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Versions: (draft-ietf-mmusic-rfc5245bis) 00
01 02 03 04 05 06 07 08 09 10 11 12
13 14 15 16 17 18 19 20 RFC 8445
ICE A. Keranen
Internet-Draft C. Holmberg
Obsoletes: 5245 (if approved) Ericsson
Intended status: Standards Track J. Rosenberg
Expires: June 11, 2017 jdrosen.net
December 8, 2016
Interactive Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal
draft-ietf-ice-rfc5245bis-07
Abstract
This document describes a protocol for Network Address Translator
(NAT) traversal for UDP-based multimedia. This protocol is called
Interactive Connectivity Establishment (ICE). ICE makes use of the
Session Traversal Utilities for NAT (STUN) protocol and its
extension, Traversal Using Relay NAT (TURN).
This document obsoletes RFC 5245.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on June 11, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 8
2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 10
2.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . 11
2.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 12
2.5. Security for Checks . . . . . . . . . . . . . . . . . . . 13
2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . 13
2.7. Lite Implementations . . . . . . . . . . . . . . . . . . 14
2.8. Usages of ICE . . . . . . . . . . . . . . . . . . . . . . 15
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 15
4. ICE Candidate Gathering and Exchange . . . . . . . . . . . . 18
4.1. Procedures for Full Implementation . . . . . . . . . . . 19
4.1.1. Gathering Candidates . . . . . . . . . . . . . . . . 19
4.1.1.1. Host Candidates . . . . . . . . . . . . . . . . . 20
4.1.1.2. Server Reflexive and Relayed Candidates . . . . . 21
4.1.1.3. Computing Foundations . . . . . . . . . . . . . . 23
4.1.1.4. Keeping Candidates Alive . . . . . . . . . . . . 23
4.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 24
4.1.2.1. Recommended Formula . . . . . . . . . . . . . . . 24
4.1.2.2. Guidelines for Choosing Type and Local
Preferences . . . . . . . . . . . . . . . . . . . 25
4.1.3. Eliminating Redundant Candidates . . . . . . . . . . 26
4.2. Lite Implementation Procedures . . . . . . . . . . . . . 26
4.3. Encoding the Candidate Information . . . . . . . . . . . 27
5. ICE Candidate Processing . . . . . . . . . . . . . . . . . . 29
5.1. Procedures for Full Implementation . . . . . . . . . . . 29
5.1.1. Verifying ICE Support . . . . . . . . . . . . . . . . 29
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5.1.2. Determining Role . . . . . . . . . . . . . . . . . . 30
5.1.3. Forming the Check Lists . . . . . . . . . . . . . . . 31
5.1.3.1. Check List State . . . . . . . . . . . . . . . . 31
5.1.3.2. Forming Candidate Pairs . . . . . . . . . . . . . 32
5.1.3.3. Computing Pair Priority and Ordering Pairs . . . 35
5.1.3.4. Pruning the Pairs . . . . . . . . . . . . . . . . 35
5.1.3.5. Removing lower-priority Pairs . . . . . . . . . . 35
5.1.3.6. Computing Candidate Pair States . . . . . . . . . 35
5.1.4. ICE State . . . . . . . . . . . . . . . . . . . . . . 40
5.1.5. Scheduling Checks . . . . . . . . . . . . . . . . . . 40
5.1.5.1. Triggered Check Queue . . . . . . . . . . . . . . 40
5.1.5.2. Timer Tc . . . . . . . . . . . . . . . . . . . . 40
5.1.5.3. Performing Connectivity Checks . . . . . . . . . 40
5.2. Lite Implementation Procedures . . . . . . . . . . . . . 41
6. Performing Connectivity Checks . . . . . . . . . . . . . . . 42
6.1. STUN Client Procedures . . . . . . . . . . . . . . . . . 42
6.1.1. Creating Permissions for Relayed Candidates . . . . . 42
6.1.2. Sending the Request . . . . . . . . . . . . . . . . . 42
6.1.2.1. PRIORITY . . . . . . . . . . . . . . . . . . . . 43
6.1.2.2. USE-CANDIDATE . . . . . . . . . . . . . . . . . . 43
6.1.2.3. ICE-CONTROLLED and ICE-CONTROLLING . . . . . . . 43
6.1.2.3.1. Forming Credentials . . . . . . . . . . . . . 43
6.1.2.3.2. DiffServ Treatment . . . . . . . . . . . . . 44
6.1.2.4. Processing the Response . . . . . . . . . . . . . 44
6.1.2.4.1. Failure Cases . . . . . . . . . . . . . . . . 44
6.1.2.4.2. Success Cases . . . . . . . . . . . . . . . . 45
6.1.2.4.3. Check List and Timer State Updates . . . . . 48
6.1.3. STUN Server Procedures . . . . . . . . . . . . . . . 48
6.1.3.1. Additional Procedures for Full Implementations . 49
6.1.3.1.1. Detecting and Repairing Role Conflicts . . . 49
6.1.3.1.2. Computing Mapped Address . . . . . . . . . . 50
6.1.3.1.3. Learning Peer Reflexive Candidates . . . . . 51
6.1.3.1.4. Triggered Checks . . . . . . . . . . . . . . 51
6.1.3.1.5. Updating the Nominated Flag . . . . . . . . . 52
6.1.3.2. Additional Procedures for Lite Implementations . 53
6.2. Concluding ICE Processing . . . . . . . . . . . . . . . . 53
6.2.1. Procedures for Full Implementations . . . . . . . . . 53
6.2.1.1. Nominating Pairs . . . . . . . . . . . . . . . . 53
6.2.1.2. Updating States . . . . . . . . . . . . . . . . . 54
6.2.2. Procedures for Lite Implementations . . . . . . . . . 55
6.2.2.1. Peer Is Full . . . . . . . . . . . . . . . . . . 56
6.2.2.2. Peer Is Lite . . . . . . . . . . . . . . . . . . 56
6.2.3. Freeing Candidates . . . . . . . . . . . . . . . . . 57
6.2.3.1. Full Implementation Procedures . . . . . . . . . 57
6.2.3.2. Lite Implementation Procedures . . . . . . . . . 57
6.3. ICE Restarts . . . . . . . . . . . . . . . . . . . . . . 57
7. ICE Option . . . . . . . . . . . . . . . . . . . . . . . . . 58
8. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 58
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9. Media Handling . . . . . . . . . . . . . . . . . . . . . . . 59
9.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 59
9.1.1. Procedures for Full Implementations . . . . . . . . . 59
9.1.2. Procedures for Lite Implementations . . . . . . . . . 60
9.1.3. Procedures for All Implementations . . . . . . . . . 60
9.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 61
10. Extensibility Considerations . . . . . . . . . . . . . . . . 61
11. Setting Ta and RTO . . . . . . . . . . . . . . . . . . . . . 62
11.1. General . . . . . . . . . . . . . . . . . . . . . . . . 62
11.2. Ta . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
11.3. RTO . . . . . . . . . . . . . . . . . . . . . . . . . . 64
12. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
13. Security Considerations . . . . . . . . . . . . . . . . . . . 69
13.1. Attacks on Connectivity Checks . . . . . . . . . . . . . 70
13.2. Attacks on Server Reflexive Address Gathering . . . . . 72
13.3. Attacks on Relayed Candidate Gathering . . . . . . . . . 73
13.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . 73
13.4.1. STUN Amplification Attack . . . . . . . . . . . . . 73
14. STUN Extensions . . . . . . . . . . . . . . . . . . . . . . . 74
14.1. New Attributes . . . . . . . . . . . . . . . . . . . . . 74
14.2. New Error Response Codes . . . . . . . . . . . . . . . . 75
15. Operational Considerations . . . . . . . . . . . . . . . . . 75
15.1. NAT and Firewall Types . . . . . . . . . . . . . . . . . 75
15.2. Bandwidth Requirements . . . . . . . . . . . . . . . . . 76
15.2.1. STUN and TURN Server Capacity Planning . . . . . . . 76
15.2.2. Gathering and Connectivity Checks . . . . . . . . . 76
15.2.3. Keepalives . . . . . . . . . . . . . . . . . . . . . 77
15.3. ICE and ICE-lite . . . . . . . . . . . . . . . . . . . . 77
15.4. Troubleshooting and Performance Management . . . . . . . 77
15.5. Endpoint Configuration . . . . . . . . . . . . . . . . . 78
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 78
16.1. STUN Attributes . . . . . . . . . . . . . . . . . . . . 78
16.2. STUN Error Responses . . . . . . . . . . . . . . . . . . 78
16.3. ICE Options . . . . . . . . . . . . . . . . . . . . . . 78
17. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 79
17.1. Problem Definition . . . . . . . . . . . . . . . . . . . 79
17.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . 80
17.3. Brittleness Introduced by ICE . . . . . . . . . . . . . 80
17.4. Requirements for a Long-Term Solution . . . . . . . . . 81
17.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . 82
18. Changes from RFC 5245 . . . . . . . . . . . . . . . . . . . . 82
19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 82
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 83
20.1. Normative References . . . . . . . . . . . . . . . . . . 83
20.2. Informative References . . . . . . . . . . . . . . . . . 83
Appendix A. Lite and Full Implementations . . . . . . . . . . . 87
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 88
B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 88
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B.2. Candidates with Multiple Bases . . . . . . . . . . . . . 90
B.3. Purpose of the Related Address and Related Port
Attributes . . . . . . . . . . . . . . . . . . . . . . . 92
B.4. Importance of the STUN Username . . . . . . . . . . . . . 92
B.5. The Candidate Pair Priority Formula . . . . . . . . . . . 94
B.6. Why Are Keepalives Needed? . . . . . . . . . . . . . . . 94
B.7. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 95
B.8. Why Are Binding Indications Used for Keepalives? . . . . 95
Appendix C. Connectivity Check Bandwidth . . . . . . . . . . . . 95
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 96
1. Introduction
Protocols establishing multimedia sessions between peers typically
involve exchanging IP addresses and ports for the media sources and
sinks. However this poses challenges when operated through Network
Address Translators (NATs) [RFC3235]. These 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 defined for allowing these protocols to
operate through NAT. These include Application Layer Gateways
(ALGs), the Middlebox Control Protocol [RFC3303], the original Simple
Traversal of UDP Through NAT (STUN) [RFC3489] specification, and
Realm Specific IP [RFC3102] [RFC3103] along with session description
extensions needed to make them work, such as the Session Description
Protocol (SDP) [RFC4566] attribute for the Real Time Control Protocol
(RTCP) [RFC3605]. 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 that is flexible enough to work well in all
situations.
This specification defines Interactive Connectivity Establishment
(ICE) as a technique for NAT traversal for UDP-based media streams
(though ICE has been extended to handle other transport protocols,
such as TCP [RFC6544]). ICE works by exchanging a multiplicity of IP
addresses and ports which are then tested for connectivity by peer-
to-peer connectivity checks. The IP addresses and ports are
exchanged via mechanisms (for example, including in a offer/answer
exchange) and the connectivity checks are performed using Session
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Traversal Utilities for NAT (STUN) specification [RFC5389]. ICE also
makes use of Traversal Using Relays around NAT (TURN) [RFC5766], an
extension to STUN. Because ICE exchanges a multiplicity of IP
addresses and ports for each media stream, it also allows for address
selection for multihomed and dual-stack hosts, and for this reason it
deprecates [RFC4091] and [RFC4092].
2. Overview of ICE
In a typical ICE deployment, we have two endpoints (known as ICE
AGENTS) that want to communicate. They are able to communicate
indirectly via some signaling protocol (such as SIP), by which they
can exchange ICE candidates. Note that ICE is not intended for NAT
traversal for the signaling protocol, which is assumed to be provided
via another mechanism. At the beginning of the ICE process, the
agents are ignorant of their own topologies. In particular, they
might or might not be behind a NAT (or multiple tiers of NATs). ICE
allows the agents to discover enough information about their
topologies to potentially find one or more paths by which they can
communicate.
Figure 1 shows a typical environment for ICE deployment. The two
endpoints are labelled L and R (for left and right, which helps
visualize call flows). Both L and R are behind their own respective
NATs though they may not be aware of it. The type of NAT and its
properties are also unknown. Agents L and R are capable of engaging
in an candidate exchange process, whose purpose is to set up a media
session between L and R. Typically, this exchange will occur through
a signaling (e.g., SIP) server.
In addition to the agents, a signaling server and NATs, ICE is
typically used in concert with STUN or TURN servers in the network.
Each agent can have its own STUN or TURN server, or they can be the
same.
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+---------+
+--------+ |Signaling| +--------+
| STUN | |Server | | STUN |
| Server | +---------+ | Server |
+--------+ / \ +--------+
/ \
/ \
/ <- Signaling -> \
/ \
+--------+ +--------+
| NAT | | NAT |
+--------+ +--------+
/ \
/ \
+-------+ +-------+
| Agent | | Agent |
| L | | R |
+-------+ +-------+
Figure 1: ICE Deployment Scenario
The basic idea behind ICE is as follows: each agent has a variety of
candidate TRANSPORT ADDRESSES (combination of IP address and port for
a particular transport protocol, which is always UDP in this
specification) it could use to communicate with the other agent.
These might include:
o A transport address on a directly attached network interface
o A translated transport address on the public side of a NAT (a
"server reflexive" address)
o A transport address allocated from a TURN server (a "relayed
address")
Potentially, any of L's candidate transport addresses can be used to
communicate with any of R's candidate transport addresses. In
practice, however, many combinations will not work. For instance, if
L and R are both behind NATs, their directly attached interface
addresses are unlikely to be able to communicate directly (this is
why ICE is needed, after all!). The purpose of ICE is to discover
which pairs of addresses will work. The way that ICE does this is to
systematically try all possible pairs (in a carefully sorted order)
until it finds one or more that work.
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2.1. Gathering Candidate Addresses
In order to execute ICE, an agent has to identify all of its address
candidates. A CANDIDATE is a transport address -- a combination of
IP address and port for a particular transport protocol (with only
UDP specified here). This document defines three types of
candidates, some derived from physical or logical network interfaces,
others discoverable via STUN and TURN. Naturally, one viable
candidate is a transport address obtained directly from a local
interface. Such a candidate is called a HOST CANDIDATE. The local
interface could be Ethernet or WiFi, or it could be one that is
obtained through a tunnel mechanism, such as a Virtual Private
Network (VPN) or Mobile IP (MIP). In all cases, such a network
interface appears to the agent as a local interface from which ports
(and thus candidates) can be allocated.
If an agent is multihomed, it obtains a candidate from each IP
address. Depending on the location of the PEER (the other agent in
the session) on the IP network relative to the agent, the agent may
be reachable by the peer through one or more of those IP addresses.
Consider, for example, an agent that has a local IP address on a
private net 10 network (I1), and a second connected to the public
Internet (I2). A candidate from I1 will be directly reachable when
communicating with a peer on the same private net 10 network, while a
candidate from I2 will be directly reachable when communicating with
a peer on the public Internet. Rather than trying to guess which IP
address will work, the initiating sends both the candidates to its
peer.
Next, the agent uses STUN or TURN to obtain additional candidates.
These come in two flavors: translated addresses on the public side of
a NAT (SERVER REFLEXIVE CANDIDATES) and addresses on TURN servers
(RELAYED CANDIDATES). When TURN servers are utilized, both types of
candidates are obtained from the TURN server. If only STUN servers
are utilized, only server reflexive candidates are obtained from
them. The relationship of these candidates to the host candidate is
shown in Figure 2. In this figure, both types of candidates are
discovered using TURN. In the figure, the notation X:x means IP
address X and UDP port x.
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To Internet
|
|
| /------------ Relayed
Y:y | / Address
+--------+
| |
| TURN |
| Server |
| |
+--------+
|
|
| /------------ Server
X1':x1'|/ Reflexive
+------------+ Address
| NAT |
+------------+
|
| /------------ Local
X:x |/ Address
+--------+
| |
| Agent |
| |
+--------+
Figure 2: Candidate Relationships
When the agent sends the TURN Allocate request from IP address and
port X:x, the NAT (assuming there is one) will create a binding
X1':x1', mapping this server reflexive candidate to the host
candidate X:x. Outgoing packets sent from the host candidate will be
translated by the NAT to the server reflexive candidate. Incoming
packets sent to the server reflexive candidate will be translated by
the NAT to the host candidate and forwarded to the agent. We call
the host candidate associated with a given server reflexive candidate
the BASE.
Note: "Base" refers to the address an agent sends from for a
particular candidate. Thus, as a degenerate case host candidates
also have a base, but it's the same as the host candidate.
When there are multiple NATs between the agent and the TURN server,
the TURN request will create a binding on each NAT, but only the
outermost server reflexive candidate (the one nearest the TURN
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server) will be discovered by the agent. If the agent is not behind
a NAT, then the base candidate will be the same as the server
reflexive candidate and the server reflexive candidate is redundant
and will be eliminated.
The Allocate request then arrives at the TURN server. The TURN
server allocates a port y from its local IP address Y, and generates
an Allocate response, informing the agent of this relayed candidate.
The TURN server also informs the agent of the server reflexive
candidate, X1':x1' by copying the source transport address of the
Allocate request into the Allocate response. The TURN server acts as
a packet relay, forwarding traffic between L and R. In order to send
traffic to L, R sends traffic to the TURN server at Y:y, and the TURN
server forwards that to X1':x1', which passes through the NAT where
it is mapped to X:x and delivered to L.
When only STUN servers are utilized, the agent sends a STUN Binding
request [RFC5389] to its STUN server. The STUN server will inform
the agent of the server reflexive candidate X1':x1' by copying the
source transport address of the Binding request into the Binding
response.
2.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them in highest
to lowest-priority and sends them to R over the signaling channel.
When R receives the candidates from L, it performs the same gathering
process and responds with its own list of candidates. At the end of
this process, each agent has a complete list of both its candidates
and its peer's candidates. It pairs them up, resulting in CANDIDATE
PAIRS. To see which pairs work, each agent schedules a series of
CHECKS. Each check is a STUN request/response transaction that the
client will perform on a particular candidate pair by sending a STUN
request from the local candidate to the remote candidate.
The basic principle of the connectivity checks is simple:
1. Sort the candidate pairs in priority order.
2. Send checks on each candidate pair in priority order.
3. Acknowledge checks received from the other agent.
With both agents performing a check on a candidate pair, the result
is a 4-way handshake:
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L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 3: Basic Connectivity Check
It is important to note that the STUN requests are sent to and from
the exact same IP addresses and ports that will be used for media
(e.g., RTP and RTCP). Consequently, agents demultiplex STUN and RTP/
RTCP using contents of the packets, rather than the port on which
they are received. Fortunately, this demultiplexing is easy to do,
especially for RTP and RTCP.
Because a STUN Binding request is used for the connectivity check,
the STUN Binding response will contain the agent's translated
transport address on the public side of any NATs between the agent
and its peer. If this transport address is different from other
candidates the agent already learned, it represents a new candidate,
called a PEER REFLEXIVE CANDIDATE, which then gets tested by ICE just
the same as any other candidate.
As an optimization, as soon as R gets L's check message, R schedules
a connectivity check message to be sent to L on the same candidate
pair. This accelerates the process of finding a valid candidate, and
is called a TRIGGERED CHECK.
At the end of this handshake, both L and R know that they can send
(and receive) messages end-to-end in both directions.
2.3. Sorting Candidates
Because the algorithm above searches all candidate pairs, if a
working pair exists it will eventually find it no matter what order
the candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The
resulting list of sorted candidate pairs is called the CHECK LIST.
