Network Working Group M. Handley Internet-Draft UCL Obsoletes: 4566 (if approved) C. Perkins Intended status: Standards Track University of Glasgow Expires: April 10, 2018 A. Begen Networked Media October 7, 2017 SDP: Session Description Protocol draft-ietf-mmusic-rfc4566bis-24 Abstract This memo defines the Session Description Protocol (SDP). SDP is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. This document obsoletes RFC 4566. 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 https://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 10, 2018. Copyright Notice Copyright (c) 2017 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 (https://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 Handley, et al. Expires April 10, 2018 [Page 1] Internet-Draft SDP October 2017 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . 4 3. Examples of SDP Usage . . . . . . . . . . . . . . . . . . . . 5 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 5 3.2. Streaming Media . . . . . . . . . . . . . . . . . . . . . 5 3.3. Email and the World Wide Web . . . . . . . . . . . . . . 5 3.4. Multicast Session Announcement . . . . . . . . . . . . . 5 4. Requirements and Recommendations . . . . . . . . . . . . . . 6 4.1. Media and Transport Information . . . . . . . . . . . . . 7 4.2. Timing Information . . . . . . . . . . . . . . . . . . . 7 4.3. Obtaining Further Information about a Session . . . . . . 8 4.4. Categorisation . . . . . . . . . . . . . . . . . . . . . 8 4.5. Internationalisation . . . . . . . . . . . . . . . . . . 8 5. SDP Specification . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Protocol Version ("v=") . . . . . . . . . . . . . . . . . 11 5.2. Origin ("o=") . . . . . . . . . . . . . . . . . . . . . . 12 5.3. Session Name ("s=") . . . . . . . . . . . . . . . . . . . 13 5.4. Session Information ("i=") . . . . . . . . . . . . . . . 13 5.5. URI ("u=") . . . . . . . . . . . . . . . . . . . . . . . 14 5.6. Email Address and Phone Number ("e=" and "p=") . . . . . 14 5.7. Connection Data ("c=") . . . . . . . . . . . . . . . . . 15 5.8. Bandwidth ("b=") . . . . . . . . . . . . . . . . . . . . 17 5.9. Timing ("t=") . . . . . . . . . . . . . . . . . . . . . . 18 5.10. Repeat Times ("r=") . . . . . . . . . . . . . . . . . . . 19 5.11. Time Zones ("z=") . . . . . . . . . . . . . . . . . . . . 19 5.12. Encryption Keys ("k=") . . . . . . . . . . . . . . . . . 20 5.13. Attributes ("a=") . . . . . . . . . . . . . . . . . . . . 20 5.14. Media Descriptions ("m=") . . . . . . . . . . . . . . . . 21 6. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . . 24 6.1. cat (category) . . . . . . . . . . . . . . . . . . . . . 24 6.2. keywds (keywords) . . . . . . . . . . . . . . . . . . . . 25 Handley, et al. Expires April 10, 2018 [Page 2] Internet-Draft SDP October 2017 6.3. tool . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.4. ptime (packet time) . . . . . . . . . . . . . . . . . . . 26 6.5. maxptime (maximum packet time) . . . . . . . . . . . . . 26 6.6. rtpmap . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.7. Media Direction Attributes . . . . . . . . . . . . . . . 28 6.7.1. recvonly (receive-only) . . . . . . . . . . . . . . . 29 6.7.2. sendrecv (send-receive) . . . . . . . . . . . . . . . 30 6.7.3. sendonly (send-only) . . . . . . . . . . . . . . . . 30 6.7.4. inactive . . . . . . . . . . . . . . . . . . . . . . 30 6.8. orient (orientation) . . . . . . . . . . . . . . . . . . 31 6.9. type (conference type) . . . . . . . . . . . . . . . . . 31 6.10. charset (character set) . . . . . . . . . . . . . . . . . 32 6.11. sdplang (SDP language) . . . . . . . . . . . . . . . . . 33 6.12. lang (language) . . . . . . . . . . . . . . . . . . . . . 34 6.13. framerate (frame rate) . . . . . . . . . . . . . . . . . 35 6.14. quality . . . . . . . . . . . . . . . . . . . . . . . . . 36 6.15. fmtp (format parameters) . . . . . . . . . . . . . . . . 36 7. Security Considerations . . . . . . . . . . . . . . . . . . . 37 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 8.1. The "application/sdp" Media Type . . . . . . . . . . . . 39 8.2. Registration of Parameters . . . . . . . . . . . . . . . 41 8.2.1. Media Types ("media") . . . . . . . . . . . . . . . . 41 8.2.2. Transport Protocols ("proto") . . . . . . . . . . . . 41 8.2.3. Media Formats ("fmt") . . . . . . . . . . . . . . . . 42 8.2.4. Attribute Names ("att-field") . . . . . . . . . . . . 42 8.2.5. Bandwidth Specifiers ("bwtype") . . . . . . . . . . . 45 8.2.6. Network Types ("nettype") . . . . . . . . . . . . . . 45 8.2.7. Address Types ("addrtype") . . . . . . . . . . . . . 46 8.2.8. Registration Procedure . . . . . . . . . . . . . . . 46 8.3. Encryption Key Access Methods . . . . . . . . . . . . . . 47 8.4. Reorganization of the nettype Registry . . . . . . . . . 47 8.5. Reorganization of the att-field Registries . . . . . . . 47 9. SDP Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 48 10. Summary of Changes from RFC 4566 . . . . . . . . . . . . . . 53 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 54 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 54 12.1. Normative References . . . . . . . . . . . . . . . . . . 54 12.2. Informative References . . . . . . . . . . . . . . . . . 55 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 58 1. Introduction When initiating multimedia teleconferences, voice-over-IP calls, streaming video, or other sessions, there is a requirement to convey media details, transport addresses, and other session description metadata to the participants. Handley, et al. Expires April 10, 2018 [Page 3] Internet-Draft SDP October 2017 SDP provides a standard representation for such information, irrespective of how that information is transported. SDP is purely a format for session description -- it does not incorporate a transport protocol, and it is intended to use different transport protocols as appropriate, including the Session Announcement Protocol (SAP) [RFC2974], Session Initiation Protocol (SIP) [RFC3261], Real Time Streaming Protocol (RTSP) [RFC7826], electronic mail using the MIME extensions, and the Hypertext Transport Protocol (HTTP). SDP is intended to be general purpose so that it can be used in a wide range of network environments and applications. However, it is not intended to support negotiation of session content or media encodings: this is viewed as outside the scope of session description. This memo obsoletes [RFC4566]. The changes relative to [RFC4566] are limited to essential corrections, and are outlined in Section 10 of this memo. 2. Glossary of Terms The following terms are used in this document and has specific meaning within the context of this document. Session Description: A well-defined format for conveying sufficient information to discover and participate in a multimedia session. Media Description: A media description starts with an "m=" line and is terminated by either the next "m=" line or by the end of the session description. Session-level Section: This refers to the parts that are not media descriptions, whereas the session description refers to the whole body that includes the session-level section and the media description(s). The terms "multimedia conference" and "multimedia session" are used in this document as defined in [RFC7656]. The terms "session" and "multimedia session" are used interchangeably in this document. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Handley, et al. Expires April 10, 2018 [Page 4] Internet-Draft SDP October 2017 3. Examples of SDP Usage 3.1. Session Initiation The Session Initiation Protocol (SIP) [RFC3261] is an application- layer control protocol for creating, modifying, and terminating sessions such as Internet multimedia conferences, Internet telephone calls, and multimedia distribution. The SIP messages used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. These session descriptions are commonly formatted using SDP. When used with SIP, the offer/answer model [RFC3264] provides a limited framework for negotiation using SDP. 3.2. Streaming Media The Real Time Streaming Protocol (RTSP) [RFC7826], is an application- level protocol for control over the delivery of data with real-time properties. RTSP provides an extensible framework to enable controlled, on-demand delivery of real-time data, such as audio and video. An RTSP client and server negotiate an appropriate set of parameters for media delivery, partially using SDP syntax to describe those parameters. 3.3. Email and the World Wide Web Alternative means of conveying session descriptions include electronic mail and the World Wide Web (WWW). For both email and WWW distribution, the media type "application/sdp" is used. This enables the automatic launching of applications for participation in the session from the WWW client or mail reader in a standard manner. Note that announcements of multicast sessions made only via email or the WWW do not have the property that the receiver of a session announcement can necessarily receive the session because the multicast sessions may be restricted in scope, and access to the WWW server or reception of email is possible outside this scope. 3.4. Multicast Session Announcement In order to assist the advertisement of multicast multimedia conferences and other multicast sessions, and to communicate the relevant session setup information to prospective participants, a distributed session directory may be used. An instance of such a session directory periodically sends packets containing a description of the session to a well-known multicast group. These advertisements are received by other session directories such that potential remote Handley, et al. Expires April 10, 2018 [Page 5] Internet-Draft SDP October 2017 participants can use the session description to start the tools required to participate in the session. One protocol used to implement such a distributed directory is the SAP [RFC2974]. SDP provides the recommended session description format for such session announcements. 4. Requirements and Recommendations The purpose of SDP is to convey information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. SDP is primarily intended for use in an internetwork, although it is sufficiently general that it can describe multimedia conferences in other network environments. Media streams can be many-to-many. Sessions need not be continually active. Thus far, multicast-based sessions on the Internet have differed from many other forms of conferencing in that anyone receiving the traffic can join the session (unless the session traffic is encrypted). In such an environment, SDP serves two primary purposes. It is a means to communicate the existence of a session, and it is a means to convey sufficient information to enable joining and participating in the session. In a unicast environment, only the latter purpose is likely to be relevant. An SDP description includes the following: o Session name and purpose o Time(s) the session is active o The media comprising the session o Information needed to receive those media (addresses, ports, formats, etc.) As resources necessary to participate in a session may be limited, some additional information may also be desirable: o Information about the bandwidth to be used by the session o Contact information for the person responsible for the session In general, SDP must convey sufficient information to enable applications to join a session (with the possible exception of encryption keys) and to announce the resources to be used to any non- participants that may need to know. (This latter feature is Handley, et al. Expires April 10, 2018 [Page 6] Internet-Draft SDP October 2017 primarily useful when SDP is used with a multicast session announcement protocol.) 