A Session Initiation Protocol (SIP) usage for Trickle ICE
draft-ietf-mmusic-trickle-ice-sip-10
The information below is for an old version of the document.
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|
|---|---|---|---|
| Authors | Emil Ivov , Thomas Stach , Enrico Marocco , Christer Holmberg | ||
| Last updated | 2017-10-16 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-mmusic-trickle-ice-sip-10
Network Working Group E. Ivov
Internet-Draft Jitsi
Intended status: Standards Track T. Stach
Expires: April 18, 2018 Unaffiliated
E. Marocco
Telecom Italia
C. Holmberg
Ericsson
October 15, 2017
A Session Initiation Protocol (SIP) usage for Trickle ICE
draft-ietf-mmusic-trickle-ice-sip-10
Abstract
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 [RFC6086].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 18, 2018.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Discovery issues . . . . . . . . . . . . . . . . . . . . 5
3.2. Relationship with the Offer/Answer Model . . . . . . . . 6
4. Incremental Signaling of ICE candidates . . . . . . . . . . . 7
4.1. Initial Offer/Answer exchange . . . . . . . . . . . . . . 8
4.1.1. Sending the initial Offer . . . . . . . . . . . . . . 8
4.1.2. Receiving the initial Offer . . . . . . . . . . . . . 9
4.1.3. Sending the initial Answer . . . . . . . . . . . . . 9
4.1.4. Receiving the initial Answer . . . . . . . . . . . . 9
4.2. Establishing the dialog . . . . . . . . . . . . . . . . . 9
4.2.1. Asserting dialog state through reliable Offer/Answer
delivery . . . . . . . . . . . . . . . . . . . . . . 10
4.2.2. Asserting dialog state through unreliable
Offer/Answer delivery . . . . . . . . . . . . . . . . 10
4.2.3. Initiating Trickle ICE without an SDP Answer . . . . 12
4.2.4. Considerations for 3PCC . . . . . . . . . . . . . . . 14
4.3. Delivering candidates in INFO messages . . . . . . . . . 15
5. Initial discovery of Trickle ICE support . . . . . . . . . . 19
5.1. Provisioning support for Trickle ICE . . . . . . . . . . 20
5.2. Trickle ICE discovery with GRUU . . . . . . . . . . . . . 20
5.3. Trickle ICE discovery through other protocols . . . . . . 21
5.4. Fall-back to Half Trickle . . . . . . . . . . . . . . . . 21
6. Considerations for RTP and RTCP multiplexing . . . . . . . . 23
7. Considerations for Media Multiplexing . . . . . . . . . . . . 25
8. SDP 'end-of-candidate' Attribute . . . . . . . . . . . . . . 27
8.1. Defintion . . . . . . . . . . . . . . . . . . . . . . . . 27
8.2. Offer/Answer procedures . . . . . . . . . . . . . . . . . 27
9. Content Type 'application/trickle-ice-sdpfrag' . . . . . . . 28
9.1. Overall Description . . . . . . . . . . . . . . . . . . . 28
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9.2. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 28
10. Info Package . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Rationale - Why INFO? . . . . . . . . . . . . . . . . . 30
10.2. Overall Description . . . . . . . . . . . . . . . . . . 31
10.3. Applicability . . . . . . . . . . . . . . . . . . . . . 31
10.4. Info Package Name . . . . . . . . . . . . . . . . . . . 31
10.5. Info Package Parameters . . . . . . . . . . . . . . . . 31
10.6. SIP Option Tags . . . . . . . . . . . . . . . . . . . . 31
10.7. Info Message Body Parts . . . . . . . . . . . . . . . . 32
10.8. Info Package Usage Restrictions . . . . . . . . . . . . 32
10.9. Rate of INFO Requests . . . . . . . . . . . . . . . . . 32
10.10. Info Package Security Considerations . . . . . . . . . . 32
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
11.1. SDP 'end-of-candidate' Attribute . . . . . . . . . . . . 32
11.2. application/trickle-ice-sdpfrag Media Type . . . . . . . 33
11.3. SIP Info Package 'trickle-ice' . . . . . . . . . . . . . 35
11.4. SIP Option Tag 'trickle-ice' . . . . . . . . . . . . . . 35
12. Security Considerations . . . . . . . . . . . . . . . . . . . 35
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 36
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
15.1. Normative References . . . . . . . . . . . . . . . . . . 38
15.2. Informative References . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
The Interactive Connectivity Establishment protocol
[I-D.ietf-mmusic-rfc5245bis] 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 [I-D.ietf-mmusic-rfc5245bis] the three
phases above happen consecutively, in a blocking way, which can
introduce undesirable latency during session establishment.
