Session Description Protocol (SDP) Offer/Answer procedures for Interactive Connectivity Establishment (ICE)
draft-ietf-mmusic-ice-sip-sdp-17
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8839.
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Authors | Marc Petit-Huguenin , Suhas Nandakumar , Ari Keränen | ||
Last updated | 2018-03-27 | ||
Replaces | draft-petithuguenin-mmusic-ice-sip-sdp | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Formats | |||
Reviews |
GENART Last Call review
(of
-36)
by Stewart Bryant
Ready w/nits
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Additional resources | Mailing list discussion | ||
Stream | WG state | WG Consensus: Waiting for Write-Up | |
Document shepherd | Flemming Andreasen | ||
IESG | IESG state | Became RFC 8839 (Proposed Standard) | |
Consensus boilerplate | Unknown | ||
Telechat date | (None) | ||
Responsible AD | (None) | ||
Send notices to | (None) |
draft-ietf-mmusic-ice-sip-sdp-17
MMUSIC M. Petit-Huguenin Internet-Draft Impedance Mismatch Obsoletes: 5245 (if approved) S. Nandakumar Intended status: Standards Track Cisco Systems Expires: September 27, 2018 A. Keranen Ericsson March 26, 2018 Session Description Protocol (SDP) Offer/Answer procedures for Interactive Connectivity Establishment (ICE) draft-ietf-mmusic-ice-sip-sdp-17 Abstract This document describes Session Description Protocol (SDP) Offer/ Answer procedures for carrying out Interactive Connectivity Establishment (ICE) between the agents. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 27, 2018. Copyright Notice Copyright (c) 2018 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 (http://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 Petit-Huguenin, et al. Expires September 27, 2018 [Page 1] Internet-Draft ICE SDP Usage March 2018 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 4 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Generic Procedures . . . . . . . . . . . . . . . . . . . 4 3.2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . 4 3.2.2. RTP/RTCP Considerations . . . . . . . . . . . . . . . 6 3.2.3. Determining Role . . . . . . . . . . . . . . . . . . 6 3.2.4. STUN Considerations . . . . . . . . . . . . . . . . . 6 3.2.5. ICE Mismatch . . . . . . . . . . . . . . . . . . . . 6 3.2.6. SDP Example . . . . . . . . . . . . . . . . . . . . . 7 3.3. Initial Offer/Answer Exchange . . . . . . . . . . . . . . 7 3.3.1. Sending the Initial Offer . . . . . . . . . . . . . . 7 3.3.2. Sending the Initial Answer . . . . . . . . . . . . . 8 3.3.3. Receiving the Initial Answer . . . . . . . . . . . . 8 3.3.4. Concluding ICE . . . . . . . . . . . . . . . . . . . 8 3.4. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 9 3.4.1. Sending Subsequent Offer . . . . . . . . . . . . . . 9 3.4.2. Sending Subsequent Answer . . . . . . . . . . . . . . 11 3.4.3. Receiving Answer for a Subsequent Offer . . . . . . . 13 4. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 15 4.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 18 4.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . 18 4.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . 19 4.5. "ice-pacing" Attribute . . . . . . . . . . . . . . . . . 20 4.6. "ice-options" Attribute . . . . . . . . . . . . . . . . . 20 5. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 21 6. SIP Considerations . . . . . . . . . . . . . . . . . . . . . 21 6.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . 21 6.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 22 Petit-Huguenin, et al. Expires September 27, 2018 [Page 2] Internet-Draft ICE SDP Usage March 2018 6.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 23 6.2. SIP Option Tags and Media Feature Tags . . . . . . . . . 23 6.3. Interactions with Forking . . . . . . . . . . . . . . . . 24 6.4. Interactions with Preconditions . . . . . . . . . . . . . 24 6.5. Interactions with Third Party Call Control . . . . . . . 24 7. Relationship with ANAT . . . . . . . . . . . . . . . . . . . 25 8. Setting Ta and RTO for RTP Media Streams . . . . . . . . . . 25 9. Security Considerations . . . . . . . . . . . . . . . . . . . 25 9.1. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 25 9.2. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 26 9.2.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 26 9.2.2. Interactions with Application Layer Gateways and SIP 26 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 10.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 27 10.1.1. candidate Attribute . . . . . . . . . . . . . . . . 28 10.1.2. remote-candidates Attribute . . . . . . . . . . . . 28 10.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . 29 10.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . 29 10.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . 30 10.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . 30 10.1.7. ice-options Attribute . . . . . . . . . . . . . . . 31 10.1.8. ice-pacing Attribute . . . . . . . . . . . . . . . . 31 10.2. Interactive Connectivity Establishment (ICE) Options Registry . . . . . . . . . . . . . . . . . . . . . . . . 32 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 12.1. Normative References . . . . . . . . . . . . . . . . . . 33 12.2. Informative References . . . . . . . . . . . . . . . . . 35 12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 37 Appendix B. The remote-candidates Attribute . . . . . . . . . . 38 Appendix C. Why Is the Conflict Resolution Mechanism Needed? . . 39 Appendix D. Why Send an Updated Offer? . . . . . . . . . . . . . 40 Appendix E. Contributors . . . . . . . . . . . . . . . . . . . . 41 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 1. Introduction This document describes how Interactive Connectivity Establishment (ICE) is used with Session Description Protocol (SDP) offer/answer [RFC3264]. 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 SDP offer/answer. Petit-Huguenin, et al. Expires September 27, 2018 [Page 3] Internet-Draft ICE SDP Usage March 2018 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]. Readers should be familiar with the terminology defined in [RFC3264], in [ICE-BIS] and the following: Default Destination/Candidate: The default destination for a component of a data stream is the transport address that would be used by an agent that is not ICE aware. A default candidate for a component is one whose transport address matches the default destination for that component. For the RTP component, the default IP address is in the "c=" line of the SDP, and the port is in the "m=" line. For the RTCP component, the address and port are indicated using the "a=rtcp" attribute defined in [RFC3605], if present; otherwise, the RTCP component address is same as the address of the RTP component, and its port is one greater than the port of the RTP component. 