MMUSIC J. Rosenberg
Internet-Draft Cisco Systems
Expires: January 18, 2006 July 17, 2005
Interactive Connectivity Establishment (ICE): A Methodology for Network
Address Translator (NAT) Traversal for Offer/Answer Protocols
draft-ietf-mmusic-ice-05
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Abstract
This document describes a methodology for Network Address Translator
(NAT) traversal for multimedia session signaling protocols, such as
the Session Initiation Protocol (SIP). This methodology is called
Interactive Connectivity Establishment (ICE). ICE makes use of
existing protocols, such as Simple Traversal of UDP Through NAT
(STUN) and Traversal Using Relay NAT (TURN). ICE makes use of STUN
in peer-to-peer cooperative fashion, allowing participants to
discover, create and verify mutual connectivity.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 6
4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 8
5. Receipt of the Offer and Generation of the Answer . . . . . . 9
6. Processing the Answer . . . . . . . . . . . . . . . . . . . . 9
7. Common Procedures . . . . . . . . . . . . . . . . . . . . . . 10
7.1 Gathering Candidates . . . . . . . . . . . . . . . . . . . 10
7.2 Encoding Candidates into SDP . . . . . . . . . . . . . . . 13
7.3 Prioritizing the Transport Addresses and Choosing an
Active One . . . . . . . . . . . . . . . . . . . . . . . . 15
7.4 Connectivity Checks . . . . . . . . . . . . . . . . . . . 17
7.4.1 UDP Connectivity Checks . . . . . . . . . . . . . . . 19
7.4.1.1 Send Validation . . . . . . . . . . . . . . . . . 19
7.4.1.2 Receive Validation . . . . . . . . . . . . . . . . 20
7.4.1.3 Learning New Candidates from Connectivity
Checks . . . . . . . . . . . . . . . . . . . . . . 22
7.4.1.3.1 On Receipt of a Binding Request . . . . . . . 23
7.4.1.3.2 On Receipt of a Binding Response . . . . . . . 26
7.4.2 TCP Connectivity Checks . . . . . . . . . . . . . . . 26
7.4.2.1 Connection Establishment . . . . . . . . . . . . . 26
7.4.2.2 Sending STUN Binding Requests . . . . . . . . . . 27
7.4.2.3 Receiving STUN Requests . . . . . . . . . . . . . 29
7.5 Promoting a Valid Candidate to Active . . . . . . . . . . 30
7.5.1 Minimum Requirements . . . . . . . . . . . . . . . . . 30
7.5.2 Suggested Algorithm . . . . . . . . . . . . . . . . . 31
7.6 Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 33
7.6.1 Sending of an Offer . . . . . . . . . . . . . . . . . 33
7.6.2 Receiving the Offer and Sending an Answer . . . . . . 34
7.6.3 Receiving the Answer . . . . . . . . . . . . . . . . . 36
7.7 Binding Keepalives . . . . . . . . . . . . . . . . . . . . 37
7.8 Sending Media . . . . . . . . . . . . . . . . . . . . . . 38
8. Interactions with Forking . . . . . . . . . . . . . . . . . . 38
9. Interactions with Preconditions . . . . . . . . . . . . . . . 38
10. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 39
11. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . 39
12. Security Considerations . . . . . . . . . . . . . . . . . . 40
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 42
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 42
14.1 Problem Definition . . . . . . . . . . . . . . . . . . . . 42
14.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 43
14.3 Brittleness Introduced by ICE . . . . . . . . . . . . . . 43
14.4 Requirements for a Long Term Solution . . . . . . . . . . 44
14.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . . 45
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 45
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
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16.1 Normative References . . . . . . . . . . . . . . . . . . . 45
16.2 Informative References . . . . . . . . . . . . . . . . . . 46
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . 48
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1. Introduction
A multimedia session signaling protocol is a protocol that exchanges
control messages between a pair of agents for the purposes of
establishing the flow of media traffic between them. This media flow
is distinct from the flow of control messages, and may take a
different path through the network. Examples of such protocols are
the Session Initiation Protocol (SIP) [3], the Real Time Streaming
Protocol (RTSP) [16] and the International Telecommunications Union
(ITU) H.323.
These protocols, by nature of their design, are difficult to operate
through Network Address Translators (NAT). Because their purpose in
life is to establish a flow of packets, they tend to carry IP
addresses within their messages, which is known to be problematic
through NAT [17]. 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 [18], Simple Traversal of UDP
through NAT (STUN) [1], Traversal Using Relay NAT [14], and Realm
Specific IP [19] [20] along with session description extensions
needed to make them work, such as the Session Description Protocol
(SDP) [7] 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 protocols based on the
offer-answer model, RFC 3264 [4]. It is called Interactive
Connectivity Establishment, or ICE. ICE makes use of STUN and TURN,
but uses them in a specific methodology which avoids many of the
pitfalls of using any one alone.
2. Terminology
Several new terms are introduced in this specification:
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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 offeror.
Transport Address: The combination of an IP address and port.
Local Transport Address: A local transport address a transport
address that has been allocated from the operating system on the
host. This includes transport addresses obtained through Virtual
Private Networks (VPNs) and transport addresses obtained through
Realm Specific IP (RSIP) [19] (which lives at the operating system
level). Transport addresses are typically obtained by binding to
an interface.
m/c line: The media and connection lines in the SDP, which together
hold the transport address used for the receipt of media.
Derived Transport Address: A derived transport address is a transport
address which is derived from a local transport address. The
derived transport address is related to the associated local
transport address in that packets sent to the derived transport
address are received on the socket bound to its associated local
transport address. Derived addresses are obtained using protocols
like STUN and TURN, and more generally, any UNSAF protocol [21].
Candidate Transport Address: A transport address advertised by a
agent in an offer or answer. A candidate transport address can
either by a local transport address or a derived transport
address.
Peer Derived Transport Address: A peer derived transport address is a
derived transport address learned from a STUN server running
within a peer in a media session.
TURN Derived Transport Address: A derived transport address obtained
from a TURN server.
STUN Derived Transport Address: A derived transport address obtained
from a STUN server whose address has been provisioned into the UA.
This, by definition, excludes Peer Derived Transport Addresses.
Candidate: A sequence of candidate transport addresses that form an
atomic set for usage with a particular media stream. In the case
of RTP, there are two candidate transport addresses per candidate:
one for RTP, and another for RTCP. Connectivity is verified to
all of the candidate transport addresses within a candidate before
that candidate is used. The transport addresses that compose a
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candidate are all of the same type - local, STUN derived, TURN
derived or peer derived.
Local Candidate: A candidate whose transport addresses are local
transport addresses.
STUN Candidate: A candidate whose transport addresses are STUN
derived transport addresses.
TURN Candidate: A candidate whose transport addresses are TURN
derived transport addresses.
Peer Candidate: A candidate whose transport addresses are peer
derived transport addresses.
Active Candidate: The candidate that is in use for exchange of media.
This is the one that an agent places in the m/c line of an offer
or answer.
3. Overview of ICE
ICE makes the fundamental assumption that clients exist in a network
of segmented connectivity. This segmentation is the result of a
number of addressing realms in which a client can simultaneously be
connected. We use "realms" here in the broadest sense. A realm is
defined purely by connectivity. Two clients are in the same realm
if, when they exchange the addresses each has in that realm, they are
able to send packets to each other. This includes IPv6 and IPv4
realms, which actually use different address spaces, in addition to
private networks connected to the public Internet through NAT.
The key assumption in ICE is that a client cannot know, apriori,
which address realms it shares with any peer it may wish to
communicate with. Therefore, in order to communicate, it has to try
connecting to addresses in all of the realms.
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Agent A TURN,STUN Servers Agent B
|(1) Gather Addresses | |
|-------------------->| |
|(2) Offer | |
|------------------------------------------>|
| |(3) Gather Addresses |
| |<--------------------|
|(4) Answer | |
|<------------------------------------------|
|(5) Media | |
|<------------------------------------------|
|(6) Media | |
|------------------------------------------>|
|(7) STUN Checks | |
|<------------------------------------------|
|(8) STUN Checks | |
|------------------------------------------>|
|(9) Offer | |
|------------------------------------------>|
|(10) Answer | |
|<------------------------------------------|
|(11) Media | |
|<------------------------------------------|
|(12) Media | |
|------------------------------------------>|
Figure 1
The basic flow of operation for ICE is shown in Figure 1. Before the
offeror establishes a session, it obtains local transport addresses
from its operating system on as many interfaces as it has access to.
These interfaces can include IPv4 and IPv6 interfaces, in addition to
Virtual Private Network (VPN) interfaces or ones associated with
RSIP. For media protocols that support both UDP and TCP (such as the
Real Time Transport Protocol (RTP) [22], which can run over either),
it obtains both TCP and UDP transport addresses. In addition, the
agent obtains derived transport addresses from each local transport
address using protocols such as STUN and TURN. Each local and
derived transport address becomes a candidate for receipt of media
traffic.
The agent will choose one of its candidate transport addresses as its
initial media transport address for inclusion in the connection and
media lines in the offer. This transport address will be utilized
for media traffic while connectivity is verified to all of the
candidates. Since these checks may take time to execute, media
clipping will occur if the media transport address is not reachable
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by the peer. To minimize the probability of clipping, the transport
address that is most likely to work is chosen. This is normally a
TURN-derived tranport address, but others can be utilized based on
local policy.
