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", that
can be used with the Session Description Protocol (SDP) Offer/Answer mechanism
to negotiate the usage of bundled media, which refers to the usage of a single 5-tuple for
sending and receiving media associated with multiple SDP media descriptions ("m=" lines).
This specification also updates sections 5.1, 8.1 and 8.2 of RFC 3264, in order to allow
an answerer to in an SDP answer assign a non-zero port value to an "m=" line, even if
the offerer in the associated SDP offer had assigned a zero port value to the "m=" line.
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. Normally different media types (audio, video etc) will be described
using different media descriptions.
This specification defines a new SDP Grouping Framework extension, "BUNDLE", that can be used with the Session Description
Protocol (SDP) Offer/Answer mechanism
to negotiate the usage of bundled media, which refers to the usage of a single 5-tuple for
sending and receiving media associated with multiple SDP media descriptions ("m=" lines).
The offerer and answerer use
the BUNDLE mechanism to negotiate 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.
SDP bodies can contain multiple BUNDLE groups. Each BUNDLE group MUST use a unique 5-tuple.
Any given "m=" line can 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 mechanism is backward compatible. Endpoints that do not support the BUNDLE mechanism
are expected to generate SDP offers and SDP answers without an SDP 'group:BUNDLE' attribute, and
are expected to assign a unique address to each "m=" line within an SDP offer and SDP answer, according
to the procedures in and
This specification also updates sections 5.1, 8.1 and 8.2 of
, in order to allow
an answerer to assign a non-zero port value to an "m=" line in an SDP answer, even if
the offerer in the associated SDP offer had assigned a zero port value to the "m=" line.
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
a single "m=" line in an SDP offer or SDP answer.
Shared address: An IP address and IP port combination that is assigned to
multiple "m=" lines in an SDP offer or SDP answer.
Offerer suggested BUNDLE mid: The first mid value in a given
SDP 'group:BUNDLE' attribute mid list in an SDP offer.
Answerer selected BUNDLE mid: The first mid value in a given
SDP 'group:BUNDLE' attribute mid list in an SDP 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 a single 5-tuple is used 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, in an SDP offer or SDP answer, associated with
a BUNDLE group.
Bundle-only "m=" line: An "m=" line, to which an SDP 'bundle-only' attribute
has been assigned.
Bundled media: All media associated with a BUNDLE group.
Initial SDP offer: The first SDP offer, within an SDP session, in which
the offerer indicates that it wants to create a given BUNDLE group.
Subsequent SDP offer: An SDP 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 indicated using an SDP session-level 'group'
attribute. Each SDP Media Description ("m=" line) that is grouped together,
using SDP media-level mid attributes, belongs to a given BUNDLE
group.
This section describes usage of the SDP offer/answer mechanism
for negotiating
usage of the BUNDLE mechanism, for creating a BUNDLE group, for selecting the
BUNDLE addresses (offerer BUNDLE address and answerer BUNDLE address),
for adding an "m=" line to a BUNDLE group, for moving an "m=" line out of a
BUNDLE group, and for disabling an "m=" line within a BUNDLE group.
The generic rules and procedures defined in and
also apply to the BUNDLE mechanism. For example, if an SDP 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 SDP offer in which the offerer wants to create a BUNDLE group,
and the answerer rejects the SDP 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 .
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 total proposed bandwidth is the sum of the proposed bandwidth for each
bundled "m=" line.
defines
rules and restrictions for assigning different types of SDP attributes
to a bundled "m=" line.
When an offerer generates an initial SDP offer, in order to create a
BUNDLE group, the offerer MUST in the SDP offer assign a unique address
to each "m=" line with a non-zero port value, following the procedures
in .
The offerer MUST in the SDP offer insert an SDP session level 'group:BUNDLE'
attribute, associated with the BUNDLE group, and assign an SDP 'mid'
attribute to each
"m=" line that the offerer wants to be within the BUNDLE group,
and place the 'mid' attribute value in the 'group:BUNDLE' attribute mid list.
[] shows
an example of an initial SDP offer.
When an offerer generates an initial SDP offer, in order to create a
BUNDLE group, the offerer MUST in the SDP offer indicate which unique address,
associated with one of the "m=" lines that the offerer wants to be within the
BUNDLE group, that the offerer wants the answerer to select as the offerer
BUNDLE address [].
