Negotiating Media Multiplexing Using the Session Description Protocol (SDP)
EricssonHirsalantie 1102420JorvasFinlandchrister.holmberg@ericsson.comGoogleKungsbron 2Stockholm11122Swedenharald@alvestrand.noCisco400 3rd Avenue SW, Suite 350CalgaryABT2P 4H2Canadafluffy@iii.ca
Transport
MMUSIC Working GroupRTPSDPBundleMultiplexingRTCWEBCLUERTCWEBMMUSICAVTWEBBrowser
This specification defines a new SDP Grouping Framework extension, 'BUNDLE'. The
BUNDLE extension can be used with the Session Description Protocol (SDP) Offer/Answer
mechanism to negotiate the usage of a single 5-tuple for sending and receiving media,
referred to as bundled media, associated with multiple SDP media descriptions ("m=" lines).
This specification also defines a new SDP attribute, 'bundle-only', which can be used to
request that specific media is only used if bundled.
This specification also updates sections 5.1, 8.1 and 8.2 of RFC 3264. The update
allows an answerer to assign a non-zero port value to an "m=" line in an answer, even
if the "m=" line in the associated offer contained a zero port value.
In the IETF RTCWEB WG, a need to use a single 5-tuple for sending and receiving media
associated with multiple SDP media descriptions ("m=" lines) has been identified. This
would e.g. allow the usage of a single set of Interactive Connectivity Establishment
(ICE) candidates for multiple
media descriptions.
This specification defines a new SDP Grouping Framework extension, 'BUNDLE'. The BUNDLE extension can be used with the Session Description
Protocol (SDP) Offer/Answer mechanism
to negotiate the usage of a single 5-tuple for sending and receiving media, referred to as
bundled media, associated with multiple SDP media descriptions ("m="). This specification also
defines a new SDP attribute, 'bundle-only', which can be used to request that specific media is
only used if bundled.
The offerer and answerer use
the BUNDLE extension to negotiate the 5-tuples (BUNDLE addresses), one for the offerer (offerer BUNDLE
address) and one for the answerer (answerer BUNDLE address) to be used for the bundled
media associated with a BUNDLE group.
Once the offerer and the answerer have negotiated a BUNDLE group, and the associated BUNDLE addresses,
each endpoint can assign its BUNDLE address to each "m=" line within, and use the address to send and receive
all media associated with, the BUNDLE group.
NOTE: As defined in RFC 4566 , the
semantics of assigning the same port value to multiple "m=" lines are undefined, and there is no
grouping defined by such means. Instead, an explicit grouping mechanism needs to be used to
express the intended semantics. This specification provides such an extension.
This specification also updates sections 5.1, 8.1 and 8.2 of RFC 3264 . The update allows an answerer to assign a non-zero port value to an "m=" line in an
answer, even if the "m=" line in the associated offer contained a zero port value.
SDP bodies can contain multiple BUNDLE groups. A given BUNDLE address MUST only be associated
with a single BUNDLE group.
The procedures in this specification apply independently to a given BUNDLE group.
All Real-time Transport Protocol (RTP) based media flows
associated with a single BUNDLE group belong to a single RTP session
.
The BUNDLE extension is backward compatible. Endpoints that do not support the extension
are expected to generate offers and answers without an SDP 'group:BUNDLE' attribute, and
are expected to assign a unique address to each "m=" line within an offer and answer, according
to the procedures in and
5-tuple: A collection of the following values: source address, source port,
destination address, destination port and protocol.
Unique address: An IP address and IP port combination that is assigned to
only one "m=" line in an offer or answer.
Shared address: An IP address and IP port combination that is assigned to
multiple "m=" lines within an offer or answer.
Offerer suggested BUNDLE mid: The first mid value in a given
SDP 'group:BUNDLE' attribute mid list in an offer.
Answerer selected BUNDLE mid: The first mid value in a given
SDP 'group:BUNDLE' attribute mid list in an answer.
Offerer BUNDLE address: Within a given BUNDLE group, an IP address and IP
port combination used by an offerer to receive all media associated
with each "m=" line within the BUNDLE group.
Answerer BUNDLE address: Within a given BUNDLE group, an IP address and IP
port combination used by an answerer to receive all media associated
with each "m=" line within the BUNDLE group.
BUNDLE group: A set of "m=" lines, created using an SDP Offer/Answer
exchange, for which each endpoint use a single 5-tuple is to send and
receive media. Each endpoint uses its BUNDLE address,
associated with the BUNDLE group, to send and receive the media.
Bundled "m=" line: An "m=" line, which SDP 'mid' attribute value
is placed in a SDP 'group:BUNDLE' attribute mid value list in an
offer or answer.
Bundle-only "m=" line: A bundled "m=" line, to which an SDP 'bundle-only' attribute
has been assigned.
Bundled media: All media associated with a given BUNDLE group.
Initial offer: The first offer, within an SDP session, in which
the offerer indicates that it wants to create a given BUNDLE group.
Subsequent offer: An offer which contains a BUNDLE group that
has been created as part of a previous SDP Offer/Answer exchange.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 .
The mechanism in this specification only applies to the Session Description Protocol (SDP)
, when used together with the
SDP Offer/Answer mechanism .
This section defines a new SDP Grouping Framework extension
, 'BUNDLE'. The
BUNDLE extension can be used with the Session Description Protocol (SDP) Offer/Answer
mechanism to negotiate the usage of a single 5-tuple for sending and receiving media,
referred to as bundled media, associated with multiple SDP media descriptions ("m=" lines).
Within a successfully created BUNDLE group, media described with "m=" lines associated
with the BUNDLE group will be sent and received using a single 5-tuple.
The BUNDLE extension is indicated using an SDP 'group' attribute
with a "BUNDLE" semantics value . An SDP "mid" attribute is assigned to each
bundled "m=" line, and the "mid" attribute value is listed in the
'group:BUNDLE' attribute mid value list. Each "m=" line, which mid
value is listed in the mid value list, is associated with a given BUNDLE
group.