3. SDP Offer/Answer Procedures 3.1. Introduction [ICE-BIS] defines ICE candidate exchange as the process for ICE agents (Initiator and Responder) to exchange their candidate information required for ICE processing at the agents. For the purposes of this specification, the candidate exchange process corresponds to the [RFC3264] Offer/Answer protocol and the terminologies offerer and answerer correspond to the initiator and responder terminologies from [ICE-BIS] respectively. Once the initiating agent has gathered, pruned and prioritized its set of candidates [ICE-BIS], the candidate exchange with the peer agent begins. 3.2. Generic Procedures 3.2.1. Encoding Section 4 provides detailed rules for constructing various SDP attributes defined in this specification. Petit-Huguenin, et al. Expires September 27, 2018 [Page 4] Internet-Draft ICE SDP Usage March 2018 3.2.1.1. Data Streams Each data stream [ICE-BIS] is represented by an SDP media description ("m=" section). 3.2.1.2. Candidates With in a "m=" section, each candidate (including the default candidate) associated with the data stream is represented by an SDP candidate attribute. Prior to nomination, the "c=" line associated with an "m=" section contains the IP address of the default candidate, while the "m=" line contains the port and transport of the default candidate for that "m=" section. After nomination, the "c=" line for a given "m=" section contains the IP address of the nominated candidate (the local candidate of the nominated candidate pair) and the "m=" line contains the port and transport corresponding to the nominated candidate for that "m=" section. 3.2.1.3. Username and Password The ICE username is represented by an SDP ice-ufrag attribute and the ICE password is represented by an SDP ice-pwd attribute. 3.2.1.4. Lite Implementations An ICE lite implementation [ICE-BIS] MUST include an SDP ice-lite attribute. A full implementation MUST NOT include that attribute. 3.2.1.5. ICE Extensions An agent uses the SDP ice-options attribute to indicate support of ICE extensions. An agent compliant to this specification MUST include an SDP ice- options attribute with an "ice2" attribute value. If an agent receives an SDP offer or answer that does not include the attribute value, the agent assumes that the peer is compliant to [RFC5245]. 3.2.1.6. Inactive and Disabled Data Streams If an "m=" section is marked as inactive [RFC4566], or has a bandwidth value of zero [RFC4566], the agent MUST still include ICE related SDP attributes. Petit-Huguenin, et al. Expires September 27, 2018 [Page 5] Internet-Draft ICE SDP Usage March 2018 If the port value associated with an "m=" section is set to zero (implying a disabled stream), the agent SHOULD NOT include ICE related SDP candidate attributes in that "m=" section, unless an SDP extension specifying otherwise is used. 3.2.2. RTP/RTCP Considerations If an agent utilizes both RTP and RTCP, the agent MUST include SDP candidate attributes for both the RTP and RTCP components in the "m=" section. If an agent uses separate ports for RTP and RTCP, the agent MUST include an SDP rtcp attribute in the "m=" section, as described in [RFC3605]. In the cases where the port number for the RTCP is one higher than the RTP port and RTCP component address is same as the address of the RTP component, the SDP rtcp attribute MUST not be included. If the agent does not utilize RTCP, it indicates that by including b=RS:0 and b=RR:0 SDP attributes, as described in [RFC3556]. 3.2.3. Determining Role The offerer acts as the Initiating agent. The answerer acts as the Responding agent. The ICE roles (controlling and controlled) are determined using the procedures in [ICE-BIS]. 3.2.4. STUN Considerations Once an agent has provided its local candidates to its peer, within an SDP offer or answer, the agent MUST be prepared to receive STUN connectivity check Binding requests on those candidates. 3.2.5. ICE Mismatch The agent will proceed with the ICE procedures defined in [ICE-BIS] and this specification if, for each data stream in the SDP it received, the default destination for each component of that data 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 [RFC3264] procedures, without using any of the ICE mechanisms described in the remainder of this specification with the following exceptions: Petit-Huguenin, et al. Expires September 27, 2018 [Page 6] Internet-Draft ICE SDP Usage March 2018 1. The agent MUST follow the rules of section 11 of [ICE-BIS], which describe keepalive procedures for all agents. 2. If the agent is not proceeding with ICE because there were a=candidate attributes, but none that matched the default destination of the data stream, the agent MUST include an a=ice- mismatch attribute in its answer and may omit a=candidate attributes for such data streams. See Section 9.2.2 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. 3. If the default candidates were relayed candidates learned through a TURN server, the agent MUST create permissions in the TURN server for the IP addresses learned from its peer in the SDP it just received. If this is not done, initial packets in the data stream from the peer may be lost. 3.2.6. SDP Example The following is an example SDP message that includes ICE attributes (lines folded for readability): v=0 o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1 s= c=IN IP4 192.0.2.3 t=0 0 a=ice-options:ice2 a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY m=audio 45664 RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 10.0.1.1 rport 8998 3.3. Initial Offer/Answer Exchange 3.3.1. Sending the Initial Offer When an offerer generates the initial offer, in each "m=" section it MUST include SDP candidate attributes for each available candidate associated with the "m=" section. In addition, the offerer MUST include an SDP ice-ufrag and an SDP ice-pwd attribute in the offer. Petit-Huguenin, et al. Expires September 27, 2018 [Page 7] Internet-Draft ICE SDP Usage March 2018 Note: Within the scope of this document, "Initial Offer" refers to the first SDP offer that is sent in order to negotiate usage of ICE. It might, or might not, be the initial SDP offer of the SDP session. Note: The procedures in this document only consider "m=" sections associated with data streams where ICE is used. 3.3.2. Sending the Initial Answer When an answerer receives an initial offer that indicates that the offerer supports ICE, and if the answerer accepts the offer and the usage of ICE, in each "m=" section within the answer, it MUST include SDP candidate attributes for each available candidate associated with the "m=" section. In addition, the answerer MUST include an SDP ice- ufrag and an SDP ice-pwd attribute in the answer. Once the answerer has sent the answer, it can start performing connectivity checks towards the peer candidates that were provided in the offer. If the offer does not indicate support of ICE, the answerer MUST NOT accept the usage of ICE. If the answerer still accepts the offer, the answerer MUST NOT include any ICE related SDP attributes in the answer. Instead the answerer will generate the answer according to normal offer/answer procedures [RFC3264]. If the answerer detects a possibility of the ICE mismatch, procedures described in (Section 3.2.5) are followed. 