In the SDP offer, the offerer BUNDLE mid value represents that address.
When an answerer generates an SDP answer, the following restrictions, defined
in , also apply a
BUNDLE group:
1) The answerer MUST NOT in the SDP answer include a BUNDLE group, unless the
offerer in the associated SDP offer requested the BUNDLE group to be created; and
2) The answerer MUST NOT in the SDP answer include an "m=" line within a BUNDLE
group, unless the offerer in the associated SDP offer requested the "m=" line to
be within the BUNDLE group.
When an answerer generates an SDP answer, it MUST select a BUNDLE address for the offerer, referred to
as the offerer BUNDLE address. The answerer MUST select an address which the offerer
in the associated SDP offer requested to be within the BUNDLE group.
In the SDP offer, the offerer suggested BUNDLE mid represents the "m=" line
to which the offerer in the SDP offer has assigned the address that it wants
the answerer to select as the offerer BUNDLE address []. The answerer MUST first select the "m=" line
associated with the offerer suggested BUNDLE mid, and check whether it fulfils the
following criteria:
The answerer will in the SDP answer create the BUNDLE group;
The answerer will not in the SDP answer move the "m=" line out of the BUNDLE group
[];
The answerer will not in the SDP answer reject the "m=" line []; and
The offerer did not in the associated SDP offer assign a zero port value to the "m=" line.
If all of the criteria above is fulfilled, the answerer MUST select the address
associated with the "m=" line as 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 the mid value.
In the SDP answer, the answerer selected BUNDLE mid value represents the "m=" line
which address (in the associated SDP offer) the answerer has selected as the offerer
BUNDLE address.
[] shows an
example of an offerer BUNDLE address selection.
When an answerer generates an SDP answer, the answerer MUST select a BUNDLE address for itself,
referred to as the answerer BUNDLE address, and in the SDP answer assign the answerer
BUNDLE address to each "m=" line within the created BUNDLE group.
The answerer MUST NOT in the SDP answer assign the answerer BUNDLE address to an "m="
line that is not associated with the BUNDLE group, or to an "m=" line that is
associated with another BUNDLE group.
The answerer is allowed to select a new answerer BUNDLE address in every SDP answer that
the answerer generates.
[] shows an example of an
answerer BUNDLE address selection.
When an answerer generates an SDP answer, in which the answerer moves a
bundled "m=" line out a BUNDLE group, the answerer assigns an address
to the moved "m=" line based on the type of address that the
offerer in the associated SDP offer assigned to the "m=" line.
If the offerer in the SDP offer has assigned a shared address (e.g. a previously
selected offerer BUNDLE address) to the "m=" line, the answerer MUST in the SDP answer reject the moved
"m=" line, according to the procedures in [].
If the offerer in the SDP offer assigned an SDP 'bundle-only' attribute to the "m=" line,
the answerer MUST in the SDP answer reject the moved "m=" line, according to the procedures
in [].
If the offerer in the SDP offer assigned a unique address to the "m=" line, the answerer
MUST in the SDP answer assign a unique address to the moved "m=" line.
In addition, in either case above, the answerer MUST NOT in the SDP answer
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 generates an SDP answer, in which the answerer rejects
an "m=" line, the answerer MUST in the SDP answer assign an address with a zero port
value to the rejected "m=" line, according to the procedures in
.
In addition, the answerer MUST NOT in the SDP answer
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 SDP answer, the offerer MUST apply the selected
offerer BUNDLE address to each bundled "m=" line. If the offerer generates
a subsequent SDP offer, the offerer MUST in the SDP offer assign the offerer
BUNDLE address to each bundled "m=" line (including any 'bundle-only' "m=" line)
[].
If the SDP answer does not contain a BUNDLE group, the offerer MUST cease
to use any procedure associated with the BUNDLE mechanism.
If the selected offerer BUNDLE address is different than the address that the offerer
in the associated SDP offer assigned to a bundled "m=" line (including an "m=" line that the
offerer in the SDP offer added to an existing BUNDLE group []), and the bundled "m=" line was not rejected [], or moved out of the BUNDLE group [] by the answerer, the offerer SHOULD as soon as possible generate a
subsequent SDP offer, in which the offerer assigns the offerer BUNDLE address to each bundled "m=" line.
