A Session Initiation Protocol (SIP) usage for Trickle ICE
JitsiStrasbourg67000France+33 6 72 81 15 55emcho@jitsi.orgUnaffiliatedVienna1130Austriathomass.stach@gmail.comTelecom ItaliaVia G. Reiss Romoli, 274Turin10148Italyenrico.marocco@telecomitalia.itEricssonHirsalantie 1102420JorvasFinlandchrister.holmberg@ericsson.com
The Interactive Connectivity Establishment (ICE) protocol
describes a Network Address Translator (NAT) traversal mechanism
for UDP-based multimedia sessions established with the
Offer/Answer model. The ICE extension for Incremental
Provisioning of Candidates (Trickle ICE) defines a mechanism
that allows ICE Agents to shorten session establishment delays
by making the candidate gathering and connectivity checking
phases of ICE non-blocking and by executing them in parallel.
This document defines usage semantics for Trickle ICE with the
Session Initiation Protocol (SIP) and defines
a new Info Package as specified in .
The Interactive Connectivity Establishment protocol
describes
a mechanism for NAT traversal that consists of three
main phases: a phase where an agent gathers a set of candidate
transport addresses (source IP address, port and transport
protocol), a second phase where these candidates are sent to a
remote agent. There, this gathering procedure is repeated and,
finally, a third phase starts where connectivity between all candidates
in both sets is checked (connectivity checks). Once these phases
have been completed, and only then, both agents can begin
communication. According to
the three phases above happen consecutively, in a blocking way,
which can introduce undesirable latency during session
establishment.
The Trickle ICE extension
defines generic
semantics required for these ICE phases to happen
simultaneously, in a non-blocking way and hence speed up session
establishment.
This specification defines a usage of Trickle ICE with
the Session Initiation Protocol (SIP).
It describes how ICE
candidates are to be exchanged incrementally with SIP INFO
requests
and how the Half Trickle and Full Trickle modes defined in
are to be used by
SIP User Agents (UAs) depending on their expectations for
support of Trickle ICE by a remote agent.
This document defines a new Info Package as specified in
for use with Trickle ICE.
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 .
This specification makes use of all terminology defined by the
protocol for Interactive Connectivity Establishment in
and its Trickle ICE extension
. It is assumed that
the reader will be familiar with the terminology from both documents.
When using ICE for SIP according to
the ICE candidates are exchanged solely via
SDP Offer/Answer as per .
This specification defines an additional mechanism
where candidates can be exchanged using SIP INFO messages
and a newly defined Info Package .
This allows ICE
candidates also to be sent in parallel to an ongoing Offer/Answer
negotiation and/or after the completion of the Offer/Answer
negotiation.
Typically, in cases where Trickle ICE is fully supported,
the Offerer would send an INVITE request
containing a subset of candidates.
Once an early dialog is established
the Offerer can continue sending
candidates in INFO requests within that dialog.
Similarly, an Answerer can send
ICE candidates using INFO requests within
the dialog established by its 18x provisional response.
shows such a sample
exchange:
In order to benefit from Trickle ICE's full potential and
reduce session establishment latency to a minimum, Trickle ICE
agents need to generate SDP Offers and Answers that contain
incomplete, potentially empty sets of candidates. Such Offers
and Answers can only be handled meaningfully by agents that
actually support incremental candidate provisioning, which
implies the need to confirm such support before actually using
it.
Contrary to other protocols, like XMPP
, where "in advance" capability
discovery is widely implemented, the mechanisms that allow this
for SIP (i.e., a combination of UA Capabilities
and GRUU )
have only seen low levels of adoption. This presents an issue
for Trickle ICE implementations as SIP UAs do not have an
obvious means of verifying that their peer will support
incremental candidate provisioning.
The Half Trickle mode of operation defined in the Trickle
ICE specification
provides one way around this, by requiring the first Offer to
contain a complete set of local ICE candidates
and only using
incremental provisioning of remote candidates
for the rest of the session.
While using Half Trickle does provide a working solution it
also comes at the price of increased latency.
therefore makes several alternative
suggestions that enable SIP UAs to engage in Full Trickle
right from their first Offer:
discusses the use of on-line provisioning as a means of
allowing use of Trickle ICE for all endpoints in controlled
environments. describes
anticipatory discovery for implementations that actually do
support GRUU and UA Capabilities and
discusses the implementation
and use of Half Trickle by SIP UAs where none of the above
are an option.
From the
perspective of all SIP middle boxes and proxies, and with the
exception of the actual INFO messages, signaling in general
and Offer/Answer exchanges in particular would look the same
way for Trickle ICE as they would for ICE for SIP
.
From an architectural viewpoint, as displayed in
, exchanging candidates
through SIP INFO requests could be represented as signaling
between ICE Agents and not between Offer/Answer modules of
SIP User Agents. Then, such INFO requests
do not impact the state of the Offer/Answer transaction other
than providing additional candidates.