The Trickle ICE extension [I-D.ietf-ice-trickle] 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)[RFC3261]. It describes how ICE candidates
are to be exchanged incrementally with SIP INFO requests [RFC6086]
and how the Half Trickle and Full Trickle modes defined in
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[I-D.ietf-ice-trickle] 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 [RFC6086]
for use with Trickle ICE.
2. Terminology
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 [RFC2119].
This specification makes use of all terminology defined by the
protocol for Interactive Connectivity Establishment in
[I-D.ietf-mmusic-rfc5245bis] and its Trickle ICE extension
[I-D.ietf-ice-trickle]. It is assumed that the reader will be
familiar with the terminology from both documents.
3. Protocol Overview
When using ICE for SIP according to [I-D.ietf-mmusic-ice-sip-sdp] the
ICE candidates are exchanged solely via SDP Offer/Answer as per
[RFC3264]. This specification defines an additional mechanism where
candidates can be exchanged using SIP INFO messages and a newly
defined Info Package [RFC6086]. 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.
Figure 1 shows such a sample exchange:
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STUN/Turn STUN/TURN
Servers Alice Bob Servers
| | | |
| STUN Bi.Req. | INVITE (Offer) | |
|<--------------|------------------------>| |
| | 183 (Answer) | TURN Alloc Req |
| STUN Bi.Resp. |<------------------------|--------------->|
|-------------->| INFO/OK (SRFLX Cand.) | |
| |------------------------>| TURN Alloc Resp|
| | INFO/OK (Relay Cand.) |<---------------|
| |<------------------------| |
| | | |
| | More Cands & ConnChecks| |
| |<=======================>| |
| | | |
| | 200 OK | |
| |<------------------------| |
| | ACK | |
| |------------------------>| |
| | | |
| |<===== MEDIA FLOWS =====>| |
| | | |
Figure 1: Sample Trickle ICE scenario with SIP
3.1. Discovery issues
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 [RFC6120], where "in advance"
capability discovery is widely implemented, the mechanisms that allow
this for SIP (i.e., a combination of UA Capabilities [RFC3840] and
GRUU [RFC5627]) 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 [I-D.ietf-ice-trickle] 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.
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While using Half Trickle does provide a working solution it also
comes at the price of increased latency. Section 5 therefore makes
several alternative suggestions that enable SIP UAs to engage in Full
Trickle right from their first Offer: Section 5.1 discusses the use
of on-line provisioning as a means of allowing use of Trickle ICE for
all endpoints in controlled environments. Section 5.2 describes
anticipatory discovery for implementations that actually do support
GRUU and UA Capabilities and Section 5.4 discusses the implementation
and use of Half Trickle by SIP UAs where none of the above are an
option.
3.2. Relationship with the Offer/Answer Model
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
[I-D.ietf-mmusic-ice-sip-sdp].
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+-------------------------------+ +-------------------------------+
| Alice +--------------+ | | +--------------+ Bob |
| | Offer/Answer | | | | Offer/Answer | |
| +-------+ | Module | | | | Module | +-------+ |
| | ICE | +--------------+ | | +--------------+ | ICE | |
| | Agent | | | | | | Agent | |
| +-------+ | | | | +-------+ |
+-------------------------------+ +-------------------------------+
| | | |
| | INVITE (Offer) | |
| |--------------------->| |
| | 183 (Answer) | |
| |<---------------------| |
| | | |
| |
| SIP INFO (more candidates) |
|----------------------------------------------------->|
| SIP INFO (more candidates) |
|<-----------------------------------------------------|
| |
| STUN Binding Requests/Responses |
|----------------------------------------------------->|
| STUN Binding Requests/Responses |
|<-----------------------------------------------------|
| |
Figure 2: Distinguishing between Trickle ICE and traditional
signaling.
From an architectural viewpoint, as displayed in Figure 2, 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 [RFC3264].
4. Incremental Signaling of ICE candidates
Trickle ICE Agents will construct Offers and Answers with ICE
descriptions compliant to [I-D.ietf-ice-trickle] and the following
additional SIP-specific additions:
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1. 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.
2. Trickle ICE Agents MUST indicate support for Trickle ICE by
including the ice-option 'trickle' within all SDP Offers and
Answers in accordance to [I-D.ietf-ice-trickle].
3. 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.
4. Trickle ICE Agents MAY exchange additional ICE candidates using
INFO requests within an existing INVITE dialog usage (including
an early dialog) as specified in [RFC6086]. 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.
5. 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.
