Network Working Group P. Thatcher Internet-Draft Google Intended status: Standards Track M. Zanaty Expires: April 21, 2016 S. Nandakumar Cisco Systems B. Burman Ericsson A. Roach B. Campen Mozilla October 19, 2015 RTP Payload Format Constraints draft-pthatcher-mmusic-rid-02 Abstract In this specification, we define a framework for identifying Source RTP Streams with the constraints on its payload format in the Session Description Protocol. This framework uses "rid" SDP attribute to: a) effectively identify the Source RTP Streams within a RTP Session, b) constrain their payload format parameters in a codec-agnostic way beyond what is provided with the regular Payload Types and c) enable unambiguous mapping between the Source RTP Streams to their media format specification in the SDP. Note-1: The name 'rid' is not yet finalized. Please refer to Section 12 for more details on the naming. 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 April 21, 2016. Thatcher, et al. Expires April 21, 2016 [Page 1] Internet-Draft rid October 2015 Copyright Notice Copyright (c) 2015 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Key Words for Requirements . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. SDP 'rid' Media Level Attribute . . . . . . . . . . . . . . . 5 6. 'rid-level' constraints . . . . . . . . . . . . . . . . . . . 6 7. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 7 7.1. Generating the Initial SDP Offer . . . . . . . . . . . . 7 7.2. Answerer processing the SDP Offer . . . . . . . . . . . . 8 7.2.1. 'rid' unaware Answerer . . . . . . . . . . . . . . . 8 7.2.2. 'rid' aware Answerer . . . . . . . . . . . . . . . . 8 7.3. Generating the SDP Answer . . . . . . . . . . . . . . . . 9 7.4. Offering Processing of the SDP Answer . . . . . . . . . . 10 7.5. Modifying the Session . . . . . . . . . . . . . . . . . . 10 8. Usage of 'rid' in RTP and RTCP . . . . . . . . . . . . . . . 10 8.1. RTCP 'RID' SDES Extension . . . . . . . . . . . . . . . . 11 8.2. RTP 'rid' Header Extension . . . . . . . . . . . . . . . 11 9. Interaction with Other Techniques . . . . . . . . . . . . . . 12 10. Formal Grammar . . . . . . . . . . . . . . . . . . . . . . . 12 11. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . 14 11.1. Many Bundled Streams using Many Codecs . . . . . . . . . 14 11.2. Scalable Layers . . . . . . . . . . . . . . . . . . . . 16 12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 16 12.1. Name of the identifier . . . . . . . . . . . . . . . . . 16 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 13.1. New RTP Header Extension URI . . . . . . . . . . . . . . 17 13.2. New SDES item . . . . . . . . . . . . . . . . . . . . . 17 13.3. New SDP Media-Level attribute . . . . . . . . . . . . . 18 13.4. Registry for RID-Level Parameters . . . . . . . . . . . 18 14. Security Considerations . . . . . . . . . . . . . . . . . . . 19 15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 Thatcher, et al. Expires April 21, 2016 [Page 2] Internet-Draft rid October 2015 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 16.1. Normative References . . . . . . . . . . . . . . . . . . 20 16.2. Informative References . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 1. Introduction Payload Type (PT) in RTP provides mapping between the format of the RTP payload and the media format description specified in the signaling. For applications that use SDP for signaling, the constructs rtpmap and/or fmtp describe the characteristics of the media that is carried in the RTP payload, mapped to a given PT. Recent advances in standards such as RTCWEB and NETVC have given rise to rich multimedia applications requiring support for multiple RTP Streams with in a RTP session [I-D.ietf-mmusic-sdp-bundle-negotiation], [I-D.ietf-mmusic-sdp-simulcast] or having to support multiple codecs, for example. These demands have unearthed challenges inherent with: o The restricted RTP PT space in specifying the various payload configurations, o The codec-specific constructs for the payload formats in SDP, o Missing or underspecied payload format parameters, o Ambiguity in mapping between the individual Source RTP Streams and their equivalent format specification in the SDP. This specification defines a new SDP framework for constraining Source RTP Streams (Section 2.1.10 [I-D.ietf-avtext-rtp-grouping-taxonomy]), called "Restriction Identifier (rid)", along with the SDP attributes to constrain their payload formats in a codec-agnostic way. The "rid" framework can be thought of as complementary