4.1. Media and Transport Information An SDP description includes the following media information: o The type of media (video, audio, etc.) o The media transport protocol (RTP/UDP/IP, H.320, etc.) o The format of the media (H.261 video, MPEG video, etc.) In addition to media format and transport protocol, SDP conveys address and port details. For an IP multicast session, these comprise: o The multicast group address for media o The transport port for media This address and port are the destination address and destination port of the multicast stream, whether being sent, received, or both. For unicast IP sessions, the following are conveyed: o The remote address for media o The remote transport port for media The semantics of this address and port depend on the media and transport protocol defined. By default, this SHOULD be the remote address and remote port to which data is sent. Some media types may redefine this behaviour, but this is NOT RECOMMENDED since it complicates implementations (including middleboxes that must parse the addresses to open Network Address Translation (NAT) or firewall pinholes). 4.2. Timing Information Sessions may be either bounded or unbounded in time. Whether or not they are bounded, they may be only active at specific times. SDP can convey: o An arbitrary list of start and stop times bounding the session o For each bound, repeat times such as "every Wednesday at 10am for one hour" Handley, et al. Expires April 10, 2018 [Page 7] Internet-Draft SDP October 2017 This timing information is globally consistent, irrespective of local time zone or daylight saving time (see Section 5.9). 4.3. Obtaining Further Information about a Session A session description could convey enough information to decide whether or not to participate in a session. SDP may include additional pointers in the form of Uniform Resource Identifiers (URIs) for more information about the session. 4.4. Categorisation When many session descriptions are being distributed by SAP, or any other advertisement mechanism, it may be desirable to filter session announcements that are of interest from those that are not. SDP supports a categorisation mechanism for sessions that is capable of being automated (the "a=cat:" attribute; see Section 6). 4.5. Internationalisation The SDP specification recommends the use of the ISO 10646 character set in the UTF-8 encoding [RFC3629] to allow many different languages to be represented. However, to assist in compact representations, SDP also allows other character sets such as ISO 8859-1 to be used when desired. Internationalisation only applies to free-text fields (session name and background information), and not to SDP as a whole. 5. SDP Specification An SDP description is denoted by the media type "application/sdp" (See Section 8). An SDP description is entirely textual. SDP field names and attribute names use only the US-ASCII subset of UTF-8, but textual fields and attribute values MAY use the full ISO 10646 character set in UTF-8 encoding, or some other character set defined by the "a=charset:" attribute. Field and attribute values that use the full UTF-8 character set are never directly compared, hence there is no requirement for UTF-8 normalisation. The textual form, as opposed to a binary encoding such as ASN.1 or XDR, was chosen to enhance portability, to enable a variety of transports to be used, and to allow flexible, text-based toolkits to be used to generate and process session descriptions. However, since SDP may be used in environments where the maximum permissible size of a session description is limited, the encoding is deliberately compact. Also, since announcements may be transported via very unreliable means or damaged by an intermediate caching server, the encoding was designed with strict order and formatting rules so that most errors would Handley, et al. Expires April 10, 2018 [Page 8] Internet-Draft SDP October 2017 result in malformed session announcements that could be detected easily and discarded. This also allows rapid discarding of encrypted session announcements for which a receiver does not have the correct key. An SDP description consists of a number of lines of text of the form: = where MUST be exactly one case-significant character and is structured text whose format depends on . In general, is either a number of fields delimited by a single space character or a free format string, and is case-significant unless a specific field defines otherwise. Whitespace separators MUST NOT be used on either side of the "=" sign, however, if the value can contain a leading whitespace as part of its syntax, i.e., that whitespace is part of the value. An SDP description consists of a session-level section followed by zero or more media descriptions. The session-level section starts with a "v=" line and continues to the first media description (or the end of the whole description, whichever comes first). Each media description starts with an "m=" line and continues to the next media description or the end of the whole session description - whichever comes first. In general, session-level values are the default for all media unless overridden by an equivalent media-level value. Some lines in each description are REQUIRED and some are OPTIONAL, but all MUST appear in exactly the order given here (the fixed order greatly enhances error detection and allows for a simple parser). OPTIONAL items are marked with a "*". Handley, et al. Expires April 10, 2018 [Page 9] Internet-Draft SDP October 2017 Session description v= (protocol version) o= (originator and session identifier) s= (session name) i=* (session information) u=* (URI of description) e=* (email address) p=* (phone number) c=* (connection information -- not required if included in all media descriptions) b=* (zero or more bandwidth information lines) One or more time descriptions ("t=" and "r=" lines; see below) z=* (time zone adjustments) k=* (encryption key) a=* (zero or more session attribute lines) Zero or more media descriptions Time description t= (time the session is active) r=* (zero or more repeat times) Media description, if present m= (media name and transport address) i=* (media title) c=* (connection information -- optional if included at session level) b=* (zero or more bandwidth information lines) k=* (encryption key) a=* (zero or more media attribute lines) The set of type letters is deliberately small and not intended to be extensible -- an SDP parser MUST completely ignore any session description that contains a type letter that it does not understand. The attribute mechanism ("a=" described below) is the primary means for extending SDP and tailoring it to particular applications or media. Some attributes (the ones listed in Section 6 of this memo) have a defined meaning, but others may be added on an application-, media-, or session-specific basis. An SDP parser MUST ignore any attribute it doesn't understand. An SDP description may contain URIs that reference external content in the "u=", "k=", and "a=" lines. These URIs may be dereferenced in some cases, making the session description non-self- contained. The connection ("c=") information in the session-level section applies to all the media of that session unless overridden by connection information in the media description. For instance, in Handley, et al. Expires April 10, 2018 [Page 10] Internet-Draft SDP October 2017 the example below, each audio media description behaves as if it were given a "c=IN IP4 233.252.0.2". An example SDP description is: v=0 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.example.com/seminars/sdp.pdf e=j.doe@example.com (Jane Doe) c=IN IP4 233.252.0.2 t=2873397496 2873404696 a=recvonly m=audio 49170 RTP/AVP 0 m=audio 49180 RTP/AVP 0 m=video 51372 RTP/AVP 99 c=IN IP4 233.252.0.1/127 a=rtpmap:99 h263-1998/90000 Text fields such as the session name and information are octet strings that may contain any octet with the exceptions of 0x00 (Nul), 0x0a (ASCII newline), and 0x0d (ASCII carriage return). The sequence CRLF (0x0d0a) is used to end a record, although parsers SHOULD be tolerant and also accept records terminated with a single newline character. If the "a=charset" attribute is not present, these octet strings MUST be interpreted as containing ISO-10646 characters in UTF-8 encoding (the presence of the "a=charset" attribute may force some fields to be interpreted differently). A session description can contain domain names in the "o=", "u=", "e=", "c=", and "a=" lines. Any domain name used in SDP MUST comply with [RFC1034], [RFC1035]. Internationalised domain names (IDNs) MUST be represented using the ASCII Compatible Encoding (ACE) form defined in [RFC5890] and MUST NOT be directly represented in UTF-8 or any other encoding (this requirement is for compatibility with [RFC2327] and other early SDP-related standards, which predate the development of internationalised domain names). 5.1. Protocol Version ("v=") v=0 The "v=" line gives the version of the Session Description Protocol. This memo defines version 0. There is no minor version number. Handley, et al. Expires April 10, 2018 [Page 11] Internet-Draft SDP October 2017 5.2. Origin ("o=") o= The "o=" line gives the originator of the session (her username and the address of the user's host) plus a session identifier and version number: is the user's login on the originating host, or it is "-" if the originating host does not support the concept of user IDs. The MUST NOT contain spaces. is a numeric string such that the tuple of , , , , and forms a globally unique identifier for the session. The method of allocation is up to the creating tool, but it has been suggested that a Network Time Protocol (NTP) format timestamp be used to ensure uniqueness [RFC5905]. is a version number for this session description. Its usage is up to the creating tool, so long as is increased when a modification is made to the session data. Again, it is RECOMMENDED that an NTP format timestamp is used. is a text string giving the type of network. Initially "IN" is defined to have the meaning "Internet", but other values MAY be registered in the future (see Section 8). is a text string giving the type of the address that follows. Initially "IP4" and "IP6" are defined, but other values MAY be registered in the future (see Section 8). is an address of the machine from which the session was created. For an address type of IP4, this is either a fully qualified domain name of the machine or the dotted-decimal representation of an IP version 4 address of the machine. For an address type of IP6, this is either a fully qualified domain name of the machine or the compressed textual representation of an IP version 6 address of the machine. For both IP4 and IP6, the fully qualified domain name is the form that SHOULD be given unless this is unavailable, in which case a globally unique address MAY be substituted. Unless an SDP extension for NAT traversal is used (e.g., ICE [RFC5245], ICE TCP [RFC6544]), a local IP address MUST NOT be used in any context where the SDP description might leave the scope in which the address is meaningful (for example, a local address MUST NOT be included in an application-level referral that might leave the scope). Handley, et al. Expires April 10, 2018 [Page 12] Internet-Draft SDP October 2017 In general, the "o=" line serves as a globally unique identifier for this version of this session description, and the sub-fields excepting the version taken together identify the session irrespective of any modifications. For privacy reasons, it is sometimes desirable to obfuscate the username and IP address of the session originator. If this is a concern, an arbitrary and private MAY be chosen to populate the "o=" line, provided that these are selected in a manner that does not affect the global uniqueness of the field. 5.3. Session Name ("s=") s= The "s=" line is the textual session name. There MUST be one and only one "s=" line per session description. The "s=" line MUST NOT be empty and SHOULD contain ISO 10646 characters (but see also the "a=charset" attribute). If a session has no meaningful name, the value "s= " SHOULD be used (i.e., a single space as the session name). 