4.1. Initial Offer/Answer exchange
4.1.1. Sending the initial Offer
If the Offerer includes candidates in its initial Offer, it MUST
encode these candidates as specified in
[I-D.ietf-mmusic-ice-sip-sdp].
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.
o The media field is set to 'audio'.
o The port value is set to '9'.
o The proto value is set to 'RTP/AVP'.
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In this case, the Offerer obviously cannot know the RTCP transport
address and, thus, MUST NOT include the "a=rtcp" attribute [RFC6086].
This avoids potential ICE mismatch (see
[I-D.ietf-mmusic-ice-sip-sdp]) for the RTCP transport address.
If the Offerer wants to use RTCP multiplexing [RFC5761] and/or
exclusive RTCP multiplexing [I-D.ietf-mmusic-mux-exclusive], 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 [I-D.ietf-ice-trickle].
4.1.2. Receiving the initial Offer
If the initial Offer included candidates, the Answerer MUST treat
these candidates as specified in [I-D.ietf-mmusic-ice-sip-sdp].
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
[I-D.ietf-mmusic-ice-sip-sdp]).
4.1.3. Sending the initial Answer
Section Section 4.1.1 applies to the Answerer with the roles of
Offerer and Answer being swapped.
4.1.4. Receiving the initial Answer
Section Section 4.1.2 applies to the Answerer with the roles of
Offerer and Answer being swapped.
4.2. Establishing the dialog
In order to be able to start trickling, the following two conditions
need to be satisfied at the SIP UAs:
o Trickle ICE support at the peer agent MUST be confirmed.
o The dialog at both peers MUST be in early or confirmed state.
Section 5 discusses in detail the various options for satisfying the
first of the above conditions. Regardless of those mechanisms,
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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
Figure 3).
4.2.1. Asserting dialog state through reliable Offer/Answer delivery
Alice Bob
| |
| INVITE (Offer) |
|------------------------>|
| 183 (Answer) |
|<------------------------|
| PRACK/OK |
|------------------------>|
| |
+----------------------------------------+
|Alice and Bob know that both can trickle|
|and know that the dialog is in the early|
|state. Send INFO! |
+----------------------------------------+
| |
| INFO/OK (+SRFLX Cand.) |
|------------------------>|
| INFO/OK (+SRFLX Cand.) |
|<------------------------|
| |
Figure 3: SIP Offerer can freely trickle as soon as it receives an
Answer.
As shown in Figure 3 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.
4.2.2. Asserting dialog state through unreliable Offer/Answer delivery
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 (Figure 4).
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Alice Bob
| |
| INVITE (Offer) |
|------------------------>|
| 183 (Answer) |
|<------------------------|
| |
| +----------------------+
| |Bob: I don't know if |
| |Alice got my 183 or if|
| |her dialog is already |
| |in the early state. |
| | Can I send INFO??? |
| +----------------------+
| |
Figure 4: A SIP UA that sent an Answer in an unreliable provisional
response does not know if it was received and if the dialog at the
side of the Offerer has entered the early state
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 [RFC3262]. 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 [I-D.ietf-mmusic-ice-sip-sdp] 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 (Figure 5).
Note: The +SRFLX in Figure 5 indicates that additionally newly
learned server-reflexive candidates are included.
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Alice Bob
| |
| INVITE (Offer) |
|------------------------>|
| 183 (Answer) |
|<------------------------|
| INFO/OK (+SRFLX Cand.) |
|------------------------>|
| |
| +----------------------+
| |Bob: Now I know Alice|
| | is ready. Send INFO! |
| +----------------------+
| INFO/OK (+SRFLX Cand.) |
|<------------------------|
| |
| 200/ACK (Answer) |
|<------------------------|
Figure 5: A SIP UA that received an INFO request after sending an
unreliable provisional response knows that the dialog at the side of
the receiver has entered the early state
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 [RFC3264]. 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.
4.2.3. Initiating Trickle ICE without an SDP Answer
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 [RFC3262]. 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 [I-D.ietf-mmusic-ice-sip-sdp] except that an Answer
is not yet provided.
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[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 Figure 6 indicates that additionally newly
learned server-reflexive candidates are included.
Alice Bob
| |
| INVITE (Offer) |
|------------------------>|
| 183 (-) |
|<------------------------|
| INFO/OK (SRFLX Cand.) |
|------------------------>|
| |
| +----------------------+
| |Bob: Now I know again|
| | that Alice is ready. |
| | Send INFO! |
| +----------------------+
| INFO/OK (SRFLX Cand.) |
|<------------------------|
| 183 (Answer) opt. |
|<------------------------|
| INFO/OK (SRFLX Cand.) |
|<------------------------|
| 200/ACK (Answer) |
|<------------------------|
Figure 6: A SIP UA sends an unreliable provisional response without
an Answer for establishing an early dialog
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 [RFC3264]. 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.