extension to the way the media format parameters are specified in SDP today, via the "a=fmtp" attribute. This specification also proposes a new RTCP SDES item to carry the "rid" value, to provide correlation between the RTP Packets and their format specification in the SDP. This SDES item also uses the header extension mechanism [I-D.ietf-avtext-sdes-hdr-ext] to provide correlation at stream startup, or stream changes where RTCP isn't sufficient. Note that the "rid" parameters only serve to further constrain the parameters that are established on a PT format. They do not relax any existing constraints. Thatcher, et al. Expires April 21, 2016 [Page 3] Internet-Draft rid October 2015 As described in Section 7.2.1, this mechanism achieves backwards compatibility via the normal SDP processing rules, which require unknown a= parameters to be ignored. This means that implementations need to be prepared to handle successful offers and answers from other implementations that neither indicate nor honor the constraints requested by this mechanism. Further, as described in Section 7 and its subsections, this mechanism achieves extensibility by: (a) having offerers include all supported constraints in their offer, abd (b) having answerers ignore a=rid lines that specify unknown constraints. 2. Key Words for Requirements 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. Terminology The terms Source RTP Stream, Endpoint, RTP Session, and RTP Stream are used as defined in [I-D.ietf-avtext-rtp-grouping-taxonomy]. [RFC4566] and [RFC3264] terminology is also used where appropriate. 4. Motivation This section summarizes several motivations for proposing the "rid" framework. 1. RTP PT Space Exhaustion: [RFC3550] defines payload type (PT) that identifies the format of the RTP payload and determine its interpretation by the application. [RFC3550] assigns 7 bits for the PT in the RTP header. However, the assignment of static mapping of payload codes to payload formats and multiplexing of RTP with other protocols (such as RTCP) could result in limited number of payload type numbers available for the application usage. In scenarios where the number of possible RTP payload configurations exceed the available PT space within a RTP Session, there is need a way to represent the additional constraints on payload configurations and to effectively map a Source RTP Stream to its corresponding constraints. 1. Multi-source and Multi-stream Use Cases: Recently, there is a rising trend with real-time multimedia applications supporting multiple sources per endpoint with various temporal resolutions (Scalable Video Codec) and spatial resolutions (Simulcast) per source. These applications are being challenged by the limited Thatcher, et al. Expires April 21, 2016 [Page 4] Internet-Draft rid October 2015 RTP PT space and/or by the underspecified SDP constructs for exercising granular control on configuring the individual Source RTP Streams. 5. SDP 'rid' Media Level Attribute This section defines new SDP media-level attribute [RFC4566], "a=rid". Roughly speaking, this attribute takes the following form (see Section 10 for a formal definition). a=rid: pt=;=... A given "a=rid" SDP media attribute specifies constraints defining an unique RTP payload configuration identified via the "rid-identifier". A set of codec-agnostic "rid-level" constraints are defined (Section 6) that describe the media format specification applicable to one or more Payload Types speicified by the "a=rid" line. The 'rid' framework MAY be used in combination with the 'a=fmtp' SDP attribute for describing the media format parameters for a given RTP Payload Type. However in such scenarios, the 'rid-level' constraints (Section 6) further constrains the equivalent 'fmtp' attributes. The 'direction' identifies the either 'send', 'recv' directionality of the Source RTP Stream. A given SDP media description MAY have zero or more "a=rid" lines describing various possible RTP payload configurations. A given 'rid-identifier' MUST NOT be repeated in a given media description. The 'rid' media attribute MAY be used for any RTP-based media transport. It is not defined for other transports. Though the 'rid-level' attributes specified by the 'rid' property follow the syntax similar to session-level and media-level attributes, they are defined independently. All 'rid-level' attributes MUST be registered with IANA, using the registry defined in Section 13 Section 10 gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for the "rid" attribute. The "a=rid" media attribute is not dependent on charset. Thatcher, et al. Expires April 21, 2016 [Page 5] Internet-Draft rid October 2015 6. 