Network Working Group M. Handley Request for Comments: 4566 UCL Obsoletes: 2327, 3266 V. Jacobson Category: Standards Track Packet Design C. Perkins University of Glasgow July 2006 SDP: Session Description Protocol Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006).
AbstractThis 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. 1. Introduction ....................................................3 2. Glossary of Terms ...............................................3 3. Examples of SDP Usage ...........................................4 3.1. Session Initiation .........................................4 3.2. Streaming Media ............................................4 3.3. Email and the World Wide Web ...............................4 3.4. Multicast Session Announcement .............................4 4. Requirements and Recommendations ................................5 4.1. Media and Transport Information ............................6 4.2. Timing Information .........................................6 4.3. Private Sessions ...........................................7 4.4. Obtaining Further Information about a Session ..............7 4.5. Categorisation .............................................7 4.6. Internationalisation .......................................7
5. SDP Specification ...............................................7 5.1. Protocol Version ("v=") ...................................10 5.2. Origin ("o=") .............................................11 5.3. Session Name ("s=") .......................................12 5.4. Session Information ("i=") ................................12 5.5. URI ("u=") ................................................13 5.6. Email Address and Phone Number ("e=" and "p=") ............13 5.7. Connection Data ("c=") ....................................14 5.8. Bandwidth ("b=") ..........................................16 5.9. Timing ("t=") .............................................17 5.10. Repeat Times ("r=") ......................................18 5.11. Time Zones ("z=") ........................................19 5.12. Encryption Keys ("k=") ...................................19 5.13. Attributes ("a=") ........................................21 5.14. Media Descriptions ("m=") ................................22 6. SDP Attributes .................................................24 7. Security Considerations ........................................31 8. IANA Considerations ............................................33 8.1. The "application/sdp" Media Type ..........................33 8.2. Registration of Parameters ................................34 8.2.1. Media Types ("media") ..............................34 8.2.2. Transport Protocols ("proto") ......................34 8.2.3. Media Formats ("fmt") ..............................35 8.2.4. Attribute Names ("att-field") ......................36 8.2.5. Bandwidth Specifiers ("bwtype") ....................37 8.2.6. Network Types ("nettype") ..........................37 8.2.7. Address Types ("addrtype") .........................38 8.2.8. Registration Procedure .............................38 8.3. Encryption Key Access Methods .............................39 9. SDP Grammar ....................................................39 10. Summary of Changes from RFC 2327 ..............................44 11. Acknowledgements ..............................................45 12. References ....................................................45 12.1. Normative References .....................................45 12.2. Informative References ...................................46
14], Session Initiation Protocol , Real Time Streaming Protocol , electronic mail using the MIME extensions, and the Hypertext Transport Protocol. 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 RFC 2327  and RFC 3266 . Section 10 outlines the changes introduced in this memo. RFC 2119 .
15] 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  provides a limited framework for negotiation using SDP. 16], 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.
One protocol used to implement such a distributed directory is the Session Announcement Protocol (SAP) . SDP provides the recommended session description format for such session announcements.
14] and SIP , and others may be defined in the future. If a session announcement is private, it is possible to use that private announcement to convey encryption keys necessary to decode each of the media in a conference, including enough information to know which encryption scheme is used for each media. Section 6). 5] 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. Section 8). An SDP session description is entirely textual using the ISO 10646 character set in UTF-8 encoding. 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. 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 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 session description consists of a number of lines of text of the form: <type>=<value> where <type> MUST be exactly one case-significant character and <value> is structured text whose format depends on <type>. In general, <value> 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 MUST NOT be used on either side of the "=" sign. An SDP session description consists of a session-level section followed by zero or more media-level sections. The session-level part starts with a "v=" line and continues to the first media-level section. Each media-level section starts with an "m=" line and continues to the next media-level section or end of the whole session description. 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 "*".
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) 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 session 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=") and attribute ("a=") information in the session-level section applies to all the media of that session unless overridden by connection information or an attribute of the same name in the media description. For instance, in the example below, each media behaves as if it were given a "recvonly" attribute. An example SDP description is: v=0 o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.example.com/seminars/sdp.pdf firstname.lastname@example.org (Jane Doe) c=IN IP4 220.127.116.11/127 t=2873397496 2873404696 a=recvonly m=audio 49170 RTP/AVP 0 m=video 51372 RTP/AVP 99 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 , . Internationalised domain names (IDNs) MUST be represented using the ASCII Compatible Encoding (ACE) form defined in  and MUST NOT be directly represented in UTF-8 or any other encoding (this requirement is for compatibility with RFC 2327 and other SDP-related standards, which predate the development of internationalised domain names).