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4.2.4. Considerations for 3PCC
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
Figure 7).
Alice Bob
| |
| INVITE |
|------------------------>|
| 183 (Offer) |
|<------------------------|
| PRACK (Answer) |
|------------------------>|
| |
| +----------------------+
| |Bob: I know Alice can|
| |trickle and I know her|
| |dialog is in the early|
| |state. Send INFO! |
| +----------------------+
| |
| INFO/OK (SRFLX Cand.) |
|<------------------------|
| |
| INFO/OK (SRFLX Cand.) |
|------------------------>|
| 200 OK/ACK |
|<------------------------|
Figure 7: A SIP Offerer in a 3PCC scenario can also freely start
trickling as soon as it receives an Answer.
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
Section 4.2.3. According to [RFC3261], 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 [RFC3262].
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 Figure 8).
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Alice Bob
| |
| INVITE |
|------------------------>|
| 183 (-) |
|<------------------------|
| INFO/OK (SRFLX Cand.) |
|------------------------>|
| |
| +-----------------------+
| |Bob: I know Alice can |
| |trickle and I know her |
| |dialog is in the early |
| |state. |
| |INFO can be sent. |
| +-----------------------+
| |
| INFO/OK (SRFLX Cand.) |
|<------------------------|
| |
| 200 (Offer) |
|<------------------------|
| ACK (Answer) |
|------------------------>|
| |
Figure 8: A SIP UAC in a 3PCC scenario can also freely start
trickling as soon as it receives an unreliable provisional response.
4.3. Delivering candidates in INFO messages
Whenever new ICE candidates become available for sending, agents
would encode them in "a=candidate" lines as described by
[I-D.ietf-mmusic-ice-sip-sdp]. For example:
a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 5000 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 8998
The use of SIP INFO requests happens within the context of the Info
Package as defined Section 10. The Media Type [RFC6838] for their
payload MUST be set to 'application/trickle-ice-sdpfrag' as defined
in Section 9.
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="
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lines and identification tags in "a=mid:" attributes as defined in
[RFC5888]. Pseudo "m=" lines follow the SDP syntax for "m=" lines as
defined in [RFC4566], 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 [RFC3264].
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.
o The media field is set to 'audio'.
o The port value is set to '9'.
o The proto value is set to 'RTP/AVP'.
o 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 Figure 9 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 [RFC5888], it does not define any grouping semantics.
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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 [I-D.ietf-ice-trickle] 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
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request body. The ICE Agents will then process the new candidates
according to the rules described in [I-D.ietf-ice-trickle].
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 [I-D.ietf-ice-trickle].
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:
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INFO sip:alice@example.com SIP/2.0
...
Info-Package: trickle-ice
Content-type: application/trickle-ice-sdpfrag
Content-Disposition: Info-Package
Content-length: ...
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 9 RTP/AVP 0
a=mid:1
a=candidate:1 1 UDP 2130706431 2001:db8:a0b:12f0::1 5000 typ host
a=candidate:1 2 UDP 2130706431 2001:db8:a0b:12f0::1 5001 typ host
a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 5000 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 8998
a=candidate:2 2 UDP 1694498815 2001:db8:a0b:12f0::3 5001 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 8998
a=end-of-candidates
m=audio 9 RTP/AVP 0
a=mid:2
a=candidate:1 1 UDP 2130706431 2001:db8:a0b:12f0::1 6000 typ host
a=candidate:1 2 UDP 2130706431 2001:db8:a0b:12f0::1 6001 typ host
a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 6000 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 9998
a=candidate:2 2 UDP 1694498815 2001:db8:a0b:12f0::3 6001 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 9998
a=end-of-candidates
Figure 9: An Example for the Content of an INFO Request
5. Initial discovery of Trickle ICE support
SIP User Agents (UAs) that support and intend to use trickle ICE are
REQUIRED by [I-D.ietf-ice-trickle] 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.
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5.1. Provisioning support for Trickle ICE
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.
5.2. Trickle ICE discovery with GRUU
[RFC3840] 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 Section 10.6 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 (Figure 10).