'rid-level' constraints This section defines the 'rid-level' constraints that can be used to constrain the RTP payload encoding format in a codec-agnostic way. The following constraints are intended to apply to video codecs in a codec-independent fashion. o max-width, for spatial resolution in pixels. In the case that stream orientation signaling is used to modify the intended display orientation, this attribute refers to the width of the stream when a rotation of zero degrees is encoded. o max-height, for spatial resolution in pixels. In the case that stream orientation signaling is used to modify the intended display orientation, this attribute refers to the width of the stream when a rotation of zero degrees is encoded. o max-fps, for frame rate in frames per second. For encoders that do not use a fixed framerate for encoding, this value should constrain the minimum amount of time between frames: the time between any two consecutive frames SHOULD NOT be less than 1/max- fps seconds. o max-fs, for frame size in pixels per frame. This is the product of frame width and frame height, in pixels, for rectangular frames. o max-br, for bit rate in bits per second. The restriction applies to the media payload only, and does not include overhead introduced by other layers (e.g., RTP, UDP, IP, or Ethernet). The exact means of keeping within this limit are left up to the implementation, and instantaneous excursions outside the limit are permissible. For any given one-second sliding window, however, the total number of bits in the payload portion of RTP SHOULD NOT exceed the value specified in "max-br." o max-pps, for pixel rate in pixels per second. This value SHOULD be handled identically to max-fps, after performing the following conversion: max-fps = max-pps / (width * height). If the stream resolution changes, this value is recalculated. Due to this recalculation, excursions outside the specified maximum are possible during near resolution change boundaries. All the constraints are optional and are subjected to negotiation based on the SDP Offer/Answer rules described in Section 7 Thatcher, et al. Expires April 21, 2016 [Page 6] Internet-Draft rid October 2015 This list is intended to be an initial set of constraints; future documents may define additional constraints; see Section 13.4. While this document doesn't define constraints for audio codecs, there is no reason such constraints should be precluded from definition and registration by other documents. Section 10 provides formal Augmented Backus-Naur Form(ABNF) [RFC5234] grammar for each of the "rid-level" attributes defined in this section. 7. SDP Offer/Answer Procedures This section describes the SDP Offer/Answer [RFC3264] procedures when using the 'rid' framework. Note that 'rid's are only required to be unique within a media section ("m-line"); they do not necessarily need to be unique within an entire RTP session. In traditional usage, each media section is sent on its own unique 5-tuple, which provides an unambiguous scope. Similarly, when using BUNDLE [I-D.ietf-mmusic-sdp-bundle-negotiation], MID values associate RTP streams uniquely to a single media description. 7.1. Generating the Initial SDP Offer For each media description in the offer, the offerer MAY choose to include one or more "a=rid" lines to specify a configuration profile for the given set of RTP Payload Types. In order to construct a given "a=rid" line, the offerer must follow the below steps: 1. It MUST generate a 'rid-identifier' that is unique within a media description 2. It MUST set the direction for the 'rid-identifier' to one of 'send' or 'recv' 3. It MAY include a listing of SDP format tokens (usually corresponding to RTP payload types) to which the constraints expressed by the 'rid-level' attributes apply. Any Payload Types chosen MUST be a valid payload type for the media section (that is, it must be listed on the "m=" line). 