Consequently, INFO requests are not considered Offers or Answers.
Nevertheless, candidates that have been exchanged using INFO
SHALL be included in subsequent Offers or Answers.
The version number in the "o=" line of that subsequent offer would
need to be incremented by 1 per the rules
in .
Trickle ICE Agents will construct Offers and Answers with
ICE descriptions compliant to
and
the following additional SIP-specific additions:
Trickle ICE Agents MUST indicate support for Trickle ICE by
including the SIP option-tag 'trickle-ice' in a SIP Supported: header field
within all SIP INVITE requests and responses.
Trickle ICE Agents MUST indicate support for Trickle ICE by
including the ice-option 'trickle'
within all SDP Offers and Answers in accordance to
.
Trickle ICE Agents MAY include any number of ICE candidates,
i.e. from zero to the complete set of candidates,
in their initial Offer or Answer.
If the complete candidate set is included already
in the initial Offer, this is called Half-Trickle.
Trickle ICE Agents MAY exchange additional ICE candidates using INFO requests
within an existing INVITE dialog usage (including an early dialog)
as specified in .
The INFO requests carry an Info-Package: trickle-ice.
Trickle ICE Agents MUST be prepared to receive INFO requests
within that same dialog usage,
containing additional candidates or an indication for the end of such candidates.
Trickle ICE Agents MAY exchange additional ICE candidates
before the Answerer has sent the Answer provided that
an invite dialog usage is established at both Trickle ICE Agents.
Note that in case of forking multiple early dialogs will exist.
The following sections provide further details on how
Trickle ICE Agents perform the initial Offers/Answers exchange
and establish the INVITE dialog usage such that they can trickle candidates.
If the Offerer includes candidates in its initial Offer,
it MUST encode these candidates as specified in
.
If the Offerer wants to send its initial Offer
before knowing any candidate of one or more media descriptions,
it MUST include the following default values
in the corresponding "m=" line.
The media field is set to 'audio'.
The port value is set to '9'.
The proto value is set to 'RTP/AVP'.
In this case, the Offerer obviously cannot know the RTCP transport address and,
thus, MUST NOT include the "a=rtcp" attribute .
This avoids potential ICE mismatch
(see ) for the RTCP transport address.
If the Offerer wants to use RTCP multiplexing
and/or exclusive RTCP multiplexing
,
it still MUST include the "a=rtcp-mux" and/or "a=rctp-mux-only" attribute
in the initial Offer.
In any case, the Offerer MUST include
the attribute a=ice-options:trickle in accordance to
.
If the initial Offer included candidates,
the Answerer MUST treat these candidates as specified in
.
If the initial Offer included the attribute a=ice-options:trickle,
the Answerer MUST be prepared for receiving trickled candidates later on.
In case of a "m=" lines with default values
neither of the eventually trickled candidates
will match the default destination.
This situation MUST NOT cause an ICE mismatch
(see ).
Section applies to the Answerer
with the roles of Offerer and Answer being swapped.
Section applies to the Answerer
with the roles of Offerer and Answer being swapped.
In order to be able to start trickling, the
following two conditions need to be satisfied at the SIP UAs:
Trickle ICE support at the peer agent MUST be confirmed.
The dialog at both peers MUST be in early or confirmed state.
discusses in detail the various options
for satisfying the first of the above conditions. Regardless
of those mechanisms, however, agents are certain to have a
clear understanding of whether their peers support trickle
ICE once an Offer and an Answer have been exchanged,
which also allows for ICE processing to commence
(see ).
As shown in
satisfying both conditions is relatively trivial for
ICE Agents that have sent an Offer in an INVITE and that have
received an Answer in a reliable provisional response.
It is guaranteed to have confirmed support for
Trickle ICE at the Answerer (or lack thereof) and to have
fully initialized the SIP dialog at both ends.
Offerers and Answerers (after receipt of the PRACK request)
in the above situation can therefore
freely commence trickling within the newly established dialog.
The situation is a bit more delicate for agents that have
received an Offer in an INVITE request and have sent an Answer
in an unreliable provisional response because, once the
response has been sent, the Answerer does not
know when or if it has been received
().
In order to clear this ambiguity as soon as possible,
the Answerer needs to retransmit the provisional response
with the exponential back-off timers described in
.
These retransmissions MUST cease on receipt of an INFO request
or on transmission of the Answer in a 2xx response.
This is similar to the procedure described in section
8.1.1 of except that
the STUN binding Request is replaced by the INFO request.
[RFC EDITOR NOTE: The section 8.1.1 in above sentence is correct for version 14 of said I-D.
Please cross-check since it could have have changed in the meantime.]
The Offerer MUST send a Trickle ICE INFO request as soon as
it receives an SDP Answer in an unreliable provisional
response. This INFO request MUST repeat the candidates
that were already provided in the Offer (as would be the case
when Half Trickle is performed or when new candidates have not
been learned since then) and/or they MAY also deliver
newly learned candidates (if available).