5.4. Session Information ("i=") i= The "i=" line provides textual information about the session. There MUST be at most one session-level "i=" line per session description, and at most one "i=" line per media description/definition. Unless a media level "i="" line is used, the session-level "i="" line applies to that media description. If the "a=charset" attribute is present, it specifies the character set used in the "i=" line. If the "a=charset" attribute is not present, the "i=" line MUST contain ISO 10646 characters in UTF-8 encoding. A single "i=" line can be used for each media definition. In media definitions, "i=" lines are primarily intended for labelling media streams. As such, they are most likely to be useful when a single session has more than one distinct media stream of the same media type. An example would be two different whiteboards, one for slides and one for feedback and questions. The "i=" line is intended to provide a free-form human-readable description of the session or the purpose of a media stream. It is not suitable for parsing by automata. Handley, et al. Expires April 10, 2018 [Page 13] Internet-Draft SDP October 2017 5.5. URI ("u=") u= A URI is a Uniform Resource Identifier as used by WWW clients [RFC3986]. The URI should be a pointer to additional information about the session. This line is OPTIONAL. No more than one URI line is allowed per session description. 5.6. Email Address and Phone Number ("e=" and "p=") e= p= The "e=" and "p=" lines specify contact information for the person responsible for the session. This is not necessarily the same person that created the session description. Inclusion of an email address or phone number is OPTIONAL. If an email address or phone number is present, it MUST be specified before the first media field. More than one email or phone line can be given for a session description. Phone numbers SHOULD be given in the form of an international public telecommunication number (see ITU-T Recommendation E.164) preceded by a "+". Spaces and hyphens may be used to split up a phone field to aid readability if desired. For example: p=+1 617 555-6011 Both email addresses and phone numbers can have an OPTIONAL free text string associated with them, normally giving the name of the person who may be contacted. This MUST be enclosed in parentheses if it is present. For example: e=j.doe@example.com (Jane Doe) The alternative [RFC5322] name quoting convention is also allowed for both email addresses and phone numbers. For example: e=Jane Doe The free text string SHOULD be in the ISO-10646 character set with UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if the appropriate session-level "a=charset" attribute is set. Handley, et al. Expires April 10, 2018 [Page 14] Internet-Draft SDP October 2017 5.7. Connection Data ("c=") c= The "c=" line contains connection data. A session description MUST contain either at least one "c=" line in each media description or a single "c=" line at the session level. It MAY contain a single session-level "c=" line and additional "c=" line(s) per media description, in which case the per-media values override the session-level settings for the respective media. The first sub-field ("") is the network type, which is a text string giving the type of network. Initially, "IN" is defined to have the meaning "Internet", but other values MAY be registered in the future (see Section 8). The second sub-field ("") is the address type. This allows SDP to be used for sessions that are not IP based. This memo only defines IP4 and IP6, but other values MAY be registered in the future (see Section 8). The third sub-field ("") is the connection address. OPTIONAL sub-fields MAY be added after the connection address depending on the value of the field. When the is IP4 and IP6, the connection address is defined as follows: o If the session is multicast, the connection address will be an IP multicast group address. If the session is not multicast, then the connection address contains the unicast IP address of the expected data source or data relay or data sink as determined by additional attribute fields. It is not expected that unicast addresses will be given in a session description that is communicated by a multicast announcement, though this is not prohibited. o Sessions using an IP4 multicast connection address MUST also have a time to live (TTL) value present in addition to the multicast address. The TTL and the address together define the scope with which multicast packets sent in this session will be sent. TTL values MUST be in the range 0-255. Although the TTL MUST be specified, its use to scope multicast traffic is deprecated; applications SHOULD use an administratively scoped address instead. Handley, et al. Expires April 10, 2018 [Page 15] Internet-Draft SDP October 2017 The TTL for the session is appended to the address using a slash as a separator. An example is: c=IN IP4 233.252.0.1/127 IP6 multicast does not use TTL scoping, and hence the TTL value MUST NOT be present for IP6 multicast. It is expected that IP6 scoped addresses will be used to limit the scope of multimedia conferences. Hierarchical or layered encoding schemes are data streams where the encoding from a single media source is split into a number of layers. The receiver can choose the desired quality (and hence bandwidth) by only subscribing to a subset of these layers. Such layered encodings are normally transmitted in multiple multicast groups to allow multicast pruning. This technique keeps unwanted traffic from sites only requiring certain levels of the hierarchy. For applications requiring multiple multicast groups, we allow the following notation to be used for the connection address: [/]/ If the number of addresses is not given, it is assumed to be one. Multicast addresses so assigned are contiguously allocated above the base address, so that, for example: c=IN IP4 233.252.0.1/127/3 would state that addresses 233.252.0.1, 233.252.0.2, and 233.252.0.3 are to be used at a TTL of 127. This is semantically identical to including multiple "c=" lines in a media description: c=IN IP4 233.252.0.1/127 c=IN IP4 233.252.0.2/127 c=IN IP4 233.252.0.3/127 Similarly, an IP6 example would be: c=IN IP6 FF15::101/3 which is semantically equivalent to: c=IN IP6 FF15::101 c=IN IP6 FF15::102 c=IN IP6 FF15::103 (remembering that the TTL field is not present in IP6 multicast). Handley, et al. Expires April 10, 2018 [Page 16] Internet-Draft SDP October 2017 Multiple addresses or "c=" lines MAY be specified on a per-media basis only if they provide multicast addresses for different layers in a hierarchical or layered encoding scheme. They MUST NOT be specified for a session-level "c=" line. The slash notation for multiple addresses described above MUST NOT be used for IP unicast addresses. 5.8. Bandwidth ("b=") b=: This OPTIONAL line denotes the proposed bandwidth to be used by the session or media. The is an alphanumeric modifier giving the meaning of the figure. Two values are defined in this specification, but other values MAY be registered in the future (see Section 8 and [RFC3556], [RFC3890]): CT If the bandwidth of a session is different from the bandwidth implicit from the scope, a "b=CT:..." line SHOULD be supplied for the session giving the proposed upper limit to the bandwidth used (the "conference total" bandwidth). Similarly, if the bandwidth of bundled media streams in an m line is different from the implicit value from the scope, a "b=CT:..." line SHOULD be supplied in the media level. The primary purpose of this is to give an approximate idea as to whether two or more sessions (or bundled media streams) can coexist simultaneously. Note that CT gives a total bandwidth figure for all the media at all endpoints. AS The bandwidth is interpreted to be application specific (it will be the application's concept of maximum bandwidth). Normally, this will coincide with what is set on the application's "maximum bandwidth" control if applicable. For RTP-based applications, AS gives the RTP "session bandwidth" as defined in Section 6.2 of [RFC3550]. Note that AS gives a bandwidth figure for a single media at a single endpoint, although there may be many endpoints sending simultaneously. A prefix "X-" is defined for names. This is intended for experimental purposes only. For example: b=X-YZ:128 Use of the "X-" prefix is NOT RECOMMENDED: instead new modifiers SHOULD be registered with IANA in the standard namespace. SDP parsers MUST ignore bandwidth fields with unknown modifiers. Modifiers MUST be alphanumeric and, although no length limit is given, it is recommended that they be short. Handley, et al. Expires April 10, 2018 [Page 17] Internet-Draft SDP October 2017 The is interpreted as kilobits per second by default. The definition of a new modifier MAY specify that the bandwidth is to be interpreted in some alternative unit (the "CT" and "AS" modifiers defined in this memo use the default units). 5.9. Timing ("t=") t= The "t=" lines specify the start and stop times for a session. Multiple "t=" lines MAY be used if a session is active at multiple irregularly spaced times; each additional "t=" line specifies an additional period of time for which the session will be active. If the session is active at regular times, an "r=" line (see below) should be used in addition to, and following, a "t=" line -- in which case the "t=" line specifies the start and stop times of the repeat sequence. The first and second sub-fields give the start and stop times, respectively, for the session. These values are the decimal representation of Network Time Protocol (NTP) time values in seconds since 1900 [RFC5905]. To convert these values to UNIX time, subtract decimal 2208988800. NTP timestamps are elsewhere represented by 64-bit values, which wrap sometime in the year 2036. Since SDP uses an arbitrary length decimal representation, this should not cause an issue (SDP timestamps MUST continue counting seconds since 1900, NTP will use the value modulo the 64-bit limit). If the is set to zero, then the session is not bounded, though it will not become active until after the . If the is also zero, the session is regarded as permanent. User interfaces SHOULD strongly discourage the creation of unbounded and permanent sessions as they give no information about when the session is actually going to terminate, and so make scheduling difficult. The general assumption may be made, when displaying unbounded sessions that have not timed out to the user, that an unbounded session will only be active until half an hour from the current time or the session start time, whichever is the later. If behaviour other than this is required, an end-time SHOULD be given and modified as appropriate when new information becomes available about when the session should really end. Handley, et al. Expires April 10, 2018 [Page 18] Internet-Draft SDP October 2017 Permanent sessions may be shown to the user as never being active unless there are associated repeat times that state precisely when the session will be active. 5.10. Repeat Times ("r=") r= "r=" line specifies repeat times for a session. For example, if a session is active at 10am on Monday and 11am on Tuesday for one hour each week for three months, then the in the corresponding "t=" line would be the NTP representation of 10am on the first Monday, the would be 1 week, the would be 1 hour, and the offsets would be zero and 25 hours. The corresponding "t=" line stop time would be the NTP representation of the end of the last session three months later. By default, all fields are in seconds, so the "r=" and "t=" lines might be the following: t=3034423619 3042462419 r=604800 3600 0 90000 To make the description more compact, times may also be given in units of days, hours, or minutes. The syntax for these is a number immediately followed by a single case-sensitive character. Fractional units are not allowed -- a smaller unit should be used instead. The following unit specification characters are allowed: d - days (86400 seconds) h - hours (3600 seconds) m - minutes (60 seconds) s - seconds (allowed for completeness) Thus, the above session announcement could also have been written: r=7d 1h 0 25h Monthly and yearly repeats cannot be directly specified with a single SDP repeat time; instead, separate "t=" lines should be used to explicitly list the session times. 