13]. <sess-version> is a version number for this session description. Its usage is up to the creating tool, so long as <sess-version> is increased when a modification is made to the session data. Again, it is RECOMMENDED that an NTP format timestamp is used. <nettype> 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). <addrtype> 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). <unicast-address> is the address of the machine from which the session was created. For an address type of IP4, this is either the fully qualified domain name of the machine or the dotted- decimal representation of the IP version 4 address of the machine. For an address type of IP6, this is either the fully qualified domain name of the machine or the compressed textual representation of the 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 the globally unique address MAY be substituted. 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).
In general, the "o=" field serves as a globally unique identifier for this version of this session description, and the subfields 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 <username> and private <unicast-address> MAY be chosen to populate the "o=" field, provided that these are selected in a manner that does not affect the global uniqueness of the field.
7]. The URI should be a pointer to additional information about the session. This field is OPTIONAL, but if it is present it MUST be specified before the first media field. No more than one URI field is allowed per session description. RFC 2822  name quoting convention is also allowed for both email addresses and phone numbers. For example: e=Jane Doe <email@example.com>
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. Section 8). The second sub-field ("<addrtype>") 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 ("<connection-address>") is the connection address. OPTIONAL sub-fields MAY be added after the connection address depending on the value of the <addrtype> field. When the <addrtype> 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 IPv4 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 conference 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. The TTL for the session is appended to the address using a slash as a separator. An example is: c=IN IP4 18.104.22.168/127 IPv6 multicast does not use TTL scoping, and hence the TTL value MUST NOT be present for IPv6 multicast. It is expected that IPv6 scoped addresses will be used to limit the scope of 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: <base multicast address>[/<ttl>]/<number of addresses> 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 22.214.171.124/127/3 would state that addresses 126.96.36.199, 188.8.131.52, and 184.108.40.206 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 220.127.116.11/127 c=IN IP4 18.104.22.168/127 c=IN IP4 22.214.171.124/127 Similarly, an IPv6 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 IPv6 multicast). 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=" field. The slash notation for multiple addresses described above MUST NOT be used for IP unicast addresses. Section 8 and , ): CT If the bandwidth of a session or media in 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). The primary purpose of this is to give an approximate idea as to whether two or more sessions can coexist simultaneously. When using the CT modifier with RTP, if several RTP sessions are part of the conference, the conference total refers to total bandwidth of all RTP sessions. 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 . Note that CT gives a total bandwidth figure for all the media at all sites. AS gives a bandwidth figure for a single media at a single site, although there may be many sites sending simultaneously. A prefix "X-" is defined for <bwtype> 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. The <bandwidth> is interpreted as kilobits per second by default. The definition of a new <bwtype> 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). 13]. 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 <stop-time> is set to zero, then the session is not bounded, though it will not become active until after the <start-time>. If the <start-time> 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. 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.
A key field is permitted before the first media entry (in which case it applies to all media in the session), or for each media entry as required. The format of keys and their usage are outside the scope of this document, and the key field provides no way to indicate the encryption algorithm to be used, key type, or other information about the key: this is assumed to be provided by the higher-level protocol using SDP. If there is a need to convey this information within SDP, the extensions mentioned previously SHOULD be used. Many security protocols require two keys: one for confidentiality, another for integrity. This specification does not support transfer of two keys. The method indicates the mechanism to be used to obtain a usable key by external means, or from the encoded encryption key given. The following methods are defined: k=clear:<encryption key> The encryption key is included untransformed in this key field. This method MUST NOT be used unless it can be guaranteed that the SDP is conveyed over a secure channel. The encryption key is interpreted as text according to the charset attribute; use the "k=base64:" method to convey characters that are otherwise prohibited in SDP. k=base64:<encoded encryption key> The encryption key is included in this key field but has been base64 encoded  because it includes characters that are prohibited in SDP. This method MUST NOT be used unless it can be guaranteed that the SDP is conveyed over a secure channel. k=uri:<URI to obtain key> A Uniform Resource Identifier is included in the key field. The URI refers to the data containing the key, and may require additional authentication before the key can be returned. When a request is made to the given URI, the reply should specify the encoding for the key. The URI is often an Secure Socket Layer/Transport Layer Security (SSL/TLS)-protected HTTP URI ("https:"), although this is not required. k=prompt No key is included in this SDP description, but the session or media stream referred to by this key field is encrypted. The user should be prompted for the key when attempting to join the session, and this user-supplied key should then be used to
decrypt the media streams. The use of user-specified keys is NOT RECOMMENDED, since such keys tend to have weak security properties. The key field MUST NOT be used unless it can be guaranteed that the SDP is conveyed over a secure and trusted channel. An example of such a channel might be SDP embedded inside an S/MIME message or a TLS-protected HTTP session. It is important to ensure that the secure channel is with the party that is authorised to join the session, not an intermediary: if a caching proxy server is used, it is important to ensure that the proxy is either trusted or unable to access the SDP.