The Offerer MAY include an end-of-candidates attribute
in case candidate discovery has ended in the mean time.
As soon as an Answerer has received such an INFO request,
the Answerer has an indication that a dialog is established
at both ends and MAY begin trickling
().
Note: The +SRFLX in
indicates that additionally newly learned server-reflexive candidates are included.
When sending the Answer in the 200 OK response to the INVITE request,
the Answerer MUST repeat
exactly the same Answer that was previously sent
in the unreliable provisional
response in order to fulfill the corresponding requirements in
.
Thus, the Offerer needs to be prepared
for receiving a different number of candidates
in that repeated Answer than previously exchanged via trickling
and MUST ignore the candidate information
in that 200 OK response.
The possibility to convey arbitrary candidates in INFO
message bodies allows ICE Agents to initiate trickling
without actually sending an Answer.
Trickle ICE Agents MAY therefore respond to an INVITE request
with provisional responses without an SDP Answer.
Such provisional responses serve for establishing an early dialog.
Agents that choose to establish the dialog in this way,
MUST retransmit these responses
with the exponential back-off timers described in
.
These retransmissions MUST cease on receipt of an INFO request
or on transmission of the Answer in a 2xx response.
This is again similar to the procedure described in section
8.1.1 of
except that an Answer is not yet provided.
[RFC EDITOR NOTE: The section 8.1.1 in above sentence is correct for version 14 of said I-D.
Please cross-check since it could have have changed in the meantime.]
Note: The +SRFLX in
indicates that additionally newly learned server-reflexive candidates are included.
When sending the Answer, the agent MUST repeat all currently
known and used candidates, if any,
and MAY include all newly gathered candidates since the last INFO request was sent.
If that Answer was sent in a unreliable provisional response,
the Answerers MUST repeat
exactly the same Answer in the 200 OK response to the INVITE request
in order to fulfill the corresponding requirements in
.
In case that trickling continued,
an Offerer needs to be prepared for receiving fewer candidates
in that repeated Answer than previously exchanged via trickling
and MUST ignore the candidate information in that 200 OK response.
Agents that have sent an Offer in a reliable provisional
response and that receive an Answer in a PRACK
are also in a situation where support for
Trickle ICE is confirmed and the SIP dialog is guaranteed
to be in a state that would allow in-dialog INFO requests
(see ).
Trickle Agents that send an Offer in a 200 OK and
receive an Answer in an ACK can still create a
dialog and confirm support for Trickle ICE
by sending an unreliable provisional response
similar to .
According to , this unreliable response
MUST NOT contain an Offer.
The Trickle Agent (at the UAS) retransmits the provisional response
with the exponential back-off timers described in
.
Retransmits MUST cease on receipt of an INFO request
or on transmission of the Answer in a 2xx response.
The peer Trickle Agent (at the UAC) MUST send a Trickle ICE INFO request
as soon as they receive an unreliable provisional response
(see ).
Whenever new ICE candidates become available for sending,
agents would encode them in "a=candidate" lines as described
by . For example:
The use of SIP INFO requests happens within the context of the
Info Package as defined .
The Media Type
for their payload MUST be set to
'application/trickle-ice-sdpfrag' as defined in
.
Since neither the "a=candidate" nor the "a=end-of-candidates"
attributes contain information that would allow correlating them to
a specific "m=" line,
this is handled through the use of
pseudo "m=" lines and
identification tags in "a=mid:" attributes as defined in
.
Pseudo "m=" lines follow the SDP syntax for "m=" lines as defined in
, but provide no semantics other than
indicating to which "m=" line a candidate belongs.
Consequently, the receiving agent MUST ignore any remaining content of the pseudo "m=" line,
which is not defined in this document.
This guarantees that the 'application/trickle-ice-sdpfrag' bodies do not interfere with the Offer/Answer
procedures as specified in .
When sending the INFO request, the agent MAY,
if already known to the agent, include the same content into
the pseudo "m=" line as for the "m=" line in the corresponding Offer or Answer.
However, since Trickle-ICE might be decoupled from the Offer/Answer negotiation this content might
be unknown to the agent. In this case, the agent MUST include the following default values.
The media field is set to 'audio'.
The port value is set to '9'.
The proto value is set to 'RTP/AVP'.
The fmt SHOULD appear only once and is set to '0'
Agents MUST include a pseudo "m=" line and an
identification tag in a "a=mid:" attribute for every "m=" line
whose candidate list they intend to update.
Such "a=mid:" attributes MUST
immediately precede the list of candidates for that specific
"m=" line. All "a=candidate" or "a=end-of-candidates" attributes
following an "a=mid:" attribute, up until (and excluding) the next
occurrence of a pseudo "m=" line, pertain to the "m=" line
identified by that identification tag.