The algorithm is described in Section 4.1.2 but follows two general
principles:
o Each agent gives its candidates a numeric priority, which is sent
along with the candidate to the peer.
o The local and remote priorities are combined so that each agent
has the same ordering for the candidate pairs.
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The second property is important for getting ICE to work when there
are NATs in front of L and R. Frequently, NATs will not allow
packets in from a host until the agent behind the NAT has sent a
packet towards that host. Consequently, ICE checks in each direction
will not succeed until both sides have sent a check through their
respective NATs.
The agent works through this check list by sending a STUN request for
the next candidate pair on the list periodically. These are called
ORDINARY CHECKS.
In general, the priority algorithm is designed so that candidates of
similar type get similar priorities and so that more direct routes
(that is, through fewer media relays and through fewer NATs) are
preferred over indirect ones (ones with more media relays and more
NATs). Within those guidelines, however, agents have a fair amount
of discretion about how to tune their algorithms.
2.4. Frozen Candidates
The previous description only addresses the case where the agents
wish to establish a media session with one COMPONENT (a piece of a
media stream requiring a single transport address; a media stream may
require multiple components, each of which has to work for the media
stream as a whole to be work). Sometimes (e.g., with RTP and RTCP in
separate components), the agents actually need to establish
connectivity for more than one flow.
The network properties are likely to be very similar for each
component (especially because RTP and RTCP are sent and received from
the same IP address). It is usually possible to leverage information
from one media component in order to determine the best candidates
for another. ICE does this with a mechanism called "frozen
candidates".
Each candidate is associated with a property called its FOUNDATION.
Two candidates have the same foundation when they are "similar" -- of
the same type and obtained from the same host candidate and STUN/TURN
server using the same protocol. Otherwise, their foundation is
different. A candidate pair has a foundation too, which is just the
concatenation of the foundations of its two candidates. Initially,
only the candidate pairs with unique foundations are tested. The
other candidate pairs are marked "frozen". When the connectivity
checks for a candidate pair succeed, the other candidate pairs with
the same foundation are unfrozen. This avoids repeated checking of
components that are superficially more attractive but in fact are
likely to fail.
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While we've described "frozen" here as a separate mechanism for
expository purposes, in fact it is an integral part of ICE and the
ICE prioritization algorithm automatically ensures that the right
candidates are unfrozen and checked in the right order. However, if
the ICE usage does not utilize multiple components or media streams,
it does not need to implement this algorithm.
2.5. Security for Checks
Because ICE is used to discover which addresses can be used to send
media between two agents, it is important to ensure that the process
cannot be hijacked to send media to the wrong location. Each STUN
connectivity check is covered by a message authentication code (MAC)
computed using a key exchanged in the signaling channel. This MAC
provides message integrity and data origin authentication, thus
stopping an attacker from forging or modifying connectivity check
messages. Furthermore, if for example a SIP [RFC3261] caller is
using ICE, and their call forks, the ICE exchanges happen
independently with each forked recipient. In such a case, the keys
exchanged in the signaling help associate each ICE exchange with each
forked recipient.
2.6. Concluding ICE
ICE checks are performed in a specific sequence, so that high-
priority candidate pairs are checked first, followed by lower-
priority ones. One way to conclude ICE is to declare victory as soon
as a check for each component of each media stream completes
successfully. Indeed, this is a reasonable algorithm, and details
for it are provided below. However, it is possible that a packet
loss will cause a higher-priority check to take longer to complete.
In that case, allowing ICE to run a little longer might produce
better results. More fundamentally, however, the prioritization
defined by this specification may not yield "optimal" results. As an
example, if the aim is to select low-latency media paths, usage of a
relay is a hint that latencies may be higher, but it is nothing more
than a hint. An actual round-trip time (RTT) measurement could be
made, and it might demonstrate that a pair with lower priority is
actually better than one with higher priority.
Consequently, ICE assigns one of the agents in the role of the
CONTROLLING AGENT, and the other of the CONTROLLED AGENT. The
controlling agent gets to nominate which candidate pairs will get
used for media amongst the ones that are valid.
When nominating, the controlling agent lets the checks continue until
at least one valid candidate pair for each media stream is found.
Then, it picks amongst those that are valid, and sends a second STUN
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request on its NOMINATED candidate pair, but this time with a flag
set to tell the peer that this pair has been nominated for use. This
is shown in Figure 4.
L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
STUN request + flag -> \ L's
<- STUN response / check
Figure 4: Nomination
Once the STUN transaction with the flag completes, both sides cancel
any future checks for that media stream. ICE will now send media
using this pair. The pair an ICE agent is using for media is called
the SELECTED PAIR.
Once ICE is concluded, it can be restarted at any time for one or all
of the media streams by either agent. This is done by sending an
updated candidate information indicating a restart.
2.7. Lite Implementations
In order for ICE to be used in a call, both agents need to support
it. However, certain agents will always be connected to the public
Internet and have a public IP address at which it can receive packets
from any correspondent. To make it easier for these devices to
support ICE, ICE defines a special type of implementation called LITE
(in contrast to the normal FULL implementation). A lite
implementation doesn't gather candidates; it includes only host
candidates for any media stream. Lite agents do not generate
connectivity checks or run the state machines, though they need to be
able to respond to connectivity checks. When a lite implementation
connects with a full implementation, the full agent takes the role of
the controlling agent, and the lite agent takes on the controlled
role. When two lite implementations connect, no checks are sent.
For guidance on when a lite implementation is appropriate, see the
discussion in Appendix A.
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It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet, a full implementation is preferable if achievable.
2.8. Usages of ICE
This document specifies generic use of ICE with protocols that
provide means to exchange candidate information between the ICE
Peers. The specific details of (i.e how to encode candidate
information and the actual candidate exchange process) for different
protocols using ICE are described in separate usage documents. One
possible way the agents can exchange the candidate information is to
use [RFC3264] based Offer/Answer semantics as part of the SIP
[RFC3261] protocol [I-D.ietf-mmusic-ice-sip-sdp].
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
Readers should be familiar with the terminology defined in the STUN
[RFC5389], and NAT Behavioral requirements for UDP [RFC4787].
This specification makes use of the following additional terminology:
ICE Agent: An agent is the protocol implementation involved in the
ICE candidate exchange. There are two agents involved in a
typical candidate exchange.
Initiating Peer, Initiating Agent, Initiator: An initiating agent is
the protocol implementation involved in the ICE candidate exchange
that initiates the ICE candidate exchange process.
Responding Peer, Responding Agent, Responder: A receiving agent is
the protocol implementation involved in the ICE candidate exchange
that receives and responds to the candidate exchange process
initiated by the Initiator.
ICE Candidate Exchange, Candidate Exchange: The process where the
ICE agents exchange information (e.g., candidates and passwords)
that is needed to perform ICE. [RFC3264] Offer/Answer with SDP
encoding is one example of a protocol that can be used for
exchanging the candidate information.
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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 initiating agent, the peer is the responding agent. From the
perspective of the responding agent, the peer is the initiating
agent.
Transport Address: The combination of an IP address and transport
protocol (such as UDP or TCP) port.
Media, Media Stream, Media Session: When ICE is used to setup
multimedia sessions, the media is usually transported over RTP,
and a media stream composes of a stream of RTP packets. When ICE
is used with other than multimedia sessions, the terms "media",
"media stream", and "media session" are still used in this
specification to refer to the IP data packets that are exchanged
between the peers on the path created and tested with ICE.
Candidate, Candidate Information: A transport address that is a
potential point of contact for receipt of media. Candidates also
have properties -- their type (server reflexive, relayed, or
host), priority,foundation, and base.
Component: A component is a piece of a media stream requiring a
single transport address; a media stream may require multiple
components, each of which has to work for the media stream as a
whole to work. For media streams based on RTP, unless RTP and
RTCP are multiplexed in the same port, there are two components
per media stream -- one for RTP, and one for RTCP.
Host Candidate: A candidate obtained by binding to a specific port
from an IP address on the host. This includes IP addresses on
physical interfaces and logical ones, such as ones obtained
through Virtual Private Networks (VPNs) and Realm Specific IP
(RSIP) [RFC3102] (which lives at the operating system level).
Server Reflexive Candidate: A candidate whose IP address and port
are a binding allocated by a NAT for an agent when it sent a
packet through the NAT to a server. Server reflexive candidates
can be learned by STUN servers using the Binding request, or TURN
servers, which provides both a relayed and server reflexive
candidate.
Peer Reflexive Candidate: A candidate whose IP address and port are
a binding allocated by a NAT for an agent when it sent a STUN
Binding request through the NAT to its peer.
Relayed Candidate: A candidate obtained by sending a TURN Allocate
request from a host candidate to a TURN server. The relayed
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candidate is resident on the TURN server, and the TURN server
relays packets back towards the agent.
Base: The transport address that an agent sends from for a
particular candidate. For host-, server reflexive- and peer
reflexive candidates the base is the same as the host candidate.
For relayed candidates the base is the same as the relayed
candidate (i.e., the transport address used by the TURN server to
send from).
Foundation: An arbitrary string that is the same for two candidates
that have the same type, base IP address, protocol (UDP, TCP,
etc.), and STUN or TURN server. If any of these are different,
then the foundation will be different. Two candidate pairs with
the same foundation pairs are likely to have similar network
characteristics. Foundations are used in the frozen algorithm.
Local Candidate: A candidate that an agent has obtained and shared
with the peer.
Remote Candidate: A candidate that an agent received from its peer.
Default Destination/Candidate: The default destination for a
component of a media stream is the transport address that would be
used by an agent that is not ICE aware. A default candidate for a
component is one whose transport address matches the default
destination for that component.
Candidate Pair: A pairing containing a local candidate and a remote
candidate.
Check, Connectivity Check, STUN Check: A STUN Binding request
transaction for the purposes of verifying connectivity. A check
is sent from the local candidate to the remote candidate of a
candidate pair.
Check List: An ordered set of candidate pairs that an agent will use
to generate checks.
Ordinary Check: A connectivity check generated by an agent as a
consequence of a timer that fires periodically, instructing it to
send a check.
Triggered Check: A connectivity check generated as a consequence of
the receipt of a connectivity check from the peer.
Valid List: An ordered set of candidate pairs for a media stream
that have been validated by a successful STUN transaction.
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Full: An ICE implementation that performs the complete set of
functionality defined by this specification.
Lite: An ICE implementation that omits certain functions,
implementing only as much as is necessary for a peer
implementation that is full to gain the benefits of ICE. Lite
implementations do not maintain any of the state machines and do
not generate connectivity checks.
Controlling Agent: The ICE agent that is responsible for selecting
the final choice of candidate pairs and signaling them through
STUN. In any session, one agent is always controlling. The other
is the controlled agent.
Controlled Agent: An ICE agent that waits for the controlling agent
to select the final choice of candidate pairs.
Nomination, Regular Nomination: The process of picking a valid
candidate pair for media traffic by validating the pair with one
STUN request, and then picking it by sending a second STUN request
with a flag indicating its nomination.
Nominated: If a valid candidate pair has its nominated flag set, it
means that it may be selected by ICE for sending and receiving
media.
Selected Pair, Selected Candidate: The candidate pair selected by
ICE for sending and receiving media is called the selected pair,
and each of its candidates is called the selected candidate.
Before a candidiate pair has been selected, any valid candidiate
pair can be used for sending and receiving media (only one
candidiate pair at any given time).
Using Protocol, ICE Usage: The protocol that uses ICE for NAT
traversal. A usage specification defines the protocol specific
details on how the procedures defined here are applied to that
protocol.
4. ICE Candidate Gathering and Exchange
As part of ICE processing, both the initiating and responding agents
exchange encoded candidate information as defined by the Usage
Protocol (ICE Usage). Specifics of encoding mechanism and the
semantics of candidate information exchange is out of scope of this
specification.
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However at a higher level, the below diagram captures ICE processing
sequence in the agents (initiator and responder) for exchange of
their respective candidate(s) information.
Initiating Responding
Agent Agent
(I) (R)
Gather, | |
prioritize, | |
eliminate | |
redundant | |
candidates, | |
Encode | |
candidates | |
| I's Candidate Information |
|------------------------------>|
| | Gather,
| | prioritize,
| | eliminate
| | redundant
| | candidates,
| | Encode
| | candidates
| R's Candidate Information |
|<------------------------------|
| |
Figure 5: Candidate Gathering and Exchange Sequence
As shown, the agents involved in the candidate exchange perform (1)
candidate gathering, (2) candidate prioritization, (3) eliminating
redundant candidates, (4) (possibly) choose default candidates, and
then (5) formulate and send the candidates to the Peer ICE agent.
All but the last of these five steps differ for full and lite
implementations.
4.1. Procedures for Full Implementation
4.1.1. Gathering Candidates
An agent gathers candidates when it believes that communication is
imminent. An initiating agent can do this based on a user interface
cue, or based on an explicit request to initiate a session. Every
candidate is a transport address. It also has a type and a base.
Four types are defined and gathered by this specification -- host
candidates, server reflexive candidates, peer reflexive candidates,
and relayed candidates. The server reflexive candidates are gathered
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using STUN or TURN, and relayed candidates are obtained through TURN.
Peer reflexive candidates are obtained in later phases of ICE, as a
consequence of connectivity checks.
The process for gathering candidates at the responding agent is
identical to the process for the initiating agent. It is RECOMMENDED
that the responding agent begins this process immediately on receipt
of the candidate information, prior to alerting the user. Such
gathering MAY begin when an agent starts.
4.1.1.1. Host Candidates
The first step is to gather host candidates. Host candidates are
obtained by binding to ports (typically ephemeral) on a IP address
attached to an interface (physical or virtual, including VPN
interfaces) on the host.
For each UDP media stream the agent wishes to use, the agent SHOULD
obtain a candidate for each component of the media stream on each IP
address that the host has, with the exceptions listed below. The
agent obtains each candidate by binding to a UDP port on the specific
IP address. A host candidate (and indeed every candidate) is always
associated with a specific component for which it is a candidate.
Each component has an ID assigned to it, called the component ID.
For RTP-based media streams, unless both RTP and RTCP are multiplexed
in the same UDP port (RTP/RTCP multiplexing), the RTP itself has a
component ID of 1, and RTCP a component ID of 2. In case of RTP/RTCP
multiplexing, a component ID of 1 is used for both RTP and RTCP.
When candidates are obtained, unless the agent knows for sure that
RTP/RTCP multiplexing will be used (i.e. the agent knows that the
other agent also supports, and is willing to use, RTP/RTCP
multiplexing), or unless the agent only supports RTP/RTCP
multiplexing, the agent MUST obtain a separate candidate for RTCP.
If an agent has obtained a candidate for RTCP, and ends up using RTP/
RTCP multiplexing, the agent does not need to perform connectivity
checks on the RTCP candidate.
If an agent is using separate candidates for RTP and RTCP, it will
end up with 2*K host candidates if an agent has K IP addresses.
Note that the responding agent, when obtaining its candidates, will
typically know if the other agent supports RTP/RTCP multiplexing, in
which case it will not need to obtain a separate candidate for RTCP.
However, absence of a component ID 2 as such does not imply use of
RTCP/RTP multiplexing, as it could also mean that RTCP is not used.
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For other than RTP-based streams, use of multiple components is
discouraged since using them increases the complexity of ICE
processing. If multiple components are needed, the component IDs
SHOULD start with 1 and increase by 1 for each component.
The base for each host candidate is set to the candidate itself.
The host candidates are gathered from all IP addresses with the
following exceptions:
o Addresses from a loopback interface MUST NOT be included in the
candidate addresses.
o Deprecated IPv4-compatible IPv6 addresses [RFC4291] and IPv6 site-
local unicast addresses [RFC3879] MUST NOT be included in the
address candidates.
o IPv4-mapped IPv6 addresses SHOULD NOT be included in the offered
candidates unless the application using ICE does not support IPv4
(i.e., is an IPv6-only application [RFC4038]).
o If one or more host candidates corresponding to an IPv6 address
generated using a mechanism that prevents location tracking
[RFC7721] are gathered, host candidates corresponding to IPv6
addresses that do allow location tracking, that are configured on
the same interface, and are part of the same network prefix MUST
NOT be gathered; and host candidates corresponding to IPv6 link-
local addresses MUST NOT be gathered.
4.1.1.2. Server Reflexive and Relayed Candidates
Agents SHOULD obtain relayed candidates and SHOULD obtain server
reflexive candidates. These requirements are at SHOULD strength to
allow for provider variation. Use of STUN and TURN servers may be
unnecessary in closed networks where agents are never connected to
the public Internet or to endpoints outside of the closed network.
In such cases, a full implementation would be used for agents that
are dual-stack or multihomed, to select a host candidate. Use of
TURN servers is expensive, and when ICE is being used, they will only
be utilized when both endpoints are behind NATs that perform address
and port dependent mapping. Consequently, some deployments might
consider this use case to be marginal, and elect not to use TURN
servers. If an agent does not gather server reflexive or relayed
candidates, it is RECOMMENDED that the functionality be implemented
and just disabled through configuration, so that it can be re-enabled
through configuration if conditions change in the future.
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If an agent is gathering both relayed and server reflexive
candidates, it uses a TURN server. If it is gathering just server
reflexive candidates, it uses a STUN server.
The agent next pairs each host candidate with the STUN or TURN server
with which it is configured or has discovered by some means. If a
STUN or TURN server is configured, it is RECOMMENDED that a domain
name be configured, and the DNS procedures in [RFC5389] (using SRV
records with the "stun" service) be used to discover the STUN server,
and the DNS procedures in [RFC5766] (using SRV records with the
"turn" service) be used to discover the TURN server.
This specification only considers usage of a single STUN or TURN
server. When there are multiple choices for that single STUN or TURN
server (when, for example, they are learned through DNS records and
multiple results are returned), an agent SHOULD use a single STUN or
TURN server (based on its IP address) for all candidates for a
particular session. This improves the performance of ICE. The
result is a set of pairs of host candidates with STUN or TURN
servers. The agent then chooses one pair, and sends a Binding or
Allocate request to the server from that host candidate. Binding
requests to a STUN server are not authenticated, and any ALTERNATE-
SERVER attribute in a response is ignored. Agents MUST support the
backwards compatibility mode for the Binding request defined in
[RFC5389]. Allocate requests SHOULD be authenticated using a long-
term credential obtained by the client through some other means.
Every Ta milliseconds thereafter, the agent can generate another new
STUN or TURN transaction. This transaction can either be a retry of
a previous transaction that failed with a recoverable error (such as
authentication failure), or a transaction for a new host candidate
and STUN or TURN server pair. The agent SHOULD NOT generate
transactions more frequently than one every Ta milliseconds. See
Section 11 for guidance on how to set Ta and the STUN retransmit
timer, RTO.
The agent will receive a Binding or Allocate response. A successful
Allocate response will provide the agent with a server reflexive
candidate (obtained from the mapped address) and a relayed candidate
in the XOR-RELAYED-ADDRESS attribute. If the Allocate request is
rejected because the server lacks resources to fulfill it, the agent
SHOULD instead send a Binding request to obtain a server reflexive
candidate. A Binding response will provide the agent with only a
server reflexive candidate (also obtained from the mapped address).
The base of the server reflexive candidate is the host candidate from
which the Allocate or Binding request was sent. The base of a
relayed candidate is that candidate itself. If a relayed candidate
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is identical to a host candidate (which can happen in rare cases),
the relayed candidate MUST be discarded.