4. The Offerer then chooses the 'rid-level' constraints (Section 6) to be applied for the rid, and adds them to the "a=rid" line. If it wishes the answer to have the ability to specify a constraint, but does not wish to set a value itself, it MUST include the name Thatcher, et al. Expires April 21, 2016 [Page 7] Internet-Draft rid October 2015 of the constraint in the "a=rid" line, but without any indicated value. Note: If an 'a=fmtp' attribute is also used to provide media-format- specific parameters, then the 'rid-level' attributes will further constrain the equivalent 'fmtp' parameters for the given Payload Type for those streams associated with the 'rid'. If a given codec would require "a=fmtp" line when used without "a=rid" then the offer MUST include a valid corresponding "a=fmtp" line even when using RID. 7.2. Answerer processing the SDP Offer For each media description in the offer, and for each "a=rid" attribute in the media description, the receiver of the offer will perform the following steps: 7.2.1. 'rid' unaware Answerer If the receiver doesn't support the 'rid' framework proposed in this specification, the entire "a=rid" line is ignored following the standard [RFC3264] Offer/Answer rules. Section 7.1 requires the offer to include a valid "a=fmtp" line for any codecs that otherwise require it (in other words, the "a=rid" line cannot be used to replace "a=fmtp" configuration). As a result, ignoring the "a=rid" line is always guaranteed to result in a valid session description. 7.2.2. 'rid' aware Answerer If the answerer supports 'rid' framework, the following steps are executed, in order, for each "a=rid" line in a given media description: 1. Extract the rid-identifier from the "a=rid" line and verify its uniqueness. In the case of a duplicate, the entire "a=rid" line, and all "a=rid" lines with rid-identifiers that duplicate this line, are rejected and MUST NOT be included in the SDP Answer. 2. If the "a=rid" line contains a "pt=" parameter, the list of payload types is verified against the list of valid payload types for the media section (that is, those listed on the "m=" line). If there is no match for the Payload Type listed in the "a=rid" line, then remove the "a=rid" line. Thatcher, et al. Expires April 21, 2016 [Page 8] Internet-Draft rid October 2015 3. The answerer ensures that "rid-level" parameters listed are supported and syntactically well formed. In the case of a syntax error or an unsupported parameter, the "a=rid" line is removed. 4. If the 'depend' rid-level attribute is included, the answerer MUST make sure that the rid-identifiers listed unambiguously match the rid-identifiers in the SDP offer. Any lines that do not are removed. 5. if the "a=rid" line contains a "pt=" parameter, the answerer verifies that the attribute values provided in the "rid-level" attributes are consistent with the corrsponding codecs and their other parameters. See Section 9 for more detail. If the rid- level parameters are incompatible with the other codec properties, then the "a=rid" line is removed. 7.3. Generating the SDP Answer Having performed the verification of the SDP offer as described, the answerer shall perform the following steps to generate the SDP answer. For each "a=rid" line: 1. The answerer MAY choose to modify specific 'rid-level' attribute value in the answer SDP. In such a case, the modified value MUST be more constrained than the ones specified in the offer. The answer MUST NOT include any constraints that were not present in the offer. 2. The answerer MUST NOT modify the 'rid-identifier' present in the offer. 3. The answerer is allowed to remove one or more media formats from a given 'a=rid' line. If the answerer chooses to remove all the media format tokens from an "a=rid" line, the answerer MUST remove the entire "a=rid" line. 4. In cases where the answerer is unable to support the payload configuration specified in a given "a=rid" line in the offer, the answerer MUST remove the corresponding "a=rid" line. This includes situations in which the answerer does not understand one or more of the constraints in the "a=rid" line that has an associated value. Note: in the case that the answerer uses different PT values to represent a codec than the offerer did, the "a=rid" values in the answer use the PT values that were sent in the offer. Thatcher, et al. Expires April 21, 2016 [Page 9] Internet-Draft rid October 2015 7.4. Offering Processing of the SDP Answer The offerer shall follow the steps similar to answerer's offer processing with the following exceptions 1. The offerer MUST ensure that the 'rid-identifiers' aren't changed between the offer and the answer. If so, the offerer MUST consider the corresponding 'a=rid' line as rejected. 2. If there exist changes in the 'rid-level' attribute values, the offerer MUST ensure that the modifications can be supported or else consider the "a=rid" line as rejected. 