3.3.3. Receiving the Initial Answer When an offerer receives an initial answer that indicates that the answerer supports ICE, it can start performing connectivity checks towards the peer candidates that were provided in the answer. If the answer does not indicate that the answerer supports ICE, or if the offerer detects an ICE mismatch in the answer, the offerer MUST terminate the usage of ICE. The subsequent actions taken by the offerer are implementation dependent and are out of the scope of this specification. 3.3.4. Concluding ICE Once the state of each check list is Completed, and if the agent is the controlling, it nominates a candidate pair [ICE-BIS], it checks for each data stream whether the nominated pair matches the default candidate pair. If there are one or more data streams with a match, Petit-Huguenin, et al. Expires September 27, 2018 [Page 8] Internet-Draft ICE SDP Usage March 2018 the controlling agent MUST generate a subsequent offer (Section 3.4.1), in which the IP address, port and transport in the "c=" and "m=" lines associated with each data stream match the corresponding local information of the nominated pair for that data stream. However, If the support for 'ice2' ice-option is in use, the nominated candidate is noted and sent in the subsequent offer/answer exchange as the default candidate and no updated offer is needed to fix the default candidate. Also as described in [ICE-BIS], once the controlling agent has nominated a candidate pair for a data stream, the agent MUST NOT nominate another pair for that data stream during the lifetime of the ICE session. 3.4. Subsequent Offer/Answer Exchanges Either agent MAY generate a subsequent offer at any time allowed by [RFC3264]. This section defines rules for construction of subsequent offers and answers. Should a subsequent offer fail, ICE processing continues as if the subsequent offer had never been made. 3.4.1. Sending Subsequent Offer 3.4.1.1. Procedures for All Implementations 3.4.1.1.1. ICE Restarts An agent MAY restart ICE processing for an existing data stream [ICE-BIS]. The rules governing the ICE restart 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 data 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. Petit-Huguenin, et al. Expires September 27, 2018 [Page 9] Internet-Draft ICE SDP Usage March 2018 An agent sets the rest of the ice related fields in the SDP for this data stream as it would in an initial offer of this data stream (see Section 3.2.1). Consequently, the set of candidates MAY include some, none, or all of the previous candidates for that data stream and MAY include a totally new set of candidates. 3.4.1.1.2. Removing a Data Stream If an agent removes a data stream by setting its port to zero, it MUST NOT include any candidate attributes for that data stream and SHOULD NOT include any other ICE-related attributes defined in Section 4 for that data stream. 3.4.1.1.3. Adding a Data Stream If an agent wishes to add a new data stream, it sets the fields in the SDP for this data stream as if this was an initial offer for that data stream (see Section 3.2.1). This will cause ICE processing to begin for this data stream. 3.4.1.2. Procedures for Full Implementations This section describes additional procedures for full implementations, covering existing data streams. 3.4.1.3. Before Nomination When an offerer sends a subsequent offer, each "m=" section for which a candidate pair has not yet been nominated, the offer MUST include the same set of ICE-related information that the offerer included in the previous offer or answer. 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. 3.4.1.4. After Nomination Once a candidate pair has been nominated for a data stream, the IP address, port and transport in each "c=" and "m=" line associated with that data stream MUST match the data associated with the nominated pair for that data stream. In addition, the offerer only includes SDP candidates representing the local candidates of the nominated candidate pair. The offerer MUST NOT include any other SDP candidate attributes in the subsequent offer. Petit-Huguenin, et al. Expires September 27, 2018 [Page 10] Internet-Draft ICE SDP Usage March 2018 In addition, if the agent is controlling, it MUST include the a=remote-candidates attribute for each data stream whose check list is in the completed state. The attribute contains the remote candidates corresponding to the nominated pair in the valid list for each component of that data 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. 3.4.1.5. Procedures for Lite Implementations If the ICE state is running, a lite implementation MUST include all of its candidates for each component of each data stream in a=candidate attribute in any subsequent offer. The candidates are formed identical to the procedures for initial offers. A lite implementation MUST NOT add additional host candidates or change username fragments, password in a subsequent offer. Otherwise, it MUST restart ICE. If ICE has completed for a data stream and if the agent is controlled, the default destination for that data 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 data stream. Additionally, the agent MUST include a candidate attribute for each default destination. If ICE state is completed and if the agent is controlling (which only happens when both agents are lite), the agent MUST include the a=remote-candidates attribute for each data stream. The attribute contains the remote candidates from the candidate pairs in the valid list (one pair for each component of each data stream). 3.4.2. Sending Subsequent Answer If ICE is Completed for a data stream, and the offer for that data stream lacked the a=remote-candidates attribute, the rules for construction of the answer are identical to those for the offerer, except that the answerer MUST NOT include the a=remote-candidates attribute in the answer. A controlled agent will receive an offer with the a=remote-candidates attribute for a data stream when its peer has concluded ICE processing for that data 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 Petit-Huguenin, et al. Expires September 27, 2018 [Page 11] Internet-Draft ICE SDP Usage March 2018 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 forms a candidate pair for each component of the data stream by: o Setting the remote candidate equal to the offerer's default destination for that component (e.g., the contents of the "m=" and "c=" lines for RTP, and the a=rtcp attribute for RTCP) o Setting the local candidate equal to the transport address for that same component in the a=remote-candidates attribute in the offer. 