Using Interactive Connectivity Establishment (ICE) with Session Description Protocol (SDP) offer/answer and Session Initiation Protocol (SIP)
draft-ietf-mmusic-ice-sip-sdp-02

Abstract

This document describes how Interactive Connectivity Establishment (ICE) is used with Session Description Protocol (SDP) offer/answer and Session Initiation Protocol (SIP).

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1. Introduction

[NOTE: this version of the document shows merely which parts of the original ICE document could be split to a separate document if the split of SDP is accepted by the WG. Later versions will define the additional procedures needed]

This document describes how Interactive Connectivity Establishment (ICE) is used with Session Description Protocol (SDP) offer/answer and Session Initiation Protocol (SIP). The ICE specification [ICE-BIS] describes procedures that are common to all usages of ICE and this document gives the additional details needed to use ICE with SIP and SDP offer/answer.

Note that ICE is not intended for NAT traversal for SIP, which is assumed to be provided via another mechanism [RFC5626].

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 RFC 2119 [RFC2119].

3. Sending the Initial Offer

3.1. Choosing Default Candidates

A candidate is said to be default if it would be the target of media from a non-ICE peer; that target is called the DEFAULT DESTINATION. If the default candidates are not selected by the ICE algorithm when communicating with an ICE-aware peer, an updated offer/answer will be required after ICE processing completes in order to "fix up" the SDP so that the default destination for media matches the candidates selected by ICE. If ICE happens to select the default candidates, no updated offer/answer is required.

An agent MUST choose a set of candidates, one for each component of each in-use media stream, to be default. A media stream is in-use if it does not have a port of zero (which is used in RFC 3264 to reject a media stream). Consequently, a media stream is in-use even if it is marked as a=inactive [RFC4566] or has a bandwidth value of zero.

It is RECOMMENDED that default candidates be chosen based on the likelihood of those candidates to work with the peer that is being contacted. It is RECOMMENDED that the default candidates are the relayed candidates (if relayed candidates are available), server reflexive candidates (if server reflexive candidates are available), and finally host candidates.

3.2. Encoding the SDP

The process of encoding the SDP is identical between full and lite implementations.

The agent will include an m line for each media stream it wishes to use. The ordering of media streams in the SDP is relevant for ICE. ICE will perform its connectivity checks for the first m line first, and consequently media will be able to flow for that stream first. Agents SHOULD place their most important media stream, if there is one, first in the SDP.

There will be a candidate attribute for each candidate for a particular media stream. Section 8 provides detailed rules for constructing this attribute.

STUN connectivity checks between agents are authenticated using the short-term credential mechanism defined for STUN [RFC5389]. This mechanism relies on a username and password that are exchanged through protocol machinery between the client and server. The username fragment and password are exchanged in the ice-ufrag and ice-pwd attributes, respectively.

If an agent is a lite implementation, it MUST include an "a=ice-lite" session-level attribute in its SDP to indicate this. If an agent is a full implementation, it MUST NOT include this attribute.

The default candidates are added to the SDP as the default destination for media. For streams based on RTP, this is done by placing the IP address and port of the RTP candidate into the c and m lines, respectively. If the agent is utilizing RTCP, it MUST encode the RTCP candidate using the a=rtcp attribute as defined in RFC 3605 [RFC3605]. If RTCP is not in use, the agent MUST signal that using b=RS:0 and b=RR:0 as defined in RFC 3556 [RFC3556].

The transport addresses that will be the default destination for media when communicating with non-ICE peers MUST also be present as candidates in one or more a=candidate lines.

ICE provides for extensibility by allowing an offer or answer to contain a series of tokens that identify the ICE extensions used by that agent. If an agent supports an ICE extension, it MUST include the token defined for that extension in the ice-options attribute.

The following is an example SDP message that includes ICE attributes (lines folded for readability):

Once an agent has sent its offer or its answer, that agent MUST be prepared to receive both STUN and media packets on each candidate. As discussed in Section 10.1 of [ICE-BIS], media packets can be sent to a candidate prior to its appearance as the default destination for media in an offer or answer.

4. Receiving the Initial Offer

4.1. Choosing Default Candidates

The process for selecting default candidates at the answerer is identical to the process followed by the offerer, as described in Section 3.1 for full implementations and 4.2 of [ICE-BIS] for lite implementations.

4.2. Verifying ICE Support

The agent will proceed with the ICE procedures defined in [ICE-BIS] and this specification if, for each media stream in the SDP it received, the default destination for each component of that media stream appears in a candidate attribute. For example, in the case of RTP, the IP address and port in the c and m lines, respectively, appear in a candidate attribute and the value in the rtcp attribute appears in a candidate attribute.

If this condition is not met, the agent MUST process the SDP based on normal RFC 3264 procedures, without using any of the ICE mechanisms described in the remainder of this specification with the following exceptions:

4.3. Determining Role

In unusual cases, described in Appendix C, it is possible for both agents to mistakenly believe they are controlled or controlling. To resolve this, each agent MUST select a random number, called the tie-breaker, uniformly distributed between 0 and (2**64) - 1 (that is, a 64-bit positive integer). This number is used in connectivity checks to detect and repair this case, as described in Section 7.1.2.2 of [ICE-BIS].

5. Receipt of the Initial Answer

When ICE is used with SIP, forking may result in a single offer generating a multiplicity of answers. In that case, ICE proceeds completely in parallel and independently for each answer, treating the combination of its offer and each answer as an independent offer/answer exchange, with its own set of pairs, check lists, states, and so on. The only case in which processing of one pair impacts another is freeing of candidates, discussed below in Section 7.2.

5.1. Verifying ICE Support

The logic at the offerer is identical to that of the answerer as described in section 5.1 of [ICE-BIS], with the exception that an offerer would not ever generate a=ice-mismatch attributes in an SDP.