If an IPv6-only agent is in a network that utilizes NAT64 [RFC6146]
and DNS64 [RFC6147] technologies, it may gather also IPv4 server
reflexive and/or relayed candidates from IPv4-only STUN or TURN
servers. IPv6-only agents SHOULD also utilize IPv6 prefix discovery
[RFC7050] to discover the IPv6 prefix used by NAT64 (if any) and
generate server reflexive candidates for each IPv6-only interface
accordingly. The NAT64 server reflexive candidates are prioritized
like IPv4 server reflexive candidates.
4.1.1.3. Computing Foundations
Finally, the agent assigns each candidate a foundation. The
foundation is an identifier, scoped within a session. Two candidates
MUST have the same foundation ID when all of the following are true:
o they are of the same type (host, relayed, server reflexive, or
peer reflexive)
o their bases have the same IP address (the ports can be different)
o for reflexive and relayed candidates, the STUN or TURN servers
used to obtain them have the same IP address
o they were obtained using the same transport protocol (TCP, UDP,
etc.)
Similarly, two candidates MUST have different foundations if their
types are different, their bases have different IP addresses, the
STUN or TURN servers used to obtain them have different IP addresses,
or their transport protocols are different.
4.1.1.4. Keeping Candidates Alive
Once server reflexive and relayed candidates are allocated, they MUST
be kept alive until ICE processing has completed, as described in
Section 6.2.3. For server reflexive candidates learned through a
Binding request, the bindings MUST be kept alive by additional
Binding requests to the server. Refreshes for allocations are done
using the Refresh transaction, as described in [RFC5766]. The
Refresh requests will also refresh the server reflexive candidate.
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4.1.2. Prioritizing Candidates
The prioritization process results in the assignment of a priority to
each candidate. Each candidate for a media stream MUST have a unique
priority that MUST be a positive integer between 1 and (2**31 - 1).
This priority will be used by ICE to determine the order of the
connectivity checks and the relative preference for candidates.
An agent SHOULD compute this priority using the formula in
Section 4.1.2.1 and choose its parameters using the guidelines in
Section 4.1.2.2. If an agent elects to use a different formula, ICE
will take longer to converge since both agents will not be
coordinated in their checks.
The process for prioritizing candidates is common across the
initiating and the responding agent.
4.1.2.1. Recommended Formula
When using the formula, an agent computes the priority by determining
a preference for each type of candidate (server reflexive, peer
reflexive, relayed, and host), and, when the agent is multihomed,
choosing a preference for its IP addresses. These two preferences
are then combined to compute the priority for a candidate. That
priority is computed using the following formula:
priority = (2^24)*(type preference) +
(2^8)*(local preference) +
(2^0)*(256 - component ID)
The type preference MUST be an integer from 0 to 126 inclusive, and
represents the preference for the type of the candidate (where the
types are local, server reflexive, peer reflexive, and relayed). A
126 is the highest preference, and a 0 is the lowest. Setting the
value to a 0 means that candidates of this type will only be used as
a last resort. The type preference MUST be identical for all
candidates of the same type and MUST be different for candidates of
different types. The type preference for peer reflexive candidates
MUST be higher than that of server reflexive candidates. Note that
candidates gathered based on the procedures of Section 4.1.1 will
never be peer reflexive candidates; candidates of these type are
learned from the connectivity checks performed by ICE.
The local preference MUST be an integer from 0 to 65535 inclusive.
It represents a preference for the particular IP address from which
the candidate was obtained. 65535 represents the highest preference,
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and a zero, the lowest. When there is only a single IP address, this
value SHOULD be set to 65535. More generally, if there are multiple
candidates for a particular component for a particular media stream
that have the same type, the local preference MUST be unique for each
one. In this specification, this only happens for multihomed hosts
or if an agent is using multiple TURN servers. If a host is
multihomed because it is dual-stack, the local preference should be
set according to the current best practice described in
[I-D.ietf-ice-dualstack-fairness].
The component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive.
4.1.2.2. Guidelines for Choosing Type and Local Preferences
One criterion for selection of the type and local preference values
is the use of a media intermediary, such as a TURN server, a tunnel
service such as VPN server, or NAT. With a media intermediary, if
media is sent to that candidate, it will first transit the media
intermediary before being received. Relayed candidates are one type
of candidate that involves a media intermediary. Another are host
candidates obtained from a VPN interface. When media is transited
through a media intermediary, it can have a positive or negative
effect on the latency between transmission and reception. It may or
may not 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 media
intermediary run by a provider. If these concerns are important, the
type preference for relayed candidates must be carefully chosen. The
RECOMMENDED values are 126 for host candidates, 100 for server
reflexive candidates, 110 for peer reflexive candidates, and 0 for
relayed candidates.
Furthermore, if an agent is multihomed and has multiple IP addresses,
the recommendation in [I-D.ietf-ice-dualstack-fairness] should be
followed. If multiple TURN servers are used, local priorities for
the candidates obtained from the TURN servers are chosen in a similar
fashion as for multihomed local candidates: the local preference
value is used to indicate a preference among different servers but
the preference MUST be unique for each one.
Another criterion for selection of preferences is IP address family.
ICE works with both IPv4 and IPv6. It 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. Implementation should follow the guidelines from
[I-D.ietf-ice-dualstack-fairness] to avoid excessively delays in the
connectivity check phase if broken paths exist.
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Another criterion for selecting preferences is topological awareness.
This is most useful for candidates that make use of intermediaries.
In those cases, if an agent has preconfigured or dynamically
discovered knowledge of the topological proximity of the
intermediaries to itself, it can use that to assign higher local
preferences to candidates obtained from closer intermediaries.
Another criterion for selecting preferences might be security or
privacy. If a user is a telecommuter, and therefore connected to a
corporate network and a local home network, the user may prefer their
voice traffic to be routed over the VPN or similar tunnel in order to
keep it on the corporate network when communicating within the
enterprise, but use the local network when communicating with users
outside of the enterprise. In such a case, a VPN address would have
a higher local preference than any other address.
4.1.3. Eliminating Redundant Candidates
Next, the agent eliminates redundant candidates. A candidate is
redundant if its transport address equals another candidate, and its
base equals the base of that other candidate. Note that two
candidates can have the same transport address yet have different
bases, and these would not be considered redundant. Frequently, a
server reflexive candidate and a host candidate will be redundant
when the agent is not behind a NAT. The agent SHOULD eliminate the
redundant candidate with the lower priority.
This process is common across the initiating and responding agents.
4.2. Lite Implementation Procedures
Lite implementations only utilize host candidates. A lite
implementation MUST, for each component of each media stream,
allocate zero or one IPv4 candidates. It MAY allocate zero or more
IPv6 candidates, but no more than one per each IPv6 address utilized
by the host. Since there can be no more than one IPv4 candidate per
component of each media stream, if an agent has multiple IPv4
addresses, it MUST choose one for allocating the candidate. If a
host is dual-stack, it is RECOMMENDED that it allocate one IPv4
candidate and one global IPv6 address. With the lite implementation,
ICE cannot be used to dynamically choose amongst candidates.
Therefore, including more than one candidate from a particular scope
is NOT RECOMMENDED, since only a connectivity check can truly
determine whether to use one address or the other.
Each component has an ID assigned to it, called the component ID.
For RTP-based media streams, unless RTCP is multiplexed in the same
port with RTP, the RTP itself has a component ID of 1, and RTCP a
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component ID of 2. If an agent is using RTCP without multiplexing,
it MUST obtain candidates for it. However, absence of a component ID
2 as such does not imply use of RTCP/RTP multiplexing, as it could
also mean that RTCP is not used.
Each candidate is assigned a foundation. The foundation MUST be
different for two candidates allocated from different IP addresses,
and MUST be the same otherwise. A simple integer that increments for
each IP address will suffice. In addition, each candidate MUST be
assigned a unique priority amongst all candidates for the same media
stream. This priority SHOULD be equal to:
priority = (2^24)*(126) +
(2^8)*(IP precedence) +
(2^0)*(256 - component ID)
If a host is v4-only, it SHOULD set the IP precedence to 65535. If a
host is v6 or dual-stack, the IP precedence SHOULD be the precedence
value for IP addresses described in RFC 6724 [RFC6724].
Next, an agent chooses a default candidate for each component of each
media stream. If a host is IPv4-only, there would only be one
candidate for each component of each media stream, and therefore that
candidate is the default. If a host is IPv6 or dual-stack, the
selection of default is a matter of local policy. This default
SHOULD be chosen such that it is the candidate most likely to be used
with a peer. For IPv6-only hosts, this would typically be a globally
scoped IPv6 address. For dual-stack hosts, the IPv4 address is
RECOMMENDED.
The procedures in this section is common across the initiating and
responding agents.
4.3. Encoding the Candidate Information
Regardless of the agent being an Initiator or Responder Agent, the
following parameters and their data types needs to be conveyed as
part of the candidate exchange process. The specifics of syntax for
encoding the candidate information is out of scope of this
specification.
Candidate attribute There will be one or more of these for each
"media stream". Each candidate is composed of:
Connection Address: The IP address and transport protocol port of
the candidate.
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Transport: An indicator of the transport protocol for this
candidate. This need not be present if the using protocol will
only ever run over a single transport protocol. If it runs
over more than one, or if others are anticipated to be used in
the future, this should be present.
Foundation: A sequence of up to 32 characters.
Component-ID: This would be present only if the using protocol
were utilizing the concept of components. If it is, it would
be a positive integer that indicates the component ID for which
this is a candidate.
Priority: An encoding of the 32-bit priority value.
Candidate Type: The candidate type, as defined in ICE.
Related Address and Port: The related IP address and port for
this candidate, as defined by ICE. These MAY be omitted or set
to invalid values if the agent does not want to reveal them,
e.g., for privacy reasons.
Extensibility Parameters: The using protocol should define some
means for adding new per-candidate ICE parameters in the
future.
Lite Flag: If ICE lite is used by the using protocol, it needs to
convey a boolean parameter which indicates whether the
implementation is lite or not.
Connectivity check pacing value: If an agent wants to use other than
the default pacing values for the connectivity checks, it MUST
indicate this in the ICE exchange.
Username Fragment and Password: The using protocol has to convey a
username fragment and password. The username fragment MUST
contain at least 24 bits of randomness, and the password MUST
contain at least 128 bits of randomness.
ICE extensions: In addition to the per-candidate extensions above,
the using protocol should allow for new media-stream or session-
level attributes (ice-options).
If the using protocol is using the ICE mismatch feature, a way is
needed to convey this parameter in answers. It is a boolean flag.
The exchange of parameters is symmetric; both agents need to send the
same set of attributes as defined above.
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The using protocol may (or may not) need to deal with backwards
compatibility with older implementations that do not support ICE. If
the fallback mechanism is being used, then presumably the using
protocol provides a way of conveying the default candidate (its IP
address and port) in addition to the ICE parameters.
STUN connectivity checks between agents are authenticated using the
short-term credential mechanism defined for STUN [RFC5389]. This
mechanism relies on a username and password that are exchanged
through protocol machinery between the client and server. The
username part of this credential is formed by concatenating a
username fragment from each agent, separated by a colon. Each agent
also provides a password, used to compute the message integrity for
requests it receives. The username fragment and password are
exchanged between the peers. In addition to providing security, the
username provides disambiguation and correlation of checks to media
streams. See Appendix B.4 for motivation.
If the initiating agent is a lite implementation, it MUST indicate
this when sending its candidates .
ICE provides for extensibility by allowing an agent to include a
series of tokens that identify ICE extensions as part of the
candidate exchange process.
Once an agent has sent its candidate information, that agent MUST be
prepared to receive both STUN and media packets on each candidate.
As discussed in Section 9.1, media packets can be sent to a candidate
prior to its appearance as the default destination for media.
5. ICE Candidate Processing
Once an agent has candidates from it's peer, it will check if the
peer supports ICE, determine its own role, exchanges candidates
(Section 4) and for full implementations, forms the check lists and
begins connectivity checks as explained in this section.
5.1. Procedures for Full Implementation
5.1.1. Verifying ICE Support
Certain middleboxes, such as ALGs, may alter the ICE candidate
information that breaks ICE. If the using protocol is vulnerable to
this kind of changes, called ICE mismatch, the responding agent needs
to detect this and signal this back to the initiating agent. The
details on whether this is needed and how it is done is defined by
the usage specifications. One exception to the above is that an
initiating agent would never indicate ICE mismatch.
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5.1.2. Determining Role
For each session, each agent (Initiating and Responding) takes on a
role. There are two roles -- controlling and controlled. The
controlling agent is responsible for the choice of the final
candidate pairs used for communications. For a full agent, this
means nominating the candidate pairs that can be used by ICE for each
media stream, and for updating the peer with the ICE's selection,
when needed. The controlled agent is told which candidate pairs to
use for each media stream, and does not require updating the peer to
signal this information. The sections below describe in detail the
actual procedures followed by controlling and controlled nodes.
The rules for determining the role and the impact on behavior are as
follows:
Both agents are full: The Initiating Agent which started the ICE
processing MUST take the controlling role, and the other MUST take
the controlled role. Both agents will form check lists, run the
ICE state machines, and generate connectivity checks. The
controlling agent will execute the logic in Section 6.2.1 to
nominate pairs that will be selected by ICE, and then both agents
end ICE as described in Section 6.2.1.2.
One agent full, one lite: The full agent MUST take the controlling
role, and the lite agent MUST take the controlled role. The full
agent will form check lists, run the ICE state machines, and
generate connectivity checks. That agent will execute the logic
in Section 6.2.1 to nominate pairs that will be selected by ICE,
and use the logic in Section 6.2.1.2 to end ICE. The lite
implementation will just listen for connectivity checks, receive
them and respond to them, and then conclude ICE as described in
Section 6.2.2. For the lite implementation, the state of ICE
processing for each media stream is considered to be Running, and
the state of ICE overall is Running.
Both lite: The Initiating Agent which started the ICE processing
MUST take the controlling role, and the other MUST take the
controlled role. In this case, no connectivity checks are ever
sent. Rather, once the candidates are exchanged, each agent
performs the processing described in Section 6.2 without
connectivity checks. It is possible that both agents will believe
they are controlled or controlling. In the latter case, the
conflict is resolved through glare detection capabilities in the
signaling protocol enabling the candidate exchange. The state of
ICE processing for each media stream is considered to be Running,
and the state of ICE overall is Running.
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Once the roles are determined for a session, they persist througout
the lifetime of the session. The roles can be re-determined as part
of an ICE restart (Section 6.3), but an ICE agent MUST NOT re-
determine the role as part of an ICE restart unless one or more of
the following criteria is fulfilled:
Full becomes lite: If the controlling agent is full, and switches to
lite, the roles MUST be re-determined if the peer agent is also
full.
Role conflict: If the ICE restart causes a role conflict, the roles
might be re-determined due to the role conflict procedures in
Section 6.1.3.1.1.
NOTE: There are certain 3PCC scenarios where an ICE restart might
cause a role conflict.
NOTE: The ICE agents needs to inform each other whether they are full
or lite before the roles are determined. The mechanism for that is
signalling protocol specific, and outside the scope of the document.
An ICE agent MUST be prepared that the peer might re-determine the
roles as part of any ICE restart, even if the criteria for doing so
are not fulfilled. This can happen if the peer is compliant with an
older version of this specification.
5.1.3. Forming the Check Lists
There is one check list per in-use media stream resulting from the
candidate exchange. To form the check list for a media stream, the
agent forms candidate pairs, computes a candidate pair priority,
orders the pairs by priority, prunes them, removes lower-priority
candidates and sets their states. These steps are described in this
section. If a check list is updated (e.g, due to detection of peer
reflexive candidates), the agent will re-perform the steps for the
updated check list.
5.1.3.1. Check List State
Each check list has a state, which captures the state of ICE checks
for the media stream associated with the check list. The states are:
Running: In this state, ICE checks are still in progress for this
media stream. Check lists are initially set to the Running state.
Completed: In this state, ICE checks have produced selected pairs
for each component of the media stream.
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Failed: In this state, the ICE checks have not completed
successfully for this media stream.
A check list with at least one pair that is Waiting is called an
active check list, and a check list with all pairs Frozen is called a
frozen check list.
5.1.3.2. Forming Candidate Pairs
First, the agent takes each of its candidates for a media stream
(called LOCAL CANDIDATES) and pairs them with the candidates it
received from its peer (called REMOTE CANDIDATES) for that media
stream. A local candidate is paired with a remote candidate if and
only if the two candidates have the same component ID and have the
same IP address version. It is possible that some of the local
candidates won't get paired with remote candidates, and some of the
remote candidates won't get paired with local candidates. This can
happen if one agent doesn't include candidates for the all of the
components for a media stream. If this happens, the number of
components for that media stream is effectively reduced, and
considered to be equal to the minimum across both agents of the
maximum component ID provided by each agent across all components for
the media stream.
In the case of RTP, this would happen when one agent provides
candidates for RTCP, and the other does not. As another example, the
initiating agent can multiplex RTP and RTCP on the same port
[RFC5761]. However, since the initiating agent doesn't know if the
peer agent can perform such multiplexing, it includes candidates for
RTP and RTCP on separate ports. If the peer agent can perform such
multiplexing, it would include just a single component for each
candidate -- for the combined RTP/RTCP mux. ICE would end up acting
as if there was just a single component for this candidate.
With IPv6 it is common for a host to have multiple host candidates
for each interface. To keep the amount of resulting candidate pairs
reasonable and to avoid candidate pairs that are highly unlikely to
work, IPv6 link-local addresses [RFC4291] MUST NOT be paired with
other than link-local addresses.
The candidate pairs whose local and remote candidates are both the
default candidates for a particular component is called the default
candidate pair for that component. This is the pair that would be
used to transmit media if both agents had not been ICE aware.
In order to aid understanding, Figure 6 shows the relationships
between several key concepts -- transport addresses, candidates,
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candidate pairs, and check lists, in addition to indicating the main
properties of candidates and candidate pairs.
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+--------------------------------------------+
| |
| +---------------------+ |
| |+----+ +----+ +----+ | +Type |
| || IP | |Port| |Tran| | +Priority |
| ||Addr| | | | | | +Foundation |
| |+----+ +----+ +----+ | +Component ID |
| | Transport | +Related Address |
| | Addr | |
| +---------------------+ +Base |
| Candidate |
+--------------------------------------------+
* *
* *************************************
* *
+-------------------------------+
.| |
| Local Remote |
| +----+ +----+ +default? |
| |Cand| |Cand| +valid? |
| +----+ +----+ +nominated?|
| +State |
| |
| |
| Candidate Pair |
+-------------------------------+
* *
* ************
* *
+------------------+
| Candidate Pair |
+------------------+
+------------------+
| Candidate Pair |
+------------------+
+------------------+
| Candidate Pair |
+------------------+
Check
List
Figure 6: Conceptual Diagram of a Check List
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5.1.3.3. Computing Pair Priority and Ordering Pairs
Once the pairs are formed, a candidate pair priority is computed.
Let G be the priority for the candidate provided by the controlling
agent. Let D be the priority for the candidate provided by the
controlled agent. The priority for a pair is computed as:
pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)
Where G>D?1:0 is an expression whose value is 1 if G is greater than
D, and 0 otherwise. Once the priority is assigned, the agent sorts
the candidate pairs in decreasing order of priority. If two pairs
have identical priority, the ordering amongst them is arbitrary.
