FEC Framework A. Begen Internet-Draft Cisco Intended status: Standards Track September 26, 2010 Expires: March 30, 2011 Session Description Protocol Elements for FEC Framework draft-ietf-fecframe-sdp-elements-09 Abstract This document specifies the use of Session Description Protocol (SDP) to describe the parameters required to signal the Forward Error Correction (FEC) Framework Configuration Information between the sender(s) and receiver(s). This document also provides examples that show the semantics for grouping multiple source and repair flows together for the applications that simultaneously use multiple instances of the FEC Framework. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on March 30, 2011. Copyright Notice Copyright (c) 2010 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 Begen Expires March 30, 2011 [Page 1] Internet-Draft SDP Elements for FEC Framework September 2010 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4 3. Forward Error Correction (FEC) and FEC Framework . . . . . . . 4 3.1. Forward Error Correction (FEC) . . . . . . . . . . . . . . 4 3.2. FEC Framework . . . . . . . . . . . . . . . . . . . . . . 5 3.3. FEC Framework Configuration Information . . . . . . . . . 5 4. SDP Elements . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Transport Protocol Identifiers . . . . . . . . . . . . . . 6 4.2. Media Stream Grouping . . . . . . . . . . . . . . . . . . 7 4.3. Source IP Addresses . . . . . . . . . . . . . . . . . . . 7 4.4. Source Flows . . . . . . . . . . . . . . . . . . . . . . . 7 4.5. Repair Flows . . . . . . . . . . . . . . . . . . . . . . . 8 4.6. Repair Window . . . . . . . . . . . . . . . . . . . . . . 10 4.7. Bandwidth Specification . . . . . . . . . . . . . . . . . 11 5. Scenarios and Examples . . . . . . . . . . . . . . . . . . . . 11 5.1. Declarative Considerations . . . . . . . . . . . . . . . . 11 5.2. Offer/Answer Model Considerations . . . . . . . . . . . . 12 6. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.1. One Source Flow, One Repair Flow and One FEC Scheme . . . 12 6.2. Two Source Flows, One Repair Flow and One FEC Scheme . . . 13 6.3. Two Source Flows, Two Repair Flows and Two FEC Schemes . . 14 6.4. One Source Flow, Two Repair Flows and Two FEC Schemes . . 15 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8.1. Registration of Transport Protocols . . . . . . . . . . . 16 8.2. Registration of SDP Attributes . . . . . . . . . . . . . . 17 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 10.1. Normative References . . . . . . . . . . . . . . . . . . . 18 10.2. Informative References . . . . . . . . . . . . . . . . . . 19 Begen Expires March 30, 2011 [Page 2] Internet-Draft SDP Elements for FEC Framework September 2010 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Begen Expires March 30, 2011 [Page 3] Internet-Draft SDP Elements for FEC Framework September 2010 1. Introduction The Forward Error Correction (FEC) Framework, described in [I-D.ietf-fecframe-framework], outlines a general framework for using FEC-based error recovery in packet flows carrying media content. While a continuous signaling between the sender(s) and receiver(s) is not required for a Content Delivery Protocol (CDP) that uses the FEC Framework, a set of parameters pertaining to the FEC Framework has to be initially communicated between the sender(s) and receiver(s). A signaling protocol (such as the one described in [I-D.ietf-fecframe-config-signaling]) is required to enable such communication and the parameters need to be appropriately encoded so that they can be carried by the signaling protocol. One format to encode the parameters is the Session Description Protocol (SDP) [RFC4566]. SDP provides a simple text-based format for announcements and invitations to describe multimedia sessions. These SDP announcements and invitations include sufficient information for the sender(s) and receiver(s) to participate in the multimedia sessions. SDP also provides a framework for capability negotiation, which can be used to negotiate all or a subset of the parameters pertaining to the individual sessions. The purpose of this document is to introduce the SDP elements that are used by the CDPs using the FEC Framework that choose SDP [RFC4566] as their session description protocol. 