Internet Engineering Task Force (IETF) E. Ivov

Internet Engineering Task Force (IETF) E. Ivov
Request for Comments: 8840 Jitsi
Category: Standards Track T. Stach
ISSN: 2070-1721 Unaffiliated
E. Marocco
Telecom Italia
C. Holmberg
Ericsson
January 2021

A Session Initiation Protocol (SIP) Usage for Incremental Provisioning
of Candidates for the Interactive Connectivity Establishment (Trickle
ICE)

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). The document also defines a new
SIP Info Package to support this usage together with the
corresponding media type. Additionally, a new Session Description
Protocol (SDP) "end-of-candidates" attribute and a new SIP option tag
"trickle-ice" are defined.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8840.

Copyright Notice

Copyright (c) 2021 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
2. Terminology
3. Protocol Overview
3.1. Discovery Issues
3.2. Relationship with the Offer/Answer Model
4. Incremental Signaling of ICE Candidates
4.1. Initial Offer/Answer Exchange
4.1.1. Sending the Initial Offer
4.1.2. Receiving the Initial Offer
4.1.3. Sending the Initial Answer
4.1.4. Receiving the Initial Answer
4.2. Subsequent Offer/Answer Exchanges
4.3. Establishing the Dialog
4.3.1. Establishing Dialog State through Reliable Offer/Answer
Delivery
4.3.2. Establishing Dialog State through Unreliable Offer/
Answer Delivery
4.3.3. Initiating Trickle ICE without an SDP Answer
4.4. Delivering Candidates in INFO Requests
5. Initial Discovery of Trickle ICE Support
5.1. Provisioning Support for Trickle ICE
5.2. Trickle ICE Discovery with Globally Routable User Agent
URIs (GRUUs)
5.3. Fall Back to Half Trickle
6. Considerations for RTP and RTCP Multiplexing
7. Considerations for Media Multiplexing
8. SDP "end-of-candidates" Attribute
8.1. Definition
8.2. Offer/Answer Procedures
9. Content Type "application/trickle-ice-sdpfrag"
9.1. Overall Description
9.2. Grammar
10. Info Package
10.1. Rationale -- Why INFO?
10.2. Overall Description
10.3. Applicability
10.4. Info Package Name
10.5. Info Package Parameters
10.6. SIP Option Tags
10.7. INFO Request Body Parts
10.8. Info Package Usage Restrictions
10.9. Rate of INFO Requests
10.10. Info Package Security Considerations
11. Deployment Considerations
12. IANA Considerations
12.1. SDP "end-of-candidates" Attribute
12.2. Media Type "application/trickle-ice-sdpfrag"
12.3. SIP Info Package "trickle-ice"
12.4. SIP Option Tag "trickle-ice"
13. Security Considerations
14. References
14.1. Normative References
14.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

The Interactive Connectivity Establishment (ICE) protocol [RFC8445]
describes a mechanism for Network Address Translator (NAT) traversal
that consists of three main phases.

During the first phase, an agent gathers a set of candidate transport
addresses (source IP, port, and transport protocol). This is
followed by a second phase where these candidates are sent to a
remote agent within the Session Description Protocol (SDP) body of a
SIP message. At the remote agent, the gathering procedure is
repeated and candidates are sent to the first agent. Once the
candidate information is available, a third phase starts in parallel
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 [RFC8445], the three phases above happen consecutively,
in a blocking way, which can introduce undesirable setup delay during
session establishment. The Trickle ICE extension [RFC8838] defines
generic semantics required for these ICE phases to happen in a
parallel, non-blocking way and hence speeds 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 using SIP INFO requests [RFC6086]
and how the Half Trickle and Full Trickle modes defined in [RFC8838]
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 together with the corresponding media type,
SDP attribute, and SIP option tag.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

This specification makes use of terminology defined by the ICE
protocol in [RFC8445] and by its Trickle ICE extension in [RFC8838].
It is assumed that the reader is familiar with the terminology from
both documents.

[RFC8445] also describes how ICE makes use of the Session Traversal
Utilities for NAT (STUN) protocol [RFC5389] and its extension
Traversal Using Relays around NAT (TURN) [RFC5766].

3. Protocol Overview

When using ICE for SIP according to [RFC8839], 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 also allows ICE candidates 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
sends 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:

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 =====>| |
| | | |

Note: "SRFLX" denotes server-reflexive candidates

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 and
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 using it.