5.1.3.4. Pruning the Pairs
This sorted list of candidate pairs is used to determine a sequence
of connectivity checks that will be performed. Each check involves
sending a request from a local candidate to a remote candidate.
Since an agent cannot send requests directly from a reflexive
candidate (server reflexive or peer reflexive), but only from its
base, the agent next goes through the sorted list of candidate pairs.
For each pair where the local candidate is reflexive, the candidate
MUST be replaced by its base. Once this has been done, the agent
MUST prune the list. This is done by removing a pair if its local
and remote candidates are identical to the local and remote
candidates of a pair higher up on the priority list. The result is a
sequence of ordered candidate pairs, called the check list for that
media stream.
5.1.3.5. Removing lower-priority Pairs
In order to limit the attacks described in Section 13.4.1, an agent
MUST limit the total number of connectivity checks the agent performs
across all check lists to a specific value, and this value MUST be
configurable. A default of 100 is RECOMMENDED. This limit is
enforced by discarding the lower-priority candidate pairs until there
are less than 100. It is RECOMMENDED that a lower value be utilized
when possible, set to the maximum number of plausible checks that
might be seen in an actual deployment configuration. The requirement
for configuration is meant to provide a tool for fixing this value in
the field if, once deployed, it is found to be problematic.
5.1.3.6. Computing Candidate Pair States
Each candidate pair in the check list has a foundation and a state.
The foundation is the combination of the foundations of the local and
remote candidates in the pair. The state is assigned once the check
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list for each media stream has been computed. There are five
potential values that the state can have:
Waiting: A check has not been sent for this pair, but can be sent as
soon as the pair is chosen based on the criteria for selecting for
which candidate pair a check is to be sent.
In-Progress: A check has been sent for this pair, but the
transaction is in progress.
Succeeded: A check has been sent for this pair, and produced a
successful result.
Failed: A check has been sent for this pair, and failed (a response
to the check was never received, or a failure response was
received).
Frozen: A check for this pair has not been sent, and it can not be
sent until the pair is unfrozen and moved into the Waiting state.
As ICE runs, the pairs will move between states as shown in Figure 7.
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+-----------+
| |
| |
| Frozen |
| |
| |
+-----------+
|
|unfreeze
|
V
+-----------+ +-----------+
| | | |
| | perform | |
| Waiting |-------->|In-Progress|
| | | |
| | | |
+-----------+ +-----------+
/ |
// |
// |
// |
/ |
// |
failure // |success
// |
/ |
// |
// |
// |
V V
+-----------+ +-----------+
| | | |
| | | |
| Failed | | Succeeded |
| | | |
| | | |
+-----------+ +-----------+
Figure 7: Pair State FSM
The initial states for each pair in a check list are computed by
performing the following sequence of steps:
1. The check lists are placed in an ordered list (the order is
determined by each ICE usage).
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2. The agent sets all of the pairs in each check list to the Frozen
state.
3. The agent sets all of the check lists to the Running state.
4. The agent examines each check list, starting from the first check
lists in the ordered list, in the following way:
* For each foundation, the candidate pair with the lowest
component ID (in case of multiple such pairs, the pair with
the highest priority) is placed in the Waiting state, unless a
candidate pair associated with the same foundation has already
been put in the Waiting state in one of the other examined
check lists. This will ensure that, within the ordered list,
only one pair with a given foundation is initially placed in
the Waiting state, while other pairs with the same foundation
remain in the Frozen state.
* When one or more candidate pairs within a given check list are
placed in the Waiting state. A check list with at least one
pair that is Waiting is called an active check list, and a
check list with all pairs Frozen is called a frozen check
list.
NOTE: The procedures above are different from RFC5245, where only
candidate pairs in the first check list of the ordered list were
initially placed in the Waiting state.
The table in Figure 8 illustrates how the initial states of the
candidiate pairs in the ordered list of check lists are set.
Table legend:
Each row (m1, m2,...) represents a check list associated with a media
stream. m1 represents the first check list in the ordered list of check
lists.
Each column (f1, f2,...) represents a foundation. Every candidiate pair
within a given column share the same foundation.
f-cp represents a candidate pair in the Frozen state.
w-cp represents a candidate pair in the Waiting state.
1. The agent sets all of the pairs in each check list to the Frozen
state.
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f1 f2 f3 f4 f5
-----------------------------
m1 | f-cp f-cp f-cp
|
m2 | f-cp f-cp f-cp f-cp
|
m3 | f-cp f-cp
2. For each foundation, the candidate pair with the lowest component ID
is placed in the Waiting state, unless a candidate pair associated with
the same foundation has already been put in the Waiting state in one of
the other examined check lists.
f1 f2 f3 f4 f5
-----------------------------
m1 | w-cp w-cp w-cp
|
m2 | f-cp f-cp f-cp w-cp
|
m3 | f-cp w-cp
In the first check list (m1) the candidate pair for each foundation is
placed in the Waiting state, as no pairs for the same foundations have
yet been placed in the Waiting state.
In the second check list (m2) the candidate pair for foundation f4 is
placed in the Waiting state. The candidate pair for foundations f1, f2
and f3 are kept in the Frozen state, as candidate pairs for those
foundations have already been placed in the Waiting state (within check
list m1).
In the third check list (m3) the candidate pair for foundation f5 is
placed in the Waiting state. The candidate pair for foundation f1 is
kept in the Frozen state, as a candidate pair for that foundation have
already been placed in the Waiting state (within check list m1).
Once each check list have been processed, one candidate pair for each
foundation has been placed in the Waiting state.
Figure 8: Initial Pair State
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5.1.4. ICE State
ICE processing across all check lists has a state associated with it.
This state is set to Running while ICE processing is under way. The
state is set to Completed when ICE processing is complete and set to
Failed if it failed without success.
5.1.5. Scheduling Checks
5.1.5.1. Triggered Check Queue
Once the agent has computed the check lists as described in
Section 5.1.3, the agent will begin performing ordinary checks and
triggered checks. For triggered checks, the agent maintains a FIFO
queue, triggered check queue, which contains candidate pairs for
which checks are to be sent at the next available opportunity.
5.1.5.2. Timer Tc
The generation of ordinary and triggered checks is govererned by a
timer, Tc. Each active check list is associated with an instance of
Tc, and whenever Tc for a given check list fires a check is performed
for a candidate pair within that check list.
The value of Tc is Ta*N seconds, where N is the number of active
check lists. Whenver the number of active check lists change, the
agent SHOULD re-calculate the Tc value. Multiplying by N allows this
aggregate check throughput to be split between all active check
lists. Tc associated with the first check list fires immediately,
causing the agent to start performing connectivity checks as soon as
the intitial states of the candidate pairs in each check list have
been calculated.
Implementations SHOULD spread out the starting of the Tc timers
associated with each check list, so that Tc for each check list do
not fire at the same time.
Based on local policy, an agent MAY set the Tc value to a number
bigger than described above, in order for Tc to fire less frequently.
5.1.5.3. Performing Connectivity Checks
When Tc for a given check list fires, the agent will perform a check
for a candidate pair within that check list as follows:
o If the triggered check queue contains one or more candidate pairs,
the agent removes the top pair from the queue, performs a
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connectivity check on that pair and puts the candidate pair state
to In-Progress; or
o If the triggered check queue is empty, and if there are one or
more candidate pairs in the Waiting state, the agent selects the
highest- priority candidate pair in the Waiting state, performs a
connectivity check on that pair and puts the candidate pair par
state to In-Progress; or
o If there is no candidate pair in the Waiting state, in any of the
check lists, and if there are one or more candidate pairs in the
Frozen state, the agent selects the highest-priority candidate
pair in the Frozen state, performs a connectivity check on that
pair and puts the candidate pair par state to In-Progress; or
o If there is no candidate in the Waiting or Frozen state, the agent
MUST terminate timer Tc for that check list and re-calculate Tc
for the remaining active check lists.
Once a candidate pair has been selected, the agent performs the check
by sending a STUN request from the base associated with the local
candiditate of the pair to the remote candidiate of the pair, as
described in Section 6.1.2.
Based on local policy, an agent MAY choose to terminate perfoming the
connectivity checks for one or more active checks lists (and
terminate the Tc associated with those check lists) at any time.
To compute the message integrity for the check, the agent uses the
remote username fragment and password learned from the candidate
information obtained from its peer. The local username fragment is
known directly by the agent for its own candidate.
The Initiator performs the ordinary checks on receiving the candidate
information from the Peer (responder) and having formed the
checklists. On the other hand the responding agent either performs
the triggered or ordinary checks as described above.
5.2. Lite Implementation Procedures
Lite implementations skips most of the steps in Section 5 except for
verifying the peer's ICE support and determining its role in the ICE
processing.
On determining the role for a lite implementation being the
controlling agent means selecting a candidate pair based on the ones
in the candidate exchange (for IPv4, there is only ever one pair),
and then updating the peer with the new candidate information
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reflecting that selection, when needed (it is never needed for an
IPv4-only host). The controlled agent is told which candidate pairs
to use for each media stream, and no further candidate updates are
needed to signal this information.
6. Performing Connectivity Checks
This section describes how connectivity checks are performed. All
ICE implementations are required to be compliant to [RFC5389], as
opposed to the older [RFC3489]. However, whereas a full
implementation will both generate checks (acting as a STUN client)
and receive them (acting as a STUN server), a lite implementation
will only receive checks, and thus will only act as a STUN server.
6.1. STUN Client Procedures
These procedures define how an agent sends a connectivity check,
whether it is an ordinary or a triggered check. These procedures are
only applicable to full implementations.
6.1.1. Creating Permissions for Relayed Candidates
If the connectivity check is being sent using a relayed local
candidate, the client MUST create a permission first if it has not
already created one previously. It would have created one previously
if it had told the TURN server to create a permission for the given
relayed candidate towards the IP address of the remote candidate. To
create the permission, the agent follows the procedures defined in
[RFC5766]. The permission MUST be created towards the IP address of
the remote candidate. It is RECOMMENDED that the agent defer
creation of a TURN channel until ICE completes, in which case
permissions for connectivity checks are normally created using a
CreatePermission request. Once established, the agent MUST keep the
permission active until ICE concludes.
6.1.2. Sending the Request
A connectivity check is generated by sending a Binding request from a
local candidate to a remote candidate. [RFC5389] describes how
Binding requests are constructed and generated. A connectivity check
MUST utilize the STUN short-term credential mechanism. Support for
backwards compatibility with RFC 3489 MUST NOT be used or assumed
with connectivity checks. The FINGERPRINT mechanism MUST be used for
connectivity checks.
ICE extends STUN by defining several new attributes, including
PRIORITY, USE-CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING. These
new attributes are formally defined in Section 14.1, and their usage
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is described in the subsections below. These STUN extensions are
applicable only to connectivity checks used for ICE.
6.1.2.1. PRIORITY
An agent MUST include the PRIORITY attribute in its Binding request.
The attribute MUST be set equal to the priority that would be
assigned, based on the algorithm in Section 4.1.2, to a peer
reflexive candidate, should one be learned as a consequence of this
check (see Section 6.1.2.4.2.1 for how peer reflexive candidates are
learned). This priority value will be computed identically to how
the priority for the local candidate of the pair was computed, except
that the type preference is set to the value for peer reflexive
candidate types.
6.1.2.2. USE-CANDIDATE
The controlling agent includes the USE-CANDIDATE attribute in order
to nominate a candidate pair Section 6.2.1.1. The controlled agent
MUST NOT include the USE-CANDIDATE attribute in its Binding request.
6.1.2.3. ICE-CONTROLLED and ICE-CONTROLLING
The agent MUST include the ICE-CONTROLLED attribute in the request if
it is in the controlled role, and MUST include the ICE-CONTROLLING
attribute in the request if it is in the controlling role.
The content of either attribute are used as tie-breaker values when
an ICE role conflict occurs Section 6.1.3.1.1.
The ICE-CONTROLLED and ICE-CONTROLLING attributes are defined in
Section 14.1.
6.1.2.3.1. Forming Credentials
A Binding request serving as a connectivity check MUST utilize the
STUN short-term credential mechanism. The username for the
credential is formed by concatenating the username fragment provided
by the peer with the username fragment of the agent sending the
request, separated by a colon (":"). The password is equal to the
password provided by the peer. For example, consider the case where
agent L is the initiating , agent and agent R is the responding
agent. Agent L included a username fragment of LFRAG for its
candidates and a password of LPASS. Agent R provided a username
fragment of RFRAG and a password of RPASS. A connectivity check from
L to R utilizes the username RFRAG:LFRAG and a password of RPASS. A
connectivity check from R to L utilizes the username LFRAG:RFRAG and
a password of LPASS. The responses utilize the same usernames and
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passwords as the requests (note that the USERNAME attribute is not
present in the response).
6.1.2.3.2. DiffServ Treatment
If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its
connectivity checks.
6.1.2.4. Processing the Response
When a Binding response is received, it is correlated to its Binding
request using the transaction ID, as defined in [RFC5389], which then
ties it to the candidate pair for which the Binding request was sent.
This section defines additional procedures for processing Binding
responses specific to this usage of STUN.
6.1.2.4.1. Failure Cases
If the STUN transaction generates a 487 (Role Conflict) error
response, the agent checks whether it included an ICE-CONTROLLED or
ICE-CONTROLLING attribute in the associated Binding request. If the
request contained an ICE-CONTROLLED attribute, the agent MUST switch
to the controlling role. If the request contained an ICE-CONTROLLING
attribute, the agent MUST switch to the controlled role.
Once the agent has switched its role, the agent MUST enqueue the
candidate pair whose check generated the 487 into the triggered check
queue. The state of that pair is set to Waiting. When the triggered
check is sent, it will contain an ICE-CONTROLLING or ICE-CONTROLLED
attribute reflecting its new role. The agent MUST NOT change the
tie-breaker value.
A change in roles will require an agent to recompute pair priorities
(Section 5.1.3.3), since those priorities are a function of
controlling and controlled roles. The change in role will also
impact whether the agent is responsible for selecting nominated pairs
and generating updated candidate information for sharing upon
conclusion of ICE.
Agents MAY support receipt of ICMP errors for connectivity checks.
If the STUN transaction generates an ICMP error, the agent sets the
state of the pair to Failed. If the STUN transaction generates a
STUN error response that is unrecoverable (as defined in [RFC5389])
or times out, the agent sets the state of the pair to Failed.
The agent MUST check that the source IP address and port of the
response equal the destination IP address and port to which the
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Binding request was sent, and that the destination IP address and
port of the response match the source IP address and port from which
the Binding request was sent. In other words, the source and
destination transport addresses in the request and responses are
symmetric. If they are not symmetric, the agent sets the state of
the pair to Failed.
6.1.2.4.2. Success Cases
A check is considered to be a success if all of the following are
true:
o The STUN transaction generated a success response.
o The source IP address and port of the response equals the
destination IP address and port to which the Binding request was
sent.
o The destination IP address and port of the response match the
source IP address and port from which the Binding request was
sent.
6.1.2.4.2.1. Discovering Peer Reflexive Candidates
The agent checks the mapped address from the STUN response. If the
transport address does not match any of the local candidates that the
agent knows about, the mapped address represents a new candidate -- a
peer reflexive candidate. Like other candidates, it has a type,
base, priority, and foundation. They are computed as follows:
o Its type is equal to peer reflexive.
o Its base is set equal to the local candidate of the candidate pair
from which the STUN check was sent.
o Its priority is set equal to the value of the PRIORITY attribute
in the Binding request.
o Its foundation is selected as described in Section 4.1.1.3.
This peer reflexive candidate is then added to the list of local
candidates for the media stream. Its username fragment and password
are the same as all other local candidates for that media stream.
However, the peer reflexive candidate is not paired with other remote
candidates. This is not necessary; a valid pair will be generated
from it momentarily based on the procedures in Section 6.1.2.4.2.2.
If an agent wishes to pair the peer reflexive candidate with other
remote candidates besides the one in the valid pair that will be
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generated, the agent MAY generate an update the peer with the
candidate information that includes the peer reflexive candidate.
This will cause it to be paired with all other remote candidates.
6.1.2.4.2.2. Constructing a Valid Pair
The agent constructs a candidate pair whose local candidate equals
the mapped address of the response, and whose remote candidate equals
the destination address to which the request was sent. This is
called a valid pair, since it has been validated by a STUN
connectivity check. The valid pair may equal the pair that generated
the check, may equal a different pair in the check list, or may be a
pair not currently on any check list. If the pair equals the pair
that generated the check or is on a check list currently, it is also
added to the VALID LIST, which is maintained by the agent for each
media stream. This list is empty at the start of ICE processing, and
fills as checks are performed, resulting in valid candidate pairs.
It will be very common that the pair will not be on any check list.
Recall that the check list has pairs whose local candidates are never
reflexive; those pairs had their local candidates converted to the
base of the reflexive candidates, and then pruned if they were
redundant. When the response to the STUN check arrives, the mapped
address will be reflexive if there is a NAT between the two. In that
case, the valid pair will have a local candidate that doesn't match
any of the pairs in the check list.
If the pair is not on any check list, the agent computes the priority
for the pair based on the priority of each candidate, using the
algorithm in Section 5.1.3. The priority of the local candidate
depends on its type. If it is not peer reflexive, it is equal to the
priority signaled for that candidate in the candidate exchange. If
it is peer reflexive, it is equal to the PRIORITY attribute the agent
placed in the Binding request that just completed. The priority of
the remote candidate is taken from the candidate information of the
peer. If the candidate does not appear there, then the check must
have been a triggered check to a new remote candidate. In that case,
the priority is taken as the value of the PRIORITY attribute in the
Binding request that triggered the check that just completed. The
pair is then added to the VALID LIST.
6.1.2.4.2.3. Updating Pair States
The agent sets the state of the pair that *generated* the check to
Succeeded. Note that, the pair which *generated* the check may be
different than the valid pair constructed in Section 6.1.2.4.2.2 as a
consequence of the response. The success of this check might also
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cause the state of other checks to change as well. The agent MUST
perform the following two steps:
1. The agent changes the states for all other Frozen pairs for the
same media stream and same foundation to Waiting. Typically, but
not always, these other pairs will have different component IDs.
2. If there is a pair in the valid list for every component of this
media stream (where this is the actual number of components being
used, in cases where the number of components signaled in the
candidate exchange differs from initiating to responding agent),
the success of this check may unfreeze checks for other media
streams. Note that this step is followed not just the first time
the valid list under consideration has a pair for every
component, but every subsequent time a check succeeds and adds
yet another pair to that valid list. The agent examines the
check list for each other media stream in turn:
* If the check list is active, the agent changes the state of
all Frozen pairs in that check list whose foundation matches a
pair in the valid list under consideration to Waiting.
* If the check list is frozen, and there is at least one pair in
the check list whose foundation matches a pair in the valid
list under consideration, the state of all pairs in the check
list whose foundation matches a pair in the valid list under
consideration is set to Waiting. This will cause the check
list to become active, and ordinary checks will begin for it,
as described in Section 5.1.5.
* If the check list is frozen, and there are no pairs in the
check list whose foundation matches a pair in the valid list
under consideration, the agent
+ groups together all of the pairs with the same foundation,
and
+ for each group, sets the state of the pair with the lowest
component ID to Waiting. If there is more than one such
pair, the one with the highest-priority is used.
