request with identical content including the same transaction_id MUST be repeated.
The transaction_id on a request can be reused if and only if all of the content is identical, including date/time information. Details of the retry process (including time intervals to pause, number of retries to attempt, and timeouts for retrying) are implementation dependent.
The tracker MUST be prepared to receive a request with a repeated transaction_id.
Error situations resulting from normal operation or from abnormal conditions (Section 2.3.2) MUST be responded to with response_type set to 0x01 and with the adequate error_code, as described here:
o If the message is found to be incorrectly formed, the receiver MUST respond with a 01 (Bad Request) error_code with an empty message-body (no peer_addr and swarm_result attributes).
o If the version number of the protocol is for a version the receiver does not support, the receiver MUST respond with a 02 (Unsupported Version Number) error_code with an empty message-body (no peer_addr and swarm_result attributes).
o In the PEER REGISTERED and TRACKING states of the tracker, certain requests are not allowed (Section 2.3.2). The tracker MUST respond with a 03 (Forbidden Action) error_code with an empty message-body (no peer_addr and swarm_result attributes).
o If the tracker is unable to process a request message due to an unexpected condition, it SHOULD respond with a 04 (Internal Server Error) error_code with an empty message-body (no peer_addr and swarm_result attributes).
o If the tracker is unable to process a request message because it is in an overloaded state, it SHOULD respond with a 05 (Service Unavailable) error_code with an empty message-body (no peer_addr and swarm_result attributes).
o If authentication is required for the peer to make the request, the tracker SHOULD respond with a 06 (Authentication Required) error_code with an empty message-body (no peer_addr and swarm_result attributes).
This document only details object members used by this specification. Extensions may include additional members within JSON objects defined in this document. PPSTP implementations MUST ignore unknown members when processing PPSTP messages.
5. Operations and Manageability
This section provides the operational and management aspects that are required to be considered in implementations of PPSTP. These aspects follow the recommendations expressed in [RFC5706].
PPSTP 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 content.
The "server" 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 content.
Content providers wishing to use PPSP for content distribution should set up 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 content sources. Content and 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 flexibility for implementers.
The swarm IDs of available content, as well as the addresses of the PPSP tracking facility, can be distributed to end users in various ways, but it is common practice to include both the swarm ID and the corresponding PPSP tracker addresses (as URLs) in the MPD of the content, which is obtainable (a link) from the service portal.
The available content could have different importance attribute values to indicate whether the content is popular or not. However, it is a totally implementation design and outside the scope of this
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specification. For example, the importance attribute values of the content could be set by content providers when distributing them or could be determined by the tracker based on the statistics of the requests from the peers that request the content. The tracker could set an upper threshold to decide that the content is popular enough when the importance attribute value is higher than the upper threshold. The tracker could also set a lower threshold to decide that the content is uncommon enough when the importance attribute value is lower than the lower threshold.
End users browse and search for desired content in the service portal and select by clicking the links of the corresponding MPDs. This action typically requires security certificates or authorization tokens from an enrollment service (end-user registration) and then launches the Client Media Player (with PPSP awareness), which will then, using PPSTP, 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.
There is no previous standard protocol providing functionality similar to PPSTP. However, some popular proprietary protocols, e.g., BitTorrent, are used in existing systems. There is no way for PPSTP to migrate to proprietary protocols like the BitTorrent tracker protocol. Because PPSTP is an application-level protocol, there is no harm in PPSTP having no migration path. However, proprietary protocols migrating to standard protocols like PPSTP can solve the problems raised in [RFC6972]. It is also possible for systems to use PPSTP as the management protocol to work with exiting propriety peer protocols like the BitTorrent peer protocol.
5.1.3. Requirements on Other Protocols and Functional Components
For security reasons, when using the Peer-to-Peer Streaming Peer Protocol (PPSPP) with PPSTP, the mechanisms described in Section 6.1 should be observed.
The correct operation of PPSTP can be verified both at the tracker and at the peer by logging the behavior of PPSTP. Additionally, the PPSP tracker collects the status of the peers, including the peers' activity; such information can be used to monitor and obtain the global view of the operation.
The management considerations for PPSTP 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.
Interoperability refers to allowing information sharing and operations between multiple devices and multiple management applications. For PPSTP, distinct types of devices host PPSTP trackers and peers. Therefore, support for multiple standard schema languages, management protocols, and information models, suited to different purposes, was considered in the PPSTP design. Specifically, management functionality for PPSTP devices can be achieved with the Simple Network Management Protocol (SNMP) [RFC3410], syslog [RFC5424], and the Network Configuration Protocol (NETCONF) [RFC6241].
PPSP trackers may implement SNMP management interfaces, namely, the Application Management MIB [RFC2564], without the need to instrument the tracker application itself. The channel, connections, and transaction objects of the Application Management MIB can be used to report the basic behavior of the PPSP tracker service.