Contrary to other protocols, where "in advance" capability discovery
is widely implemented, the mechanisms that allow this for SIP (i.e.,
a combination of UA capabilities [RFC3840] and Globally Routable User
Agent URIs (GRUUs) [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 [RFC8838] provides one way around this, by requiring
the first Offer to contain a complete set of local ICE candidates and
using only 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, Section 5 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 online provisioning as a means of allowing the 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 5.3 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 SIP middleboxes and proxies, the Offer/Answer
exchange for Trickle ICE looks partly similar to the Offer/Answer
exchange for regular ICE for SIP [RFC8839]. However, in order to
have the full picture of the candidate exchange, the newly introduced
INFO messages need to be considered as well.

+-------------------------------+ +-------------------------------+
| Alice +--------------+ | | +--------------+ Bob |
| | Offer/Answer | | | | Offer/Answer | |
| +--------+ | Module | | | | Module | +--------+ |
| | ICE | +--------------+ | | +--------------+ | ICE | |
| | Module | | | | | | Module | |
| +--------+ | | | | +--------+ |
+-------------------------------+ +-------------------------------+
| | | |
| | 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 modules and not between Offer/Answer modules of
SIP UAs. 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 requests
SHALL be included in subsequent Offers or Answers. The version
number in the "o=" line of that subsequent Offer needs to be
incremented by 1 per the rules in [RFC3264].

4. Incremental Signaling of ICE Candidates

Trickle ICE Agents will exchange ICE descriptions compliant to
[RFC8838] via Offer/Answer procedures and/or INFO request bodies.
This requires the following SIP-specific extensions:

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 [RFC8838].

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
already included in the initial Offer, it 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 and/or an indication that
trickling of such candidates has ended.

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 may exist.

The following sections provide further details on how Trickle ICE
Agents perform the initial Offer/Answer exchange (Section 4.1),
perform subsequent Offer/Answer exchanges (Section 4.2), and
establish the INVITE dialog usage (Section 4.3) such that they can
incrementally trickle candidates (Section 4.4).

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 [RFC8839].

If the Offerer wants to send its initial Offer before knowing any
candidate for one or more media descriptions, it MUST set the port to
the default value '9' for these media descriptions. If the Offerer
does not want to include the host IP address in the corresponding
"c="line, e.g., due to privacy reasons, it SHOULD include a default
address in the "c="line, which is set to the IPv4 address 0.0.0.0 or
to the IPv6 equivalent ::.

In this case, the Offerer obviously cannot know the RTP Control
Protocol (RTCP) transport address; thus, it MUST NOT include the
"rtcp" attribute [RFC3605]. This avoids potential ICE mismatch (see
[RFC8839]) for the RTCP transport address.

If the Offerer wants to use RTCP multiplexing [RFC5761] and/or
exclusive RTCP multiplexing [RFC8858], it still will include the
"rtcp-mux" and/or "rctp-mux-only" attribute in the initial Offer.

In any case, the Offerer MUST include the "ice-options:trickle"
attribute in accordance to [RFC8838] and MUST include in each "m="
line a "mid" attribute in accordance to [RFC5888]. The "mid"
attribute identifies the "m=" line to which a candidate belongs and
helps in case of multiple "m=" lines, when candidate gathering could
occur in an order different from the order of the "m=" lines.

4.1.2. Receiving the Initial Offer

If the initial Offer included candidates, the Answerer uses these
candidates to start ICE processing as specified in [RFC8838].

If the initial Offer included the "ice-options:trickle" attribute,
the Answerer MUST be prepared for receiving trickled candidates later
on.

In case of a "m/c=" line with default values, none of the eventually
trickled candidates will match the default destination. This
situation MUST NOT cause an ICE mismatch (see [RFC8839]).

4.1.3. Sending the Initial Answer

If the Answerer includes candidates in its initial Answer, it MUST
encode these candidates as specified in [RFC8839].

If the Answerer wants to send its initial Answer before knowing any
candidate for one or more media descriptions, it MUST set the port to
the default value '9' for these media descriptions. If the Answerer
does not want to include the host IP address in the corresponding
"c="line, e.g., due to privacy reasons, it SHOULD include a default
address in the "c="line, which is set to the IPv4 address 0.0.0.0 or
to the IPv6 equivalent ::.

In this case, the Answerer obviously cannot know the RTCP transport
address; thus, it MUST NOT include the "rtcp" attribute [RFC6086].
This avoids potential ICE mismatch (see [RFC8839]) for the RTCP
transport address.

If the Answerer accepts the use of RTCP multiplexing [RFC5761] and/or
exclusive RTCP multiplexing [RFC8858], it will include the "rtcp-mux"
attribute in the initial Answer.

In any case, the Answerer MUST include the "ice-options:trickle"
attribute in accordance to [RFC8838] and MUST include in each "m="
line a "mid" attribute in accordance to [RFC5888].