6.1.2.4.2.4. Updating the Nominated Flag
If the agent was a controlling agent, and it had included a USE-
CANDIDATE attribute in the Binding request, the valid pair generated
from that check has its nominated flag set to true. This flag
indicates that this valid pair SHOULD be used for media, unless the
sending agent detects that the candidiate pair does not work. This
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concludes the ICE processing for this media stream or all media
streams; see Section 6.2.
If the agent is the controlled agent, the response may be the result
of a triggered check that was sent in response to a request that
itself had the USE-CANDIDATE attribute. This case is described in
Section 6.1.3.1.5, and may now result in setting the nominated flag
for the pair learned from the original request.
An agent MUST NOT select a candidate pair until it has sent a Binding
request, and received the corresponding Binding response, associated
with the candidiate pair.
6.1.2.4.3. Check List and Timer State Updates
Regardless of whether the check was successful or failed, the
completion of the transaction may require updating of check list and
timer states.
If all of the pairs in the check list are now either in the Failed or
Succeeded state:
o If there is not a pair in the valid list for each component of the
media stream, the state of the check list is set to Failed.
o For each frozen check list, the agent
* groups together all of the pairs with the same foundation, and
* for each group, sets the state of the pair with the lowest
component ID to Waiting. If there is more than one such pair,
the one with the highest-priority is used.
If none of the pairs in the check list are in the Waiting or Frozen
state, the check list is no longer considered active, and will not
count towards the value of N in the computation of timers for
ordinary checks as described in Section 5.1.5.
6.1.3. STUN Server Procedures
An agent MUST be prepared to receive a Binding request on the base of
each candidate it included in its most recent candidate exchange.
This requirement holds even if the peer is a lite implementation.
The agent MUST use the short-term credential mechanism (i.e., the
MESSAGE-INTEGRITY attribute) to authenticate the request and perform
a message integrity check. Likewise, the short-term credential
mechanism MUST be used for the response. The agent MUST consider the
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username to be valid if it consists of two values separated by a
colon, where the first value is equal to the username fragment
generated by the agent in an candidate exchange for a session in-
progress. It is possible (and in fact very likely) that the
initiating agent will receive a Binding request prior to receiving
the candidates from its peer. If this happens, the agent MUST
immediately generate a response (including computation of the mapped
address as described in Section 6.1.3.1.2). The agent has sufficient
information at this point to generate the response; the password from
the peer is not required. Once the answer is received, it MUST
proceed with the remaining steps required, namely, Section 6.1.3.1.3,
Section 6.1.3.1.4, and Section 6.1.3.1.5 for full implementations.
In cases where multiple STUN requests are received before the answer,
this may cause several pairs to be queued up in the triggered check
queue.
An agent MUST NOT utilize the ALTERNATE-SERVER mechanism, and MUST
NOT support the backwards-compatibility mechanisms to RFC 3489. It
MUST utilize the FINGERPRINT mechanism.
If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its responses
to Binding requests. The same would apply to any layer 2 markings
the endpoint might be applying to media packets.
6.1.3.1. Additional Procedures for Full Implementations
This subsection defines the additional server procedures applicable
to full implementations.
6.1.3.1.1. Detecting and Repairing Role Conflicts
Normally, the rules for selection of a role in Section 5.1.2 will
result in each agent selecting a different role -- one controlling
and one controlled. However, in unusual call flows, typically
utilizing third party call control, it is possible for both agents to
select the same role. This section describes procedures for checking
for this case and repairing it. These procedures apply only to
usages of ICE that require conflict resolution. The usage document
MUST specify whether this mechanism is needed.
An agent MUST examine the Binding request for either the ICE-
CONTROLLING or ICE-CONTROLLED attribute. It MUST follow these
procedures:
o If neither ICE-CONTROLLING nor ICE-CONTROLLED is present in the
request, the peer agent may have implemented a previous version of
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this specification. There may be a conflict, but it cannot be
detected.
o If the agent is in the controlling role, and the ICE-CONTROLLING
attribute is present in the request:
* If the agent's tie-breaker value is larger than or equal to the
contents of the ICE-CONTROLLING attribute, the agent generates
a Binding error response and includes an ERROR-CODE attribute
with a value of 487 (Role Conflict) but retains its role.
* If the agent's tie-breaker value is less than the contents of
the ICE-CONTROLLING attribute, the agent switches to the
controlled role.
o If the agent is in the controlled role, and the ICE-CONTROLLED
attribute is present in the request:
* If the agent's tie-breaker value is larger than or equal to the
contents of the ICE-CONTROLLED attribute, the agent switches to
the controlling role.
* If the agent's tie-breaker value is less than the contents of
the ICE-CONTROLLED attribute, the agent generates a Binding
error response and includes an ERROR-CODE attribute with a
value of 487 (Role Conflict) but retains its role.
o If the agent is in the controlled role and the ICE-CONTROLLING
attribute was present in the request, or the agent was in the
controlling role and the ICE-CONTROLLED attribute was present in
the request, there is no conflict.
A change in roles will require an agent to recompute pair priorities
(Section 5.1.3.3), since those priorities are a function of
controlling and controlled roles. The change in role will also
impact whether the agent is responsible for selecting nominated pairs
and initiating exchange with updated candidate information upon
conclusion of ICE.
The remaining sections in Section 6.1.3.1 are followed if the server
generated a successful response to the Binding request, even if the
agent changed roles.
6.1.3.1.2. Computing Mapped Address
For requests being received on a relayed candidate, the source
transport address used for STUN processing (namely, generation of the
XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the
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TURN server. That source transport address will be present in the
XOR-PEER-ADDRESS attribute of a Data Indication message, if the
Binding request was delivered through a Data Indication. If the
Binding request was delivered through a ChannelData message, the
source transport address is the one that was bound to the channel.
6.1.3.1.3. Learning Peer Reflexive Candidates
If the source transport address of the request does not match any
existing remote candidates, it represents a new peer reflexive remote
candidate. This candidate is constructed as follows:
o The priority of the candidate is set to the PRIORITY attribute
from the request.
o The type of the candidate is set to peer reflexive.
o The foundation of the candidate is set to an arbitrary value,
different from the foundation for all other remote candidates. If
any subsequent candidate exchanges contain this peer reflexive
candidate, it will signal the actual foundation for the candidate.
o The component ID of this candidate is set to the component ID for
the local candidate to which the request was sent.
This candidate is added to the list of remote candidates. However,
the agent does not pair this candidate with any local candidates.
6.1.3.1.4. Triggered Checks
Next, the agent constructs a pair whose local candidate is equal to
the transport address on which the STUN request was received, and a
remote candidate equal to the source transport address where the
request came from (which may be the peer reflexive remote candidate
that was just learned). The local candidate will either be a host
candidate (for cases where the request was not received through a
relay) or a relayed candidate (for cases where it is received through
a relay). The local candidate can never be a server reflexive
candidate. Since both candidates are known to the agent, it can
obtain their priorities and compute the candidate pair priority.
This pair is then looked up in the check list. There can be one of
several outcomes:
o If the pair is already on the check list:
* If the state of that pair is Waiting or Frozen, a check for
that pair is enqueued into the triggered check queue if not
already present.
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* If the state of that pair is In-Progress, the agent cancels the
in-progress transaction. Cancellation means that the agent
will not retransmit the request, will not treat the lack of
response to be a failure, but will wait the duration of the
transaction timeout for a response. In addition, the agent
MUST create a new connectivity check for that pair
(representing a new STUN Binding request transaction) by
enqueueing the pair in the triggered check queue. The state of
the pair is then changed to Waiting.
* If the state of the pair is Failed, it is changed to Waiting
and the agent MUST create a new connectivity check for that
pair (representing a new STUN Binding request transaction), by
enqueueing the pair in the triggered check queue.
* If the state of that pair is Succeeded, nothing further is
done.
These steps are done to facilitate rapid completion of ICE when both
agents are behind NAT.
o If the pair is not already on the check list:
* The pair is inserted into the check list based on its priority.
* Its state is set to Waiting.
* The pair is enqueued into the triggered check queue.
When a triggered check is to be sent, it is constructed and processed
as described in Section 6.1.2. These procedures require the agent to
know the transport address, username fragment, and password for the
peer. The username fragment for the remote candidate is equal to the
part after the colon of the USERNAME in the Binding request that was
just received. Using that username fragment, the agent can check the
candidates received from its peer (there may be more than one in
cases of forking), and find this username fragment. The
corresponding password is then selected.
6.1.3.1.5. Updating the Nominated Flag
If the Binding request received by the agent had the USE-CANDIDATE
attribute set, and the agent is in the controlled role, the agent
looks at the state of the pair computed in Section 6.1.3.1.4:
o If the state of this pair is Succeeded, it means that the check
generated by this pair produced a successful response. This would
have caused the agent to construct a valid pair when that success
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response was received (see Section 6.1.2.4.2.2). The agent now
sets the nominated flag in the valid pair to true. This may end
ICE processing for this media stream; see Section 6.2.
o If the state of this pair is In-Progress, if its check produces a
successful result, the resulting valid pair has its nominated flag
set when the response arrives. This may end ICE processing for
this media stream when it arrives; see Section 6.2.
6.1.3.2. Additional Procedures for Lite Implementations
If the check that was just received contained a USE-CANDIDATE
attribute, the agent constructs a candidate pair whose local
candidate is equal to the transport address on which the request was
received, and whose remote candidate is equal to the source transport
address of the request that was received. This candidate pair is
assigned an arbitrary priority, and placed into a list of valid
candidates called the valid list. The agent sets the nominated flag
for that pair to true. ICE processing is considered complete for a
media stream if the valid list contains a candidate pair for each
component.
6.2. Concluding ICE Processing
This section describes how an agent completes ICE.
6.2.1. Procedures for Full Implementations
Concluding ICE involves nominating pairs by the controlling agent and
updating of state machinery.
6.2.1.1. Nominating Pairs
When nominating, the controlling agent lets some number of checks
complete, each of which omit the USE-CANDIDATE attribute. Once one
or more checks complete successfully for a component of a media
stream, valid pairs are generated and added to the valid list. The
agent lets the checks continue until some stopping criterion is met,
and then picks amongst the valid pairs based on an evaluation
criterion. The criteria for stopping the checks and for evaluating
the valid pairs is entirely a matter of local optimization.
When the controlling agent selects the valid pair, it repeats the
check that produced this valid pair (by enqueueing the pair that
generated the check into the triggered check queue), this time with
the USE-CANDIDATE attribute. This check should succeed (since the
previous did), causing the nominated flag of that and only that pair
to be set. Consequently, there will be only a single nominated pair
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in the valid list for each component, and when the state of the check
list moves to completed, that exact pair is selected by ICE for
sending and receiving media for that component.
The controlling agent has control over the stopping and selection
criteria for checks. The only requirement is that the agent MUST
eventually pick one and only one candidate pair and generate a check
for that pair with the USE-CANDIDATE attribute present.
The controlled agent SHOULD select the nominated candidate pair if
the agent is receiving Binding responses associated with that
candidiate pair. Before the agent has received Binding responses
associated with the candidiate pair, the agent can send media on any
candidiate for which it has received Binding responses. If more than
one candidate pair is nominated by the controlling agent, the
controlled agent SHOULD select the candidate pair with the highest
priority.
NOTE: A controlling agent that does not support this speification
(i.e. it is implemented according to RFC 5245) might nominate more
than one candidiate pair. This was referred to as aggressive
nomination in RFC 5245. The usage of the 'ice2' ice option by
endpoints supporting this specifcation should prevent such
controlling agents from using aggressive nomination.
6.2.1.2. Updating States
For both controlling and controlled agents, the state of ICE
processing depends on the presence of nominated candidate pairs in
the valid list and on the state of the check list. Note that, at any
time, more than one of the following cases can apply:
o If there are no nominated pairs in the valid list for a media
stream and the state of the check list is Running, ICE processing
continues.
o If there is at least one nominated pair in the valid list for a
media stream and the state of the check list is Running:
* The agent MUST remove all Waiting and Frozen pairs in the check
list and triggered check queue for the same component as the
nominated pairs for that media stream.
* If an In-Progress pair in the check list is for the same
component as a nominated pair, the agent SHOULD cease
retransmissions for its check if its pair priority is lower
than the lowest-priority nominated pair for that component.
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o Once there is at least one nominated pair in the valid list for
every component of at least one media stream and the state of the
check list is Running:
* The agent MUST change the state of processing for its check
list for that media stream to Completed.
* The agent MUST continue to respond to any checks it may still
receive for that media stream, and MUST perform triggered
checks if required by the processing of Section 6.1.3.
* The agent MUST continue retransmitting any In-Progress checks
for that check list.
* The agent MAY begin transmitting media for this media stream as
described in Section 9.1.
o Once the state of each check list is Completed:
* The agent sets the state of ICE processing overall to
Completed.
o If the state of the check list is Failed, ICE has not been able to
complete for this media stream. The correct behavior depends on
the state of the check lists for other media streams:
* If all check lists are Failed, ICE processing overall is
considered to be in the Failed state, and the agent SHOULD
consider the session a failure, SHOULD NOT restart ICE, and the
controlling agent SHOULD terminate the entire session.
* If at least one of the check lists for other media streams is
Completed, the controlling agent SHOULD remove the failed media
stream from the session while sending updated candidate list to
its peer.
* If none of the check lists for other media streams are
Completed, but at least one is Running, the agent SHOULD let
ICE continue.
6.2.2. Procedures for Lite Implementations
Concluding ICE for a lite implementation is relatively
straightforward. There are two cases to consider:
The implementation is lite, and its peer is full.
The implementation is lite, and its peer is lite.
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The effect of ICE concluding is that the agent can free any allocated
host candidates that were not utilized by ICE, as described in
Section 6.2.3.
6.2.2.1. Peer Is Full
In this case, the agent will receive connectivity checks from its
peer. When an agent has received a connectivity check that includes
the USE-CANDIDATE attribute for each component of a media stream, the
state of ICE processing for that media stream moves from Running to
Completed. When the state of ICE processing for all media streams is
Completed, the state of ICE processing overall is Completed.
The lite implementation will never itself determine that ICE
processing has failed for a media stream; rather, the full peer will
make that determination and then remove or restart the failed media
stream as part of subsequent candidate exchange process.
6.2.2.2. Peer Is Lite
Once the candidate exchange has completed, both agents examine their
candidates and those of its peer. For each media stream, each agent
pairs up its own candidates with the candidates of its peer for that
media stream. Two candidates are paired up when they are for the
same component, utilize the same transport protocol (UDP in this
specification), and are from the same IP address family (IPv4 or
IPv6).
o If there is a single pair per component, that pair is added to the
Valid list. If all of the components for a media stream had one
pair, the state of ICE processing for that media stream is set to
Completed. If all media streams are Completed, the state of ICE
processing is set to Completed overall. This will always be the
case for implementations that are IPv4-only.
o If there is more than one pair per component:
* The agent MUST select a pair based on local policy. Since this
case only arises for IPv6, it is RECOMMENDED that an agent
follow the procedures of RFC 6724 [RFC6724] to select a single
pair.
* The agent adds the selected pair for each component to the
valid list. As described in Section 9.1, this will permit
media to begin flowing. However, it is possible (and in fact
likely) that both agents have chosen different pairs.
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* To reconcile this, the controlling agent MUST send updated
candidate list which will include the remote-candidates
attribute.
* The agent MUST NOT update the state of ICE processing until
after the candidate exchange completes. Then the controlling
agent MUST change the state of ICE processing to Completed for
all media streams, and the state of ICE processing overall to
Completed.
6.2.3. Freeing Candidates
6.2.3.1. Full Implementation Procedures
The procedures in Section 6.2 require that an agent continue to
listen for STUN requests and continue to generate triggered checks
for a media stream, even once processing for that stream completes.
The rules in this section describe when it is safe for an agent to
cease sending or receiving checks on a candidate that was not
selected by ICE, and then free the candidate.
Once ICE processing has reached the Completed state for all peers for
media streams using those candidates, the agent SHOULD wait an
additional three seconds, and then it MAY cease responding to checks
or generating triggered checks on that candidate. It MAY free the
candidate at that time. Freeing of server reflexive candidates is
never explicit; it happens by lack of a keepalive. The three-second
delay handles cases when aggressive nomination is used, and the
selected pairs can quickly change after ICE has completed.
6.2.3.2. Lite Implementation Procedures
A lite implementation MAY free candidates not selected by ICE as soon
as ICE processing has reached the Completed state for all peers for
all media streams using those candidates.
6.3. ICE Restarts
An agent MAY restart ICE processing for an existing media stream. An
ICE restart will cause all previous states, excluding the roles of
the agents, of ICE processing to be flushed and checks to start anew.
The only difference between an ICE restart and a brand new media
session is that during the restart media can continue to be sent to
the previously validated pair, and that a new media session always
requires the roles to be determined.
An agent MUST restart ICE for a media stream if:
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o The candidate(s) is being generated for the purposes of changing
the target of the media stream. In other words, if an agent wants
to generate an updated candidate information that, had ICE not
been in use, would result in a new value for the destination of a
media component.
o An agent is changing its implementation level. This typically
only happens in third party call control use cases, where the
entity performing the signaling is not the entity receiving the
media, and it has changed the target of media mid-session to
another entity that has a different ICE implementation.
To restart ICE, an agent MUST change both the password and the user
name fragment for the media stream when exchanging the candidates.
The new candidate set MAY include some, none, or all of the previous
candidates for that stream and MAY include a totally new set of
candidates.
As described in Section 5.1.2, ICE agents MUST NOT re-determine the
roles as part as an ICE restart, unless certain criteria that require
the roles to be re-determined is fulfilled.
7. ICE Option
This section defines a new ICE option, 'ice2'. The ICE option
indicates that the ICE agent that includes it (in an ice-options
attribute) is compliant to this specification. For example, the ICE
agent will not use the aggressive nomination procedure defined in
[RFC5245].
An ICE agent compliant to this specification MUST inform the peer
about the compliance using the 'ice2' ICE option.
NOTE: The encoding of the 'ice2' ICE option, and the message(s) used
to carry it to the peer, are protocol specific. The encoding for the
Session Description Protocol (SDP) [RFC4566] is defined in
[I-D.ietf-mmusic-ice-sip-sdp].
8. Keepalives
All endpoints MUST send keepalives for each media session. These
keepalives serve the purpose of keeping NAT bindings alive for the
media session. The keepalives SHOULD be sent using a format that is
supported by its peer. ICE endpoints allow for STUN-based keepalives
for UDP streams, and as such, STUN keepalives MUST be used when an
agent is a full ICE implementation and is communicating with a peer
that supports ICE (lite or full).
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If there has been no packet sent on the candidate pair ICE is using
for a media component for Tr seconds (where packets include those
defined for the component (RTP or RTCP) and previous keepalives), an
agent MUST generate a keepalive on that pair. ICE endpoints SHOULD
use a Tr value of 15 seconds, but MAY use another value, e.g. based
on configuration or network/NAT characteristics. For example, if an
agent has a dynamic way to discover the binding lifetimes of the
intervening NATs, it can use that value to determine Tr.
Administrators deploying ICE in more controlled networking
environments SHOULD set Tr to the longest duration possible in their
environment. ICE endpoints MUST NOT use a Tr value smaller than 15
seconds.
When STUN is being used for keepalives, a STUN Binding Indication is
used [RFC5389]. The Indication MUST NOT utilize any authentication
mechanism. It SHOULD contain the FINGERPRINT attribute to aid in
demultiplexing, but SHOULD NOT contain any other attributes. It is
used solely to keep the NAT bindings alive. The Binding Indication
is sent using the same local and remote candidates that are being
used for media. Though Binding Indications are used for keepalives,
an agent MUST be prepared to receive a connectivity check as well.