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Alice Bob
| |
| OPTIONS sip:b1@example.com SIP/2.0 |
|-------------------------------------------------->|
| |
| 200 OK |
| Contact: sip:b1@example.com;gr=hha9s8d-999a |
| ;audio;video|;trickle-ice;... |
|<--------------------------------------------------|
| |
| INVITE sip:b1@example.com;gr=hha9s8d-999a SIP/2.0 |
|-------------------------------------------------->|
| |
| 183 (Answer) |
|<--------------------------------------------------|
| INFO/OK (Trickling) |
|<------------------------------------------------->|
| |
| ... |
| |
Figure 10: Trickle ICE support discovery with OPTIONS and GRUU
Confirming support for Trickle ICE through [RFC3840] 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.
5.3. Trickle ICE discovery through other protocols
Protocols like XMPP [RFC6120] define advanced discovery mechanisms
that allow specific features to be queried priory to actually
attempting to use them. Solutions like [RFC7081] 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.
5.4. Fall-back to Half Trickle
In cases where none of the other mechanisms in this section are
acceptable, SIP UAs should use the Half Trickle mode defined in
[I-D.ietf-ice-trickle]. With Half Trickle, agents initiate sessions
the same way they would when using ICE for SIP
[I-D.ietf-mmusic-ice-sip-sdp]. 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
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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 [RFC3264] endpoints.
STUN/Turn STUN/TURN
Servers Alice Bob Servers
| | | |
|<--------------| | |
| | | |
| | | |
| Candidate | | |
| | | |
| | | |
| Discovery | | |
| | | |
| | | |
|-------------->| INVITE (Offer) | |
| |---------------------------->| |
| | 183 (Answer) |-------------->|
| |<----------------------------| |
| | INFO (repeated candidates) | |
| |---------------------------->| |
| | | |
| | INFO (more candidates) | Candidate |
| |<----------------------------| |
| | Connectivity Checks | |
| |<===========================>| Discovery |
| | INFO (more candidates) | |
| |<----------------------------| |
| | Connectivity Checks |<--------------|
| |<===========================>| |
| | | |
| | 200 OK | |
| |<----------------------------| |
| | | |
| |<======= MEDIA FLOWS =======>| |
| | | |
Figure 11: Example - A typical (Half) Trickle ICE exchange with SIP
It is worth reminding that once a single Offer or Answer had been
exchanged within a specific dialog, support for Trickle ICE will have
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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.
6. Considerations for RTP and RTCP multiplexing
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 [RFC3605] and the "a=rtcp-mux"
attribute for RTP/RTCP multiplexing [RFC5761] is already considered
in section 5.6.1. of [I-D.ietf-mmusic-rfc5245bis] and as well in
[RFC5761] 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 [I-D.ietf-mmusic-mux-exclusive], 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
[I-D.ietf-mmusic-ice-sip-sdp] and [RFC5761] 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 [RFC5761] 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
[I-D.ietf-mmusic-ice-sip-sdp] and [RFC3605]. 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.
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This behavior is illustrated by the following example offer that
indicates support for RTP and RTCP multiplexing.
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=ice-pwd:777uzjYhagZgasd88fgpdd
a=ice-ufrag:Yhh8
m=audio 10000 RTP/AVP 0
a=mid:1
a=rtcp-mux
a=candidate:1 1 UDP 1658497328 2001:db8:a0b:12f0::3 5000 typ host
Once the dialog is established as described in section Section 4.2
the Answerer sends the following INFO request.
INFO sip:alice@example.com SIP/2.0
...
Info-Package: trickle-ice
Content-type: application/sdp
Content-Disposition: Info-Package
Content-length: ...
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 9 RTP/AVP 0
a=mid:1
a=rtcp-mux
a=candidate:1 1 UDP 1658497328 2001:db8:a0b:12f0::3 5000 typ host
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.
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7. Considerations for Media 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 [I-D.ietf-mmusic-sdp-bundle-negotiation]. It is assumed
that the reader is familiar with
[I-D.ietf-mmusic-sdp-bundle-negotiation].
ICE candidate exchange is already considered in section 11 of
[I-D.ietf-mmusic-sdp-bundle-negotiation]. 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
[I-D.ietf-mmusic-sdp-bundle-negotiation] 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.
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v=0
o=alice 2890844526 2890844526 IN IP6 atlanta.example.com
s=
c=IN IP6 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
a=ice-pwd:777uzjYhagZgasd88fgpdd
a=ice-ufrag:Yhh8
m=audio 10000 RTP/AVP 0
a=mid:foo
a=rtpmap:0 PCMU/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31
a=mid:bar
a=rtpmap:31 H261/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
Once the dialog is established as described in section Section 4.2
the Answerer sends the following INFO request.
INFO sip:alice@example.com SIP/2.0
...