4.1.4. Receiving the Initial Answer

If the initial Answer included candidates, the Offerer uses these
candidates to start ICE processing as specified in [RFC8838].

In case of a "m/c=" line with default values, none of the eventually
trickled candidates will match the default destination. This
situation MUST NOT cause an ICE mismatch (see [RFC8839]).

4.2. Subsequent Offer/Answer Exchanges

Subsequent Offer/Answer exchanges are handled the same as regular ICE
(see Section 4.4 of [RFC8839]).

If an Offer or Answer needs to be sent while the ICE Agents are in
the middle of trickling, Section 4.4 of [RFC8839] applies. This
means that an ICE Agent includes candidate attributes for all local
candidates it had trickled previously for a specific media stream.

4.3. Establishing the Dialog

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.

* A dialog MUST have been created between the peers.

Section 5 discusses in detail the various options for satisfying the
first of the above conditions. However, regardless of those
mechanisms, 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).

4.3.1. Establishing 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.) |
|<------------------------|
| |

Note: "SRFLX" denotes server-reflexive candidates

Figure 3: A 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 (or lack thereof) for Trickle
ICE at the Answerer 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.

4.3.2. Establishing 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).

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 or 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
backoff timers described in [RFC3262]. These retransmissions MUST
cease on receipt of an INFO request carrying a "trickle-ice" Info
Package body, on receipt of any other in-dialog request from the
Offerer, or on transmission of the Answer in a 2xx response. The
Offerer cannot send in-dialog requests until it receives a response,
so the arrival of such a request proves that the response has
arrived. Using the INFO request for dialog confirmation is similar
to the procedure described in Section 7.1.1 of [RFC8839], except that
the STUN binding request is replaced by the INFO request.

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). The first
case could happen when Half Trickle is used and all candidates are
already in the initial offer. The second case could happen when Full
Trickle is used and the Offerer is currently gathering additional
candidates but did not yet get them. Also, if the initial Offer did
not contain any candidates, depending on how the Offerer gathers its
candidates and how long it takes to do so, this INFO could still
contain no candidates.

When Full Trickle is used and if newly learned candidates are
available, the Offerer SHOULD also deliver these candidates in said
INFO request, unless it wants to hold back some candidates in
reserve, e.g., in case these candidates are expensive to use and
would only be trickled if all other candidates failed.

The Offerer SHOULD include an "end-of-candidates" attribute in case
candidate discovery has ended in the meantime and no further
candidates are to be trickled.

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 trickling can begin (Figure 5).

Note: The "+SRFLX" in Figure 5 indicates that additional newly
learned server-reflexive candidates are included.

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) |
|<------------------------|

Note: "SRFLX" denotes server-reflexive candidates

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 needs to 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.

4.3.3. Initiating Trickle ICE without an SDP Answer

The ability to convey arbitrary candidates in INFO message bodies
allows ICE Agents to initiate trickling without actually sending an
Answer. Trickle ICE Agents can therefore respond to an INVITE
request with provisional responses without an SDP Answer [RFC3261].
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 backoff timers
described in [RFC3262]. These retransmissions MUST cease on receipt
of an INFO request carrying a "trickle-ice" Info Package body, on
receipt of any in-dialog requests from the Offerer, or on
transmission of the Answer in a 2xx response. The Offerer cannot
send in-dialog requests until it receives a response, so the arrival
of such a request proves that the response has arrived. This is
again similar to the procedure described in Section 6.1.1 of
[RFC8839], except that an Answer is not yet provided.

Note: The "+SRFLX" in Figure 6 indicates that additional 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) |
|<------------------------|

Note: "SRFLX" denotes server-reflexive candidates

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. However, if that
Answer was already sent in an 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.

4.4. Delivering Candidates in INFO Requests

Whenever new ICE candidates become available for sending, agents
encode them in "candidate" attributes as described by [RFC8839]. For
example:

a=candidate:1 1 UDP 2130706432 200a0b:12f0::1 5000 typ host

The use of SIP INFO requests happens within the context of the Info
Package as defined in Section 10. The media type [RFC6838] for their
payload MUST be set to "application/trickle-ice-sdpfrag" as defined
in Section 9. The INFO request body adheres to the grammar as
specified in Section 9.2.

Since neither the "candidate" nor the "end-of-candidates" attributes
contain information that would allow correlating them to a specific
"m=" line, it is handled through the use of pseudo "m=" lines.

Pseudo "m=" lines follow the SDP syntax for "m=" lines as defined in
[RFC4566] and are linked to the corresponding "m=" line in the SDP
Offer or Answer via the identification tag in a "mid" attribute
[RFC5888]. A pseudo "m=" line does not provide 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, the
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 field MUST appear only once and is set to '0'.