Any given "m=" line MUST NOT be associated with more than one BUNDLE group.
defines the
detailed SDP Offer/Answer procedures for the BUNDLE extension.
This section defines a new SDP media-level attribute , 'bundle-only'.
The 'bundle-only' attribute can be assigned to a bundled "m=" line in an offer, to
request that the answerer only accepts the "m=" line if the answerer keeps the "m=" line
within the associated BUNDLE group.
In order to ensure that an answerer that does not supports the BUNDLE extension always
rejects a 'bundle-only' "m=" line, the offerer can assign a zero port value to the "m="
line to which the 'bundle-only' attribute has been assigned. According to
an answerer will reject such
"m=" line.
The usage of the 'bundle-only' attribute is only defined for a bundled "m=" line within
an offer. Other usage is unspecified.
defines the detailed SDP
Offer/Answer procedures for the 'bundle-only' attribute.
This section defines the Augmented Backus-Naur Form (ABNF)
for the SDP 'bundle-only' attribute, based on the SDP
grammar.
This section describes restrictions associated with the usage of
SDP parameters within a BUNDLE group. It also describes, when parameter
and attribute values have been assigned to each bundled "m=" line, how to
calculate a value for the whole BUNDLE group.
The "c=" line nettype value assigned to a bundled "m=" line MUST be 'IN'.
The "c=" line addrtype value assigned to a bundled "m=" line MUST be 'IP4' or
'IP6'. The same value MUST be assigned to each "m=" line.
NOTE: Extensions to this specification can specify usage of the BUNDLE
mechanism for other nettype and addrtype values than the ones listed above.
The proposed bandwidth for a bundled "m=" line SHOULD be calculated
in the same way as for a non-bundled "m=" line.
The total proposed bandwidth for a BUNDLE group is the sum of the
proposed bandwidth for each bundled "m=" line.
The total proposed bandwidth for an offer or answer is the sum of
the proposed bandwidth for each "m=" line (bundled and non-bundled)
within the offer or answer.
defines
rules and restrictions for assigning different types of SDP attributes
to a bundled "m=" line.
This section describes the SDP Offer/Answer procedures for:
Negotiating and creating of a BUNDLE group;
Selecting the BUNDLE addresses (offerer BUNDLE address and answerer BUNDLE address);
Adding an "m=" line to a BUNDLE group;
Moving an "m=" line out of a BUNDLE group; and
Disabling an "m=" line within a BUNDLE group.
The generic rules and procedures defined in and
also apply to the BUNDLE extension. For example, if an offer is rejected
by the answerer, the previously negotiated SDP parameters and characteristics
(including those associated with a BUNDLE group) apply. Hence, if an offerer
generates an offer in which the offerer wants to create a BUNDLE group,
and the answerer rejects the offer, the BUNDLE group is not created.
The procedures in this section are independent of the media type or transport
protocol represented by a bundled "m=" line. [] defines additional considerations for RTP based media.
[] defines
additional considerations for the usage of the SDP 'bundle-only' attribute.
[] defines additional
considerations for the usage of Interactive Connectivity Establishment (ICE)
mechanism .
The offerer and answerer MUST follow the rules and restrictions defined
in when
creating offers and answers.
When an offerer generates an initial offer, in order to create a
BUNDLE group, it MUST:
Assign a unique address to each "m=" line within the offer, following the procedures
in ;
Assign an SDP 'group:BUNDLE' attribute to the offer;
Place the SDP 'mid' attribute value
of each bundled "m=" line to the SDP 'group:BUNDLE' attribute mid value list; and
Indicate which unique address the offerer wants the answerer to select as the
offerer BUNDLE address .
If the offerer wants to request that the answerer accepts a given "m=" line only if
the the answerer keeps the "m=" line within the BUNDLE group, the offerer MUST:
Assign an SDP 'bundle-only' attribute to the "m=" line; and
Assign a zero port value to the "m=" line.
NOTE: If the offerer assigns a zero port value to an "m=" line, but does not also assign an
SDP 'bundle-only' attribute to the "m=" line, it is an indication that the offerer wants to disable the "m="
line [].
[] shows
an example of an initial offer.
In the offer, the address assigned to the "m=" line associated with the offerer suggested BUNDLE
mid indicates the address that the offerer wants the answer to select as the
offerer BUNDLE address [].
When an answerer generates an answer, which contains a BUNDLE group, the following general
SDP grouping framework restrictions, defined in , also apply to the BUNDLE group:
The answerer MUST NOT include a BUNDLE group in the answer, unless the
offerer requested the BUNDLE group to be created in the associated offer; and
The answerer MUST NOT include an "m=" line within a BUNDLE group, unless
the offerer requested to "m=" line to be within a BUNDLE group in the
associated offer.
If the answer contains a BUNDLE group, the answerer MUST:
Select an Offerer BUNDLE Address ; and
Select an Answerer BUNDLE Address ;
The answerer is allowed to select a new Answerer BUNDLE address each time it
generates an answer to an offer.
If the answerer does not want to keep an "m=" line within a BUNDLE group, it MUST:
Move the "m=" line out of the BUNDLE group ; or
Reject the "m=" line ;
If a bundled "m=" line in an offer contains an SDP 'bundle-only' attribute,
and if the answerer keeps the "m=" line within the BUNDLE group, the answerer MUST
process the "m=" line as any other bundled "m=" line in the offer. The answerer
MUST NOT assign a 'bundle-only' attribute to any "m=" line in an answer (not even if the
"m=" line in the associated offer contains a 'bundle-only' attribute).
NOTE: If a bundled "m=" line in an offer contains a port zero value, but the "m=" line does not contain
an SDP 'bundle-only' attribute, it is an indication that the offerer wants to disable the "m="
line [].