An "a=end-of-candidates" attribute, preceding
any pseudo "m=" line, indicates the end of all trickling from that
agent,
as opposed to end of trickling for a specific "m=" line,
which would be indicated by a media level
"a=end-of-candidates" attribute.
Refer to
for an example of the INFO request content.
The use of pseudo "m=" lines allows for a structure similar to
the one in SDP Offers and Answers where
separate media-level and session-level sections can be distinguished.
In the current case, lines preceding any pseudo "m=" line are considered
to be session-level. Lines appearing in between or after
pseudo "m=" lines will be interpreted as media-level.
Note that while this specification uses the "a=mid:"
attribute from , it does not
define any grouping semantics. Consequently, the
"a=group:" attribute from that same specification is neither
needed nor used in Trickle ICE for SIP.
All INFO requests MUST carry the "a=ice-pwd:" and "a=ice-ufrag:"
attributes that would allow mapping them to a specific ICE generation.
An agent MUST discard any received INFO requests containing "a=ice-pwd:" and "a=ice-ufrag:"
attributes that do not match those of the current ICE Negotiation Session.
The "a=ice-pwd:" and "a=ice-ufrag:" attributes MUST appear at the same level
as the ones in the Offer/Answer exchange. In other words, if they were present
as session-level attributes, they will also appear
at the beginning of all INFO request payloads, i.e. preceding all
pseudo "m=" lines. If they were originally exchanged as media
level attributes, potentially overriding session-level values,
then they will also be included in INFO request payloads
following the corresponding pseudo "m=" lines.
Note that requires that
when candidates are trickled, each candidate MUST be delivered
to the receiving Trickle ICE implementation not more than once
and in the same order as it was conveyed.
If the signaling protocol provides any candidate retransmissions,
they need to be hidden from the ICE implementation.
This requirement is fulfilled as follows.
Since the agent is not fully aware of the state of the ICE Negotiation Session at its peer
it MUST include all currently known and used local candidates in every INFO request.
I.e. the agent MUST repeat in the INFO request body
all candidates that were previously sent under the same
combination of "a=ice-pwd:" and "a=ice-ufrag:"
in the same order as they were gathered.
In other words, the sequence of a previously sent
list of candidates MUST NOT change in subsequent INFO requests
and newly gathered candidates MUST be added at the end of that list.
Although repeating all candidates creates some overhead, it also allows easier handling of problems
that could arise from unreliable transports, like e.g. loss of messages and reordering,
which can be detected through the CSeq: header field in the INFO request.
When receiving INFO requests carrying any candidates, agents
will therefore first identify and discard the attribute lines
containing candidates they have already received in previous
INFO requests or in the Offer/Answer exchange preceding them.
Two candidates are considered to be equal if their IP address
port, transport and component ID are the same.
After identifying and discarding known candidates,
the agents MUST forward the actually new candidates to the ICE Agents
in the same order as they were received in the INFO request body.
The ICE Agents will then process the new candidates
according to the rules described in .
Receiving an "a=end-of-candidates" attribute in an INFO request body
- with the "a=ice-ufrag" and "a=ice-pwd" attributes matching the current ICE generation -
is an indication of the peer agent that it will not send any further candidates.
When included at session level, i.e. before any pseudo "m=" line,
this indication applies to the whole session;
when included at media level the indication applies
only to the corresponding "m=" line.
Handling of such end-of-candidate indications is defined in
.
Note: At the time of writing this specification there were ongoing discussions
if a functionality for removing already exchanged candidates would be useful.
Such a functionality is out of the scope of this specification
and most likely needs to be signaled by means of a yet to be defined ICE extension,
although it could in principle be achieved quite easily,
e.g. without anticipating any solution by simply omitting a previously sent candidate from a subsequent INFO request.
However, if an implementation according to this specification receives such an INFO request with a missing candidate
it MAY treat that as an exceptional case.
Implementing appropriate recovery procedures at the receiving side is RECOMMENDED for this situation.
Ignoring that a candidate was missing might be a sensible strategy.
The following example shows the content of one sample
candidate delivering INFO request:
SIP User Agents (UAs) that support and intend to use trickle
ICE are REQUIRED by
to indicate
that in their Offers and Answers using the following
attribute: "a=ice-options:trickle". This makes discovery
fairly straightforward for Answerers or for cases where
Offers need to be generated within existing dialogs (i.e.,
when sending re-INVITE requests). In both scenarios prior
SDP would have provided the necessary information.
Obviously, prior SDP is not available at the time a first
Offer is being constructed and it is therefore impossible
for ICE Agents to determine support for incremental
provisioning that way. The following options are suggested as
ways of addressing this issue.
In certain situations it may be possible for integrators
deploying Trickle ICE to know in advance that some or all
endpoints reachable from within the deployment will support
Trickle ICE. This is likely to be the case, for example,
for WebRTC clients that will always be communicating with
other WebRTC clients or known Session Border Controllers
(SBC) with support for this specification.
