MMUSIC J. Rosenberg
Internet-Draft Cisco Systems
Expires: July 20, 2007 January 16, 2007
Interactive Connectivity Establishment (ICE): A Methodology for Network
Address Translator (NAT) Traversal for Offer/Answer Protocols
draft-ietf-mmusic-ice-13
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Abstract
This document describes a protocol for Network Address Translator
(NAT) traversal for multimedia session signaling protocols based on
the offer/answer model, such as the Session Initiation Protocol
(SIP). This protocol is called Interactive Connectivity
Establishment (ICE). ICE makes use of the Session Traversal
Utilities for NAT (STUN) protocol, applying its binding discovery and
relay usages, in addition to defining a new usage for checking
connectivity between peers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 7
2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 9
2.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . . 10
2.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 11
2.5. Security for Checks . . . . . . . . . . . . . . . . . . . 11
2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . 12
2.7. Lite Implementations . . . . . . . . . . . . . . . . . . . 13
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13
4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 16
4.1. Full Implementation Requirements . . . . . . . . . . . . . 16
4.1.1. Gathering Candidates . . . . . . . . . . . . . . . . . 16
4.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 18
4.1.3. Choosing In-Use Candidates . . . . . . . . . . . . . . 20
4.2. Lite Implementation . . . . . . . . . . . . . . . . . . . 20
4.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 21
5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 22
5.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 23
5.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 23
5.3. Gathering Candidates . . . . . . . . . . . . . . . . . . . 24
5.4. Prioritizing Candidates . . . . . . . . . . . . . . . . . 24
5.5. Choosing In Use Candidates . . . . . . . . . . . . . . . . 24
5.6. Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 24
5.7. Forming the Check Lists . . . . . . . . . . . . . . . . . 24
5.8. Performing Periodic Checks . . . . . . . . . . . . . . . . 27
6. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 28
6.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 28
6.2. Determining Role . . . . . . . . . . . . . . . . . . . . . 28
6.3. Forming the Check List . . . . . . . . . . . . . . . . . . 28
6.4. Performing Periodic Checks . . . . . . . . . . . . . . . . 28
7. Connectivity Checks . . . . . . . . . . . . . . . . . . . . . 28
7.1. Client Procedures . . . . . . . . . . . . . . . . . . . . 29
7.1.1. Sending the Request . . . . . . . . . . . . . . . . . 29
7.1.2. Processing the Response . . . . . . . . . . . . . . . 30
7.2. Server Procedures . . . . . . . . . . . . . . . . . . . . 31
7.2.1. Additional Procedures for Full Implementations . . . . 32
7.2.2. Additional Procedures for Lite Implementations . . . . 34
8. Concluding ICE . . . . . . . . . . . . . . . . . . . . . . . . 34
9. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 35
9.1. Generating the Offer . . . . . . . . . . . . . . . . . . . 35
9.1.1. Additional Procedures for Full Implementations . . . . 36
9.1.2. Additional Procedures for Lite Implementations . . . . 37
9.2. Receiving the Offer and Generating an Answer . . . . . . . 37
9.2.1. Additional Procedures for Full Implementations . . . . 38
9.3. Updating the Check and Valid Lists . . . . . . . . . . . . 38
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9.3.1. Additional Procedures for Full Implementations . . . . 38
10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 40
11. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 41
11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 41
11.1.1. Procedures for Full Implementations . . . . . . . . . 41
11.1.2. Procedures for Lite Implementations . . . . . . . . . 42
11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 42
12. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . . 42
12.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . . 42
12.2. SIP Option Tags and Media Feature Tags . . . . . . . . . . 44
12.3. Interactions with Forking . . . . . . . . . . . . . . . . 44
12.4. Interactions with Preconditions . . . . . . . . . . . . . 45
12.5. Interactions with Third Party Call Control . . . . . . . . 45
13. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
14. Extensibility Considerations . . . . . . . . . . . . . . . . . 48
15. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
16. Security Considerations . . . . . . . . . . . . . . . . . . . 54
16.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 54
16.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 57
16.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 57
16.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 57
16.4.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 58
16.4.2. STUN Amplification Attack . . . . . . . . . . . . . . 58
16.5. Interactions with Application Layer Gateways and SIP . . . 59
17. Definition of Connectivity Check Usage . . . . . . . . . . . . 59
17.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 60
17.2. Client Discovery of Server . . . . . . . . . . . . . . . . 60
17.3. Server Determination of Usage . . . . . . . . . . . . . . 60
17.4. New Requests or Indications . . . . . . . . . . . . . . . 60
17.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 60
17.6. New Error Response Codes . . . . . . . . . . . . . . . . . 61
17.7. Client Procedures . . . . . . . . . . . . . . . . . . . . 61
17.8. Server Procedures . . . . . . . . . . . . . . . . . . . . 61
17.9. Security Considerations for Connectivity Check . . . . . . 61
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 61
18.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . 61
18.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 61
18.1.2. remote-candidates Attribute . . . . . . . . . . . . . 62
18.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . . 62
18.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . . 63
18.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 63
18.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 63
18.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 64
18.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . . 64
19. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 65
19.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 65
19.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 65
19.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 66
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19.4. Requirements for a Long Term Solution . . . . . . . . . . 67
19.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 67
20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68
21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 68
21.1. Normative References . . . . . . . . . . . . . . . . . . . 68
21.2. Informative References . . . . . . . . . . . . . . . . . . 69
Appendix A. Lite and Full Implementations . . . . . . . . . . . . 71
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 71
B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 72
B.2. Candidates with Multiple Bases . . . . . . . . . . . . . . 72
B.3. Purpose of the Translation . . . . . . . . . . . . . . . . 74
B.4. Importance of the STUN Username . . . . . . . . . . . . . 74
B.5. The Candidate Pair Sequence Number Formula . . . . . . . . 75
B.6. The Frozen State . . . . . . . . . . . . . . . . . . . . . 76
B.7. The remote-candidates attribute . . . . . . . . . . . . . 76
B.8. Why are Keepalives Needed? . . . . . . . . . . . . . . . . 77
B.9. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 78
B.10. Why Send an Updated Offer? . . . . . . . . . . . . . . . . 78
B.11. Why are Binding Indications Used for Keepalives? . . . . . 78
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 80
Intellectual Property and Copyright Statements . . . . . . . . . . 81
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1. Introduction
RFC 3264 [4] defines a two-phase exchange of Session Description
Protocol (SDP) messages [10] for the purposes of establishment of
multimedia sessions. This offer/answer mechanism is used by
protocols such as the Session Initiation Protocol (SIP) [3].
Protocols using offer/answer are difficult to operate through Network
Address Translators (NAT). Because their purpose is to establish a
flow of media packets, they tend to carry IP addresses within their
messages, which is known to be problematic through NAT [15]. The
protocols also seek to create a media flow directly between
participants, so that there is no application layer intermediary
between them. This is done to reduce media latency, decrease packet
loss, and reduce the operational costs of deploying the application.
However, this is difficult to accomplish through NAT. A full
treatment of the reasons for this is beyond the scope of this
specification.
Numerous solutions have been proposed for allowing these protocols to
operate through NAT. These include Application Layer Gateways
(ALGs), the Middlebox Control Protocol [16], Simple Traversal
Underneath NAT (STUN) [14] and its revision, retitled Session
Traversal Utilities for NAT [11], the STUN Relay Usage [12], and
Realm Specific IP [18] [19] along with session description extensions
needed to make them work, such as the Session Description Protocol
(SDP) [10] attribute for the Real Time Control Protocol (RTCP) [2].
Unfortunately, these techniques all have pros and cons which make
each one optimal in some network topologies, but a poor choice in
others. The result is that administrators and implementors are
making assumptions about the topologies of the networks in which
their solutions will be deployed. This introduces complexity and
brittleness into the system. What is needed is a single solution
which is flexible enough to work well in all situations.
This specification provides that solution for media streams
established by signaling protocols based on the offer-answer model.
It is called Interactive Connectivity Establishment, or ICE. ICE
makes use of STUN and its relay extension, commonly called TURN, but
uses them in a specific methodology which avoids many of the pitfalls
of using any one alone.
2. Overview of ICE
In a typical ICE deployment, we have two endpoints (known as agents
in RFC 3264 terminology) which want to communicate. They are able to
communicate indirectly via some signaling system such as SIP, by
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which they can perform an offer/answer exchange of SDP [4] messages.
Note that ICE is not intended for NAT traversal for SIP, which is
assumed to be provided via some other mechanism [32]. At the
beginning of the ICE process, the agents are ignorant of their own
topologies. In particular, they might or might not be behind a NAT
(or multiple tiers of NATs). ICE allows the agents to discover
enough information about their topologies to find a path or paths by
which they can communicate.
Figure Figure 1 shows a typical environment for ICE deployment. The
two endpoints are labelled L and R (for left and right, which helps
visualize call flows). Both L and R are behind NATs -- though as
mentioned before, they don't know that. The type of NAT and its
properties are also unknown. Agents L and R are capable of engaging
in an offer/answer exchange by which they can exchange SDP messages,
whose purpose is to set up a media session between L and R.
Typically, this exchange will occur through a SIP server.
In addition to the agents, a SIP server and NATs, ICE is typically
used in concert with STUN servers in the network. Each agent can
have its own STUN server, or they can be the same.
+-------+
| SIP |
+-------+ | Srvr | +-------+
| STUN | | | | STUN |
| Srvr | +-------+ | Srvr |
| | / \ | |
+-------+ / \ +-------+
/ \
/ \
/ \
/ \
/ <- Signalling -> \
/ \
/ \
+--------+ +--------+
| NAT | | NAT |
+--------+ +--------+
/ \
/ \
/ \
+-------+ +-------+
| Agent | | Agent |
| L | | R |
| | | |
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+-------+ +-------+
Figure 1
The basic idea behind ICE is as follows: each agent has a variety of
candidate transport addresses it could use to communicate with the
other agent. These might include:
o It's directly attached network interface (or interfaces in the
case of a multihomed machine
o A translated address on the public side of a NAT (a "server
reflexive" address)
o The address of a media relay the agent is using.
Potentially, any of L's candidate transport addresses can be used to
communicate with any of R's candidate transport addresses. In
practice, however, many combinations will not work. For instance, if
L and R are both behind NATs then their directly interface addresses
are unlikely to be able to communicate directly (this is why ICE is
needed, after all!). The purpose of ICE is to discover which pairs
of addresses will work. The way that ICE does this is to
systematically try all possible pairs (in a carefully sorted order)
until it finds one or more that works.
2.1. Gathering Candidate Addresses
In order to execute ICE, an agent has to identify all of its address
candidates. Naturally, one viable candidate is one obtained directly
from a local interface the client has towards the network. Such a
candidate is called a HOST CANDIDATE. The local interface could be
one on a local layer 2 network technology, such as ethernet or WiFi,
or it could be one that is obtained through a tunnel mechanism, such
as a Virtual Private Network (VPN) or Mobile IP (MIP). In all cases,
these appear to the agent as a local interface from which ports (and
thus a candidate) can be allocated.
If an agent is multihomed, it can obtain a candidate from each
interface. Depending on the location of the peer on the IP network
relative to the agent, the agent may be reachable by the peer through
one of those interfaces, or through another. Consider, for example,
an agent which has a local interface to a private net 10 network, and
also to the public Internet. A candidate from the net10 interface
will be directly reachable when communicating with a peer on the same
private net 10 network, while a candidate from the public interface
will be directly reachable when communicating with a peer on the
public Internet. Rather than trying to guess which interface will
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work prior to sending an offer, the offering agent includes both
candidates in its offer.
Once the agent has obtained host candidates, it uses STUN to obtain
additional candidates. These come in two flavors: translated
addresses on the public side of a NAT (SERVER REFLEXIVE CANDIDATES)
and addresses of media relays (RELAYED CANDIDATES). The relationship
of these candidates to the host candidate is shown in Figure 2. Both
types of candidates are discovered using STUN.
To Internet
|
|
| /------------ Relayed
| / Candidate
+--------+
| |
| STUN |
| Server |
| |
+--------+
|
|
| /------------ Server
|/ Reflexive
+------------+ Candidate
| NAT |
+------------+
|
| /------------ Host
|/ Candidate
+--------+
| |
| Agent |
| |
+--------+
Figure 2
To find a server reflexive candidate, the agent sends a STUN Binding
Request, using the Binding Discovery Usage [11] from each host
candidate, to its STUN server. (It is assumed that the address of
the STUN server is configured, or learned in some way.) When the
agent sends the Binding Request, the NAT (assuming there is one) will
allocate a binding, mapping this server reflexive candidate to the
host candidate. Outgoing packets sent from the host candidate will
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be translated by the NAT to the server reflexive candidate. Incoming
packets sent to the server relexive candidate will be translated by
the NAT to the host candidate and forwarded to the agent. We call
the host candidate associated with a given server reflexive candidate
the BASE.
Note
"Base" refers to the address you'd send from for a particular
candidate. Thus, as a degenerate case host candidates also have a
base, but it's the same as the host candidate.
When there are multiple NATs between the agent and the STUN server,
the STUN request will create a binding on each NAT, but only the
outermost server reflexive candidate will be discovered by the agent.
If the agent is not behind a NAT, then the base candidate will be the
same as the server reflexive candidate and the server reflexive
candidate can be ignored.
The final type of candidate is a RELAYED candidate. The STUN Relay
Usage [12] allows a STUN server to act as a media relay, forwarding
traffic between L and R. In order to send traffic to L, R sends
traffic to the media relay which forwards it to L and vice versa.
The same thing happens in the other direction.
Traffic from L to R has its addresses rewritten twice: first by the
NAT and second by the STUN relay server. Thus, the address that R
knows about and the one that it wants to send to is the one on the
STUN relay server. This address is the final kind of candidate,
which we call a RELAYED CANDIDATE.
2.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them highest to
lowest priority and sends them to R over the signalling channel. The
candidates are carried in attributes in the SDP offer. When R
receives the offer, it performs the same gathering process and
responds with its own list of candidates. At the end of this
process, each agent has a complete list of both its candidates and
its peer's candidates and is ready to perform connectivity checks by
pairing up the candidates to see which pair works.
The basic principle of the connectivity checks is simple:
1. Sort the candidate pairs in priority order.
2. Send checks on each candidate pair in priority order.
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3. Acknowledge checks received from the other agent.