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: o If none of the pairs are In-Progress, and at least one is Failed, it is most likely that a network failure, such as a network partition or serious packet loss, has occurred. The agent SHOULD generate an answer for this data stream as if the remote- candidates attribute had not been present, and then restart ICE for this stream. o If at least one of the pairs is In-Progress, the agent SHOULD wait for those checks to complete, and as each completes, redo the processing in this section until there are no losing pairs. 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. 3.4.2.1. Detecting ICE Restart If the offerer in a subsequent offer requested an ICE restart for a data stream, and if the answerer accepts the offer, the answerer follows the procedures for generating an initial answer. Petit-Huguenin, et al. Expires September 27, 2018 [Page 12] Internet-Draft ICE SDP Usage March 2018 For a given data stream, the answerer MAY include the same candidates that were used in the previous ICE session, but it MUST change the SDP ice-pwd and ice-ufrag attribute values. 3.4.2.2. Lite Implementation specific procedures If the received offer contains the remote-candidates attribute for a data stream, the agent forms a candidate pair for each component of the data stream by: o Setting the remote candidate equal to the offerer's default destination for that component (e.g., the contents of the "m=" and "c=" lines for RTP, and the a=rtcp attribute for RTCP). o Setting the local candidate equal to the transport address for that same component in the a=remote-candidates attribute in the offer. The state of ICE processing for that data stream is set to Completed. Furthermore, if the agent believed it was controlling, but the offer contained the a=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 controlling, 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. 3.4.3. Receiving Answer for a Subsequent Offer 3.4.3.1. Procedures for Full Implementations There may be certain situations where the offerer might receive SDP answer that lacks ICE candidates although the initial answer did. One example of such an "unexpected" answer might be happen when a ICE-unaware B2BUA introduces a media server during call hold using Petit-Huguenin, et al. Expires September 27, 2018 [Page 13] Internet-Draft ICE SDP Usage March 2018 3rd party call-control procedures. Omitting further details how this is done, this could result in an answer being received at the holding UA that was constructed by the B2BUA. With the B2BUA being ICE- unaware, that answer would not include ICE candidates. Receiving an answer without ICE attributes in this situation might be unexpected, but would not necessarily impair the user experience. When the offerer receives an answer indicating support for ICE, the offer performs on of the following actions: o If the offer was a restart, the agent MUST perform ICE restart procedures as specified in Section 3.4.3.1.1 o If the offer/answer exchange removed a data stream, or an answer rejected an offered data stream, an agent MUST flush the Valid list for that data stream. It MUST also terminate any STUN transactions in progress for that data stream. o If the offer/answer exchange added a new data stream, the agent MUST create a new check list for it (and an empty Valid list to start of course) which in turn triggers the candidate processing procedures [ICE-BIS]. o If ICE state is running for a given data stream, the agent recomputes the check list. 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), appropriate procedures in [ICE-BIS] are performed to move candidate(s) to the Waiting state to further continue ICE processing. o If ICE state is completed and the SDP answer conforms to Section 3.4.2, the agent MUST reman in the ICE completed state. However, if the ICE support is no longer indicated in the SDP answer, the agent MUST fall-back to [RFC3264] procedures and SHOULD NOT drop the dialog because of the missing ICE support or unexpected answer. Once the agent sends a new offer later on, it MUST perform an ICE restart. 3.4.3.1.1. ICE Restarts The agent MUST remember the nominated pair in the Valid list for each component of the data stream, called the previous selected pair prior to the restart. The agent will continue to send media using this Petit-Huguenin, et al. Expires September 27, 2018 [Page 14] Internet-Draft ICE SDP Usage March 2018 pair, as described in section 12 of [ICE-BIS]. Once these destinations are noted, the agent MUST flush the valid and check lists, and then recompute the check list and its states, thus triggering the candidate processing procedures [ICE-BIS] 3.4.3.2. Procedures for Lite Implementations If ICE is restarting for a data stream, the agent MUST start a new Valid list for that data stream. It MUST remember the nominated pair in the previous Valid list for each component of the data stream, called the previous selected pairs, and continue to send media there as described in section 12 of [ICE-BIS]. The state of ICE processing for each data stream MUST change to Running, and the state of ICE processing MUST change to Running 4. Grammar This specification defines eight new SDP attributes -- the "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice- ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes. This section also provides non-normative examples of the attributes defined. The syntax for the attributes follow Augmented BNF as defined in [RFC5234]. 