If a connectivity check is received, a response is generated as
discussed in [RFC5389], but there is no impact on ICE processing
otherwise.
An agent MUST begin the keepalive processing once ICE has selected
candidates for usage with media, or media begins to flow, whichever
happens first. Keepalives end once the session terminates or the
media stream is removed.
9. Media Handling
9.1. Sending Media
Procedures for sending media differ for full and lite
implementations.
9.1.1. Procedures for Full Implementations
Agents always send media using a candidate pair, using candidate
pairs in the Valid list. Once a candidiate pair has been selected
only that candidiate pair (referred to as selected pair) is used for
sending media. An agent will send media to the remote candidate
(i.e., setting the destination address and port of the packet equal
to that remote candidate), and will send it from the base associated
with the candidiate pair used for sending media. In case of a
relayed candidate, media is sent from the agent and forwarded through
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the base (located in the TURN server), using the procedures defined
in [RFC5766].
If the local candidate is a relayed candidate, it is RECOMMENDED that
an agent creates a channel on the TURN server towards the remote
candidate. This is done using the procedures for channel creation as
defined in Section 11 of [RFC5766].
The selected pair for a component of a media stream is:
o empty if the state of the check list for that media stream is
Running, and there is no previous selected pair for that component
due to an ICE restart
o equal to the previous selected pair for a component of a media
stream if the state of the check list for that media stream is
Running, and there was a previous selected pair for that component
due to an ICE restart
Unless an agent is able to produce a selected pair for all components
associated with a media stream, the agent MUST NOT continue sending
media for any component associated with that media stream.
9.1.2. Procedures for Lite Implementations
A lite implementation MUST NOT send media until it has a Valid list
that contains a candidate pair for each component of that media
stream. Once that happens, the agent MAY begin sending media
packets. To do that, it sends media to the remote candidate in the
pair (setting the destination address and port of the packet equal to
that remote candidate), and will send it from the base associated
with the candidiate pair used for sending media. In case of a
relayed candidate, media is sent from the agent and forwarded through
the base (located in the TURN server), using the procedures defined
in [RFC5766].
9.1.3. Procedures for All Implementations
ICE has interactions with jitter buffer adaptation mechanisms. An
RTP stream can begin using one candidate, and switch to another one,
though this happens rarely with ICE. The newer candidate may result
in RTP packets taking a different path through the network -- one
with different delay characteristics. As discussed below, agents are
encouraged to re-adjust jitter buffers when there are changes in
source or destination address of media packets. Furthermore, many
audio codecs use the marker bit to signal the beginning of a
talkspurt, for the purposes of jitter buffer adaptation. For such
codecs, it is RECOMMENDED that the sender set the marker bit
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[RFC3550] when an agent switches transmission of media from one
candidate pair to another.
9.2. Receiving Media
Even though ICE agents are only allowed to send media using valid
candidiate pairs (and, once a candidiate pair has been selected, only
on the selected pair) ICE implementations SHOULD by default be
prepared to receive media on any of the candidiates provided in the
most recent candidiate exchange with the peer. Specific ICE usages
MAY define rules that differs from this, e.g., by defining that media
must not be sent until selected pairs have been procduced for each
component associated with that media.
It is RECOMMENDED that, when an agent receives an RTP packet with a
new source or destination IP address for a particular media stream,
that the agent re-adjust its jitter buffers.
RFC 3550 [RFC3550] describes an algorithm in Section 8.2 for
detecting synchronization source (SSRC) collisions and loops. These
algorithms are based, in part, on seeing different source transport
addresses with the same SSRC. However, when ICE is used, such
changes will sometimes occur as the media streams switch between
candidates. An agent will be able to determine that a media stream
is from the same peer as a consequence of the STUN exchange that
proceeds media transmission. Thus, if there is a change in source
transport address, but the media packets come from the same peer
agent, this SHOULD NOT be treated as an SSRC collision.
10. Extensibility Considerations
This specification makes very specific choices about how both agents
in a session coordinate to arrive at the set of candidate pairs that
are selected for media. It is anticipated that future specifications
will want to alter these algorithms, whether they are simple changes
like timer tweaks or larger changes like a revamp of the priority
algorithm. When such a change is made, providing interoperability
between the two agents in a session is critical.
First, ICE provides the ice-options attribute. Each extension or
change to ICE is associated with a token. When an agent supporting
such an extension or change triggers candidate exchange, it MUST
include the token for that extension in this attribute. This allows
each side to know what the other side is doing. This attribute MUST
NOT be present if the agent doesn't support any ICE extensions or
changes.
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One of the complications in achieving interoperability is that ICE
relies on a distributed algorithm running on both agents to converge
on an agreed set of candidate pairs. If the two agents run different
algorithms, it can be difficult to guarantee convergence on the same
candidate pairs. The regular nomination procedure described in
Section 6.2 eliminates some of the tight coordination by delegating
the selection algorithm completely to the controlling agent.
Consequently, when a controlling agent is communicating with a peer
that supports options it doesn't know about, the agent MUST run a
regular nomination algorithm. When regular nomination is used, ICE
will converge perfectly even when both agents use different pair
prioritization algorithms. One of the keys to such convergence is
triggered checks, which ensure that the nominated pair is validated
by both agents. Consequently, any future ICE enhancements MUST
preserve triggered checks.
ICE is also extensible to other media streams beyond RTP, and for
transport protocols beyond UDP. Extensions to ICE for non-RTP media
streams need to specify how many components they utilize, and assign
component IDs to them, starting at 1 for the most important component
ID. Specifications for new transport protocols must define how, if
at all, various steps in the ICE processing differ from UDP.
11. Setting Ta and RTO
11.1. General
During the ICE gathering phase (Section 4.1.1) and while ICE is
performing connectivity checks (Section 6), an agent triggers STUN
and TURN transactions. These transactions are paced at a rate
indicated by Ta, and the retransmission interval for each transaction
is calculated based on the the retransmission timer for the STUN
transactions (RTO) [RFC5389].
This section describes how the Ta and RTO values are computed during
the ICE gathering phase and while ICE is performing connectivity
checks.
NOTE: Previously, in RFC 5245, different formulas were defined for
computing Ta and RTO, depending on whether ICE was used for a real-
time media stream (e.g. RTP) or not.
The formulas below result in a behavior whereby an agent will send
its first packet for every single connectivity check before
performing a retransmit. This can be seen in the formulas for the
RTO (which represents the retransmit interval). Those formulas scale
with N, the number of checks to be performed. As a result of this,
ICE maintains a nicely constant rate, but becomes more sensitive to
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packet loss. The loss of the first single packet for any
connectivity check is likely to cause that pair to take a long time
to be validated, and instead, a lower-priority check (but one for
which there was no packet loss) is much more likely to complete
first. This results in ICE performing sub-optimally, choosing lower-
priority pairs over higher-priority pairs. Implementors should be
aware of this consequence, but still should utilize the timer values
described here.
11.2. Ta
ICE agents SHOULD use the default Ta value, 50 ms, but MAY use
another value based on the characteristics of the associated media.
If an ICE agent wants to use another Ta value than the default value,
the agent MUST indicate the proposed value to its peer during the ICE
exchange. Both agents MUST use the higher value of the proposed
values. If an agent does not propose a value, the default value is
used for that agent when comparing which value is higher.
Regardless of the Ta value chosen for each ICE agent, the combination
of all transactions from all ICE agents (if a given implementation
runs several concurrent ICE agents) MUST NOT be sent more often than
once every 5ms (as though there were one global Ta value for pacing
all ICE agents).
This mechanism of a global minimum pacing interval of 5ms is not
generally applicable to transport protocols, but is applicable to ICE
based on the following reasoning.
o Start with the following rules which would be generally applicable
to transport protocols:
1. Let MaxBytes be the maximum number of bytes allowed to be
outstanding in the network at start-up, which SHOULD be 14600
bytes per RFC 6928.
2. Let HTO be the transaction timeout, which SHOULD be 2*RTT if
RTT is known and 500ms otherwise. This is based on the RTO
for STUN messages from RFC 5389 and the the TCP initial RTO,
which is 1 sec in RFC 6298.
3. Let MinPacing be the minimum pacing interval between
transactions, which SHOULD be 5ms.
o Observe that ICE agents typically do not know the RTT for ICE
transactions (connectivity checks in particular), meaning that HTO
will almost always be 500ms.
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o Observe that a MinPacing of 5ms and HTO of 500ms gives at most 100
packets/HTO, which for a typical ICE check of less than 120 bytes
means a maximum of 12000 outstanding bytes in the network, which
is less than the maximum expressed by rule 1.
o Thus, for ICE, the rule set reduces down to just the MinPacing
rule, which is equivalant to having a global Ta value.
NOTE: Appendix C shows examples of required bandwidth, using
different Ta values.
11.3. RTO
During the ICE gathering phase, ICE agents SHOULD calculate the RTO
value using the following formula:
RTO = MAX (500ms, Ta * (Num-Of-Pairs))
Num-Of-Pairs: the number of pairs of candidates
with STUN or TURN servers.
For connectivity checks, ICE agents SHOULD calculate the RTO value
using the following formula:
RTO = MAX (500ms, Ta*N * (Num-Waiting + Num-In-Progress))
Num-Waiting: the number of checks in the check list in the
Waiting state.
Num-In-Progress: the number of checks in the In-Progress state.
Note that the RTO will be different for each transaction as the
number of checks in the Waiting and In-Progress states change.
ICE agents MAY calculate the RTO value using other mechanisms than
those described above. ICE agents MUST NOT use a RTO value smaller
than 500 ms.
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12. Example
The example is based on the simplified topology of Figure 9.
+-------+
|STUN |
|Server |
+-------+
|
+---------------------+
| |
| Internet |
| |
+---------------------+
| |
| |
+---------+ |
| NAT | |
+---------+ |
| |
| |
+-----+ +-----+
| L | | R |
+-----+ +-----+
Figure 9: Example Topology
Two agents, L and R, are using ICE. Both are full-mode ICE
implementations and use aggressive nomination when they are
controlling. Both agents have a single IPv4 address. For agent L,
it is 10.0.1.1 in private address space [RFC1918], and for agent R,
192.0.2.1 on the public Internet. Both are configured with the same
STUN server (shown in this example for simplicity, although in
practice the agents do not need to use the same STUN server), which
is listening for STUN Binding requests at an IP address of 192.0.2.2
and port 3478. TURN servers are not used in this example. Agent L
is behind a NAT, and agent R is on the public Internet. The NAT has
an endpoint independent mapping property and an address dependent
filtering property. The public side of the NAT has an IP address of
192.0.2.3.
To facilitate understanding, transport addresses are listed using
variables that have mnemonic names. The format of the name is
entity-type-seqno, where entity refers to the entity whose IP address
the transport address is on, and is one of "L", "R", "STUN", or
"NAT". The type is either "PUB" for transport addresses that are
public, and "PRIV" for transport addresses that are private.
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Finally, seq-no is a sequence number that is different for each
transport address of the same type on a particular entity. Each
variable has an IP address and port, denoted by varname.IP and
varname.PORT, respectively, where varname is the name of the
variable.
The STUN server has advertised transport address STUN-PUB-1 (which is
192.0.2.2:3478).
In the call flow itself, STUN messages are annotated with several
attributes. The "S=" attribute indicates the source transport
address of the message. The "D=" attribute indicates the destination
transport address of the message. The "MA=" attribute is used in
STUN Binding response messages and refers to the mapped address.
"USE-CAND" implies the presence of the USE-CANDIDATE attribute.
The call flow examples omit STUN authentication operations and RTCP,
and focus on RTP for a single media stream between two full
implementations.
L NAT STUN R
|RTP STUN alloc. | |
|(1) STUN Req | | |
|S=$L-PRIV-1 | | |
|D=$STUN-PUB-1 | | |
|------------->| | |
| |(2) STUN Req | |
| |S=$NAT-PUB-1 | |
| |D=$STUN-PUB-1 | |
| |------------->| |
| |(3) STUN Res | |
| |S=$STUN-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<-------------| |
|(4) STUN Res | | |
|S=$STUN-PUB-1 | | |
|D=$L-PRIV-1 | | |
|MA=$NAT-PUB-1 | | |
|<-------------| | |
|(5) L's Candidate Information| |
|------------------------------------------->|
| | | | RTP STUN
| | | | alloc.
| | |(6) STUN Req |
| | |S=$R-PUB-1 |
| | |D=$STUN-PUB-1 |
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| | |<-------------|
| | |(7) STUN Res |
| | |S=$STUN-PUB-1 |
| | |D=$R-PUB-1 |
| | |MA=$R-PUB-1 |
| | |------------->|
|(8) R's Candidate Information| |
|<-------------------------------------------|
| |(9) Bind Req | |Begin
| |S=$R-PUB-1 | |Connectivity
| |D=L-PRIV-1 | |Checks
| |<----------------------------|
| |Dropped | |
|(10) Bind Req | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|USE-CAND | | |
|------------->| | |
| |(11) Bind Req | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |USE-CAND | |
| |---------------------------->|
| |(12) Bind Res | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<----------------------------|
|(13) Bind Res | | |
|S=$R-PUB-1 | | |
|D=$L-PRIV-1 | | |
|MA=$NAT-PUB-1 | | |
|<-------------| | |
|RTP flows | | |
| |(14) Bind Req | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |<----------------------------|
|(15) Bind Req | | |
|S=$R-PUB-1 | | |
|D=$L-PRIV-1 | | |
|<-------------| | |
|(16) Bind Res | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|MA=$R-PUB-1 | | |
|------------->| | |
| |(17) Bind Res | |
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| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |MA=$R-PUB-1 | |
| |---------------------------->|
| | | |RTP flows
Figure 10: Example Flow
First, agent L obtains a host candidate from its local IP address
(not shown), and from that, sends a STUN Binding request to the STUN
server to get a server reflexive candidate (messages 1-4). Recall
that the NAT has the address and port independent mapping property.
Here, it creates a binding of NAT-PUB-1 for this UDP request, and
this becomes the server reflexive candidate for RTP.
Agent L sets a type preference of 126 for the host candidate and 100
for the server reflexive. The local preference is 65535. Based on
this, the priority of the host candidate is 2130706431 and for the
server reflexive candidate is 1694498815. The host candidate is
assigned a foundation of 1, and the server reflexive, a foundation of
2. These are sent to the peer.
This candidate information is received at agent R. Agent R will
obtain a host candidate, and from it, obtain a server reflexive
candidate (messages 6-7). Since R is not behind a NAT, this
candidate is identical to its host candidate, and they share the same
base. It therefore discards this redundant candidate and ends up
with a single host candidate. With identical type and local
preferences as L, the priority for this candidate is 2130706431. It
chooses a foundation of 1 for its single candidate. Then R's
candidates are then sent to L.
Since neither side indicated that it is lite, the initiating agent
that began ICE processing (agent L) becomes the controlling agent.
Agents L and R both pair up the candidates. They both initially have
two pairs. However, agent L will prune the pair containing its
server reflexive candidate, resulting in just one. At agent L, this
pair has a local candidate of $L_PRIV_1 and remote candidate of
$R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note that
an implementation would represent this as a 64-bit integer so as not
to lose precision). At agent R, there are two pairs. The highest
priority has a local candidate of $R_PUB_1 and remote candidate of
$L_PRIV_1 and has a priority of 4.57566E+18, and the second has a
local candidate of $R_PUB_1 and remote candidate of $NAT_PUB_1 and
priority 3.63891E+18.
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Agent R begins its connectivity check (message 9) for the first pair
(between the two host candidates). Since R is the controlled agent
for this session, the check omits the USE-CANDIDATE attribute. The
host candidate from agent L is private and behind a NAT, and thus
this check won't be successful, because the packet cannot be routed
from R to L.
When agent L gets the R's candidates, it performs its one and only
connectivity check (messages 10-13). It implements the aggressive
nomination algorithm, and thus includes a USE-CANDIDATE attribute in
this check. Since the check succeeds, agent L creates a new pair,
whose local candidate is from the mapped address in the Binding
response (NAT-PUB-1 from message 13) and whose remote candidate is
the destination of the request (R-PUB-1 from message 10). This is
added to the valid list. In addition, it is marked as selected since
the Binding request contained the USE-CANDIDATE attribute. Since
there is a selected candidate in the Valid list for the one component
of this media stream, ICE processing for this stream moves into the
Completed state. Agent L can now send media if it so chooses.
Soon after receipt of the STUN Binding request from agent L (message
11), agent R will generate its triggered check. This check happens
to match the next one on its check list -- from its host candidate to
agent L's server reflexive candidate. This check (messages 14-17)
will succeed. Consequently, agent R constructs a new candidate pair
using the mapped address from the response as the local candidate (R-
PUB-1) and the destination of the request (NAT-PUB-1) as the remote
candidate. This pair is added to the Valid list for that media
stream. Since the check was generated in the reverse direction of a
check that contained the USE-CANDIDATE attribute, the candidate pair
is marked as selected. Consequently, processing for this stream
moves into the Completed state, and agent R can also send media.
13. Security Considerations
There are several types of attacks possible in an ICE system. This
section considers these attacks and their countermeasures. These
countermeasures include:
o Using ICE in conjunction with secure signaling techniques, such as
SIPS.
o Limiting the total number of connectivity checks to 100, and
optionally limiting the number of candidates they'll accept in an
candidate exchange.
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13.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks.
Ultimately, all of these attacks fool an agent into thinking
something incorrect about the results of the connectivity checks.
The possible false conclusions an attacker can try and cause are:
False Invalid: An attacker can fool a pair of agents into thinking a
candidate pair is invalid, when it isn't. This can be used to
cause an agent to prefer a different candidate (such as one
injected by the attacker) or to disrupt a call by forcing all
candidates to fail.
False Valid: An attacker can fool a pair of agents into thinking a
candidate pair is valid, when it isn't. This can cause an agent
to proceed with a session, but then not be able to receive any
media.
False Peer Reflexive Candidate: An attacker can cause an agent to
discover a new peer reflexive candidate, when it shouldn't have.
This can be used to redirect media streams to a Denial-of-Service
(DoS) target or to the attacker, for eavesdropping or other
purposes.
False Valid on False Candidate: An attacker has already convinced an
agent that there is a candidate with an address that doesn't
actually route to that agent (for example, by injecting a false
peer reflexive candidate or false server reflexive candidate). It
must then launch an attack that forces the agents to believe that
this candidate is valid.
If an attacker can cause a false peer reflexive candidate or false
valid on a false candidate, it can launch any of the attacks
described in [RFC5389].
To force the false invalid result, the attacker has to wait for the
connectivity check from one of the agents to be sent. When it is,
the attacker needs to inject a fake response with an unrecoverable
error response, such as a 400. However, since the candidate is, in
fact, valid, the original request may reach the peer agent, and
result in a success response. The attacker needs to force this
packet or its response to be dropped, through a DoS attack, layer 2
network disruption, or other technique. If it doesn't do this, the
success response will also reach the originator, alerting it to a
possible attack. Fortunately, this attack is mitigated completely
through the STUN short-term credential mechanism. The attacker needs
to inject a fake response, and in order for this response to be
processed, the attacker needs the password. If the candidate
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exchange signaling is secured, the attacker will not have the
password and its response will be discarded.