A complete connectivity check for a single candidate pair is a simple
4-message handshake:
L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 3
As an optimization, as soon as R gets L's check message he
immediately sends his own check message to L on the same candidate
pair. This accelerates the process of finding a valid candidate, and
is called a triggered check.
At the end of this handshake, both L and R know that they can send
(and receive) messages end-to-end in both directions.
2.3. Sorting Candidates
Because the algorithm above searches all candidate pairs, if a
working pair exists it will eventually find it no matter what order
the candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The
algorithm is described in Section 4.1.2 but follows two general
principles:
o Each agent gives its candidates a numeric priority which is sent
along with the candidate to the peer
o The local and remote priorities are combined so that each agent
has the same ordering for the candidate pairs.
The second property is important for getting ICE to work when there
are NATs in front of A and B. Frequently, NATs will not allow packets
in from a host until the agent behind the NAT has sent a packet
towards that host. Consequently, ICE checks in each direction will
not succeed until both sides have sent a check through their
respective NATs.
In general the priority algorithm is designed so that candidates of
similar type get similar priorities and so that more direct routes
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are preferred over indirect ones. Within those guidelines, however,
agents have a fair amount of discretion about how to tune their
algorithms.
2.4. Frozen Candidates
The previous description only addresses the case where the agents
wish to establish a single media component--i.e., a single flow with
a single host-port quartet. However, in many cases (in particular
RTP and RTCP) the agents actually need to establish connectivity for
more than one flow.
The naive way to attack this problem would be to simply do
independent ICE exchanges for each media component. This is
obviously inefficient because the network properties are likely to be
very similar for each component (especially because RTP and RTCP are
typically run on adjacent ports). Thus, it should be possible to
leverage information from one media component in order to determine
the best candidates for another. ICE does this with a mechanism
called "frozen candidates."
The basic principle behind frozen candidates is that initially only
the candidates for a single media component are tested. The other
media components are marked "frozen". When the connectivity checks
for the first component succeed, the corresponding candidates for the
other components are unfrozen and checked immediately. This avoids
repeated checking of components which are superficially more
attractive but in fact are likely to fail.
While we've described "frozen" here as a separate mechanism for
expository purposes, in fact it is an integral part of ICE and the
the ICE prioritization algorithm automatically ensures that the right
candidates are unfrozen and checked in the right order.
2.5. Security for Checks
Because ICE is used to discover which addresses can be used to send
media between two agents, it is important to ensure that the process
cannot be hijacked to send media to the wrong location. Each STUN
connectivity check is covered by a message authentication code (MAC)
computed using a key exchanged in the signalling channel. This MAC
provides message integrity and data origin authentication, thus
stopping an attacker from forging or modifying connectivity check
messages. The MAC also aids in disambiguating ICE exchanges from
forked calls.
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2.6. Concluding ICE
ICE checks are performed in a specific sequence, so that high
priority pairs are checked first, followed by lower priority ones.
One way to conclude ICE is to declare victory as soon as a check for
each component of each media stream completes successfully. Indeed,
this is a reasonable algorithm, and details for it are provided
below. However, it is possible that packet losses will cause a
higher priority check to take longer to complete, and allowing ICE to
run a little longer might produce better results. More
fundamentally, however, the prioritization defined by this
specification may not yield "optimal" results. As an example, if the
aim is to select low latency media paths, usage of a relay is a hint
that latencies may be higher, but it is nothing more than a hint. An
actual RTT measurement could be made, and it might demonstrate that a
pair with lower priority is actually better than one with higher
priority.
Consequently, ICE assigns one of the agents in the role of the
controlling agent, and the other of the controlled agent. The
controlling agent runs a selection algorithm, through which it can
decide when to conclude ICE checks, and which pairs get selected.
The one that is selected is called the favored candidate pair. When
a controlling agent selects a pair for a particular component of a
media stream, it generates a check for that pair and includes a flag
in the check indicating that the pair has been selected. If the
controlled agent has already performed in a check in the reverse
direction that succeeded, the controlled agent considers ICE
processing to be concluded for that component. Once there is a
selected pair for each component of a media stream, the ICE checks
for that media stream are considered to be completed. At this point,
further checks stop for that media stream - ICE is considered to be
done. Consequently, media can flow in each direction for that
stream, as shown in Figure 4. Once all of the media streams are
completed, the controlling endpoint sends an updated offer if the
currently in-use candidates don't match the ones it selected.
L R
- -
STUN request + flag -> \ L's
<- STUN response / check
-> RTP Data
<- RTP Data
Figure 4
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Once ICE is concluded, it can be restarted at any time for one or all
of the media streams by each agent. This is done by sending an
updated offer indicating a restart.
2.7. Lite Implementations
In order for ICE to be used in a call, both agents need to support
it. However, certain agents, such as those in gateways to the PSTN,
media servers, conferencing servers, and voicemail servers, are known
to not be behind a NAT or firewall. To make it easier for these
devices to support ICE, ICE defines a special type of implementation
called "lite" (in contrast to the normal "full" implementation). A
lite implementation doesn't gather candidates; it includes only its
host candidate for any media stream. When a lite implementation
connects with a full implementation, the full agent takes the role of
the controlling agent, and the lite agent takes on the controlled
role. In addition, lite agents do not need to generate connectivity
checks, run the state machines, or compute candidate pairs. For an
informational summary of ICE processing as seen by a lite agent, see
[33].
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
This specification makes use of the following terminology:
Agent: As defined in RFC 3264, an agent is the protocol
implementation involved in the offer/answer exchange. There are
two agents involved in an offer/answer exchange.
Peer: From the perspective of one of the agents in a session, its
peer is the other agent. Specifically, from the perspective of
the offerer, the peer is the answerer. From the perspective of
the answerer, the peer is the offerer.
Transport Address: The combination of an IP address and port.
Candidate: A transport address that is to be tested by ICE procedures
in order to determine its suitability for usage for receipt of
media.
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Component: A component is a single transport address that is used to
support a media stream. For media streams based on RTP, there are
two components per media stream - one for RTP, and one for RTCP.
Host Candidate: A candidate obtained by binding to a specific port
from an interface on the host. This includes both physical
interfaces and logical ones, such as ones obtained through Virtual
Private Networks (VPNs) and Realm Specific IP (RSIP) [18] (which
lives at the operating system level).
Server Reflexive Candidate: A candidate obtained by sending a STUN
request from a host candidate to a STUN server, distinct from the
peer, whose address is configured or learned by the client prior
to an offer/answer exchange.
Peer Reflexive Candidate: A candidate obtained by sending a STUN
request from a host candidate to the STUN server running on a
peer's candidate.
Relayed Candidate: A candidate obtained by sending a STUN Allocate
request from a host candidate to a STUN server. The relayed
candidate is resident on the STUN server, and the STUN server
relays packets back towards the agent.
Translation: The translation of a relayed candidate is the transport
address that the relay will forward a packet to, when one is
received at the relayed candidate. For relayed candidates learned
through the STUN Allocate request, the translation of the relayed
candidate is the server reflexive candidate returned by the
Allocate response.
Base: The base of a server reflexive candidate is the host candidate
from which it was derived. A host candidate is also said to have
a base, equal to that candidate itself. Similarly, the base of a
relayed candidate is that candidate itself.
Foundation: Each candidate has a foundation, which is an identifier
that is distinct for two candidates that have different types,
different interface IP addresses for their base, and different IP
addresses for their STUN servers. Two candidates have the same
foundation when they are of the same type, their bases have the
same IP address, and, for server reflexive or relayed candidates,
they come from the same STUN server. Foundations are used to
correlate candidates, so that when one candidate is found to be
valid, candidates sharing the same foundation can be tested next,
as they are likely to also be valid.
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Local Candidate: A candidate that an agent has obtained and included
in an offer or answer it sent.
Remote Candidate: A candidate that an agent received in an offer or
answer from its peer.
In-Use Candidate: A candidate is in-use when it appears in the m/c-
line of an active media stream.
Candidate Pair: A pairing containing a local candidate and a remote
candidate.
Check: A candidate pair where the local candidate is a transport
address from which an agent can send a STUN connectivity check.
Check List: An ordered set of STUN checks that an agent is to
generate towards a peer.
Periodic Check: A connectivity check generated by an agent as a
consequence of a timer that fires periodically, instructing it to
send a check.
Triggered Check: A connectivity check generated as a consequence of
the receipt of a connectivity check from the peer.
Valid List: An ordered set of candidate pairs for a media stream that
have been validated by a successful STUN transaction.
Full: An ICE implementation that performs the complete set of
functionality defined by this specification.
Lite: An ICE implementation that omits certain functions,
implementing only as much as is necessary for a peer
implementation that is full to gain the benefits of ICE. Lite
implementations can only act as the controlled agent in a session,
and do not gather candidates.
Controlling Agent: The STUN agent which is responsible for selecting
the final choice of candidate pairs and signaling them through
STUN and an updated offer, if needed. In any session, one agent
is always controlling. The other is the controlled agent.
Controlled Agent: A STUN agent which waits for the controlling agent
to select the final choice of candidate pairs.
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4. Sending the Initial Offer
In order to send the initial offer in an offer/answer exchange, an
agent must gather candidates, priorize them, choose ones for
inclusion in the m/c-line, and then formulate and send the SDP. The
first of these three steps differ for full and lite implementations.
4.1. Full Implementation Requirements
4.1.1. Gathering Candidates
An agent gathers candidates when it believes that communications is
imminent. An offerer can do this based on a user interface cue, or
based on an explicit request to initiate a session. Every candidate
is a transport address. It also has a type and a base. Three types
are defined and gathered by this specification - host candidates,
server reflexive candidates, and relayed candidates. The base of a
candidate is the candidate that an agent must send from when using
that candidate.
The first step is to gather host candidates. Host candidates are
obtained by binding to ports (typically ephemeral) on an interface
(physical or virtual, including VPN interfaces) on the host. The
process for gathering host candidates depends on the transport
protocol. Procedures are specified here for UDP.
For each UDP media stream the agent wishes to use, the agent SHOULD
obtain a candidate for each component of the media stream on each
interface that the host has. It obtains each candidate by binding to
a UDP port on the specific interface. A host candidate (and indeed
every candidate) is always associated with a specific component for
which it is a candidate. Each component has an ID assigned to it,
called the component ID. For RTP-based media streams, the RTP itself
has a component ID of 1, and RTCP a component ID of 2. If an agent
is using RTCP it MUST obtain a candidate for it. If an agent is
using both RTP and RTCP, it would end up with 2*K host candidates if
an agent has K interfaces.
The base for each host candidate is set to the candidate itself.
Agents SHOULD obtain relayed candidates and MUST obtain server
reflexive candidates. The requirement to obtain relayed candidates
is at SHOULD strength to allow for provider variation. If they are
not used, it is RECOMMENDED that it be implemented and just disabled
through configuration, so that it can re-enabled through
configuration if conditions change in the future.
The agent next pairs each host candidate with the STUN server with
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which it is configured or has discovered by some means. This
specification only considers usage of a single STUN server. Every Ta
seconds, the agent chooses another such pair (the order is
inconsequential), and sends a STUN request to the server from that
host candidate. If the agent is using both relayed and server
reflexive candidates, this request MUST be a STUN Allocate request
from the relay usage [12]. If the agent is using only server
reflexive candidates, the request MUST be a STUN Binding request
using the binding discovery usage [11].
The value of Ta SHOULD be configurable, and SHOULD have a default of
20ms. Note that this pacing applies only to starting STUN
transactions with source and destination transport addresses (i.e.,
the host candidate and STUN server respectively) for which a STUN
transaction has not previously been sent. Consequently,
retransmissions of a STUN request are governed entirely by the
retransmission rules defined in [11]. Similarly, retries of a
request due to recoverable errors (such as an authentication
challenge) happen immediately and are not paced by timer Ta. 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 amount of candidates
which are gathered.
An Allocate Response will provide the agent with a server reflexive
candidate (obtained from the mapped address) and a relayed candidate
in the RELAY-ADDRESS attribute. A Binding Response will provide the
agent with only a server reflexive candidate (also obtained from the
mapped address). The base of the server reflexive candidate is the
host candidate from which the Allocate or Binding request was sent.
The base of a relayed candidate is that candidate itself. A server
reflexive candidate obtained from an Allocate response is the called
the "translation" of the relayed candidate obtained from the same
response. The agent will need to remember the translation for the
relayed candidate, since it is placed into the SDP. If a relayed
candidate is identical to a host candidate (which can happen in rare
cases), the relayed candidate MUST be discarded. Proper operation of
ICE depends on each base being unique.
Next, the agent eliminates redundant candidates. A candidate is
redundant if its transport address equals another candidate, and its
base equals the base of that other candidate. Note that two
candidates can have the same transport address yet have different
bases, and these would not be considered redundant.
Finally, the agent assigns each candidate a foundation. The
foundation is an identifier, scoped within a session. Two candidates
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MUST have the same foundation ID when they are of the same type
(host, relayed, server reflexive, peer reflexive or relayed), their
bases have the same IP address (the ports can be different), and, for
reflexive and relayed candidates, the STUN servers used to obtain
them have the same IP address. Similarly, two candidates MUST have
different foundations if their types are different, their bases have
different IP addresses, or the STUN servers used to obtain them have
different IP addresses.
4.1.2. Prioritizing Candidates
The prioritization process results in the assignment of a priority to
each candidate. Each candidate for a media stream MUST have a unique
priority. An agent SHOULD compute the priority by determining a
preference for each type of candidate (server reflexive, peer
reflexive, relayed and host), and, when the agent is multihomed,
choosing a preference for its interfaces. These two preferences are
then combined to compute the priority for a candidate. That priority
SHOULD be computed using the following formula:
priority = (2^24)*(type preference) +
(2^8)*(local preference) +
(2^0)*(256 - component ID)
The type preference MUST be an integer from 0 to 126 inclusive, and
represents the preference for the type of the candidate (where the
types are local, server reflexive, peer reflexive and relayed). A
126 is the highest preference, and a 0 is the lowest. Setting the
value to a 0 means that candidates of this type will only be used as
a last resort. The type preference MUST be identical for all
candidates of the same type and MUST be different for candidates of
different types. The type preference for peer reflexive candidates
MUST be higher than that of server reflexive candidates. Note that
candidates gathered based on the procedures of Section 4.1.1 will
never be peer reflexive candidates; candidates of these type are
learned from the STUN connectivity checks performed by ICE. The
component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive. The local preference MUST be an integer
from 0 to 65535 inclusive. It represents a preference for the
particular interface from which the candidate was obtained, in cases
where an agent is multihomed. 65535 represents the highest
preference, and a zero, the lowest. When there is only a single
interface, this value SHOULD be set to 65535. Generally speaking, if
there are multiple candidates for a particular component for a
particular media stream which have the same type, the local
preference MUST be unique for each one. In this specification, this
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only happens for multi-homed hosts.