Each candidate transport address (including the one being used as the
media transport address) is listed in an a=candidate attribute in the
offer. Each candidate is given a preference. Preference is a matter
of local policy, but typically, lowest preference would be given to
transport addresses learned from a TURN server (i.e., TURN derived
transport addresses). Each candidate is also assigned a distinct ID,
called a transport ID (tid).
The offer is then sent to the answerer. This specification does not
address the issue of how the signaling messages themselves traverse
NAT. It is assumed that signaling protocol specific mechanisms are
used for that purpose. The answerer follows a similar process as the
offeror followed; it obtains addresses from local interfaces, obtains
derived transport addresses from those, and the combination becomes
its set of candidate transport addresses. It picks one as its
initial media transport address and places it into the m/c line in
the answer, and then lists all of them in the a=candidate attributes
in the answer, along with a preference and tid.
Once the offer/answer exchange has completed, each agent sends media
from its media transport address to the media transport address of
its peer. This media stream may or may not work, depending on
whether or not the media transport address is reachable. In parallel
with the transmission of media, a connectivity check begins. This
check makes use of STUN messages sent from each candidate to each
other candidate. These checks will allow each agent to determine
whether it can send packets from a particular candidate to a
candidate from its peer, and whether packets can be sent back. If,
after a certain period of time, an agent determines that a pair of
candidates works, and has a higher priority than the transport
addresses currently in use for media (perhaps because the ones in use
don't work), it sends a new offer that "promotes" its candidate into
the m/c line. This causes the media traffic to switch to this new
transport address.
4. Sending the Initial Offer
When an agent wishes to begin a session by sending an initial offer,
it starts by gathering transport addresses, as described in
Section 7.1. This will produce a set of candidates, including local
ones, STUN-derived ones, and TURN-derived ones.
This process of gathering candidates can actually happen at any time
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before sending the initial offer. A agent can pre-gather transport
addresses, using a user interface cue (such as picking up the phone,
or entry into an address book) as a hint that communications is
imminent. Doing so eliminates any additional perceivable call setup
delays due to address gathering.
When it comes time to offer communications, it determines a priority
for each candidate and identifies the active candidate that will be
used for receipt of media, as described in Section 7.3.
The next step is to construct the offer message. For each media
stream, it places its candidates into a=candidate attributes in the
offer and puts its active candidate into the m/c line. The process
for doing this is described in Section 7.2. The offer is then sent.
5. Receipt of the Offer and Generation of the Answer
Upon receipt of the offer message, the agent checks if the offer
contains any a=candidate attributes. If it does, the offeror
supports ICE. In that case, it starts gathering candidates, as
described in Section 7.1, and prioritizes them Section 7.3. This
processing is done immediately on receipt of the offer, to prepare
for the case where the user should accept the call, or early media
needs to be generated. By gathering candidates while the user is
being alerted to the request for communications, session
establishment delays due to that gathering can be eliminated.
At some point, the answerer will decide to accept or reject the
communications. A rejection terminates ICE processing. In the case
of acceptance, the answer is constructed, and if the offeror
supported ICE, the candidates are encoded into the SDP as described
in Section 7.2. The answer is then sent. If the offeror supported
ICE, the answerer begins its connectivity checks as described in
Section 7.4.
In addition, and regardless if the offeror supported ICE, the
answerer can begin sending media packets as it normally would. It
sends media according to the procedures in Section 7.8.
6. Processing the Answer
There are two possible cases for processing of the answer. If the
answerer did not support ICE, the answer will not contain any
a=candidate attributes. As a result, the offeror knows that it
cannot perform its connectivity checks. In this case, it proceeds
with normal media processing as if ICE was not in use. The
procedures for sending media, described in Section 7.8, MUST be
followed however.
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If the answer contains candidates, it implies that the answerer
supported ICE. In that case, the offeror begins connectivity checks
as described in Section 7.4. It also starts sending media, using the
candidate in the m/c line, based on the procedures described in
Section 7.8.
7. Common Procedures
This section discusses procedures that are common between offeror and
answerer.
7.1 Gathering Candidates
An agent gathers candidates when it believes that communications is
imminent. For offerors, this occurs before sending an offer
(Section 4). For answerers, it occurs before sending an answer
(Section 5).
Each candidate is composed of a series of transport addresses of the
same type. In the case of RTP, the candidate is composed of either
one or two transport addresses. Normally there are two - one for
RTP, and one for RTCP. However, if RTCP is not in use, a candidate
will only contain a single transport address.
The first step is to gather local candidates. Local candidates are
obtained by binding to ephemeral ports on an interface (physical or
virtual, including VPN interfaces) on the host. Specifically, for
each UDP-only media stream the agent wishes to use, the agent SHOULD
obtain a set of candidates (one for each interface) by binding to N
ephemeral UDP ports on each interface, where N is the number of
transport addresses needed for the candidate. For RTP, N is
typically two. For each TCP-only media stream the agent wishes to
use, the agent SHOULD obtain a set of candidates by binding to N
ephemeral TCP ports on each interface, where N is the number of
transport addresses needed for the candidate. For media streams that
can support either UDP or TCP, the agent SHOULD obtain a set of
candidates by binding to N ephemeral UDP and N ephemeral TCP ports on
each interface, where N is the number of transport addresses needed
for the candidate.
If a host has K local interfaces, this will result in K candidates
for each UDP stream (requiring K*N transport addresses), K candidates
for each TCP stream (requiring K*N transport addresses), and 2K
candidates for streams that support UDP and TCP (requiring 2*K*N
transport addresses).
Media streams carried using the Real Time Transport Protocol (RTP)
[22] can run over TCP [27]. As such, it is RECOMMENDED that both UDP
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and TCP candidates be obtained. Transmission of real time media over
UDP is generally preferred to TCP. However, many network
environments, for better or for worse, permit only TCP traffic.
Obtaining a TCP candidate, and then using it in conjunction with a
TURN relay as described below, allows for ICE to make use of the TCP
media only when UDP connectivity is non-existent, as it may be in
these restricted environments. However, providers of real-time
communications services may decide that it is preferable to have no
media at all than it is to have media over TCP. To allow for choice,
it is RECOMMENDED that agents be configurable with whether they
obtain TCP candidates for real time media.
Having it be configurable, and then configuring it to be off, is
far better than not having the capability at all. An important
goal of this specification is to provide a single mechanism that
can be used across all types of endpoints. As such, it is
preferable to account for provider and network variation through
configuration, instead of hard-coded limitations in an
implementation. Furthermore, network characteristics and
connectivity assumptions can, and will change over time. Just
because a agent is communicating with a server on the public
network today, doesn't mean that it won't need to communicate with
one behind a NAT tomorrow. Just because a agent is behind a full
cone NAT today, doesn't mean that tomorrow they won't pick up
their agent and take it to a public network access point where
there is a symmetric NAT or one that only allows outbound TCP.
The way to handle these cases and build a reliable system is for
agents to implement a diverse set of techniques for allocating
addresses, so that at least one of them is almost certainly going
to work in any situation. Implementors should consider very
carefully any assumptions that they make about deployments before
electing not to implement one of the mechanisms for address
allocation. In particular, implementors should consider whether
the elements in the system may be mobile, and connect through
different networks with different connectivity. They should also
consider whether endpoints which are under their control, in terms
of location and network connectivity, would always be under their
control. Only in cases where there isn't now, and never will be,
endpoint mobility or nomadicity of any sort, should a technique be
omitted.
Once the agent has obtained local candidates, it obtains candidates
with derived transport addresses. Agents which serve end users
directly, such as softphones, hardphones, terminal adaptors and so
on, MUST implement STUN and SHOULD use it to obtain STUN candidates.
These devices SHOULD implement and SHOULD use TURN to obtain TURN
candidates. They MAY implement and MAY use other protocols that
provide derived transport addresses, such as TEREDO [25]. As with
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TCP, usage of STUN and TURN is at SHOULD strength to allow for
provider variation. If it is not to be used, it is also RECOMMENDED
that it be implemented and just disabled through configuration, so
that it can re-enabled through configuration if conditions change in
the future.
Agents which represent network servers under the control of a service
provider, such as gateways to the telephone network, media servers,
or conferencing servers that are targeted at deployment only in
networks with public IP addresses MAY use STUN, TURN or other similar
protocols to obtain candidates.
Why would these types of endpoints even bother to implement ICE?
The answer is that such an implementation greatly facilitates NAT
traversal for endpoints that connect to it. The ability to
process STUN connectivity checks allows for the network server to
obtain peer-derived transport addresses that can be used to
provide relay-free traversal of symmetric NAT for endpoints that
connect to it. Furthermore, implementation of the STUN
connectivity checks allows for NAT bindings along the way to be
kept open. ICE also provides numerous security properties that
are independent of NAT traversal, and would benefit any multimedia
endpoint. See Section 12 for a discussion on these benefits.
To obtain STUN candidates (which are always UDP), the client takes a
local UDP candidate, and for each configured STUN server, produces a
STUN candidate. It is anticipated that clients may have a
multiplicity of STUN servers configured in network environments where
there are multiple layers of NAT, and that layering is known to the
provider of the client. To produce the STUN candidate from the local
candidate, it follows the procedures of Section 9 of RFC 3489 for
each local transport address in the local candidate. It obtains a
shared secret from the STUN server and then initiates a Binding
Request transaction from the local transport address to that server.
The Binding Response will provide the client with its STUN derived
transport address in the MAPPED-ADDRESS attribute. If the client had
K local candidates, this will produce S*K STUN candidates, where S is
the number of configured STUN servers.