4.1. "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. Petit-Huguenin, et al. Expires September 27, 2018 [Page 15] Internet-Draft ICE SDP Usage March 2018 candidate-attribute = "candidate" ":" foundation SP component-id SP transport SP priority SP connection-address SP ;from RFC 4566 port ;port from RFC 4566 SP cand-type [SP rel-addr] [SP rel-port] *(SP extension-att-name SP extension-att-value) foundation = 1*32ice-char component-id = 1*5DIGIT transport = "UDP" / transport-extension transport-extension = token ; from RFC 3261 priority = 1*10DIGIT cand-type = "typ" SP candidate-types candidate-types = "host" / "srflx" / "prflx" / "relay" / token rel-addr = "raddr" SP connection-address rel-port = "rport" SP port extension-att-name = token extension-att-value = *VCHAR ice-char = ALPHA / DIGIT / "+" / "/" 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: <connection-address>: is taken from RFC 4566 [RFC4566]. It is the IP address of the candidate. When parsing this field, an agent can differentiate an IPv4 address and an IPv6 address by presence of a colon in its value -- the presence of a colon indicates IPv6. An agent MUST ignore candidate lines that include candidates with IP address versions that are not supported or recognized. An IP address SHOULD be used, but an FQDN MAY be used in place of an IP address. In that case, when receiving an offer or answer containing an FQDN in an a=candidate attribute, the FQDN is looked up in the DNS first using an AAAA record (assuming the agent supports IPv6), and if no result is found or the agent only supports IPv4, using an A record. The rules from section 6 of [RFC6724] is followed by fixing the source address to be one from the candidate pair to be matched against destination addresses reported by FQDN, in cases where the DNS query returns more than one IP address. <port>: is also taken from RFC 4566 [RFC4566]. It is the port of the candidate. Petit-Huguenin, et al. Expires September 27, 2018 [Page 16] Internet-Draft ICE SDP Usage March 2018 <transport>: indicates the transport protocol for the candidate. This specification only defines UDP. However, extensibility is provided to allow for future transport protocols to be used with ICE by extending the sub-registry "ICE Transport Protocols" under "Interactive Connectivity Establishment (ICE)" registry. <foundation>: is composed of 1 to 32 <ice-char>s. It is an identifier that is equivalent for two candidates that are of the same type, share the same base, and come from the same STUN server. The foundation is used to optimize ICE performance in the Frozen algorithm as described in [ICE-BIS] <component-id>: is a positive integer between 1 and 256 (inclusive) that identifies the specific component of the dta stream for which this is a candidate. It MUST start at 1 and MUST increment by 1 for each component of a particular candidate. For data streams based on RTP, candidates for the actual RTP media MUST have a component ID of 1, and candidates for RTCP MUST have a component ID of 2. See section 13 in [ICE-BIS] for additional discussion on extending ICE to new data streams. <priority>: is a positive integer between 1 and (2**31 - 1) inclusive. The procedures for computing candidate's priority is described in section 5.1.2 of [ICE-BIS]. <cand-type>: encodes the type of candidate. This specification defines the values "host", "srflx", "prflx", and "relay" for host, server reflexive, peer reflexive, and relayed candidates, respectively. Specifications for new candidate types MUST define how, if at all, various steps in the ICE processing differ from the ones defined by this specification. <rel-addr> and <rel-port>: convey transport addresses related to the candidate, useful for diagnostics and other purposes. <rel-addr> and <rel-port> MUST be present for server reflexive, peer reflexive, and relayed candidates. If a candidate is server or peer reflexive, <rel-addr> and <rel-port> are equal to the base for that server or peer reflexive candidate. If the candidate is relayed, <rel-addr> and <rel-port> are equal to the mapped address in the Allocate response that provided the client with that relayed candidate (see section Appendix B.3 of [ICE-BIS] for a discussion of its purpose). If the candidate is a host candidate, <rel-addr> and <rel-port> MUST be omitted. In some cases, e.g., for privacy reasons, an agent may not want to reveal the related address and port. In this case the address MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6 candidates) and the port to zero. Petit-Huguenin, et al. Expires September 27, 2018 [Page 17] Internet-Draft ICE SDP Usage March 2018 The candidate attribute can itself be extended. The grammar allows for new name/value pairs to be added at the end of the attribute. Such extensions MUST be made through IETF Review or IESG Approval [RFC5226] and the assignments MUST contain the specific extension and a reference to the document defining the usage of the extension An implementation MUST ignore any name/value pairs it doesn't understand. Example: SDP line for UDP server reflexive candidate attribute for the RTP component a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr 10.0.1.1 rport 8998 4.2. "remote-candidates" Attribute The syntax of the "remote-candidates" attribute is defined using Augmented BNF as defined in [RFC5234]. The remote-candidates attribute is a media-level attribute only. remote-candidate-att = "remote-candidates:" remote-candidate 0*(SP remote-candidate) remote-candidate = component-ID SP connection-address SP port 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 data stream. This attribute MUST be included in an offer by a controlling agent for a data stream that is Completed, and MUST NOT be included in any other case. Example: Remote candidates SDP lines for the RTP and RTCP components: a=remote-candidates:1 192.0.2.3 45664 a=remote-candidates:2 192.0.2.3 45665 4.3. "ice-lite" and "ice-mismatch" Attributes The syntax of the "ice-lite" and "ice-mismatch" attributes, both of which are flags, is: ice-lite = "ice-lite" ice-mismatch = "ice-mismatch" "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 Petit-Huguenin, et al. Expires September 27, 2018 [Page 18] Internet-Draft ICE SDP Usage March 2018 offer arrived with a default destination for a media component that didn't have a corresponding candidate attribute. 4.4. "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: ice-pwd-att = "ice-pwd:" password ice-ufrag-att = "ice-ufrag:" ufrag password = 22*256ice-char ufrag = 4*256ice-char 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 data 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 data stream. If two data 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 same applies when ICE is restarting for an agent.). 