These rules guarantee that there is a unique priority for each
candidate. This priority will be used by ICE to determine the order
of the connectivity checks and the relative preference for
candidates. Consequently, what follows are some guidelines for
selection of these values.
One criteria for selection of the type and local preference values is
the use of an intermediary. That is, if media is sent to that
candidate, will the media first transit an intermediate server before
being received? Relayed candidates are clearly one type of
candidates that involve an intermediary. Another are host candidates
obtained from a VPN interface. When media is transited through an
intermediary, it can increase the latency between transmission and
reception. It can increase the packet losses, because of the
additional router hops that may be taken. It may increase the cost
of providing service, since media will be routed in and right back
out of an intermediary run by the provider. If these concerns are
important, the type preference for relayed candidates can be set
lower than the type preference for reflexive and host candidates.
Indeed, it is RECOMMENDED that in this case, host candidates have a
type preference of 126, server reflexive candidates have a type
preference of 100, peer reflexive have a type prefence of 110, and
relayed candidates have a type preference of zero. Furthermore, if
an agent is multi-homed and has multiple interfaces, the local
preference for host candidates from a VPN interface SHOULD have a
priority of 0.
Another criteria for selection of preferences is IP address family.
ICE works with both IPv4 and IPv6. It therefore provides a
transition mechanism that allows dual-stack hosts to prefer
connectivity over IPv6, but to fall back to IPv4 in case the v6
networks are disconnected (due, for example, to a failure in a 6to4
relay) [23]. It can also help with hosts that have both a native
IPv6 address and a 6to4 address. In such a case, lower local
preferences could be assigned to the v6 interface, followed by the
6to4 interfaces, followed by the v4 interfaces. This allows a site
to obtain and begin using native v6 addresses immediately, yet still
fallback to 6to4 addresses when communicating with agents in other
sites that do not yet have native v6 connectivity.
Another criteria for selecting preferences is security. If a user is
a telecommuter, and therefore connected to their corporate network
and a local home network, they may prefer their voice traffic to be
routed over the VPN in order to keep it on the corporate network when
communicating within the enterprise, but use the local network when
communicating with users outside of the enterprise. In such a case,
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a VPN interface would have a higher local preference than any other
interface.
Another criteria for selecting preferences is topological awareness.
This is most useful for candidates that make use of relays. In those
cases, if an agent has preconfigured or dynamically discovered
knowledge of the topological proximity of the relays to itself, it
can use that to assign higher local preferences to candidates
obtained from closer relays.
4.1.3. Choosing In-Use Candidates
A candidate is said to be "in-use" if it appears in the m/c-line of
an offer or answer. When communicating with an ICE peer, being in-
use implies that, should these candidates be selected by the ICE
algorithm, a re-INVITE will not be required after ICE processing
completes. When communicating with a peer that is not ICE-aware, the
in-use candidates will be used exclusively for the exchange of media,
as defined in normal offer/answer procedures.
An agent MUST choose a set of candidates, one for each component of
each active media stream, to be in-use. A media stream is active if
it does not contain the a=inactive SDP attribute.
It is RECOMMENDED that in-use candidates be chosen based on the
likelihood of those candidates to work with the peer that is being
contacted. Unfortunately, it is difficult to ascertain which
candidates that might be. As an example, consider a user within an
enterprise. To reach non-ICE capable agents within the enterprise,
host candidates have to be used, since the enterprise policies may
prevent communication between elements using a relay on the public
network. However, when communicating to peers outside of the
enterprise, relayed candidates from a publically accessible STUN
server are needed.
Indeed, the difficulty in picking just one transport address that
will work is the whole problem that motivated the development of this
specification in the first place. As such, it is RECOMMENDED that
agents select relayed candidates to be in-use.
4.2. Lite Implementation
For each media stream, the agent allocates a single candidate for
each component of the media stream from one of its interfaces. If an
agent is multi-homed, it MUST choose one of its interfaces for a
particular media stream; ICE cannot be used to dynamically choose
one. Each component has an ID assigned to it, called the component
ID. For RTP-based media streams, the RTP itself has a component ID
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of 1, and RTCP a component ID of 2. If an agent is using RTCP it
MUST obtain a candidate for it.
Each candidate is assigned a foundation. The foundation MUST be
different for two candidates from different interfaces (which can
occur if media streams are on different interfaces), and MUST be the
same otherwise. A simple integer that increments for each interface
will suffice. In addition, each candidate MUST be assigned a unique
priority amongst all candidates for the same media stream. This
priority SHOULD be equal to 2^24*(126) + 2^8*(65535) + 256 minus the
component ID, which is 2130706432 minus the component ID. Each of
these candidates is also considered to be "in-use", since they will
be included in the m/c-line of an offer or answer.
4.3. Encoding the SDP
The process of encoding the SDP is identical between full and lite
implementations.
The agent includes a single a=candidate media level attribute in the
SDP for each candidate for that media stream. The a=candidate
attribute contains the IP address, port and transport protocol for
that candidate. A Fully Qualified Domain Name (FQDN) for a host MAY
be used in place of a unicast 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 using an A or AAAA record, and the
resulting IP address is used for the remainder of ICE processing.
The candidate attribute also includes the component ID for that
candidate. For media 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. Other types of media streams which require
multiple components MUST develop specifications which define the
mapping of components to component IDs, and these component IDs MUST
be between 1 and 256.
The candidate attribute also includes the priority and the
foundation. The agent SHOULD include a type for each candidate by
populating the candidate-types production with the appropriate value
- "host" for host candidates, "srflx" for server reflexive
candidates, "prflx" for peer reflexive candidates (though these never
appear in an initial offer/answer exchange), and "relay" for relayed
candidates. The related address MUST NOT be included if a type was
not included. If a type was included, the related address SHOULD be
present for server reflexive, peer reflexive and relayed candidates.
If a candidate is server or peer reflexive, the related address is
equal to the base for that server or peer reflexive candidate. If
the candidate is relayed, the related address is equal to the
translation of the relayed address. If the candidiate is a host
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candidate, there is no related address and the rel-addr production
MUST be omitted.
STUN connectivity checks between agents make use of a short term
credential that is exchanged in the offer/answer process. The
username part of this credential is formed by concatenating a
username fragment from each agent, separated by a colon. Each agent
also provides a password, used to compute the message integrity for
requests it receives. As such, an SDP MUST contain the ice-ufrag and
ice-pwd attributes, containing the username fragment and password
respectively. These can be either session or media level attributes,
and thus common across all candidates for all media streams, or all
candidates for a particular media stream, respectively. However, 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.
If an agent is a lite implementation, it MUST include an "a=ice-lite"
session level attribute in its SDP. If an agent is a full
implementation, it MUST NOT include this attribute.
The m/c-line is populated with the candidates that are in-use. For
streams based on RTP, this is done by placing the RTP candidate into
the m and c lines respectively. If the agent is utilizing RTCP, it
MUST encode the RTCP candidate into the m/c-line using the a=rtcp
attribute as defined in RFC 3605 [2]. If RTCP is not in use, the
agent MUST signal that using b=RS:0 and b=RR:0 as defined in RFC 3556
[5].
There MUST be a candidate attribute for each component of the media
stream in the m/c-line.
Once an offer or answer are sent, an agent MUST be prepared to
receive both STUN and media packets on each candidate. As discussed
in Section 11.1, media packets can be sent to a candidate prior to
its appearence in the m/c-line.
5. Receiving the Initial Offer
When an agent receives an initial offer, it will check if the offeror
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supports ICE, determine its role, gather candidates, prioritize them,
choose one for in-use, encode and send an answer, and for full
implementations, form the check lists and begin connectivity checks.
5.1. Verifying ICE Support
The answerer will proceed with the ICE procedures defined in this
specification if the following are true:
o There is at least one a=candidate attribute for each media stream
in the offer it just received.
o For each media stream, at least one of the candidates is a match
for its respective in-use component in the m/c-line.
If both of these conditions are 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 two
exceptions. First, in all cases, the agent MUST follow the rules of
Section 10, which describe keepalive procedures for all agents.
Secondly, if the agent is not proceeding with ICE because there were
a=candidate attributes, but none that matched the m/c-line of the
media stream, the agent MUST include an a=ice-mismatch attribute in
its answer. This mismatch occurs in cases where intermediary
elements modify the m/c-line, but don't modify candidate attributes.
By including this attribute in the response, diagnostic information
on the ICE failure is provided to the offeror and any intermediate
signaling entities.
In addition, if the offer contains the "a=ice-lite" attribute, and
the answerer is also lite, the agent MUST process the SDP based on
normal RFC 3264 procedures, as if it didn't support ICE, with the
exception of Section 10, which describes keepalive procedures.
5.2. Determining Role
For each session, each agent takes on a role. There are two roles -
controlling, and controlled. The controlling agent is responsible
for selecting the candidate pairs to be used for each media stream,
and for generating the updated offer based on that selection, when
needed. The controlled agent is told which candidate pairs to use
for each media stream, and does not generate an updated offer to
signal this information in SIP.
If one of the agents is a lite implementation, it MUST assume the
controlled role, and its peer (which will be full) MUST assume the
controlling role. If the agent and its peer are both full
implementations, the agent which generated the offer which started
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the ICE processing takes on the controlling role, and the other takes
the controlled role.
Based on this definition, once roles are determined for a session,
they persist unless ICE is restarted, as discussed below. A restart
causes a new selection of roles.
5.3. Gathering Candidates
The process for gathering candidates at the answerer is identical to
the process for the offerer as described in Section 4.1.1 for full
implementations and Section 4.2 for lite implementations. It is
RECOMMENDED that this process begin immediately on receipt of the
offer, prior to user acceptance of a session. Such gathering MAY
even be done pre-emptively when an agent starts.
5.4. Prioritizing Candidates
The process for prioritizing candidates at the answerer is identical
to the process followed by the offerer, as described in Section 4.1.2
for full implementations and Section 4.2 for lite implementations.
5.5. Choosing In Use Candidates
The process for selecting in-use candidates at the answerer is
identical to the process followed by the offerer, as described in
Section 4.1.3 for full implementations and Section 4.2 for lite
implementations.
5.6. Encoding the SDP
The process for encoding the SDP at the answerer is identical to the
process followed by the offerer, as described in Section 4.3.
5.7. Forming the Check Lists
Forming check lists is done only by full implementations. Lite
implementations MUST skip the steps defined in this section.
There is one check list per in-use media stream resulting from the
offer/answer exchange. A media stream is in-use as long as its port
is non-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 or has a bandwidth value of zero. Each check list is a
sequence of STUN connectivity checks that are performed by the agent.
To form the check list for a media stream, the agent forms candidate
pairs, computes a candidate pair priority, orders the pairs by
priority, prunes them, and sets their states. These steps are
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described in this section.
First, the agent takes each of its candidates for a media stream
(called local candidates) and pairs them with the candidates it
received from its peer (called remote candidates) for that media
stream. A local candidate is paired with a remote candidate if and
only if the two candidates have the same component ID and have the
same IP address version. It is possible that some of the local
candidates don't get paired with a remote candidate, and some of the
remote candidates don't get paired with local candidates. This can
happen if one agent didn't include candidates for the all of the
components for a media stream. In the case of RTP, for example, this
would happen when one agent provided candidates for RTCP, and the
other did not. If this happens, the number of components for that
media stream is effectively reduced, and considered to be equal to
the minimum across both agents of the maximum component ID provided
by each agent across all components for the media stream.
Once the pairs are formed, a candidate pair priority is computed.
Let O-P be the priority for the candidate provided by the offerer.
Let A-P be the priority for the candidate provided by the answerer.
The priority for a pair is computed as:
pair priority = 2^32*MIN(O-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P?1:0)
Where O-P>A-P?1:0 is an expression whose value is 1 if O-P is greater
than A-P, and 0 otherwise. This formula ensures a unique priority
for each pair in most cases. One the priority is assigned, the agent
sorts the candidate pairs in decreasing order of priority. If two
pairs have identical priority, the ordering amongst them is
arbitrary.
This sorted list of candidate pairs is used to determine a sequence
of connectivity checks that will be performed. Each check involves
sending a request from a local candidate to a remote candidate.
Since an agent cannot send requests directly from a reflexive
candidate, but only from its base, the agent next goes through the
sorted list of candidate pairs. For each pair where the local
candidate is server reflexive, the server reflexive candidate MUST be
replaced by its base. Once this has been done, the agent MUST remove
redundant pairs. A pair is redundant if its local and remote
candidates are identical to the local and remote candidates of a pair
higher up on the priority list. The result is called the check list
for that media stream, and each candidate pair on it is called a
check.
Each check is also said to have a foundation, which is merely the
combination of the foundations of the local and remote candidates in
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the check.
Each check in the check list is associated with a state. This state
is assigned once the check list for each media stream has been
computed. There are five potential values that the state can have:
Waiting: This check has not been performed, and can be performed as
soon as it is the highest priority Waiting check on the check
list.
In-Progress: A request has been sent for this check, but the
transaction is in progress.
Succeeded: This check was already done and produced a successful
result.
Failed: This check was already done and failed, either never
producing any response or producing an unrecoverable failure
response.
Frozen: This check hasn't been performed, and it can't yet be
performed until some other check succeeds, allowing it to move
into the Waiting state.
First, the agent sets all of the checks in each check list to the
Frozen state. Then, it takes the first check in the check list for
the first media stream (a media stream is the first media stream when
it is described by the first m-line in the SDP offer and answer), and
sets its state to Waiting. It then finds all of the other checks in
that check list with the same component ID, but different
foundations, and sets all of their states to Waiting as well. Once
this is done, one of the check lists will have some number of checks
in the Waiting state, and the other check lists will have all of
their checks in the Frozen state. A check list with at least one
check that is not Frozen is called an active check list.
The check list itself is associated with a state, which captures the
state of ICE checks for that media stream. There are two states:
Running: In this state, ICE checks are still in progress for this
media stream.
Completed: In this state, the controlling agent has signaled that a
candidate pair has been selected for each component.
Consequently, no further ICE checks are performed.
When a check list is first constructed as the consequence of an
offer/answer exchange, it is placed in the Running state.