Info-Package: trickle-ice
Content-type: application/sdp
Content-Disposition: Info-Package
Content-length: ...
a=group:BUNDLE foo bar
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 9 RTP/AVP 0
a=mid:foo
a=rtcp-mux
a=candidate:1 1 UDP 1658497328 2001:db8:a0b:12f0::3 5000 typ host
m=audio 9 RTP/AVP 0
a=mid:bar
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 Section 4.3, e.g. media set
'audio' although 'video' was offered. The receiving ICE Agents MUST
ignore these default values in the pseudo "m=" lines.
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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 [I-D.ietf-mmusic-sdp-mux-attributes] apply to both, Offerer and
Answerer, when putting attributes as specified in Section 9.2 in the
application/trickle-ice-sdpfrag body.
8. SDP 'end-of-candidate' Attribute
8.1. Defintion
This section defines a new SDP media-level and session-level
attribute [RFC4566] 'end-of-candidate'. 'end-of-candidate' is a
property attribute [RFC4566], 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
8.2. Offer/Answer procedures
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 [I-D.ietf-ice-trickle]. When included at session level this
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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 [I-D.ietf-ice-trickle].
9. Content Type 'application/trickle-ice-sdpfrag'
9.1. Overall Description
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 [RFC4566]. 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.
9.2. Grammar
The grammar of an 'application/trickle-ice-sdpfrag' body is based on
the following ABNF [RFC5234]. 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
[RFC7405], 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.
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; Syntax
trickle-ice-sdpfrag = session-level-fields
pseudo-media-descriptions
session-level-fields = [bundle-group-attribute CRLF]
[ice-lite-attribute CRLF]
ice-pwd-attribute CRLF
ice-ufrag-attribute CRLF
[ice-options-attribute CRLF]
[ice-pacing-attribute CRLF]
[end-of-candidates-attribute CRLF]
extension-attribute-fields
; for future extensions
ice-lite-attribute = %s"a" "=" ice-lite
ice-pwd-attribute = %s"a" "=" ice-pwd-att
ice-ufrag-attribute = %s"a" "=" ice-ufrag-att
ice-pacing-attribute = %s"a" "=" ice-pacing-att
ice-options-attribute = %s"a" "=" ice-options
bundle-group-attribute = %s"a" "=" %s"group:" bundle-semantics
*(SP identification-tag)
bundle-semantics = "BUNDLE"
end-of-candidates-attribute = %s"a" "=" end-of-candidates
end-of-candidates = %s"end-of-candidates"
extension-attribute-fields = attribute-fields
pseudo-media-descriptions = *( media-field
trickle-ice-attribute-fields
[extension-attribute-fields] )
; for future extensions
trickle-ice-attribute-fields = %s"a" "=" mid-attribute CRLF
[%s"a" "=" %s"rtcp" CRLF]
[%s"a" "=" %s"rtcp-mux" CRLF]
[%s"a" "=" %s"rtcp-mux-only" CRLF]
*(candidate-attributes CRLF)
[ice-pwd-attribute CRLF]
[ice-ufrag-attribute CRLF]
[remote-candidate-attribute CRLF]
[end-of-candidates-attribute CRLF]
remote-candidate-attribute = %s"a" "=" remote-candidate-att
candidate-attributes = %s"a" "=" candidate-attribute
with ice-lite, ice-pwd-att, remote-candidate-att, ice-ufrag-att, ice-
pacing-att, ice-options, candidate-attribute remote-candidate-att
from [I-D.ietf-mmusic-ice-sip-sdp], identification-tag, mid-attribute
; from [RFC5888], media-field, attribute-fields from [RFC4566]. The
"a=rtcp" attribute is defined in [RFC3605], the "a=rtcp-mux"
attribute in [RFC5761] and the "a=rtcp-mux-only" attribute in
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[I-D.ietf-mmusic-mux-exclusive]. 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.
10. Info Package
10.1. Rationale - Why INFO?
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 [RFC3311]
introduces the following complications:
Blocking of messages: [RFC3264] 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.
Elevated risk of glare: 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. [RFC3261], 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
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Routable UA URI (GRUU) [RFC5627] which, given GRUUs relatively low
adoption levels, would have constituted too strong of a constraint to
the adoption of Trickle ICE.
10.2. Overall Description
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.
10.3. Applicability
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.
10.4. Info Package Name
This document defines a SIP Info Package as per [RFC6086]. The Info
Package token name for this package is "trickle-ice"
10.5. Info Package Parameters
This document does not define any Info Package parameters.
10.6. SIP Option Tags
[RFC6086] allows Info Package specifications to define SIP option-
tags. This specification extends the option-tag construct of the SIP
grammar as follows:
option-tag /= "trickle-ice"
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.