To obtain UDP TURN candidates, the client takes a local UDP
candidate, and for each configured TURN server, produces a TURN
candidate. It is anticipated that clients may have a multiplicity of
TURN servers configured in network environments where there are
multiple layers of NAT, and that layering is known to the provider of
the client. To produce the TURN candidate from the local candidate,
it follows the procedures of Section 8 of [14] for each local
transport address in the local candidate. It initiates an Allocate
Request transaction from the local transport address to that server.
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The Allocate Response will provide the client with its TURN derived
transport address in the MAPPED-ADDRESS attribute. If the client had
K local candidates, this will produce S*K UDP TURN candidates, where
S is the number of configured TURN servers.
To obtain a TURN-derived TCP candidates, the client takes a local TCP
candidate, and for each configured TURN server, produces a TCP TURN
candidate. It is anticipated that clients may have a multiplicity of
TURN servers configured in network environments where there are
multiple layers of NAT, and that layering is known to the provider of
the client. To produce the TURN candidate from the local candidate,
it iterates through the local transport addresses in the local
candidate, and for for each one, initiates a TCP connection from the
same interface the local transport address to the TURN server. It is
not neccesary to initiate the connection from the actual port in the
local transport address. Following the procedures of Section 8 of
[14], it initiates an Allocate Request transaction over the
connection. The Allocate Response will provide the client with its
TCP TURN derived transport address in the MAPPED-ADDRESS attribute.
If the client had K local TCP candidates, this will produce S*K TCP
TURN candidates, where S is the number of configured TURN servers.
7.2 Encoding Candidates into SDP
For each candidate to be placed into the SDP, the agent includes a
series of a=candidate attributes as media-level attributes, one for
each transport address in the candidate. Each of the transport
addresses for the same candidate MUST have the same value of the
candidate-id attribute. The a=candidate attributes for different
candidates MUST be unique within that media stream. Using a simple
sequence number, incrementing by one for each candidate for a media
stream, meets these requirements. The transport, unicast-address and
port of the attribute are set to those for the candidate. The qvalue
is set to the priority of this candidate (note that, for RTP, the RTP
and RTCP transport addresses MUST have equal priority values). The
tid MUST be chosen randomly with 128 bits of randomness. The tid is
chosen only when the transport address is placed into the SDP for the
first time; subsequent offers or answers within the same session
containing that same transport address would use the same tid used
previously.
The tid serves as a unique identifier for each transport address. It
also gets combined, through concatenation, with the tid of a peer
candidate to form the username and password that is placed in the
STUN checks between the peers. This allows the STUN message to
uniquely identify the pairing whose connectivity it is checking. The
tid is needed as a unique identifier because the IP address within
the candidate fails to provide that uniqueness as a consequence of
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NAT.
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 transport addresses. In this case, thats 10.0.1.1:8866
and 8877. As it turns out, B is in a session at that same time, and
is also using 10.0.1.1:8866 and 8877. 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 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, tid
takes on the role of a unique identifier. C provides A with an
identifier for its transport address, and A provides one to C. A
concatenates these two identifiers and uses the result as the
username and password in its STUN query to 10.0.1.1:8866. This STUN
query arrives at B. However, the username is unknown to B, and so the
request is rejected. A treats the rejected STUN request as if there
were no connectivity to C (which is actually true). Therefore, the
error is avoided.
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 effected. Fortunately, since
the ports exchanged in SDP are ephemeral and ususally 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, because there are separate transport addresses for RTP and
RTCP, each will have a distinct tid.
The active candidate is placed into the m/c lines of the SDP. For
RTP streams, this is done by placing the RTP address and port into
the c and m lines in the SDP respectively. If the agent it utilizing
RTCP, it MUST encode its address and port 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 [8].
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For media streams that are inherently TCP-based (as opposed to ones
where TCP is a fallback and would be listed as a candidate but not
the initial active address), the connections MUST be signaled using
comedia [13], and those connections MUST be in "holdconn" mode. This
has the effect of suspending connection attempts via the comedia
mechanisms, allowing ICE to open the connections instead. These
connections then get removed from holdconn mode when the ICE
procedures complete and an updated offer/answer exchange takes place
that promotes one of the existing ICE-established connections to
active. Note that this has the result of increasing the post-dial-
delay for TCP-oriented media, but brings with it substantial security
and NAT traversal properties.
7.3 Prioritizing the Transport Addresses and Choosing an Active One
The prioritization process takes the set of candidates and associates
each with a priority. This priority reflects the desire that the
agent has to receive media on that address, and is assigned as a
value from 0 to 1 (1 being most preferred). Priorities are ordinal,
so that their significance is only meaningful relative to other
candidates for a particular media stream.
This specification makes no normative recommendations on how the
prioritization is done. However, some useful guidelines are
suggested on how such a prioritization can be determined.
One criteria for choosing one candidate over another is whether or
not that candidate involves the use of a relay. That is, if media is
sent to that candidate, will the media first transit a relay before
being received. TURN candidates make use of relays (the TURN
server), as do any local candidates associated with a VPN server.
When media is transited through a relay, 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 a relay run by the provider. If
these concerns are important, candidates with this property can be
listed with lower priority.
Another criteria for choosing one candidate over another 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) [24]. It can also help with hosts that have both a
native IPv6 address and a 6to4 address. In such a case, higher
priority could be afforded to the native v6 address, followed by the
6to4 address, followed by a native v4 address. This allows a site to
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obtain and begin using native v6 addresss 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 choosing one candidate over another 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.
Another criteria for choosing one address over another is topological
awareness. This is most useful for candidates which make use of
relays (including TURN and VPN). In those cases, if a agent has
preconfigured or dynamically discovered knowledge of the topological
proximity of the relays to itself, it can use that to select closer
relays with higher priority.
Finally, the transport protocol itself is a criteria for choosing one
candidate over another. If a particular media stream can run over
UDP or TCP, the UDP candidates might be preferred over the TCP
candidates. This allows ICE to use the lower latency UDP
connectivity if it exists, but fallback to TCP if UDP doesn't work.
Once the candidates have been prioritized, one is selected as the
active one. This is the candidate that will be used for actual
exchange of media, until replaced by an updated offer or answer.
Since the ICE connectivity checks can take a few seconds to execute,
media clipping can occur is this candidate doesn't work. The active
candidate will also be used to receive media from ICE-unaware peers.
As such, it is RECOMMENDED that one be chosen based on the likelihood
of that candidate to work with the peer that is being contacted.
Unfortunately, it is difficult to ascertain which candidate that
might be. As an example, consider a user within an enterprise. To
reach non-ICE capable agents within the enterprise, a local candidate
has 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, a
TURN-based candidate from a publically accessible TURN server is
needed.
Indeed, the difficulty in picking just one 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
the default address be a TURN candidate from a TURN server providing
public IP addresses. Furthermore, ICE is only truly effective when
it is supported on both sides of the session. It is therefore most
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prudent to deploy it to close-knit communities as a whole, rather
than piecemeal. In the example above, this would mean that ICE would
ideally be deployed completely within the enterprise, rather than
just to parts of it.
7.4 Connectivity Checks
Once the offer/answer exchange has completed, both agents will have a
set of candidates for each media stream. Each agent forms a set of
pairings for each media stream by combining each of its UDP
candidates with each of the UDP candidates of its peer, and by
combining each of its TCP candidates with each of the TCP candidates
of its peer. If candidates for other transport protocols were
signaled through the offer/answer exchange, a pairing is performed
between each of those as well. If an offer/answer exchange took
place for a session comprised of an audio and a video stream, and
each stream had two UDP and two TCP candidates from each agent, there
would be 16 pairings, 8 for audio and 8 for video. Each of those
eight would be comprised of four UDP and four TCP. Note that there
is no requirement that the number of candidates from each peer be the
same. One agent can offer two UDP candidates for a media stream, and
the answer can contain three UDP candidates for the same media
stream. In that case, there would be six UDP pairings.
Each candidate has a number of transport addresses. In the case of
RTP, there are either one or two. Within the pairing, the transport
addresses of each candidate are linked together one-to-one to form a
transport address pair. In the case of RTP, the result will either
be one or two transport address pairs - one for RTP, and possibly
another for RTCP. The relationship between a candidate, transport
address, pairing and transport address pair are shown in Figure 2.
This figure shows the pairing as seen by the agent that owns the
candidate {A,B}. The candidate owned by that agent is called the
native candidate, and the one owned by its peer is the remote
candidate. As the figure shows, there is one pairing between two
candidates, and two transport address pairs ({A,C} and {B,D}). If
one of the candidates only had one transport address (in the case
where RTCP was not being used by one agent), there would only be one
transport address pair, {A,C}. Each transport address is associated
with a tid. Furthermore, each transport address pair is associated
with an ID, the transport address pair ID. This ID is equal to the
concatenation of the tid of the native transport address with the tid
of the remote transport address. This means that the identifiers are
different for each agent. For the agent that owns {A,B}, the
transport address pair ID is WY for the first transport address pair,
and XZ for the second. For the agent that owns {C,D}, it would be
reversed - YW for the first transport address pair, and ZX for the
second.
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...........................................
. .
.......... . . ..........
. . . ............. ............. . . .
. . . . . . . . . .
. -- . . . -- . . -- . . . -- .
. | A|<<<<<<<<<<| A|--------------------| C|>>>>>>>>>>>>| K| .
. -- . . . -- . Transport . -- . . . -- .
. . . . Transport . Address . Transport . . . .
. . . . Address . Pair . Address . . . .