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ICE processing across all media streams also has a state associated
with it. This state is equal to Running while checks are in
progress. The state is Completed when all checks have been
completed. Rules for transitioning between states are described
below.
5.8. Performing Periodic Checks
Checks are generated only by full implementations. Lite
implementations MUST skip the steps described in this section.
An agent performs two types of checks. The first type are periodic
checks. These checks occur periodically for each media stream, and
involve choosing the highest priority check in the Waiting state from
each check list, and performing it. The other type of check is
called a triggered check. This is a check that is performed on
receipt of a connectivity check from the peer. This section
describes how periodic checks are performed.
Once the agent has computed the check lists as described in
Section 5.7, it sets a timer for each active check list. The timer
fires every Ta/N seconds, where N is the number of active check lists
(initially, there is only one active check list). Implementations
MAY set the timer to fire less frequently than this. Ta is the same
value used to pace the gathering of candidates, as described in
Section 4.1.1. The first timer for each active check list fires
immediately, so that the agent performs a connectivity check the
moment the offer/answer exchange has been done, followed by the next
periodic check Ta seconds later.
When the timer fires, the agent MUST find the highest priority check
in that check list that is in the Waiting state. The agent then
sends a STUN check from the local candidate of that check to the
remote candidate of that check. The procedures for forming the STUN
request for this purpose are described in Section 7.1.1. If none of
the checks in that check list are in the Waiting state, but there are
checks in the Frozen state, the highest priority check in the Frozen
state is moved into the Waiting state, and that check is performed.
When a check is performed, its state is set to In-Progress. If there
are no checks in either the Waiting or Frozen state, then the timer
for that check list is stopped.
Performing the connectivity check requires the agent to know the
username fragment for the local and remote candidates, and the
password for the remote candidate. For periodic checks, the remote
username fragment and password are learned directly from the SDP
received from the peer, and the local username fragment is known by
the agent.
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6. Receipt of the Initial Answer
This section describes the procedures that an agent follows when it
receives the answer from the peer. It verifies that its peer
supports ICE, determines its role, and for full implementations,
forms the check list and begins performing periodic checks.
6.1. Verifying ICE Support
The answerer will proceed with the ICE procedures defined in this
specification if there is at least one a=candidate attribute for each
media stream in the answer it just received. 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 exception of Section 10,
which describes keepalive procedures.
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 an intermediary modified the m/c-
lines without modifying the candidate attributes. See Section 16 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.
6.2. Determining Role
The offerer follows the same procedures described for the answerer in
Section 5.2.
6.3. Forming the Check List
Formation of check lists is performed only by full implementations.
The offerer follows the same procedures described for the answerer in
Section 5.7.
6.4. Performing Periodic Checks
Periodic checks are performed only by full implementations. The
offerer follows the same procedures described for the answerer in
Section 5.8.
7. Connectivity Checks
This section describes how connectivity checks are performed. All
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ICE implementations are required to be compliant to [11], as opposed
to the older [14]. However, whereas a full implementation will both
generate checks (acting as a STUN client) and receive them (acting as
a STUN server), a lite implementation will only ever receive checks,
and thus will only act as a STUN server.
7.1. Client Procedures
These procedures define how an agent sends a connectivity check,
whether it is a periodic or a triggered check. These procedures are
only applicable to full implementations.
7.1.1. Sending the Request
The agent acting as the client generates a connectivity check either
periodically, or triggered. In either case, the check is generated
by sending a Binding Request from a local candidate, to a remote
candidate. The agent must know the username fragment for both
candidates and the password for the remote candidate.
A Binding Request serving as a connectivity check MUST utilize a STUN
short term credential. Rather than being learned from a Shared
Secret request, the short term credential is exchanged in the offer/
answer procedures. In particular, the username is formed by
concatenating the username fragment provided by the peer with the
username fragment of the agent sending the request, separated by a
colon (":"). The password is equal to the password provided by the
peer. For example, consider the case where agent A is the offerer,
and agent B is the answerer. Agent A included a username fragment of
AFRAG for its candidates, and a password of APASS. Agent B provided
a username fragment of BFRAG and a password of BPASS. A connectivity
check from A to B (and its response of course) utilize the username
BFRAG:AFRAG and a password of BPASS. A connectivity check from B to
A (and its response) utilize the username AFRAG:BFRAG and a password
of APASS.
An agent MUST include the PRIORITY attribute in its Binding Request.
The attribute MUST be set equal to the priority that would be
assigned, based on the algorithm in Section 4.1.2, to a peer
reflexive candidate learned from this check. Such a peer reflexive
candidate has a stream ID, component ID and local preference that are
equal to the host candidate from which the check is being sent, but a
type preference equal to the value associated with peer reflexive
candidates.
The Binding Request sent by an agent MUST include the USERNAME and
MESSAGE-INTEGRITY attributes. That is, an agent MUST NOT wait to be
challenged for short term credentials. Rather, it MUST provide them
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in the Binding Request right away.
The controlling agent MAY include the USE-CANDIDATE attribute in the
Binding Request. The controlled agent MUST NOT include it in its
Binding Request. This attribute signals that the controlling agent
wishes to cease checks for this component, and use the candidate pair
resulting from the check for this component. Section 8 provides
guidance on determining when to include it.
If the agent is using Diffserv Codepoint markings [26] in its media
packets, it SHOULD apply those same markings to its connectivity
checks.
7.1.2. Processing the Response
If the STUN transaction generates an unrecoverable failure response
or times out, the agent sets the state of the check to Failed. The
remainder of this section applies to processing of successful
responses (any response from 200 to 299).
The agent MUST check that the source IP address and port of the
response equals the destination IP address and port that the Binding
Request was sent to, and that the destination IP address and port of
the response match the source IP address and port that the Binding
Request was sent from. If these do not match, the processing
described in the remainder of this section MUST NOT be performed. In
addition, an agent sets the state of the check to Failed.
If the check succeeds, processing continues. The agent creates a
candidate pair whose local candidate equals the mapped address of the
response, and whose remote candidate equals the destination address
to which the request was sent. This is called a validated pair,
since it has been validated by a STUN connectivity check. It is very
important to note that this validated pair will often not be
identical to the check itself; in many cases, the local candidate
(learned through the mapped address in the response) will be
different than the local candidate the request was sent from.
Next, the agent computes the priority for the pair based on the
priority of each candidate, using the algorithm in Section 5.7. The
priority of the local candidate depends on its type. If it is not
peer reflexive, it is equal to the priority signaled for that
candidate in the SDP. If it is peer reflexive, it is equal to the
PRIORITY attribute the agent placed in the Binding Request which just
completed. The priority of the remote candidate is taken from the
SDP of the peer. If the candidate does not appear there, then the
check must have been a triggered check to a new remote candidate. In
that case, the priority is taken as the value of the PRIORITY
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attribute in the Binding Request which triggered the check that just
completed.
Once the priority of the candidate pair has been computed, the pair
is added to the valid list for that media stream. If the agent was a
controlling agent, and the check had included a USE-CANDIDATE
attribute, the candidate pair is marked as "favored". If the agent
was a controlled agent, and the check was a triggered check, and the
request which caused the triggered check included the USE-CANDIDATE
attribute, the candidate pair is marked as "favored".
Next, the agent updates its ICE states. The agent checks the mapped
address from the STUN response. If the transport address does not
match any of the local candidates that the agent knows about, the
mapped address represents a new peer reflexive candidate. Its type
is equal to peer reflexive. Its base is set equal to the candidate
from which the STUN check was sent. Its username fragment and
password are identical to the candidate from which the check was
sent. It is assigned the priority value that was placed in the
PRIORITY attribute of the request. Its foundation is selected as
described in Section 4.1.1. The peer reflexive candidate is then
added to the list of local candidates known by the agent (though it
is not paired with other remote candidates at this time).
Next, the agent changes the state for this check to Succeeded. The
agent sees if the success of this check can cause other checks to be
unfrozen. If the check had a component ID of one, the agent MUST
change the states for all other Frozen checks for the same media
stream and same foundation, but different component IDs, to Waiting.
If the component ID for the check was equal to the number of
components for the media stream (where this is the actual number of
components being used, in cases where the number of components
signaled in the SDP differs from offerer to answerer), the agent MUST
change the state for all other Frozen checks for the first component
of different media streams (and thus in different check lists) but
the same foundation, to Waiting.
7.2. Server Procedures
An agent MUST be prepared to receive a Binding Request on the base of
each candidate it included in its most recent offer or answer.
Receipt of a Binding Request on a transport address that the agent
had included in a candidate attribute is an indication that the
connectivity check usage applies to the request.
The agent MUST use a short term credential to authenticate the
request and perform a message integrity check. The agent MUST accept
a credential if the username consists of two values separated by a
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colon, where the first value is equal to the username fragment
generated by the agent in an offer or answer for a session in-
progress, and the password is equal to the password for that username
fragment. It is possible (and in fact very likely) that an offeror
will receive a Binding Request prior to receiving the answer from its
peer. However, the request can be processed without receiving this
answer, and a response generated.
If the agent is using Diffserv Codepoint markings [26] in its media
packets, it SHOULD apply those same markings to its responses to
Binding Requests.
7.2.1. Additional Procedures for Full Implementations
This subsection defines the additional server procedures applicable
to full implementations.
For requests being received on a relayed candidate, the source
transport address used for STUN processing (namely, generation of the
XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the
relay. That source transport address will be present in the REMOTE-
ADDRESS attribute of a STUN Data Indication message, if the Binding
Request was delivered through a Data Indication. If the Binding
Request was not encapsulated in a Data Indication, that source
address is equal to the current active destination for the STUN relay
session.
If the STUN request resulted in an error response, no further
processing is performed.
Assuming a success response, if the source transport address of the
request does not match any existing remote candidates, it represents
a new peer reflexive remote candidate. The full-mode agent gives the
candidate a priority equal to the PRIORITY attribute from the
request. The type of the candidate is equal to peer reflexive. Its
foundation is set to an arbitrary value, different from the
foundation for all other remote candidates. Note that any subsequent
offer/answer exchanges will contain this new peer reflexive candidate
in the SDP, and will signal the actual foundation for the candidate.
This candidate is then added to the list of remote candidates.
However, the agent does not pair this candidate with any local
candidates.
Next, the agent constructs a tentative check in the reverse
direction, called a triggered check. The triggered check has a local
candidate equal to the candidate on which the STUN request was
received, and a remote candidate equal to the source transport
address where the request came from (which may be a new peer-
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reflexive remote candidate). Since both candidates are known to the
agent, it can obtain their priorities and compute the candidate pair
priority. This tentative check is then looked up in the check list.
There can be one of several outcomes:
o If there is already a check on the check list with this same local
and remote candidates, and the state of that check is Waiting or
Frozen, its state is changed to In-Progress and the tentative
check is performed.
o If there is already a check on the check list with this same local
and remote candidates, and its state was In-Progress, the agent
SHOULD abandon the new tentative check and instead generate an
immediate retransmit of the Binding Request for the check in
progress. This is to facilitate rapid completion of ICE when both
agents are behind NAT.
o If there is already a check on the check list with this same local
and remote candidates, and its state was Succeeded, the new
tentative check is abandoned. If the Binding Request just
received contained the USE-CANDIDATE attribute, it means that the
pair resulting from that previous check is favored by the peer
controlling agent. The agent MUST take the candidate pair in the
valid list that was learned from that previous successful check,
and mark it as favored.
o If there is already a check on the check list with this same local
and remote candidates, and its state was Failed, the new tentative
check is abandoned.
o If there is no matching check on the check list, the new tentative
check is inserted into the check list based on its priority, and
its state is set to In-Progress.
If the tentative check is to be performed, it is constructed and
processed as described in Section 7.1.1. These procedures require
the agent to know the username fragment and password for the peer.
They are readily determined from the SDP and from the check that was
just received. The username fragment for the remote candidate is
equal to the bottom half (the part after the colon) of the USERNAME
in the Binding Request that was just received. Using that username
fragment, the agent can check the SDP messages received from its peer
(there may be more than one in cases of forking), and find this
username fragment. The corresponding password is then selected. If
agent has not yet received this SDP (a likely case for the offerer in
the initial offer/answer exchange), it MUST wait for the SDP to be
received, and then proceed with the triggered check.
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7.2.2. Additional Procedures for Lite Implementations
If the check that was just received contained a USE-CANDIDATE
attribute, the agent constructs a candidate pair whose local
candidate is equal to the transport address on which the request was
received, and whose remote candidate is equal to the source transport
address of the request that was received. This candidate pair is
assigned an arbitrary priority, and placed into a list of valid
candidates for that component of that media stream called the valid
list. In addition, it is marked as favored, since the peer agent has
indicated that it is to be used. ICE processing is considered
complete for a media stream if the valid list contains a candidate
pair for each component.
8. Concluding ICE
The processing rules in this section apply only to full
implementations.
Concluding ICE involves selection of pairs by the controlling agent,
updating of state machinery, and possibly the generation of an
updated offer by the controlling agent.
The controlling agent can use any algorithm it likes for deciding
when to select a candidate pair, called the favored pair, as the one
that will be used for media. However, it MUST eventually include a
USE-CANDIDATE attribute in at least one successful check for each
component of each media stream.
The most apparent way to utilize the USE-CANDIDATE attribute is to
run through a series of checks, each of which omit the flag. Once
one or more checks complete successfully for a component of a media
stream, the agent evaluates the choices based on some criteria, and
picks a candidate pair. The criteria for evaluation is a matter of
implementation and it allows for localized optimizations. The check
that yielded this pair is then repeated, this time with the USE-
CANDIDATE flag. This approach provides the most flexibility in terms
of algorithms, and also improves ICE's resilience to variations in
implementation (see Section 14. This approach is called
"introspective selection". The drawback of introspective selection
is that it is guaranteed to increase latencies because it requires an
additional check to be done.
An alternative is called "proactive selection". In this approach,
the controlling agent includes the USE-CANDIDATE attribute in every
check it sends. Once the first check for a component succeeds, it is
used by ICE. In this mode, the agent will end up using the candidate
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pair which is highest priority based on ICE's prioritization
algorithm, instead of some other local optimization. It is possible
with proactive selection that multiple checks might succeed with the
flag set; this is why ICE still applies its prioritization algorithm
to pick amongst those pairs that have been favored.
If an agent is controlling and its peer has a lite implementation, an
agent MUST use an introspective selection algorithm. Of course, it
MAY select a favored pair based on ICE's prioritization. The key
requirement is that the agent must complete a successful check before
redoing it with the USE-CANDIDATE attribute.