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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.
10.7. Info Message Body Parts
Entities implementing this specification MUST include a payload of
type 'application/trickle-ice-sdpfrag' as defined in Section 9.2 all
SIP INFO requests. The payload is used to convey SDP encoded ICE
candidates.
10.8. Info Package Usage Restrictions
This document does not define any Info Package Usage Restrictions.
10.9. Rate of INFO Requests
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.
10.10. Info Package Security Considerations
See Section 12
11. IANA Considerations
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document. ]
11.1. SDP 'end-of-candidate' Attribute
This section defines a new SDP media-level and session-level
attribute [RFC4566] , 'end-of-candidate'. 'end-of-candidate' is a
property attribute [RFC4566] , and hence has no value.
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Name: end-of-candidate
Value: N/A
Usage Level: media and session
Charset Dependent: no
Purpose: The sender indicates that it will not trickle
further candidates.
O/A Procedures: RFCXXX defines the detailed
SDP Offer/Answer procedures for
the 'end-of-candidate' attribute.
Mux Category: IDENTICAL
Reference: RFCXXXX
Example:
a=end-of-candidate
11.2. application/trickle-ice-sdpfrag Media Type
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
[RFC4566]). Arbitrary binary content cannot be directly
represented in SDP or a trickle-ice-sdpfrag body.
Security considerations:
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See [RFC4566]) 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/A
File extension(s): N/A
Macintosh 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:
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IETF MMUSIC working group mmusic@ietf.org
Provisional registration? (standards tree only): N/A
11.3. SIP Info Package 'trickle-ice'
This document defines a new SIP Info Package named 'trickle-ice' and
updates the Info Packages Registry with the following entry.
+-------------+-----------+
| Name | Reference |
+-------------+-----------+
| trickle-ice | [RFCXXXX] |
| | |
+-------------+-----------+
11.4. SIP Option Tag 'trickle-ice'
This specification registers a new SIP option tag 'trickle-ice' as
per the guidelines in Section 27.1 of [RFC3261] and updates the
"Option Tags" section of the SIP Parameter Registry with the
following entry:
+-------------+-------------------------------------+-----------+
| Name | Description | Reference |
+-------------+-------------------------------------+-----------+
| trickle-ice | This option tag is used to indicate | [RFCXXXX] |
| | that a UA supports and understands | |
| | Trickle-ICE. | |
+-------------+-------------------------------------+-----------+
12. Security Considerations
The Security Considerations of [I-D.ietf-mmusic-ice-sip-sdp],
[RFC6086] and [I-D.ietf-ice-trickle] apply. This document clarifies
how the above specifications are used together for trickling
candidates and does not create addtitional security risks.
13. Acknowledgements
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.
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14. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing].
Changes from draft-ietf-mmusic-trickle-ice-sip-01
o Editorial Clean up
o IANA Consideration added
o Security Consideration added
o RTCP and BUNDLE Consideration added with rules for including
"a=rtcp-mux" and "a=group: BUNDLLE" attributes
o 3PCC Consideration added
o Clarified that 18x w/o answer is sufficient to create a dialog
that allows for trickling to start
o Added remaining Info Package definition sections as outlined in
section 10 of [RFC6086]
o Added definition of application/sdpfrag making draft-ivov-mmusic-
sdpfrag obsolete
o Added pseudo m-lines as additional separator in sdpfrag bodies for
Trickle ICE
o Added ABNF for sdp-frag bodies and Trickle-ICE package
Changes from draft-ietf-mmusic-trickle-ice-sip-02
o Removed definition of application/sdpfrag
o Replaced with new type application/trickle-ice-sdpfrag
o RTCP and BUNDLE Consideration enhanced with some examples
o draft-ietf-mmusic-sdp-bundle-negotiation and RFC5761 changed to
normative reference
o Removed reference to 4566bis
o Addressed review comment from Simon Perreault
Changes from draft-ietf-mmusic-trickle-ice-sip-03
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o replaced reference to RFC5245 with draft-ietf-mmusic-rfc5245bis
and draft-ietf-mmusic-ice-sip-sdp
o Corrected Figure 10, credits to Ayush Jain for finding the bug
o Referencing a=rtcp and a=rtcp-mux handling from draft-ietf-mmusic-
ice-sip-sdp
o Referencing a=rtcp-mux-exclusive handling from draft-ietf-mmusic-
mux-exclusive, enhanced ABNF to support a=rtcp-mux-exclusive
o 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
o considered comments from Christer Holmberg
o corrected grammar for INFO package, such that ice-ufrag/pwd are
also allowed on media-level as specified in
[I-D.ietf-mmusic-ice-sip-sdp]
o Added new ice-pacing-attribute fom [I-D.ietf-mmusic-ice-sip-sdp]
o Added formal definition for the end-of-candidates attribute
Changes from draft-ietf-mmusic-trickle-ice-sip-05
o considered further comments from Christer Holmberg
o editorial comments on section 3 addressed
o moved section 3.1 to section 10.1 and applied some edits
o replaced the term "previously sent candidates" with "currently
known and used candidates".