Agents MUST include a pseudo "m=" line and an identification tag in a
"mid" attribute for every "m=" line whose candidate list they intend
to update. Such "mid" attributes MUST immediately precede the list
of candidates for that specific "m=" line.

All "candidate" or "end-of-candidates" attributes following 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.

Note, that there is no requirement that the INFO request body
contains as many pseudo "m=" lines as the Offer/Answer contains "m="
lines, nor that the pseudo "m=" lines be in the same order as the
"m=" lines that they pertain to. The correspondence can be made via
the "mid" attributes since candidates are grouped in sections headed
by "pseudo" "m=" lines. These sections contain "mid" attribute
values that point back to the true "m=" line.

An "end-of-candidates" attribute, preceding the first 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 "end-of-candidates" attribute.

Refer to Figure 7 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 the first 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 "mid" attribute
| from [RFC5888], it does not define any grouping semantics.

All INFO requests MUST carry the "ice-pwd" and "ice-ufrag" attributes
that allow mapping them to a specific ICE generation. An agent MUST
discard any received INFO requests containing "ice-pwd" and "ice-
ufrag" attributes that do not match those of the current ICE
Negotiation Session.

The "ice-pwd" and "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 the
first pseudo "m=" line. 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 when candidates are trickled, [RFC8838] requires that 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. That is, the agent
MUST repeat in the INFO request body all candidates that were
previously sent under the same combination of "ice-pwd" and "ice-
ufrag" in the same order as they were sent before. 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.

In addition, an ICE Agent needs to adhere to Section 17 of [RFC8838]
on preserving candidate order while trickling.

When receiving INFO requests carrying any candidates, agents MUST
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.

Such candidates are considered to be equal if their IP address port,
transport, and component ID are the same. After identifying and
discarding the known candidates, the agents MUST forward the actual
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 [RFC8838].

Receiving an "end-of-candidates" attribute in an INFO request body --
with the "ice-ufrag" and "ice-pwd" attributes matching the current
ICE generation -- is an indication from the peer agent that it will
not send any further candidates. When included at the session level,
i.e., before any pseudo "m=" line, this indication applies to the
whole session; when included at the media level, the indication
applies only to the corresponding "m=" line. Handling of such end-
of-candidates indications is defined in [RFC8838].

The example in Figure 7 shows the content of a candidate delivering
INFO request. In the example, the "end-of-candidates" attributes
indicate that the candidate gathering is finished and that no further
INFO requests follow.

INFO sip:[email protected] SIP/2.0
...
Info-Package: trickle-ice
Content-type: application/trickle-ice-sdpfrag
Content-Disposition: Info-Package
Content-length: 862

a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 9 RTP/AVP 0
a=mid:1
a=candidate:1 1 UDP 2130706432 2001:db8:a0b:12f0::1 5000 typ host
a=candidate:1 2 UDP 2130706432 2001:db8:a0b:12f0::1 5001 typ host
a=candidate:1 1 UDP 2130706431 192.0.2.1 5010 typ host
a=candidate:1 2 UDP 2130706431 192.0.2.1 5011 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 5010 typ srflx
raddr 192.0.2.1 rport 8998
a=candidate:2 2 UDP 1694498815 192.0.2.3 5011 typ srflx
raddr 192.0.2.1 rport 8998
a=end-of-candidates
m=audio 9 RTP/AVP 0
a=mid:2
a=candidate:1 1 UDP 2130706432 2001:db8:a0b:12f0::1 6000 typ host
a=candidate:1 2 UDP 2130706432 2001:db8:a0b:12f0::1 6001 typ host
a=candidate:1 1 UDP 2130706431 192.0.2.1 6010 typ host
a=candidate:1 2 UDP 2130706431 192.0.2.1 6011 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 6010 typ srflx
raddr 192.0.2.1 rport 9998
a=candidate:2 2 UDP 1694498815 192.0.2.3 6011 typ srflx
raddr 192.0.2.1 rport 9998
a=end-of-candidates

Note: In a real INFO request, there will be no line breaks
in the "candidate" attributes

Figure 7: An Example for the Content of an INFO Request

5. Initial Discovery of Trickle ICE Support

SIP UAs are required by [RFC8838] to indicate their support of and
intent to use Trickle ICE in their Offers and Answers by using the
"ice-options:trickle" attribute, and they MUST include the SIP
option-tag "trickle-ice" in a SIP Supported: or Require: header
field. This makes discovery fairly straightforward for Answerers or
for cases where Offers need to be generated within existing dialogs
(i.e., when sending UPDATE or re-INVITE requests). In both
scenarios, prior SDP bodies will have provided the necessary
information.

Obviously, such information 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.