In some cases, the answer may omit a=candidate attributes for the media streams, and instead include an a=ice-mismatch attribute for one or more of the media streams in the SDP. This signals to the offerer that the answerer supports ICE, but that ICE processing was not used for the session because a signaling intermediary modified the default destination for media components without modifying the corresponding candidate attributes. See Section 15.2.2 for a discussion of cases where this can happen. This specification provides no guidance on how an agent should proceed in such a failure case.

6. Performing Connectivity Checks

The possibility for role conflicts described in Section 7.2.1.1 of [ICE-BIS] applies to this usage and hence all full agents MUST implement the role conflict repairing mechanism. Also both full and lite agents MUST utilize the ICE-CONTROLLED and ICE-CONTROLLING attributes as described in Section 7.1.2.2 of [ICE-BIS].

7. Concluding ICE

Once all of the media streams are completed, the controlling endpoint sends an updated offer if the candidates in the m and c lines for the media stream (called the DEFAULT CANDIDATES) don't match ICE's SELECTED CANDIDATES.

7.1. Procedures for Full Implementations

7.1.1. Updating states

Once the state of each check list is Completed, If an agent is controlling, it examines the highest-priority nominated candidate pair for each component of each media stream. If any of those candidate pairs differ from the default candidate pairs in the most recent offer/answer exchange, the controlling agent MUST generate an updated offer as described in Section 9.

7.2. Freeing Candidates

7.2.1. Full Implementation Procedures

When ICE is used with SIP, and an offer is forked to multiple recipients, ICE proceeds in parallel and independently with each answerer, all using the same local candidates. Once ICE processing has reached the Completed state for all peers for media streams using those candidates, the agent SHOULD wait an additional three seconds, and then it MAY cease responding to checks or generating triggered checks on that candidate. It MAY free the candidate at that time. Freeing of server reflexive candidates is never explicit; it happens by lack of a keepalive. The three-second delay handles cases when aggressive nomination is used, and the selected pairs can quickly change after ICE has completed.

8. Grammar

This specification defines seven new SDP attributes -- the "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-ufrag", "ice-pwd", and "ice-options" attributes.

8.1. "candidate" Attribute

The candidate attribute is a media-level attribute only. It contains a transport address for a candidate that can be used for connectivity checks.

The syntax of this attribute is defined using Augmented BNF as defined in [RFC5234]:

This grammar encodes the primary information about a candidate: its IP address, port and transport protocol, and its properties: the foundation, component ID, priority, type, and related transport address:

The candidate attribute can itself be extended. The grammar allows for new name/value pairs to be added at the end of the attribute. An implementation MUST ignore any name/value pairs it doesn't understand.

8.2. "remote-candidates" Attribute

The syntax of the "remote-candidates" attribute is defined using Augmented BNF as defined in RFC 5234 [RFC5234]. The remote-candidates attribute is a media-level attribute only.

The attribute contains a connection-address and port for each component. The ordering of components is irrelevant. However, a value MUST be present for each component of a media stream. This attribute MUST be included in an offer by a controlling agent for a media stream that is Completed, and MUST NOT be included in any other case.

8.3. "ice-lite" and "ice-mismatch" Attributes

The syntax of the "ice-lite" and "ice-mismatch" attributes, both of which are flags, is:

"ice-lite" is a session-level attribute only, and indicates that an agent is a lite implementation. "ice-mismatch" is a media-level attribute only, and when present in an answer, indicates that the offer arrived with a default destination for a media component that didn't have a corresponding candidate attribute.

8.4. "ice-ufrag" and "ice-pwd" Attributes

The "ice-ufrag" and "ice-pwd" attributes convey the username fragment and password used by ICE for message integrity. Their syntax is:

The "ice-pwd" and "ice-ufrag" attributes can appear at either the session-level or media-level. When present in both, the value in the media-level takes precedence. Thus, the value at the session-level is effectively a default that applies to all media streams, unless overridden by a media-level value. Whether present at the session or media-level, there MUST be an ice-pwd and ice-ufrag attribute for each media stream. If two media streams have identical ice-ufrag's, they MUST have identical ice-pwd's.

The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the beginning of a session. The ice-ufrag attribute MUST contain at least 24 bits of randomness, and the ice-pwd attribute MUST contain at least 128 bits of randomness. This means that the ice-ufrag attribute will be at least 4 characters long, and the ice-pwd at least 22 characters long, since the grammar for these attributes allows for 6 bits of randomness per character. The attributes MAY be longer than 4 and 22 characters, respectively, of course, up to 256 characters. The upper limit allows for buffer sizing in implementations. Its large upper limit allows for increased amounts of randomness to be added over time. For compatibility with the 512 character limitation for the STUN username attribute value and for bandwidth conservation considerations, the ice-ufrag attribute MUST NOT be longer than 32 characters when sending, but an implementation MUST accept up to 256 characters when receiving.

8.5. "ice-options" Attribute

The "ice-options" attribute is a session- and media-level attribute. It contains a series of tokens that identify the options supported by the agent. Its grammar is:

9. Subsequent Offer/Answer Exchanges

Either agent MAY generate a subsequent offer at any time allowed by RFC 3264 [RFC3264]. The rules in Section 7 will cause the controlling agent to send an updated offer at the conclusion of ICE processing when ICE has selected different candidate pairs from the default pairs. This section defines rules for construction of subsequent offers and answers.

Should a subsequent offer be rejected, ICE processing continues as if the subsequent offer had never been made.

9.1. Generating the Offer

9.1.1. Procedures for All Implementations

9.1.1.1. ICE Restarts

An agent MAY restart ICE processing for an existing media stream. An ICE restart, as the name implies, will cause all previous states of ICE processing to be flushed and checks to start anew. The only difference between an ICE restart and a brand new media session is that, during the restart, media can continue to be sent to the previously validated pair.