Forcing the fake valid result works in a similar way. The agent
needs to wait for the Binding request from each agent, and inject a
fake success response. The attacker won't need to worry about
disrupting the actual response since, if the candidate is not valid,
it presumably wouldn't be received anyway. However, like the fake
invalid attack, this attack is mitigated by the STUN short-term
credential mechanism in conjunction with a secure candidate exchange.
Forcing the false peer reflexive candidate result can be done either
with fake requests or responses, or with replays. We consider the
fake requests and responses case first. It requires the attacker to
send a Binding request to one agent with a source IP address and port
for the false candidate. In addition, the attacker must wait for a
Binding request from the other agent, and generate a fake response
with a XOR-MAPPED-ADDRESS attribute containing the false candidate.
Like the other attacks described here, this attack is mitigated by
the STUN message integrity mechanisms and secure candidate exchanges.
Forcing the false peer reflexive candidate result with packet replays
is different. The attacker waits until one of the agents sends a
check. It intercepts this request, and replays it towards the other
agent with a faked source IP address. It must also prevent the
original request from reaching the remote agent, either by launching
a DoS attack to cause the packet to be dropped, or forcing it to be
dropped using layer 2 mechanisms. The replayed packet is received at
the other agent, and accepted, since the integrity check passes (the
integrity check cannot and does not cover the source IP address and
port). It is then responded to. This response will contain a XOR-
MAPPED-ADDRESS with the false candidate, and will be sent to that
false candidate. The attacker must then receive it and relay it
towards the originator.
The other agent will then initiate a connectivity check towards that
false candidate. This validation needs to succeed. This requires
the attacker to force a false valid on a false candidate. Injecting
of fake requests or responses to achieve this goal is prevented using
the integrity mechanisms of STUN and the candidate exchange. Thus,
this attack can only be launched through replays. To do that, the
attacker must intercept the check towards this false candidate, and
replay it towards the other agent. Then, it must intercept the
response and replay that back as well.
This attack is very hard to launch unless the attacker is identified
by the fake candidate. This is because it requires the attacker to
intercept and replay packets sent by two different hosts. If both
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agents are on different networks (for example, across the public
Internet), this attack can be hard to coordinate, since it needs to
occur against two different endpoints on different parts of the
network at the same time.
If the attacker itself is identified by the fake candidate, the
attack is easier to coordinate. However, if the media path is
secured (e.g., using SRTP [RFC3711]), the attacker will not be able
to play the media packets, but will only be able to discard them,
effectively disabling the media stream for the call. However, this
attack requires the agent to disrupt packets in order to block the
connectivity check from reaching the target. In that case, if the
goal is to disrupt the media stream, it's much easier to just disrupt
it with the same mechanism, rather than attack ICE.
13.2. Attacks on Server Reflexive Address Gathering
ICE endpoints make use of STUN Binding requests for gathering server
reflexive candidates from a STUN server. These requests are not
authenticated in any way. As a consequence, there are numerous
techniques an attacker can employ to provide the client with a false
server reflexive candidate:
o An attacker can compromise the DNS, causing DNS queries to return
a rogue STUN server address. That server can provide the client
with fake server reflexive candidates. This attack is mitigated
by DNS security, though DNS-SEC is not required to address it.
o An attacker that can observe STUN messages (such as an attacker on
a shared network segment, like WiFi) can inject a fake response
that is valid and will be accepted by the client.
o An attacker can compromise a STUN server by means of a virus, and
cause it to send responses with incorrect mapped addresses.
A false mapped address learned by these attacks will be used as a
server reflexive candidate in the ICE exchange. For this candidate
to actually be used for media, the attacker must also attack the
connectivity checks, and in particular, force a false valid on a
false candidate. This attack is very hard to launch if the false
address identifies a fourth party (neither the initiator, responder,
nor attacker), since it requires attacking the checks generated by
each agent in the session, and is prevented by SRTP if it identifies
the attacker themself.
If the attacker elects not to attack the connectivity checks, the
worst it can do is prevent the server reflexive candidate from being
used. However, if the peer agent has at least one candidate that is
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reachable by the agent under attack, the STUN connectivity checks
themselves will provide a peer reflexive candidate that can be used
for the exchange of media. Peer reflexive candidates are generally
preferred over server reflexive candidates. As such, an attack
solely on the STUN address gathering will normally have no impact on
a session at all.
13.3. Attacks on Relayed Candidate Gathering
An attacker might attempt to disrupt the gathering of relayed
candidates, forcing the client to believe it has a false relayed
candidate. Exchanges with the TURN server are authenticated using a
long-term credential. Consequently, injection of fake responses or
requests will not work. In addition, unlike Binding requests,
Allocate requests are not susceptible to replay attacks with modified
source IP addresses and ports, since the source IP address and port
are not utilized to provide the client with its relayed candidate.
However, TURN servers are susceptible to DNS attacks, or to viruses
aimed at the TURN server, for purposes of turning it into a zombie or
rogue server. These attacks can be mitigated by DNS-SEC and through
good box and software security on TURN servers.
Even if an attacker has caused the client to believe in a false
relayed candidate, the connectivity checks cause such a candidate to
be used only if they succeed. Thus, an attacker must launch a false
valid on a false candidate, per above, which is a very difficult
attack to coordinate.
13.4. Insider Attacks
In addition to attacks where the attacker is a third party trying to
insert fake candidate information or stun messages, there are attacks
possible with ICE when the attacker is an authenticated and valid
participant in the ICE exchange.
13.4.1. STUN Amplification Attack
The STUN amplification attack is similar to the voice hammer.
However, instead of voice packets being directed to the target, STUN
connectivity checks are directed to the target. The attacker sends
an a large number of candidates, say, 50. The responding agent
receives the candidate information, and starts its checks, which are
directed at the target, and consequently, never generate a response.
The answerer will start a new connectivity check every Ta ms (say,
Ta=20ms). However, the retransmission timers are set to a large
number due to the large number of candidates. As a consequence,
packets will be sent at an interval of one every Ta milliseconds, and
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then with increasing intervals after that. Thus, STUN will not send
packets at a rate faster than media would be sent, and the STUN
packets persist only briefly, until ICE fails for the session.
Nonetheless, this is an amplification mechanism.
It is impossible to eliminate the amplification, but the volume can
be reduced through a variety of heuristics. Agents SHOULD limit the
total number of connectivity checks they perform to 100.
Additionally, agents MAY limit the number of candidates they'll
accept.
Frequently, protocols that wish to avoid these kinds of attacks force
the initiator to wait for a response prior to sending the next
message. However, in the case of ICE, this is not possible. It is
not possible to differentiate the following two cases:
o There was no response because the initiator is being used to
launch a DoS attack against an unsuspecting target that will not
respond.
o There was no response because the IP address and port are not
reachable by the initiator.
In the second case, another check should be sent at the next
opportunity, while in the former case, no further checks should be
sent.
14. STUN Extensions
14.1. New Attributes
This specification defines four new attributes, PRIORITY, USE-
CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING.
The PRIORITY attribute indicates the priority that is to be
associated with a peer reflexive candidate, should one be discovered
by this check. It is a 32-bit unsigned integer, and has an attribute
value of 0x0024.
The USE-CANDIDATE attribute indicates that the candidate pair
resulting from this check should be used for transmission of media.
The attribute has no content (the Length field of the attribute is
zero); it serves as a flag. It has an attribute value of 0x0025.
The ICE-CONTROLLED attribute is present in a Binding request and
indicates that the client believes it is currently in the controlled
role. The content of the attribute is a 64-bit unsigned integer in
network byte order, which contains a random number. The number is
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used for solving role conflicts, when it is referred to as the tie-
breaker value. An ICE agent MUST use the same number for all Binding
requests, for all streams, within an ICE session. The ICE agent MAY
change the number when an ICE restart occurs.
The ICE-CONTROLLING attribute is present in a Binding request and
indicates that the client believes it is currently in the controlling
role. The content of the attribute is a 64-bit unsigned integer in
network byte order, which contains a random number. The number is
used for solving role conflicts, when it is referred to as the tie-
breaker value. An ICE agent MUST use the same number for all Binding
requests, for all streams, within an ICE session. The ICE agent MAY
change the number when an ICE restart occurs.
14.2. New Error Response Codes
This specification defines a single error response code:
487 (Role Conflict): The Binding request contained either the ICE-
CONTROLLING or ICE-CONTROLLED attribute, indicating an ICE role
that conflicted with the server. The server compared the tie-
breaker values of the client and the server and determined that
the client needs to switch roles.
15. Operational Considerations
This section discusses issues relevant to network operators looking
to deploy ICE.
15.1. NAT and Firewall Types
ICE was designed to work with existing NAT and firewall equipment.
Consequently, it is not necessary to replace or reconfigure existing
firewall and NAT equipment in order to facilitate deployment of ICE.
Indeed, ICE was developed to be deployed in environments where the
Voice over IP (VoIP) operator has no control over the IP network
infrastructure, including firewalls and NAT.
That said, ICE works best in environments where the NAT devices are
"behave" compliant, meeting the recommendations defined in [RFC4787]
and [RFC5382]. In networks with behave-compliant NAT, ICE will work
without the need for a TURN server, thus improving voice quality,
decreasing call setup times, and reducing the bandwidth demands on
the network operator.
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15.2. Bandwidth Requirements
Deployment of ICE can have several interactions with available
network capacity that operators should take into consideration.
15.2.1. STUN and TURN Server Capacity Planning
First and foremost, ICE makes use of TURN and STUN servers, which
would typically be located in the network operator's data centers.
The STUN servers require relatively little bandwidth. For each
component of each media stream, there will be one or more STUN
transactions from each client to the STUN server. In a basic voice-
only IPv4 VoIP deployment, there will be four transactions per call
(one for RTP and one for RTCP, for both caller and callee). Each
transaction is a single request and a single response, the former
being 20 bytes long, and the latter, 28. Consequently, if a system
has N users, and each makes four calls in a busy hour, this would
require N*1.7bps. For one million users, this is 1.7 Mbps, a very
small number (relatively speaking).
TURN traffic is more substantial. The TURN server will see traffic
volume equal to the STUN volume (indeed, if TURN servers are
deployed, there is no need for a separate STUN server), in addition
to the traffic for the actual media traffic. The amount of calls
requiring TURN for media relay is highly dependent on network
topologies, and can and will vary over time. In a network with 100%
behave-compliant NAT, it is exactly zero. At time of writing, large-
scale consumer deployments were seeing between 5 and 10 percent of
calls requiring TURN servers. Considering a voice-only deployment
using G.711 (so 80 kbps in each direction), with .2 erlangs during
the busy hour, this is N*3.2 kbps. For a population of one million
users, this is 3.2 Gbps, assuming a 10% usage of TURN servers.
15.2.2. Gathering and Connectivity Checks
The process of gathering of candidates and performing of connectivity
checks can be bandwidth intensive. ICE has been designed to pace
both of these processes. The gathering phase and the connectivity
check phase are meant to generate traffic at roughly the same
bandwidth as the media traffic itself. This was done to ensure that,
if a network is designed to support multimedia traffic of a certain
type (voice, video, or just text), it will have sufficient capacity
to support the ICE checks for that media. Of course, the ICE checks
will cause a marginal increase in the total utilization; however,
this will typically be an extremely small increase.
Congestion due to the gathering and check phases has proven to be a
problem in deployments that did not utilize pacing. Typically,
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access links became congested as the endpoints flooded the network
with checks as fast as they can send them. Consequently, network
operators should make sure that their ICE implementations support the
pacing feature. Though this pacing does increase call setup times,
it makes ICE network friendly and easier to deploy.
15.2.3. Keepalives
STUN keepalives (in the form of STUN Binding Indications) are sent in
the middle of a media session. However, they are sent only in the
absence of actual media traffic. In deployments that are not
utilizing Voice Activity Detection (VAD), the keepalives are never
used and there is no increase in bandwidth usage. When VAD is being
used, keepalives will be sent during silence periods. This involves
a single packet every 15-20 seconds, far less than the packet every
20-30 ms that is sent when there is voice. Therefore, keepalives
don't have any real impact on capacity planning.
15.3. ICE and ICE-lite
Deployments utilizing a mix of ICE and ICE-lite interoperate
perfectly. They have been explicitly designed to do so, without loss
of function.
However, ICE-lite can only be deployed in limited use cases. Those
cases, and the caveats involved in doing so, are documented in
Appendix A.
15.4. Troubleshooting and Performance Management
ICE utilizes end-to-end connectivity checks, and places much of the
processing in the endpoints. This introduces a challenge to the
network operator -- how can they troubleshoot ICE deployments? How
can they know how ICE is performing?
ICE has built-in features to help deal with these problems. SIP
servers on the signaling path, typically deployed in the data centers
of the network operator, will see the contents of the candidate
exchanges that convey the ICE parameters. These parameters include
the type of each candidate (host, server reflexive, or relayed),
along with their related addresses. Once ICE processing has
completed, an updated candidate exchange takes place, signaling the
selected address (and its type). This updated re-INVITE is performed
exactly for the purposes of educating network equipment (such as a
diagnostic tool attached to a SIP server) about the results of ICE
processing.
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As a consequence, through the logs generated by the SIP server, a
network operator can observe what types of candidates are being used
for each call, and what address was selected by ICE. This is the
primary information that helps evaluate how ICE is performing.
15.5. Endpoint Configuration
ICE relies on several pieces of data being configured into the
endpoints. This configuration data includes timers, credentials for
TURN servers, and hostnames for STUN and TURN servers. ICE itself
does not provide a mechanism for this configuration. Instead, it is
assumed that this information is attached to whatever mechanism is
used to configure all of the other parameters in the endpoint. For
SIP phones, standard solutions such as the configuration framework
[RFC6080] have been defined.
16. IANA Considerations
The original ICE specification registered four new STUN attributes,
and one new STUN error response. The STUN attributes and error
response are reproduced here. In addition, this specification
registers a new ICE option.
16.1. STUN Attributes
IANA has registered four STUN attributes:
0x0024 PRIORITY
0x0025 USE-CANDIDATE
0x8029 ICE-CONTROLLED
0x802A ICE-CONTROLLING
16.2. STUN Error Responses
IANA has registered following STUN error response code:
487 Role Conflict: The client asserted an ICE role (controlling or
controlled) that is in conflict with the role of the server.
16.3. ICE Options
IANA is requested to register the following ICE option in the "ICE
Options" sub-registry of the "Interactive Connectivity Establishment
(ICE) registry", following the procedures defined in [RFC6336].
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ICE Option name:
ice2
Contact:
Name: Christer Holmberg
E-mail: christer.holmberg(at)ericsson(dot)com
Address: Oy LM Ericsson Ab, 02420 Jorvas, FINLAND
Change control:
IESG
Description:
The ICE option indicates that the ICE agent using the ICE option
is compliant and implemented according to RFC XXXX.
Reference:
RFC XXXX
17. 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 [RFC3424]. 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 significant impact on the issues raised by
IAB. Indeed, ICE can be considered a B-SAF (Bilateral Self-Address
Fixing) protocol, rather than an UNSAF protocol. Regardless, the IAB
has mandated that any protocols developed for this purpose document a
specific set of considerations. This section meets those
requirements.
17.1. 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 that can be used for communication.
Provide a means for a agent to determine an address that is
reachable by another peer with which it wishes to communicate.
17.2. Exit Strategy
>From RFC 3424, any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better
short-term fixes are the ones that will naturally see less and
less use as the appropriate technology is deployed.
ICE itself doesn't easily get phased out. However, it is useful even
in a globally connected Internet, to serve as a means for detecting
whether a router failure has temporarily disrupted connectivity, for
example. ICE also helps prevent certain security attacks that have
nothing to do with NAT. However, what ICE does is help phase out
other UNSAF mechanisms. ICE effectively selects amongst those
mechanisms, prioritizing ones that are better, and deprioritizing
ones that are worse. Local IPv6 addresses can be preferred. As NATs
begin to dissipate as IPv6 is introduced, server reflexive and
relayed candidates (both forms of UNSAF addresses) simply never get
used, because higher-priority connectivity exists to the native host
candidates. Therefore, the servers get used less and less, and can
eventually be remove when their usage goes to zero.
Indeed, ICE can assist in the transition from IPv4 to IPv6. It can
be used to determine whether to use IPv6 or IPv4 when two dual-stack
hosts communicate with SIP (IPv6 gets used). It can also allow a
network with both 6to4 and native v6 connectivity to determine which
address to use when communicating with a peer.
17.3. Brittleness Introduced by ICE
>From RFC 3424, any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
ICE actually removes brittleness from existing UNSAF mechanisms. In
particular, classic STUN (as described in RFC 3489 [RFC3489]) has
several points of brittleness. One of them is the discovery process
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that requires an agent to try to classify the type of NAT it is
behind. This process is error-prone. With ICE, that discovery
process is simply not used. Rather than unilaterally assessing the
validity of the address, its validity is dynamically determined by
measuring connectivity to a peer. The process of determining
connectivity is very robust.
Another point of brittleness in classic STUN and any other unilateral
mechanism is its absolute reliance on an additional server. ICE
makes use of a server for allocating unilateral addresses, but allows
agents to directly connect if possible. Therefore, in some cases,
the failure of a STUN server would still allow for a call to progress
when ICE is used.
Another point of brittleness in classic 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 classic STUN is that it
doesn't work in all network topologies. In cases where there is a
shared NAT between each agent and the STUN server, traditional STUN
may not work. With ICE, that restriction is removed.
Classic STUN also introduces some security considerations.
Fortunately, those security considerations are also mitigated by ICE.
Consequently, ICE serves to repair the brittleness introduced in
classic STUN, and does not introduce any additional brittleness into
the system.
The penalty of these improvements is that ICE increases session
establishment times.
17.4. Requirements for a Long-Term Solution
From RFC 3424, any UNSAF proposal must provide:
... requirements for longer term, sound technical solutions --
contribute to the process of finding the right longer term
solution.
Our conclusions from RFC 3489 remain unchanged. However, we feel ICE
actually helps because we believe it can be part of the long-term
solution.
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17.5. 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 that try
to provide "generic" ALG functionality. These generic ALGs hunt for
IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This interferes with classic
STUN. However, the update to STUN [RFC5389] uses an encoding that
hides these binary addresses from generic ALGs.
Existing NAPT boxes have non-deterministic and typically short
expiration times for UDP-based bindings. This requires
implementations to send periodic keepalives to maintain those
bindings. ICE uses a default of 15 s, which is a very conservative
estimate. Eventually, over time, as NAT boxes become compliant to
behave [RFC4787], this minimum keepalive will become deterministic
and well-known, and the ICE timers can be adjusted. Having a way to
discover and control the minimum keepalive interval would be far
better still.
18. Changes from RFC 5245
Following is the list of changes from RFC 5245
o The specification was generalized to be more usable with any
protocol and the parts that are specific to SIP and SDP were moved
to a SIP/SDP usage document [I-D.ietf-mmusic-ice-sip-sdp].
o Default candidates, multiple components, ICE mismatch detection,
subsequent offer/answer, and role conflict resolution were made
optional since they are not needed with every protocol using ICE.
o With IPv6, the precedence rules of RFC 6724 are used instead of
the obsoleted RFC 3483 and using address preferences provided by
the host operating system is recommended.
o Candidate gathering rules regarding loopback addresses and IPv6
addresses were clarified.