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 the
case, for example, if Session Border Controllers (SBCs) with support
for this specification are used to connect to UAs that do not support
Trickle ICE.

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.

However, an Offerer assuming Trickle ICE support MUST include a SIP
Require: trickle-ice header field. That way, if the provisioned
assumption of Trickle ICE support ends up being incorrect, the
failure is (a) operationally easy to track down and (b) recoverable
by the client, i.e., they can resend the request without the SIP
Require: header field and without the assumption of Trickle ICE
support.

5.2. Trickle ICE Discovery with Globally Routable User Agent URIs
(GRUUs)

[RFC3840] provides a way for SIP UAs to query for support of specific
capabilities using, among others, OPTIONS requests. On the other
hand, support for GRUU according to [RFC5627] 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 SIP UA instances and then
addressing them directly with INVITE requests that require Trickle
ICE support.

Such learning of capabilities may happen in different ways. One
option for a SIP UA is to learn the GRUU instance ID of a peer
through presence and then to query its capabilities with an OPTIONS
request. Alternatively, it can also just send an OPTIONS request to
the Address of Record (AOR) it intends to contact and then inspect
the returned response(s) for support of both GRUU and Trickle ICE
(Figure 8). It is noted that using the GRUU means that the INVITE
request can go only to that particular device. This prevents the use
of forking for that request.

Alice Bob
| |
| OPTIONS sip:[email protected] SIP/2.0 |
|-------------------------------------------------->|
| |
| 200 OK |
| Contact: sip:[email protected];gr=hha9s8d-999a |
| ;audio;video|;trickle-ice;... |
|<--------------------------------------------------|
| |
| INVITE sip:[email protected];gr=hha9s8d-999a SIP/2.0 |
| Supported: trickle-ice |
| (Offer) |
|-------------------------------------------------->|
| |
| 183 (Answer) |
|<--------------------------------------------------|
| INFO/OK (Trickling) |
|<------------------------------------------------->|
| |
| ... |
| |

Figure 8: Trickle ICE Support Discovery with OPTIONS and GRUU

Confirming support for Trickle ICE through [RFC3840] gives SIP UAs
the option to engage in Full Trickle negotiation (as opposed to the
more lengthy Half Trickle) from the very first Offer they send.

5.3. 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
[RFC8838]. With Half Trickle, agents initiate sessions the same way
they would when using ICE for SIP [RFC8839]. This means that, prior
to actually sending an Offer, agents first gather ICE candidates in a
blocking way and then send them all in that Offer. The blocking
nature of the process implies that some amount of latency will be
accumulated, and it is advised that agents try to anticipate it where
possible, 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 results in Offers that are compatible with both
ICE SIP endpoints [RFC8839] and legacy endpoints [RFC3264].

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 9: Example of a Typical (Half) Trickle ICE Exchange with SIP

As a reminder, once a single Offer or Answer has 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.

6. Considerations for RTP and RTCP Multiplexing

The following consideration describes options for Trickle ICE in
order to give some guidance to implementers on how trickling can be
optimized with respect to providing RTCP candidates.

Handling of the "rtcp" attribute [RFC3605] and the "rtcp-mux"
attribute for RTP/RTCP multiplexing [RFC5761] is already considered
in Section 5.1.1.1 of [RFC8445] and in [RFC5761]. These
considerations are still valid for Trickle ICE; however, trickling
provides more flexibility for the sequence of candidate exchange in
case of RTCP multiplexing.

If the Offerer supports RTP/RTCP multiplexing exclusively as
specified in [RFC8858], the procedures in that document apply for the
handling of the "rtcp-mux-only", "rtcp", and "rtcp-mux" attributes.

While a Half Trickle Offerer has to send an Offer compliant to
[RFC8839] and [RFC5761] including candidates for all components, the
flexibility of a Full Trickle Offerer allows the sending of only RTP
candidates (component 1) in the initial Offer assuming 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 "rtcp-mux" attribute [RFC5761] in
the "application/trickle-ice-sdpfrag" body if it supports and uses
RTP and RTCP multiplexing. The Trickle Answerer needs to follow the
guidance on the usage of the "rtcp" attribute as given in [RFC8839]
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 the 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.

v=0
o=alice 2890844526 2890844526 IN IP6 atlanta.example.com
s=
c=IN IP6 2001:db8:a0b:12f0::3
t=0 0
a=ice-pwd:777uzjYhagZgasd88fgpdd
a=ice-ufrag:Yhh8
m=audio 5000 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 4.3, the
Answerer sends the following INFO request.