In an offer, the address (unique or shared) assigned to the bundled "m=" line associated with the offerer
suggested BUNDLE mid indicates the address that the offerer wants the answer to select as the offerer
BUNDLE address []. The answerer
MUST check whether the "m=" line fulfills the following criteria:
The answerer will not move the "m=" line out of the BUNDLE group
[];
The answerer will not reject the "m=" line []; and
The "m=" line does not contain a zero port value.
If all of the criteria above is fulfilled, the answerer MUST select the address
associated with the "m=" line as the offerer BUNDLE address. In the answer, the
answerer selected BUNDLE mid represents the "m=" line, and the address associated
with the "m=" line in the offer becomes the offerer BUNDLE address.
If all of the criteria is not fulfilled, the answerer MUST select the next mid
value in the mid list, and perform the same criteria check for the "m=" line
associated with that mid value. If there are no more mid values in the mid list,
the answerer MUST NOT create the BUNDLE group.
[] shows an
example of an offerer BUNDLE address selection.
When the answerer selects a BUNDLE address for itself, referred to as the answerer
BUNDLE address, it MUST assign the address to each bundled "m=" line within the
created BUNDLE group in the answer.
The answerer MUST NOT assign the answerer BUNDLE address to an "m=" line that is
not within the BUNDLE group, or to an "m=" line that is within another BUNDLE group.
[] shows an example of an
answerer BUNDLE address selection.
When an answerer moves a "m=" line out of a BUNDLE group, it assigns an
address to the "m=" line in the answer based on the following rules:
In the associated offer, if the "m=" line contains a shared address (e.g. a
previously selected offerer BUNDLE address), the answerer MUST reject the
moved "m=" line [];
In the associated offer, if the "m=" line contains a unique address, the
answerer MUST assign a unique address to the "m=" line in the answer; or
In the associated offer, if the "m=" line contains an SDP 'bundle-only'
attribute the answerer MUST reject the "m=" line [].
In addition, in either case above, the answerer MUST NOT include a mid value, associated
with the moved "m=" line, in the SDP 'group:BUNDLE' attribute mid list associated
with the BUNDLE group.
When an answerer rejects an "m=" line, it MUST assign an address with a zero port
value to the "m=" line in the answer, according to the procedures in .
In addition, the answerer MUST NOT include a mid value, associated
with the rejected "m=" line, in the SDP 'group:BUNDLE' attribute mid list
associated with the BUNDLE group.
When an offerer receives an answer, if the answer contains a BUNDLE group, the offerer
MUST check that any bundled "m=" line in the answer was indicated as bundled in the
associated offer. If there is no mismatch, the offerer MUST apply the offerer BUNDLE
address, selected by the answerer [], to each bundled "m=" line. The offerer MUST assign the
address to each bundled "m=" line (excluding a bundled "m=" line added to a BUNDLE group,
to which the offerer MAY assign a unique address []) in any subsequent offer.
NOTE: As the answerer might reject one or more bundled "m=" lines, or move a bundled
"m=" line out of a BUNDLE group, each bundled "m=" line in the offer might not be
indicated as bundled in the answer.
If the answer does not contain a BUNDLE group, the offerer MUST process the answer
as a normal answer.
When an offerer receives an answer, if the answer contains a BUNDLE group, the offerer MUST
check whether the offerer BUNDLE address, selected by the answerer [], matches was assigned to each bundled "m=" line (excluding any
bundled "m=" line that was rejected, or moved out of the BUNDLE group, by the answer)
in the associated offer. If there is a mismatch, the offerer SHOULD as soon as possible
generate a subsequent offer, in which it assigns the offerer BUNDLE address to each
bundled "m=" line. Such offer is referred to as a Bundle Address Synchronization (BAS) offer.
A BAS offer is typically sent in the following scenarios:
The offerer receives an answer to an initial offer, as the bundled "m=" lines
in the initial offer always contain unique addresses []; or
The offerer receives an answer to an offer, in which a new bundled "m=" line has
been added to the BUNDLE group [], and the offerer assigned a unique address to the bundled
"m=" line in the offer.
The offerer is allowed to modify any SDP parameter in the BAS offer.
NOTE: It is important that the BAS offer gets accepted by the
answerer. For that reason the offerer needs to consider the
necessity to modify SDP parameters in the BAS offer, in such a way
that could trigger the answerer to reject the BAS offer.
Disabling "m=" lines, or reducing the number of codecs, in a BAS
offer is considered to have a low risk of being rejected.
NOTE: The main purpose of the BAS offer is to ensure that
intermediaries, that might not support the BUNDLE extension, have
correct information regarding the address is going to be used to
transport the bundled media.
[]
shows an example of a BAS offer.
When an offerer generates a subsequent offer, it MUST assign
the previously selected offerer BUNDLE address [], to each bundled "m=" line
(including any bundle-only "m=" line), with the following exceptions:
The offerer wants to request the answerer to select a new offerer
BUNDLE address [];
The offerer wants to add a bundled "m=" line to the BUNDLE group
[];
The offerer wants to move a bundled "m=" line out of the BUNDLE group
[]; or
The offerer wants to disable the bundled "m=" line [].
In addition, the offerer MUST select an offerer suggested BUNDLE mid
[], even
if the offerer does not want the answerer to select a new offerer BUNDLE
address.
If the offerer assigns an SDP 'bundle-only' to a bundled "m=" line
in the subsequent offer, if MUST assign the offerer BUNDLE address to the
"m=" line. The offerer MUST NOT assign a unique address, or a zero port
value, to a bundle-only "m=" line in a subsequent offer.
NOTE: The offerer can in assign an SDP 'bundle-only' attribute to a
bundled "m=" line in a subsequent offer, even if the offerer did not
assign a 'bundle-only' attribute to the "m=" line in a previous offer.