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. <media> is the media type. Currently defined media are "audio", "video", "text", "application", and "message", although this list may be extended in the future (see Section 8). <port> 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=" field, and on the transport protocol defined in the <proto> sub-field of the media field. Other ports used by the media application (such as the RTP Control Protocol (RTCP) port ) MAY be derived algorithmically from the base media port or MAY be specified in a separate attribute (for example, "a=rtcp:" as defined in ). 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 <port> 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 <port> 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=" field:
m=<media> <port>/<number of ports> <proto> <fmt> ... 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 <number of ports> 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 ). If multiple addresses are specified in the "c=" field and multiple ports are specified in the "m=" field, a one-to-one mapping from port to the corresponding address is implied. For example: c=IN IP4 126.96.36.199/127/2 m=video 49170/2 RTP/AVP 31 would imply that address 188.8.131.52 is used with ports 49170 and 49171, and address 184.108.40.206 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, ) should instead be used to express the intended semantics. <proto> is the transport protocol. The meaning of the transport protocol is dependent on the address type field in the relevant "c=" field. 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  used under the RTP Profile for Audio and Video Conferences with Minimal Control  running over UDP. * RTP/SAVP: denotes the Secure Real-time Transport Protocol  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. <fmt> 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 <proto> sub-field. If the <proto> sub-field is "RTP/AVP" or "RTP/SAVP" the <fmt> 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:" attribute MAY be used to specify format parameters (see Section 6). If the <proto> sub-field is "udp" the <fmt> 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 <fmt> field is protocol specific. Rules for interpretation of the <fmt> sub- field MUST be defined when registering new protocols (see Section 8.2.2). Section 8.2.4. a=cat:<category> 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. It is a session-level attribute, and it is not dependent on charset.
a=keywds:<keywords> 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. It is a session-level attribute. It is a charset-dependent attribute, meaning that 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. a=tool:<name and version of tool> This gives the name and version number of the tool used to create the session description. It is a session-level attribute, and it is not dependent on charset. a=ptime:<packet time> 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. It is a media-level attribute, and it is not dependent on charset. a=maxptime:<maximum packet time> 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. It is a media-level attribute, and it is not dependent on charset. Note that this attribute was introduced after RFC 2327, and non-updated implementations will ignore this attribute. a=rtpmap:<payload type> <encoding name>/<clock rate> [/<encoding parameters>] 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. It is a media-level attribute that is not dependent on charset.
Although an RTP profile may 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 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, <encoding parameters> 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. a=recvonly This specifies that the tools should be started in receive-only mode where applicable. It can be either a session- or media- level attribute, and it is not dependent on charset. 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). a=sendrecv This specifies that the tools should be started in send and receive mode. This is necessary for interactive conferences with tools that default to receive-only mode. It can be either a session or media-level attribute, and it is not dependent on charset. If none of the attributes "sendonly", "recvonly", "inactive", and "sendrecv" is present, "sendrecv" SHOULD be assumed as the default for sessions that are not of the conference type "broadcast" or "H332" (see below). 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. It can be either a session- or media-level attribute, but would normally only be used as a media attribute. It is not dependent on charset. Note that sendonly applies only to the media, and any associated control protocol (e.g., RTCP) SHOULD still be received and processed as normal. a=inactive This specifies that the tools should be started in inactive mode. This is necessary for interactive conferences where users can put other users on hold. No media is sent over an
inactive media stream. Note that an RTP-based system SHOULD still send RTCP, even if started inactive. It can be either a session or media-level attribute, and it is not dependent on charset. a=orient:<orientation> Normally this is only used for a whiteboard or presentation tool. It specifies the orientation of a the workspace on the screen. It is a media-level attribute. Permitted values are "portrait", "landscape", and "seascape" (upside-down landscape). It is not dependent on charset. a=type:<conference type> This specifies the type of the 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 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 . 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. The type attribute is a session-level attribute, and it is not dependent on charset. a=charset:<character set> 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