INFO sip:[email protected] SIP/2.0
...
Info-Package: trickle-ice
Content-type: application/trickle-ice-sdpfrag
Content-Disposition: Info-Package
Content-length: 161

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 1658497382 2001:db8:a0b:12f0::4 6000 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 RTCP candidates or releasing already gathered RTCP
candidates. If the INFO request did not contain the "rtcp-mux"
attribute, the Offerer has 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. In case the Offerer
already sent RTCP candidates in a previous INFO request, it still
needs to repeat them in subsequent INFO requests, even when that
support for RTCP multiplexing was confirmed by the Answerer and the
Offerer has released its RTCP candidates.

7. Considerations for Media Multiplexing

The following considerations describe options for Trickle ICE in
order to give some guidance to implementers on how trickling can be
optimized with respect to providing candidates in case of Media
Multiplexing [RFC8843]. It is assumed that the reader is familiar
with [RFC8843].

ICE candidate exchange is already considered in Section 10 of
[RFC8843]. 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 has 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.

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 that is
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 a "group:BUNDLE" attribute
[RFC8843] at session level 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 behavior is illustrated by the following example Offer that
indicates support for Media Multiplexing.

If the Offerer already sent candidates for "m=" lines in a bundle in
a previous INFO request, it still needs to repeat them in subsequent
INFO requests, even when that support for bundling was confirmed by
the Answerer and the Offerer has released candidates that are no
longer needed.

v=0
o=alice 2890844526 2890844526 IN IP6 atlanta.example.com
s=
c=IN IP6 2001:db8:a0b:12f0::3
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=rtcp-mux
a=rtpmap:0 PCMU/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
a=candidate:1 1 UDP 1658497328 2001:db8:a0b:12f0::3 10000 typ host
m=video 10002 RTP/AVP 31
a=mid:bar
a=rtcp-mux
a=rtpmap:31 H261/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid

The example Offer indicates support for RTP and RTCP multiplexing and
contains a "candidate" attribute only for the "m=" line with the
suggested Offerer BUNDLE address. Once the dialog is established as
described in Section 4.3, the Answerer sends the following INFO
request.

INFO sip:[email protected] SIP/2.0
...
Info-Package: trickle-ice
Content-type: application/trickle-ice-sdpfrag
Content-Disposition: Info-Package
Content-length: 219

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

This INFO request indicates that the Answerer supports and uses Media
Multiplexing as well. Note that the Answerer only includes a single
pseudo "m=" line since candidates matching those from the second "m="
line in the offer are not needed from the Answerer.

The INFO request also indicates that the Answerer accepted the
suggested Offerer BUNDLE address. This allows the Offerer to omit
gathering RTP and RTCP candidates for the other "m=" lines or
releasing already gathered candidates. If the INFO request did not
contain the "group:BUNDLE" attribute, the Offerer has 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 [RFC8859] 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-candidates" Attribute

8.1. Definition

This section defines the new SDP media-level and session-level
[RFC4566] "end-of-candidates" attribute. "end-of-candidates" is a
property attribute [RFC4566]; hence, it 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 the
session level, this indication applies to the whole session; when
included at the media level, the indication applies only to the
corresponding media description.

Name: end-of-candidates

Value: N/A

Usage Level: media and session level

Charset Dependent: no

Mux Category: IDENTICAL

Example: a=end-of-candidates

8.2. Offer/Answer Procedures

The Offerer or Answerer MAY include an "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 "end-of-
candidates" attribute together with the "ice-ufrag" and "ice-pwd"
attributes of the current ICE generation as required by [RFC8838].
When included at the session level, this indication applies to the
whole session; when included at the media level, the indication
applies only to the corresponding media description.

Receipt of an "end-of-candidates" attribute at an Offerer or Answerer
-- with the "ice-ufrag" and "ice-pwd" attributes matching the current
ICE generation -- indicates that the gathering of candidates has
ended at the peer, for either the session or only 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 [RFC8838].

9. Content Type "application/trickle-ice-sdpfrag"

9.1. Overall Description

An "application/trickle-ice-sdpfrag" body is used exclusively by the
"trickle-ice" Info Package. Other SDP-related applications need to
define their own media type. The INFO request body uses a subset of
the possible SDP lines as defined by the grammar 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 is needed or useful for trickling candidates.
The grammar uses the indicator for case-sensitive %s, as defined in
[RFC7405], but it also imports grammar for other SDP attributes that
precede the production of [RFC7405]. A sender SHOULD use lower case
for attributes from such earlier grammar, but a receiver MUST treat
them as case insensitive.