While the exact mechanism for allowing such provisioning
is out of scope here, this specification encourages trickle
ICE implementations to allow the option in the way they find
most appropriate.
provides a way for SIP User Agents
to query for support of specific capabilities using, among
others, OPTIONS requests. GRUU support on the other hand
allows SIP requests to be addressed to specific UAs (as
opposed to arbitrary instances of an address of record).
Combining the two and using the "trickle-ice" option tag
defined in provides SIP UAs with
a way of learning the capabilities of specific US instances
and then addressing them directly with INVITE requests that
require SIP support.
Such targeted trickling may happen in different ways. One
option would be for a SIP UA to learn the GRUU
instance ID of a peer through presence and to then query
its capabilities direction with an OPTIONS request.
Alternately, it can also just send an OPTIONS request to
the AOR it intends to contact and then inspect the returned
response(s) for support of both GRUU and Trickle ICE
().
Confirming support for Trickle ICE through
gives SIP UAs the options to engage
in Full Trickle negotiation (as opposed to the more lengthy
Half Trickle) from the very first Offer they send.
Protocols like XMPP define advanced
discovery mechanisms that allow specific features to be
queried priory to actually attempting to use them. Solutions
like define ways of using SIP and
XMPP together which also provides a way for dual stack
SIP+XMPP endpoints to make use of such features and verify
Trickle ICE support for a specific SIP endpoint through
XMPP.
However, such discovery mechanisms are out of the scope of
this document.
In cases where none of the other mechanisms in this section
are acceptable, SIP UAs should use the Half Trickle mode
defined in .
With Half Trickle, agents initiate sessions the same way
they would when using ICE for SIP
.
This means that, prior to actually sending an Offer, agents
would first gather ICE candidates in a blocking way and then
send them all in that Offer. The blocking nature of the
process would likely imply that some amount of latency will
be accumulated and it is advised that agents try to
anticipate it where possible, like for example, when user
actions indicate a high likelihood for an imminent call
(e.g., activity on a keypad or a phone going off-hook).
Using Half Trickle would result in Offers that are
compatible with both ICE SIP endpoints and legacy
endpoints.
It is worth reminding that once a single Offer or Answer had
been exchanged within a specific dialog, support for
Trickle ICE will have been determined.
No further use of Half Trickle will therefore be necessary
within that same dialog
and all subsequent exchanges can use the Full Trickle mode
of operation.
The following consideration describe options for Trickle-ICE
in order to give some guidance to implementors on how trickling
can be optimized with respect to providing RTCP candidates.
Handling of the "a=rtcp" attribute
and the "a=rtcp-mux" attribute for RTP/RTCP multiplexing
is already considered in section 5.6.1.
of and
as well in itself.
These considerations are still valid for Trickle ICE, however,
trickling provides more flexibility for the sequence of candidate exchange in case of RTCP multiplexing.
[RFC EDITOR NOTE: The section 5.6.1 in above sentence is correct for version 05 of said I-D. Please cross-check since it could have have changed in the meantime.]
If the Offerer supports RTP/RTCP multiplexing exclusively as specified
in ,
the procedures in that document apply for the handling of the "a=rtcp-mux-only", "a=rtcp" and the "a=rtcp-mux" attributes.
While a Half Trickle Offerer would have to send an offer compliant to
and including candidates for all components,
this flexibility allows a Full Trickle Offerer
to send only RTP candidates (component 1) in the initial Offer
if it assumes that RTCP multiplexing is supported by the Answerer.
A Full Trickle Offerer would need to start gathering and trickling
RTCP candidates (component 2)
only after having received an indication in the Answer that
the Answerer unexpectedly does not support RTCP multiplexing.
A Trickle Answerer MAY include an "a=rtcp-mux" attribute
in the application/trickle-ice-sdpfrag body
if it supports and uses RTP and RTCP multiplexing.
The Trickle Answerer MUST follow the guidance on the usage of the "a=rtcp" attribute as given in
and
.
Receipt of this attribute at the Offerer in an INFO request prior to the Answer
indicates that the Answerer supports and uses RTP and RTCP multiplexing.
The Offerer can use this information e.g. for stopping gathering of RTCP candidates
and/or for freeing corresponding resources.
This behavior is illustrated by the following example offer that indicates support for RTP and RTCP multiplexing.
Once the dialog is established as described in section the Answerer
sends the following INFO request.
This INFO request indicates that the Answerer supports and uses RTP and RTCP multiplexing as well.
It allows the Offerer to omit gathering of RTCP candidates or releasing already gathered RTCP candidates.
If the INFO request did not contain the a=rtcp-mux attribute, the Offerer would have to gather RTCP candidates
unless it wants to wait until receipt of an Answer that eventually confirms
support or non-support for RTP and RTCP multiplexing.