This procedure is referred to as Bundle Address Synchronization (BAS), and the SDP offer is referred
to as a BAS Offer.
The offerer MAY in the BAS offer modify any SDP parameter.
NOTE: It is important that the BAS offer gets accepted by the
answerer. For that reason the offerer needs to consider the
necessity to in the BAS offer modify SDP parameters that could
get 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 mechanism, have
correct information regarding the address is going to be used to
transport the bundled media.
[]
shows an example where an offerer sends a BAS offer.
When an offerer generates a subsequent SDP offer,
the offerer MUST in the SDP offer assign the previously
selected offerer BUNDLE address [] to each bundled "m=" line (including any
bundle-only "m=" line), unless the offerer in the SDP offer
moves the "m=" line out of the BUNDLE group [], or disables the "m=" line
[].
If the SDP offerer in the SDP offer adds an "m=" line to the
BUNDLE group [], the offerer MAY assign the
previously selected offerer BUNDLE address to the added "m=" line.
In addition, the offerer MUST in the SDP offer indicate which
address (unique or previously selected offerer BUNDLE address)
it wants the answerer to select as the offerer BUNDLE
address, following the procedures in []. The offerer MUST
do this even if the offerer in the SDP offer assigns a
previously selected offerer BUNDLE address to each bundled
"m=" line.
When an offerer generates an SDP offer, in which the offerer wants to add an
"m=" line to a BUNDLE group, the offerer assigns in the SDP offer an address
(unique or previously selected offerer BUNDLE address) to the "m=" line, assigns an SDP 'mid'
attribute to the "m=" line, and places the mid value in the SDP 'group:BUNDLE' attribute mid
list associated with the BUNDLE group [].
NOTE: If the offerer wants the answerer to select the address
associated with the added "m=" as the offerer BUNDLE address, the
offerer suggested BUNDLE mid MUST represent the added "m=" line
[].
[]
shows an example where an offerer sends an SDP offer in order to
add an "m=" line to a BUNDLE group.
When an offerer generates an SDP offer, in which the offerer wants to move
a bundled "m=" line out of a BUNDLE group, the offerer
MUST assign a unique address to the "m=" line.
In addition, the offerer MUST NOT place a mid value associated
with the "m=" line in the SDP 'group:BUNDLE' attribute mid
list associated with the BUNDLE group.
NOTE: The offerer MAY keep a previously assigned SDP 'mid' attribute in an "m=" line
that it wants to move out of a BUNDLE group, e.g. if the mid value is
used for some other SDP grouping extension than BUNDLE.
[]
shows an example where an offerer sends an SDP offer in order to
move an "m=" line out of a BUNDLE group.
When an offerer generates an SDP offer, in which the offerer wants to
disable a bundled "m=" line, the offerer MUST assign an address with
a zero port alue to the "m=" line, following the procedures
in .
In addition, the offerer MUST NOT place a mid value associated
with the "m=" line in the SDP 'group:BUNDLE' attribute mid
list associated with the BUNDLE group.
NOTE: The offerer MAY assign an SDP 'mid' attribute to an "m=" line
that it wants to disable, e.g. if the mid value is
used for some other SDP grouping extension than BUNDLE.
[]
shows an example where an offerer sends an SDP offer in order to
disable an "m=" line within a BUNDLE group.
This section defines a new SDP media-level attribute , 'bundle-only'. An offerer can in an SDP offer assign a 'bundle-only'
"m=" line to a bundled "m=" line (including an "m=" line that the offerer wants
to add to the BUNDLE group []), in order to ensure that the answerer only accepts the "m=" line
if the answerer supports the BUNDLE mechanism, and if the answerer in the SDP
answer keeps the "m=" line within the BUNDLE group.
When an offerer generates an initial SDP offer, in order to create a BUNDLE group, the offerer
can in the SDP offer assign an SDP 'bundle-only' attribute to an "m=" line
that the offerer wants to be within the BUNDLE group.
The offerer MUST in the SDP offer assign a zero port value the bundle-only "m="
line.