The Application Performance Measurement MIB (APM-MIB) [RFC3729] and the Transport Performance Metrics MIB (TPM-MIB) [RFC4150] can be used with PPSTP to provide adequate metrics for the analysis of performance for transaction flows in the network, in direct relationship to the transport of PPSTP.
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.
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The TCP-MIB [RFC4022] can additionally be considered for network monitoring.
Logging is an important functionality for PPSTP trackers and peers; it is done via syslog [RFC5424].
As PPSP tracker failures can be mainly attributed to host or network conditions, the facilities previously described for verifying the correct operation of PPSTP and the management of PPSP tracker servers appear sufficient for PPSTP fault monitoring.
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].
PPSTP implementations, primarily in content provider environments, can benefit from accounting standardization efforts as described in [RFC2975], which indicates that accounting management is "concerned with the collection of resource consumption data for the purposes of capacity and trend analysis, cost allocation, auditing, and billing".
Standard SNMP notifications for PPSP tracker management [RFC5590] and syslog messages [RFC5424] can be used to alert operators to the conditions identified in the security considerations (Section 6).
The statistics collected about the operation of PPSTP can be used for detecting attacks (e.g., the receipt of malformed messages, messages out of order, or messages with invalid timestamps). However, collecting such endpoint properties may also raise some security issues. For example, the statistics collected by the tracker may be disclosed to an unauthorized third party that has malicious intentions. To address such risk, the provider of the tracker should
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evaluate how much information is revealed and the associated risks. A confidentiality mechanism must be provided by HTTP over TLS to guarantee the confidentiality of PPSTP.
P2P streaming systems are subject to attacks by malicious or unfriendly peers/trackers that may eavesdrop on signaling, forge/deny information/knowledge about streaming content and/or its availability, impersonate a valid participant, or launch DoS attacks on a chosen victim.
No security system can guarantee complete security in an open P2P streaming system where participants may be malicious or uncooperative. The goal of the security considerations described here is to provide sufficient protection for maintaining some security properties during tracker-peer communication even in the face of a large number of malicious peers and/or eventual distrustful trackers (under the distributed tracker deployment scenario).
Since the protocol uses HTTP to transfer signaling, most of the security considerations described in [RFC7230] and [RFC7231] also apply. Due to the transactional nature of the communication between peers and tracker, the method for adding authentication and data security services can be the OAuth 2.0 Authorization [RFC6749] with a bearer token, which provides the peer with the information required to successfully utilize an access token to make protected requests to the tracker.
To protect PPSTP 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 the scope of this specification.
Transport Layer Security (TLS) [RFC5246] MUST be used in the communication between peers and tracker to provide privacy and data integrity. Software engineers developing and service providers deploying the tracker should make themselves familiar with the Best Current Practices (BCP) on configuring HTTP over TLS [RFC7525].
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OAuth 2.0 Authorization [RFC6749] SHOULD also be considered when digest authentication [RFC7616] and HTTPS client certificates are required.
6.2. Content Integrity Protection against Polluting Peers/Trackers
Malicious peers may claim ownership of popular content to the tracker and try to serve polluted (i.e., decoy content or even virus/trojan- infected content) to other peers. Since trackers do not exchange content information among peers, it is difficult to detect whether or not a peer is polluting the content. Usually, this kind of pollution can be detected by the Peer-to-Peer Streaming Peer Protocol (PPSPP) [RFC7574] with requiring the use of Merkle Hash Tree scheme for protecting the integrity of the content. More details can be seen in Section 5 of [RFC7574].
Some attackers that disrupt P2P streaming on behalf of content providers may provide false or modified content or peer list information to achieve certain malicious goals. Peers connecting to those portals or trackers provided by the attackers may be redirected to some corrupted malicious content. However, there is no standard way for peers to avoid this kind of situation completely. Peers can have mechanisms to detect undesirable content or results themselves. For example, if a peer finds that the portal returned some undesired content information or the tracker returned some malicious peer lists, the peer may choose to quit the swarm or switch to other P2P streaming services provided by other content providers.
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 in the trust mechanism by collecting evaluations, computing credit values, and providing them to joining peers.
Property types for STAT_REPORT messages may consider additional pro- incentive parameters (see the 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, the amount of both uploaded and downloaded data should be reported to the tracker to allow checking for inconsistencies between the upload and download report and to 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 and 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].
PPSTP provides mechanisms in which the peers can send messages containing IP addresses, ports, and other information to the tracker. A tracker or a third party who is able to intercept such messages can store and process the obtained information in order to analyze peers' behaviors and communication patterns. Such analysis can lead to privacy risks. For example, an unauthorized party may snoop on the data transmission from the peer to a tracker in order to introduce some corrupted chunks.
The Peer-to-Peer Streaming Peer Protocol (PPSPP) [RFC7574] has already introduced some mechanisms to protect streamed content; see Sections 12.3 and 12.4 of [RFC7574]. For PPSTP, peer implementations as well as tracker implementations MUST support the "https" URI scheme [RFC2818] and Transport Layer Security (TLS) [RFC5246]. In addition, a peer should be cognizant about potential trackers tracking through queries of peers, e.g., by using HTTP cookies. PPSTP as specified in this document does not rely on HTTP cookies. Thus, peers may decide not to return cookies received from the tracker, in order to make additional tracking more difficult.