These rules imply that setting the IP address in the c line to 0.0.0.0 will cause an ICE restart. Consequently, ICE implementations MUST NOT utilize this mechanism for call hold, and instead MUST use a=inactive and a=sendonly as described in [RFC3264].

To restart ICE, an agent MUST change both the ice-pwd and the ice-ufrag for the media stream in an offer. Note that it is permissible to use a session-level attribute in one offer, but to provide the same ice-pwd or ice-ufrag as a media-level attribute in a subsequent offer. This is not a change in password, just a change in its representation, and does not cause an ICE restart.

An agent sets the rest of the fields in the SDP for this media stream as it would in an initial offer of this media stream (see Section 3.2). Consequently, the set of candidates MAY include some, none, or all of the previous candidates for that stream and MAY include a totally new set of candidates.

9.1.1.2. Removing a Media Stream

If an agent removes a media stream by setting its port to zero, it MUST NOT include any candidate attributes for that media stream and SHOULD NOT include any other ICE-related attributes defined in Section 8 for that media stream.

9.1.1.3. Adding a Media Stream

If an agent wishes to add a new media stream, it sets the fields in the SDP for this media stream as if this was an initial offer for that media stream (see Section 3.2). This will cause ICE processing to begin for this media stream.

9.1.2. Procedures for Full Implementations

This section describes additional procedures for full implementations, covering existing media streams.

The username fragments, password, and implementation level MUST remain the same as used previously. If an agent needs to change one of these, it MUST restart ICE for that media stream.

9.1.2.1. Existing Media Streams with ICE Running

If an agent generates an updated offer including a media stream that was previously established, and for which ICE checks are in the Running state, the agent follows the procedures defined here.

An agent MUST include candidate attributes for all local candidates it had signaled previously for that media stream. The properties of that candidate as signaled in SDP -- the priority, foundation, type, and related transport address -- SHOULD remain the same. The IP address, port, and transport protocol, which fundamentally identify that candidate, MUST remain the same (if they change, it would be a new candidate). The component ID MUST remain the same. The agent MAY include additional candidates it did not offer previously, but which it has gathered since the last offer/answer exchange, including peer reflexive candidates.

The agent MAY change the default destination for media. As with initial offers, there MUST be a set of candidate attributes in the offer matching this default destination.

9.1.2.2. Existing Media Streams with ICE Completed

If an agent generates an updated offer including a media stream that was previously established, and for which ICE checks are in the Completed state, the agent follows the procedures defined here.

The default destination for media (i.e., the values of the IP addresses and ports in the m and c lines used for that media stream) MUST be the local candidate from the highest-priority nominated pair in the valid list for each component. This "fixes" the default destination for media to equal the destination ICE has selected for media.

The agent MUST include candidate attributes for candidates matching the default destination for each component of the media stream, and MUST NOT include any other candidates.

In addition, if the agent is controlling, it MUST include the a=remote-candidates attribute for each media stream whose check list is in the Completed state. The attribute contains the remote candidates from the highest-priority nominated pair in the valid list for each component of that media stream. It is needed to avoid a race condition whereby the controlling agent chooses its pairs, but the updated offer beats the connectivity checks to the controlled agent, which doesn't even know these pairs are valid, let alone selected. See Appendix B for elaboration on this race condition.

9.1.3. Procedures for Lite Implementations

9.1.3.1. Existing Media Streams with ICE Running

This section describes procedures for lite implementations for existing streams for which ICE is running.

A lite implementation MUST include all of its candidates for each component of each media stream in an a=candidate attribute in any subsequent offer. These candidates are formed identically to the procedures for initial offers, as described in section 4.2 of [ICE-BIS].

A lite implementation MUST NOT add additional host candidates in a subsequent offer. If an agent needs to offer additional candidates, it MUST restart ICE.

The username fragments, password, and implementation level MUST remain the same as used previously. If an agent needs to change one of these, it MUST restart ICE for that media stream.

9.1.3.2. Existing Media Streams with ICE Completed

If ICE has completed for a media stream, the default destination for that media stream MUST be set to the remote candidate of the candidate pair for that component in the valid list. For a lite implementation, there is always just a single candidate pair in the valid list for each component of a media stream. Additionally, the agent MUST include a candidate attribute for each default destination.

Additionally, if the agent is controlling (which only happens when both agents are lite), the agent MUST include the a=remote-candidates attribute for each media stream. The attribute contains the remote candidates from the candidate pairs in the valid list (one pair for each component of each media stream).

9.2. Receiving the Offer and Generating an Answer

9.2.1. Procedures for All Implementations

When receiving a subsequent offer within an existing session, an agent MUST reapply the verification procedures in Section 4.2 without regard to the results of verification from any previous offer/answer exchanges. Indeed, it is possible that a previous offer/answer exchange resulted in ICE not being used, but it is used as a consequence of a subsequent exchange.

9.2.1.1. Detecting ICE Restart

If the offer contained a change in the a=ice-ufrag or a=ice-pwd attributes compared to the previous SDP from the peer, it indicates that ICE is restarting for this media stream. If all media streams are restarting, then ICE is restarting overall.

An agent sets the rest of the fields in the SDP for this media stream as it would in an initial answer to this media stream (see Section 3.2). Consequently, the set of candidates MAY include some, none, or all of the previous candidates for that stream and MAY include a totally new set of candidates.

9.2.1.2. New Media Stream

If the offer contains a new media stream, the agent sets the fields in the answer as if it had received an initial offer containing that media stream (see Section 3.2). This will cause ICE processing to begin for this media stream.

9.2.1.3. Removed Media Stream

If an offer contains a media stream whose port is zero, the agent MUST NOT include any candidate attributes for that media stream in its answer and SHOULD NOT include any other ICE-related attributes defined in Section 8 for that media stream.