Changes from draft-ietf-mmusic-trickle-ice-sip-06
o editorial fixes
o additional text on the content of the INFO messages.
o recommendation on what to do if a previously sent candidate is
unexpectedly missing in a subsequent INFO
o terminology alignment with draft-ietf-ice-trickle-07
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Changes from draft-ietf-mmusic-trickle-ice-sip-07
o editorial fixes
o clarification on ordering of candidates for alignment with draft-
ietf-ice-trickle-12
o O/A procedures for end-of-candidates attribute described here
after corresponding procedures have been removed from draft-ietf-
ice-trickle-11
o using IPv6 addresses in examples
Changes from draft-ietf-mmusic-trickle-ice-sip-08
o editorial fixes/clarification based on Flemmings review
o 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
o editorial fixes/correction of references
o adding missing Ref to RFC3605 in section 6, 5th para
o replaced remaining IPv4 adresses with IPv6
o Added text for handling a=rtcp in case of default RTP address
0.0.0.0:9 based on comment from Roman Shpount.
15. References
15.1. Normative References
[I-D.ietf-ice-trickle]
Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
"Trickle ICE: Incremental Provisioning of Candidates for
the Interactive Connectivity Establishment (ICE)
Protocol", draft-ietf-ice-trickle-14 (work in progress),
September 2017.
[I-D.ietf-mmusic-ice-sip-sdp]
Petit-Huguenin, M., Keranen, A., and S. Nandakumar,
"Session Description Protocol (SDP) Offer/Answer
procedures for Interactive Connectivity Establishment
(ICE)", draft-ietf-mmusic-ice-sip-sdp-14 (work in
progress), October 2017.
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[I-D.ietf-mmusic-mux-exclusive]
Holmberg, C., "Indicating Exclusive Support of RTP/RTCP
Multiplexing using SDP", draft-ietf-mmusic-mux-
exclusive-12 (work in progress), May 2017.
[I-D.ietf-mmusic-rfc5245bis]
Keranen, A. and J. Rosenberg, "Interactive Connectivity
Establishment (ICE): A Protocol for Network Address
Translator (NAT) Traversal", draft-ietf-mmusic-
rfc5245bis-05 (work in progress), September 2015.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-39 (work in progress), August 2017.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
(work in progress), December 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,
<https://www.rfc-editor.org/info/rfc3262>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003,
<https://www.rfc-editor.org/info/rfc3605>.
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[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<https://www.rfc-editor.org/info/rfc5761>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<https://www.rfc-editor.org/info/rfc5888>.
[RFC6086] Holmberg, C., Burger, E., and H. Kaplan, "Session
Initiation Protocol (SIP) INFO Method and Package
Framework", RFC 6086, DOI 10.17487/RFC6086, January 2011,
<https://www.rfc-editor.org/info/rfc6086>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
15.2. Informative References
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
2002, <https://www.rfc-editor.org/info/rfc3311>.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840,
DOI 10.17487/RFC3840, August 2004,
<https://www.rfc-editor.org/info/rfc3840>.
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[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent URIs (GRUUs) in the Session Initiation Protocol
(SIP)", RFC 5627, DOI 10.17487/RFC5627, October 2009,
<https://www.rfc-editor.org/info/rfc5627>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC7081] Ivov, E., Saint-Andre, P., and E. Marocco, "CUSAX:
Combined Use of the Session Initiation Protocol (SIP) and
the Extensible Messaging and Presence Protocol (XMPP)",
RFC 7081, DOI 10.17487/RFC7081, November 2013,
<https://www.rfc-editor.org/info/rfc7081>.
Authors' Addresses
Emil Ivov
Jitsi
Strasbourg 67000
France
Phone: +33 6 72 81 15 55
Email: emcho@jitsi.org
Thomas Stach
Unaffiliated
Vienna 1130
Austria
Email: thomass.stach@gmail.com
Enrico Marocco
Telecom Italia
Via G. Reiss Romoli, 274
Turin 10148
Italy
Email: enrico.marocco@telecomitalia.it
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Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
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