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 information 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. Example shows sample ice-ufrag and ice-pwd SDP lines: a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY Petit-Huguenin, et al. Expires September 27, 2018 [Page 19] Internet-Draft ICE SDP Usage March 2018 4.5. "ice-pacing" Attribute The "ice-pacing" is a session level attribute that indicates the desired connectivity check pacing, in milliseconds, for this agent (see section 14 of [ICE-BIS]). The syntax is: ice-pacing-att = "ice-pacing:" pacing-value pacing-value = 1*10DIGIT Following the procedures defined in [ICE-BIS], a default value of 50ms is used for an agent when ice-pacing attribute is omitted in the offer or the answer. The same rule applies for ice-pacing attribute values lower than 50ms. This mandates that, if an agent includes the ice-pacing attribute, its value MUST be greater than 50ms or else a value of 50ms is considered by default for that agent. Also the larger of the ice-pacing attribute values between the offer and the answer (determined either by the one provided in the ice- pacing attribute or by picking the default value) MUST be considered for a given ICE session. The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'ice- pacing' attribute is 'TRANSPORT'. Example shows ice-pacing value of 5 ms: a=ice-pacing:5 4.6. "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: ice-options = "ice-options:" ice-option-tag 0*(SP ice-option-tag) ice-option-tag = 1*ice-char The existence of an ice-option in an offer indicates that a certain extension is supported by the agent and is willing to use it, if the peer agent also includes the same extension in the answer. There might be further extension specific negotiations needed between the agents that determine how the extensions gets used in a given session. The details of the negotiation procedures, if present, MUST be defined by the specification defining the extension (see Section 10.2). Petit-Huguenin, et al. Expires September 27, 2018 [Page 20] Internet-Draft ICE SDP Usage March 2018 Example shows 'rtp+ecn' ice-option SDP line from <<RFC6679>>: a=ice-options:rtp+ecn 5. Keepalives All the ICE agents MUST follow the procedures defined in section 11 of [ICE-BIS] for sending keepalives. The keepalives MUST be sent regardless of whether the data 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. 6. SIP Considerations Note that ICE is not intended for NAT traversal for SIP, which is assumed to be provided via another mechanism [RFC5626]. 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 local candidates, pairs, check lists, states, and so on. 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. 6.1. 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 Petit-Huguenin, et al. Expires September 27, 2018 [Page 21] Internet-Draft ICE SDP Usage March 2018 consequence of having successfully started alerting the called user agent. Two cases can be considered -- one where the offer is present in the initial INVITE and one where it is in a response. 6.1.1. 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. On the receipt of the offer, the answerer SHOULD generate an answer in a provisional response once it has completed candidate gathering. 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 ICE specific optimization, wherein, the agent retransmits the provisional response with the exponential backoff timers described in [RFC3262]. Such retransmissions MUST cease on receipt of a STUN Binding request for one of the data streams signaled in that SDP or on transmission of the answer in a 2xx response. If no Binding request is received prior to the last retransmit, the agent does not consider the session terminated. For the ICE lite peers, 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). It should be noted that the ICE specific optimization is very specific to provisional response carrying answers that start ICE processing and it is not a general technique for 1xx reliability. Also such an optimization SHOULD NOT be used if both agents support PRACK. 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 [RFC3262]. 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. Alternatively, an agent MAY delay sending an answer until the 200 OK; however, this results in a poor user experience and is NOT RECOMMENDED. Petit-Huguenin, et al. Expires September 27, 2018 [Page 22] Internet-Draft ICE SDP Usage March 2018 Once the answer has been sent, the agent SHOULD begin its connectivity checks. Once candidate pairs for each component of a data stream enter the valid list, the answerer can begin sending media on that data 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 data 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]. 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. 6.1.2. 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 [RFC3262]), 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. 6.2. 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 Petit-Huguenin, et al. Expires September 27, 2018 [Page 23] Internet-Draft ICE SDP Usage March 2018 specification, which uses a feature tag in registrations to facilitate interoperability through signaling intermediaries. 6.3. 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 data streams, it cannot determine which data 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. 6.4. Interactions with Preconditions Quality of Service (QoS) preconditions, which are defined in [RFC3312] and [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 6.1 describe their own type of "preconditions", albeit with less functionality than those provided by the explicit preconditions in [RFC5898]. 6.5. 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. Petit-Huguenin, et al. Expires September 27, 2018 [Page 24] Internet-Draft ICE SDP Usage March 2018 The flows for continued operation, as described in Section 7 of [RFC3725], 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 data stream and go through the process of gathering new candidates. Furthermore, that list of candidates SHOULD include the ones currently being used for media. 7. Relationship with ANAT [RFC4091], the Alternative Network Address Types (ANAT) Semantics for the SDP grouping framework, and [RFC4092], its usage with SIP, define a mechanism for indicating that an agent can support both IPv4 and IPv6 for a data 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. It is RECOMMENDED that ICE be used in realizing the dual-stack use- cases in agents that support ICE. 8. Setting Ta and RTO for RTP Media Streams During the gathering phase of ICE and while ICE is performing connectivity checks, an agent sends STUN and TURN transactions. These transactions are paced at a rate of one every Ta milliseconds, and utilize a specific RTO. See Section 14 of [ICE-BIS] for details on how the values of Ta and RTO are computed with a real-time media stream of known maximum bandwidth to rate-control the ICE exchanges. 9. Security Considerations 9.1. 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 data stream, and so on. These are similar to the general security considerations for offer/answer exchanges, and the security considerations in [RFC3264] apply. These require techniques for message integrity and encryption for offers and answers, which are satisfied by the TLS mechanism [RFC3261] when SIP is used. As such, the usage of TLS with ICE is RECOMMENDED. Petit-Huguenin, et al. Expires September 27, 2018 [Page 25] Internet-Draft ICE SDP Usage March 2018 9.2. 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. 9.2.1. 