When the answerer selects the offerer BUNDLE address [], the answerer MUST also take a bundle-only "m=" line
with a non-zero port value into consideration.
If the offerer in the SDP offer has assigned a zero port value to a bundle-only "m="
line, and if the answerer accepts the "m=" line, the answerer will treat the
"m=" line as any other bundle "m=" line when the answerer generates the SDP answer
[].
NOTE: If the offerer in the SDP offer has assigned a zero port value to a bundled "m=" line,
but the offerer has not assigned a 'bundle-only' SDP attribute to the "m=" line, it is an
indication that the offerer wants to disable the "m=" line
[].
If the answerer in the SDP answer does not keep the bundle-only "m=" line within
the BUNDLE group, the answerer MUST in the SDP answer reject the "m=" line
[].
The answerer MUST NOT in the SDP answer assign an SDP 'bundle-only' attribute to
an "m=" line (even if the offerer in the associated SDP offer has assigned a 'bundle-only'
attribute to the "m="line).
When the offerer receives an SDP answer, the offerer follows the procedures in
[]. If the offerer
in the associated SDP offer assigned an SDP 'bundle-only' attribute to an "m=" line,
and the "m=" line was accepted (and was kept within the BUNDLE group) by the answerer,
the selected offerer BUNDLE address also applies to the "m=" line.
When an offerer creates a subsequent SDP offer, the offerer follows the
procedures in [].
If the offerer in the SDP offer assigns an SDP 'bundle-only' attribute to
a bundled "m=" line, in order to ensure that the answerer accepts the "m=" line
only if the answerer keeps the "m=" line within the BUNDLE group, the offerer
MUST NOT assign a zero port value to the "m=" line. Instead, the offerer MUST
in the SDP offer assign the offerer BUNDLE address or, if the "m=" line is added to the
BUNDLE group [],
either the offerer BUNDLE address or a unique address, to the "m=" line.
NOTE: The offerer can in a subsequent SDP offer assign an
SDP 'bundle-only' attribute to a bundled "m=" line even if the
offerer did not assign a 'bundle-only' attribute to the
"m=" line in a previous SDP offer.
If the offerer in the SDP offer wants to move a bundled
"m=" line out of a BUNDLE group [], the offerer MUST NOT in
the SDP offer assign a 'bundle-only' attribute to the "m=" line.
If the offerer in the SDP offer wants to disable a
bundled "m=" line [], the offerer MUST NOT in the SDP offer assign
a 'bundle-only' attribute to the "m=" line.
If bundled "m=" lines represent different transport protocols, there MUST
exist a 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 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 SDP 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 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 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 SDP offer and answer inform each other which SSRC values they will use
inside sent RTP 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 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 SDP answer
providing that information. Due to this, before the offerer has received the SDP answer, the offerer will
not be able to associate received RTP 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 packets with the correct
"m=" line, the offerer and answerer MUST in an SDP offer and answer assign an SDP "receiver-id" attribute
[receiver-id-reference-to-be-added] to each bundled "m=" line which contains a payload type value that
is also used inside another bundled "m=" line. If an answerer accepts such "m=" line, and keeps it within
the BNDLE group, the answerer MUST insert the 'receiver-id' attribute value in RTP packets, associated
with the "m=" line, sent towards the offerer.
OPEN ISSUE: We need a mechanism that implements the 'receiver-id' mechanism and the associated SDP attribute.
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 SDP offer, if the offerer wants to negotiate usage
of RTP/RTCP multiplexing within a BUNDLE group, the offerer MUST in the SDP offer assign
an SDP 'rtcp-mux' attribute to
each bundled "m=" line (including any bundle-only "m=" line). In addition, the offerer
MUST in the SDP offer 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.
If the offerer does not want to negotiate usage of RTP/RTCP multiplexing, the offerer MUST
NOT assign the SDP attributes above to any bundled "m=" line.
When the answerer generates an SDP answer to an initial SDP offer,
if the offerer in the associated SDP offer indicated support of RTP/RTCP
multiplexing within
a BUNDLE group, the answerer MUST in the SDP answer either accept or reject usage
of RTP/RTCP multiplexing.