. . . . tid=W . ID=WY . tid=Y . . . .
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
. -- . . . -- . . -- . . . -- .
. | J|<<<<<<<<<<| B|--------------------| D|>>>>>>>>>>>>| D| .
. -- . . . -- . Transport . -- . . . -- .
.......... . . Transport . Address . Transport . . ..........
Associated . . Address . Pair . Address . . Associated
Local . . tid=X . ID=XZ . tid=Z . . Local
Transport . . . . . . Transport
Addresses . ............. ............. . Addresses
. Native Remote .
. Candidate Candidate .
. and and .
. Transport Addresses Transport Addresses .
. .
...........................................
Pairing
Figure 2
The figure also shows that each transport address has an associated
local transport address. The associated local transport address is
the local transport address at which the agent will receive packets
sent to the transport address. For a local transport address, its
associated local transport address is the same. That is the case of
transport address A and D in the diagram. For STUN derived and TURN
derived transport addresses, however, they are not the same. The
associated local transport address is the one from which the STUN or
TURN transport was derived.
Next, each agent begins sending connectivity checks for each
transport address pair. The procedure differs for UDP and TCP.
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7.4.1 UDP Connectivity Checks
An agent considers a UDP pairing validated when all of its transport
address pairs have been validated. Each transport address pair is
validated if an agent successfully completed a STUN Binding Request
transaction from its native transport address to the corresponding
remote transport address, and when it has received a STUN Binding
Request transaction on its native transport address, sent from the
remote transport address. This ensures that packets can flow in each
direction.
Because validation of a transport address pair involves a STUN
transaction in each direction, a pair can be in one of five states -
unknown, invalid, send-valid, receive-valid and valid. Each
transport address pair starts in the unknown state.
7.4.1.1 Send Validation
To validate a transport address pair in the send direction, an agent
needs to complete a successful STUN Binding Request transaction.
This means it needs to send a Binding Request from its native
transport address to the remote transport address, and receive a
successful Binding Response back.
For UDP-based transport addresses, an agent initiates a STUN Binding
Request transaction by sending from its native transport address, and
sends it to the remote transport address. The meaning of "sending
from its native transport address" is clear in the case of a local
transport address - the request is sent such that the source IP
address and port of the packet is equal to that local transport
address. However, the meaning is different for STUN and TURN derived
transport addresses. For STUN derived transport address, it is sent
by sending from the local transport address used to derive that STUN
address. For TURN derived transport addresses, it is sent by using
TURN mechanisms to send the request through the TURN server (using
the SEND primitive). Sending the request through the TURN server
neccesarily requires that the request be sent from the client, using
the local transport address used to derive the TURN transport
address.
The Binding Request sent by the agent MUST contain the USERNAME
attribute. This attribute MUST be set to the transport address pair
ID of the corresponding transport address pair as seen by its peer.
Thus, for the first transport address pair in the example above, if
the agent on the left sends the STUN Binding Request, the USERNAME
will have the value YW. The request MAY contain the MESSAGE-
INTEGRITY attribute, computed according to RFC 3489 procedures. The
MESSAGE-INTEGRITY The Binding Request MUST NOT contain the CHANGE-
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REQUEST or ANSWER-ADDRESS attribute.
Each of these STUN transactions will generate either a timeout, or a
response. If the response is a 420, 500, or 401, the agent should
try again as described in RFC 3489. Either initially, or after such
a retry, the STUN transaction might produce a non-recoverable failure
response (error codes 400, 431, or 600) or a failure result
inapplicable to this usage of STUN and thus unrecoverable (432, 433).
If this happens the transport address pair and its corresponding
candidate is considered invalid. If the STUN transaction produces a
430 error or times out, the client SHOULD retry with a new STUN
Binding Request transaction. The 430 response code, as described
below, is generated when the server doesn't recognize the STUN
username because the BindingRequest was sent received prior to the
receipt of the answer. Its ocurrence is a result of a failed race
between the BindingRequest and the answer. This is remedied by
retrying, which allows the "slower" answer to be received. These
retry transactions carry the same USERNAME value as the original
Binding Request, and differ only in their STUN transaction ID. If
these retries have not produced a success response after Tg seconds,
the transport address pair is considered invalid. Tg SHOULD be
configurable. It is RECOMMENDED that it default to 50 seconds. This
is a reasonable approximation of the maximum SIP transaction
duration.
If the STUN transaction succeeds for a UDP transport address pair
(producing a success response), and the pair was previously in the
receive-valid state, it is considered valid. If the pair was
previously in the unknown state, it is considered send-valid.
If a transport address pair is send-valid or valid, an agent MUST
generate a new STUN Binding Request transaction every Tr seconds.
This transaction ensures that NAT bindings for the transport address
pair remain open while the candidate is under consideration. They
can also be used to keep the bindings alive when the candidate is
promoted to active, as described in Section 7.7. Tr SHOULD be
configurable, and SHOULD default to 15 seconds. Each new Binding
Request transaction is processed according to the procedures in this
Section. It is possible for a previously valid candidate to later be
invalidated by a subsequent STUN transaction. This happens in cases
where the NAT bindings expire.
7.4.1.2 Receive Validation
As a result of providing a list of candidates in its offer or answer,
an ICE implementation will receive STUN Binding Request messages. An
agent MUST be prepared to receive STUN Binding Requests on each local
transport address from the moment it sends an offer or answer that
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contains a candidate with that local transport address. Similarly,
it MUST be prepared to receive STUN Binding Requests on a local
transport address the moment it sends an offer or answer that
contains a STUN or TURN candidate derived from a local candidate
containing that local transport address. It can cease listening for
STUN messages on that local transport address after reliably sending
an updated offer or answer which does not include any candidates
equal to or derived from that local transport address. Here,
"reliably" means that the agent knows that the offer or answer was
received by its peer. This knowledge is based on the protocol
carrying the offer/answer exchanges. In the case of SIP, if the
offer is in an INVITE, the agent knows this was received by its peer
when a 200 OK or reliable provisional response [9] is received with
the answer. If the offer is in a reliable provisional response, the
agent knows it was reliably received when the PRACK arrives. If an
answer is in a 200 OK response, the agent knows this was received
when the ACK is received.
The agent does not need to provide STUN service on any other IP
address or port, unlike the STUN usage described in [1]. The need to
run the service on multiple ports is to support the change flags.
However, those flags are not needed with ICE, and the server SHOULD
reject, with a 400 answer, any STUN requests with these flags set.
The CHANGED-ADDRESS attribute in a BindingAnswer is set to the
transport address on which the server is running.
Furthermore, there is no need to support TLS or to be prepared to
receive SharedSecret request messages. Those messages are used to
obtain shared secrets to be used with BindingRequests. However, with
ICE, a shared secret is not needed. The tid's that are exchanged and
used to form the STUN USERNAME attribute do not actually require the
security properties associated with a shared secret in order for ICE
to operate securely; this is because ICE security is bootstrapped off
of the protocol carrying the offer/answer exchanges.
One of the candidates will be in use as the active candidate. For
the transport addresses comprising that candidate, the agent will
receive both STUN requests and media packets on its associated local
transport addresses. The agent MUST be able to disambiguate them.
In the case of RTP/RTCP, this disambiguation is easy. RTP and RTCP
packets start with the bits 0b10 (v=2). The first two bits in STUN
are always 0b00. This disambiguation also works for packets sent
using Secure RTP [23], since the RTP header is in the clear.
Disambiguating STUN with other media stream protocols may be more
complicated. However, it can always be possible with arbitrarily
high probabilities by selecting an appropriately random username (see
below).
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The STUN Binding Request can only be usefully processed once an
offer/answer exchange has completed. As a result, if an offeror
receives a STUN Binding Request message prior to the receipt of an
answer to its offer, it MUST reject the request with a 430 response.
This will cause the answerer to retry, and give time for the answer
(which is in transit) to arrive at the offerer.
If the offer/answer exchange has completed, the agent MUST follow the
procedures defined in RFC 3489 and verify that the USERNAME attribute
is known to the server. Here, this is done by taking the USERNAME
attribute, and comparing it against the transport address pair
identifiers for each transport address pair as seen by that agent.
If there is no match, the STUN Binding Request generates a 400. If
there is a match, the resulting transport address pair is called the
matching transport address pair. The user agent proceeds with the
processing of the request and generation of a response as per RFC
3489. In addition, the if the state of that transport address pair
was previously unknown, it changes to receive-valid. If the state
was previously send-valid, it moves to valid.
An agent will continue to receive periodic STUN transactions as long
as it had listed its transport address in an a=candidate attribute.
It MUST process those transactions according to this section. It is
possible that a transport address pair that was previously valid may
become invalidated as a result of a subsequent failed STUN
transaction.
7.4.1.3 Learning New Candidates from Connectivity Checks
ICE makes use of candidate addresses learned through protocols like
STUN, as described in Section 7.1. These addresses are learned when
STUN requests are sent to configured STUN servers. However, the
peer-to-peer STUN connectivity checks can themselves provide
additional candidates that ICE can make use of. This happens when
two agents are separated by a symmetric NAT. When the agent behind
the symmetric NAT sends a Binding Request to the other agent (which
can have a public address or be behind any type of NAT except for
symmetric), the symmetric NAT will create a new NAT binding for this
Binding Request. Because of the properties of symmetric NAT, that
binding can be used be the agent on the public side of the symmetric
NAT to send packets back to the agent behind the symmetric NAT.