When an offerer generates an offer, in which it wants the answerer to select a new
offerer BUNDLE address ,
the offerer MUST:
Assign a unique address, which the offerer wants the answerer to select as
the offerer BUNDLE address, to a bundled "m=" line (added to the BUNDLE group
in a previous offer/answer transaction, or requested to be added to the
BUNDLE group in the current offer); and
Indicate that the offerer wants the answerer to select the unique address as the
offerer BUNDLE address []
NOTE: The offerer can assign a unique address to each bundled "m=" line in the offer, or
it can assign the previously negotiated offerer BUNDLE address to each "m=" line
(except the "m=" line to which it assigns the unique address that it wants the answerer to
select as the new offerer BUNDLE address).
When an offerer generates an offer, in which it wants to add a bundled "m=" line
to BUNDLE group, the offerer MUST:
Assign a unique address (excluding bundle-only "m=" lines), or the offerer
BUNDLE address (selected by the answerer in a previous offer/answer transaction),
to the "m=" line;
Place the SDP 'mid' attribute value associated with the "m=" line in the SDP 'group:BUNDLE'
attribute mid list associated with the BUNDLE group [].
If the offerer wants the answerer to select the address
associated with the added "m=" line as the new offerer BUNDLE
address, the offerer suggested BUNDLE mid MUST represent the added "m="
line [].
If the offerer assigns an SDP 'bundle-only' attribute to the added "m=" line,
the offerer MUST assign the offerer BUNDLE address (selected by the answerer in
a previous offer/answer transaction) to the "m=" line.
[]
shows an example where an offerer sends an offer in order to
add a bundled "m=" line to a BUNDLE group.
When an offerer generates an offer, in which it wants to move a bundled "m=" line (added to
the BUNDLE group in a previous offer/answer transaction), the offerer:
MUST assign a unique address to the "m=" line;
MUST NOT place a mid value associated with the "m=" line in the SDP 'group:BUNDLE'
attribute mid list associated with the BUNDLE group; and
MUST NOT assign an SDP 'bundle-only' attribute to the "m=" line.
[]
shows an example of an offer for moving an "m=" line out of a BUNDLE group.
When an offerer generates an offer, in which it wants to disable a bundled "m=" line (added
to the BUNDLE group in a previous offer/answer transaction), the offerer:
MUST assign an address with a zero port value to the "m=" line,
following the procedures in ;
MUST NOT place a mid value associated with the "m=" line in the SDP 'group:BUNDLE'
attribute mid list associated with the BUNDLE group; and
MUST NOT assign an SDP 'bundle-only' attribute to the "m=" line.
[]
shows an example of an offer for disabling an "m=" line within a
BUNDLE group.
If bundled "m=" lines represent different transport protocols, there MUST
exist a publically available specification which describes a mechanism, for this
specific transport protocol combination, how to associate a received packet with
the correct transport protocol.
In addition, if a received packet can be associated with more than one
bundled "m=" line, there MUST exist a publically available specification which
describes a mechanism how to associated the received packet with the correct "m=" line.
Section 5.1.2 of describes a
mechanism how to identify the protocol among the STUN, DTLS and SRTP protocols (in any combination).
If an offer or answerer in offers or answers include bundled "m=" lines that represent these protocols,
the offerer or answerer MUST support the mechanism described in , and no explicit negotiation is required in order to indicate support
and usage of the mechanism.
does not describe how to identify
different protocols transported on DTLS, only how to identify the DTLS protocol itself. If
multiple protocols are transported on DTLS, there MUST exist a specification describing a
mechanism how to identify each individual protocol. In addition, if a received DTLS packet
can be associated with more than one "m=" line, there MUST exist a specification which
describes a mechanism how to associate the received DTLS packet with the correct "m=" line.
[] describes how to associate a received
(S)RTP packet with the correct "m=" line.
All RTP-based media within a single BUNDLE group belong to a
single RTP session . Disjoint BUNDLE groups will form multiple
RTP sessions, one per BUNDLE group.
Since a single RTP session is used for each bundle group, all
"m=" lines representing RTP-based media in a bundle group will
share a single SSRC numbering space .
The following rules and restrictions apply for a single RTP
session:
A specific payload type value can be used in multiple bundled "m=" lines
if each codec associated with the payload type number shares an identical
codec configuration [].The "proto" value in each bundled "m=" line MUST be identical (e.g. RTP/AVPF).A given SSRC SHOULD NOT transmit RTP packets using payload types that
originates from different bundled "m=" lines.
NOTE: The last bullet above is to avoid sending multiple media types from the same SSRC.
If transmission of multiple media types are done with time overlap RTP and RTCP fails
to function. Even if done in proper sequence this causes RTP Timestamp rate switching
issues [ref to draft-ietf-avtext-multiple-clock-rates].
Multiple bundled "m=" lines might represent RTP based media. As all RTP based
media associated with a BUNDLE group belong to the same RTP session, in order for a given payload type
value to used inside more than one bundled "m=" line, all codecs associated with the payload type
numbers MUST share an identical codec configuration. This means that the codecs MUST share the same media type,
encoding name, clock rate and any parameter that can affect the codec configuration and packetization.
lists SDP
attributes, which attribute values must be identical for all codecs that use the same payload type value.
In general, there are multiple mechanisms that can be used by an endpoint in order to associate received
RTP/RTCP packets with the bundled "m=" line representing the RTP packets. Such mechanisms include using the local
address:port combination on which the RTP packets are received, the payload type value carried inside the RTP packets,
the SSRC values carried inside the RTP packets, and other "m=" line specific information carried inside
the RTP packets.
As all RTP/RTCP packets associated with a BUNDLE group are sent and received using the same 5-tuple, the local address:port
combination cannot be used to associate received RTP packets with the correct "m=" line.
As described in [], the same payload
type value might be used inside RTP packets described by multiple "m=" lines. In such cases, the payload
type value cannot be used to associate received RTP packets with the correct "m=" line.