5.11. Time Zones ("z=") z= .... To schedule a repeated session that spans a change from daylight saving time to standard time or vice versa, it is necessary to specify offsets from the base time. This is required because Handley, et al. Expires April 10, 2018 [Page 19] Internet-Draft SDP October 2017 different time zones change time at different times of day, different countries change to or from daylight saving time on different dates, and some countries do not have daylight saving time at all. Thus, in order to schedule a session that is at the same time winter and summer, it must be possible to specify unambiguously by whose time zone a session is scheduled. To simplify this task for receivers, we allow the sender to specify the NTP time that a time zone adjustment happens and the offset from the time when the session was first scheduled. The "z=" line allows the sender to specify a list of these adjustment times and offsets from the base time. An example might be the following: z=2882844526 -1h 2898848070 0 This specifies that at time 2882844526, the time base by which the session's repeat times are calculated is shifted back by 1 hour, and that at time 2898848070, the session's original time base is restored. Adjustments are always relative to the specified start time -- they are not cumulative. Adjustments apply to all "t=" and "r=" lines in a session description. If a session is likely to last several years, it is expected that the session description will be modified periodically rather than transmit several years' worth of adjustments in one session description. 5.12. Encryption Keys ("k=") k= k=: The "k=" line is obsolete and MUST NOT be used. It is included in this document for legacy reasons. One MUST NOT include a "k=" line in an SDP, and MUST discard it if it is received in an SDP. 5.13. Attributes ("a=") a= a=: Attributes are the primary means for extending SDP. Attributes may be defined to be used as "session-level" attributes, "media-level" attributes, or both. A media description may have any number of attributes ("a=" lines) that are media specific. These are referred to as "media-level" Handley, et al. Expires April 10, 2018 [Page 20] Internet-Draft SDP October 2017 attributes and add information about the media stream. Attribute lines can also be added before the first media field; these "session- level" attributes convey additional information that applies to the session as a whole rather than to individual media. Attribute lines may be of two forms: o A property attribute is simply of the form "a=". These are binary attributes, and the presence of the attribute conveys that the attribute is a property of the session. An example might be "a=recvonly". o A value attribute is of the form "a=:". For example, a whiteboard could have the value attribute "a=orient:landscape" Attribute interpretation depends on the media tool being invoked. Thus receivers of session descriptions should be configurable in their interpretation of session descriptions in general and of attributes in particular. Attribute names MUST use the US-ASCII subset of ISO-10646/UTF-8. Attribute values are octet strings, and MAY use any octet value except 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values are to be interpreted as in ISO-10646 character set with UTF-8 encoding. Unlike other text fields, attribute values are NOT normally affected by the "charset" attribute as this would make comparisons against known values problematic. However, when an attribute is defined, it can be defined to be charset dependent, in which case its value should be interpreted in the session charset rather than in ISO-10646. Attributes MUST be registered with IANA (see Section 8). If an attribute is received that is not understood, it MUST be ignored by the receiver. 5.14. Media Descriptions ("m=") m= ... A session description may contain a number of media descriptions. Each media description starts with an "m=" line and is terminated by either the next "m=" line or by the end of the session description. A media field has several sub-fields: is the media type. This document defines the values "audio", "video", "text", "application", and "message". This list Handley, et al. Expires April 10, 2018 [Page 21] Internet-Draft SDP October 2017 is extended and may be further extended by other memos registering media types in the future (see Section 8). For example, [RFC6466] defined the "image" media type. is the transport port to which the media stream is sent. The meaning of the transport port depends on the network being used as specified in the relevant "c=" line, and on the transport protocol defined in the sub-field of the media field. Other ports used by the media application (such as the RTP Control Protocol (RTCP) port [RFC3550]) MAY be derived algorithmically from the base media port or MAY be specified in a separate attribute (for example, "a=rtcp:" as defined in [RFC3605]). If non-contiguous ports are used or if they don't follow the parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:" attribute MUST be used. Applications that are requested to send media to a that is odd and where the "a=rtcp:" is present MUST NOT subtract 1 from the RTP port: that is, they MUST send the RTP to the port indicated in and send the RTCP to the port indicated in the "a=rtcp" attribute. For applications where hierarchically encoded streams are being sent to a unicast address, it may be necessary to specify multiple transport ports. This is done using a similar notation to that used for IP multicast addresses in the "c=" line: m= / ... In such a case, the ports used depend on the transport protocol. For RTP, the default is that only the even-numbered ports are used for data with the corresponding one-higher odd ports used for the RTCP belonging to the RTP session, and the denoting the number of RTP sessions. For example: m=video 49170/2 RTP/AVP 31 would specify that ports 49170 and 49171 form one RTP/RTCP pair and 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the transport protocol and 31 is the format (see below). If non- contiguous ports are required, they must be signalled using a separate attribute (for example, "a=rtcp:" as defined in [RFC3605]). If multiple addresses are specified in the "c=" line and multiple ports are specified in the "m=" line, a one-to-one mapping from port to the corresponding address is implied. For example: Handley, et al. Expires April 10, 2018 [Page 22] Internet-Draft SDP October 2017 c=IN IP4 233.252.0.1/127/2 m=video 49170/2 RTP/AVP 31 would imply that address 233.252.0.1 is used with ports 49170 and 49171, and address 233.252.0.2 is used with ports 49172 and 49173. The semantics of multiple "m=" lines using the same transport address are undefined. This implies that, unlike limited past practice, there is no implicit grouping defined by such means and an explicit grouping framework (for example, [RFC5888]) should instead be used to express the intended semantics. is the transport protocol. The meaning of the transport protocol is dependent on the address type field in the relevant "c=" line. Thus a "c=" field of IP4 indicates that the transport protocol runs over IP4. The following transport protocols are defined, but may be extended through registration of new protocols with IANA (see Section 8): * udp: denotes an unspecified protocol running over UDP. * RTP/AVP: denotes RTP [RFC3550] used under the RTP Profile for Audio and Video Conferences with Minimal Control [RFC3551] running over UDP. * RTP/SAVP: denotes the Secure Real-time Transport Protocol [RFC3711] running over UDP. The main reason to specify the transport protocol in addition to the media format is that the same standard media formats may be carried over different transport protocols even when the network protocol is the same -- a historical example is vat Pulse Code Modulation (PCM) audio and RTP PCM audio; another might be TCP/RTP PCM audio. In addition, relays and monitoring tools that are transport-protocol-specific but format-independent are possible. is a media format description. The fourth and any subsequent sub-fields describe the format of the media. The interpretation of the media format depends on the value of the sub-field. If the sub-field is "RTP/AVP" or "RTP/SAVP" the sub- fields contain RTP payload type numbers. When a list of payload type numbers is given, this implies that all of these payload formats MAY be used in the session, but the first of these formats SHOULD be used as the default format for the session. For dynamic payload type assignments the "a=rtpmap:" attribute (see Section 6) SHOULD be used to map from an RTP payload type number to a media encoding name that identifies the payload format. The "a=fmtp:" Handley, et al. Expires April 10, 2018 [Page 23] Internet-Draft SDP October 2017 attribute MAY be used to specify format parameters (see Section 6). If the sub-field is "udp" the sub-fields MUST reference a media type describing the format under the "audio", "video", "text", "application", or "message" top-level media types. The media type registration SHOULD define the packet format for use with UDP transport. For media using other transport protocols, the field is protocol specific. Rules for interpretation of the sub- field MUST be defined when registering new protocols (see Section 8.2.2). Section 3 of [RFC4855] states that the payload format (encoding) names defined in the RTP Profile are commonly shown in upper case, while media subtype names are commonly shown in lower case. It also states that both of these names are case-insensitive in both places, similar to parameter names which are case-insensitive both in media type strings and in the default mapping to the SDP a=fmtp attribute. 6. SDP Attributes The following attributes are defined. Since application writers may add new attributes as they are required, this list is not exhaustive. Registration procedures for new attributes are defined in Section 8.2.4. 6.1. cat (category) Name: cat Value: cat-value Usage Level: session Charset Dependent: no Syntax: cat-value = category category = non-ws-string Example: a=cat:foo.bar Handley, et al. Expires April 10, 2018 [Page 24] Internet-Draft SDP October 2017 This attribute gives the dot-separated hierarchical category of the session. This is to enable a receiver to filter unwanted sessions by category. There is no central registry of categories. This attribute is obsoleted. 6.2. keywds (keywords) Name: keywds Value: keywds-value Usage Level: session Charset Dependent: yes Syntax: keywds-value = keywords keywords = text Example: a=keywds:SDP session description protocol Like the cat attribute, this is to assist identifying wanted sessions at the receiver. This allows a receiver to select interesting session based on keywords describing the purpose of the session; there is no central registry of keywords. Its value should be interpreted in the charset specified for the session description if one is specified, or by default in ISO 10646/UTF-8. This attribute is obsoleted. 6.3. tool Name: tool Value: tool-value Usage Level: session Charset Dependent: no Syntax: tool-value = tool-name-and-version tool-name-and-version = text Handley, et al. Expires April 10, 2018 [Page 25] Internet-Draft SDP October 2017 Example: a=tool:foobar V3.2 This gives the name and version number of the tool used to create the session description. 6.4. ptime (packet time) Name: ptime Value: ptime-value Usage Level: media Charset Dependent: no Syntax: ptime-value = non-zero-int-or-real Example: a=ptime:20 This gives the length of time in milliseconds represented by the media in a packet. This is probably only meaningful for audio data, but may be used with other media types if it makes sense. It should not be necessary to know ptime to decode RTP or vat audio, and it is intended as a recommendation for the encoding/packetisation of audio. 