Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal
draft-ietf-ice-rfc5245bis-14
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8445.
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|
|---|---|---|---|
| Authors | Ari Keränen , Christer Holmberg , Jonathan Rosenberg | ||
| Last updated | 2017-10-30 | ||
| Replaces | draft-ietf-mmusic-rfc5245bis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
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| Document shepherd | Peter Thatcher | ||
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| Send notices to | "Peter Thatcher" <pthatcher@google.com> |
draft-ietf-ice-rfc5245bis-14
ICE A. Keranen
Internet-Draft C. Holmberg
Obsoletes: 5245 (if approved) Ericsson
Intended status: Standards Track J. Rosenberg
Expires: May 3, 2018 jdrosen.net
October 30, 2017
Interactive Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal
draft-ietf-ice-rfc5245bis-14
Abstract
This document describes a protocol for Network Address Translator
(NAT) traversal for UDP-based communication. 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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on May 3, 2018.
Copyright Notice
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Gathering Candidates . . . . . . . . . . . . . . . . . . 8
2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 10
2.3. Nominating Candidate Pairs And Concluding ICE . . . . . . 12
2.4. ICE Restart . . . . . . . . . . . . . . . . . . . . . . . 12
2.5. Lite Implementations . . . . . . . . . . . . . . . . . . 13
3. ICE Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. ICE Candidate Gathering and Exchange . . . . . . . . . . . . 17
5.1. Full Implementation . . . . . . . . . . . . . . . . . . . 18
5.1.1. Gathering Candidates . . . . . . . . . . . . . . . . 18
5.1.1.1. Host Candidates . . . . . . . . . . . . . . . . . 18
5.1.1.2. Server Reflexive and Relayed Candidates . . . . . 20
5.1.1.3. Computing Foundations . . . . . . . . . . . . . . 21
5.1.1.4. Keeping Candidates Alive . . . . . . . . . . . . 21
5.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 22
5.1.2.1. Recommended Formula . . . . . . . . . . . . . . . 22
5.1.2.2. Guidelines for Choosing Type and Local
Preferences . . . . . . . . . . . . . . . . . . . 23
5.1.3. Eliminating Redundant Candidates . . . . . . . . . . 23
5.2. Lite Implementation Procedures . . . . . . . . . . . . . 23
5.3. Exchanging Candidate Information . . . . . . . . . . . . 25
5.4. ICE Mismatch . . . . . . . . . . . . . . . . . . . . . . 26
6. ICE Candidate Processing . . . . . . . . . . . . . . . . . . 26
6.1. Procedures for Full Implementation . . . . . . . . . . . 26
6.1.1. Determining Role . . . . . . . . . . . . . . . . . . 26
6.1.2. Forming the Check Lists . . . . . . . . . . . . . . . 28
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6.1.2.1. Check List State . . . . . . . . . . . . . . . . 28
6.1.2.2. Forming Candidate Pairs . . . . . . . . . . . . . 29
6.1.2.3. Computing Pair Priority and Ordering Pairs . . . 31
6.1.2.4. Pruning the Pairs . . . . . . . . . . . . . . . . 31
6.1.2.5. Removing lower-priority Pairs . . . . . . . . . . 31
6.1.2.6. Computing Candidate Pair States . . . . . . . . . 32
6.1.3. ICE State . . . . . . . . . . . . . . . . . . . . . . 35
6.1.4. Scheduling Checks . . . . . . . . . . . . . . . . . . 35
6.1.4.1. Triggered Check Queue . . . . . . . . . . . . . . 35
6.1.4.2. Performing Connectivity Checks . . . . . . . . . 36
6.2. Lite Implementation Procedures . . . . . . . . . . . . . 37
7. Performing Connectivity Checks . . . . . . . . . . . . . . . 37
7.1. STUN Extensions . . . . . . . . . . . . . . . . . . . . . 38
7.1.1. PRIORITY . . . . . . . . . . . . . . . . . . . . . . 38
7.1.2. USE-CANDIDATE . . . . . . . . . . . . . . . . . . . . 38
7.1.3. ICE-CONTROLLED and ICE-CONTROLLING . . . . . . . . . 38
7.2. STUN Client Procedures . . . . . . . . . . . . . . . . . 38
7.2.1. Creating Permissions for Relayed Candidates . . . . . 38
7.2.2. Forming Credentials . . . . . . . . . . . . . . . . . 39
7.2.3. DiffServ Treatment . . . . . . . . . . . . . . . . . 39
7.2.4. Sending the Request . . . . . . . . . . . . . . . . . 39
7.2.5. Processing the Response . . . . . . . . . . . . . . . 39
7.2.5.1. Role Conflict . . . . . . . . . . . . . . . . . . 40
7.2.5.2. Failure . . . . . . . . . . . . . . . . . . . . . 40
7.2.5.2.1. Non-Symmetric Transport Addresses . . . . . . 40
7.2.5.2.2. ICMP Error . . . . . . . . . . . . . . . . . 41
7.2.5.2.3. Timeout . . . . . . . . . . . . . . . . . . . 41
7.2.5.2.4. Unrecoverable STUN Response . . . . . . . . . 41
7.2.5.3. Success . . . . . . . . . . . . . . . . . . . . . 41
7.2.5.3.1. Discovering Peer Reflexive Candidates . . . . 41
7.2.5.3.2. Constructing a Valid Pair . . . . . . . . . . 42
7.2.5.3.3. Updating Candidate Pair States . . . . . . . 43
7.2.5.3.4. Updating the Nominated Flag . . . . . . . . . 43
7.2.5.4. Check List State Updates . . . . . . . . . . . . 44
7.3. STUN Server Procedures . . . . . . . . . . . . . . . . . 44
7.3.1. Additional Procedures for Full Implementations . . . 45
7.3.1.1. Detecting and Repairing Role Conflicts . . . . . 45
7.3.1.2. Computing Mapped Address . . . . . . . . . . . . 46
7.3.1.3. Learning Peer Reflexive Candidates . . . . . . . 46
7.3.1.4. Triggered Checks . . . . . . . . . . . . . . . . 47
7.3.1.5. Updating the Nominated Flag . . . . . . . . . . . 48
7.3.2. Additional Procedures for Lite Implementations . . . 49
8. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 49
8.1. Procedures for Full Implementations . . . . . . . . . . . 49
8.1.1. Nominating Pairs . . . . . . . . . . . . . . . . . . 49
8.1.2. Updating States . . . . . . . . . . . . . . . . . . . 50
8.2. Procedures for Lite Implementations . . . . . . . . . . . 51
8.3. Freeing Candidates . . . . . . . . . . . . . . . . . . . 52
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8.3.1. Full Implementation Procedures . . . . . . . . . . . 52
8.3.2. Lite Implementation Procedures . . . . . . . . . . . 52
9. ICE Restarts . . . . . . . . . . . . . . . . . . . . . . . . 52
10. ICE Option . . . . . . . . . . . . . . . . . . . . . . . . . 53
11. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 53
12. Data Handling . . . . . . . . . . . . . . . . . . . . . . . . 54
12.1. Sending Data . . . . . . . . . . . . . . . . . . . . . . 54
12.2. Procedures for Lite Implementations . . . . . . . . . . 55
12.3. Procedures for All Implementations . . . . . . . . . . . 55
13. Receiving Data . . . . . . . . . . . . . . . . . . . . . . . 56
14. Extensibility Considerations . . . . . . . . . . . . . . . . 56
15. Setting Ta and RTO . . . . . . . . . . . . . . . . . . . . . 57
15.1. General . . . . . . . . . . . . . . . . . . . . . . . . 57
15.2. Ta . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
15.3. RTO . . . . . . . . . . . . . . . . . . . . . . . . . . 59
16. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
17. Security Considerations . . . . . . . . . . . . . . . . . . . 64
17.1. Attacks on Connectivity Checks . . . . . . . . . . . . . 65
17.2. Attacks on Server Reflexive Address Gathering . . . . . 67
17.3. Attacks on Relayed Candidate Gathering . . . . . . . . . 68
17.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . 68
17.4.1. STUN Amplification Attack . . . . . . . . . . . . . 68
18. STUN Extensions . . . . . . . . . . . . . . . . . . . . . . . 69
18.1. New Attributes . . . . . . . . . . . . . . . . . . . . . 69
18.2. New Error Response Codes . . . . . . . . . . . . . . . . 70
19. Operational Considerations . . . . . . . . . . . . . . . . . 70
19.1. NAT and Firewall Types . . . . . . . . . . . . . . . . . 70
19.2. Bandwidth Requirements . . . . . . . . . . . . . . . . . 71
19.2.1. STUN and TURN Server Capacity Planning . . . . . . . 71
19.2.2. Gathering and Connectivity Checks . . . . . . . . . 71
19.2.3. Keepalives . . . . . . . . . . . . . . . . . . . . . 72
19.3. ICE and ICE-lite . . . . . . . . . . . . . . . . . . . . 72
19.4. Troubleshooting and Performance Management . . . . . . . 72
19.5. Endpoint Configuration . . . . . . . . . . . . . . . . . 73
20. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 73
20.1. STUN Attributes . . . . . . . . . . . . . . . . . . . . 73
20.2. STUN Error Responses . . . . . . . . . . . . . . . . . . 73
20.3. ICE Options . . . . . . . . . . . . . . . . . . . . . . 73
21. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 74
21.1. Problem Definition . . . . . . . . . . . . . . . . . . . 74
21.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . 75
21.3. Brittleness Introduced by ICE . . . . . . . . . . . . . 75
21.4. Requirements for a Long-Term Solution . . . . . . . . . 76
21.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . 77
22. Changes from RFC 5245 . . . . . . . . . . . . . . . . . . . . 77
23. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 77
24. References . . . . . . . . . . . . . . . . . . . . . . . . . 78
24.1. Normative References . . . . . . . . . . . . . . . . . . 78
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24.2. Informative References . . . . . . . . . . . . . . . . . 78
Appendix A. Lite and Full Implementations . . . . . . . . . . . 82
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 83
B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 83
B.2. Candidates with Multiple Bases . . . . . . . . . . . . . 85
B.3. Purpose of the Related Address and Related Port
Attributes . . . . . . . . . . . . . . . . . . . . . . . 87
B.4. Importance of the STUN Username . . . . . . . . . . . . . 87
B.5. The Candidate Pair Priority Formula . . . . . . . . . . . 89
B.6. Why Are Keepalives Needed? . . . . . . . . . . . . . . . 89
B.7. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 90
B.8. Why Are Binding Indications Used for Keepalives? . . . . 90
B.9. Selecting Candidate Type Preference . . . . . . . . . . . 90
Appendix C. Connectivity Check Bandwidth . . . . . . . . . . . . 91
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 92
1. Introduction
Protocols establishing communication sessions between peers typically
involve exchanging IP addresses and ports for the data sources and
sinks. However this poses challenges when operated through Network
Address Translators (NATs) [RFC3235]. These protocols also seek to
create a data flow directly between participants, so that there is no
application layer intermediary between them. This is done to reduce
data latency, decrease packet loss, and reduce the operational costs
of deploying the application. However, this is difficult to
accomplish through NATs. 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 NATs. 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 data streams
(though ICE has been extended to handle other transport protocols,
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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