3. If the SDP answer contains any "rid-identifier" that doesn't match with the offer, the offerer MUST ignore the corresponding "a=rid" line. 4. If the "a=rid" line contains a "pt=" parameter, the offerer verifies that the list of payload types is a subset of those sent in the corresponding "a=rid" line in the offer. 5. If the "a=rid" line contains a "pt=" parameter, the offerer verifies that the attribute values provided in the "rid-level" attributes are consistent with the corrsponding codecs and their other parameters. See Section 9 for more detail. If the rid- level parameters are incompatible with the other codec properties, then the "a=rid" line is removed. 7.5. Modifying the Session Offers and answers inside an existing session follow the rules for initial session negotiation. Such an offer MAY propose a change the number of RIDs in use. To avoid race conditions with media, any RIDs with proposed changes SHOULD use a new ID, rather than re-using one from the previous offer/answer exchange. RIDs without proposed changes SHOULD re-use the ID from the previous exchange. 8. Usage of 'rid' in RTP and RTCP The RTP fixed header includes the payload type number and the SSRC values of the RTP stream. RTP defines how you de-multiplex streams within an RTP session, but in some use cases applications need further identifiers in order to effectively map the individual RTP Streams to their equivalent payload configurations in the SDP. This specification defines a new RTCP SDES item [RFC3550], 'RID', which is used to carry rids within RTCP SDES packets. This makes it possible for a receiver to associate received RTP packets Thatcher, et al. Expires April 21, 2016 [Page 10] Internet-Draft rid October 2015 (identifying the Source RTP Stream) with a media description having the format constraint specified. This specification also uses the RTP header extension for RTCP SDES items [I-D.ietf-avtext-sdes-hdr-ext] to allow carrying RID information in RTP packets to provide correlation at stream startup, or after stream changes where the use of RTCP may not be sufficiently responsive. 8.1. RTCP 'RID' SDES Extension 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RID=TBD | length | rid ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The rid payload is UTF-8 encoded and is not null-terminated. RFC EDITOR NOTE: Please replace TBD with the assigned SDES identifier value. 8.2. RTP 'rid' Header Extension Because recipients of RTP packets will typically need to know which "a=rid" constraints they correspond to immediately upon receipt, this specification also defines a means of carrying RID identifiers in RTP extension headers, using the technique described in [I-D.ietf-avtext-sdes-hdr-ext]. As described in that document, the header extension element can be encoded using either the one-byte or two-byte header, and the identification-tag payload is UTF-8 encoded, as in SDP. As the identification-tag is included in an RTP header extension, there should be some consideration about the packet expansion caused by the identification-tag. To avoid Maximum Transmission Unit (MTU) issues for the RTP packets, the header extension's size needs to be taken into account when the encoding media. Note that set of header extensions included in the packet needs to be padded to the next 32-bit boundary using zero bytes [RFC5285] It is RECOMMENDED that the identification-tag is kept short. Due to the properties of the RTP header extension mechanism, when using the one-byte header, a tag that is 1-3 bytes will result in that a minimal number of 32-bit words are used for the RTP header extension, in case no other header extensions are included at the same time. In Thatcher, et al. Expires April 21, 2016 [Page 11] Internet-Draft rid October 2015 many cases, a one-byte tag will be sufficient; it is RECOMMENDED that implementations use the shortest identifier that fits their purposes. 9. Interaction with Other Techniques Historically, a number of other approaches have been defined that allow constraining media streams via SDP parameters. These include: o Codec-specific configuration set via format parameters ("a=fmtp"); for example, the H.264 "max-fs" format parameter o Size restrictions imposed by image attribute attributes ("a=imgattr") [RFC6236] When the mechanism described in this document is used in conjunction with these other restricting mechanisms, it is intended to impose additional restrictions beyond those communicated in other techniques. In an offer, this means that a=rid lines, when combined with other restrictions on the media stream, are expected to result in a non- empty union. For example, if image