To do this, ICE agents dynamically learn new candidates by examining
the source IP addresses and MAPPED-ADDRESS attributes in STUN Binding
Requests and Responses respectively. If they don't match any
existing candidates, a new candidate is added. This candidate
corresponds to the new IP address and port created by the symmetric
NAT, and is a new point of contact for the agent behind the symmetric
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NAT. Since that candidate is only reachable from the very specific
IP address and port where the STUN request was sent to, the new
candidate is paired up with that transport address on the other
agent. Since all candidates need to have properties, such as tids,
priorities and candidate IDs, these are all computed algorithmically,
so that they can be determined by both agents just from the STUN
message.
The specific procedures on receipt of a Binding Request and Response
for accomplishing this are described here.
7.4.1.3.1 On Receipt of a Binding Request
When a STUN Binding Request is received which generates a success
response, the source IP address and port of that request is compared
all existing remote transport addresses. If there is no match, the
agent creates a new remote candidate, and adds a transport address to
it. It sets the IP address and port of this new remote transport
address to the IP address and port that was present in the incoming
Binding Request. Since this is a new candidate transport address, it
requires a new tid. The agent creates one algorithmically, by
concatenating the tid of the remote transport address in the matching
transport address pair (recall that the matching transport address
pair is the one whose transport address pair ID matched the username
of the incoming Binding Request) with the string representation of
the source IP address and port from the incoming Binding Request.
This string representation is defined using the grammar for
"hostport" from RFC 3261 [3], which defines the familiar notation of
the IP address and port separated by a colon.
The priority of the new candidate MUST be set to the priority of the
remote candidate in the matching transport address pair. There is no
need to compute the candidate ID for this new candidate.
Though this is a valid transport address, the agent does not pair it
up with each of its own transport addresses. Rather, it pairs it up
only with the native transport address from the matching transport
address pair. This creates a new transport address pair. Since
connectivity has been verified in the receive direction, the agent
sets its state to receive-valid. As with all other transport address
pairs, the agent will attempt to validate send capabilities by
sending a STUN Binding Request according to the procedures in
Section 7.4.1.1.
It is important to note that this process creates a new remote
transport address, not a whole new remote candidate. For a whole
remote candidate to come into existence, all of its component
transport addresses must come into existence, and all must have been
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obtained as a result of a STUN Binding Requests between transport
address pairs in the same pairing. As an example, consider the
pairing in Figure 2. If the peer is behind a symmetric NAT, the
Binding Request sent from C to A might produce a new remote transport
address for RTP. To create a full candidate, a STUN Binding Request
from D to B has to also create a new remote transport address, to be
used for RTCP. If this were to happen, the resulting set of
relationships is shown in Figure 3. To simplify the diagram,
associated local transport address relationships have been omitted.
Notice how the tids of the new remote candidate have been constructed
by concatenating the tids of the original remote candidate with the
newly discovered transport addresses, here, {R,S}.
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............. .............
. . . .
. -- . . -- .
. | A|---------------------------------------| C| .
. -- -----------+ Transport . -- .
. Transport . | Address . Transport .
. Address . | Pair . Address .
. tid=W . | ID=WY . tid=Y .
. . | . .
. . | . .
. . | . .
. -- . | . -- .
. | B|-----------C---------------------------| D| .
. -- ---------+ | Transport . -- .
. Transport . | | Address . Transport .
. Address . | | Pair . Address .
. tid=X . | | ID=XZ . tid=Z .
. . | | . .
............. | | .............
| | remote
native | | candidate
candidate | |
| | .............
| | . .
| | . -- .
| +---------------------------| R| .
| Transport . -- .
| Address . Transport .
| Pair . Address .
| ID=WYR . tid=YR .
| . .
| . .
| . .
| . -- .
+-----------------------------| S| .
Transport . -- .
Address . Transport .
Pair . Address .
ID=XZS . tid=ZS .
. .
.............
peer-derived
remote candidate
Figure 3
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7.4.1.3.2 On Receipt of a Binding Response
When an agent receives a successful Binding Response, it examines the
MAPPED-ADDRESS attribute in that response. If the MAPPED-ADDRESS
does match any of the existing candidate transport addresses, this
represents a new peer-derived transport address.
The agent creates a new local candidate, and adds a transport address
to it. It sets the IP address and port of this new native transport
address to the IP address and port that was present in the MAPPED-
ADDRESS attribute of the Binding Response. Since this is a new
candidate transport address, it requires a new tid. The agent
creates one algorithmically, by concatenating the tid of the native
transport address in the transport address pair that was being
validated by the Binding Request with the string representation of
the source IP address and port from the MAPPED-ADDRESS attribute.
This string representation is defined using the grammar for
"hostport" from RFC 3261 [3], which defines the familiar notation of
the IP address and port separated by a colon.
The priority of the new candidate MUST be set to the priority of the
native candidate that was being validated by the Binding Request.
The agent SHOULD assign a new candidate ID to this candidate.
Though this is a valid transport address, the agent does not pair it
up with each of the remote transport addresses. Rather, it pairs it
up only with the remote transport address from the transport address
pair that was being validated. This creates a new transport address
pair. Since connectivity has been verified in the send direction,
the agent sets its state to send-valid. As with all other transport
address pairs, the agent will attempt to validate receive
capabilities by waiting for a a STUN Binding Request according to the
procedures in Section 7.4.1.2.
It is important to note that this process creates a new native
transport address, not a whole new candidate. For a whole native
candidate to come into existence, all of its component transport
addresses must come into existence, and all must have been obtained
as a result of a STUN Binding Requests between transport address
pairs in the same pairing.
7.4.2 TCP Connectivity Checks
7.4.2.1 Connection Establishment
Because of the connection-oriented nature of TCP, the connectivity
checks work differently. After the offer/answer exchange completes,
each agent will have a set of TCP candidates at which it is waiting
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to receive a connection on, and it will have a similar set from its
peer. Thus, a pairing of TCP candidates allows for the possibility
of TCP connections in each direction. Unlike the UDP checks, where
the STUN packets are sent from the native transport addresses to the
remote ones, the TCP connections are not opened from the native TCP
transport addresses to the remote ones. This would represent a
simultaneous open, and represent an unusual condition that would
either fail, or at best result in a single TCP connection. Rather,
ICE desires to attempt two connections, one in each direction, and
use one of them if both happen to succeed.
To accomplish this, each agent will attempt to open a connection to
each remote transport address in the transport address pair, and do
so "from" its native transport address. Here, however, "from" means
something different than the UDP case. If the native transport
address is a local transport address, the agent opens the TCP
connection from the same IP interface used to obtain the local
transport address, but from a different and ephemeral port. Indeed,
that port MUST NOT be the same as the port in the local transport
address. If the native transport address is a TURN-derived TCP
transport address, no attempt is made to open a connection at all.
TURN-derived TCP transport addresses can only be used in passive
mode.
As such, for each TCP transport address pair, there will be either
zero, one, or two connection attempts. If the transport address
pairs are both TURN-derived, there will be zero (both sides passive).
If one of the transport addresses is local, and the other TURN
derived, there will be one connection attempt. The agent owning the
local transport address will be in active mode, and the agent owning
the TURN-derived one will be in passive mode. If both are local
transport address, there will be two attempts, and each agent will
act in active mode.
Because a transport address pair can produce multiple connections,
validity becomes a property of the TCP connection itself. A
transport address pair is considered valid if at least one valid
connection has been established within it. An entire pairing is
valid if all transport address pairs are valid.
7.4.2.2 Sending STUN Binding Requests
Once the connection is established, the agent which opened the
connection (that is, acted in active mode) sends a STUN Binding
Request over that connection. STUN Binding Requests as described in
RFC 3489 are not normally sent over UDP, but when used in conjunction
with ICE for connectivity checks, they are sent over TCP.
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This unusual operation requires some explanation. At first glance, a
successful TCP connection ought to be sufficient. Clearly,
connectivity is established, as TCP packets were exchanged in both
directions via the TCP handshake. While that is true, the STUN
Binding Requests serve many purposes, only one of which is to
literally test connectivity. The STUN requests also serve as a
correlation vehicle, allowing the agent to match the source of a
connection attempt with the offer/answer signaling driving the entire
mechanism. For example, in the case of a forked SIP INVITE carrying
an offer, the UAC may receive two connection attempts to each of its
passive TCP addresses, one from each branch of the fork. These are
readily disambiguated by the STUN Binding Request which will follow,
as the tid in the USERNAME tells the UAC which branch has initiated
the connection.
More importantly, however, the STUN Binding Request is an essential
part of the security properties of ICE. Without it, an entity
eavesdropping the signaling messages would be able to deny service or
hijack media connections, and such attacks would require encryption
of the offer/answer exchanges (using a mechanism like SIPS [3]) to
prevent. However, when a STUN Binding Request exchange is added,
these attacks are completely foiled without the need for SIPS,
raising the overall security of ICE substantially with minimal cost.
These properties of ICE are discussed thoroughly in Section 12.
As such, once an agent has actively opened a TCP connection to the
remote agent, it sends a STUN Binding Request over that connection.
Recall that STUN messages include length indicators, allowing them to
be framed over a connection-oriented transport protocol. The Binding
Request MUST contain the USERNAME attribute. This attribute MUST be
set to the transport address pair ID of the corresponding transport
address pair as seen by its peer. Thus, for the first transport
address pair in Figure 2, if the agent on the left sends the STUN
Binding Request, the USERNAME will have the value YW. The request
MAY contain the MESSAGE-INTEGRITY attribute, computed according to
RFC 3489 procedures. The MESSAGE-INTEGRITY The Binding Request MUST
NOT contain the CHANGE-REQUEST or ANSWER-ADDRESS attribute. The STUN
BindingRequest message SHOULD NOT be retransmitted over the
connection.