2. Requirements Notation 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]. 3. Forward Error Correction (FEC) and FEC Framework This section gives a brief overview of FEC and the FEC Framework. 3.1. Forward Error Correction (FEC) Any application that needs a reliable transmission over an unreliable packet network has to cope with packet losses. FEC is an effective approach that provides reliable transmission particularly in multicast and broadcast applications where the feedback from the receiver(s) is either not available or quite limited. In a nutshell, FEC groups source packets into blocks and applies Begen Expires March 30, 2011 [Page 4] Internet-Draft SDP Elements for FEC Framework September 2010 protection to generate a desired number of repair packets. These repair packets can be sent on demand or independently of any receiver feedback. The choice depends on the FEC scheme or the Content Delivery Protocol used by the application, the packet loss characteristics of the underlying network, the transport scheme (e.g., unicast, multicast and broadcast) and the application. At the receiver side, lost packets can be recovered by erasure decoding provided that a sufficient number of source and repair packets have been received. 3.2. FEC Framework The FEC Framework [I-D.ietf-fecframe-framework] outlines a general framework for using FEC codes in multimedia applications that stream audio, video or other types of multimedia content. It defines the common components and aspects of Content Delivery Protocols (CDP). The FEC Framework also defines the requirements for the FEC schemes that need to be used within a CDP. However, the details of the FEC schemes are not specified within the FEC Framework. For example, the FEC Framework defines what configuration information has to be known at the sender and receiver(s) at minimum, but the FEC Framework neither specifies how the FEC repair packets are generated and used to recover missing source packets, nor dictates how the configuration information is communicated between the sender and receiver(s). These are rather specified by the individual FEC schemes or CDPs. 3.3. FEC Framework Configuration Information The FEC Framework [I-D.ietf-fecframe-framework] defines a minimum set of information that has to be communicated between the sender and receiver(s) for a proper operation of an FEC scheme. This information is called the FEC Framework Configuration Information. This information includes unique identifiers for the source and repair flows that carry the source and repair packets, respectively. It also specifies how the sender applies protection to the source flow(s) and how the repair flow(s) can be used to recover lost data. Multiple instances of the FEC Framework can simultaneously exist at the sender and the receiver(s) for different source flows, for the same source flow, or for various combinations of the source flows. Each instance of the FEC Framework provides the following FEC Framework Configuration Information: 1. Identification of the repair flows. Begen Expires March 30, 2011 [Page 5] Internet-Draft SDP Elements for FEC Framework September 2010 2. For each source flow protected by the repair flow(s): a. Definition of the source flow. b. An integer identifier for this flow definition (i.e., tuple). This identifier MUST be unique amongst all source flows that are protected by the same FEC repair flow. The identifiers SHOULD be allocated starting from zero and increasing by one for each flow. A source flow identifier need not be carried in source packets since source packets are directly associated with a flow by virtue of their packet headers. 3. The FEC Encoding ID, identifying the FEC scheme. 4. The length of the Explicit Source FEC Payload ID (in octets). 