For both controlling and controlled agents, once a candidate pair in
the Valid list is marked as favored, an agent MUST NOT generate any
further periodic checks for that component of that media stream, and
SHOULD cease any retransmissions in progress for checks for that
component of that media stream. Once there is at least one candidate
pair for each component of a media stream that is favored, a full-
mode agent MUST change the state of processing for its check list to
Completed. Once all of the check lists for the media streams enter
the Completed state, the controlling agent takes the highest priority
favored candidate pair for each component of each media stream. If
any of those candidate pairs differ from the in-use candidates in
m/c-lines of the most recent offer/answer exchange, the controlling
agent MUST generate an updated offer as described in Section 9.
9. Subsequent Offer/Answer Exchanges
An agent MAY generate a subsequent offer at any time. However, the
rules in Section 8 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 in-use pairs. This
section defines rules for construction of subsequent offers and
answers.
9.1. Generating the Offer
An agent MAY change the ice-pwd and/or ice-ufrag for a media stream
in an offer. Doing so is a signal to restart ICE processing for that
media stream. When an agent restarts ICE for a media stream, it MUST
NOT include the a=remote-candidates attribute, since the state of the
media stream would not be Completed at this point. Note that it is
permissible to use a session-level attribute in one offer, but to
provide the same password as a media-level attribute in a subsequent
offer. This is not a change in password, just a change in its
representation.
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An agent MUST restart ICE processing if the offer is being generated
for the purposes of changing the target of the media stream. In
other words, if an agent wants to generated an updated offer which,
had ICE not been in use, would result in a new value for the
transport address in the m/c-line, the agent MUST restart ICE for
that media stream. This implies that setting the IP address in the c
line to 0.0.0.0 will cause an ICE restart. Consequently, ICE
implementations SHOULD NOT utilize this mechanism for call hold, and
instead use a=inactive as described in [4]
If an agent removes a media stream by setting its port to zero, it
MUST NOT include any candidate attributes for that media stream.
An agent MUST NOT signal a change in its implementation level (full
or lite) by adding or removing the a=ice-lite attribute from an
updated offer, unless ICE processing is being restarted for all media
streams in the offer. Of course, in normal cases the implementation
level is not dynamic and there would be no need to signal a change.
However, in applications like third party call control, which involve
a mid-session change in remote correspondent, this can happen and it
is permitted by ICE with a restart.
Note that an agent can add a new media stream at any time, even if
ICE has long finished for the existing media streams. Based on the
rules described here, checks will begin for this new stream as if it
was in an initial offer.
9.1.1. Additional Procedures for Full Implementations
This section describes additional procedures for full
implementations.
When an agent generates an updated offer, the set of candidate
attributes to include for each media stream depend on the state of
ICE processing for that media stream. If the processing for that
media stream is in the Completed state, a full-mode agent MUST
include a candidate attribute for the local candidate of each pair
that has been chosen for use by ICE for that media stream. A pair is
chosen if it is the highest priority favored pair in the valid list
for a component of that media stream. An agent SHOULD NOT include
any other candidate attributes for that media stream. If ICE
processing for a media stream is in the Running state, the agent MUST
include all current candidates (including peer reflexive candidates
learned through ICE processing) for that media stream. It MAY
include candidates it did not offer previously, but which it has
gathered since the last offer/answer exchange. If a media stream is
new or ICE checks are restarting for that stream, an agent includes
the set of candidates it wishes to utilize. This MAY include some,
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none, or all of the previous candidates for that stream in the case
of a restart, and MAY include a totally new set of candidates
gathered as described in Section 4.1.1.
If a candidate was sent in a previous offer/answer exchange, it
SHOULD have the same priority. For a peer reflexive candidate, the
priority SHOULD be the same as determined by the processing in
Section 7.1.2. The foundation SHOULD be the same. The username
fragments and passwords for a media stream SHOULD remain the same as
the previous offer or answer.
Population of the m/c-lines also depends on the state of ICE
processing. If ICE processing for a media stream is in the Completed
state, the m/c-line MUST use the local candidate from the highest
priority favored pair in the valid list for each component of that
media stream. If ICE processing is in the Running state, a full-mode
agent SHOULD populate the m/c-line for that media stream based on the
considerations in Section 4.1.3.
In addition, if the agent is controlling, it MUST include the
a=remote-candidates attribute for each media stream that is in the
Completed state. The attribute contains the remote candidates from
the highest priority favored pair in the valid list for each
component of that media stream.
9.1.2. Additional Procedures for Lite Implementations
A passive-only agent includes its one and only candidate for each
component of each media stream in an a=candidate attribute in any
subsequent offer. This candidate is formed identically to the
procedures for initial offers, as described in Section 4.2.
9.2. Receiving the Offer and Generating an Answer
When receiving a subsequent offer within an existing session, an
agent MUST re-apply the verification procedures in Section 5.1
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.
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 is a signal
that ICE is restarting for this media stream. If all media streams
are restarting, than ICE is restarting overall. Procedures for ICE
restarts are discussed below. Unless ICE is restarting for that
media stream, an agent MUST NOT change the a=ice-ufrag or a=ice-pwd
attributes in an answer relative to the last SDP it provided. Such a
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change can only take place in an offer. If ICE is restarting, the
a=ice-ufrag and a=ice-pwd attributes MUST be changed.
An agent MUST NOT change its implementation level from its previous
SDP unless, based on the offer, ICE procedures are being restarted
for all media streams in the offer. In that case, it MAY change its
level.
An agent MUST NOT include the a=remote-candidates attribute in an
answer.
When the answerer generates its answer, it must decide what
candidates to include in the answer, how to populate the m/c-line,
and how to adjust the states of ICE processing. The rules for
inclusion of candidate attributes in an answer are identical to the
rules followed by the offerer as described in Section 9.1 for both
full and lite implementations. For lite implementations, those rules
also apply for setting the m/c-line. However, additional
considerations apply to full implementations.
9.2.1. Additional Procedures for Full Implementations
The computation of the m/c-line additionally depends on the presence
or absence of the a=remote-candidates attribute in a media stream.
If present, it means that the offerer (acting as the controlling
agent) believed that ICE processing has completed for that media
stream. In this case, the remote-candidates attribute contains the
candidates that the answerer is supposed to use. It is possible that
the agent doesn't even know of these candidates yet; they will be
discovered shortly through a response to an in-progress check. The
full-mode agent MUST populate the m/c-line with the candidates from
the a=remote-candidates attribute.
If the offer did not contain the a=remote-candidates attribute, the
agent follows the same procedures for populating the m/c-line as
described for the offerer in Section 9.1.
9.3. Updating the Check and Valid Lists
If ICE is restarting for a media stream, the agent MUST start a new
Valid list for that media stream. However, it retains the old Valid
list for the purposes of sending media until ICE processing
completes, at which point the old Valid list is discarded and the new
one is utilized to determine media and keepalive targets.
9.3.1. Additional Procedures for Full Implementations
The procedures in this section are applicable only to full
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implementations.
Once the subsequent offer/answer exchange has completed, each agent
needs to determine the impact, if any, on the Check and Valid lists.
Unless there is an ICE restart, an offer/answer exchange has no
impact on the state of ICE processing for each media stream; that is
determined entirely by the checks themselves.
When ICE restarts, an agent MUST flush the check list for the
affected media streams, and then recompute the check list and its
states as described in Section 5.7.
The remainder of this section describes processing when ICE is not
restarting.
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 5.7.
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 periodic checks for it.
If a media stream existed previously, and remains after the offer/
answer exchange, the agent MUST NOT modify the Valid list for that
media stream. However, if an agent is in the Running state for that
media stream, the check list is updated. To do that, the agent
recomputes the check lists using the procedures described in
Section 5.7. If a check on the new check lists was also on the
previous check lists, and its state was Waiting, In-Progress,
Succeeded or Failed, its state is copied over. If a check on the new
check lists does not have a state (because it's a new check on an
existing check list, or a check on a new check list, or the check was
on an old check list but its state was not copied over) its state is
set to Frozen.
If none of the check lists are active (meaning that the checks in
each check list are Frozen), the full-mode agent sets the first check
in the check list for the first media stream to Waiting, and then
sets the state of all other checks 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 check. If a check has a state of Succeeded, and it
has a component ID of 1, then all Frozen checks in the same check
list with the same foundation whose component IDs are not one, have
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their state set to Waiting. If, for a particular check list, there
are checks for each component of that media stream in the Succeeded
state, the agent moves the state of all Frozen checks for the first
component of all other media streams (and thus in different check
lists) with the same foundation to Waiting.
10. Keepalives
All endpoints MUST send keepalives for each media session. These
keepalives serve the purpose of keeping NAT bindings active for the
media session. These 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. These keepalives MUST be sent even if ICE is not being
utilized for the session at all. The keepalive SHOULD be sent using
a format which is supported by its peer. ICE endpoints allow for
STUN-based keepalives for UDP streams, and as such, STUN keepalives
MUST be used when an agent is communicating with a peer that supports
ICE. An agent can determine that its peer supports ICE by the
presence of a=candidate attributes for each media session. If the
peer does not support ICE, the choice of a packet format for
keepalives is a matter of local implementation. A format which
allows packets to easily be sent in the absence of actual media
content is RECOMMENDED. Examples of formats which readily meet this
goal are RTP No-Op [28] and RTP comfort noise [24]. If the peer
doesn't support any formats that are particularly well suited for
keepalives, an agent SHOULD send RTP packets with an incorrect
version number, or some other form of error which would cause them to
be discarded by the peer.
If there has been no packet sent on a candidate pair being used for
media for Tr seconds (where packets include media and previous
keepalives), an agent MUST generate a keepalive on that pair. Tr
SHOULD be configurable and SHOULD have a default of 15 seconds.
If STUN is being used for keepalives, a STUN Binding Indication is
used [11]. The Binding Indication SHOULD NOT contain integrity
checks; since the messages are simply discarded on receipt regardless
of contents. The Indication SHOULD NOT contain the PRIORITY or USE-
CANDIDATE attributes defined here. The Binding Indication is sent
using the same local and remote candidates that are being used for
media. An agent receipt a Binding Indication MUST discard it
silently. Though Binding Indications are used for keepalives, an
agent MUST be prepared to receive Binding Requests as well. If a
Binding Request is received, a response is generated as discussed in
[11], but there is no impact on ICE processing otherwise.
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An agent MUST begin the keepalive processing once ICE has selected
candidates for usage with media, or media begins to flow, whichever
happens first. Keepalives end once the session terminates or the
media stream is removed.
11. Media Handling
11.1. Sending Media
Procedures for sending media differ for full and lite
implementations.
11.1.1. Procedures for Full Implementations
Agents always send media using a candidate pair. An agent will send
media to the remote candidate in the pair (setting the destination
address and port of the packet equal to that remote candidate), and
will send it from the local candidate. When the local candidate is
server or peer reflexive, media is originated from the base. Media
sent from a relayed candidate is sent through that relay, using
procedures defined in [12].
If the state of a media stream is Running, there is no old Valid list
for that media stream (which would be due to an ICE restart), an
agent MUST NOT send media.
When an agent sends media, it MUST send it using the highest priority
selected pair for each component in either the old Valid list for a
media stream (if it exists), else the new Valid list for that media
stream. In several cases, this will not be the same candidate pairs
present in the m/c-line. When ICE first completes, if the selected
pairs aren't a match for the m/c-line, an updated offer/answer
exchange will take place to remedy this disparity. However, until
that update offer arrives, there will not be a match. Furthermore,
in very unusual cases, the m/c-lines in the updated offer/answer will
not be a match.
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. 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 change the marker bit when an agent
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switches transmission of media from one candidate pair to another.
11.1.2. Procedures for Lite Implementations
A lite implementation MUST NOT send media until it has a Valid list
that contains a candidate pair for each component of that media
stream. Once that happens, the agent MAY begin sending media
packets. To do that, it sends media to the remote candidate in the
pair (setting the destination address and port of the packet equal to
that remote candidate), and will send it from the local candidate.
In cases where there has been an ICE restart, there will be an old
and a new Valid list. The old Valid list MUST be used by the agent
for sending media until the new one is complete, at which point the
new one MUST be used, and the old one discarded.
11.2. Receiving Media
ICE implementations MUST be prepared to receive media on any
candidates provided in the most recent offer/answer exchange.
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 [21] describes an algorithm in Section 8.2 for detecting
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 effect
perceived user latency. Two latency figures are of particular
interest. These are the post-pickup delay and the post-dial delay.
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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 succesfully started ringing the phone of the
called party.
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 offhook.
If an offer is received in an INVITE request, the callee SHOULD
immediately gather its candidates and then generate an answer in a
provisional response. ICE requires that a provisional response with
an SDP be transmitted reliably. This can be done through the
existing PRACK mechanism [9], 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 3264. To do this, the
agent retransmits the provisional response with th 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 or on transmission of a 2xx response. 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 callee 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 [25] cannot be transmitted. For
this reason, implementations SHOULD delay alerting the called party
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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 [6], since its 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.
In addition to uses where the offer is in an INVITE, and the answer
is in the provisional and/or 200 OK, ICE works with cases where the
offer appears in the response. In such cases, which are common in
third party call control, ICE agents SHOULD generate their offers in
a reliable provisional response (which MUST utilize RFC 3262). In
that case, the answer will arrive in a PRACK. This allows for ICE
processing to take place prior to alerting. Once ICE completes, the
agent can alert the user and then generate a 200 OK. The 200 OK
would contain no SDP, since the offer/answer exchange has completed.
Agents MAY place the offer in a 2xx instead (in which case the answer
comes in the ACK). This flow is simpler but results in a poorer user
experience.
As discussed in Section 16, offer/answer exchanges SHOULD be secured
against eavesdropping and man-in-the-middle attacks. To do that, the
usage of SIPS [3] is RECOMMENDED when used in concert with ICE.
12.2. SIP Option Tags and Media Feature Tags
[13] 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 gateways.
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 which arrive on the same 5-tuple as the connectivity check
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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
[6] and RFC 4032 [7], apply only to the transport addresses listed in
the m/c lines in an offer/answer. If ICE changes the transport
address where media is received, this change is reflected in the m/c
lines of a new offer/answer. As such, it appears like any other re-
INVITE would, and is fully treated in RFC 3312 and 4032, which apply
without regard to the fact that the m/c lines are changing due to ICE
negotiations ocurring "in the background".