The STUN will generate either a timeout, or a response. If the
response is a 420, 500, or 401, the agent should try again as
described in RFC 3489. Either initially, or after such a retry, the
STUN transaction might produce a non-recoverable failure response
(error codes 400, 431, or 600) or a failure result inapplicable to
this usage of STUN and thus unrecoverable (432, 433). If this
happens the connection is considered invalid. If the STUN
transaction produces a 430 error or times out, the client SHOULD
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retry with a new STUN Binding Request transaction. The 430 response
code is a result of a failed race between the BindingRequest and the
answer. This is remedied by retrying, which allows the "slower"
answer to be received. These retry transactions carry the same
USERNAME value as the original Binding Request, and differ only in
their STUN transaction ID. If these retries have not produced a
success response after Tg seconds, the connection is considered
invalid. Tg SHOULD be configurable. It is RECOMMENDED that it
default to 50 seconds. This is a reasonable approximation of the
maximum SIP transaction duration.
If the STUN Binding Request generates a successful response, the
connection over which it was sent is considered valid. Furthermore,
the agent stores the IP address and port from the MAPPED-ADDRESS
response in the STUN Binding Response. This is called the "apparent"
native transport address for the active side of the connection. It
will be used later if this connection is used for media transport.
Once a connection is valid, the agent which initiated the connection
MUST generate a new STUN Binding Request transaction every Tr
seconds. This transaction ensures that NAT bindings for the
connection remain open while the connection is under consideration as
a candidate. Tr SHOULD be configurable, and SHOULD default to 15
seconds. Each new Binding Request transaction is processed according
to the procedures in this section. It is possible for a previously
valid candidate to later be invalidated by a subsequent STUN
transaction. This happens in cases where the NAT bindings expire.
Note that, unlike the UDP case, STUN is sent only while a connection
is is not active for media. If the connection is used as the active
connection for media, STUN MUST NOT be sent.
7.4.2.3 Receiving STUN Requests
When an agent acted as the passive side of a TCP connection, it will
receive a STUN Binding Request over that connection.
One of the candidates will be in use as the active candidate. For
the transport addresses comprising that candidate, the agent will
receive both STUN requests and media packets on its associated local
transport addresses. The agent MUST be able to disambiguate them.
In the case of RTP/RTCP, this disambiguation is easy. RTP and RTCP
packets start with the bits 0b10 (v=2). The first two bits in STUN
are always 0b00. This disambiguation also works for packets sent
using Secure RTP [23], since the RTP header is in the clear.
Disambiguating STUN with other media stream protocols may be more
complicated. However, it can always be possible with arbitrarily
high probabilities by selecting an appropriately random username (see
below).
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The STUN Binding Request can only be usefully processed once an
offer/answer exchange has completed. As a result, if an offeror
receives a STUN Binding Request message prior to the receipt of an
answer to its offer, it MUST reject the request with a 430 response.
This will cause the answerer to retry, and give time for the answer
(which is in transit) to arrive at the offerer.
If the offer/answer exchange has completed, the agent MUST follow the
procedures defined in RFC 3489 and verify that the USERNAME attribute
is known to the server. Here, this is done by taking the USERNAME
attribute, and comparing it against the transport address pair
identifiers for each transport address pair as seen by that agent.
If there is no match, the STUN Binding Request generates a 400. If
there is a match, the resulting transport address pair is called the
matching transport address pair. The user agent proceeds with the
processing of the request and generation of a response as per RFC
3489. In addition, the agent stores the source IP address and port
of the Binding Request, and associates it with the connection. This
address is called the "apparent" remote transport address for this
connection.
An agent will continue to receive periodic STUN transactions as long
as it had listed its transport address in an a=candidate attribute.
It MUST process those transactions according to this section. It is
possible that a transport address pair that was previously valid may
become invalidated as a result of a subsequent failed STUN
transaction.
Note that, unlike the UDP case, there will never be simultaneous
transmission of media and STUN packets over TCP connections. This is
because the connection is listed as on hold according to comedia
procedures, and no media will be transmitted. ICE will establish the
connections as described here. Once established, an updated offer/
answer exchange can promote those connections to active usage through
the comedia "exist" mechanism, as described below. The additional
offer/answer exchange provides a barrier synchronization point at
which a TCP connection switches from ICE control to control by the
media source and sinks. Once it is active, STUN packets will no
longer be sent on the connection.
7.5 Promoting a Valid Candidate to Active
7.5.1 Minimum Requirements
As the STUN connectivity checks run, they will result in the
validation of pairings. Once validated, a pairing can be used by
promoting it to active. This promotion occurs by placing the
transport addresses for the native candidate of the pairing into the
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m/c line and sending an updated offer. It MAY promote a candidate
associated with any validated pairing at any time, as long as the
candidate had been provided in series of a=candidate attributes in
the most recent offer (in other words, an agent can't validate a
candidate, omit that candidate from the a=candidate attribute of an
offer, and then later on, generate a new offer that promotes the
candidate to active). The procedures for doing so are described
here.
Any candidates which the agent would like to retain as valid
candidates are also included in a=candidate lines in the offer. It
SHOULD include any candidates learned from the peer-to-peer discovery
processing of Section 7.4.1.3, and SHOULD include any candidates of
higher priority than the one just promoted to active. It SHOULD omit
candidates of lower priority than the one being promoted to active.
It SHOULD omit any for whom all pairings that include that candidate
have become invalid.
If a candidate is omitted, and that candidate was a TURN-derived
transport address, the agent SHOULD de-allocate the address from the
TURN server. If a local candidate was omitted, along with all of its
derived transport addresses, local operating system resources for
that candidate SHOULD be de-allocated.
Once it has decided on the set of candidates to provide in the
updated offer, the agent constructs the offer and follows the
procedures in Section 7.6 which defines general subsequent offer/
answer processing.
7.5.2 Suggested Algorithm
ICE leaves substantial variability to implementors around when an
agent decides to generate a new offer. However, there are good ways
to do this, and bad ways. Perhaps the worst algorithm possible would
be to generate a new offer every time a candidate with higher
priority than the active one becomes valid. This algorithm will
likely result in a large number of offer/answer exchanges in rapid
succession, many of which will produce "glare" as each agent will
independently initiate an exchange. This will consume CPU and
network resources for little benefit. Rather, the ideal algorithm
strikes a balance between usage of network resources and the desire
to use the ideal pair of candidates.
The following algorithm provides a good tradeoff, and usage of this
algorithm is RECOMMENDED. The algorithm results in a bounded number
of additional offer/answer exchanges after the initial one - never
more than two, and frequently one or zero. The algorithm almost
never produces a glare condition.
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Once the initial offer/answer exchange completes, media flow will
happen, though not optimally (where optimal is defined by the
policies used to set the priorities of the candidates), as long as
the candidate that is active has been validated. Thus, the objective
of the algorithm is to quickly make sure that there is a valid path
for media (to avoid clipping), and then do a single offer/answer
exchange to use the highest priority pairing that was validated.
After the initial offer/answer exchange, each agent sets a timer Tu.
This timer SHOULD have a configurable baseline value, which SHOULD
default to 3 seconds. The actual timer is set to this baseline, plus
a time value chosen uniformly beween -1 and 1 seconds. This causes
the actual timer to be randomized so that the timer doesnt fire
simultaneously at each agent. In addition, each agent monitors the
status of the active pairing. If the active media stream is UDP-
based, the status of the active candidates is equal to the status of
the pairing with matching transport addresses. In the case of TCP-
based media, the active media stream is never active initially, since
it always begins with the "holdconn" state.
If, when Tu fires, the active pairing has not been validated, and
there exists at least one pairing that has been validated, the agent
generates a new offer. This offer promotes its highest priority
candidate with a validated pairing to the active candidate. If there
are no pairings that have been validated when the timer fires, the
agent waits until one is validated, and once that happens, sets a
timer to fire randomly between 0 and 2 seconds. When the timer
fires, a new offer is generated that promotes the candidate from this
validating pairing to active. If the active pairing is validated
when the timer fires, the agent does nothing at this time.
If new offer is to be sent, the agent includes the new active
candidate in the a=candidate attribute list. It also includes all
candidates with higher priority than the one that is active,
including ones it learned from the connectivity checks themselves.
At this point, media is flowing successfully, since a valid candidate
is active. However, it may not be optimal. So, the next stage of
the algorithm is to let the connectivity checks continue. If those
checks indicate that a pairing between the two highest priority
candidates from both agents has been validated, each agent sets a
timer whose value is randomly set between 0 and 2 seconds. When the
timer fires, a new offer is generated that promotes the candidate
from this validating pairing to active. Otherwise, when the
connectivity checks have all concluded, such that no pairing exists
in the invalid state, each agent sets a timer whose value is randomly
set between 0 and 2 seconds. When the timer fires, a new offer is
generated that promotes the candidate from the valid pairing with the
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highest priority to active.
7.6 Subsequent Offer/Answer Exchanges
An offer/answer exchange within a session can occur at any time,
whether it is the result of the algorithm described in Section 7.5.2,
or because one of the agents wishes to add or remove a media stream,
or add a codec, and so on.