9.2.2. Procedures for Full Implementations

Unless the agent has detected an ICE restart from the offer, the username fragments, password, and implementation level MUST remain the same as used previously. If an agent needs to change one of these it MUST restart ICE for that media stream by generating an offer; ICE cannot be restarted in an answer.

Additional behaviors depend on the state of ICE processing for that media stream.

9.2.2.1. Existing Media Streams with ICE Running and no remote-candidates

If ICE is running for a media stream, and the offer for that media stream lacked the remote-candidates attribute, the rules for construction of the answer are identical to those for the offerer as described in Section 9.1.2.1.

9.2.2.2. Existing Media Streams with ICE Completed and no remote-candidates

If ICE is Completed for a media stream, and the offer for that media stream lacked the remote-candidates attribute, the rules for construction of the answer are identical to those for the offerer as described in Section 9.1.2.2, except that the answerer MUST NOT include the a=remote-candidates attribute in the answer.

9.2.2.3. Existing Media Streams and remote-candidates

A controlled agent will receive an offer with the a=remote-candidates attribute for a media stream when its peer has concluded ICE processing for that media stream. This attribute is present in the offer to deal with a race condition between the receipt of the offer, and the receipt of the Binding response that tells the answerer the candidate that will be selected by ICE. See Appendix B for an explanation of this race condition. Consequently, processing of an offer with this attribute depends on the winner of the race.

The agent then sees if each of these candidate pairs is present in the valid list. If a particular pair is not in the valid list, the check has "lost" the race. Call such a pair a "losing pair".

The agent finds all the pairs in the check list whose remote candidates equal the remote candidate in the losing pair:

Once there are no losing pairs, the agent can generate the answer. It MUST set the default destination for media to the candidates in the remote-candidates attribute from the offer (each of which will now be the local candidate of a candidate pair in the valid list). It MUST include a candidate attribute in the answer for each candidate in the remote-candidates attribute in the offer.

9.2.3. Procedures for Lite Implementations

If the received offer contains the remote-candidates attribute for a media stream, the agent forms a candidate pair for each component of the media stream by:

It then places those candidates into the Valid list for the media stream. The state of ICE processing for that media stream is set to Completed.

Furthermore, if the agent believed it was controlling, but the offer contained the remote-candidates attribute, both agents believe they are controlling. In this case, both would have sent updated offers around the same time. However, the signaling protocol carrying the offer/answer exchanges will have resolved this glare condition, so that one agent is always the 'winner' by having its offer received before its peer has sent an offer. The winner takes the role of controlled, so that the loser (the answerer under consideration in this section) MUST change its role to controlled. Consequently, if the agent was going to send an updated offer since, based on the rules in section 8.2 of [ICE-BIS], it was controlling, it no longer needs to.

Besides the potential role change, change in the Valid list, and state changes, the construction of the answer is performed identically to the construction of an offer as described in Section 9.1.3.

9.3. Updating the Check and Valid Lists

9.3.1. Procedures for Full Implementations

9.3.1.1. ICE Restarts

The agent MUST remember the highest-priority nominated pairs in the Valid list for each component of the media stream, called the previous selected pairs, prior to the restart. The agent will continue to send media using these pairs, as described in Section 11.1. Once these destinations are noted, the agent MUST flush the valid and check lists, and then recompute the check list and its states as described in section 6.3 of [ICE-BIS].

9.3.1.2. New Media Stream

If the offer/answer exchange added a new media stream, the agent MUST create a new check list for it (and an empty Valid list to start of course), as described in section 6.3 of [ICE-BIS].

9.3.1.3. Removed Media Stream

If the offer/answer exchange removed a media stream, or an answer rejected an offered media stream, an agent MUST flush the Valid list for that media stream. It MUST terminate any STUN transactions in progress for that media stream. An agent MUST remove the check list for that media stream and cancel any pending ordinary checks for it.

9.3.1.4. ICE Continuing for Existing Media Stream

The valid list is not affected by an updated offer/answer exchange unless ICE is restarting.

If an agent is in the Running state for that media stream, the check list is updated (the check list is irrelevant if the state is completed). To do that, the agent recomputes the check list using the procedures described in section 6.3 of [ICE-BIS]. If a pair on the new check list was also on the previous check list, and its state was Waiting, In-Progress, Succeeded, or Failed, its state is copied over. Otherwise, its state is set to Frozen.

If none of the check lists are active (meaning that the pairs in each check list are Frozen), the full-mode agent sets the first pair in the check list for the first media stream to Waiting, and then sets the state of all other pairs in that check list for the same component ID and with the same foundation to Waiting as well.

Next, the agent goes through each check list, starting with the highest-priority pair. If a pair has a state of Succeeded, and it has a component ID of 1, then all Frozen pairs in the same check list with the same foundation whose component IDs are not 1 have their state set to Waiting. If, for a particular check list, there are pairs for each component of that media stream in the Succeeded state, the agent moves the state of all Frozen pairs for the first component of all other media streams (and thus in different check lists) with the same foundation to Waiting.

9.3.2. Procedures for Lite Implementations

If ICE is restarting for a media stream, the agent MUST start a new Valid list for that media stream. It MUST remember the pairs in the previous Valid list for each component of the media stream, called the previous selected pairs, and continue to send media there as described in Section 11.1. The state of ICE processing for each media stream MUST change to Running, and the state of ICE processing MUST change to Running.

10. Keepalives

The keepalives MUST be sent regardless of whether the media stream is currently inactive, sendonly, recvonly, or sendrecv, and regardless of the presence or value of the bandwidth attribute. An agent can determine that its peer supports ICE by the presence of a=candidate attributes for each media session.

11. Media Handling

11.1. Sending Media

Note that the selected pair for a component of a media stream may not equal the default pair for that same component from the most recent offer/answer exchange. When this happens, the selected pair is used for media, not the default pair. When ICE first completes, if the selected pairs aren't a match for the default pairs, the controlling agent sends an updated offer/answer exchange to remedy this disparity. However, until that updated offer arrives, there will not be a match. Furthermore, in very unusual cases, the default candidates in the updated offer/answer will not be a match.