An offerer and answerer can in an offer and answer inform each other which SSRC values they will use
inside sent RTP/RTCP packets by, by assigning an SDP 'ssrc' attribute to each bundled "m=" line which contains a payload type value that is also used inside
another bundled "m=" line. As the SSRC values will be carried inside the RTP/RTCP packets, the offerer and
answerer can then use that information to associate received RTP packets with the correct "m=" line. However, an
offerer will not know which SSRC values the answerer will use until it has received the answer
providing that information. Due to this, before the offerer has received the answer, the offerer will
not be able to associate received RTP/RTCP packets with the correct "m=" line using the SSRC values.
In order for an offerer and answerer to always be able to associate received RTP and RTCP packets with the
correct "m=" line, an offerer and answerer using the BUNDLE extension MUST use the mechanism defined in
, where the remote endpoint inserts the
SDP 'mid' attribute value of an "m=" line in RTP and RTCP packets associated with that "m=" line.
When a BUNDLE group, which contains RTP based media, is created, the offerer
and answerer MUST negotiate whether to enable RTP/RTCP multiplexing
for the RTP based media associated with the BUNDLE group .
If RTP/RTCP multiplexing is not enabled, separate 5-tuples will be used for sending
and receiving the RTP packets and the RTCP packets.
This section describes how an offerer and answerer can use the SDP 'rtcp-mux' attribute
and the SDP 'rtcp' attribute
to negotiate usage of RTP/RTCP
multiplexing for RTP based associated with a BUNDLE group.
When an offerer generates an initial offer, if the offerer wants to negotiate usage
of RTP/RTCP multiplexing within a BUNDLE group, the offerer MUST assign
an SDP 'rtcp-mux' attribute to
each bundled "m=" line (including any bundle-only "m=" line) in the offer. In addition, the offerer
MUST assign an SDP 'rtcp' attribute to each
bundled "m=" line (including any bundle-only "m=" line), with an attribute value that is identical to
the port value assigned to the "m=" line itself, in the offer.
If the offerer does not want to negotiate usage of RTP/RTCP multiplexing, it MUST
NOT assign the SDP attributes above to any bundled "m=" line.
When an answerer generates an answer, if the offerer indicated support of
RTP/RTCP multiplexing
within a BUNDLE group in the associated offer, the answerer MUST either accept
or reject the usage of RTP/RTCP multiplexing in the answer.
If the answerer accepts usage of RTP/RTCP multiplexing within the BUNDLE group, it
MUST assign an SDP 'rtcp-mux' attribute to each bundled "m=" line in the answer. The
answerer MUST NOT assign an SDP 'rtcp' attribute to any bundled "m=" line in the answer.
If the answerer rejects usage of RTP/RTCP multiplexing within the BUNDLE group, it
MUST NOT assign an SDP 'rtcp-mux' or SDP 'rtcp' attribute to any bundled "m=" line in
the answer.
If the usage of RTP/RTCP multiplexing has been negotiated in a previous offer/answer
transaction, and the offerer indicates that it wants to continue using RTP/RTCP
multiplexing in a subsequent offer, the answerer MUST assign an SDP 'rtcp-mux' attribute
to each bundled "m=" line in the answer. I.e. the answerer MUST NOT disable the usage of
RTP/RTCP multiplexing.
When the offerer receives an answer, if the answerer accepts the
usage of RTP/RTCP multiplexing, by including an SDP 'rtcp-mux' attribute
to each bundled "m=" line in the answer , the answerer follows the procedures
for RTP/RTCP multiplexing defined in .
If the answerer does not accept the usage of RTP/RTCP multiplexing
,
the offerer MUST use separate 5-tuples for RTP and RTCP.
When an offerer generates a subsequent offer, if it wants
to negotiate usage of RTP/RTCP multiplexing within a BUNDLE group, or
if it wants to continue usage of RTP/RTCP multiplexing (negotiated in
a previous offer/answer transaction), it MUST assign SDP 'rtcp-mux'
and 'rtcp' attributes to each bundled "m=" line (including bundle-only
"m=" lines, and a bundled "m=" line that the offerer wants to add to the
BUNDLE group), unless the offerer wants to disable or remove the "m=" line
from the BUNDLE group.
If the offerer does not want to negotiate usage of RTP/RTCP multiplexing
within the BUNDLE group, or if it wants to disable usage of RTP/RTCP
multiplexing (negotiated in a previous offer/answer transaction), the
offerer MUST NOT assign SDP 'rtcp-mux' and 'rtcp' attributes to any
bundled "m=" line in the subsequent offer.
NOTE: It is RECOMMENDED that, once usage of RTP/RTCP multiplexing has been
negotiated within a BUNDLE group, that the usage is not disabled. Disabling
RTP/RTCP multiplexing means that the offerer and answerer need to
reserve new IP ports, to be used for sending and receiving RTCP packets.
This section describes how to use the BUNDLE grouping extension together
with the Interactive Connectivity Establishment (ICE) mechanism .
The procedures defined in also apply
to usage of ICE with BUNDLE, with the following exception:
When BUNDLE addresses for a BUNDLE group have been selected for both endpoints, ICE
connectivity checks and keep-alives only need to be performed for the whole BUNDLE group,
instead of per bundled "m=" line.
Support and usage of ICE mechanism together with the BUNDLE extension is OPTIONAL.
When an offerer or answerer assigns a unique address to a bundled "m=" line (excluding
bundle-only "m=" lines), it MUST also assign unique ICE candidates to the "m=" line.
When an offerer or answerer assigns a shared address (i.e. a previously selected BUNDLE
address) to one or more bundled "m=" line (including bundle-only "m=" lines), and when it assigns an
address with a zero port value to one or more bundle-only "m=" lines, it MUST assign identical
ICE candidates (referred to as shared ICE candidates) to each of those "m=" lines.
When an offerer generates an initial offer, it assigns unique or shared ICE candidates
to the bundled "m=" lines, according to .