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 PPSTP, this means that an extension MUST NOT alter an existing protocol schema as the changes would result in a new version of an existing schema, not an extension of an existing schema, typically non-backwards-compatible.
Additionally, a designer MUST remember that extensions themselves may also be extensible.
Extensions MUST adhere to the principles described in this section in order to be considered valid.
Extensions MUST be documented in Standards Track RFCs if there are requirements for coordination, interoperability, and broad distribution.
In PPSTP, two extension mechanisms can be used: a Request-Response Extension or a Protocol-Level Extension.
o Request-Response Extension: Adding elements or attributes to an existing element mapping in the schema is the simplest form of extension. This form should be explored before any other. This task can be accomplished by extending an existing element mapping.
For example, an element mapping for the Statistics Group can be extended to include additional elements needed to express status information about the activity of the peer, such as online time for the stat element.
o Protocol-Level Extension: If there is no existing element mapping that can be extended to meet the requirements and the existing PPSTP request and response message structures are insufficient, then extending the protocol should be considered in order to define new operational requests and responses.
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For example, to enhance the level of control and the granularity of the operations, a new version of the protocol with new messages (JOIN, DISCONNECT), a retro-compatible change in semantics of an existing CONNECT request/response, and an extension in STAT_REPORT could be considered.
As illustrated in Figure 6, the peer would use an enhanced CONNECT request to perform the initial registration in the system. Then it would join a first swarm as SEEDER, later join a second swarm as LEECH, and then disconnect from the latter swarm but remain as SEEDER for the first one. When deciding to leave the system, the peer disconnects gracefully from it:
There are several issues that all extensions should take into consideration.
o Overview of the Extension: It is RECOMMENDED that extensions to PPSTP 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.
o Backward Compatibility: The new extension MUST be backward compatible with the base PPSTP specified in this document.
o 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 PPSTP. Method names SHOULD be shorter than 16 characters and SHOULD attempt to convey the general meaning of the request or response.
o Semantic Issues: PPSTP 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.
o Security Issues: As security is an important component of any protocol, designers of PPSTP extensions need to carefully consider security requirements, e.g., authorization requirements and requirements for end-to-end integrity.
o 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.
IANA has created the "PPSTP Version Number Registry". Values are integers in the range 0-255, with initial assignments and reservations given in Table 2. New PPSTP version types are assigned after IETF Review [RFC5226] to ensure that proper documentation regarding the new version types and their usage has been provided.
IANA has created the "PPSTP Request Type Registry". Values are strings listed in Table 8. New PPSTP request types are assigned after IETF Review [RFC5226] to ensure that proper documentation regarding the new request types and their usage has been provided.
+----------------------+-------------------------------------------+ | request_type | Description | +----------------------+-------------------------------------------+ | "CONNECT" | Returns information about the successful | | | registration of the peer and/or of each | | | swarm action requested. May additionally | | | return the list of peers corresponding to | | | the action attribute | | | requested. | | | | | "FIND" | Returns the list of peers corresponding | | | to the requested scope. | | | | | "STAT_REPORT" | Confirms the success of the requested | | | operation. | +----------------------+-------------------------------------------+
IANA has created the "PPSTP Error Code Registry". Values are the strings listed in Table 9. New PPSTP error codes are assigned after IETF Review [RFC5226] to ensure that proper documentation regarding the new error codes and their usage has been provided.
+---------------+-------------------------------------------+ | error_code | Description | +---------------+-------------------------------------------+ | 00 | No Error | | 01 | Bad Request | | 02 | Unsupported Version Number | | 03 | Forbidden Action | | 04 | Internal Server Error | | 05 | Service Unavailable | | 06 | Authentication Required | +---------------+-------------------------------------------+
[Contracts] Piatek, M., Krishnamurthy, A., Venkataramani, A., Yang, R., Zhang, D., and A. Jaffe, "Contracts: Practical Contribution Incentives for P2P Live Streaming", NSDI: USENIX Symposium on Networked Systems Design and Implementation, April 2010.
The authors appreciate the contributions made by Yingjie Gu in the early stages of the specification. Also, they thank the following people for their help and comments: Zhang Yunfei, Liao Hongluan, Roni Even, Dave Cottlehuber, Bhumip Khasnabish, Wu Yichuan, Peng Jin, Chi Jing, Zong Ning, Song Haibin, Chen Wei, Zhijia Chen, Christian Schmidt, Lars Eggert, David Harrington, Henning Schulzrinne, Kangheng Wu, Martin Stiemerling, Jianyin Zhang, Johan Pouwelse, Riccardo Petrocco, and Arno Bakker.
The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the SARACEN project [SARACEN], the European Commission, Huawei, or China Mobile.