5. Zero or more FEC-Scheme-Specific Information (FSSI) elements, each consisting of a name and a value where the valid element names and value ranges are defined by the FEC scheme. FSSI includes the information that is specific to the FEC scheme used by the CDP. FSSI is used to communicate the information that cannot be adequately represented otherwise and is essential for proper FEC encoding and decoding operations. The motivation behind separating the FSSI required only by the sender (which is carried in Sender-Side FEC-Scheme-Specific Information (SS-FSSI) container) from the rest of the FSSI is to provide the receiver or the third party entities a means of controlling the FEC operations at the sender. Any FSSI other than the one solely required by the sender MUST be communicated via the FSSI container. The variable-length SS-FSSI and FSSI containers transmit the information in textual representation and contain zero or more distinct elements, whose descriptions are provided by the fully- specified FEC schemes. 4. SDP Elements This section defines the SDP elements that MUST be used to describe the FEC Framework Configuration Information in multimedia sessions by the CDPs that choose SDP [RFC4566] as their session description protocol. Example SDP descriptions can be found in Section 6. 4.1. Transport Protocol Identifiers This specification defines a class of new transport protocol identifiers for SDP media descriptions. For all existing identifiers Begen Expires March 30, 2011 [Page 6] Internet-Draft SDP Elements for FEC Framework September 2010 (listed in the table for the 'proto' field in the Session Description Protocol (SDP) Parameters registry), this specification defines the identifier 'FEC/'. This identifier MAY be used as the transport protocol identifier in the media descriptions for the source data to indicate that the FEC Source Packet format defined in Section 5.3 of [I-D.ietf-fecframe-framework] is used, where the original transport payload field is formatted according to . However, if the FEC scheme does not use the Explicit Source FEC Payload ID as described in Section 4.1 of [I-D.ietf-fecframe-framework], then the original transport protocol identifier MUST be used to support backward compatibility with the receivers that do not support FEC at all. This specification also defines another transport protocol identifier, 'UDP/FEC', to indicate the FEC Repair Packet format defined in Section 5.4 of [I-D.ietf-fecframe-framework]. 4.2. Media Stream Grouping In FEC Framework, the 'group' attribute and the FEC grouping semantics defined in [RFC5888] and [I-D.ietf-mmusic-rfc4756bis], respectively are used to associate source and repair flows together. 4.3. Source IP Addresses The 'source-filter' attribute of SDP ("a=source-filter") as defined in [RFC4570] is used to express the source addresses or fully qualified domain names in the FEC Framework. 4.4. Source Flows The FEC Framework allows that multiple source flows MAY be grouped and protected together by a single or multiple FEC Framework instances. For this reason, as described in Section 3.3, individual source flows MUST be identified with unique identifiers. For this purpose, we introduce the attribute 'fec-source-flow'. The syntax for the new attribute in ABNF [RFC5234] is as follows: Begen Expires March 30, 2011 [Page 7] Internet-Draft SDP Elements for FEC Framework September 2010 fec-source-flow-line = "a=fec-source-flow:" SP source-id [";" SP tag-length] CRLF source-id = "id=" src-id src-id = 1*DIGIT tag-length = "tag-len=" tlen tlen = %x31-39 *DIGIT The REQUIRED parameter 'id' is used to identify the source flow. Parameter 'id' MUST be an integer. The 'tag-len' parameter is used to specify the length of the Explicit Source FEC Payload ID field (in octets). In the case that an Explicit Source FEC Payload ID is used, the 'tag-len' parameter MUST exist and indicate its length. Otherwise, the 'tag-len' parameter MUST NOT exist. 