The following considerations describe options for Trickle-ICE
in order to give some guidance to implementors on how trickling
can be optimized with respect to providing candidates in case of Media Multiplexing
.
It is assumed that the reader is familiar with .
ICE candidate exchange is already considered
in section 11 of
.
These considerations are still valid for Trickle ICE, however,
trickling provides more flexibility for the sequence of candidate exchange,
especially in Full Trickle mode.
Except for bundle-only "m=" lines, a Half Trickle Offerer would have to
send an offer with candidates for all bundled "m=" lines.
The additional flexibility, however, allows a Full Trickle Offerer
to initially send only candidates for the "m=" line with the
suggested Offerer BUNDLE address.
Latest on receipt of the Answer, the Offerer will detect
if BUNDLE is supported by the Answerer and if the suggested Offerer BUNDLE address was selected.
In this case, the Offerer does not need to trickle further candidates for the remaining "m=" lines in a bundle.
However, if BUNDLE is not supported, the Full Trickle Offerer needs to gather and trickle candidates
for the remaining "m=" lines as necessary.
If the Answerer selects an Offerer BUNDLE address different from the suggested Offerer BUNDLE address,
the Full Trickle Offerer needs to gather and trickle candidates
for the "m=" line that carries the selected Offerer BUNDLE address.
A Trickle Answerer SHOULD include an "a=group: BUNDLE" attribute
in the application/trickle-ice-sdpfrag body
if it supports and uses bundling.
When doing so, the Answerer MUST include all identification-tags in the same order that is used or will be used in the Answer.
Receipt of this attribute at the Offerer in an INFO request prior to the Answer indicates that the Answerer
supports and uses bundling.
The Offerer can use this information e.g. for stopping the gathering of candidates
for the remaining "m=" lines in a bundle and/or for freeing corresponding resources.
This behaviour is illustrated by the following example offer that indicates support for Media Multiplexing.
Once the dialog is established as described in section the Answerer
sends the following INFO request.
This INFO request indicates that the Answerer supports and uses Media Multiplexing as well.
Note, that the second "m=" line shows the default values as specified in section ,
e.g. media set 'audio' although 'video' was offered.
The receiving ICE Agents MUST ignore these default values in the pseudo "m=" lines.
The INFO request also indicates that the Answerer accepted the suggested Offerer Bundle Address.
This allows the Offerer to omit gathering of RTP and RTCP candidates for the other "m=" lines
or releasing already gathered candidates.
If the INFO request did not contain the a=group:BUNDLE attribute, the Offerer would have to gather
RTP and RTCP candidates for the other "m=" lines unless it wants to wait until receipt
of an Answer that eventually confirms
support or non-support for Media Multiplexing.
Independent of using Full Trickle or Half Trickle mode, the rules from
apply to both, Offerer and Answerer,
when putting attributes as specified in
in the application/trickle-ice-sdpfrag body.
This section defines a new SDP media-level and session-level attribute
'end-of-candidate'. 'end-of-candidate' is a property attribute
, and hence has no value.
By including this attribute in an Offer or Answer the sending agent indicates
that it will not trickle further candidates.
When included at session level this indication applies to the whole session,
when included at media level the indication applies only to the corresponding media desciption.
Name: end-of-candidate
Value: N/A
Usage Level: media and session-level
Charset Dependent: no
Mux Category: IDENTICAL
Example: a=end-of-candidate
The Offerer or Answerer MAY include an "a=end-of-candidates" attribute
in case candidate discovery has ended
and no further candidates are to be trickled.
The Offerer or Answerer MUST provide the "a=end-of-candidates" attribute
together with the "a=ice-ufrag" and "a=ice-pwd" attributes of the current
ICE generation as required by
.
When included at session level
this indication applies to the whole session;
when included at media level the indication applies
only to the corresponding media description.
Receipt of an "a=end-of-candidates" attribute at an
Offerer or Anwerer
- with the "a=ice-ufrag" and "a=ice-pwd" attributes matching the current ICE generation -
indicates that gathering of candidates
has ended at the peer, either for the session or only for the
corresponding media description as specified above.
The receiving agent forwards an end-of-candidates indication
to the ICE Agent, which in turn acts as specified in
.
A application/trickle-ice-sdpfrag body is used by the 'trickle-ice' Info Package.
It uses a subset of the possible SDP lines
as defined by the grammar defined in .
A valid body uses only pseudo "m=" lines and certain attributes
that are needed and/or useful for trickling candidates.
The content adheres to the following grammar.
The grammar of an 'application/trickle-ice-sdpfrag' body is based on the following
ABNF .
It specifies the subset of existing SDP attributes, that are needed or useful for trickling candidates.
The grammar uses the indicator for case-sensitivity %s is defined in ,
but also imports grammars for other SDP attributes that precede the production of that RFC.