6.5. maxptime (maximum packet time) Name: maxptime Value: maxptime-value Usage Level: media Charset Dependent: no Syntax: maxptime-value = non-zero-int-or-real Example: a=maxptime:20 Handley, et al. Expires April 10, 2018 [Page 26] Internet-Draft SDP October 2017 This gives the maximum amount of media that can be encapsulated in each packet, expressed as time in milliseconds. The time SHALL be calculated as the sum of the time the media present in the packet represents. For frame-based codecs, the time SHOULD be an integer multiple of the frame size. This attribute is probably only meaningful for audio data, but may be used with other media types if it makes sense. Note that this attribute was introduced after [RFC2327], and non-updated implementations will ignore this attribute. 6.6. rtpmap Name: rtpmap Value: rtpmap-value Usage Level: media Charset Dependent: no Syntax: rtpmap-value = payload-type SP encoding-name "/" clock-rate [ "/" encoding-params ] payload-type = zero-based-integer encoding-name = token clock-rate = integer encoding-params = channels channels = integer This attribute maps from an RTP payload type number (as used in an "m=" line) to an encoding name denoting the payload format to be used. It also provides information on the clock rate and encoding parameters. Note that the payload type number is indicated in a 7-bit field, limiting the values to incusively between 0 and 127. Although an RTP profile can make static assignments of payload type numbers to payload formats, it is more common for that assignment to be done dynamically using "a=rtpmap:" attributes. As an example of a static payload type, consider u-law PCM coded single-channel audio sampled at 8 kHz. This is completely defined in the RTP Audio/Video profile as payload type 0, so there is no need for an "a=rtpmap:" attribute, and the media for such a stream sent to UDP port 49232 can be specified as: m=audio 49232 RTP/AVP 0 Handley, et al. Expires April 10, 2018 [Page 27] Internet-Draft SDP October 2017 An example of a dynamic payload type is 16-bit linear encoded stereo audio sampled at 16 kHz. If we wish to use the dynamic RTP/AVP payload type 98 for this stream, additional information is required to decode it: m=audio 49232 RTP/AVP 98 a=rtpmap:98 L16/16000/2 Up to one rtpmap attribute can be defined for each media format specified. Thus, we might have the following: m=audio 49230 RTP/AVP 96 97 98 a=rtpmap:96 L8/8000 a=rtpmap:97 L16/8000 a=rtpmap:98 L16/11025/2 RTP profiles that specify the use of dynamic payload types MUST define the set of valid encoding names and/or a means to register encoding names if that profile is to be used with SDP. The "RTP/AVP" and "RTP/SAVP" profiles use media subtypes for encoding names, under the top-level media type denoted in the "m=" line. In the example above, the media types are "audio/l8" and "audio/l16". For audio streams, indicates the number of audio channels. This parameter is OPTIONAL and may be omitted if the number of channels is one, provided that no additional parameters are needed. For video streams, no encoding parameters are currently specified. Additional encoding parameters MAY be defined in the future, but codec-specific parameters SHOULD NOT be added. Parameters added to an "a=rtpmap:" attribute SHOULD only be those required for a session directory to make the choice of appropriate media to participate in a session. Codec-specific parameters should be added in other attributes (for example, "a=fmtp:"). Note: RTP audio formats typically do not include information about the number of samples per packet. If a non-default (as defined in the RTP Audio/Video Profile) packetisation is required, the "ptime" attribute is used as given above. 6.7. Media Direction Attributes At most one of recvonly/sendrecv/sendonly/inactive MAY appear at session level, and at most one MAY appear in each media section. Handley, et al. Expires April 10, 2018 [Page 28] Internet-Draft SDP October 2017 If any one of these appears in a media section then it applies for that media section. If none appear in a media section then the one from session level, if any, applies to that media section. If none of the media direction attributes is present at either session level or media level, "sendrecv" SHOULD be assumed as the default for sessions that are not of the multimedia conference type "broadcast" or "H332" (see below). Within the following SDP example, the "inactive" attribute applies to audio media and the "recvonly" attribute applies to video media. v=0 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.example.com/seminars/sdp.pdf e=j.doe@example.com (Jane Doe) c=IN IP4 233.252.0.1/127 t=2873397496 2873404696 a=inactive m=audio 49170 RTP/AVP 0 m=video 51372 RTP/AVP 99 a=rtpmap:99 h263-1998/90000 a=recvonly 6.7.1. recvonly (receive-only) Name: recvonly Value: Usage Level: session, media Charset Dependent: no Example: a=recvonly This specifies that the tools should be started in receive-only mode where applicable. Note that recvonly applies to the media only, not to any associated control protocol (e.g., an RTP-based system in recvonly mode SHOULD still send RTCP packets). Handley, et al. Expires April 10, 2018 [Page 29] Internet-Draft SDP October 2017 6.7.2. sendrecv (send-receive) Name: sendrecv Value: Usage Level: session, media Charset Dependent: no Example: a=sendrecv This specifies that the tools should be started in send and receive mode. This is necessary for interactive multimedia conferences with tools that default to receive-only mode. 6.7.3. sendonly (send-only) Name: sendonly Value: Usage Level: session, media Charset Dependent: no Example: a=sendonly This specifies that the tools should be started in send-only mode. An example may be where a different unicast address is to be used for a traffic destination than for a traffic source. In such a case, two media descriptions may be used, one sendonly and one recvonly. Note that sendonly applies only to the media, and any associated control protocol (e.g., RTCP) SHOULD still be received and processed as normal. 6.7.4. inactive Name: inactive Value: Usage Level: session, media Handley, et al. Expires April 10, 2018 [Page 30] Internet-Draft SDP October 2017 Charset Dependent: no Example: a=inactive This specifies that the tools should be started in inactive mode. This is necessary for interactive multimedia conferences where users can put other users on hold. No media is sent over an inactive media stream. Note that an RTP-based system MUST still send RTCP (if RTCP is used), even if started inactive. 6.8. orient (orientation) Name: orient Value: orient-value Usage Level: media Charset Dependent: no Syntax: orient-value = portrait / landscape / seascape portrait = %s"portrait" landscape = %s"landscape" seascape = %s"seascape" ; NOTE: These names are case-sensitive. Example: a=orient:portrait Normally this is only used for a whiteboard or presentation tool. It specifies the orientation of the workspace on the screen. Permitted values are "portrait", "landscape", and "seascape" (upside-down landscape). 6.9. type (conference type) Name: type Value: type-value Usage Level: session Charset Dependent: no Handley, et al. Expires April 10, 2018 [Page 31] Internet-Draft SDP October 2017 Syntax: type-value = conference-type conference-type = broadcast / meeting / moderated / test / H332 broadcast = %s"broadcast" meeting = %s"meeting" moderated = %s"moderated" test = %s"test" H332 = %s"H332" ; NOTE: These names are case-sensitive. Example: a=type:moderated This specifies the type of the multimedia conference. Suggested values are "broadcast", "meeting", "moderated", "test", and "H332". "recvonly" should be the default for "type:broadcast" sessions, "type:meeting" should imply "sendrecv", and "type:moderated" should indicate the use of a floor control tool and that the media tools are started so as to mute new sites joining the multimedia conference. Specifying the attribute "type:H332" indicates that this loosely coupled session is part of an H.332 session as defined in the ITU H.332 specification [ITU.H332.1998]. Media tools should be started "recvonly". Specifying the attribute "type:test" is suggested as a hint that, unless explicitly requested otherwise, receivers can safely avoid displaying this session description to users. 6.10. charset (character set) Name: charset Value: charset-value Usage Level: session Charset Dependent: no Syntax: charset-value = mime-charset (as defined in [RFC 2978]) Handley, et al. Expires April 10, 2018 [Page 32] Internet-Draft SDP October 2017 This specifies the character set to be used to display the session name and information data. By default, the ISO-10646 character set in UTF-8 encoding is used. If a more compact representation is required, other character sets may be used. For example, the ISO 8859-1 is specified with the following SDP attribute: a=charset:ISO-8859-1 The charset specified MUST be one of those registered in the IANA Character Sets registry (http://www.iana.org/assignments/character- sets), such as ISO-8859-1. The character set identifier is a US- ASCII string and MUST be compared against identifiers from the "Name" or "Preferred MIME Name" field of the registry using a case- insensitive comparison. If the identifier is not recognised or not supported, all strings that are affected by it SHOULD be regarded as octet strings. Note that a character set specified MUST still prohibit the use of bytes 0x00 (Nul), 0x0A (LF), and 0x0d (CR). Character sets requiring the use of these characters MUST define a quoting mechanism that prevents these bytes from appearing within text fields. 6.11. sdplang (SDP language) Name: sdplang Value: sdplang-value Usage Level: session, media Charset Dependent: no Syntax: sdplang-value = Language-Tag ; Language-Tag defined in RFC5646 Example: a=sdplang:fr Multiple sdplang attributes can be provided either at session or media level if the session description or media use multiple languages. As a session-level attribute, it specifies the language for the session description (not the language of the media). As a media- level attribute, it specifies the language for any media-level SDP Handley, et al. Expires April 10, 2018 [Page 33] Internet-Draft SDP October 2017 information field associated with that media (again not the language of the media), overriding any sdplang attributes specified at session level. In general, sending session descriptions consisting of multiple languages is discouraged. Instead, multiple descriptions SHOULD be sent describing the session, one in each language. However, this is not possible with all transport mechanisms, and so multiple sdplang attributes are allowed although NOT RECOMMENDED. The "sdplang" attribute value must be a single [RFC5646] language tag in US-ASCII. An "sdplang" attribute SHOULD be specified when a session is distributed with sufficient scope to cross geographic boundaries, where the language of recipients cannot be assumed, or where the session is in a different language from the locally assumed norm. 6.12. lang (language) Name: lang Value: lang-value Usage Level: session, media Charset Dependent: no Syntax: lang-value = Language-Tag ; Language-Tag defined in RFC5646 Example: a=lang:de Multiple lang attributes can be provided either at session or media level if the session or media has capabilities in more than one language, in which case the order of the attributes indicates the order of preference of the various languages in the session or media, from most preferred to least preferred. As a session-level attribute, lang specifies a language capability for the session being described. As a media-level attribute, it specifies a language capability for that media, overriding any session-level language(s) specified. Handley, et al. Expires April 10, 2018 [Page 34] Internet-Draft SDP October 2017 The "lang" attribute value must