When an answerer generates an answer, which contains a BUNDLE group,
the answerer MUST assign shared ICE candidates to each bundled "m=" line
(including "m=" lines that were indicated as bundle-only in the associated offer)
in the answer.
When an offerer receives an answer, if the answerer supports and uses the ICE
mechanism and the BUNDLE extension, the offerer MUST assign the ICE candidates,
associated with the "m=" line representing the offerer BUNDLE address (selected by the
answerer) to each bundled "m=" line.
When an offerer generates a subsequent offer, it assigns unique or shared ICE
candidates to the bundled "m=" lines, according to .
This section replaces the text of the following sections
of RFC 3264:
Section 5.1 (Unicast Streams).Section 8.2 (Removing a Media Stream).Section 8.4 (Putting a Unicast Media Stream on Hold).
For recvonly and sendrecv streams, the port number and address in the
offer indicate where the offerer would like to receive the media
stream. For sendonly RTP streams, the address and port number
indirectly indicate where the offerer wants to receive RTCP reports.
Unless there is an explicit indication otherwise, reports are sent to
the port number one higher than the number indicated. The IP address
and port present in the offer indicate nothing about the source IP
address and source port of RTP and RTCP packets that will be sent by
the offerer. A port number of zero in the offer indicates that the
stream is offered but MUST NOT be used. This has no useful semantics
in an initial offer, but is allowed for reasons of completeness,
since the answer can contain a zero port indicating a rejected stream
(Section 6). Furthermore, existing streams can be terminated by
setting the port to zero (Section 8). In general, a port number of
zero indicates that the media stream is not wanted.
For recvonly and sendrecv streams, the port number and address in the
offer indicate where the offerer would like to receive the media
stream. For sendonly RTP streams, the address and port number
indirectly indicate where the offerer wants to receive RTCP reports.
Unless there is an explicit indication otherwise, reports are sent to
the port number one higher than the number indicated. The IP address
and port present in the offer indicate nothing about the source IP
address and source port of RTP and RTCP packets that will be sent by
the offerer. A port number of zero in the offer by default indicates that the
stream is offered but MUST NOT be used, but an extension mechanism
might specify different semantics for the usage of a zero port value.
Furthermore, existing streams can be terminated by setting the port to
zero (Section 8). In general, a port number of zero by default indicates
that the media stream is not wanted.
A stream that is offered with a port of zero MUST be marked with port
zero in the answer. Like the offer, the answer MAY omit all
attributes present previously, and MAY list just a single media
format from amongst those in the offer.
A stream that is offered with a port of zero MUST by default be marked
with port zero in the answer, unless an extension mechanism, which
specifies semantics for the usage of a non-zero port value, is used.
RFC 2543 [10] specified that placing a user on hold was accomplished
by setting the connection address to 0.0.0.0. Its usage for putting
a call on hold is no longer recommended, since it doesn't allow for
RTCP to be used with held streams, doesn't work with IPv6, and breaks
with connection oriented media. However, it can be useful in an
initial offer when the offerer knows it wants to use a particular set
of media streams and formats, but doesn't know the addresses and
ports at the time of the offer. Of course, when used, the port
number MUST NOT be zero, which would specify that the stream has been
disabled. An agent MUST be capable of receiving SDP with a
connection address of 0.0.0.0, in which case it means that neither
RTP nor RTCP should be sent to the peer.
RFC 2543 [10] specified that placing a user on hold was accomplished
by setting the connection address to 0.0.0.0. Its usage for putting
a call on hold is no longer recommended, since it doesn't allow for
RTCP to be used with held streams, doesn't work with IPv6, and breaks
with connection oriented media. However, it can be useful in an
initial offer when the offerer knows it wants to use a particular set
of media streams and formats, but doesn't know the addresses and
ports at the time of the offer. Of course, when used, the port
number MUST NOT be zero, if it would specify that the stream has been
disabled. However, an extension mechanism might specify different
semantics of the zero port number usage. An agent MUST be capable
of receiving SDP with a connection address of 0.0.0.0, in which case it
means that neither RTP nor RTCP should be sent to the peer.
SDP Offerers and Answerers can assign
values, mid values, to SDP Media Descriptions (m= lines) within SDP Offers and Answers, using
the procedures in . Each mid value
uniquely references an m= line.
This section defines a new RTP SDES item , 'MID',
which is used to carry mid values within RTCP SDES packets. This section also defines a new
RTP header extension , which can be used to carry
the mid value in RTP packets.
The SDES item and RTP header extension makes is possible for a receiver to associate received RTCP- and
RTP packets with a specific m= line, to which the receiver has assigned a mid value, even if those m= lines
are part of the same RTP session. The endpoint informs the remote endpoint about the mid values using the
procedures in , and the remote endpoint then inserts
the mid values in RTCP- and RTP packets sent towards the other endpoint.
NOTE: This text above defines how the mid value is carried in SDP Offers and Answers. The usage of other
signalling protocols for carrying the mid value is not prevented, but the usage of such protocols is outside
the scope of this document.
The RTP MID SDES item SHOULD be sent in the first few RTCP packets sent on joining the session, and SHOULD be
sent regularly thereafter. The exact number of RTCP packets in which this SDES item is sent is
intentionally not specified here, as it will depend on the expected packet loss rate, the RTCP reporting interval,
and the allowable overhead.
The RTP MID header extension SHOULD be included in some RTP packets at the start of the session and whenever
the SSRC changes. It might also be useful to include the header extension in RTP packets that comprise random
access points in the media (e.g., with video I-frames). The exact number of RTP packets in which this header extension
is sent is intentionally not specified here, as it will depend on expected packet loss rate and loss patterns, the
overhead the application can tolerate, and the importance of immediate receipt of the mid value.
For robustness purpose, endpoints need to be prepared for situations where the mid value is delayed, and SHOULD NOT
terminate sessions in such cases, as the mid value is likely to arrive soon.