11.1.1. Procedures for All Implementations

ICE has interactions with jitter buffer adaptation mechanisms. An RTP stream can begin using one candidate, and switch to another one, though this happens rarely with ICE. The newer candidate may result in RTP packets taking a different path through the network -- one with different delay characteristics. As discussed below, agents are encouraged to re-adjust jitter buffers when there are changes in source or destination address of media packets. Furthermore, many audio codecs use the marker bit to signal the beginning of a talkspurt, for the purposes of jitter buffer adaptation. For such codecs, it is RECOMMENDED that the sender set the marker bit [RFC3550] when an agent switches transmission of media from one candidate pair to another.

11.2. Receiving Media

ICE implementations MUST be prepared to receive media on each component on any candidates provided for that component in the most recent offer/answer exchange (in the case of RTP, this would include both RTP and RTCP if candidates were provided for both).

It is RECOMMENDED that, when an agent receives an RTP packet with a new source or destination IP address for a particular media stream, that the agent re-adjust its jitter buffers.

RFC 3550 [RFC3550] describes an algorithm in Section 8.2 for detecting synchronization source (SSRC) collisions and loops. These algorithms are based, in part, on seeing different source transport addresses with the same SSRC. However, when ICE is used, such changes will sometimes occur as the media streams switch between candidates. An agent will be able to determine that a media stream is from the same peer as a consequence of the STUN exchange that proceeds media transmission. Thus, if there is a change in source transport address, but the media packets come from the same peer agent, this SHOULD NOT be treated as an SSRC collision.

12. Usage with SIP

12.1. Latency Guidelines

ICE requires a series of STUN-based connectivity checks to take place between endpoints. These checks start from the answerer on generation of its answer, and start from the offerer when it receives the answer. These checks can take time to complete, and as such, the selection of messages to use with offers and answers can affect perceived user latency. Two latency figures are of particular interest. These are the post-pickup delay and the post-dial delay. The post-pickup delay refers to the time between when a user "answers the phone" and when any speech they utter can be delivered to the caller. The post-dial delay refers to the time between when a user enters the destination address for the user and ringback begins as a consequence of having successfully started ringing the phone of the called party.

Two cases can be considered -- one where the offer is present in the initial INVITE and one where it is in a response.

12.1.1. Offer in INVITE

To reduce post-dial delays, it is RECOMMENDED that the caller begin gathering candidates prior to actually sending its initial INVITE. This can be started upon user interface cues that a call is pending, such as activity on a keypad or the phone going off-hook.

If an offer is received in an INVITE request, the answerer SHOULD begin to gather its candidates on receipt of the offer and then generate an answer in a provisional response once it has completed that process. ICE requires that a provisional response with an SDP be transmitted reliably. This can be done through the existing Provisional Response Acknowledgment (PRACK) mechanism [RFC3262] or through an optimization that is specific to ICE. With this optimization, provisional responses containing an SDP answer that begins ICE processing for one or more media streams can be sent reliably without RFC 3262. To do this, the agent retransmits the provisional response with the exponential backoff timers described in RFC 3262. Retransmits MUST cease on receipt of a STUN Binding request for one of the media streams signaled in that SDP (because receipt of a Binding request indicates the offerer has received the answer) or on transmission of the answer in a 2xx response. If the peer agent is lite, there will never be a STUN Binding request. In such a case, the agent MUST cease retransmitting the 18x after sending it four times (ICE will actually work even if the peer never receives the 18x; however, experience has shown that sending it is important for middleboxes and firewall traversal). If no Binding request is received prior to the last retransmit, the agent does not consider the session terminated. Despite the fact that the provisional response will be delivered reliably, the rules for when an agent can send an updated offer or answer do not change from those specified in RFC 3262. Specifically, if the INVITE contained an offer, the same answer appears in all of the 1xx and in the 2xx response to the INVITE. Only after that 2xx has been sent can an updated offer/answer exchange occur. This optimization SHOULD NOT be used if both agents support PRACK. Note that the optimization is very specific to provisional response carrying answers that start ICE processing; it is not a general technique for 1xx reliability.

Alternatively, an agent MAY delay sending an answer until the 200 OK; however, this results in a poor user experience and is NOT RECOMMENDED.

Once the answer has been sent, the agent SHOULD begin its connectivity checks. Once candidate pairs for each component of a media stream enter the valid list, the answerer can begin sending media on that media stream.

However, prior to this point, any media that needs to be sent towards the caller (such as SIP early media [RFC3960]) MUST NOT be transmitted. For this reason, implementations SHOULD delay alerting the called party until candidates for each component of each media stream have entered the valid list. In the case of a PSTN gateway, this would mean that the setup message into the PSTN is delayed until this point. Doing this increases the post-dial delay, but has the effect of eliminating 'ghost rings'. Ghost rings are cases where the called party hears the phone ring, picks up, but hears nothing and cannot be heard. This technique works without requiring support for, or usage of, preconditions [RFC3312], since it's a localized decision. It also has the benefit of guaranteeing that not a single packet of media will get clipped, so that post-pickup delay is zero. If an agent chooses to delay local alerting in this way, it SHOULD generate a 180 response once alerting begins.

12.1.2. Offer in Response

In addition to uses where the offer is in an INVITE, and the answer is in the provisional and/or 200 OK response, ICE works with cases where the offer appears in the response. In such cases, which are common in third party call control [RFC3725], ICE agents SHOULD generate their offers in a reliable provisional response (which MUST utilize RFC 3262), and not alert the user on receipt of the INVITE. The answer will arrive in a PRACK. This allows for ICE processing to take place prior to alerting, so that there is no post-pickup delay, at the expense of increased call setup delays. Once ICE completes, the callee can alert the user and then generate a 200 OK when they answer. The 200 OK would contain no SDP, since the offer/answer exchange has completed.