4.5. Repair Flows A repair flow MUST contain only repair packets formatted as described in [I-D.ietf-fecframe-framework] for a single FEC Framework instance, i.e., packets belonging to source flows or other repair flows from a different FEC Framework instance cannot be sent within this flow. We introduce the attribute 'fec-repair-flow' to describe the repair flows. The syntax for the new attribute in ABNF is as follows (CHAR and CTL are defined in [RFC5234]): Begen Expires March 30, 2011 [Page 8] Internet-Draft SDP Elements for FEC Framework September 2010 fec-repair-flow-line = "a=fec-repair-flow:" SP fec-encoding-id [";" SP flow-preference] [";" SP sender-side-scheme-specific] [";" SP scheme-specific] CRLF fec-encoding-id = "encoding-id=" enc-id enc-id = 1*DIGIT ; FEC Encoding ID flow-preference = "preference-lvl=" preference-level-of-the-flow preference-level-of-the-flow = 1*DIGIT sender-side-scheme-specific = "ss-fssi=" sender-info sender-info = element *( "," element ) element = name ":" value name = token token = 1* value = * separator = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> / "/" / "[" / "]" / "?" / "=" / "{" / "}" / SP / HTAB scheme-specific = "fssi=" scheme-info scheme-info = element *( "," element ) element = name ":" value The REQUIRED parameter 'encoding-id' is used to identify the FEC scheme used to generate this repair flow. These identifiers (in the range of [0 - 255]) are registered by the FEC schemes that use the FEC Framework and are maintained by IANA. The OPTIONAL parameter 'preference-lvl' is used to indicate the preferred order of using the repair flows. The exact usage of the parameter 'preference-lvl' and the pertaining rules MAY be defined by the FEC scheme or the CDP. If the parameter 'preference-lvl' does not exist, it means that the receiver(s) MAY receive and use the repair flows in any order. However, if a preference level is assigned to the repair flow(s), the receivers are encouraged to follow the specified order in receiving and using the repair flow(s). The OPTIONAL parameters 'ss-fssi' and 'fssi' are containers to convey the FEC-Scheme-Specific Information (FSSI) that includes the information that is specific to the FEC scheme used by the CDP and is necessary for proper FEC encoding and decoding operations. The FSSI required only by the sender (called Sender-Side FSSI) MUST be communicated in the container specified by the parameter 'ss-fssi'. Any other FSSI MUST be communicated in the container specified by the Begen Expires March 30, 2011 [Page 9] Internet-Draft SDP Elements for FEC Framework September 2010 parameter 'fssi'. In both containers, FSSI is transmitted in the form of textual representation and MAY contain multiple distinct elements. If the FEC scheme does not require any specific information, the 'ss-fssi' and 'fssi' parameters MUST NOT exist. 4.6. Repair Window Repair window is the time that spans an FEC block, which consists of the source block and the corresponding repair packets. At the sender side, the FEC encoder processes a block of source packets and generates a number of repair packets. Then both the source and repair packets are transmitted within a certain duration not larger than the value of the repair window. The value of the repair window impacts the maximum number of source packets that can be included in an FEC block. At the receiver side, the FEC decoder should wait at least for the duration of the repair window after getting the first packet in an FEC block to allow all the repair packets to arrive (The waiting time can be adjusted if there are missing packets at the beginning of the FEC block). The FEC decoder can start decoding the already received packets sooner, however, it SHOULD NOT register an FEC decoding failure until it waits at least for the repair-window duration. This document specifies a new attribute to describe the size of the repair window in milliseconds and microseconds. The syntax for the attribute in ABNF is as follows: repair-window-line = "a=repair-window:" window-size unit CRLF window-size = %x31-39 *DIGIT unit = "ms" / "us" is the unit of time the repair window size is specified with. Two units are defined here: 'ms', which stands for milliseconds and 'us', which stands for microseconds. The 'a=repair-window' attribute is a media-level attribute since each repair flow MAY have a different repair window size. Specifying the repair window size in an absolute time value does not necessarily correspond to an integer number of packets or exactly match with the clock rate used in RTP (in case of RTP transport) causing mismatches among subsequent repair windows. However, in practice, this mismatch does not break anything in the FEC decoding Begen Expires March 30, 2011 [Page 10] Internet-Draft SDP Elements for FEC Framework September 2010 process. 