Indeed, an agent SHOULD NOT indicate that Qos preconditions have been
met until the ICE checks have completed and selected the candidate
pairs to be used for media.
ICE also has (purposeful) interactions with connectivity
preconditions [27]. 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 [27].
12.5. Interactions with Third Party Call Control
ICE works with Flows I, III and IV as described in [17]. 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 in-use.
13. Grammar
This specification defines seven new SDP attributes - the
"candidate", "remote-candidates", "ice-lite", "ice-ufrag", "ice-pwd"
"ice-options" and "ice-mismatch" attributes.
The candidate attribute is a media-level attribute only. It contains
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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 RFC 4234 [8]:
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*ice-char
component-id = 1*DIGIT
transport = "UDP" / transport-extension
transport-extension = token ; from RFC 3261
priority = 1*DIGIT
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 = byte-string ;from RFC 4566
extension-att-value = byte-string
ice-char = ALPHA / DIGIT / "+" / "/"
The foundation is composed of one or more ice-char. The component-id
is a positive integer, which identifies the specific component for
which the transport address is a candidate. It MUST start at 1 and
MUST increment by 1 for each component of a particular candidate.
The connect-address production is taken from RFC 4566 [10], allowing
for IPv4 addresses, IPv6 addresses and FQDNs. The port production is
also taken from RFC 4566 [10]. The token production is taken from
RFC 3261 [3]. The transport production 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, such as TCP or the Datagram Congestion
Control Protocol (DCCP) [29].
The cand-type production encodes the type of candidate. This
specification defines the values "host", "srflx", "prflx" and "relay"
for host, server reflexive, peer reflexive and relayed candidates,
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respectively. The set of candidate types is extensible for the
future. Inclusion of the candidate type is optional. The rel-addr
and rel-port productions convey information the related transport
addresses. Rules for inclusion of these values is described in
Section 4.3.
The a=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.
The syntax of the "remote-candidates" attribute is defined using
Augmented BNF as defined in RFC 4234 [8]. 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 media stream.
The syntax of the "ice-lite" and "ice-mismatch", both of which are
flags, is:
ice-lite = "ice-lite"
ice-mismatch = "ice-mismatch"
"ice-lite" is a session level attribute only, and "ice-mismatch" is a
media level attribute only. The syntax of the "ice-pwd" and "ice-
ufrag" attributes are defined as:
ice-pwd-att = "ice-pwd" ":" password
ice-ufrag-att = "ice-ufrag" ":" ufrag
password = 22*ice-char
ufrag = 4*ice-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 media streams, unless
overriden by a media-level value.
The "ice-options" attribute is a session level attribute. It
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contains a series of tokens which 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
14. Extensibility Considerations
This specification makes very specific choices about how both agents
in a session coordinate to arrive at the set of candidate pairs that
are selected for media. It is anticipated that future specifications
will want to alter these algorithms, whether they are simple changes
like timer tweaks, or larger changes like a revamp of the priority
algorithm. When such a change is made, providing interoperability
between the two agents in a session is critical.
Firstly, ICE provides the a=ice-options SDP attribute. Each
extension or change to ICE is associated with a token. When an agent
supporting such an extension or change generates an offer or an
answer, it MUST include the token for that extension in this
attribute. This allows each side to know what the other side is
doing. This attribute MUST NOT be present if the agent doesn't
support any ICE extensions or changes.
At this time, no IANA registry or registration procedures are defined
for these option tags. At time of writing, it is unclear whether ICE
changes and extensions will be sufficiently common to warrrant a
registry.
One of the complications in achieving interoperability is that ICE
relies on a distributed algorithm running on both agents to converge
on an agreed set of candidate pairs. If the two agents run different
algorithms, it can be difficult to guarantee convergence on the same
candidate pairs. The introspective selection procedure described in
Section 8 eliminates some of the tight coordination by delegating the
selection algorithm completely to the controlling agent.
Consequently, when a controlling agent is communicating with a peer
that supports options it doesn't know about, the agent MUST run an
introspective selection algorithm. When introspective selection is
used, ICE will converge perfectly even when both agents use different
pair prioritization algorithms. One of the keys to such convergence
are triggered checks, which ensure that the favored pair is validated
by both agents. Consequently, any future ICE enhancements MUST
preserve triggered checks.
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15. Example
Two agents, L and R, are using ICE. Both are full-mode ICE
implementations. Both agents have a single IPv4 interface. For
agent L, it is 10.0.1.1, and for agent R, 192.0.2.1. Both are
configured with a single STUN server each (indeed, the same one for
each), which is listening for STUN requests at an IP address of
192.0.2.2 and port 3478. This STUN server supports only the Binding
Discovery usage; relays are not used in this example. Agent L is
behind a NAT, and agent R is on the public Internet. The NAT has an
endpoint independent mapping property and an address dependent
filtering property. The public side of the NAT has an IP address of
192.0.2.3.
To facilitate understanding, transport addresses are listed using
variables that have mnemonic names. The format of the name is
entity-type-seqno, where entity refers to the entity whose interface
the transport address is on, and is one of "L", "R", "STUN", or
"NAT". The type is either "PUB" for transport addresses that are
public, and "PRIV" for transport addresses that are private.
Finally, seq-no is a sequence number that is different for each
transport address of the same type on a particular entity. Each
variable has an IP address and port, denoted by varname.IP and
varname.PORT, respectively, where varname is the name of the
variable.
The STUN server has advertised transport address STUN-PUB-1 (which is
192.0.2.2:3478) for the binding discovery usage.
In the call flow itself, STUN messages are annotated with several
attributes. The "S=" attribute indicates the source transport
address of the message. The "D=" attribute indicates the destination
transport address of the message. The "MA=" attribute is used in
STUN Binding Response messages and refers to the mapped address.
"USE-CAND" implies the presence of the USE-CANDIDATE attribute.
The call flow examples omit STUN authentication operations and RTCP,
and focus on RTP for a single media stream between two full
implementations.
L NAT STUN R
|RTP STUN alloc. | |
|(1) STUN Req | | |
|S=$L-PRIV-1 | | |
|D=$STUN-PUB-1 | | |
|------------->| | |
| |(2) STUN Req | |
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| |S=$NAT-PUB-1 | |
| |D=$STUN-PUB-1 | |
| |------------->| |
| |(3) STUN Res | |
| |S=$STUN-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<-------------| |
|(4) STUN Res | | |
|S=$STUN-PUB-1 | | |
|D=$L-PRIV-1 | | |
|MA=$NAT-PUB-1 | | |
|<-------------| | |
|(5) Offer | | |
|------------------------------------------->|
| | | |RTP STUN alloc.
| | |(6) STUN Req |
| | |S=$R-PUB-1 |
| | |D=$STUN-PUB-1 |
| | |<-------------|
| | |(7) STUN Res |
| | |S=$STUN-PUB-1 |
| | |D=$R-PUB-1 |
| | |MA=$R-PUB-1 |
| | |------------->|
|(8) answer | | |
|<-------------------------------------------|
| |(9) Bind Req | |
| |S=$R-PUB-1 | |
| |D=L-PRIV-1 | |
| |<----------------------------|
| |Dropped | |
|(10) Bind Req | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|USE-CAND | | |
|------------->| | |
| |(11) Bind Req | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |USE-CAND | |
| |---------------------------->|
| |(12) Bind Res | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<----------------------------|
|(13) Bind Res | | |
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|S=$R-PUB-1 | | |
|D=$L-PRIV-1 | | |
|MA=$NAT-PUB-1 | | |
|<-------------| | |
|RTP flows | | |
| |(14) Bind Req | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |<----------------------------|
|(15) Bind Req | | |
|S=$R-PUB-1 | | |
|D=$L-PRIV-1 | | |
|<-------------| | |
|(16) Bind Res | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|MA=$R-PUB-1 | | |
|------------->| | |
| |(17) Bind Res | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |MA=$R-PUB-1 | |
| |---------------------------->|
| | | |RTP flows
Figure 11
First, agent L obtains a host candidate from its local interface (not
shown), and from that, sends a STUN Binding Request to the STUN
server to get a server reflexive candidate (messages 1-4). Recall
that the NAT has the address and port independent mapping property.
Here, it creates a binding of NAT-PUB-1 for this UDP request, and
this becomes the server reflexive candidate for RTP.
Agent L sets a type preference of 126 for the host candidate and 100
for the server reflexive. The local preference is 65535. Based on
this, the priority of the host candidate is 2130706178 and for the
server reflexive candidate is 1694498562. The host candidate is
assigned a foundation of 1, and the server reflexive, a foundation of
2. It chooses its server reflexive candidate as the in-use
candidate, and encodes it into the m/c-line. The resulting offer
(message 5) looks like (lines folded for clarity):
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v=0
o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
s=
c=IN IP4 $NAT-PUB-1.IP
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio $NAT-PUB-1.PORT RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 $L-PRIV-1.IP $L-PRIV-1.PORT typ local
a=candidate:2 1 UDP 1694498562 $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 IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 10.0.1.1 8998 typ local
a=candidate:2 1 UDP 1694498562 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
This offer is received at agent R. Agent R will obtain a host
candidate, and from it, obtain a server reflexive candidate (messages
6-7). Since R is not behind a NAT, this candidate is identical to
its host candidate, and they share the same base. It therefore
discards this candidate and ends up with a single host candidate.
With identical type and local preferences as L, the priority for this
candidate is 2130706178. It chooses a foundation of 1 for its single
candidate. Its resulting answer looks like:
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
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a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 $R-PUB-1.IP $R-PUB-1.PORT typ local
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
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706178 192.0.2.1 3478 typ local
Since neither side indicated that they are passive-only, the agent
which sent the offer that began ICE processing (agent L) becomes the
controlling agent.
Agents L and R both pair up the candidates. They both initially have
two. However, agent L will prune the pair containing its server
reflexive candidate, resulting in just one. At agent L, this pair
(the check) has a local candidate of $L_PRIV_1 and remote candidate
of $R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note
that an implementation would represent this as a 64 bit integer so as
not to lose precision). At agent R, there are two checks. The
highest priority has a local candidate of $R_PUB_1 and remote
candidate of $L_PRIV_1 and has a priority of 4.57566E+18, and the
second has a local candidate of $R_PUB_1 and remote candidate of
$NAT_PUB_1 and priority 3.63891E+18.
Agent R begins its connectivity check (message 9) for the first pair
(between the two host candidates). Since R is the passive agent for
this session, the check omits the USE-CANDIDATE attribute. The host
candidate from agent L is private and behind a different NAT, and
thus this check is discarded.
When agent L gets the answer, it performs its one and only
connectivity check (messages 10-13). It implements the default
algorithm for candidate selection, and thus includes a USE-CANDIDATE
attribute in this check. Since the check succeeds, agent L creates a
new pair, whose local candidate is from the mapped address in the
binding response (NAT-PUB-1 from message 13) and whose remote
candidate is the destination of the request (R-PUB-1 from message
10). This is added to the valid list. In addition, it is marked as
selected since the Binding Request contained the USE-CANDIDATE
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attribute. Since there is a selected candidate in the Valid list for
the one component of this media stream, ICE processing for this
stream moves into the Completed state. Agent L can now send media if
it so chooses.
Upon receipt of the check from agent L (message 11), agent R will
generate its triggered check. This check happens to match the next
one on its check list - from its host candidate to agent L's server
reflexive candidate. This check (messages 14-17) will succeed.
Consequently, agent R constructs a new candidate pair using the
mapped address from the response as the local candidate (R-PUB-1) and
the destination of the request (NAT-PUB-1) as the remote candidate.
This pair is added to the Valid list for that media stream. Since
the check was generated in the reverse direction of a check that
contained the USE-CANDIDATE attribute, the candidate pair is marked
as selected. Consequently, processing for this stream moves into the
Completed state, and agent R can also send media.
16. Security Considerations
There are several types of attacks possible in an ICE system. This
section considers these attacks and their countermeasures.
16.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks.
Ultimately, all of these attacks fool an agent into thinking
something incorrect about the results of the connectivity checks.
The possible false conclusions an attacker can try and cause are:
False Invalid: An attacker can fool a pair of agents into thinking a
candidate pair is invalid, when it isn't. This can be used to
cause an agent to prefer a different candidate (such as one
injected by the attacker), or to disrupt a call by forcing all
candidates to fail.
False Valid: An attacker can fool a pair of agents into thinking a
candidate pair is valid, when it isn't. This can cause an agent
to proceed with a session, but then not be able to receive any
media.
False Peer-Reflexive Candidate: An attacker can cause an agent to
discover a new peer reflexive candidate, when it shouldn't have.
This can be used to redirect media streams to a DoS target or to
the attacker, for eavesdropping or other purposes.
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False Valid on False Candidate: An attacker has already convinced an
agent that there is a candidate with an address that doesn't
actually route to that agent (for example, by injecting a false
peer reflexive candidate or false server reflexive candidate). It
must then launch an attack that forces the agents to believe that
this candidate is valid.
Of the various techniques for creating faked STUN messages described
in [11], many are not applicable for the connectivity checks.
Compromises of STUN servers are not much of a concern, since the STUN
servers are embedded in endpoints and distributed throughout the
network. Thus, compromising the STUN server is equivalent to
comprimising the endpoint, and if that happens, far more problematic
attacks are possible than those against ICE. Similarly, DNS attacks
are usually irrelevant since STUN servers are not typically
discovered via DNS, they are signaled via IP addresses embedded in
SDP. Injection of fake responses and relaying modified requests all
can be handled in ICE with the countermeasures discussed below.
To force the false invalid result, the attacker has to wait for the
connectivity check from one of the agents to be sent. When it is,
the attacker needs to inject a fake response with an unrecoverable
error response, such as a 600. However, since the candidate is, in
fact, valid, the original request may reach the peer agent, and
result in a success response. The attacker needs to force this
packet or its response to be dropped, through a DoS attack, layer 2
network disruption, or other technique. If it doesn't do this, the
success response will also reach the originator, alerting it to a
possible attack. Fortunately, this attack is mitigated completely
through the STUN message integrity mechanism. The attacker needs to
inject a fake response, and in order for this response to be
processed, the attacker needs the password. If the offer/answer
signaling is secured, the attacker will not have the password.
Forcing the fake valid result works in a similar way. The agent
needs to wait for the Binding Request from each agent, and inject a
fake success response. The attacker won't need to worry about
disrupting the actual response since, if the candidate is not valid,
it presumably wouldn't be received anyway. However, like the fake
invalid attack, this attack is mitigated completely through the STUN
message integrity and offer/answer security techniques.