A sender SHOULD stick to lower-case for such grammars, but a receiver SHOULD treat them case-insensitive.
with ice-lite, ice-pwd-att, remote-candidate-att, ice-ufrag-att,
ice-pacing-att, ice-options, candidate-attribute remote-candidate-att
from , identification-tag, mid-attribute ; from ,
media-field, attribute-fields from .
The "a=rtcp" attribute is defined in ,
the "a=rtcp-mux" attribute in and
the "a=rtcp-mux-only" attribute in .
The latter attributes lack a formal grammar in their corresponding RFC and are reproduced here.
An Agent MUST ignore any received unknown extension-attribute-fields.
The decision to use SIP INFO requests as a candidate transport
method is based primarily on their lightweight nature. Once a
dialog has been established, INFO messages can be exchanged
both ways with no restrictions on timing and frequency and no
risk of collision.
On the other hand, using Offer/Answer and UPDATE requests
introduces the following complications:
defines Offer/Answer as a
strictly sequential mechanism. There can only be a maximum
of one exchange at any point of time. Both sides cannot
simultaneously send Offers nor can they generate multiple
Offers prior to receiving an Answer.
Using UPDATE requests for
candidate transport would therefore imply the
implementation of a candidate pool at every agent where
candidates can be stored until it is once again that
agent's "turn" to emit an Answer or a new Offer. Such an
approach would introduce non-negligible complexity for no
additional value.
The sequential nature of Offer/Answer also makes it
impossible for both sides to send Offers simultaneously.
What's worse is that there are no mechanisms in SIP to
actually prevent that. , where
the situation of Offers crossing on the wire is described
as "glare", only defines a procedure for addressing the
issue after it has occurred. According to that procedure
both Offers are invalidated and both sides need to retry
the negotiation after a period between 0 and 4 seconds.
The high likelihood for glare to occur and the average two
second back-off intervals would imply Trickle ICE
processing duration would not only fail to improve but
actually exceed those of regular ICE.
INFO messages decouple the exchange of candidates from the
Offer/Answer negotiation
and are subject to none of the glare issues described above,
which makes them a very convenient and lightweight mechanism
for asynchronous delivery of candidates.
Using in-dialog INFO messages also provides a way of
guaranteeing that candidates are delivered end-to-end, between
the same entities that are actually in the process of
initiating a session. Out-of-dialog alternatives would have implied
requiring support for Globally Routable UA URI (GRUU)
which, given GRUUs relatively low
adoption levels, would have constituted too strong of a
constraint to the adoption of Trickle ICE.
This specification defines an Info Package for use by
SIP User Agents implementing Trickle ICE.
INFO requests carry ICE candidates discovered after the peer user
agents have confirmed mutual support for Trickle ICE.
The purpose of the ICE protocol is to establish a media path
in the presence of NAT and firewalls.
The candidates are transported in INFO requests and are
part of this establishment.
Candidates sent by a Trickle ICE Agent after the Offer,
follow the same signaling path and reach the same
entity as the Offer itself. While it is true that GRUUs can
be used to achieve this, one of the goals of this
specification is to allow operation of Trickle ICE in as many
environments as possible including those without GRUU support.
Using out-of-dialog SUBSCRIBE/NOTIFY requests would not
satisfy this goal.
This document defines a SIP Info Package as per
. The Info Package token name for this
package is "trickle-ice"
This document does not define any Info Package parameters.
allows Info Package specifications to
define SIP option-tags. This specification extends the option-tag
construct of the SIP grammar as follows:
SIP entities that support this
specification MUST place the 'trickle-ice' option-tag in a SIP
Supported: header field within all SIP INVITE requests and responses.
When responding to, or generating a SIP OPTIONS request a SIP
entity MUST also include the 'trickle-ice' option-tag in a SIP
Supported: header field.
Entities implementing this specification MUST include a
payload of type 'application/trickle-ice-sdpfrag' as defined
in
all SIP INFO requests.
The payload is used to convey SDP encoded ICE candidates.
This document does not define any Info Package Usage Restrictions.
A Trickle ICE Agent with many network interfaces might create a
high rate of INFO requests if every newly
detected candidate is trickled individually without aggregation.
Implementors that are concerned about loss of packets in such a case
might consider aggregating ICE candidates and sending INFOs only
at some configurable intervals.
See
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document. ]
This section defines a new SDP media-level and session-level attribute
, 'end-of-candidate'. 'end-of-candidate' is a property attribute
, and hence has no value.
Type name: application Subtype name: trickle-ice-sdpfrag Required parameters: None. Optional parameters: None. Encoding considerations: SDP files are primarily UTF-8 format text.
Although the initially defined content of a trickle-ice-sdpfrag body
does only include ASCII characters, UTF-8 encoded content might be introduced via extension attributes.
The "a=charset:" attribute may be used to signal the presence of other
character sets in certain parts of a trickle-ice-sdpfrag body (see
).
Arbitrary binary content cannot be directly represented in SDP or a trickle-ice-sdpfrag body.