If the answerer accepts usage of RTP/RTCP multiplexing within the BUNDLE group, the answerer
MUST in the SDP answer assign an SDP 'rtcp-mux' attribute to each bundled "m=" line. The
answerer MUST NOT in the SDP answer assign an SDP 'rtcp' attribute to any bundled
"m=" line.
OPEN ISSUE: Do we want to include the SDP 'rtcp' attribute also in the SDP answer, eventhough it
is not needed?
If the answerer rejects usage of RTP/RTCP multiplexing within the BUNDLE group, the answerer
MUST NOT in the SDP answer assign an SDP 'rtcp-mux' or SDP 'rtcp' attribute to any bundled "m=" line.
When the answerer generates an SDP answer to a subsequent SDP offer, if the offerer in the associated SDP offer
indicated support of RTP/RTCP multiplexing within
a BUNDLE group, the answerer MUST in the SDP answer assign an SDP 'rtcp-mux' attribute and SDP 'rtcp' attribute
to each bundled "m=" line.
NOTE: The BUNDLE mechanism does not allow the answerer to, in a subsequent SDP answer, disable
usage of RTP/RTCP multiplexing, if the offerer in the associated SDP offer indicates that it
wants to continue using RTP/RTCP multiplexing.
When the offerer receives an SDP answer, it follows the procedures
defined in .
When an offerer generates a subsequent SDP offer, if the offerer wants
to negotiate usage of RTP/RTCP multiplexing within a BUNDLE group, or
if the offerer wants to continue usage of previously negotiated RTP/RTCP
multiplexing within the BUNDLE group, the offerer MUST in the SDP offer
assign 'rtcp-mux' and 'rtcp' attributes to each bundled "m=" line (including
bundle-only "m=" lines), unless the "m=" line is disabled or removed from
the BUNDLE group.
If the offerer does not want to negotiate usage of RTP/RTCP multiplexing
within the BUNDLE group, or if the offerer wants to disable previously
negotiated usage of RTP/RTCP multiplexing within a BUNDLE group, the offerer
MUST NOT in the SDP offer assign 'rtcp-mux' and 'rtcp' attributes to any
bundled "m=" line.
NOTE: It is RECOMMENDED that, once usage of RTP/RTCP multiplexing has been negotiated within a BUNDLE group,
that the usage of 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 .
Support and usage of ICE mechanism together with the BUNDLE mechanism is optional.
When an offerer generates an initial SDP offer, which contains a BUNDLE group, the
offerer MUST assign ICE candidates
to each bundled "m=" line, except to an "m=" line to which the offerer assigns
a zero port value (e.g. a bundle-only "m=" line). The offerer MUST assign unique
ICE candidate values to each "m=" line.
When an answerer generates and SDP Answer, which contains a BUNDLE group,
the answerer MUST assign ICE candidates to each bundled "m=" line. The answerer
MUST assign identical ICE candidate values to each bundled "m=" line.
When the offerer receives an SDP answer, it follows the procedures
defined in .
When an offerer generates a subsequent SDP offer, for each bundled
"m=" line to which the offerer assigns its BUNDLE address, the offerer
MUST assign identical ICE candidate values. The offerer MUST assign
the ICE candidate values associated with the "m=" line that was used
by the answerer to select the offerer BUNDLE address [ref-to-be-added].
Once it is known that both endpoints support, and accept to use, the BUNDLE grouping
extension, ICE connectivity checks and keep-alives only needs to be performed for the
whole BUNDLE group, instead of for each bundled "m=" line.
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.
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 SDP offer, in which the offerer assigns a unique address to each bundled "m=" line within the BUNDLE group.2. An SDP 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 SDP offer (BAS offer), which is used to perform a Bundle Address Synchronization (BAS).
The example below shows:
1. An SDP offer, in which the offerer assigns a unique address to each bundled "m=" line within the BUNDLE group.2. An SDP 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 SDP 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 SDP 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 SDP offer (BAS offer), which is used to perform a Bundle Address Synchronization (BAS).
The example below shows:
1. An SDP 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 SDP 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 SDP 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 SDP 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 mechanism 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 and Martin Thompson for taking the 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-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 mechanism.Q8 Closed (IETF#88): An Offerer can, before it knows whether the Answerer supports the BUNDLE mechanism,
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.