SDP详解（英文版）

SDP Specification

An SDP session description is denoted by the media type
“application/sdp” (See 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:

  =

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 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
  [email protected] (Jane Doe)
  c=IN IP4 224.2.17.12/127
  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 [1], [2]. Internationalised domain names (IDNs) MUST be
represented using the ASCII Compatible Encoding (ACE) form defined in
[11] 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).

Protocol Version (“v=”)

  v=0

The “v=” field gives the version of the Session Description Protocol.
This memo defines version 0. There is no minor version number.

Origin (“o=”)

  o=    
    

The “o=” field gives the originator of the session (her username and
the address of the user’s host) plus a session identifier and version
number:

<username> 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 <username> MUST NOT contain spaces.

<sess-id> is a numeric string such that the tuple of <username>,
<sess-id>, <nettype>, <addrtype>, and <unicast-address> forms a
globally unique identifier for the session. The method of
<sess-id> 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

<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.

Session Name (“s=”)

  s=

The “s=” field is the textual session name. There MUST be one and
only one “s=” field per session description. The “s=” field 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).

Session Information (“i=”)

  i=

The “i=” field provides textual information about the session. There
MUST be at most one session-level “i=” field per session description,
and at most one “i=” field per media. If the “a=charset” attribute
is present, it specifies the character set used in the “i=” field.
If the “a=charset” attribute is not present, the “i=” field MUST
contain ISO 10646 characters in UTF-8 encoding.

A single “i=” field MAY also be used for each media definition. In
media definitions, “i=” fields 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=” field 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.

URI (“u=”)

  u=

A URI is a Uniform Resource Identifier as used by WWW clients [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.

Email Address and Phone Number (“e=” and “p=”)

  e=
  p=

The “e=” and “p=” lines specify contact information for the person
responsible for the conference. This is not necessarily the same
person that created the conference announcement.

Inclusion of an email address or phone number is OPTIONAL. Note that
the previous version of SDP specified that either an email field or a
phone field MUST be specified, but this was widely ignored. The
change brings the specification into line with common usage.

If an email address or phone number is present, it MUST be specified
before the first media field. More than one email or phone field 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:

  [email protected] (Jane Doe)

The alternative RFC 2822 [29] 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.

Connection Data (“c=”)

  c=  

The “c=” field contains connection data.

A session description MUST contain either at least one “c=” field in
each media description or a single “c=” field at the session level.
It MAY contain a single session-level “c=” field and additional “c=”
field(s) per media description, in which case the per-media values
override the session-level settings for the respective media.

The first sub-field (“<nettype>”) 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 (“<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 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 224.2.36.42/127

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:

  [/]/

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 224.2.1.1/127/3

would state that addresses 224.2.1.1, 224.2.1.2, and 224.2.1.3 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 224.2.1.1/127
  c=IN IP4 224.2.1.2/127
  c=IN IP4 224.2.1.3/127

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.

Bandwidth (“b=”)

  b=:

This OPTIONAL field denotes the proposed bandwidth to be used by the
session or media. The <bwtype> is an alphanumeric modifier giving
the meaning of the <bandwidth> figure. Two values are defined in
this specification, but other values MAY be registered in the future
(see Section 8 and [21], [25]):

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
[19].

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).

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 [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 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.

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.

Repeat Times (“r=”)

  r=  

“r=” fields specify 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=” field would be the NTP representation of 10am on
the first Monday, the would be 1 week, the <active
duration> would be 1 hour, and the offsets would be zero and 25
hours. The corresponding “t=” field 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=” fields might
be the following:

  t=3034423619 3042462419
  r=604800 3600 0 90000

To make 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=” fields should be used to
explicitly list the session times.

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
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=” field 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 announcement will be modified periodically rather than
transmit several years’ worth of adjustments in one session
announcement.

Encryption Keys (“k=”)

  k=
  k=:

If transported over a secure and trusted channel, the Session
Description Protocol MAY be used to convey encryption keys. A simple
mechanism for key exchange is provided by the key field (“k=”),
although this is primarily supported for compatibility with older
implementations and its use is NOT RECOMMENDED. Work is in progress
to define new key exchange mechanisms for use with SDP [27] [28], and
it is expected that new applications will use those mechanisms.
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:

     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:

     The encryption key is included in this key field but has been
     base64 encoded [12] 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:

     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.

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=” fields)
that are media specific. These are referred to as “media-level”
attributes and add information about the media stream. Attribute
fields can also be added before the first media field; these
“session-level” attributes convey additional information that applies
to the conference as a whole rather than to individual media.

Attribute fields 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.

Media Descriptions (“m=”)

  m=    ...

A session description may contain a number of media descriptions.
Each media description starts with an “m=” field and is terminated by
either the next “m=” field or by the end of the session description.
A media field has several sub-fields:

<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 sub-field of the media field.
Other ports used by the media application (such as the RTP Control
Protocol (RTCP) port [19]) MAY be derived algorithmically from the
base media port or MAY be specified in a separate attribute (for
example, “a=rtcp:” as defined in [22]).

  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=" field:

     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 [22]).

  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 224.2.1.1/127/2
     m=video 49170/2 RTP/AVP 31

  would imply that address 224.2.1.1 is used with ports 49170 and
  49171, and address 224.2.1.2 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, [18]) 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 [19] used under the RTP Profile for Audio
     and Video Conferences with Minimal Control [20] running over
     UDP.

  *  RTP/SAVP: denotes the Secure Real-time Transport Protocol [23]
     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 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:"  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).