attributes are used to indicate that a PT has a minimum width of 640, then specification of "max- width=320" in an "a=rid" line that is then applied to that PT is nonsensical. According to the rules of Section 7.2.2, this will result in the corresponding "a=rid" line being ignored by the recipient. Similarly, an answer the a=rid lines, when combined with the other restrictions on the media stream, are also expected to result in a non-empty union. If the implementation generating an answer wishes to restrict a property of the stream below that which would be allowed by other parameters (e.g., those specified in "a=fmtp" or "a=imgattr"), its only recourse is to remove the "a=rid" line altogether, as described in Section 7.3. If it instead attempts to constrain the stream beyond what is allowed by other mechanisms, then the offerer will ignore the corresponding "a=rid" line, as described in Section 7.4. 10. Formal Grammar This section gives a formal Augmented Backus-Naur Form (ABNF) [RFC5234] grammar for each of the new media and rid-level attributes defined in this document. Thatcher, et al. Expires April 21, 2016 [Page 12] Internet-Draft rid October 2015 rid-syntax = "a=rid:" rid-identifier SP rid-dir [ rid-pt-param-list / rid-param-list ] rid-identifier = 1*(alpha-numeric / "-" / "_") rid-dir = "send" / "recv" rid-pt-param-list = SP rid-fmt-list *(";" rid-param) rid-param-list = SP rid-param *(";" rid-param) rid-fmt-list = "pt=" fmt *( "," fmt ) ; fmt defined in {{RFC4566}} rid-param = rid-width-param / rid-height-param / rid-fps-param / rid-fs-param / rid-br-param / rid-pps-param / rid-depend-param / rid-param-other rid-width-param = "max-width" [ "=" int-param-val ] rid-height-param = "max-height" [ "=" int-param-val ] rid-fps-param = "max-fps" [ "=" int-param-val ] rid-fs-param = "max-fs" [ "=" int-param-val ] rid-br-param = "max-br" [ "=" int-param-val ] rid-pps-param = "max-pps" [ "=" int-param-val ] rid-depend-param = "depend=" rid-list rid-param-other = 1*(alpha-numeric / "-") [ "=" param-val ] rid-list = rid-identifier *( "," rid-identifier ) int-param-val = 1*DIGIT param-val = *( %x20-58 / %x60-7E ) ; Any printable character except semicolon Thatcher, et al. Expires April 21, 2016 [Page 13] Internet-Draft rid October 2015 11. SDP Examples Note: see [I-D.ietf-mmusic-sdp-simulcast] for examples of RID used in simulcast scenarios. 11.1. Many Bundled Streams using Many Codecs In this scenario, the offerer supports the Opus, G.722, G.711 and DTMF audio codecs, and VP8, VP9, H.264 (CBP/CHP, mode 0/1), H.264-SVC (SCBP/SCHP) and H.265 (MP/M10P) for video. An 8-way video call (to a mixer) is supported (send 1 and receive 7 video streams) by offering 7 video media sections (1 sendrecv at max resolution and 6 recvonly at smaller resolutions), all bundled on the same port, using 3 different resolutions. The resolutions include: o 1 receive stream of 720p resolution is offered for the active speaker. o 2 receive streams of 360p resolution are offered for the prior 2 active speakers. o 4 receive streams of 180p resolution are offered for others in the call. Expressing all these codecs and resolutions using 32 dynamic PTs (2 audio + 10x3 video) would exhaust the primary dynamic space (96-127). RIDs are used to avoid PT exhaustion and express the resolution constraints. NOTE: The SDP given below skips few lines to keep the example short and focused, as indicated by either the "..." or the comments inserted. Example 1 Offer: ... m=audio 10000 RTP/SAVPF 96 9 8 0 123 a=rtpmap:96 OPUS/48000 a=rtpmap:9 G722/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:0 PCMU/8000 a=rtpmap:123 telephone-event/8000 a=mid:a1 ... m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107 a=rtpmap:98 VP8/90000 Thatcher, et al. Expires April 21, 2016 [Page 14] Internet-Draft rid October 2015 a=fmtp:98 max-fs=3600; max-fr=30 a=rtpmap:99 VP9/90000 a=fmtp:99 max-fs=3600; max-fr=30 a=rtpmap:100 H264/90000 a=fmtp:100 profile-level-id=42401f; packetization-mode=0 a=rtpmap:101 H264/90000 a=fmtp:101 profile-level-id=42401f; packetization-mode=1 a=rtpmap:102 H264/90000 a=fmtp:102 profile-level-id=640c1f; packetization-mode=0 a=rtpmap:103 H264/90000 a=fmtp:103 profile-level-id=640c1f; packetization-mode=1 a=rtpmap:104 H264-SVC/90000 a=fmtp:104 profile-level-id=530c1f a=rtpmap:105 H264-SVC/90000 a=fmtp:105 profile-level-id=560c1f a=rtpmap:106 H265/90000 a=fmtp:106 profile-id=1; level-id=93 a=rtpmap:107 H265/90000 a=fmtp:107 profile-id=2; level-id=93 a=sendrecv a=mid:v1 (max resolution) a=rid:1 send