Alternatively, agents MAY place the offer in a 2xx instead (in which case the answer comes in the ACK). When this happens, the callee will alert the user on receipt of the INVITE, and the ICE exchanges will take place only after the user answers. This has the effect of reducing call setup delay, but can cause substantial post-pickup delays and media clipping.

12.2. SIP Option Tags and Media Feature Tags

[RFC5768] specifies a SIP option tag and media feature tag for usage with ICE. ICE implementations using SIP SHOULD support this specification, which uses a feature tag in registrations to facilitate interoperability through signaling intermediaries.

12.3. Interactions with Forking

ICE interacts very well with forking. Indeed, ICE fixes some of the problems associated with forking. Without ICE, when a call forks and the caller receives multiple incoming media streams, it cannot determine which media stream corresponds to which callee.

With ICE, this problem is resolved. The connectivity checks which occur prior to transmission of media carry username fragments, which in turn are correlated to a specific callee. Subsequent media packets that arrive on the same candidate pair as the connectivity check will be associated with that same callee. Thus, the caller can perform this correlation as long as it has received an answer.

12.4. Interactions with Preconditions

Quality of Service (QoS) preconditions, which are defined in RFC 3312 [RFC3312] and RFC 4032 [RFC4032], apply only to the transport addresses listed as the default targets for media in an offer/answer. If ICE changes the transport address where media is received, this change is reflected in an updated offer that changes the default destination for media to match ICE's selection. As such, it appears like any other re-INVITE would, and is fully treated in RFCs 3312 and 4032, which apply without regard to the fact that the destination for media is changing due to ICE negotiations occurring "in the background".

Indeed, an agent SHOULD NOT indicate that QoS preconditions have been met until the checks have completed and selected the candidate pairs to be used for media.

ICE also has (purposeful) interactions with connectivity preconditions [RFC5898]. Those interactions are described there. Note that the procedures described in Section 12.1 describe their own type of "preconditions", albeit with less functionality than those provided by the explicit preconditions in [RFC5898].

12.5. Interactions with Third Party Call Control

ICE works with Flows I, III, and IV as described in [RFC3725]. Flow I works without the controller supporting or being aware of ICE. Flow IV will work as long as the controller passes along the ICE attributes without alteration. Flow II is fundamentally incompatible with ICE; each agent will believe itself to be the answerer and thus never generate a re-INVITE.

The flows for continued operation, as described in Section 7 of RFC 3725, require additional behavior of ICE implementations to support. In particular, if an agent receives a mid-dialog re-INVITE that contains no offer, it MUST restart ICE for each media stream and go through the process of gathering new candidates. Furthermore, that list of candidates SHOULD include the ones currently being used for media.

13. Relationship with ANAT

RFC 4091 [RFC4091], the Alternative Network Address Types (ANAT) Semantics for the SDP grouping framework, and RFC 4092 [RFC4092], its usage with SIP, define a mechanism for indicating that an agent can support both IPv4 and IPv6 for a media stream, and it does so by including two m lines, one for v4 and one for v6. This is similar to ICE, which allows for an agent to indicate multiple transport addresses using the candidate attribute. However, ANAT relies on static selection to pick between choices, rather than a dynamic connectivity check used by ICE.

This specification deprecates RFC 4091 and RFC 4092. Instead, agents wishing to support dual-stack will utilize ICE.

14. Setting Ta and RTO for RTP Media Streams

During the gathering phase of ICE (section 4.1.1 [ICE-BIS]) and while ICE is performing connectivity checks (section 7 [ICE-BIS]), an agent sends STUN and TURN transactions. These transactions are paced at a rate of one every Ta milliseconds, and utilize a specific RTO. This section describes how the values of Ta and RTO are computed with a real-time media stream (such as RTP). When ICE is used for a stream with a known maximum bandwidth, the following computation MAY be followed to rate-control the ICE exchanges.

The values of RTO and Ta change during the lifetime of ICE processing. One set of values applies during the gathering phase, and the other, for connectivity checks.

where k is the number of media streams. During the gathering phase, Ta is computed based on the number of media streams the agent has indicated in its offer or answer, and the RTP packet size and RTP ptime are those of the most preferred codec for each media stream. Once an offer and answer have been exchanged, the agent recomputes Ta to pace the connectivity checks. In that case, the value of Ta is based on the number of media streams that will actually be used in the session, and the RTP packet size and RTP ptime are those of the most preferred codec with which the agent will send.

In addition, the retransmission timer for the STUN transactions, RTO, defined in [RFC5389], SHOULD be configurable and during the gathering phase, SHOULD have a default of:

where the number of pairs refers to the number of pairs of candidates with STUN or TURN servers.

For connectivity checks, RTO SHOULD be configurable and SHOULD have a default of:

where Num-Waiting is the number of checks in the check list in the Waiting state, and Num-In-Progress is the number of checks in the In-Progress state. Note that the RTO will be different for each transaction as the number of checks in the Waiting and In-Progress states change.

These formulas are aimed at causing STUN transactions to be paced at the same rate as media. This ensures that ICE will work properly under the same network conditions needed to support the media as well. See section B.1 of [ICE-BIS] for additional discussion and motivations. Because of this pacing, it will take a certain amount of time to obtain all of the server reflexive and relayed candidates. Implementations should be aware of the time required to do this, and if the application requires a time budget, limit the number of candidates that are gathered.

The formulas result in a behavior whereby an agent will send its first packet for every single connectivity check before performing a retransmit. This can be seen in the formulas for the RTO (which represents the retransmit interval). Those formulas scale with N, the number of checks to be performed. As a result of this, ICE maintains a nicely constant rate, but becomes more sensitive to packet loss. The loss of the first single packet for any connectivity check is likely to cause that pair to take a long time to be validated, and instead, a lower-priority check (but one for which there was no packet loss) is much more likely to complete first. This results in ICE performing sub-optimally, choosing lower-priority pairs over higher-priority pairs. Implementors should be aware of this consequence, but still should utilize the timer values described here.