This is a session-level attribute and is not dependent on charset. The charset specified MUST be one of those registered with IANA, such as ISO-8859-1. The character set identifier is a US-ASCII string and MUST be compared against the IANA identifiers 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. a=sdplang:<language tag> This can be a session-level attribute or a media-level attribute. As a session-level attribute, it specifies the language for the session description. As a media-level attribute, it specifies the language for any media-level SDP information field associated with that media. Multiple sdplang attributes can be provided either at session or media level if multiple languages in the session description or media use multiple languages, in which case the order of the attributes indicates the order of importance of the various languages in the session or media from most important to least important. 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 RFC 3066 language tag in US-ASCII . It is not dependent on the charset attribute. An "sdplang" attribute SHOULD be specified when a session is of 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. a=lang:<language tag> This can be a session-level attribute or a media-level attribute. As a session-level attribute, it specifies the default language for the session being described. As a media- level attribute, it specifies the language for that media,
overriding any session-level language specified. Multiple lang attributes can be provided either at session or media level if the session description or media use multiple languages, in which case the order of the attributes indicates the order of importance of the various languages in the session or media from most important to least important. The "lang" attribute value must be a single RFC 3066 language tag in US-ASCII . It is not dependent on the charset attribute. A "lang" attribute SHOULD be specified when a session is of 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. a=framerate:<frame rate> 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 using the notation "<integer>.<fraction>" are allowed. It is a media-level attribute, defined only for video media, and it is not dependent on charset. a=quality:<quality> 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. It is a media-level attribute, and it is not dependent on charset. a=fmtp:<format> <format specific parameters> 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. It is a media-level attribute, and it is not dependent on charset. 15] using the offer/answer model  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 description, 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 Session Announcement Protocol (SAP). SAP provides both encryption and authentication mechanisms, but due to the nature of session announcements it is likely that 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). Use of the "k=" field 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.
RFC 2327 is to be updated, as defined below. To: firstname.lastname@example.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 4566). Arbitrary binary content cannot be directly represented in SDP. Security considerations: See Section 7 of RFC 4566 Interoperability considerations: See RFC 4566 Published specification: See RFC 4566 Applications which use this media type: Voice over IP, video teleconferencing, streaming media, instant messaging, among others. See also Section 3 of RFC 4566. 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: Mark Handley <M.Handley@cs.ucl.ac.uk> Colin Perkins <email@example.com> IETF MMUSIC working group <firstname.lastname@example.org>
Intended usage: COMMON Author/Change controller: Authors of RFC 4566 IETF MMUSIC working group delegated from the IESG RFC 2434 . This memo registers the media types "audio", "video", "text", "application", and "message". Note: The media types "control" and "data" were listed as valid in the previous version of this specification ; 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 RFC 3840 . 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 RFC 3840). 19] used under the RTP Profile for Audio and Video Conferences with Minimal Control 
running over UDP/IP, "RTP/SAVP" is a reference to the Secure Real- time Transport Protocol , and "udp" indicates an unspecified protocol over UDP. 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). 31] 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.
RFC 2434, provided that the specification includes the following information: o contact name, email address, and telephone number o attribute name (as it will appear in SDP) o long-form attribute name in English o type of attribute (session level, media level, or both) o whether the attribute value is subject to the charset attribute o a one-paragraph explanation of the purpose of the attribute o a specification of appropriate attribute values for this 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. IANA has registered the following initial set of attribute names ("att-field" values), with definitions as in Section 6 of this memo (these definitions update those in RFC 2327):
Name | Session or Media level? | Dependent on charset? ----------+-------------------------+---------------------- cat | Session | No keywds | Session | Yes tool | Session | No ptime | Media | No maxptime | Media | No rtpmap | Media | No recvonly | Either | No sendrecv | Either | No sendonly | Either | No inactive | Either | No orient | Media | No type | Session | No charset | Session | No sdplang | Either | No lang | Either | No framerate | Media | No quality | Media | No fmtp | Media | No Section 5.8 of this memo (these definitions update those in RFC 2327).
type. A new network type registration MUST reference an RFC that gives details of the network type and address type 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 RFC 2327). Sections 5.2 and 5.7 of this memo (these definitions update those in RFC 2327).