; Syntax
trickle-ice-sdpfrag = session-level-fields
pseudo-media-descriptions
session-level-fields = *(session-level-field CRLF)

session-level-field = ice-lite-attribute /
ice-pwd-attribute /
ice-ufrag-attribute /
ice-options-attribute /
ice-pacing-attribute /
end-of-candidates-attribute /
bundle-group-attribute /
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
end-of-candidates-attribute = %s"a" "=" end-of-candidates
end-of-candidates = %s"end-of-candidates"
bundle-group-attribute = %s"a" "=" %s"group:" bundle-semantics
*(SP identification-tag)
bundle-semantics = "BUNDLE"
extension-attribute-fields = attribute-fields

pseudo-media-descriptions = *( media-field
trickle-ice-attribute-fields )
trickle-ice-attribute-fields = *(trickle-ice-attribute-field CRLF)
trickle-ice-attribute-field = mid-attribute /
candidate-attributes /
ice-pwd-attribute /
ice-ufrag-attribute /
remote-candidate-attribute /
end-of-candidates-attribute /
rtcp-attribute /
rtcp-mux-attribute /
rtcp-mux-only-attribute /
extension-attribute-fields
; for future extensions

rtcp-attribute = %s"a" "=" %s"rtcp"
rtcp-mux-attribute = %s"a" "=" %s"rtcp-mux"
rtcp-mux-only-attribute = %s"a" "=" %s"rtcp-mux-only"
candidate-attributes = %s"a" "=" candidate-attribute
remote-candidate-attribute = %s"a" "=" remote-candidate-att

ice-lite, ice-pwd-att, remote-candidate-att, ice-ufrag-att, ice-
pacing-att, ice-options, candidate-attribute, and remote-candidate-
att are from [RFC8839]; identification-tag and mid-attribute are from
[RFC5888]; and media-field and attribute-fields are from [RFC4566].
The "rtcp" attribute is defined in [RFC3605], the "rtcp-mux"
attribute is defined in [RFC5761], and the "rtcp-mux-only" attribute
is defined in [RFC8858]. The latter attributes lack formal grammar
in their corresponding RFCs and are reproduced here.

The "ice-pwd" and "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.

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 requests can be exchanged both ways with no
restrictions on timing and frequency and no risk of collision.

A critical fact is that the sending of Trickle ICE candidates in one
direction is entirely uncoupled from sending candidates in the other
direction. Thus, the sending of candidates in each direction can be
done by a stream of INFO requests that is not correlated with the
stream of INFO requests in the other direction. And since each INFO
request cumulatively includes the contents of all previous INFO
requests in that direction, the ordering between INFO requests need
not be preserved. All of this permits using largely independent INFO
requests.

Contrarily, UPDATE or other Offer/Answer mechanisms assume that the
messages in each direction are tightly coupled with messages in the
other direction. Using Offer/Answer and UPDATE requests [RFC3311]
would introduce the following complications:

Blocking of messages: Offer/Answer is defined as a strictly
sequential mechanism in [RFC3264]. There can only be a maximum of
one active 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 and the average two-second backoff
intervals to occur implies that the duration of Trickle ICE
processing 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 GRUU
[RFC5627] that, 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 UAs
implementing Trickle ICE. INFO requests carry ICE candidates
discovered after the peer UAs 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: or Require: 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: or Require: header field.

10.7. INFO Request Body Parts

Entities implementing this specification MUST include a payload of
type "application/trickle-ice-sdpfrag" in SIP INFO requests as
defined in Section 9.2. 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

Given that IP addresses may be gathered rapidly, 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. An implementation MUST aggregate ICE candidates
in case an unreliable transport protocol such as UDP is used. A
Trickle ICE Agent MUST NOT have more than one INFO request pending at
any one time. When INFO messages are sent over an unreliable
transport, they are retransmitted according to the rules specified in
[RFC3261], Section 17.1.2.1.

If the INFO requests are sent on top of TCP, which is probably the
standard way, it is not an issue for the network anymore, but it can
remain one for SIP proxies and other intermediaries forwarding the
SIP INFO messages. Also, an endpoint may not be able to tell that it
has congestion controlled transport all the way.

10.10. Info Package Security Considerations

See Section 13.

11. Deployment Considerations

Trickle ICE uses two mechanisms for the exchange of candidate
information. This imposes new requirements to certain middleboxes
that are used in some networks, e.g., for monitoring purposes. While
the first mechanism, SDP Offers and Answers, is already used by
regular ICE and is assumed to be supported, the second mechanism,
INFO request bodies, needs to be considered by such middleboxes as
well when trickle ICE is used. Such middleboxes need to make sure
that they remain in the signaling path of the INFO requests and
understand the INFO request body.

12. IANA Considerations

12.1. SDP "end-of-candidates" Attribute

This section defines a new SDP media-level and session-level
[RFC4566] "end-of-candidates" attribute, which is a property
attribute [RFC4566] and hence has no value.