Security considerations:
See ) and RFCXXXX
Interoperability considerations:
See RFCXXXX
Published specification:
See RFCXXXX
Applications which use this Media Type:
Voice over IP, video teleconferencing, streaming media,
instant messaging, Trickle-ICE among others.
Fragment identifier considerations: N/A Additional information: Magic number(s): N/AFile extension(s): N/AMacintosh File Type Code(s): N/A Person and email address to contact for further information:
IETF MMUSIC working group mmusic@ietf.org
Intended usage:
Trickle-ICE for SIP as specified in RFCXXXX.
Restrictions on usage: N/A Author/Change controller:
IETF MMUSIC working group mmusic@ietf.org
Provisional registration? (standards tree only): N/A
This document defines a new SIP Info Package named 'trickle-ice'
and updates the Info Packages Registry with the following entry.
This specification registers a new SIP option tag 'trickle-ice'
as per the guidelines in Section 27.1 of
and updates the "Option Tags" section of the
SIP Parameter Registry with the following entry:
The Security Considerations of
,
and
apply.
This document clarifies how the above specifications are used together for trickling
candidates and does not create addtitional security risks.
The authors would like to thank
Flemming Andreasen,
Ayush Jain,
Paul Kyzivat,
Jonathan Lennox,
Simon Perreault,
Roman Shpount
and
Martin Thomson
for reviewing and/or making various suggestions for
improvements and optimizations.
[RFC EDITOR NOTE: Please remove this section when publishing].
Changes from draft-ietf-mmusic-trickle-ice-sip-01
Editorial Clean up IANA Consideration added Security Consideration added RTCP and BUNDLE Consideration added with rules for including "a=rtcp-mux" and "a=group: BUNDLLE" attributes 3PCC Consideration added Clarified that 18x w/o answer is sufficient to create a dialog that allows for trickling to start Added remaining Info Package definition sections as outlined in section 10 of Added definition of application/sdpfrag making draft-ivov-mmusic-sdpfrag obsolete Added pseudo m-lines as additional separator in sdpfrag bodies for Trickle ICE Added ABNF for sdp-frag bodies and Trickle-ICE package
Changes from draft-ietf-mmusic-trickle-ice-sip-02
Removed definition of application/sdpfrag Replaced with new type application/trickle-ice-sdpfrag RTCP and BUNDLE Consideration enhanced with some examples draft-ietf-mmusic-sdp-bundle-negotiation and RFC5761 changed to normative reference Removed reference to 4566bis Addressed review comment from Simon Perreault
Changes from draft-ietf-mmusic-trickle-ice-sip-03
replaced reference to RFC5245 with draft-ietf-mmusic-rfc5245bis and draft-ietf-mmusic-ice-sip-sdp Corrected Figure 10, credits to Ayush Jain for finding the bug Referencing a=rtcp and a=rtcp-mux handling from draft-ietf-mmusic-ice-sip-sdp Referencing a=rtcp-mux-exclusive handling from draft-ietf-mmusic-mux-exclusive, enhanced ABNF to support a=rtcp-mux-exclusive Clarifying that draft-ietf-mmusic-sdp-mux-attributes applies for the application/trickle-ice-sdpfrag body
Changes from draft-ietf-mmusic-trickle-ice-sip-04
considered comments from Christer Holmberg corrected grammar for INFO package, such that ice-ufrag/pwd are also allowed on media-level as specified in Added new ice-pacing-attribute fom Added formal definition for the end-of-candidates attribute
Changes from draft-ietf-mmusic-trickle-ice-sip-05
considered further comments from Christer Holmberg editorial comments on section 3 addressed moved section 3.1 to section 10.1 and applied some edits replaced the term "previously sent candidates" with "currently known and used candidates".
Changes from draft-ietf-mmusic-trickle-ice-sip-06
editorial fixes additional text on the content of the INFO messages. recommendation on what to do if a previously sent candidate is unexpectedly missing in a subsequent INFO terminology alignment with draft-ietf-ice-trickle-07
Changes from draft-ietf-mmusic-trickle-ice-sip-07
editorial fixes clarification on ordering of candidates for alignment with draft-ietf-ice-trickle-12 O/A procedures for end-of-candidates attribute described here after corresponding procedures
have been removed from draft-ietf-ice-trickle-11 using IPv6 addresses in examples
Changes from draft-ietf-mmusic-trickle-ice-sip-08
editorial fixes/clarification based on Flemmings review Description of Trickle specifics in O/A procedures for initial O/A exchange and specification of ICE mismatch exception
Changes from draft-ietf-mmusic-trickle-ice-sip-09
editorial fixes/correction of references adding missing Ref to RFC3605 in section 6, 5th para replaced remaining IPv4 adresses with IPv6
Added text for handling a=rtcp in case of default RTP address 0.0.0.0:9 based on comment from Roman Shpount.