15. Security Considerations

15.1. Attacks on the Offer/Answer Exchanges

An attacker that can modify or disrupt the offer/answer exchanges themselves can readily launch a variety of attacks with ICE. They could direct media to a target of a DoS attack, they could insert themselves into the media stream, and so on. These are similar to the general security considerations for offer/answer exchanges, and the security considerations in RFC 3264 [RFC3264] apply. These require techniques for message integrity and encryption for offers and answers, which are satisfied by the SIPS mechanism [RFC3261] when SIP is used. As such, the usage of SIPS with ICE is RECOMMENDED.

15.2. Insider Attacks

In addition to attacks where the attacker is a third party trying to insert fake offers, answers, or stun messages, there are several attacks possible with ICE when the attacker is an authenticated and valid participant in the ICE exchange.

15.2.1. The Voice Hammer Attack

The voice hammer attack is an amplification attack. In this attack, the attacker initiates sessions to other agents, and maliciously includes the IP address and port of a DoS target as the destination for media traffic signaled in the SDP. This causes substantial amplification; a single offer/answer exchange can create a continuing flood of media packets, possibly at high rates (consider video sources). This attack is not specific to ICE, but ICE can help provide remediation.

Specifically, if ICE is used, the agent receiving the malicious SDP will first perform connectivity checks to the target of media before sending media there. If this target is a third-party host, the checks will not succeed, and media is never sent.

Unfortunately, ICE doesn't help if its not used, in which case an attacker could simply send the offer without the ICE parameters. However, in environments where the set of clients is known, and is limited to ones that support ICE, the server can reject any offers or answers that don't indicate ICE support.

15.2.2. Interactions with Application Layer Gateways and SIP

Application Layer Gateways (ALGs) are functions present in a NAT device that inspect the contents of packets and modify them, in order to facilitate NAT traversal for application protocols. Session Border Controllers (SBCs) are close cousins of ALGs, but are less transparent since they actually exist as application layer SIP intermediaries. ICE has interactions with SBCs and ALGs.

If an ALG is SIP aware but not ICE aware, ICE will work through it as long as the ALG correctly modifies the SDP. A correct ALG implementation behaves as follows:

Unfortunately, many ALGs are known to work poorly in these corner cases. ICE does not try to work around broken ALGs, as this is outside the scope of its functionality. ICE can help diagnose these conditions, which often show up as a mismatch between the set of candidates and the m and c lines and rtcp attributes. The ice-mismatch attribute is used for this purpose.

ICE works best through ALGs when the signaling is run over TLS. This prevents the ALG from manipulating the SDP messages and interfering with ICE operation. Implementations that are expected to be deployed behind ALGs SHOULD provide for TLS transport of the SDP.

If an SBC is SIP aware but not ICE aware, the result depends on the behavior of the SBC. If it is acting as a proper Back-to-Back User Agent (B2BUA), the SBC will remove any SDP attributes it doesn't understand, including the ICE attributes. Consequently, the call will appear to both endpoints as if the other side doesn't support ICE. This will result in ICE being disabled, and media flowing through the SBC, if the SBC has requested it. If, however, the SBC passes the ICE attributes without modification, yet modifies the default destination for media (contained in the m and c lines and rtcp attribute), this will be detected as an ICE mismatch, and ICE processing is aborted for the call. It is outside of the scope of ICE for it to act as a tool for "working around" SBCs. If one is present, ICE will not be used and the SBC techniques take precedence.

16. IANA Considerations

16.1. SDP Attributes

Original ICE specification defined seven new SDP attributes per the procedures of Section 8.2.4 of [RFC4566]. The registration information is reproduced here.

16.1.1. candidate Attribute

16.1.2. remote-candidates Attribute

16.1.3. ice-lite Attribute

16.1.4. ice-mismatch Attribute

16.1.5. ice-pwd Attribute

16.1.6. ice-ufrag Attribute

16.1.7. ice-options Attribute

16.2. Interactive Connectivity Establishment (ICE) Options Registry

ICE options are of unlimited length according to the syntax in Section 8.5; however, they are RECOMMENDED to be no longer than 20 characters. This is to reduce message sizes and allow for efficient parsing.

In RFC 5245 ICE options could only be defined at the session level. ICE options can now also be defined at the media level. This can be used when aggregating between different ICE agents in the same endpoint, but future options may require to be defined at the media-level. To ensure compatibility with legacy implementation, the media-level ICE options MUST be aggregated into a session-level ICE option. Because aggregation rules depend on the specifics of each option, all new ICE options MUST also define in their specification how the media-level ICE option values are aggregated to generate the value of the session-level ICE option.

The only ICE option defined at the time of publication is "rtp+ecn" [RFC6679]. The aggregation rule for this ICE options is that if all aggregated media using ICE contain a media-level "rtp+ecn" ICE option then an "rtp+ecn" ICE option MUST be inserted at the session-level. If one of the media does not contain the option, then it MUST NOT be inserted at the session-level.

17. Acknowledgments

A large part of the text in this document was taken from RFC 5245, authored by Jonathan Rosenberg.

Some of the text in this document was taken from RFC 6336, authored by Magnus Westerlund and Colin Perkins.