4.7. Bandwidth Specification The bandwidth specification as defined in [RFC4566] denotes the proposed bandwidth to be used by the session or media. The specification of bandwidth is OPTIONAL. In the context of the FEC Framework, the bandwidth specification can be used to express the bandwidth of the repair flows or the bandwidth of the session. If included in the SDP, it SHALL adhere to the following rules: The session-level bandwidth for an FEC Framework instance or the media-level bandwidth for the individual repair flows MAY be specified. In this case, it is RECOMMENDED to use the Transport Independent Application Specific (TIAS) bandwidth modifier [RFC3890] and the 'a=maxprate' attribute unless the Application Specific (AS) bandwidth modifier [RFC4566] is used. The use of AS bandwidth modifier is NOT RECOMMENDED since TIAS allows the calculation of the bitrate according to the IP version and transport protocol, whereas AS does not. Thus, in TIAS-based bitrate calculations, the packet size SHALL include all headers and payload, excluding the IP and UDP headers. In AS-based bitrate calculations, the packet size SHALL include all headers and payload, plus the IP and UDP headers. For the ABNF syntax information of the TIAS and AS, refer to [RFC3890] and [RFC4566], respectively. 5. Scenarios and Examples This section discusses the considerations for Session Announcement and Offer/Answer Models. 5.1. Declarative Considerations In multicast-based applications, the FEC Framework Configuration Information pertaining to all FEC protection options available at the sender MAY be advertised to the receivers as a part of a session announcement. This way, the sender can let the receivers know all available options for FEC protection. Based on their needs, the receivers MAY choose protection provided by one or more FEC Framework instances and subscribe to the respective multicast session(s) to receive the repair flow(s). Unless explicitly required by the CDP, the receivers SHOULD NOT send an answer back to the sender specifying their choices since this can easily overwhelm the sender particularly in large-scale multicast applications. Begen Expires March 30, 2011 [Page 11] Internet-Draft SDP Elements for FEC Framework September 2010 5.2. Offer/Answer Model Considerations In unicast-based applications, a sender and receiver MAY adopt the Offer/Answer Model [RFC3264] to set the FEC Framework Configuration Information. In this case, the sender offers the options available to this particular receiver and the receiver answers back to the sender with its choice(s). Receivers supporting the SDP Capability Negotiation Framework [I-D.ietf-mmusic-sdp-capability-negotiation] MAY also use this framework to negotiate all or a subset of the FEC Framework parameters. The backward compatibility in Offer/Answer Model is handled as specified in [I-D.ietf-mmusic-rfc4756bis]. 6. SDP Examples This section provides SDP examples that can be used by the FEC Framework. [RFC5888] defines the media stream identification attribute ('mid') as a token in ABNF. In contrast, the identifiers for the source flows MUST be integers and SHOULD be allocated starting from zero and increasing by one for each flow. To avoid any ambiguity, using the same values for identifying the media streams and source flows is NOT RECOMMENDED, even when 'mid' values are integers. In the examples below, random FEC Encoding IDs will be used for illustrative purposes. Artificial content for the SS-FSSI and FSSI will also be provided. 