7.6.1 Sending of an Offer
The meaning of a=candidate attributes within a subsequent offer have
the same meaning they do in an initial offer. They are a request for
the peer to attempt (or continue to attempt if the candidate was
provided previously) a connectivity check using STUN from each of its
own candidates. As such, an a=candidate attribute is included in
subsequent offers when (1) connectivity checks haven't concluded yet
to that candidate, or (2) the checks have concluded, and the
candidate is currently active. In that case, STUN is used to keep
the bindings active.
If an agent sends an offer which omits candidates it had sent to its
peer previously, it MUST cease connectivity checks from that
candidate. Any pairings that include the absent native candidate are
discarded. Any STUN transactions in progress from that candidate are
immediately terminated - no further retransmissions take place, and
no further transactions from that candidate will be made. If a TCP
connection was opened to or from that candidate, and that connection
is not listed as the active one in the offer, the connection is torn
down.
The offer MAY contain a new active candidate in the m/c line. If the
new active transprot address is UDP, candidate is encoded into an
update offer as described in Section 7.2. The transport addresses
constituting the candidate SHOULD also be listed in a=candidate
attributes, so that STUN can be used as an ongoing keepalive.
If the new active transport address is TCP, it is more complicated.
Recall that each TCP connection is opened from one of the agents to
the other, such that, for each connection, one agent has the active
role, and the other, the passive. The ICE mechanisms allow the
active agent to actually choose a specific connection for use in an
offer, so long as the agent has used a different ephemeral port for
each connection it initiated (which is almost always the case). If,
however, an agent was in the passive role, it cannot choose a
specific connection. Rather, it can choose a specific native
transport address which may have been used to receive multiple
connections. This assymetric behavior brings with it some important
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security properties, which are discussed in Section 12.
If the agent was the active one and established the connection, it
includes its apparent native transport address in the m/c line of the
SDP (recall that this address was discovered via the STUN exchange
over the connection). Note that this is instead of the SHOULD-
strength recommendation in comedia, which recommends that the port
number sent by the entity which initiated the connection should be
'9'. The actual port number is present to facilitate identification
of the connection. The a=setup attribute MUST be present and MUST
contain the value "active". The a=connection attribute MUST be
present and MUST have the value of "existing".
If the agent was the passive one and was the recipient of the
connection, it includes its transport address in the m/c line of the
SDP. In this case, that address will be the same as the one it had
placed into the a=candidate line of the SDP. The a=setup attribute
MUST be present and MUST contain the value of "passive". The
a=connection attribute MUST be present and MUST have the value of
"existing".
7.6.2 Receiving the Offer and Sending an Answer
If an agent receives an updated offer with a=candidate attributes, it
checks to see if it already knows about the listed candidates. This
is done by comparing the tid with the candidates it had received in
the previous offer or answer from the peer. If the tid is already
known, processing for that candidate continues as if no offer had
been made. Any connectivity checks in progress continue, and any
ongoing STUN keepalives continue.
If a candidate which had been listed previously is no longer present
in the offer, this tells the answerer to cease connectivity checks.
Any pairings that include the absent remote candidate are discarded.
Any STUN transactions in progress to that candidate are immediately
terminated - no further retransmissions take place, and no further
transactions to that candidate will be made. If a TCP connection was
opened to or from that candidate, and that connection is not listed
as the active one in the offer, the connection is torn down.
The agent then sends its answer. Like the offerer, it can add or
remove candidates from its answer. If it removed candidates from its
answer, it ceases STUN connectivity checks from those candidates, and
any pairings that include those candidates are discarded. Any STUN
transactions in progress to that candidate are immediately terminated
- no further retransmissions take place, and no further transactions
to that candidate will be made. If a TCP connection was opened to or
from that candidate, and that connection is not listed as the active
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one in the answer, the connection is torn down.
After transmission of the answer, there may be a set of candidates
which were new in the offer, and a set that were new in the answer.
The agent begins connectivity checks as described in Section 7.4,
pairing each new candidate in its answer with all candidates in the
offer, and each new candidate in the offer with all of its candidates
in the answer.
The m/c line may have also changed, indicating a new active
candidate. If the m/c line contains a UDP stream, the agent begins
sending media to the transport addresses listed there. In addition,
it checks to see if those transport addresses correspond to a remote
candidate in a valid pairing. So long as the remote agent has
offered up a candidate that has been validated by ICE, it should be
the case. Indeed, there may be a multitude of valid pairings
containing the transport addresses in the m/c line as the remote
candidate. In that case, the agent MUST choose the pairing whose
native candidate has the highest priority. It MUST place this
candidate in the m/c line. Transmission of media occurs as defined
in Section 7.8.
If the m/c line has changed, and now indicates a new TCP candidate,
the agent examines it. The comedia "a=connection" attribute will
normally be present and normally contain the value of "existing". If
not present, or if present but with a value of "new", comedia process
is followed, as apparently the peer has abandoned ICE operation for
this media stream. Assuming it contains a value of "existing", the
agent looks at whether the a=setup attribute is present. If its
value is "active", it means that a connection that was initiated by
the remote agent is to be used. The agent examines the transport
address in the m/c line. It looks for a matching value in the
apparent remote transport addresses of existing connections. If it
matches multiple connections (though it should normally match just
one), one of those connections is chosen. The native transport
address of that connection is then placed into the m/c line of the
answer. If no existing connections where matched, an error has
occured. The agent SHOULD respond with "holdconn", and then generate
its own offer with a connection to the peer which it believes is
valid.
If the a=setup attribute had a value of "passive", it means that a
connection that was initiated by the agent itself is to be used. The
agent examines the transport address in the m/c line. It looks for a
matching value amongst the remote transport addresses in valid
pairings. If multiple pairings match, it MUST choose the one whose
native transport address has the highest priority. The apparent
native transport address associated with an active connection
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initiated by the agent is then placed into the m/c line, and that TCP
connection is used to send and receive media. If no pairings match,
an error has occured. The agent SHOULD respond with "holdconn", and
then generate its own offer with a connection to the peer which it
believes is valid.
7.6.3 Receiving the Answer
If an agent receives an answer with a=candidate attributes, it checks
to see if it already knows about the listed candidates. This is done
by comparing the tid with the candidates it had received in the
previous offer or answer from the peer. If the tid is already known,
processing for that candidate continues as if no offer had been made.
Any connectivity checks in progress continue, and any ongoing STUN
keepalives continue.
If a candidate which had been listed previously is no longer present
in the answer, this tells the offerer to cease connectivity checks.
Any pairings that include the absent remote candidate are discarded.
Any STUN transactions in progress to that candidate are immediately
terminated - no further retransmissions take place, and no further
transactions to that candidate will be made. If a TCP connection was
opened to or from that candidate, and that connection is not listed
as the active one in the answer, the connection is torn down.
Furthermore, there may be a set of candidates which were new in the
offer, and a set that were new in the answer. The agent begins
connectivity checks as described in Section 7.4, pairing each new
candidate in its offer with all candidates in the answer, and each
new candidate in the answer with all of its candidates in the offer.
The m/c line may have also changed, indicating a new active
candidate. If the m/c line contains a UDP stream, the agent begins
sending media to the transport addresses listed there as defined in
Section 7.8. It will send from the m/c line it had signaled in the
offer.
If the m/c line has changed, and now indicates a new TCP candidate,
the agent examines it. If the agent had, in its offer, indicated the
desire to use a specific connection that it had initiated, it would
have used the a=connection attribute with the value of "existing",
and the a=setup attribute with the value of "active", and have placed
its apparent native transport address in the m/c line. In that case,
the m/c line in the answer will normally have the a=connection
attribute with the value "existing", which means that the remote
agent agrees with the usage of that connection. The transport
addresses in the m/c line should correspond to the remote transport
addresses that the agent had initiated its connection to. If so,
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that connection is used.
If the agent had, in its offer, indicated the desire to use any
connection that had been established to a specific native transport
address, it would have, in its offer, used the a=connection attribute
with the value of "existing" and the a=setup attribute with the value
of "passive", and placed that address in the m/c line. In that case,
the m/c line in the answer will normally have the a=connection
attribute with the value of "existing" and the a=setup attribute with
the value of "active". The transport address in the m/c line will
correspond to the apparent remote transport address. The agent MUST
scan its existing connections to the native transport address it had
advertised in the offer, and find the one whose apparent remote
transport address matches the m/c line in the answer. If there is a
match, that connection is used for sending media. If there is no
match, an error has occurred.
7.7 Binding Keepalives
Once the candidates are promoted to active, and media begins flowing,
it is still necessary to keep the bindings alive at intermediate NATs
for the duration of the session. Normally, the RTP packets
themselves 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 [28], 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.
To prevent these problems, ICE implementations MUST continue to list
their active transport addresses as candidates in a=candidate lines.
As a consequence of this, STUN packets will be transmitted
periodically independently of the transmission (or lack thereof) of
media packets. This provides a media independent, RTP independent,
and codec independent solution for keeping the NAT bindings alive.
If an ICE implementation is communciating with one that does not
support ICE, keepalives MUST still be sent. In that case, it is
RECOMMENDED that an agent support the RTP No-Op payload format [15],
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and send it at least once every 20 seconds if media is not otherwise
being sent. This No-Op MUST be sent even if the media stream is
inactive or recvonly.
7.8 Sending Media
When an agent sends media packets, it MUST send them from the same IP
address and port it has advertised in the m/c-line. This provides a
property known as symmetry, which is an essential facet of NAT
travresal.