18. References

18.1. Normative References

18.2. Informative References

Appendix A. Examples

For the example shown in Section 13 of [ICE-BIS] the resulting offer (message 5) encoded in SDP looks like:

The offer, with the variables replaced with their values, will look like (lines folded for clarity):

Appendix B. The remote-candidates Attribute

The a=remote-candidates attribute exists to eliminate a race condition between the updated offer and the response to the STUN Binding request that moved a candidate into the Valid list. This race condition is shown in Figure 1. On receipt of message 4, agent L adds a candidate pair to the valid list. If there was only a single media stream with a single component, agent L could now send an updated offer. However, the check from agent R has not yet generated a response, and agent R receives the updated offer (message 7) before getting the response (message 9). Thus, it does not yet know that this particular pair is valid. To eliminate this condition, the actual candidates at R that were selected by the offerer (the remote candidates) are included in the offer itself, and the answerer delays its answer until those pairs validate.


       Agent A               Network               Agent B
          |(1) Offer            |                     |
          |------------------------------------------>|
          |(2) Answer           |                     |
          |<------------------------------------------|
          |(3) STUN Req.        |                     |
          |------------------------------------------>|
          |(4) STUN Res.        |                     |
          |<------------------------------------------|
          |(5) STUN Req.        |                     |
          |<------------------------------------------|
          |(6) STUN Res.        |                     |
          |-------------------->|                     |
          |                     |Lost                 |
          |(7) Offer            |                     |
          |------------------------------------------>|
          |(8) STUN Req.        |                     |
          |<------------------------------------------|
          |(9) STUN Res.        |                     |
          |------------------------------------------>|
          |(10) Answer          |                     |
          |<------------------------------------------|

Figure 1: Race Condition Flow

Appendix C. Why Is the Conflict Resolution Mechanism Needed?

When ICE runs between two peers, one agent acts as controlled, and the other as controlling. Rules are defined as a function of implementation type and offerer/answerer to determine who is controlling and who is controlled. However, the specification mentions that, in some cases, both sides might believe they are controlling, or both sides might believe they are controlled. How can this happen?

The condition when both agents believe they are controlled shows up in third party call control cases. Consider the following flow:


          A         Controller          B
          |(1) INV()     |              |
          |<-------------|              |
          |(2) 200(SDP1) |              |
          |------------->|              |
          |              |(3) INV()     |
          |              |------------->|
          |              |(4) 200(SDP2) |
          |              |<-------------|
          |(5) ACK(SDP2) |              |
          |<-------------|              |
          |              |(6) ACK(SDP1) |
          |              |------------->|

Figure 2: Role Conflict Flow

This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact, it works better than flow III since it produces fewer messages. In this flow, the controller sends an offerless INVITE to agent A, which responds with its offer, SDP1. The agent then sends an offerless INVITE to agent B, which it responds to with its offer, SDP2. The controller then uses the offer from each agent to generate the answers. When this flow is used, ICE will run between agents A and B, but both will believe they are in the controlling role. With the role conflict resolution procedures, this flow will function properly when ICE is used.

At this time, there are no documented flows that can result in the case where both agents believe they are controlled. However, the conflict resolution procedures allow for this case, should a flow arise that would fit into this category.

Appendix D. Why Send an Updated Offer?

Section 11.1 describes rules for sending media. Both agents can send media once ICE checks complete, without waiting for an updated offer. Indeed, the only purpose of the updated offer is to "correct" the SDP so that the default destination for media matches where media is being sent based on ICE procedures (which will be the highest-priority nominated candidate pair).

This begs the question -- why is the updated offer/answer exchange needed at all? Indeed, in a pure offer/answer environment, it would not be. The offerer and answerer will agree on the candidates to use through ICE, and then can begin using them. As far as the agents themselves are concerned, the updated offer/answer provides no new information. However, in practice, numerous components along the signaling path look at the SDP information. These include entities performing off-path QoS reservations, NAT traversal components such as ALGs and Session Border Controllers (SBCs), and diagnostic tools that passively monitor the network. For these tools to continue to function without change, the core property of SDP -- that the existing, pre-ICE definitions of the addresses used for media -- the m and c lines and the rtcp attribute -- must be retained. For this reason, an updated offer must be sent.

Authors' Addresses

Marc Petit-Huguenin Jive Communications 1275 West 1600 North, Suite 100 Orem, UT 84057 USA EMail: marcph@getjive.com

Ari Keranen Ericsson Jorvas, 02420 Finland EMail: ari.keranen@ericsson.com

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261]	Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.
[RFC3262]	Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002.
[RFC3264]	Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.
[RFC3312]	Camarillo, G., Marshall, W. and J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002.
[RFC3550]	Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.
[RFC3556]	Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July 2003.
[RFC3605]	Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003.
[RFC4032]	Camarillo, G. and P. Kyzivat, "Update to the Session Initiation Protocol (SIP) Preconditions Framework", RFC 4032, March 2005.
[RFC4091]	Camarillo, G. and J. Rosenberg, "The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework", RFC 4091, June 2005.
[RFC4092]	Camarillo, G. and J. Rosenberg, "Usage of the Session Description Protocol (SDP) Alternative Network Address Types (ANAT) Semantics in the Session Initiation Protocol (SIP)", RFC 4092, June 2005.
[RFC4566]	Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
[RFC5226]	Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5234]	Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5389]	Rosenberg, J., Mahy, R., Matthews, P. and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008.
[RFC5768]	Rosenberg, J., "Indicating Support for Interactive Connectivity Establishment (ICE) in the Session Initiation Protocol (SIP)", RFC 5768, April 2010.
[RFC6679]	Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P. and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, August 2012.
[ICE-BIS]	Keranen, A. and J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", Internet-Draft draft-keranen-mmusic-rfc5245bis-01, February 2013.

[RFC3725]	Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, "Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April 2004.
[RFC3960]	Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)", RFC 3960, December 2004.
[RFC4340]	Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC5626]	Jennings, C., Mahy, R. and F. Audet, "Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)", RFC 5626, October 2009.
[RFC5898]	Andreasen, F., Camarillo, G., Oran, D. and D. Wing, "Connectivity Preconditions for Session Description Protocol (SDP) Media Streams", RFC 5898, July 2010.