max-width=1280;max-height=720;max-fps=30 a=rid:2 recv max-width=1280;max-height=720;max-fps=30 ... m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107 ...same rtpmap/fmtp as above... a=recvonly a=mid:v2 (medium resolution) a=rid:3 recv max-width=640;max-height=360;max-fps=15 ... m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107 ...same rtpmap/fmtp as above... a=recvonly a=mid:v3 (medium resolution) a=rid:3 recv max-width=640;max-height=360;max-fps=15 ... m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107 ...same rtpmap/fmtp as above... a=recvonly a=mid:v4 (small resolution) a=rid:4 recv max-width=320;max-height=180;max-fps=15 ... m=video 10000 RTP/SAVPF 98 99 100 101 102 103 104 105 106 107 ...same rtpmap/fmtp as above... ...same rid:4 as above for mid:v5,v6,v7 (small resolution)... ... Thatcher, et al. Expires April 21, 2016 [Page 15] Internet-Draft rid October 2015 Answer: ...same as offer but swap send/recv... 11.2. Scalable Layers Adding scalable layers to the above simulcast example gives the SFU further flexibility to selectively forward packets from a source that best match the bandwidth and capabilities of diverse receivers. Scalable encodings have dependencies between layers, unlike independent simulcast streams. RIDs can be used to express these dependencies using the "depend" parameter. In the example below, the highest resolution is offered to be sent as 2 scalable temporal layers (using MRST). Example 3 Offer: ... m=audio ...same as Example 1 ... ... m=video ...same as Example 1 ... ...same rtpmap/fmtp as Example 1... a=sendrecv a=mid:v1 (max resolution) a=rid:0 send max-width=1280;max-height=720;max-fps=15 a=rid:1 send max-width=1280;max-height=720;max-fps=30;depend=0 a=rid:2 recv max-width=1280;max-height=720;max-fps=30 a=rid:5 send max-width=640;max-height=360;max-fps=15 a=rid:6 send max-width=320;max-height=180;max-fps=15 a=simulcast: send rid=0;1;5;6 recv rid=2 ... ...same m=video sections as Example1 for mid:v2-v7... ... Answer: ...same as offer but swap send/recv... 12. Open Issues 12.1. Name of the identifier The name 'rid' is provisionally used and is open for further discussion. Here are the few options that were considered while writing this draft Thatcher, et al. Expires April 21, 2016 [Page 16] Internet-Draft rid October 2015 o CID: Constraint ID, which is a rather precise description of what we are attempting to accomplish. o ESID: Encoded Stream ID, does not align well with taxonomy which defines Encoded Stream as before RTP packetization. o RSID or RID: RTP Stream ID, aligns better with taxonomy but very vague. o LID: Layer ID, aligns well for SVC with each layer in a separate stream, but not for other SVC layerings or independent simulcast which is awkward to view as layers. o EPT or XPT: EXtended Payload Type, conveys XPT.PT usage well, but may be confused with PT, for example people may mistakenly think they can use it in other places where PT would normally be used. 13. IANA Considerations 13.1. New RTP Header Extension URI This document defines a new extension URI in the RTP Compact Header Extensions subregistry of the Real-Time Transport Protocol (RTP) Parameters registry, according to the following data: Extension URI: urn:ietf:params:rtp-hdrext:sdes:rid Description: RTP Stream Restriction Identifier Contact: Reference: RFCXXXX 13.2. New SDES item RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document. RFC EDITOR NOTE: Please replace TBD with the assigned SDES identifier value. This document adds the MID SDES item to the IANA "RTCP SDES item types" registry as follows: Value: TBD Abbrev.: RID Name: Restriction Identification Reference: RFCXXXX Thatcher, et al. Expires April 21, 2016 [Page 17] Internet-Draft rid October 2015 13.3. New SDP Media-Level attribute This document defines "rid" as SDP media-level attribute. This attribute must be registered by IANA under "Session Description Protocol (SDP) Parameters" under "att-field (media level only)". The "rid" attribute is used to identify characteristics of RTP stream with in a RTP Session. Its format is defined in Section 10. 