In the case of a STUN-derived transport address, this means that the
RTP packets are sent from the local transport address used to obtain
the STUN address. In the case of a TURN-derived transport address,
this means that media packets are sent through the TURN server (using
the TURN SEND primitive). For local transport addresses, media is
sent from that local transport address.
This symmetric behavior MUST be followed by an agent even if its peer
in the session doesn't support ICE.
8. Interactions with Forking
SIP allows INVITE requests carrying offers to fork, which means that
they are delivered to multiple user agents. Each of those user
agents then provides an answer to the offer in the INVITE. The
result is that a single offer generated by the UAC produces multiple
answers.
ICE interacts very well with forking. Indeed, ICE fixes some of the
problems associated with forking. Once the offer/answer exchange has
completed, the UAC will have an answer from each UAS that received
the INVITE. The ICE connectivity checks that ensue will carry tids
that correlate each of those checks (and thus their corresponding
source IP address and port or TCP connection) with a specific remote
user agent. As these checks happen before any media is transmitted,
ICE allows a UAC to disambiguate subsequent media traffic, and
corelate that traffic with a particular remote UA. When SIP is used
without ICE, the incoming media traffic cannot be disambiguated
without an additional offer/answer exchange.
9. Interactions with Preconditions
Because ICE involves multiple addresses and pre-session activities,
its interactions with preconditions [10] merits further discussion.
Quality of Service (QoS) preconditions, which are defined in RFC
3312, apply only to the IP addresses and ports listed in the m/c
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lines in an offer/answer. If ICE changes the address and port 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, which applies without regard to the
fact that the m/c lines are changing due to ICE negotiations ocurring
"in the background".
ICE also has (purposeful) interactions with connectivity
preconditions [12]. As described there, the precondition is
satisfied once ICE has verified that there exists a valid path of
connectivity for each media stream to which the precondition applies.
More specifically, it is satisfied when there is at least one valid
UDP transport address pairing or TCP connection for such a media
stream. Furthermore, when a subsequent offer is made to promote one
of those valid transport address pairings or connections into the
m/c-line, the preconditions is marked as met in that same offer/
answer exchange.
10. Example
In the example that follows, messages are labeled with "message name
A,B" to mean a message from transport address A to B. For STUN
Requests, this is followed by curly brackets enclosing the username
(which is also the password). For STUN answers, this is followed by
square brackets containing the value of MAPPED ADDRESS. The example
shows a flow of two agents where one is behind a full cone NAT, and
the other is behind a symmetric NAT.
TODO: Fill in. This is a big complicated flow!
11. Grammar
This specification defines a new SDP attribute. It is called
"candidate". The candidate attribute MUST be present within a media
block of the SDP. It contains a transport address for a candidate
that can be used for connectivity checks. There MAY be multiple
candidate attributes in a media block.
The syntax of this attribute is:
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candidate-attribute = "candidate" ":" candidate-id SP tid SP
transport SP
qvalue SP ;qvalue from RFC 3261
addr SP
port SP
;addr, port from RFC 2327
transport = "UDP" / "TCP" / transport-extension
transport-extension = token
candidate-id = 1*DIGIT
id = non-ws-string
The candidate-id is used to group together the transport addresses
for a particular candidate. It MUST be a positive integer whose
value is less than (2^31 -1). It MUST have the same value for all
transport addresses within the same candidate. It MUST have a
different value for transport addresses within different candidates
for the same media stream. The tid production contains an
identifier, chosen with 128 bits of randomness, that identifies the
transport address. The tid of a pair of transport addresses is
combined to for the username and password of a STUN request from one
transport address to another. The transport production indicates the
transport protocol for the candidate. This can be either UDP or TCP.
Extensibility is provided to allow for future transport protocols to
be used with ICE, such as the Datagram Congestion Control Protocol
(DCCP) [26]. The unicast-address production is from RFC 2327, and
contains the IPv4 or IPv6 address of the candidate. The port
production contains its port.
12. Security Considerations
There are numerous threats in a system using ICE. This section
overviews these threats and discusses how they are mitigated.
STUN itself introduces many security considerations, which receive an
extensive treatment in RFC 3489. STUN is used within ICE in two ways
- one, as a technique for address gathering, and two, as a peer-to-
peer connectivity check. All of the security considerations of RFC
3489 apply directly to the former usage. However, the latter usage,
as a peer-to-peer connectivity check, is sufficiently different that
a discussion of its security considerations is appropriate.
It remains the case that many attacks are rooted in a single
primitive - an attacker attempts to inject a STUN response with an
invalid MAPPED-ADDRESS attribute. In the usages of STUN described in
RFC 3489, this injection can occur as a result of compromises of STUN
servers, attacks on the DNS, rogue NATs, injection of faked responses
coupled with a dos attack, and replaying modified requests. With
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peer-to-peer STUN, 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 irrelevant since STUN servers are
not discovered via DNS, they are signaled via SIP. Rogue NATs,
injection of fake responses and relaying modified requests all can be
handled in ICE with the countermeasures discussed below.
Consider an attacker that intercepts a STUN packet used for
connectivity checks, and replays it using its own source address. If
successful, this would fool an endpoint into thinking that this faked
source address was a valid destination for media (recall that the
source transport address of received STUN packets is used as a
potential candidate address). However, the recipient of the replayed
packet will not just send media to that candidate. It will verify it
with a STUN connectivity check. This check will be sent to that
faked source address, and if there is no answer, the address will not
be used. The attacker cannot answer the STUN request without access
to the username and password, which are exchanged as part of the
signaling. Thus, if the signaling is protected as recommended above,
the attacker cannot obtain the username or password.
If an attacker instead intercepts and replays STUN packets used for
the purposes of unilateral allocation, a similar result occurs. The
target of the attack will be fooled into thinking it has a STUN
derived transport address that it does not. Its peer will perform a
connectivity check to this address, which will fail. The attacker
cannot force this check to succeed without access to the username and
password, which are protected. Thus, this address will not be used.
In the worst case, an attacker can generate enough traffic so that
none of the valid STUN checks or unilateral allocations succeed.
This would result in a service disruption. However, this attack is
no worse than any pure packet flood disruption attack launched
against any other protocol. These attacks cannot be prevented by any
protocol means.
If an attacker could intercept and modify the contents of the Offer
or Accept messages, they could disrupt the session, divert the media,
and otherwise take control over the session. This attack is
prevented by encryption, authentication and message integrity of the
signaling channel used for ICE.
SIP-based implementations of ICE SHOULD use the sips URI scheme when
transporting SDP with ICE information, and MAY use S/MIME [3].
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13. IANA Considerations
This specification defines one new SDP attribute per the procedures
of Appendix B of RFC 2327. The required information for the
registration is:
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: candidate
Long Form: candidiate
Type of Attribute: media level
Charset Considerations: The attribute is not subject the 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 of UDP
with NAT (STUN).
Appropriate Values: See Section 11 of RFC XXXX [Note to RFC-ed:
please replace XXXX with the RFC number of this specification].
14. 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 [21]. 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. The
IAB has mandated that any protocols developed for this purpose
document a specific set of considerations. This section meets those
requirements.
14.1 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".
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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.
14.2 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. 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, derived transport addresses from
other UNSAF mechanisms simply never get used, because higher priority
connectivity exists. 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.
14.3 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.
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ICE actually removes brittleness from existing UNSAF mechanisms. In
particular, traditional STUN (the usage described in RFC 3489) 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. The only potential problem is that
bilaterally fixed addresses through STUN can expire if traffic does
not keep them alive. However, that is substantially less brittleness
than the STUN discovery mechanisms.
Another point of brittleness in STUN, TURN, 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 or TURN 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.
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 can be lifted.
Traditional STUN also introduces some security considerations.
Fortunately, those security considerations are also mitigated by ICE.
14.4 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.
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14.5 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 will interfere with
proper operation of any UNSAF mechanism, including ICE.
15. Acknowledgements
The authors would like to thank Douglas Otis, Francois Audet and
Magnus Westerland for their comments and input.
16. References
16.1 Normative References
[1] 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.
[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] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
[6] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
July 2003.
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[9] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262,
June 2002.
[10] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)",
RFC 3312, October 2002.
[11] Camarillo, G., "The Alternative Network Address Types Semantics
(ANAT) for theSession Description Protocol (SDP) Grouping
Framework", draft-ietf-mmusic-anat-02 (work in progress),
October 2004.
[12] Andreasen, F., "Connectivity Preconditions for Session
Description Protocol Media Streams",
draft-ietf-mmusic-connectivity-precon-00 (work in progress),
May 2005.
[13] Yon, D., "Connection-Oriented Media Transport in the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-comedia-10
(work in progress), November 2004.
[14] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-07 (work in progress),
February 2005.
[15] Andreasen, F., "A No-Op Payload Format for RTP",
draft-ietf-avt-rtp-no-op-00 (work in progress), May 2005.
16.2 Informative References
[16] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[17] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[18] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[19] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm
Specific IP: Framework", RFC 3102, October 2001.
[20] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm
Specific IP: Protocol Specification", RFC 3103, October 2001.
[21] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
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Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[22] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[23] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[24] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
[25] Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs",
draft-huitema-v6ops-teredo-05 (work in progress), April 2005.
[26] Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
draft-ietf-dccp-spec-11 (work in progress), March 2005.
[27] Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
Oriented Transport", draft-ietf-avt-rtp-framing-contrans-05
(work in progress), January 2005.
[28] Hellstrom, G., "RTP Payload for Text Conversation",
draft-ietf-avt-rfc2793bis-09 (work in progress), August 2004.
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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