Name: end-of-candidates

Value: N/A

Usage Level: media and session

Charset Dependent: no

Purpose: The sender indicates that it will not trickle further
ICE candidates.

O/A Procedures: RFC 8840 defines the detailed SDP Offer/Answer
procedures for the "end-of-candidates" attribute.

Mux Category: IDENTICAL

Reference: RFC 8840

Example: a=end-of-candidates

12.2. Media Type "application/trickle-ice-sdpfrag"

This document defines the new media type "application/trickle-ice-
sdpfrag" in accordance with [RFC6838].

Type name: application

Subtype name: trickle-ice-sdpfrag

Required parameters: None.

Optional parameters: None.

Encoding considerations: The media contents follow the same rules as
SDP, except as noted in this document. The media contents are
text, with the grammar specified in Section 9.2.

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 "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: See [RFC4566] and RFC 8840

Interoperability considerations: See RFC 8840

Published specification: See RFC 8840

Applications that use this media type: Trickle ICE

Fragment identifier considerations: N/A

Additional information:

Deprecated alias names for this type: N/A
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: The
IESG ([email protected])

Intended usage: Trickle ICE for SIP as specified in RFC 8840.

Restrictions on usage: N/A

Author/Change controller: The IESG ([email protected])

Provisional registration? (standards tree only): N/A

12.3. 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 | RFC 8840 |
+-------------+-----------+

Table 1

12.4. 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" subregistry of the SIP Parameters registry with the
following entry:

+=============+==============================+===========+
| Name | Description | Reference |
+=============+==============================+===========+
| trickle-ice | This option tag is used to | RFC 8840 |
| | indicate that a UA supports | |
| | and understands Trickle ICE. | |
+-------------+------------------------------+-----------+

Table 2

13. Security Considerations

The Security Considerations of [RFC6086], [RFC8838], and [RFC8839]
apply. This document clarifies how the above specifications are used
together for trickling candidates and does not create additional
security risks.

The new Info Package "trickle-ice" and the new media type
"application/trickle-ice-sdpfrag" do not introduce additional
security considerations when used in the context of Trickle ICE.
Both are not intended to be used for other applications, so any
security considerations for its use in other contexts is out of the
scope of this document

14. References

14.1. Normative References

[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>.

[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>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

[RFC8838] Ivov, E., Uberti, J., and P. Saint-Andre, "Trickle ICE:
Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol", RFC 8838,
DOI 10.17487/RFC8838, January 2021,
<https://www.rfc-editor.org/info/rfc8838>.

[RFC8839] Petit-Huguenin, M., Nandakumar, S., Holmberg, C., Keränen,
A., and R. Shpount, "Session Description Protocol (SDP)
Offer/Answer Procedures for Interactive Connectivity
Establishment (ICE)", RFC 8839, DOI 10.17487/RFC8839,
January 2021, <https://www.rfc-editor.org/info/rfc8839>.

[RFC8843] Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", RFC 8843,
DOI 10.17487/RFC8843, January 2021,
<https://www.rfc-editor.org/info/rfc8843>.

[RFC8858] Holmberg, C., "Indicating Exclusive Support of RTP and RTP
Control Protocol (RTCP) Multiplexing Using the Session
Description Protocol (SDP)", RFC 8858,
DOI 10.17487/RFC8858, January 2021,
<https://www.rfc-editor.org/info/rfc8858>.

[RFC8859] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859,
DOI 10.17487/RFC8859, January 2021,
<https://www.rfc-editor.org/info/rfc8859>.

14.2. 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>.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<https://www.rfc-editor.org/info/rfc5389>.

[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>.

[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5766>.

Acknowledgements

The authors 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.

The authors also like to thank Flemming Andreasen for shepherding
this document and Ben Campbell for his AD review and suggestions. In
addition, the authors thank Benjamin Kaduk, Adam Roach, Mirja
Kühlewind, and Eric Rescorla for their comments and/or text proposals
for improving the document during IESG review.

Many thanks to Dale Worley for the Gen-Art review and proposed
enhancements for several sections.

Many thanks to Joerg Ott for the TSV-Art review and suggested
improvements.

The authors thank Shawn Emery for the Security Directorate review.

Authors' Addresses

Emil Ivov
Jitsi
67000 Strasbourg
France

Phone: +33 6 72 81 15 55
Email: [email protected]

Thomas Stach
Unaffiliated
1130 Vienna
Austria

Email: [email protected]

Enrico Marocco
Telecom Italia
Via G. Reiss Romoli, 274
10148 Turin
Italy

Email: [email protected]

Christer Holmberg
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland

Email: [email protected]