The mid value payload is UTF-8 encoded, as in SDP.
The payload, containing the mid value, of the RTP MID header extension element
can be encoded using either the one-byte or two-byte header . The mid value payload is UTF-8 encoded, as in SDP.
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number
of this document.]
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.]
This document adds the MID SDES item to the IANA "RTP SDES item
types" registry as follows:
This document defines a new extension URI in the RTP Compact Header
Extensions subregistry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data:
This specification does not significantly change the security
considerations of SDP which can be found in Section X of TBD.
TODO: Think carefully about security analysis of reuse of same SDES
key on multiple "m=" lines when the far end does not use BUNDLE and
warn developers of any risks.
The example below shows:
1. An offer, in which the offerer assigns a unique address to each bundled "m=" line within the BUNDLE group.2. An answer, in which the answerer selects the offerer BUNDLE address, and in which selects its own BUNDLE
address (the answerer BUNDLE address) and assigns it each bundled "m=" line within the BUNDLE group.3. A subsequent offer (BAS offer), which is used to perform a Bundle Address Synchronization (BAS).
The example below shows:
1. An offer, in which the offerer assigns a unique address to each bundled "m=" line within the BUNDLE group.2. An answer, in which the answerer rejects the offered BUNDLE group, and assigns a unique addresses
to each "m=" line (following normal RFC 3264 procedures).
The example below shows:
1. An offer, in which the offerer adds a new "m=" line, represented by the "zen" mid value,
to a previously negotiated BUNDLE group, assigns a unique address to the added "m=" line, and assigns
the previously selected offerer BUNDLE address to each of the other bundled "m=" lines within the BUNDLE group.2. An answer, in which the answerer assigns the answerer BUNDLE address to each bundled "m=" line
(including the newly added "m=" line) within the BUNDLE group.3. A subsequent offer (BAS offer), which is used to perform a Bundle Address Synchronization (BAS).
The example below shows:
1. An offer, in which the offerer moves a bundled "m=" line out of a BUNDLE group, assigns a unique address to
the moved "m=" line, and assigns the offerer BUNDLE address to each other bundled "m=" line within the BUNDLE group.2. An answer, in which the answerer moves the "m=" line out of the BUNDLE group, assigns unique address to
the moved "m=" line, and assigns the answerer BUNDLE address to each other bundled "m=" line within the BUNDLE group.
The example below shows:
1. An offer, in which the offerer disables a bundled "m=" line within BUNDLE group, assigns a zero port number
the disabled "m=" line, and assigns the offerer BUNDLE address to each of the other bundled
"m=" lines within the BUNDLE group.2. An answer, in which the answerer moves the disabled "m=" line out of the BUNDLE group, assigns a zero port value
to the disabled "m=" line, and assigns the answerer BUNDLE address to each of the other bundled "m=" lines within
the BUNDLE group.
This document requests IANA to register the new SDP Grouping semantic
extension called BUNDLE.
The usage of the SDP grouping extension for negotiating bundled media is
based on a similar alternatives proposed by Harald Alvestrand and Cullen
Jennings. The BUNDLE extension described in this document is based on
the different alternative proposals, and text (e.g. SDP examples)
have been borrowed (and, in some cases, modified) from those alternative
proposals.
The SDP examples are also modified versions from the ones in the Alvestrand
proposal.
Thanks to Paul Kyzivat, Martin Thompson and Flemming Andreasen for
taking the time to read the text along the way, and providing useful
feedback.
[RFC EDITOR NOTE: Please remove this section when publishing]Changes from draft-ietf-mmusic-sdp-bundle-negotiation-08
Editorial corrections.- "of"->"if" (8.3.2.5).- "optional"->"OPTIONAL" (9.1).- Syntax/ABNF for 'bundle-only' attribute added.- SDP Offer/Answer sections merged.- 'Request new offerer BUNDLE address' section addedChanges from draft-ietf-mmusic-sdp-bundle-negotiation-07
OPEN ISSUE regarding Receiver-ID closed.- RTP MID SDES Item.- RTP MID Header Extension.OPEN ISSUE regarding insertion of SDP 'rtcp' attribute in answers closed.- Indicating that, when rtcp-mux is used, the answerer MUST NOT include
an 'rtcp' attribute in the answer, based on the procedures in section 5.1.3 of
RFC 5761.Changes from draft-ietf-mmusic-sdp-bundle-negotiation-06
Draft title changed.Added "SDP" to section names containing "Offer" or "Answer".Editorial fixes based on comments from Paul Kyzivat (http://www.ietf.org/mail-archive/web/mmusic/current/msg13314.html).Editorial fixed based on comments from Colin Perkins (http://www.ietf.org/mail-archive/web/mmusic/current/msg13318.html).- Removed text about extending BUNDLE to allow multiple RTP sessions within a BUNDLE group.Changes from draft-ietf-mmusic-sdp-bundle-negotiation-05
Major re-structure of SDP Offer/Answer sections, to align with RFC 3264 structure.Additional definitions added.- Shared address.- Bundled "m=" line.- Bundle-only "m=" line.- Offerer suggested BUNDLE mid.- Answerer selected BUNDLE mid.Q6 Closed (IETF#88): An Offerer MUST NOT assign a shared address to multiple "m=" lines until it has
received an SDP Answer indicating support of the BUNDLE extension.Q8 Closed (IETF#88): An Offerer can, before it knows whether the Answerer supports the BUNDLE extension,
assign a zero port value to a 'bundle-only' "m=" line.SDP 'bundle-only' attribute section added.Connection data nettype/addrtype restrictions added.RFC 3264 update section added.Indicating that a specific payload type value can be used in multiple "m=" lines, if the value
represents the same codec configuration in each "m=" line.Changes from draft-ietf-mmusic-sdp-bundle-negotiation-04
Updated Offerer procedures (http://www.ietf.org/mail-archive/web/mmusic/current/msg12293.html).Updated Answerer procedures (http://www.ietf.org/mail-archive/web/mmusic/current/msg12333.html).Usage of SDP 'bundle-only' attribute added.Reference to Trickle ICE document added.Changes from draft-ietf-mmusic-sdp-bundle-negotiation-02
Mechanism modified, to be based on usage of SDP Offers
with both different and identical port number values, depending
on whether it is known if the remote endpoint supports the
extension.Cullen Jennings added as co-author.Changes from draft-ietf-mmusic-sdp-bundle-negotiation-01
No changes. New version due to expiration.Changes from draft-ietf-mmusic-sdp-bundle-negotiation-00
No changes. New version due to expiration.Changes from draft-holmberg-mmusic-sdp-multiplex-negotiation-00
Draft name changed.Harald Alvestrand added as co-author."Multiplex" terminology changed to "bundle".Added text about single versus multiple RTP Sessions.Added reference to RFC 3550.