4]. ; SDP Syntax session-description = proto-version origin-field session-name-field information-field uri-field email-fields phone-fields connection-field bandwidth-fields time-fields key-field attribute-fields media-descriptions proto-version = %x76 "=" 1*DIGIT CRLF ;this memo describes version 0 origin-field = %x6f "=" username SP sess-id SP sess-version SP nettype SP addrtype SP unicast-address CRLF session-name-field = %x73 "=" text CRLF information-field = [%x69 "=" text CRLF] uri-field = [%x75 "=" uri CRLF] email-fields = *(%x65 "=" email-address CRLF) phone-fields = *(%x70 "=" phone-number CRLF) connection-field = [%x63 "=" nettype SP addrtype SP connection-address CRLF] ;a connection field must be present ;in every media description or at the ;session-level
bandwidth-fields = *(%x62 "=" bwtype ":" bandwidth CRLF) time-fields = 1*( %x74 "=" start-time SP stop-time *(CRLF repeat-fields) CRLF) [zone-adjustments CRLF] repeat-fields = %x72 "=" repeat-interval SP typed-time 1*(SP typed-time) zone-adjustments = %x7a "=" time SP ["-"] typed-time *(SP time SP ["-"] typed-time) key-field = [%x6b "=" key-type CRLF] attribute-fields = *(%x61 "=" attribute CRLF) media-descriptions = *( media-field information-field *connection-field bandwidth-fields key-field attribute-fields ) media-field = %x6d "=" 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 ;typically "IP4" or "IP6" ; sub-rules of 'u=' uri = URI-reference ; see RFC 3986
; sub-rules of 'e=', see RFC 2822 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 = %x64 / %x68 / %x6d / %x73 ; sub-rules of 'k=' key-type = %x70 %x72 %x6f %x6d %x70 %x74 / ; "prompt" %x63 %x6c %x65 %x61 %x72 ":" text / ; "clear:" %x62 %x61 %x73 %x65 "64:" base64 / ; "base64:" %x75 %x72 %x69 ":" uri ; "uri:" 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", 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 [ "/" integer ] ; IPv4 multicast addresses may be in the ; range 220.127.116.11 to 18.104.22.168 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / ("23" DIGIT ) IP6-multicast = hexpart [ "/" integer ] ; IPv6 address starting with FF 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" ; The following is consistent with RFC 2373 , Appendix B. IP6-address = hexpart [ ":" IP4-address ] hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ] hexseq = hex4 *( ":" hex4) hex4 = 1*4HEXDIG ; 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 = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39 / %x41-5A / %x5E-7E 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 ; 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 4234 ; URI-reference: from RFC 3986 ; addr-spec: from RFC 2822 RFC 2327 it is likely that some implementations interpreted that memo in ways that differ from this version of SDP. The ABNF grammar in Section 9 has been extensively revised and updated, correcting a number of mistakes and incorporating the RFC 3266 IPv6 extensions. Known inconsistencies between the grammar and the specification text have been resolved. A media type registration for SDP is included. Requirements for the registration of attributes and other parameters with IANA have been clarified and tightened (Section 8). It is noted that "text" and "message" are valid media types for use with SDP, but that "control" and "data" are under-specified and deprecated. RFC 2119 terms are now used throughout to specify requirements levels. Certain of those requirements, in particular in relation to parameter registration, are stricter than those in RFC 2327. The "RTP/SAVP" RTP profile and its "fmt" namespace are registered. The attributes "a=inactive" and "a=maxptime" have been added. RFC 2327 mandated that either "e=" or "p=" was required. Both are now optional, to reflect actual usage. The significant limitations of the "k=" field are noted, and its use is deprecated. Most uses of the "x-" prefix notation for experimental parameters are disallowed and the other uses are deprecated.
 Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.  Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005.  Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.  Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.  Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066, January 2001.  Olson, S., Camarillo, G., and A. Roach, "Support for IPv6 in Session Description Protocol (SDP)", RFC 3266, June 2002.
 Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003.  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003.  Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992.  Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000.  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998.  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.  Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne, "Grouping of Media Lines in the Session Description Protocol (SDP)", RFC 3388, December 2002.  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003.  Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July 2003.  Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003.  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.
 Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, August 2004.  Westerlund, M., "A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP)", RFC 3890, September 2004.  International Telecommunication Union, "H.323 extended for loosely coupled conferences", ITU Recommendation H.332, September 1998.  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, "Key Management Extensions for Session Description Protocol (SDP) and Real Time Streaming Protocol (RTSP)", RFC 4567, July 2006.  Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol (SDP) Security Descriptions for Media Streams", RFC 4568, July 2006.  Resnick, P., "Internet Message Format", RFC 2822, April 2001.  Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, July 1998.  Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005.
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