6.1. One Source Flow, One Repair Flow and One FEC Scheme SOURCE FLOWS | INSTANCE #1 S1: Source Flow |--------| R1: Repair Flow | Figure 1: Scenario #1 In this example, we have one source video flow (mid:S1) and one FEC repair flow (mid:R1). We form one FEC group with the "a=group:FEC-FR S1 R1" line. The source and repair flows are sent to the same port on different multicast groups. The repair window is set to 150 ms. Begen Expires March 30, 2011 [Page 12] Internet-Draft SDP Elements for FEC Framework September 2010 v=0 o=ali 1122334455 1122334466 IN IP4 fec.example.com s=FEC Framework Examples t=0 0 a=group:FEC-FR S1 R1 m=video 30000 RTP/AVP 100 c=IN IP4 233.252.0.1/127 a=rtpmap:100 MP2T/90000 a=fec-source-flow: id=0 a=mid:S1 m=application 30000 UDP/FEC c=IN IP4 233.252.0.2/127 a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5 a=repair-window:150ms a=mid:R1 6.2. Two Source Flows, One Repair Flow and One FEC Scheme SOURCE FLOWS S2: Source Flow | | INSTANCE #1 |---------| R2: Repair Flow S3: Source Flow | Figure 2: Scenario #2 In this example, we have two source video flows (mid:S2 and mid:S3) and one FEC repair flow (mid:R2), protecting both source flows. We form one FEC group with the "a=group:FEC-FR S2 S3 R2" line. The source and repair flows are sent to the same port on different multicast groups. The repair window is set to 150500 us. Begen Expires March 30, 2011 [Page 13] Internet-Draft SDP Elements for FEC Framework September 2010 v=0 o=ali 1122334455 1122334466 IN IP4 fec.example.com s=FEC Framework Examples t=0 0 a=group:FEC-FR S2 S3 R2 m=video 30000 RTP/AVP 100 c=IN IP4 233.252.0.1/127 a=rtpmap:100 MP2T/90000 a=fec-source-flow: id=0 a=mid:S2 m=video 30000 RTP/AVP 101 c=IN IP4 233.252.0.2/127 a=rtpmap:101 MP2T/90000 a=fec-source-flow: id=1 a=mid:S3 m=application 30000 UDP/FEC c=IN IP4 233.252.0.3/127 a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5 a=repair-window:150500us a=mid:R2 6.3. Two Source Flows, Two Repair Flows and Two FEC Schemes SOURCE FLOWS | INSTANCE #1 S4: Source Flow |--------| R3: Repair Flow S5: Source Flow |--------| INSTANCE #2 | R4: Repair Flow Figure 3: Scenario #3 In this example, we have two source video flows (mid:S4 and mid:S5) and two FEC repair flows (mid:R3 and mid:R4). The source flows mid:S4 and mid:S5 are protected by the repair flows mid:R3 and mid:R4, respectively. We form two FEC groups with the "a=group: FEC-FR S4 R3" and "a=group:FEC-FR S5 R4" lines. The source and repair flows are sent to the same port on different multicast groups. The repair window is set to 200 ms and 400 ms for the first and second FEC group, respectively. Begen Expires March 30, 2011 [Page 14] Internet-Draft SDP Elements for FEC Framework September 2010 v=0 o=ali 1122334455 1122334466 IN IP4 fec.example.com s=FEC Framework Examples t=0 0 a=group:FEC-FR S4 R3 a=group:FEC-FR S5 R4 m=video 30000 RTP/AVP 100 c=IN IP4 233.252.0.1/127 a=rtpmap:100 MP2T/90000 a=fec-source-flow: id=0 a=mid:S4 m=video 30000 RTP/AVP 101 c=IN IP4 233.252.0.2/127 a=rtpmap:101 MP2T/90000 a=fec-source-flow: id=1 a=mid:S5 m=application 30000 UDP/FEC c=IN IP4 233.252.0.3/127 a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5 a=repair-window:200ms a=mid:R3 m=application 30000 UDP/FEC c=IN IP4 233.252.0.4/127 a=fec-repair-flow: encoding-id=0; ss-fssi=n:14,k:10 a=repair-window:400ms a=mid:R4 6.4. One Source Flow, Two Repair Flows and Two FEC Schemes SOURCE FLOWS | INSTANCE #1 S6: Source Flow |--------| R5: Repair Flow | |--------| INSTANCE #2 | R6: Repair Flow Figure 4: Scenario #4 In this example, we have one source video flow (mid:S6) and two FEC repair flows (mid:R5 and mid:R6) with different preference levels. The source flow mid:S6 is protected by both of the repair flows. We form two FEC groups with the "a=group:FEC-FR S6 R5" and "a=group: FEC-FR S6 R6" lines. The source and repair flows are sent to the same port on different multicast groups. The repair window is set to 200 ms for both FEC groups. Begen Expires March 30, 2011 [Page 15] Internet-Draft SDP Elements for FEC Framework September 2010 v=0 o=ali 1122334455 1122334466 IN IP4 fec.example.com s=FEC Framework Examples t=0 0 a=group:FEC-FR S6 R5 a=group:FEC-FR S6 R6 m=video 30000 RTP/AVP 100 c=IN IP4 233.252.0.1/127 a=rtpmap:100 MP2T/90000 a=fec-source-flow: id=0 a=mid:S6 m=application 30000 UDP/FEC c=IN IP4 233.252.0.3/127 a=fec-repair-flow: encoding-id=0; preference-lvl=0; ss-fssi=n:7,k:5 a=repair-window:200ms a=mid:R5 m=application 30000 UDP/FEC c=IN IP4 233.252.0.4/127 a=fec-repair-flow: encoding-id=1; preference-lvl=1; ss-fssi=t:3 a=repair-window:200ms a=mid:R6 7. Security Considerations There is a weak threat if the SDP is modified in a way that it shows incorrect association and/or grouping of the source and repair flows. Such attacks can result in failure of FEC protection and/or mishandling of other media streams. It is RECOMMENDED that the receiver does integrity check on SDP to only trust SDP from trusted sources. The receiver MUST also follow the security considerations of SDP [RFC4566]. For other general security considerations related to SDP, refer to [RFC4566]. For the security considerations related to the use of source address filters in SDP, refer to [RFC4570]. The security considerations for the FEC Framework also apply. Refer to [I-D.ietf-fecframe-framework] for details. 8. IANA Considerations Note to the RFC Editor: In the following, please replace "XXXX" with the number of this document prior to publication as an RFC. 8.1. Registration of Transport Protocols This specification updates the Session Description Protocol (SDP) Parameters registry as defined in Section 8.2.2 of [RFC4566]. Begen Expires March 30, 2011 [Page 16] Internet-Draft SDP Elements for FEC Framework September 2010 Specifically, it adds the following values to the table for the 'proto' field. Type SDP Name Reference ------ ---------- ----------- proto UDP/FEC [RFCXXXX] This specification also defines a class of new transport protocol identifiers. For all existing identifiers (listed in the table for the 'proto' field in the Session Description Protocol (SDP) Parameters registry), this specification defines the identifier 'FEC/ '. 8.2. Registration of SDP Attributes This document registers new attribute names in SDP. SDP Attribute ("att-field"): Attribute name: fec-source-flow Long form: Pointer to FEC Source Flow Type of name: att-field Type of attribute: Media level Subject to charset: No Purpose: Provide parameters for an FEC source flow Reference: [RFCXXXX] Values: See [RFCXXXX] SDP Attribute ("att-field"): Attribute name: fec-repair-flow Long form: Pointer to FEC Repair Flow Type of name: att-field Type of attribute: Media level Subject to charset: No Purpose: Provide parameters for an FEC repair flow Reference: [RFCXXXX] Values: See [RFCXXXX] Begen Expires March 30, 2011 [Page 17] Internet-Draft SDP Elements for FEC Framework September 2010 SDP Attribute ("att-field"): Attribute name: repair-window Long form: Pointer to FEC Repair Window Type of name: att-field Type of attribute: Media level Subject to charset: No Purpose: Indicate the size of the repair window Reference: [RFCXXXX] Values: See [RFCXXXX] 9. Acknowledgments The author would like to thank the FEC Framework Design Team for their inputs, suggestions and contributions. 10. References 10.1. Normative References [I-D.ietf-fecframe-framework] Watson, M., "Forward Error Correction (FEC) Framework", draft-ietf-fecframe-framework-10 (work in progress), September 2010. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC4570] Quinn, B. and R. Finlayson, "Session Description Protocol (SDP) Source Filters", RFC 4570, July 2006. [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, June 2010. [I-D.ietf-mmusic-rfc4756bis] Begen, A., "Forward Error Correction Grouping Semantics in Session Description Protocol", draft-ietf-mmusic-rfc4756bis-10 (work in progress), June 2010. [RFC3890] Westerlund, M., "A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP)", RFC 3890, September 2004. Begen Expires March 30, 2011 [Page 18] Internet-Draft SDP Elements for FEC Framework September 2010 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. 10.2. Informative References [I-D.ietf-fecframe-config-signaling] Asati, R., "Methods to convey FEC Framework Configuration Information", draft-ietf-fecframe-config-signaling-03 (work in progress), June 2010. [I-D.ietf-mmusic-sdp-capability-negotiation] Andreasen, F., "SDP Capability Negotiation", draft-ietf-mmusic-sdp-capability-negotiation-13 (work in progress), March 2010. Author's Address Ali Begen Cisco 181 Bay Street Toronto, ON M5J 2T3 Canada Email: abegen@cisco.com Begen Expires March 30, 2011 [Page 19]