Forcing the false peer reflexive candidate result can be done either
with fake requests or responses, or with replays. We consider the
fake requests and responses case first. It requires the attacker to
send a Binding Request to one agent with a source IP address and port
for the false candidate. In addition, the attacker must wait for a
Binding Request from the other agent, and generate a fake response
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with a XOR-MAPPED-ADDRESS attribute containing the false candidate.
Like the other attacks described here, this attack is mitigated by
the STUN message integrity mechanisms and secure offer/answer
exchanges.
Forcing the false peer reflexive candidate result with packet replays
is different. The attacker waits until one of the agents sends a
check. It intercepts this request, and replays it towards the other
agent with a faked source IP address. It must also prevent the
original request from reaching the remote agent, either by launching
a DoS attack to cause the packet to be dropped, or forcing it to be
dropped using layer 2 mechanisms. The replayed packet is received at
the other agent, and accepted, since the integrity check passes (the
integrity check cannot and does not cover the source IP address and
port). It is then responded to. This response will contain a XOR-
MAPPED-ADDRESS with the false candidate, and will be sent to that
false candidate. The attacker must then intercept it and relay it
towards the originator.
The other agent will then initiate a connectivity check towards that
false candidate. This validation needs to succeed. This requires
the attacker to force a false valid on a false candidate. Injecting
of fake requests or responses to achieve this goal is prevented using
the integrity mechanisms of STUN and the offer/answer exchange.
Thus, this attack can only be launched through replays. To do that,
the attacker must intercept the check towards this false candidate,
and replay it towards the other agent. Then, it must intercept the
response and replay that back as well.
This attack is very hard to launch unless the attacker themself is
identified by the fake candidate. This is because it requires the
attacker to intercept and replay packets sent by two different hosts.
If both agents are on different networks (for example, across the
public Internet), this attack can be hard to coordinate, since it
needs to occur against two different endpoints on different parts of
the network at the same time.
If the attacker themself is identified by the fake candidate the
attack is easier to coordinate. However, if SRTP is used [22], the
attacker will not be able to play the media packets, they will only
be able to discard them, effectively disabling the media stream for
the call. However, this attack requires the agent to disrupt packets
in order to block the connectivity check from reaching the target.
In that case, if the goal is to disrupt the media stream, its much
easier to just disrupt it with the same mechanism, rather than attack
ICE.
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16.2. Attacks on Address Gathering
ICE endpoints make use of STUN for gathering candidates rom a STUN
server in the network. This is corresponds to the Binding Discovery
usage of STUN described in [11]. As a consequence, the attacks
against STUN itself that are described in that specification can
still be used against the binding discovery usage when utilized with
ICE.
However, the additional mechanisms provided by ICE actually
counteract such attacks, making binding discovery with STUN more
secure when combined with ICE than without ICE.
Consider an attacker which is able to provide an agent with a faked
mapped address in a STUN Binding Request that is used for address
gathering. This is the primary attack primitive described in [11].
This address will be used as a server reflexive candidate in the ICE
exchange. For this candidate to actually be used for media, the
attacker must also attack the connectivity checks, and in particular,
force a false valid on a false candidate. This attack is very hard
to launch if the false address identifies a third party, and is
prevented by SRTP if it identifies the attacker themself.
If the attacker elects not to attack the connectivity checks, the
worst it can do is prevent the server reflexive candidate from being
used. However, if the peer agent has at least one candidate that is
reachable by the agent under attack, the STUN connectivity checks
themselves will provide a peer reflexive candidate that can be used
for the exchange of media. Peer reflexive candidates are generally
preferred over server reflexive candidates. As such, an attack
solely on the STUN address gathering will normally have no impact on
a session at all.
16.3. 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 [4] apply. These require
techniques for message integrity and encryption for offers and
answers, which are satisfied by the SIPS mechanism [3] when SIP is
used. As such, the usage of SIPS with ICE is RECOMMENDED.
16.4. Insider Attacks
In addition to attacks where the attacker is a third party trying to
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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.
16.4.1. The Voice Hammer Attack
The voice hammer attack is an amplification attack. In this attack,
the attacker initiates sessions to other agents, and includes the IP
address and port of a DoS target in the m/c-line of their 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 peform connectivity checks to the target of media before
sending it 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 are known, and
limited to ones that support ICE, the server can reject any offers or
answers that don't indicate ICE support.
16.4.2. STUN Amplification Attack
The STUN amplification attack is similar to the voice hammer.
However, instead of voice packets being directed to the target, STUN
connectivity checks are directed to the target. This attack is
accomplished by having the offerer send an offer with a large number
of candidates, say 50. The answerer receives the offer, and starts
its checks, which are directed at the target, and consequently, never
generate a response. The answerer will start a new connectivity
check every 20ms, and each check is a STUN transaction consisting of
7 transmissions of a message 65 bytes in length (plus 28 bytes for
the IP/UDP header) that runs for 7.9 seconds, for a total of 58
bytes/second per transaction on average. In the worst case, there
can be 395 transactions in progress at once (7.9 seconds divided by
20ms), for a total of 182 kbps, just for STUN requests.
It is impossible to eliminate the amplification, but the volume can
be reduced through a variety of heuristics. Agents SHOULD limit the
total number of connectivity checks they perform to 100.
Additionally, agents MAY limit the number of candidates they'll
accept in an offer or answer.
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16.5. Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a NAT
device which inspect the contents of packets and modify them, in
order to facilitate NAT traversal for application protocols. Session
Border Controllers (SBC) 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 m/c-lines of SDP. In this
case, correctly means that the ALG does not modify m/c-lines with
external addresses. If the m/c-line contains internal addresses, but
ones for which a public binding exists, the ALG replaces the internal
address in the m/c-line with the public binding. Unfortunately, many
ALG 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/c-line.
The a=ice-mismatch parameter 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 which 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 they SBC has requested it. If, however, the SBC
passes the ICE attributes without modification, yet modifies the m/c-
lines, 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.
17. Definition of Connectivity Check Usage
STUN [11] requires that new usages provide a specific set of
information as part of their formal definition. This section meets
the requirements spelled out there.
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17.1. Applicability
This STUN usage provides a connectivity check between two peers
participating in an offer/answer exchange. This check serves to
validate a pair of candidates for usage of exchange of media.
Connectivity checks also allow agents to discover reflexive
candidates towards their peers, called peer reflexive candidates.
Finally, connectivity checks serve to keep NAT bindings alive.
It is fundamental to this STUN usage that the addresses and ports
used for media are the same ones used for the Binding Requests and
responses. Consequently, it will be necessary to demultiplex STUN
traffic from whatever the media traffic is. This demultiplexing is
done using the techniques described in [11].
17.2. Client Discovery of Server
The client does not follow the DNS-based procedures defined in [11].
Rather, the remote candidate of the check to be performed is used as
the transport address of the STUN server. Note that the STUN server
is a logical entity, and is not a physically distinct server in this
usage.
17.3. Server Determination of Usage
The server is aware of this usage because it signaled this port
through the offer/answer exchange. Any STUN packets received on this
port will be for the connectivity check usage.
17.4. New Requests or Indications
This usage does not define any new message types.
17.5. New Attributes
This usage defines two new attributes, PRIORITY and USE-CANDIDATE.
The PRIORITY attribute indicates the priority that is to be
associated with a peer reflexive candidate, should one be discovered
by this check. It is a 32 bit unsigned integer, and has an attribute
type of 0x0024.
The USE-CANDIDATE attribute indicates that the candidate pair
resulting from this check should be used for transmission of media.
The attribute has no content (the Length field of the attribute is
zero); it serves as a flag. It has an attribute type of 0x0025.
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17.6. New Error Response Codes
This usage does not define any new error response codes.
17.7. Client Procedures
Client procedures are defined in Section 7.1.
17.8. Server Procedures
Server procedures are defined in Section 7.2.
17.9. Security Considerations for Connectivity Check
Security considerations for the connectivity check are discussed in
Section 16.
18. IANA Considerations
This specification registers new SDP attributes and new STUN
attributes.
18.1. SDP Attributes
This specification defines seven new SDP attributes per the
procedures of Section 8.2.4 of [10]. The required information for
the registrations are included here.
18.1.1. candidate Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: candidate
Long Form: candidate
Type of Attribute: media level
Charset Considerations: The attribute is not subject to the charset
attribute.
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 Simple Traversal Underneath
NAT (STUN).
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Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
18.1.2. remote-candidates Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: remote-candidates
Long Form: remote-candidates
Type of Attribute: media level
Charset Considerations: The attribute is not subject to the charset
attribute.
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 13 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
18.1.3. ice-lite Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-lite
Long Form: ice-lite
Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset
attribute.
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 13 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
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18.1.4. ice-mismatch Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-mismatch
Long Form: ice-mismatch
Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset
attribute.
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 values in the m/c-line.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
18.1.5. ice-pwd Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-pwd
Long Form: ice-pwd
Type of Attribute: session or media level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the password used to protect
STUN connectivity checks.
Appropriate Values: See Section 13 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
18.1.6. ice-ufrag Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
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Attribute Name: ice-ufrag
Long Form: ice-ufrag
Type of Attribute: session or media level
Charset Considerations: The attribute is not subject to the charset
attribute.
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 13 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
18.1.7. ice-options Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-options
Long Form: ice-options
Type of Attribute: session level
Charset Considerations: The attribute is not subject to the charset
attribute.
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 13 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
18.2. STUN Attributes
This section registers two new STUN attributes per the procedures in
[11].
0x0024 PRIORITY
0x0025 USE-CANDIDATE
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19. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a agent attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [20]. ICE is an example
of a protocol that performs this type of function. Interestingly,
the process for ICE is not unilateral, but bilateral, and the
difference has a signficant impact on the issues raised by IAB.
Indeed, ICE can be considered a B-SAF (Bilateral Self-Address Fixing)
protocol, rather than an UNSAF protocol. Regardless, the IAB has
mandated that any protocols developed for this purpose document a
specific set of considerations. This section meets those
requirements.
19.1. Problem Definition
From RFC 3424 any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term fixes
usually aren't".
The specific problems being solved by ICE are:
Provide a means for two peers to determine the set of transport
addresses which can be used for communication.
Provide a means for resolving many of the limitations of other
UNSAF mechanisms by wrapping them in an additional layer of
processing (the ICE methodology).
Provide a means for a agent to determine an address that is
reachable by another peer with which it wishes to communicate.
19.2. Exit Strategy
From RFC 3424, any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
ICE itself doesn't easily get phased out. However, it is useful even
in a globally connected Internet, to serve as a means for detecting
whether a router failure has temporarily disrupted connectivity, for
example. ICE also helps prevent certain security attacks which have
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nothing to do with NAT. However, what ICE does is help phase out
other UNSAF mechanisms. ICE effectively selects amongst those
mechanisms, prioritizing ones that are better, and deprioritizing
ones that are worse. Local IPv6 addresses can be preferred. As NATs
begin to dissipate as IPv6 is introduced, server reflexive and
relayed candidates (both forms of UNSAF mechanisms) simply never get
used, because higher priority connectivity exists to the native host
candidates. Therefore, the servers get used less and less, and can
eventually be remove when their usage goes to zero.
Indeed, ICE can assist in the transition from IPv4 to IPv6. It can
be used to determine whether to use IPv6 or IPv4 when two dual-stack
hosts communicate with SIP (IPv6 gets used). It can also allow a
network with both 6to4 and native v6 connectivity to determine which
address to use when communicating with a peer.
19.3. Brittleness Introduced by ICE
From RFC3424, any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
ICE actually removes brittleness from existing UNSAF mechanisms. In
particular, traditional STUN (as described in RFC 3489 [14]) has
several points of brittleness. One of them is the discovery process
which requires a agent to try and classify the type of NAT it is
behind. This process is error-prone. With ICE, that discovery
process is simply not used. Rather than unilaterally assessing the
validity of the address, its validity is dynamically determined by
measuring connectivity to a peer. The process of determining
connectivity is very robust.
Another point of brittleness in traditional STUN and any other
unilateral mechanism is its absolute reliance on an additional
server. ICE makes use of a server for allocating unilateral
addresses, but allows agents to directly connect if possible.
Therefore, in some cases, the failure of a STUN server would still
allow for a call to progress when ICE is used.
Another point of brittleness in traditional STUN is that it assumes
that the STUN server is on the public Internet. Interestingly, with
ICE, that is not necessary. There can be a multitude of STUN servers
in a variety of address realms. ICE will discover the one that has
provided a usable address.
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The most troubling point of brittleness in traditional STUN is that
it doesn't work in all network topologies. In cases where there is a
shared NAT between each agent and the STUN server, traditional STUN
may not work. With ICE, that restriction is removed.
Traditional STUN also introduces some security considerations.
Fortunately, those security considerations are also mitigated by ICE.
Consequently, ICE serves to repair the brittleness introduced in
other UNSAF mechanisms, and does not introduce any additional
brittleness into the system.
19.4. Requirements for a Long Term Solution
From RFC 3424, any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our conclusions from STUN remain unchanged. However, we feel ICE
actually helps because we believe it can be part of the long term
solution.
19.5. Issues with Existing NAPT Boxes
From RFC 3424, any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market which
try and provide "generic" ALG functionality. These generic ALGs hunt
for IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This interferes with
traditional STUN. However, the update to STUN [11] uses an encoding
which hides these binary addresses from generic ALGs. Since [11] is
required for all ICE implementations, this NAPT problem does not
impact ICE.
Existing NAPT boxes have non-deterministic and typically short
expiration times for UDP-based bindings. This requires
implementations to send periodic keepalives to maintain those
bindings. ICE uses a default of 15s, which is a very conservative
estimate. Eventually, over time, as NAT boxes become compliant to
behave [31], this minimum keepalive will become deterministic and
well-known, and the ICE timers can be adjusted. Having a way to
discover and control the minimum keepalive interval would be far
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better still.
20. Acknowledgements
The authors would like to thank Flemming Andreasen, Rohan Mahy, Dean
Willis, Eric Cooper, Dan Wing, Douglas Otis, Tim Moore, and Francois
Audet for their comments and input. A special thanks goes to Bill
May, who suggested several of the concepts in this specification,
Philip Matthews, who suggested many of the key performance
optimizations in this specification, Eric Rescorla, who drafted the
text in the introduction, and Magnus Westerlund, for doing several
detailed reviews on the various revisions of this specification.
21. References
21.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
[3] 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.
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[5] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
July 2003.
[6] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)",
RFC 3312, October 2002.