A Framework for SDP Attributes when Multiplexing
Cisco
Trickle ICE: Incremental Provisioning of Candidates
for the Interactive Connectivity Establishment (ICE) Protocol
JitsiRTFM, IncGoogle
One of the main issues regarding the BUNDLE grouping extensions has been whether,
in SDP Offers and SDP Answers, the same port number value should be inserted in "m="
lines associated with a BUNDLE group, as the purpose of the extension is to negotiate
the usage of a single 5-tuple for media associated with the "m=" lines. Issues
with both approaches, discussed in the Appendix have been raised. The outcome was
to specify a mechanism which uses SDP Offers with both different and identical
port number values.
Below are the primary issues that have been considered when defining the "BUNDLE"
grouping extension:
1) Interoperability with existing UAs.2) Interoperability with intermediary B2BUA- and proxy entities.3) Time to gather, and the number of, ICE candidates.4) Different error scenarios, and when they occur.5) SDP Offer/Answer impacts, including usage of port number value zero.
NOTE: Before this document is published as an RFC, this Appendix might be removed.
Consider the following SDP Offer/Answer exchange, where Alice sends an SDP Offer to Bob:
RFC 4961 specifies a way of doing symmetric RTP but that is an a later
invention to RTP and Bob can not assume that Alice supports RFC 4961. This
means that Alice may be sending RTP from a different port than 10000 or
10002 - some implementation simply send the RTP from an ephemeral
port. When Bob's endpoint receives an RTP packet, the only way that Bob
know if it should be passed to the video or audio codec is by looking at
the port it was received on. This lead some SDP implementations to use the
fact that each "m=" line had a different port number to use that port
number as an index to find the correct m line in the SDP. As a result, some
implementations that do support symmetric RTP and ICE still use a SDP data
structure where SDP with "m=" lines with the same port such as:
will result in the second "m=" line being considered an SDP error
because it has the same port as the first line.
In an SDP Offer or SDP Answer, the media associated with an "m=" line can be
disabled/rejected by setting the port number value to zero. This is different
from e.g. using the SDP direction attributes, where RTCP traffic will
continue even if the SDP "inactive" attribute is indicated for the
associated "m=" line.
If each "m=" line associated with a BUNDLE group would contain different
port number values, and one of those port would be used for the 5-tuple,
problems would occur if an endpoint wants to disable/reject the "m=" line
associated with that port, by setting the port number value to zero. After that,
no "m=" line would contain the port number value which is used for the 5-tuple.
In addition, it is unclear what would happen to the ICE candidates associated
with the "m=" line, as they are also used for the 5-tuple.
Some back to back user agents may be configured in a mode where if
the incoming call leg contains an SDP attribute the B2BUA does not
understand, the B2BUS still generates that SDP attribute in the Offer
for the outgoing call leg. Consider an B2BUA that did not understand
the SDP "rtcp" attribute, defined in RFC 3605, yet acted this way.
Further assume that the B2BUA was configured to tear down any call
where it did not see any RTCP for 5 minutes. In this cases, if the B2BUA
received an Offer like:
It would be looking for RTCP on port 49172 but would not see any
because the RTCP would be on port 53020 and after five minutes, it would
tear down the call. Similarly, an SBC that did not understand BUNDLE yet
put BUNDLE in it's offer may be looking for media on the wrong port and
tear down the call. It is worth noting that a B2BUA that generated an
Offer with capabilities it does not understand is not compliant with the
specifications.
Sometimes intermediaries do not act as B2BUA, in the sense that
they don't modify SDP bodies, nor do they terminate SIP dialogs.
Still, however, they may use SDP information (e.g. IP address and
port) in order to control traffic gating functions, and to set
traffic policing rules. There might be rules which will trigger
a session to be terminated in case media is not sent or received
on the ports retrieved from the SDP. This typically occurs once the
session is already established and ongoing.
Sometimes intermediaries do not act as B2BUA, in the sense that
they don't modify SDP bodies, nor do they terminate SIP dialogs.
Still, however, they may use SDP information (e.g. codecs and
media types) in order to control bandwidth allocation functions.
The bandwidth allocation is done per "m=" line, which means that
it might not be enough if media associated with all "m=" lines
try to use that bandwidth. That may either simply lead to bad
user experience, or to termination of the call.
When using ICE, an candidate needs to be gathered for each port. This
takes approximately 20 ms extra for each extra "m=" line due to the NAT
pacing requirements. All of this gather can be overlapped with other
things while the page is loading to minimize the impact. If the client
only wants to generate TURN or STUN ICE candidates for one of the "m="
lines and then use trickle ICE
to get the non host ICE candidates for the rest of the "m=" lines, it MAY do
that and will not need any additional gathering time.
Some people have suggested a TURN extension to get a bunch of TURN
allocation at once. This would only provide a single STUN result so in
cases where the other end did not support BUNDLE, may cause more use of
the TURN server but would be quick in the cases where both sides
supported BUNDLE and would fall back to a successful call in the other
cases.