be a single [RFC5646] language tag in US-ASCII. A "lang" attribute SHOULD be specified when a session is of sufficient scope to cross geographic boundaries where the language of participants cannot be assumed, or where the session has capabilities in languages different from the locally assumed norm. The "lang" attribute is supposed to be used for settling the initial language(s) used in the session. Events during the session may influence which language(s) are used, and the participants are not strictly bound to only use the declared languages. Most real-time use cases start with just one language used, while other cases involve a range of languages, e.g. an interpreted or subtitled session. When more than one 'lang' attribute is specified, the "lang" attribute itself does not provide any information about if multiple languages are intended to be used during the session, or if the intention is to only select one language. Other attributes or the semantics in which the "lang" attributes are used might indicate such conditions. Without such indications of usage intent, it is assumed that for a negotiated session one of the declared languages will be selected and used. 6.13. framerate (frame rate) Name: framerate Value: framerate-value Usage Level: media Charset Dependent: no Syntax: framerate-value = non-zero-int-or-real Example: a=framerate:60 This gives the maximum video frame rate in frames/sec. It is intended as a recommendation for the encoding of video data. Decimal representations of fractional values are allowed. It is defined only for video media. Handley, et al. Expires April 10, 2018 [Page 35] Internet-Draft SDP October 2017 6.14. quality Name: quality Value: quality-value Usage Level: media Charset Dependent: no Syntax: quality-value = zero-based-integer Example: a=quality:10 This gives a suggestion for the quality of the encoding as an integer value. The intention of the quality attribute for video is to specify a non-default trade-off between frame-rate and still-image quality. For video, the value is in the range 0 to 10, with the following suggested meaning: 10 - the best still-image quality the compression scheme can give. 5 - the default behaviour given no quality suggestion. 0 - the worst still-image quality the codec designer thinks is still usable. Editor's note: The usage should be checked with the SIP Forum to see whether anybody is using this. Otherwise, the recommendation will be not to use this attribute and the receiver ignores it upon reception. 6.15. fmtp (format parameters) Name: fmtp Value: fmtp-value Usage Level: media Charset Dependent: no Handley, et al. Expires April 10, 2018 [Page 36] Internet-Draft SDP October 2017 Syntax: fmtp-value = fmt SP format-specific-params format-specific-params = byte-string ; Notes: ; - The format parameters are media type parameters and need to reflect their syntax. Example: a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600 This attribute allows parameters that are specific to a particular format to be conveyed in a way that SDP does not have to understand them. The format must be one of the formats specified for the media. Format-specific parameters may be any set of parameters required to be conveyed by SDP and given unchanged to the media tool that will use this format. At most one instance of this attribute is allowed for each format. 7. Security Considerations SDP is frequently used with the Session Initiation Protocol [RFC3261] using the offer/answer model [RFC3264] to agree on parameters for unicast sessions. When used in this manner, the security considerations of those protocols apply. SDP is a session description format that describes multimedia sessions. Entities receiving and acting upon an SDP message SHOULD be aware that a session description cannot be trusted unless it has been obtained by an authenticated transport protocol from a known and trusted source. Many different transport protocols may be used to distribute session descriptions, and the nature of the authentication will differ from transport to transport. For some transports, security features are often not deployed. In case a session description has not been obtained in a trusted manner, the endpoint SHOULD exercise care because, among other attacks, the media sessions received may not be the intended ones, the destination where media is sent to may not be the expected one, any of the parameters of the session may be incorrect, or the media security may be compromised. It is up to the endpoint to make a sensible decision taking into account the security risks of the application and the user preferences and may decide to ask the user whether or not to accept the session. One transport that can be used to distribute session descriptions is the SAP. SAP provides both encryption and authentication mechanisms, but due to the nature of session announcements it is likely that Handley, et al. Expires April 10, 2018 [Page 37] Internet-Draft SDP October 2017 there are many occasions where the originator of a session announcement cannot be authenticated because the originator is previously unknown to the receiver of the announcement and because no common public key infrastructure is available. On receiving a session description over an unauthenticated transport mechanism or from an untrusted party, software parsing the session should take a few precautions. Session descriptions contain information required to start software on the receiver's system. Software that parses a session description MUST NOT be able to start other software except that which is specifically configured as appropriate software to participate in multimedia sessions. It is normally considered inappropriate for software parsing a session description to start, on a user's system, software that is appropriate to participate in multimedia sessions, without the user first being informed that such software will be started and giving the user's consent. Thus, a session description arriving by session announcement, email, session invitation, or WWW page MUST NOT deliver the user into an interactive multimedia session unless the user has explicitly pre-authorised such action. As it is not always simple to tell whether or not a session is interactive, applications that are unsure should assume sessions are interactive. In this specification, there are no attributes that would allow the recipient of a session description to be informed to start multimedia tools in a mode where they default to transmitting. Under some circumstances it might be appropriate to define such attributes. If this is done, an application parsing a session description containing such attributes SHOULD either ignore them or inform the user that joining this session will result in the automatic transmission of multimedia data. The default behaviour for an unknown attribute is to ignore it. In certain environments, it has become common for intermediary systems to intercept and analyse session descriptions contained within other signalling protocols. This is done for a range of purposes, including but not limited to opening holes in firewalls to allow media streams to pass, or to mark, prioritize, or block traffic selectively. In some cases, such intermediary systems may modify the session description, for example, to have the contents of the session description match NAT bindings dynamically created. These behaviours are NOT RECOMMENDED unless the session description is conveyed in such a manner that allows the intermediary system to conduct proper checks to establish the authenticity of the session description, and the authority of its source to establish such communication sessions. SDP by itself does not include sufficient information to enable these checks: they depend on the encapsulating protocol (e.g., SIP or RTSP). Handley, et al. Expires April 10, 2018 [Page 38] Internet-Draft SDP October 2017 Use of the "k=" line poses a significant security risk, since it conveys session encryption keys in the clear. SDP MUST NOT be used to convey key material, unless it can be guaranteed that the channel over which the SDP is delivered is both private and authenticated. Moreover, the "k=" line provides no way to indicate or negotiate cryptographic key algorithms. As it provides for only a single symmetric key, rather than separate keys for confidentiality and integrity, its utility is severely limited. The use of the "k=" line is NOT RECOMMENDED, as discussed in Section 5.12. 8. IANA Considerations 8.1. The "application/sdp" Media Type One media type registration from [RFC4566] is to be updated, as defined below. Handley, et al. Expires April 10, 2018 [Page 39] Internet-Draft SDP October 2017 To: ietf-types@iana.org Subject: Registration of media type "application/sdp" Type name: application Subtype name: sdp Required parameters: None. Optional parameters: None. Encoding considerations: SDP files are primarily UTF-8 format text. The "a=charset:" attribute may be used to signal the presence of other character sets in certain parts of an SDP file (see Section 6 of RFC XXXX). Arbitrary binary content cannot be directly represented in SDP. Security considerations: See Section 7 of RFC XXXX. Interoperability considerations: See RFC XXXX. Published specification: See RFC XXXX. Applications which use this media type: Voice over IP, video teleconferencing, streaming media, instant messaging, among others. See also Section 3 of RFC XXXX. Additional information: Magic number(s): None. File extension(s): The extension ".sdp" is commonly used. Macintosh File Type Code(s): "sdp " Person & email address to contact for further information: IETF MMUSIC working group Intended usage: COMMON Author/Change controller: Authors of RFC XXXX IETF MMUSIC working group delegated from the IESG Handley, et al. Expires April 10, 2018 [Page 40] Internet-Draft SDP October 2017 8.2. Registration of Parameters There are seven field names that are registered with IANA. Using the terminology in the SDP specification Backus-Naur Form (BNF), they are "media", "proto", "fmt", "att-field", "bwtype", "nettype", and "addrtype". The contact address for all parameters registered below is: IETF MMUSIC working group 8.2.1. Media Types ("media") The set of media types is intended to be small and SHOULD NOT be extended except under rare circumstances. The same rules should apply for media names as for top-level media types, and where possible the same name should be registered for SDP as for MIME. For media other than existing top-level media types, a Standards Track RFC MUST be produced for a new top-level media type to be registered, and the registration MUST provide good justification why no existing media name is appropriate (the "Standards Action" policy of [RFC8126]. This memo registers the media types "audio", "video", "text", "application", and "message". Note: The media types "control" and "data" were listed as valid in an early version of this specification (RFC 2327); however, their semantics were never fully specified and they are not widely used. These media types have been removed in this specification, although they still remain valid media type capabilities for a SIP user agent as defined in [RFC3840]. If these media types are considered useful in the future, a Standards Track RFC MUST be produced to document their use. Until that is done, applications SHOULD NOT use these types and SHOULD NOT declare support for them in SIP capabilities declarations (even though they exist in the registry created by [RFC3840]). Also note that [RFC6466] defined the "image" media type. 8.2.2. Transport Protocols ("proto") The "proto" field describes the transport protocol used. This SHOULD reference a standards-track protocol RFC. This memo registers three values: "RTP/AVP" is a reference to [RFC3550] used under the RTP Profile for Audio and Video Conferences with Minimal Control [RFC3551] running over UDP/IP, "RTP/SAVP" is a reference to the Secure Real-time Transport Protocol [RFC3711], and "udp" indicates an unspecified protocol over UDP. Handley, et al. Expires April 10, 2018 [Page 41] Internet-Draft SDP October 2017 If other RTP profiles are defined in the future, their "proto" name SHOULD be specified in the same manner. For example, an RTP profile whose short name is "XYZ" would be denoted by a "proto" field of "RTP/XYZ". New transport protocols SHOULD be registered with IANA. Registrations MUST reference an RFC describing the protocol. Such an RFC MAY be Experimental or Informational, although it is preferable that it be Standards Track. Registrations MUST also define the rules by which their "fmt" namespace is managed (see below). 