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 it's 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. User Agents that are not willing to receive non-ICE answers MUST include an "ice" Option Tag in the Require Header Field in their offer. Clients that rejects non-ICE offers SHOULD use a 421 response code, together with an Option Tag "ice" in the Require Header Field in the response. 9.2.2. Interactions with Application Layer Gateways and SIP Application Layer Gateways (ALGs) are functions present in a Network Address Translation (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: Petit-Huguenin, et al. Expires September 27, 2018 [Page 26] Internet-Draft ICE SDP Usage March 2018 o The ALG does not modify the "m=" and "c=" lines or the rtcp attribute if they contain external addresses. o If the "m=" and "c=" lines contain internal addresses, the modification depends on the state of the ALG: * If the ALG already has a binding established that maps an external port to an internal IP address and port matching the values in the "m=" and "c=" lines or rtcp attribute, the ALG uses that binding instead of creating a new one. * If the ALG does not already have a binding, it creates a new one and modifies the SDP, rewriting the "m=" and "c=" lines and rtcp attribute. 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. 10. IANA Considerations 10.1. SDP Attributes The original ICE specification defined seven new SDP attributes per the procedures of Section 8.2.4 of [RFC4566]. The registration information from the original specification is included here with Petit-Huguenin, et al. Expires September 27, 2018 [Page 27] Internet-Draft ICE SDP Usage March 2018 modifications to include Mux Category and to align with the recent recommendations for populating Contact information. 10.1.1. candidate Attribute Attribute Name: candidate Type of Attribute: media-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides one of many possible candidate addresses for communication. These addresses are validated with an end-to-end connectivity check using Session Traversal Utilities for NAT (STUN). Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [1] Reference: RFCXXXX Mux Category: TRANSPORT 10.1.2. remote-candidates Attribute Attribute Name: remote-candidates Type of Attribute: media-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the identity of the remote candidates that the offerer wishes the answerer to use in its answer. Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [2] Reference: RFCXXXX Petit-Huguenin, et al. Expires September 27, 2018 [Page 28] Internet-Draft ICE SDP Usage March 2018 Mux Category: TRANSPORT 10.1.3. ice-lite Attribute Attribute Name: ice-lite Type of Attribute: session-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and indicates that an agent has the minimum functionality required to support ICE inter-operation with a peer that has a full implementation. Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [3] Reference: RFCXXXX Mux Category: NORMAL 10.1.4. ice-mismatch Attribute Attribute Name: ice-mismatch Type of Attribute: media-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and indicates that an agent is ICE capable, but did not proceed with ICE due to a mismatch of candidates with the default destination for media signaled in the SDP. Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [4] Reference: RFCXXXX Mux Category: NORMAL Petit-Huguenin, et al. Expires September 27, 2018 [Page 29] Internet-Draft ICE SDP Usage March 2018 10.1.5. ice-pwd Attribute Attribute Name: ice-pwd Type of Attribute: session- or media-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the password used to protect STUN connectivity checks. Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [5] Reference: RFCXXXX Mux Category: TRANSPORT 10.1.6. ice-ufrag Attribute Attribute Name: ice-ufrag Type of Attribute: session- or media-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides the fragments used to construct the username in STUN connectivity checks. Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [6] Reference: RFCXXXX Mux Category: TRANSPORT Petit-Huguenin, et al. Expires September 27, 2018 [Page 30] Internet-Draft ICE SDP Usage March 2018 10.1.7. ice-options Attribute Attribute Name: ice-options Long Form: ice-options Type of Attribute: session- or media-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and indicates the ICE options or extensions used by the agent. Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [7] Reference: RFCXXXX Mux Category: NORMAL 10.1.8. ice-pacing Attribute This specification also defines a new SDP attribute, "ice-pacing" according to the following data: Attribute Name: ice-pacing Type of Attribute: session-level Subject to charset: No Purpose: This attribute is used with Interactive Connectivity Establishment (ICE) to indicate desired connectivity check pacing values. Appropriate Values: See Section 4 of RFC XXXX. Contact Name: IESG Contact e-mail: iesg@ietf.org [8] Reference: RFCXXXX Mux Category: TRANSPORT Petit-Huguenin, et al. Expires September 27, 2018 [Page 31] Internet-Draft ICE SDP Usage March 2018 10.2. Interactive Connectivity Establishment (ICE) Options Registry IANA maintains a registry for ice-options identifiers under the Specification Required policy as defined in "Guidelines for Writing an IANA Considerations Section in RFCs" [RFC5226]. ICE options are of unlimited length according to the syntax in Section 4.6; however, they are RECOMMENDED to be no longer than 20 characters. This is to reduce message sizes and allow for efficient parsing. In [RFC5245] 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. [RFC6679] defines the "rtp+ecn" ICE option. The aggregation rule for this ICE option 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. Section 10 of [ICE-BIS] defines "ice2" ICE option. Since "ice2" is a session level ICE option, no aggregation rules apply. A registration request MUST include the following information: o The ICE option identifier to be registered o Name, Email, and Address of a contact person for the registration o Organization or individuals having the change control o Short description of the ICE extension to which the option relates o Reference(s) to the specification defining the ICE option and the related extensions Petit-Huguenin, et al. Expires September 27, 2018 [Page 32] Internet-Draft ICE SDP Usage March 2018 11. Acknowledgments A large part of the text in this document was taken from [RFC5245], authored by Jonathan Rosenberg. Some of the text in this document was taken from [RFC6336], authored by Magnus Westerlund and Colin Perkins. Many thanks to Christer Holmberg for providing text suggestions in Section 4 that aligns with [ICE-BIS] Thanks to Thomas Stach for text help, Roman Shpount for suggesting RTCP candidate handling and Simon Perreault for advising on IPV6 address selection when candidate-address includes FQDN. Thanks to following experts for their reviews and constructive feedback: Christer Holmberg, Adam Roach and the MMUSIC WG. 12. References 12.1. Normative References [I-D.ietf-mmusic-sdp-mux-attributes] Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17 (work in progress), February 2018, <http://www.rfc-editor.org/info/rfc7656>. [ICE-BIS] Keranen, A. and J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", draft-ietf-ice-rfc5245bis-00 (work in progress), March 2015. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://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, <http://www.rfc-editor.org/info/rfc3261>. Petit-Huguenin, et al. Expires September 27, 2018 [Page 33] Internet-Draft ICE SDP Usage March 2018 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002, <http://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, <http://www.rfc-editor.org/info/rfc3264>. [RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October 2002, <http://www.rfc-editor.org/info/rfc3312>. [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, DOI 10.17487/RFC3556, July 2003, <http://www.rfc-editor.org/info/rfc3556>. [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, DOI 10.17487/RFC3605, October 2003, <http://www.rfc-editor.org/info/rfc3605>. [RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session Initiation Protocol (SIP) Preconditions Framework", RFC 4032, DOI 10.17487/RFC4032, March 2005, <http://www.rfc-editor.org/info/rfc4032>. [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, <http://www.rfc-editor.org/info/rfc4091>. [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, <http://www.rfc-editor.org/info/rfc4092>. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, <http://www.rfc-editor.org/info/rfc4566>. Petit-Huguenin, et al. Expires September 27, 2018 [Page 34] Internet-Draft ICE SDP Usage March 2018 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc5234>. [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, DOI 10.17487/RFC5245, April 2010, <http://www.rfc-editor.org/info/rfc5245>. [RFC5768] Rosenberg, J., "Indicating Support for Interactive Connectivity Establishment (ICE) in the Session Initiation Protocol (SIP)", RFC 5768, DOI 10.17487/RFC5768, April 2010, <http://www.rfc-editor.org/info/rfc5768>. [RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for Interactive Connectivity Establishment (ICE) Options", RFC 6336, April 2010, <http://www.rfc-editor.org/info/rfc6336>. [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P., and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 2012, <http://www.rfc-editor.org/info/rfc6679>. [RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, September 2012, <http://www.rfc-editor.org/info/rfc6724>. [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources", RFC 7656, DOI 10.17487/RFC7656, November 2015, <http://www.rfc-editor.org/info/rfc7656>. 12.2. Informative References Petit-Huguenin, et al. Expires September 27, 2018 [Page 35] Internet-Draft ICE SDP Usage March 2018 [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, DOI 10.17487/RFC3725, April 2004, <http://www.rfc-editor.org/info/rfc3725>. [RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)", RFC 3960, DOI 10.17487/RFC3960, December 2004, <http://www.rfc-editor.org/info/rfc3960>. [RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed., "Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)", RFC 5626, DOI 10.17487/RFC5626, October 2009, <http://www.rfc-editor.org/info/rfc5626>. [RFC5898] Andreasen, F., Camarillo, G., Oran, D., and D. Wing, "Connectivity Preconditions for Session Description Protocol (SDP) Media Streams", RFC 5898, DOI 10.17487/RFC5898, July 2010, <http://www.rfc-editor.org/info/rfc5898>. 12.3. URIs [1] mailto:iesg@ietf.org [2] mailto:iesg@ietf.org [3] mailto:iesg@ietf.org [4] mailto:iesg@ietf.org [5] mailto:iesg@ietf.org [6] mailto:iesg@ietf.org [7] mailto:iesg@ietf.org [8] mailto:iesg@ietf.org [9] mailto:christer.holmberg@ericsson.com [10] mailto:rshpount@turbobridge.com [11] mailto:thomass.stach@gmail.com Petit-Huguenin, et al. Expires September 27, 2018 [Page 36] Internet-Draft ICE SDP Usage March 2018 Appendix A. Examples For the example shown in section 16 of [ICE-BIS] the resulting offer (message 5) encoded in SDP looks like: v=0 o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP s= c=IN IP6 $NAT-PUB-1.IP t=0 0 a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY m=audio $NAT-PUB-1.PORT RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT The offer, with the variables replaced with their values, will look like (lines folded for clarity): v=0 o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a s= c=IN IP6 2001:420:c0e0:1005::61 t=0 0 a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY m=audio 45664 RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998 typ host a=candidate:2 1 UDP 1694498815 2001:420:c0e0:1005::61 45664 typ srflx raddr fe80::6676:baff:fe9c:ee4a rport 8998 The resulting answer looks like: Petit-Huguenin, et al. Expires September 27, 2018 [Page 37] Internet-Draft ICE SDP Usage March 2018 v=0 o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP s= c=IN IP4 $R-PUB-1.IP t=0 0 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh a=ice-ufrag:9uB6 m=audio $R-PUB-1.PORT RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host With the variables filled in: v=0 o=bob 2808844564 2808844564 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh a=ice-ufrag:9uB6 m=audio 3478 RTP/AVP 0 b=RS:0 b=RR:0 a=rtpmap:0 PCMU/8000 a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host 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 data 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. Petit-Huguenin, et al. Expires September 27, 2018 [Page 38] Internet-Draft ICE SDP Usage March 2018 Agent L Network Agent R |(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: Petit-Huguenin, et al. Expires September 27, 2018 [Page 39] Internet-Draft ICE SDP Usage March 2018 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 Petit-Huguenin, et al. Expires September 27, 2018 [Page 40] Internet-Draft ICE SDP Usage March 2018 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. Appendix E. Contributors o Christer Holmberg Ericsson Email: christer.holmberg@ericsson.com [9] o Roman Shpount TurboBridge rshpount@turbobridge.com [10] o Thomas Stach thomass.stach@gmail.com [11] Authors' Addresses Marc Petit-Huguenin Impedance Mismatch Email: marc@petit-huguenin.org Suhas Nandakumar Cisco Systems 707 Tasman Dr Milpitas, CA 95035 USA Email: snandaku@cisco.com Ari Keranen Ericsson Jorvas 02420 Finland Email: ari.keranen@ericsson.com Petit-Huguenin, et al. Expires September 27, 2018 [Page 41]