19. Acknowledgements
Most of the text in this document comes from the original ICE
specification, RFC 5245. The authors would like to thank everyone
who has contributed to that document. For additional contributions
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to this revision of the specification we would like to thank Emil
Ivov, Paul Kyzivat, Pal-Erik Martinsen, Simon Perrault, Eric
Rescorla, Thomas Stach, Peter Thatcher, Martin Thomson, Justin
Uberti, and Suhas Nandakumar.
20. References
20.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<http://www.rfc-editor.org/info/rfc5766>.
[RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for
Interactive Connectivity Establishment (ICE) Options",
RFC 6336, DOI 10.17487/RFC6336, July 2011,
<http://www.rfc-editor.org/info/rfc6336>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<http://www.rfc-editor.org/info/rfc6724>.
20.2. Informative References
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003,
<http://www.rfc-editor.org/info/rfc3605>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
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[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
DOI 10.17487/RFC3489, March 2003,
<http://www.rfc-editor.org/info/rfc3489>.
[RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235,
DOI 10.17487/RFC3235, January 2002,
<http://www.rfc-editor.org/info/rfc3235>.
[RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, DOI 10.17487/RFC3303, August 2002,
<http://www.rfc-editor.org/info/rfc3303>.
[RFC3102] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
"Realm Specific IP: Framework", RFC 3102,
DOI 10.17487/RFC3102, October 2001,
<http://www.rfc-editor.org/info/rfc3102>.
[RFC3103] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi,
"Realm Specific IP: Protocol Specification", RFC 3103,
DOI 10.17487/RFC3103, October 2001,
<http://www.rfc-editor.org/info/rfc3103>.
[RFC3424] Daigle, L., Ed. and IAB, "IAB Considerations for
UNilateral Self-Address Fixing (UNSAF) Across Network
Address Translation", RFC 3424, DOI 10.17487/RFC3424,
November 2002, <http://www.rfc-editor.org/info/rfc3424>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
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[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, DOI 10.17487/RFC3056, February
2001, <http://www.rfc-editor.org/info/rfc3056>.
[RFC3879] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, DOI 10.17487/RFC3879, September
2004, <http://www.rfc-editor.org/info/rfc3879>.
[RFC4038] Shin, M-K., Ed., Hong, Y-G., Hagino, J., Savola, P., and
E. Castro, "Application Aspects of IPv6 Transition",
RFC 4038, DOI 10.17487/RFC4038, March 2005,
<http://www.rfc-editor.org/info/rfc4038>.
[RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework", RFC 4091,
DOI 10.17487/RFC4091, June 2005,
<http://www.rfc-editor.org/info/rfc4091>.
[RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session
Description Protocol (SDP) Alternative Network Address
Types (ANAT) Semantics in the Session Initiation Protocol
(SIP)", RFC 4092, DOI 10.17487/RFC4092, June 2005,
<http://www.rfc-editor.org/info/rfc4092>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<http://www.rfc-editor.org/info/rfc2475>.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<http://www.rfc-editor.org/info/rfc1918>.
[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <http://www.rfc-editor.org/info/rfc4787>.
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[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
<http://www.rfc-editor.org/info/rfc4103>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<http://www.rfc-editor.org/info/rfc5382>.
[RFC6080] Petrie, D. and S. Channabasappa, Ed., "A Framework for
Session Initiation Protocol User Agent Profile Delivery",
RFC 6080, DOI 10.17487/RFC6080, March 2011,
<http://www.rfc-editor.org/info/rfc6080>.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <http://www.rfc-editor.org/info/rfc6146>.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<http://www.rfc-editor.org/info/rfc6147>.
[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
March 2012, <http://www.rfc-editor.org/info/rfc6544>.
[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<http://www.rfc-editor.org/info/rfc7050>.
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[I-D.ietf-mmusic-ice-sip-sdp]
Petit-Huguenin, M., Keranen, A., and S. Nandakumar, "Using
Interactive Connectivity Establishment (ICE) with Session
Description Protocol (SDP) offer/answer and Session
Initiation Protocol (SIP)", draft-ietf-mmusic-ice-sip-
sdp-10 (work in progress), July 2016.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<http://www.rfc-editor.org/info/rfc7721>.
[I-D.ietf-ice-dualstack-fairness]
Martinsen, P., Reddy, T., and P. Patil, "ICE Multihomed
and IPv4/IPv6 Dual Stack Guidelines", draft-ietf-ice-
dualstack-fairness-07 (work in progress), November 2016.
Appendix A. Lite and Full Implementations
ICE allows for two types of implementations. A full implementation
supports the controlling and controlled roles in a session, and can
also perform address gathering. In contrast, a lite implementation
is a minimalist implementation that does little but respond to STUN
checks.
Because ICE requires both endpoints to support it in order to bring
benefits to either endpoint, incremental deployment of ICE in a
network is more complicated. Many sessions involve an endpoint that
is, by itself, not behind a NAT and not one that would worry about
NAT traversal. A very common case is to have one endpoint that
requires NAT traversal (such as a VoIP hard phone or soft phone) make
a call to one of these devices. Even if the phone supports a full
ICE implementation, ICE won't be used at all if the other device
doesn't support it. The lite implementation allows for a low-cost
entry point for these devices. Once they support the lite
implementation, full implementations can connect to them and get the
full benefits of ICE.
Consequently, a lite implementation is only appropriate for devices
that will *always* be connected to the public Internet and have a
public IP address at which it can receive packets from any
correspondent. ICE will not function when a lite implementation is
placed behind a NAT.
ICE allows a lite implementation to have a single IPv4 host candidate
and several IPv6 addresses. In that case, candidate pairs are
selected by the controlling agent using a static algorithm, such as
the one in RFC 6724, which is recommended by this specification.
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However, static mechanisms for address selection are always prone to
error, since they cannot ever reflect the actual topology and can
never provide actual guarantees on connectivity. They are always
heuristics. Consequently, if an agent is implementing ICE just to
select between its IPv4 and IPv6 addresses, and none of its IP
addresses are behind NAT, usage of full ICE is still RECOMMENDED in
order to provide the most robust form of address selection possible.
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet with just a single IPv4 address, a full
implementation is preferable if achievable. Full implementations
also obtain the security benefits of ICE unrelated to NAT traversal;
in particular, the voice hammer attack described in Section 13 is
prevented only for full implementations, not lite. Finally, it is
often the case that a device that finds itself with a public address
today will be placed in a network tomorrow where it will be behind a
NAT. It is difficult to definitively know, over the lifetime of a
device or product, that it will always be used on the public
Internet. Full implementation provides assurance that communications
will always work.
Appendix B. Design Motivations
ICE contains a number of normative behaviors that may themselves be
simple, but derive from complicated or non-obvious thinking or use
cases that merit further discussion. Since these design motivations
are not necessary to understand for purposes of implementation, they
are discussed here in an appendix to the specification. This section
is non-normative.
B.1. Pacing of STUN Transactions
STUN transactions used to gather candidates and to verify
connectivity are paced out at an approximate rate of one new
transaction every Ta milliseconds. Each transaction, in turn, has a
retransmission timer RTO that is a function of Ta as well. Why are
these transactions paced, and why are these formulas used?
Sending of these STUN requests will often have the effect of creating
bindings on NAT devices between the client and the STUN servers.
Experience has shown that many NAT devices have upper limits on the
rate at which they will create new bindings. Experiments have shown
that once every 5 ms is well supported. This is why Ta has a lower
bound of 5 ms. Furthermore, transmission of these packets on the
network makes use of bandwidth and needs to be rate limited by the
agent. Deployments based on earlier draft versions of [RFC5245]
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tended to overload rate-constrained access links and perform poorly
overall, in addition to negatively impacting the network. As a
consequence, the pacing ensures that the NAT device does not get
overloaded and that traffic is kept at a reasonable rate.
The definition of a "reasonable" rate is that STUN should not use
more bandwidth than the RTP itself will use, once media starts
flowing. The formula for Ta is designed so that, if a STUN packet
were sent every Ta seconds, it would consume the same amount of
bandwidth as RTP packets, summed across all media streams. Of
course, STUN has retransmits, and the desire is to pace those as
well. For this reason, RTO is set such that the first retransmit on
the first transaction happens just as the first STUN request on the
last transaction occurs. Pictorially:
First Packets Retransmits
| |
| |
-------+------ -------+------
/ \ / \
/ \ / \
+--+ +--+ +--+ +--+ +--+ +--+
|A1| |B1| |C1| |A2| |B2| |C2|
+--+ +--+ +--+ +--+ +--+ +--+
---+-------+-------+-------+-------+-------+------------ Time
0 Ta 2Ta 3Ta 4Ta 5Ta
In this picture, there are three transactions that will be sent (for
example, in the case of candidate gathering, there are three host
candidate/STUN server pairs). These are transactions A, B, and C.
The retransmit timer is set so that the first retransmission on the
first transaction (packet A2) is sent at time 3Ta.
Subsequent retransmits after the first will occur even less
frequently than Ta milliseconds apart, since STUN uses an exponential
back-off on its retransmissions.
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B.2. Candidates with Multiple Bases
Section 4.1.3 talks about eliminating candidates that have the same
transport address and base. However, candidates with the same
transport addresses but different bases are not redundant. When can
an agent have two candidates that have the same IP address and port,
but different bases? Consider the topology of Figure 11:
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+----------+
| STUN Srvr|
+----------+
|
|
-----
// \\
| |
| B:net10 |
| |
\\ //
-----
|
|
+----------+
| NAT |
+----------+
|
|
-----
// \\
| A |
|192.168/16 |
| |
\\ //
-----
|
|
|192.168.1.100 -----
+----------+ // \\ +----------+
| | | | | |
| Initiator|---------| C:net10 |-----------| Responder|
| |10.0.1.100| | 10.0.1.101 | |
+----------+ \\ // +----------+
-----
Figure 11: Identical Candidates with Different Bases
In this case, the initiating agent is multihomed. It has one IP
address, 10.0.1.100, on network C, which is a net 10 private network.
The responding agent is on this same network. The initiating agent
is also connected to network A, which is 192.168/16 and has an IP
address of 192.168.1.100 on this network. There is a NAT on this
network, natting into network B, which is another net 10 private
network, but not connected to network C. There is a STUN server on
network B.
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The initiating agent obtains a host candidate on its IP address on
network C (10.0.1.100:2498) and a host candidate on its IP address on
network A (192.168.1.100:3344). It performs a STUN query to its
configured STUN server from 192.168.1.100:3344. This query passes
through the NAT, which happens to assign the binding 10.0.1.100:2498.
The STUN server reflects this in the STUN Binding response. Now, the
initiating agent has obtained a server reflexive candidate with a
transport address that is identical to a host candidate
(10.0.1.100:2498). However, the server reflexive candidate has a
base of 192.168.1.100:3344, and the host candidate has a base of
10.0.1.100:2498.
B.3. Purpose of the Related Address and Related Port Attributes
The candidate attribute contains two values that are not used at all
by ICE itself -- related address and related port. Why are they
present?
There are two motivations for its inclusion. The first is
diagnostic. It is very useful to know the relationship between the
different types of candidates. By including it, an agent can know
which relayed candidate is associated with which reflexive candidate,
which in turn is associated with a specific host candidate. When
checks for one candidate succeed and not for others, this provides
useful diagnostics on what is going on in the network.
The second reason has to do with off-path Quality of Service (QoS)
mechanisms. When ICE is used in environments such as PacketCable
2.0, proxies will, in addition to performing normal SIP operations,
inspect the SDP in SIP messages, and extract the IP address and port
for media traffic. They can then interact, through policy servers,
with access routers in the network, to establish guaranteed QoS for
the media flows. This QoS is provided by classifying the RTP traffic
based on 5-tuple, and then providing it a guaranteed rate, or marking
its Diffserv codepoints appropriately. When a residential NAT is
present, and a relayed candidate gets selected for media, this
relayed candidate will be a transport address on an actual TURN
server. That address says nothing about the actual transport address
in the access router that would be used to classify packets for QoS
treatment. Rather, the server reflexive candidate towards the TURN
server is needed. By carrying the translation in the SDP, the proxy
can use that transport address to request QoS from the access router.
B.4. Importance of the STUN Username
ICE requires the usage of message integrity with STUN using its
short-term credential functionality. The actual short-term
credential is formed by exchanging username fragments in the
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candidate exchange. The need for this mechanism goes beyond just
security; it is actually required for correct operation of ICE in the
first place.
Consider agents L, R, and Z. L and R are within private enterprise
1, which is using 10.0.0.0/8. Z is within private enterprise 2,
which is also using 10.0.0.0/8. As it turns out, R and Z both have
IP address 10.0.1.1. L sends candidates to Z. Z, in responds L with
its host candidates. In this case, those candidates are
10.0.1.1:8866 and 10.0.1.1:8877. As it turns out, R is in a session
at that same time, and is also using 10.0.1.1:8866 and 10.0.1.1:8877
as host candidates. This means that R is prepared to accept STUN
messages on those ports, just as Z is. L will send a STUN request to
10.0.1.1:8866 and another to 10.0.1.1:8877. However, these do not go
to Z as expected. Instead, they go to R! If R just replied to them,
L would believe it has connectivity to Z, when in fact it has
connectivity to a completely different user, R. To fix this, the
STUN short-term credential mechanisms are used. The username
fragments are sufficiently random that it is highly unlikely that R
would be using the same values as Z. Consequently, R would reject
the STUN request since the credentials were invalid. In essence, the
STUN username fragments provide a form of transient host identifiers,
bound to a particular session established as part of the candidate
exchange.
An unfortunate consequence of the non-uniqueness of IP addresses is
that, in the above example, R might not even be an ICE agent. It
could be any host, and the port to which the STUN packet is directed
could be any ephemeral port on that host. If there is an application
listening on this socket for packets, and it is not prepared to
handle malformed packets for whatever protocol is in use, the
operation of that application could be affected. Fortunately, since
the ports exchanged are ephemeral and usually drawn from the dynamic
or registered range, the odds are good that the port is not used to
run a server on host R, but rather is the agent side of some
protocol. This decreases the probability of hitting an allocated
port, due to the transient nature of port usage in this range.
However, the possibility of a problem does exist, and network
deployers should be prepared for it. Note that this is not a problem
specific to ICE; stray packets can arrive at a port at any time for
any type of protocol, especially ones on the public Internet. As
such, this requirement is just restating a general design guideline
for Internet applications -- be prepared for unknown packets on any
port.
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B.5. The Candidate Pair Priority Formula
The priority for a candidate pair has an odd form. It is:
pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)
Why is this? When the candidate pairs are sorted based on this
value, the resulting sorting has the MAX/MIN property. This means
that the pairs are first sorted based on decreasing value of the
minimum of the two priorities. For pairs that have the same value of
the minimum priority, the maximum priority is used to sort amongst
them. If the max and the min priorities are the same, the
controlling agent's priority is used as the tie-breaker in the last
part of the expression. The factor of 2*32 is used since the
priority of a single candidate is always less than 2*32, resulting in
the pair priority being a "concatenation" of the two component
priorities. This creates the MAX/MIN sorting. MAX/MIN ensures that,
for a particular agent, a lower-priority candidate is never used
until all higher-priority candidates have been tried.
B.6. Why Are Keepalives Needed?
Once media begins flowing on a candidate pair, it is still necessary
to keep the bindings alive at intermediate NATs for the duration of
the session. Normally, the media stream packets themselves (e.g.,
RTP) meet this objective. However, several cases merit further
discussion. Firstly, in some RTP usages, such as SIP, the media
streams can be "put on hold". This is accomplished by using the SDP
"sendonly" or "inactive" attributes, as defined in RFC 3264
[RFC3264]. 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 [RFC4103], may send packets so infrequently that
the interval exceeds the NAT binding timeouts.
Thirdly, if silence suppression is in use, long periods of silence
may cause media transmission to cease sufficiently long for NAT
bindings to time out.
For these reasons, the media packets themselves cannot be relied
upon. ICE defines a simple periodic keepalive utilizing STUN Binding
indications. This makes its bandwidth requirements highly
predictable, and thus amenable to QoS reservations.
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B.7. Why Prefer Peer Reflexive Candidates?
Section 4.1.2 describes procedures for computing the priority of
candidate based on its type and local preferences. That section
requires that the type preference for peer reflexive candidates
always be higher than server reflexive. Why is that? The reason has
to do with the security considerations in Section 13. It is much
easier for an attacker to cause an agent to use a false server
reflexive candidate than it is for an attacker to cause an agent to
use a false peer reflexive candidate. Consequently, attacks against
address gathering with Binding requests are thwarted by ICE by
preferring the peer reflexive candidates.
B.8. Why Are Binding Indications Used for Keepalives?
Media keepalives are described in Section 8. These keepalives make
use of STUN when both endpoints are ICE capable. However, rather
than using a Binding request transaction (which generates a
response), the keepalives use an Indication. Why is that?
The primary reason has to do with network QoS mechanisms. Once media
begins flowing, network elements will assume that the media stream
has a fairly regular structure, making use of periodic packets at
fixed intervals, with the possibility of jitter. If an agent is
sending media packets, and then receives a Binding request, it would
need to generate a response packet along with its media packets.
This will increase the actual bandwidth requirements for the 5-tuple
carrying the media packets, and introduce jitter in the delivery of
those packets. Analysis has shown that this is a concern in certain
layer 2 access networks that use fairly tight packet schedulers for
media.
Additionally, using a Binding Indication allows integrity to be
disabled, allowing for better performance. This is useful for large-
scale endpoints, such as PSTN gateways and SBCs.
Appendix C. Connectivity Check Bandwidth
The tables below show, for IPv4 and IPv6, the bandwidth required for
performing connectivity checks, using different Ta values (given in
ms) and different ufrag sizes (given in bytes).
The results were provided by Jusin Uberti (Google) 11th April 2016.
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IP version: IPv4
Packet len (bytes): 108 + ufrag
|
ms | 4 8 12 16
-----|------------------------
500 | 1.86k 1.98k 2.11k 2.24k
200 | 4.64k 4.96k 5.28k 5.6k
100 | 9.28k 9.92k 10.6k 11.2k
50 | 18.6k 19.8k 21.1k 22.4k
20 | 46.4k 49.6k 52.8k 56.0k
10 | 92.8k 99.2k 105k 112k
5 | 185k 198k 211k 224k
2 | 464k 496k 528k 560k
1 | 928k 992k 1.06M 1.12M
IP version: IPv6
Packet len (bytes): 128 + ufrag
|
ms | 4 8 12 16
-----|------------------------
500 | 2.18k 2.3k 2.43k 2.56k
200 | 5.44k 5.76k 6.08k 6.4k
100 | 10.9k 11.5k 12.2k 12.8k
50 | 21.8k 23.0k 24.3k 25.6k
20 | 54.4k 57.6k 60.8k 64.0k
10 | 108k 115k 121k 128k
5 | 217k 230k 243k 256k
2 | 544k 576k 608k 640k
1 | 1.09M 1.15M 1.22M 1.28M
Figure 12: Connectivity Check Bandwidth
Authors' Addresses
Ari Keranen
Ericsson
Hirsalantie 11
02420 Jorvas
Finland
Email: ari.keranen@ericsson.com
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Christer Holmberg
Ericsson
Hirsalantie 11
02420 Jorvas
Finland
Email: christer.holmberg@ericsson.com
Jonathan Rosenberg
jdrosen.net
Monmouth, NJ
US
Email: jdrosen@jdrosen.net
URI: http://www.jdrosen.net
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