8.2.3. Media Formats ("fmt") Each transport protocol, defined by the "proto" field, has an associated "fmt" namespace that describes the media formats that may be conveyed by that protocol. Formats cover all the possible encodings that could be transported in a multimedia session. RTP payload formats under the "RTP/AVP" and "RTP/SAVP" profiles MUST use the payload type number as their "fmt" value. If the payload type number is dynamically assigned by this session description, an additional "rtpmap" attribute MUST be included to specify the format name and parameters as defined by the media type registration for the payload format. It is RECOMMENDED that other RTP profiles that are registered (in combination with RTP) as SDP transport protocols specify the same rules for the "fmt" namespace. For the "udp" protocol, new formats SHOULD be registered. Use of an existing media subtype for the format is encouraged. If no media subtype exists, it is RECOMMENDED that a suitable one be registered through the IETF process [RFC6838] by production of, or reference to, a standards-track RFC that defines the transport protocol for the format. For other protocols, formats MAY be registered according to the rules of the associated "proto" specification. Registrations of new formats MUST specify which transport protocols they apply to. 8.2.4. Attribute Names ("att-field") 8.2.4.1. New Attributes Attribute field names ("att-field") MUST be registered with IANA and documented, because of noticeable issues due to conflicting attributes under the same name. Unknown attributes in SDP are simply Handley, et al. Expires April 10, 2018 [Page 42] Internet-Draft SDP October 2017 ignored, but conflicting ones that fragment the protocol are a serious problem. New attribute registrations are accepted according to the "Specification Required" policy of [RFC8126], provided that the specification includes the following information: o Contact Name. o Contact Email Address. o Attribute Name: The name of the attribute that will appear in SDP). This MUST conform to the definition of . o Attribute Syntax: For a value attribute (see clause 5.13), an ABNF definition of the attribute value syntax (See Section Section 9) MUST be provided. The syntax MUST follow the rule form as per Section 2.2 of [RFC5234]. This SHALL define the allowable values that the attribute might take. It MAY also define an extension method for the addition of future values. For a property attribute, the ABNF definition is omitted as the property attribute takes no values. o Attribute Semantics: For a value attribute, a semantic description of the values that the attribute might take MUST be provided. The usage of a property attribute is described under purpose below. o Attribute Value: The name of an ABNF syntax rule defining the syntax of the value. Absence of a rule name indicates that the attribute takes no values. Enclosing the rule name in "[" and "]" indicates that a value is optional. o Usage Level: Usage level(s) of the attribute. One or more of: session, media, source, dcsa, dcsa(subprotocol). For a definition of source level attributes, see [RFC5576]. For a definition of dcsa attributes see: [I-D.ietf-mmusic-data-channel-sdpneg]. o Charset Dependent: Whether the attribute value is subject to the charset attribute or not (Yes/No). o Purpose: An explanation of the purpose and usage of the attribute. o O/A Procedures: Offer/Answer procedures as explained in [RFC3264]. o Mux Category: Indication of which multiplexing "category" [I-D.ietf-mmusic-sdp-mux-attributes] an attribute is associated with. Handley, et al. Expires April 10, 2018 [Page 43] Internet-Draft SDP October 2017 o Reference: A reference to the specification defining the attribute. The above is the minimum that IANA will accept. Attributes that are expected to see widespread use and interoperability SHOULD be documented with a standards-track RFC that specifies the attribute more precisely. Submitters of registrations should ensure that the specification is in the spirit of SDP attributes, most notably that the attribute is platform independent in the sense that it makes no implicit assumptions about operating systems and does not name specific pieces of software in a manner that might inhibit interoperability. Submitters of registrations should also carefully choose the attribute usage level. They should not choose only "session" when the attribute can have different values when media is disaggregated, i.e., when each m= section has its own IP address on a different endpoint. In that case the attribute type chosen should be "session, media". The default rule is that for all new SDP attributes that can occur both in session and media level, the media level overrides the session level. When this is not the case for a new SDP attribute, it MUST be explicitly stated. IANA has registered the initial set of attribute names ("att-field" values), with definitions as in Section 6 of this memo (these definitions replace those in [RFC4566]). 8.2.4.2. Updates to Existing Attributes Updated attribute registrations are accepted according to the "Specification Required" policy of [RFC8126], provided that the specification updating the attribute (for example, by adding a new value) considers the registration information items from Section Section 8.2.4.1 according to the following bullets: o Contact Name: A name MUST be provided. o Contact Email Address: An email address MUST be provided. o Attribute Name: MUST be provided and MUST NOT be changed. Otherwise it is a new attribute. o Attribute Syntax: The existing rule syntax with the syntax extensions MUST be provided if there is a change to the syntax. A revision to an existing attribute usage MAY extend the syntax of an attribute, but MUST be backward compatible. Handley, et al. Expires April 10, 2018 [Page 44] Internet-Draft SDP October 2017 o Attribute Semantics: A semantic description of new additional attributes values or a semantic extension of existing values. Existing attribute values semantics MUST only be extended in a backward compatible manner. o Usage Level: Updates MAY only add additional levels. o Charset Dependent: MUST NOT be changed. o Purpose: MAY be extended according to the updated usage. o O/A Procedures: MAY be updated in a backward compatible manner and/or it applies to a new usage level only. o Mux Category: No change unless from TBD to another value. It MAY also change if 'media' level is being added to the definition of an attribute that previously did not include it. o Reference: A new reference MUST be provided. Items SHOULD be omitted if there is no impact to them as a result of the attribute update. 8.2.5. Bandwidth Specifiers ("bwtype") A proliferation of bandwidth specifiers is strongly discouraged. New bandwidth specifiers ("bwtype" fields) MUST be registered with IANA. The submission MUST reference a standards-track RFC specifying the semantics of the bandwidth specifier precisely, and indicating when it should be used, and why the existing registered bandwidth specifiers do not suffice. IANA has registered the bandwidth specifiers "CT" and "AS" with definitions as in Section 5.8 of this memo (these definitions update those in [RFC4566]). 8.2.6. Network Types ("nettype") New network types (the "nettype" field) MUST be registered with IANA if SDP needs to be used in the context of non-Internet environments. The registration is subject to the RFC Required - RFC publication policy of [RFC8126]. Although these are not normally the preserve of IANA, there may be circumstances when an Internet application needs to interoperate with a non-Internet application, such as when gatewaying an Internet telephone call into the Public Switched Telephone Network (PSTN). The number of network types should be small and should be rarely extended. A new network type cannot be Handley, et al. Expires April 10, 2018 [Page 45] Internet-Draft SDP October 2017 registered without registering at least one address type to be used with that network type. A new network type registration MUST reference an RFC that gives details of the network type and address type(s) and specifies how and when they would be used. IANA has registered the network type "IN" to represent the Internet, with definition as in Sections 5.2 and 5.7 of this memo (these definitions update those in [RFC4566]). 8.2.7. Address Types ("addrtype") New address types ("addrtype") MUST be registered with IANA. The registration is subject to the RFC Required - RFC publication policy of [RFC8126]. An address type is only meaningful in the context of a network type, and any registration of an address type MUST specify a registered network type or be submitted along with a network type registration. A new address type registration MUST reference an RFC giving details of the syntax of the address type. Address types are not expected to be registered frequently. IANA has registered the address types "IP4" and "IP6" with definitions as in Sections 5.2 and 5.7 of this memo (these definitions update those in [RFC4566]). 8.2.8. Registration Procedure In the RFC documentation that registers SDP "media", "proto", "fmt", "bwtype", "nettype", and "addrtype" fields, the authors MUST include the following information for IANA to place in the appropriate registry: o contact name, email address, and telephone number o name being registered (as it will appear in SDP) o long-form name in English o type of name ("media", "proto", "fmt", "bwtype", "nettype", or "addrtype") o a one-paragraph explanation of the purpose of the registered name o a reference to the specification for the registered name (this will typically be an RFC number) In the case of a new addrtype registration, the author has to check whether the new address type is usable with the existing network types. If yes, the "nettype" registry MUST be updated accordingly. Handley, et al. Expires April 10, 2018 [Page 46] Internet-Draft SDP October 2017 In the case of a new nettype registration, the author MUST specify the usable address type(s). IANA may refer any registration to the IESG for review, and may request revisions to be made before a registration will be made. 8.3. Encryption Key Access Methods The IANA previously maintained a table of SDP encryption key access method ("enckey") names. This table is obsolete, since the "k=" line is not extensible. New registrations MUST NOT be accepted. 8.4. Reorganization of the nettype Registry This document adds a new column in the "nettype" registry with the title "Usable addrtype Values" and updates the "nettype" registry as follows: -------------------------------------------------------------------- |Type | SDP Name | Usable addrtype Values | Reference | -------------------------------------------------------------------- |nettype | IN | IP4, IP6 | [RFC4566] | |nettype | TN | RFC2543 | [RFC2848] | |nettype | ATM | NSAP, GWID, E164 | [RFC3108] | |nettype | PSTN | E164 | [RFC7195] | -------------------------------------------------------------------- Note that both [RFC7195] and [RFC3108] registered "E164" as an address type, although [RFC7195] mentions that the "E164" address type has a different context for ATM and PSTN networks. 