13.4. Registry for RID-Level Parameters This specification creates a new IANA registry named "att-field (rid level)" within the SDP parameters registry. The rid-level parameters MUST be registered with IANA and documented under the same rules as for SDP session-level and media-level attributes as specified in [RFC4566]. Parameters for "a=rid" lines that modify the nature of encoded media MUST be of the form that the result of applying the modification to the stream results in a stream that still complies with the other parameters that affect the media. In other words, parameters always have to restrict the definition to be a subset of what is otherwise allowable, and never expand it. New parameter registrations are accepted according to the "Specification Required" policy of [RFC5226], provided that the specification includes the following information: o contact name, email address, and telephone number o parameter name (as it will appear in SDP) o long-form parameter name in English o whether the parameter value is subject to the charset attribute o an explanation of the purpose of the parameter o a specification of appropriate attribute values for this parameter o an ABNF definition of the parameter The initial set of rid-level parameter names, with definitions in Section 6 of this document, is given below: Thatcher, et al. Expires April 21, 2016 [Page 18] Internet-Draft rid October 2015 Type SDP Name Reference ---- ------------------ --------- att-field (rid level) max-width [RFCXXXX] max-height [RFCXXXX] max-fps [RFCXXXX] max-fs [RFCXXXX] max-br [RFCXXXX] max-pps [RFCXXXX] depend [RFCXXXX] It is conceivable that a future document wants to define a RID-level parameter that contains string values. These extensions need to take care to conform to the ABNF defined for rid-param-other. In particular, this means that such extensions will need to define escaping mechanisms if they want to allow semicolons, unprintable characters, or byte values greater than 127 in the string. OPEN ITEM: Do we need to do more than this regarding escaping? 14. Security Considerations As with most SDP parameters, a failure to provide integrity protection over the a=rid attributes provides attackers a way to modify the session in potentially unwanted ways. This could result in an implementation sending greater amounts of data than a recipient wishes to receive. In general, however, since the "a=rid" attribute can only restrict a stream to be a subset of what is otherwise allowable, modification of the value cannot result in a stream that is of higher bandwidth than would be sent to an implementation that does not support this mechanism. The actual identifiers used for RIDs are expected to be opaque. As such, they are not expected to contain information that would be sensitive, were it observed by third-parties. 15. Acknowledgements Many thanks to review from Cullen Jennings, Magnus Westerlund, and Paul Kyzivat. 16. References Thatcher, et al. Expires April 21, 2016 [Page 19] Internet-Draft rid October 2015 16.1. Normative References [I-D.ietf-avtext-sdes-hdr-ext] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP Header Extension for RTCP Source Description Items", draft-ietf-avtext-sdes-hdr-ext-02 (work in progress), July 2015. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002, . [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003, . [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ RFC5234, January 2008, . [RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July 2008, . 16.2. Informative References [I-D.ietf-avtext-rtp-grouping-taxonomy] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and B. Burman, "A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources", draft-ietf- avtext-rtp-grouping-taxonomy-08 (work in progress), July 2015. Thatcher, et al. Expires April 21, 2016 [Page 20] Internet-Draft rid October 2015 [I-D.ietf-mmusic-sdp-bundle-negotiation] Holmberg, C., Alvestrand, H., and C. Jennings, "Negotiating Media Multiplexing Using the Session Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- negotiation-23 (work in progress), July 2015. [I-D.ietf-mmusic-sdp-simulcast] Burman, B., Westerlund, M., Nandakumar, S., and M. Zanaty, "Using Simulcast in SDP and RTP Sessions", draft-ietf- mmusic-sdp-simulcast-02 (work in progress), October 2015. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, . [RFC6236] Johansson, I. and K. Jung, "Negotiation of Generic Image Attributes in the Session Description Protocol (SDP)", RFC 6236, DOI 10.17487/RFC6236, May 2011, . Authors' Addresses Peter Thatcher Google Email: pthatcher@google.com Mo Zanaty Cisco Systems Email: mzanaty@cisco.com Suhas Nandakumar Cisco Systems Email: snandaku@cisco.com Bo Burman Ericsson Email: bo.burman@ericsson.com Thatcher, et al. Expires April 21, 2016 [Page 21] Internet-Draft rid October 2015 Adam Roach Mozilla Email: adam@nostrum.com Byron Campen Mozilla Email: bcampen@mozilla.com Thatcher, et al. Expires April 21, 2016 [Page 22]