[7] Camarillo, G. and P. Kyzivat, "Update to the Session Initiation
Protocol (SIP) Preconditions Framework", RFC 4032, March 2005.
[8] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[9] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262,
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June 2002.
[10] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[11] Rosenberg, J., "Simple Traversal Underneath Network Address
Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05
(work in progress), October 2006.
[12] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in
progress), October 2006.
[13] Rosenberg, J., "Indicating Support for Interactive Connectivity
Establishment (ICE) in the Session Initiation Protocol (SIP)",
draft-ietf-sip-ice-option-tag-00 (work in progress),
January 2007.
21.2. Informative References
[14] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
- Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
[15] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[16] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[17] 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.
[18] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm
Specific IP: Framework", RFC 3102, October 2001.
[19] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm
Specific IP: Protocol Specification", RFC 3103, October 2001.
[20] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[21] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
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RFC 3550, July 2003.
[22] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[23] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
[24] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
[25] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
Generation in the Session Initiation Protocol (SIP)", RFC 3960,
December 2004.
[26] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W.
Weiss, "An Architecture for Differentiated Services", RFC 2475,
December 1998.
[27] Andreasen, F., "Connectivity Preconditions for Session
Description Protocol Media Streams",
draft-ietf-mmusic-connectivity-precon-02 (work in progress),
June 2006.
[28] Andreasen, F., "A No-Op Payload Format for RTP",
draft-ietf-avt-rtp-no-op-00 (work in progress), May 2005.
[29] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, March 2006.
[30] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
[31] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress),
October 2006.
[32] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-07 (work in progress), January 2007.
[33] Rescorla, E., "Overview of the Lite Implementation of
Interactive Connectivity Establishment (ICE)",
draft-ietf-mmusic-ice-lite-00.txt (work in progress),
January 2007.
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Appendix A. Lite and Full Implementations
ICE allows for two types of implementations. A full implementation
supports the controlling and controlled roles in a session, and can
also perform address gathering. In contrast, a lite implementation
is a minimalist implementation that does little but respond to STUN
checks.
Because ICE requires both endpoints to support it in order to bring
benefits to either endpoint, incremental deployment of ICE in a
network is more complicated. Many sessions involve an endpoint which
is, by itself, not behind a NAT and not one that would worry about
NAT traversal. Examples include gateways to the PSTN, media servers,
conference bridges, and application servers. A very common case is
to have one endpoint that requires NAT traversal (such as a VoIP hard
phone or soft phone) make a call through one of these devices. Even
if the phone supports a full ICE implementation, ICE won't be used at
all if the other device doesn't support it. The lite implementation
allows for a low-cost entry point for these devices. Once they
support the lite implementation, full implementations can connect to
them and get the full benefits of ICE.
Consequently, a lite implementation is only appropriate for devices
that will always be connected to the public Internet and have a
public IP address at which it can receive packets from any
correspondent. ICE will not function when a lite implementation is
placed behind a NAT.
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet, a full implementation is preferable if achievable.
A full implementation will reduce call setup times. Full
implementations also obtain the security benefits of ICE unrelated to
NAT traversal; in particular, the voice hammer attack described in
Section 16 is prevented only for full implementations, not lite.
Finally, it is often the case that a device which finds itself with a
public address today will be placed in a network tomorrow where it
will be behind a NAT. It is difficult to definitively know, over the
lifetime of a device or product, that it will always be used on the
public Internet. Full implementation provides assurance that
communications will always work.
Appendix B. Design Motivations
ICE contains a number of normative behaviors which may themselves be
simple, but derive from complicated or non-obvious thinking or use
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cases which merit further discussion. Since these design motivations
are not neccesary to understand for purposes of implementation, they
are discussed here in an appendix to the specification. This section
is non-normative.
B.1. Pacing of STUN Transactions
STUN transactions used to gather candidates and to verify
connectivity are paced out at an approximate rate of one new
transaction every Ta seconds, where Ta has a default of 20ms. Why
are these transactions paced, and why was 20ms chosen as default?
Sending of these STUN requests will often have the effect of creating
bindings on NAT devices between the client and the STUN servers.
Experience has shown that many NAT devices have upper limits on the
rate at which they will create new bindings. Furthermore,
transmission of these packets on the network makes use of bandwidth
and needs to be rate limited by the agent. As a consequence, the
pacing ensures that the NAT devices does not get overloaded and that
traffic is kept at a reasonable rate.
B.2. Candidates with Multiple Bases
Section 4.1.1 talks about merging together candidates that are
identical but have different bases. When can an agent have two
candidates that have the same IP address and port, but different
bases? Consider the topology of Figure 17:
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+----------+
| STUN Srvr|
+----------+
|
|
-----
// \\
| |
| B:net10 |
| |
\\ //
-----
|
|
+----------+
| NAT |
+----------+
|
|
-----
// \\
| A |
|192.168/16 |
| |
\\ //
-----
|
|
|192.168.1.1 -----
+----------+ // \\ +----------+
| | | | | |
| Offerer |---------| C:net10 |---------| Answerer |
| |10.0.1.1 | | 10.0.1.2 | |
+----------+ \\ // +----------+
-----
Figure 17
In this case, the offerer is multi-homed. It has one interface,
10.0.1.1, on network C, which is a net 10 private network. The
Answerer is on this same network. The offerer is also connected to
network A, which is 192.168/16. The offerer has an interface of
192.168.1.1 on this network. There is a NAT on this network, natting
into network B, which is another net10 private network, but not
connected to network C. There is a STUN server on network B.
The offerer obtains a host candidate on its interface on network C
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(10.0.1.1:2498) and a host candidate on its interface on network A
(192.168.1.1:3344). It performs a STUN query to its configured STUN
server from 192.168.1.1:3344. This query passes through the NAT,
which happens to assign the binding 10.0.1.1:2498. The STUN server
reflects this in the STUN Binding Response. Now, the offerer has
obtained a server reflexive candidate with a transport address that
is identical to a host candidate (10.0.1.1:2498). However, the
server reflexive candidate has a base of 192.168.1.1:3344, and the
host candidate has a base of 10.0.1.1:2498.
B.3. Purpose of the Translation
When a candidate is relayed, the SDP offer or answer contain both the
relayed candidate and its translation. However, the translation is
never used by ICE itself. Why is it present in the message?
There are two motivations for its inclusion. The first is
diagnostic. It is very useful to know the relationship between the
different types of candidates. By including the translation, an
agent can know which relayed candidate is associated with which
reflexive candidate, which in turn is associated with a specific host
candidate. When checks for one candidate succeed and not the others,
this provides useful diagnostics on what is going on in the network.
The second reason has to do with off-path Quality of Service (QoS)
mechanisms. When ICE is used in environments such as PacketCable
2.0, proxies will, in addition to performing normal SIP operations,
inspect the SDP in SIP messages, and extract the IP address and port
for media traffic. They can then interact, through policy servers,
with access routers in the network, to establish guaranteed QoS for
the media flows. This QoS is provided by classifying the RTP traffic
based on 5-tuple, and then providing it a guaranteed rate, or marking
its Diffserv codepoints appropriately. When a residential NAT is
present, and a relayed candidate gets selected for media, this
relayed candidate will be a transport address on an actual STUN
relay. That address says nothing about the actual transport address
in the access router that would be used to classify packets for QoS
treatment. Rather, the translation of that relayed address is
needed. By carrying the translation in the SDP, the proxy can use
that transport address to request QoS from the access router.
B.4. Importance of the STUN Username
ICE requires the usage of message integrity with STUN using its short
term credential functionality. The actual short term credential is
formed by exchanging username fragments in the SDP offer/answer
exchange. The need for this mechanism goes beyond just security; it
is actual required for correct operation of ICE in the first place.
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Consider agents A, B, and C. A and B are within private enterprise 1,
which is using 10.0.0.0/8. C is within private enterprise 2, which
is also using 10.0.0.0/8. As it turns out, B and C both have IP
address 10.0.1.1. A sends an offer to C. C, in its answer, provides
A with its host candidates. In this case, those candidates are
10.0.1.1:8866 and 10.0.1.1:8877. As it turns out, B is in a session
at that same time, and is also using 10.0.1.1:8866 and 10.0.1.1:8877
as host candidates. This means that B is prepared to accept STUN
messages on those ports, just as C is. A will send a STUN request to
10.0.1.1:8866 and and another to 10.0.1.1:8877. However, these do
not go to C as expected. Instead, they go to B! If B just replied
to them, A would believe it has connectivity to C, when in fact it
has connectivity to a completely different user, B. To fix this, the
STUN short term credential mechanisms are used. The username
fragments are sufficiently random that it is highly unlikely that B
would be using the same values as A. Consequently, B would reject the
STUN request since the credentials were invalid. In essence, the
STUN username fragments provide a form of transient host identifiers,
bound to a particular offer/answer session.
An unfortunate consequence of the non-uniqueness of IP addresses is
that, in the above example, B might not even be an ICE agent. It
could be any host, and the port to which the STUN packet is directed
could be any ephemeral port on that host. If there is an application
listening on this socket for packets, and it is not prepared to
handle malformed packets for whatever protocol is in use, the
operation of that application could be affected. Fortunately, since
the ports exchanged in SDP are ephemeral and usually drawn from the
dynamic or registered range, the odds are good that the port is not
used to run a server on host B, but rather is the agent side of some
protocol. This decreases the probability of hitting a port in-use,
due to the transient nature of port usage in this range. However,
the possibility of a problem does exist, and network deployers should
be prepared for it. Note that this is not a problem specific to ICE;
stray packets can arrive at a port at any time for any type of
protocol, especially ones on the public Internet. As such, this
requirement is just restating a general design guideline for Internet
applications - be prepared for unknown packets on any port.
B.5. The Candidate Pair Sequence Number Formula
The sequence number for a candidate pair has an odd form. It is:
pair priority = 2^32*MIN(O-P,A-P) + 2*MAX(O-P,A-P) + (O-P>A-P?1:0)
Why is this? When the candidate pairs are sorted based on this
value, the resulting sorting has the MAX/MIN property. This means
that the pairs are first sorted based on decreasing value of the
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maximum of the two sequence numbers. For pairs that have the same
value of the maximum sequence number, the minimum sequence number is
used to sort amongst them. If the max and the min sequence numbers
are the same, the offerers priority is used as the tie breaker in the
last part of the expression. The factor of 2*32 is used since the
priority of a single candidate is always less than 2*32, resulting in
the pair priority being a "concatenation" of the two component
priorities. This creates the desired sorting property.
B.6. The Frozen State
The Frozen state is used for two purposes. Firstly, it allows ICE to
first perform checks for the first component of a media stream. Once
a successful check has completed for the first component, the other
components of the same type and local preference will get performed.
Secondly, when there are multiple media streams, it allows ICE to
first check candidates for a single media stream, and once a set of
candidates has been found, candidates of that same type for other
media streams can be checked first. This effectively 'caches' the
results of a check for one media stream, and applies them to another.
For example, if only the relayed candidates for audio (which were the
last resort candidates) succeed, ICE will check the relayed
candidates for video first.
B.7. 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 18. On receipt of message 4, agent
A adds a candidate pair to the valid list. If there was only a
single media stream with a single component, agent A could now send
an updated offer. However, the check from agent B has not yet
generated a response, and agent B receives the updated offer (message
7) before getting the response (message 10). Thus, it does not yet
know that this particular pair is valid. To eliminate this
condition, the actual candidates at B that were selected by the
offerer (the remote candidates) are included in the offer itself.
Note, however, that agent B will not send media until it has received
this STUN response.
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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) Answer | |
|<------------------------------------------|
|(9) STUN Req. | |
|<------------------------------------------|
|(10) STUN Res. | |
|------------------------------------------>|
Figure 18
B.8. Why are Keepalives Needed?
Once media begins flowing on a candidate pair, it is still necessary
to keep the bindings alive at intermediate NATs for the duration of
the session. Normally, the media stream packets themselves (e.g.,
RTP) meet this objective. However, several cases merit further
discussion. Firstly, in some RTP usages, such as SIP, the media
streams can be "put on hold". This is accomplished by using the SDP
"sendonly" or "inactive" attributes, as defined in RFC 3264 [4]. RFC
3264 directs implementations to cease transmission of media in these
cases. However, doing so may cause NAT bindings to timeout, and
media won't be able to come off hold.
Secondly, some RTP payload formats, such as the payload format for
text conversation [30], may send packets so infrequently that the
interval exceeds the NAT binding timeouts.
Thirdly, if silence suppression is in use, long periods of silence
may cause media transmission to cease sufficiently long for NAT
bindings to time out.
For these reasons, the media packets themselves cannot be relied
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upon. ICE defines a simple periodic keepalive that operates
independently of media transmission. This makes its bandwidth
requirements highly predictable, and thus amenable to QoS
reservations.
B.9. Why Prefer Peer Reflexive Candidates?
Section 4.1.2 describes procedures for computing the priority of
candidate based on its type and local preferences. That section
requires that the type preference for peer reflexive candidates
always be lower than server reflexive. Why is that? The reason has
to do with the security considerations in Section 16. It is much
easier for an attacker to cause an agent to use a false server
reflexive candidate than it is for an attacker to cause an agent to
use a false peer reflexive candidate. Consequently, attacks against
the STUN binding discovery usage are thwarted by ICE by preferring
the peer reflexive candidates.
B.10. 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
m/c-line so that it matches where media is being sent, based on ICE
procedures.
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 m/c-
lines represent the addresses used for media - must be retained. For
this reason, an updated offer must be sent.
B.11. Why are Binding Indications Used for Keepalives?
Media keepalives are described in Section 10. These keepalives make
use of STUN when both endpoints are ICE capable. However, rather
than using a Binding Request transaction (which generates a
response), the keepalives use an Indication. Why is that?
The primary reason has to do with network QoS mechanisms. Once media
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begins flowing, network elements will assume that the media stream
has a fairly regular structure, making use of periodic packets at
fixed intervals, with the possibility of jitter. If an agent is
sending media packets, and then receives a Binding Request, it would
need to generate a response packet along with its media packets.
This will increase the actual bandwidth requirements for the 5-tuple
carrying the media packets, and introduce jitter in the delivery of
those packets. Analysis has shown that this is a concern in certain
layer 2 access networks that use fairly tight packet schedulers for
media.
Additionally, using a Binding Indication allows integrity to be
disabled, allowing for better performance. This is useful for large
scale endpoints, such as PSTN gateways.
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Author's Address
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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