8.5. Reorganization of the att-field Registries This document combines all of the (currently) five "att-field" registries into one registry called "att-field" registry, and updates the columns to reflect the name, usage level(s), charset dependency and reference. As such IANA is requested to create a new combined registry using the following columns: Name | Usage Level | Dependent on Charset? | Mux Category | Reference The "Name" column reflects the attribute name (as it will appear in the SDP). The "Usage Level" column MUST indicate one or more of the following: session, media, source, dcsa and dcsa(subprotocol). The "Dependent on Charset?" column MUST indicate "Yes" or "No" depending on whether the attribute value is subject to the charset attribute. The "Mux Category" column MUST indicate one of the following categories: NORMAL, NOT RECOMMENDED, IDENTICAL, SUM, TRANSPORT, Handley, et al. Expires April 10, 2018 [Page 47] Internet-Draft SDP October 2017 INHERIT, IDENTICAL-PER-PT, SPECIAL or TBD as defined by [I-D.ietf-mmusic-sdp-mux-attributes]. Finally, the "Reference" column indicates the specification(s) where the attribute is defined. For example, the attribute "setup" which is defined for both session and media level, will be listed in the new registry as follows: Name | Usage Level | Dependent on Charset?|Mux Category| Reference | setup | session,media, | No |IDENTICAL | [RFC4145] | | dcsa,dcsa(msrp)| | | [RFC6135] | | | | | [I-D.mmusic | | | |-msrp-usage- | | | |data-channel | | | |] | 9. SDP Grammar This section provides an Augmented BNF grammar for SDP. ABNF is defined in [RFC5234] and [RFC7405]. ; SDP Syntax session-description = proto-version origin-field session-name-field [information-field] [uri-field] *email-field *phone-field [connection-field] *bandwidth-field 1*time-field [key-field] *attribute-field *media-description proto-version = %s"v" "=" 1*DIGIT CRLF ;this memo describes version 0 origin-field = %s"o" "=" username SP sess-id SP sess-version SP nettype SP addrtype SP unicast-address CRLF session-name-field = %s"s" "=" text CRLF information-field = %s"i" "=" text CRLF uri-field = %s"u" "=" uri CRLF email-field = %s"e" "=" email-address CRLF Handley, et al. Expires April 10, 2018 [Page 48] Internet-Draft SDP October 2017 phone-field = %s"p" "=" phone-number CRLF connection-field = %s"c" "=" nettype SP addrtype SP connection-address CRLF ;a connection field must be present ;in every media description or at the ;session-level bandwidth-field = %s"b" "=" bwtype ":" bandwidth CRLF time-field = %s"t" "=" start-time SP stop-time *(CRLF repeat-fields) CRLF [zone-adjustments CRLF] repeat-fields = %s"r" "=" repeat-interval SP typed-time 1*(SP typed-time) zone-adjustments = %s"z" "=" time SP ["-"] typed-time *(SP time SP ["-"] typed-time) key-field = %s"k" "=" key-type CRLF attribute-field = %s"a" "=" attribute CRLF media-description = media-field [information-field] *connection-field *bandwidth-field [key-field] *attribute-field media-field = %s"m" "=" media SP port ["/" integer] SP proto 1*(SP fmt) CRLF ; sub-rules of 'o=' username = non-ws-string ;pretty wide definition, but doesn't ;include space sess-id = 1*DIGIT ;should be unique for this username/host sess-version = 1*DIGIT nettype = token ;typically "IN" addrtype = token Handley, et al. Expires April 10, 2018 [Page 49] Internet-Draft SDP October 2017 ;typically "IP4" or "IP6" ; sub-rules of 'u=' uri = URI-reference ; see RFC 3986 ; sub-rules of 'e=', see RFC 5322 for definitions email-address = address-and-comment / dispname-and-address / addr-spec address-and-comment = addr-spec 1*SP "(" 1*email-safe ")" dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">" ; sub-rules of 'p=' phone-number = phone *SP "(" 1*email-safe ")" / 1*email-safe "<" phone ">" / phone phone = ["+"] DIGIT 1*(SP / "-" / DIGIT) ; sub-rules of 'c=' connection-address = multicast-address / unicast-address ; sub-rules of 'b=' bwtype = token bandwidth = 1*DIGIT ; sub-rules of 't=' start-time = time / "0" stop-time = time / "0" time = POS-DIGIT 9*DIGIT ; Decimal representation of NTP time in ; seconds since 1900. The representation ; of NTP time is an unbounded length field ; containing at least 10 digits. Unlike the ; 64-bit representation used elsewhere, time ; in SDP does not wrap in the year 2036. ; sub-rules of 'r=' and 'z=' repeat-interval = POS-DIGIT *DIGIT [fixed-len-time-unit] typed-time = 1*DIGIT [fixed-len-time-unit] fixed-len-time-unit = %s"d" / %s"h" / %s"m" / %s"s" ; NOTE: These units are case-sensitive. Handley, et al. Expires April 10, 2018 [Page 50] Internet-Draft SDP October 2017 ; sub-rules of 'k=' key-type = %s"prompt" / %s"clear:" text / %s"base64:" base64 / %s"uri:" uri ; NOTE: These names are case-sensitive. base64 = *base64-unit [base64-pad] base64-unit = 4base64-char base64-pad = 2base64-char "==" / 3base64-char "=" base64-char = ALPHA / DIGIT / "+" / "/" ; sub-rules of 'a=' attribute = (att-field ":" att-value) / att-field att-field = token att-value = byte-string ; sub-rules of 'm=' media = token ;typically "audio", "video", "text", "image" ;or "application" fmt = token ;typically an RTP payload type for audio ;and video media proto = token *("/" token) ;typically "RTP/AVP" or "udp" port = 1*DIGIT ; generic sub-rules: addressing unicast-address = IP4-address / IP6-address / FQDN / extn-addr multicast-address = IP4-multicast / IP6-multicast / FQDN / extn-addr IP4-multicast = m1 3( "." decimal-uchar ) "/" ttl [ "/" numaddr ] ; IP4 multicast addresses may be in the ; range 224.0.0.0 to 239.255.255.255 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / ("23" DIGIT ) IP6-multicast = IP6-address [ "/" numaddr ] Handley, et al. Expires April 10, 2018 [Page 51] Internet-Draft SDP October 2017 ; IP6 address starting with FF numaddr = integer ttl = (POS-DIGIT *2DIGIT) / "0" FQDN = 4*(alpha-numeric / "-" / ".") ; fully qualified domain name as specified ; in RFC 1035 (and updates) IP4-address = b1 3("." decimal-uchar) b1 = decimal-uchar ; less than "224" IP6-address = 6( h16 ":" ) ls32 / "::" 5( h16 ":" ) ls32 / [ h16 ] "::" 4( h16 ":" ) ls32 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 / [ *4( h16 ":" ) h16 ] "::" ls32 / [ *5( h16 ":" ) h16 ] "::" h16 / [ *6( h16 ":" ) h16 ] "::" h16 = 1*4HEXDIG ls32 = ( h16 ":" h16 ) / IP4-address ; Generic for other address families extn-addr = non-ws-string ; generic sub-rules: datatypes text = byte-string ;default is to interpret this as UTF8 text. ;ISO 8859-1 requires "a=charset:ISO-8859-1" ;session-level attribute to be used byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) ;any byte except NUL, CR, or LF non-ws-string = 1*(VCHAR/%x80-FF) ;string of visible characters token-char = ALPHA / DIGIT / "!" / "#" / "$" / "%" / "&" / "'" ; (single quote) / "*" / "+" / "-" / "." / "^" / "_" Handley, et al. Expires April 10, 2018 [Page 52] Internet-Draft SDP October 2017 / "`" ; (Grave accent) / "{" / "|" / "}" / "~" token = 1*(token-char) email-safe = %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF ;any byte except NUL, CR, LF, or the quoting ;characters ()<> integer = POS-DIGIT *DIGIT zero-based-integer = "0" / integer non-zero-int-or-real = integer / non-zero-real non-zero-real = zero-based-integer "." *DIGIT POS-DIGIT ; generic sub-rules: primitives alpha-numeric = ALPHA / DIGIT POS-DIGIT = %x31-39 ; 1 - 9 decimal-uchar = DIGIT / POS-DIGIT DIGIT / ("1" 2*(DIGIT)) / ("2" ("0"/"1"/"2"/"3"/"4") DIGIT) / ("2" "5" ("0"/"1"/"2"/"3"/"4"/"5")) ; external references: ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 5234 ; URI-reference: from RFC 3986 ; addr-spec: from RFC 5322 10. Summary of Changes from RFC 4566 The ABNF rule for IP6-address has been corrected. As a result, the ABNF rule for IP6-multicast has changed, and the (now unused) rules for hexpart, hexseq, and hex4 have been removed. IP4 unicast and multicast addresses in the example SDP descriptions have been revised per RFCs 5735 and 5771. Text in Section 5.2 has been revised to clarify the use of local addresses in case of ICE-like SDP extensions. Normative and informative references have been updated. Handley, et al. Expires April 10, 2018 [Page 53] Internet-Draft SDP October 2017 The text regarding the session vs. media-level attribute usage has been clarified. The case-insensitivity rules from RFC 4855 have been included in this document. 11. Acknowledgements Many people in the IETF Multiparty Multimedia Session Control (MMUSIC) working group have made comments and suggestions contributing to this document. 12. References 12.1. Normative References [I-D.ietf-mmusic-data-channel-sdpneg] Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R., and J. Marcon, "SDP-based Data Channel Negotiation", draft-ietf-mmusic-data-channel-sdpneg-13 (work in progress), September 2017. [I-D.ietf-mmusic-sdp-mux-attributes] Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16 (work in progress), December 2016. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, . [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978, October 2000, . [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, . Handley, et al. Expires April 10, 2018 [Page 54] Internet-Draft SDP October 2017 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, . [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, . [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009, . [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009, . [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . 12.2. Informative References [ITU.H332.1998] International Telecommunication Union, "H.323 extended for loosely coupled conferences", ITU Recommendation H.332, September 1998. [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, DOI 10.17487/RFC2327, April 1998, . [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, October 2000, . Handley, et al. Expires April 10, 2018 [Page 55] Internet-Draft SDP October 2017 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, . [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, . [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, DOI 10.17487/RFC3551, July 2003, . [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, DOI 10.17487/RFC3556, July 2003, . [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, DOI 10.17487/RFC3605, October 2003, . [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004, . [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, DOI 10.17487/RFC3840, August 2004, . [RFC3890] Westerlund, M., "A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP)", RFC 3890, DOI 10.17487/RFC3890, September 2004, . Handley, et al. Expires April 10, 2018 [Page 56] Internet-Draft SDP October 2017 [RFC4855] Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007, . [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, DOI 10.17487/RFC5245, April 2010, . [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, . [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, DOI 10.17487/RFC5888, June 2010, . [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, . [RFC6466] Salgueiro, G., "IANA Registration of the 'image' Media Type for the Session Description Protocol (SDP)", RFC 6466, DOI 10.17487/RFC6466, December 2011, . [RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach, "TCP Candidates with Interactive Connectivity Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544, March 2012, . [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, . [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, . [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources", RFC 7656, DOI 10.17487/RFC7656, November 2015, . Handley, et al. Expires April 10, 2018 [Page 57] Internet-Draft SDP October 2017 [RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., and M. Stiemerling, Ed., "Real-Time Streaming Protocol Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December 2016, . Authors' Addresses Mark Handley University College London Department of Computer Science London WC1E 6BT UK EMail: M.Handley@cs.ucl.ac.uk Colin Perkins University of Glasgow School of Computing Science University of Glasgow Glasgow G12 8QQ UK EMail: csp@csperkins.org Ali Begen Networked Media Konya Turkey EMail: ali.begen@networked.media Handley, et al. Expires April 10, 2018 [Page 58]