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 data 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, there are two endpoints (ICE agents)
that want to communicate. Note that ICE is not intended for NAT
traversal for the signaling protocol, which is assumed to be provided
via another mechanism. ICE assumes that the agents are able to
establish a signaling connection between each other.
Initially, the agents are ignorant of their own topologies. In
particular, the agents may or may not be behind NATs (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 establish a data session.
Figure 1 shows a typical ICE deployment. The agents are labelled L
and R. 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. L and R are capable of engaging in an candidate exchange
process, whose purpose is to set up a data session between L and R.
Typically, this exchange will occur through a signaling server (e.g.,
SIP proxy).
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 Candidates
In order to execute ICE, an ICE agent has to identify all of its
address candidates. A candidate has a transport address -- a
combination of IP address and port for a particular transport
protocol (with only UDP specified here). There are different types
of candidates, some derived from physical or logical network
interfaces, others discoverable via STUN and TURN. Naturally, one
viable candidate has 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.
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. The host
candidate associated with a given server reflexive candidate is 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 ICE 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 connectivity 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 data
(e.g., RTP, RTCP, or other protocols). Consequently, agents
demultiplex STUN and data using the contents of the packets, rather
than the port on which they are received.
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 that of
other candidates the agent already learned, it represents a new
candidate (peer reflexive candidate), which then gets tested by ICE
just the same as any other candidate.
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 agent works through the check list by sending a STUN request for
the next candidate pair on the list periodically. These are called
ordinary checks.
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.
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 data relays and through fewer NATs) are
preferred over indirect ones (ones with more data relays and more
NATs). Within those guidelines, however, agents have a fair amount
of discretion about how to tune their algorithms.
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A data stream might consist of multiple components (pieces of a data
stream that require their own set of candidates, e.g., RTP and RTCP).
2.3. Nominating Candidate Pairs And Concluding ICE
ICE assigns one of the ICE agents in the role of the controlling
agent, and the other of the controlled agent. For each component of
a data stream, the controlling agent nominates a candidate pair from
the valid candidate pairs to be used for data. The exact timing of
the nomination is based on local policy.
When nominating, the controlling agent lets the checks continue until
at least one valid candidate pair for each component of a data stream
is found and then picks a candidate pair from the valid candidate
pairs and sends a STUN request on the pair, using an attribute to
indicate to the controlled peer that it has nominated the pair. 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 controlled agent receives the STUN request with the
attribute, it will check (unless the check has already been done) the
same pair. If the transactions above succeed, the agents will set
the nominated flag for the pairs, and will cancel any future checks
for that component of the data stream. Once an agent has set the
nominated flag for each component of a data stream, the pairs become
the selected pairs. After that, only the selected pairs will be used
for sending and receiving data associated with that data stream.
2.4. ICE Restart
Once ICE is concluded, it can be restarted at any time for one or all
of the data streams by either ICE agent. This is done by sending an
updated candidate information indicating a restart.
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2.5. Lite Implementations
Certain ICE 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). Lite agents only use host
candidates and do not generate connectivity checks or run the state
machines, though they need to be able to respond to connectivity
checks.
3. ICE Usage
This document specifies generic use of ICE with protocols that
provide means to exchange candidate information between the ICE
agents. The specific details of (i.e how to encode candidate
information and the actual candidate exchange process) for different
protocols using ICE (referred to as using protocol) are described in
separate usage documents.
One mechanism for agents to exchange the candidate information by
using [RFC3264] based Offer/Answer semantics as part of the SIP
[RFC3261] protocol [I-D.ietf-mmusic-ice-sip-sdp].
4. 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 Session: An ICE session consists of all ICE-related actions
starting with the candidate gathering, followed by the
interactions (candidate exchange, connectivity checks, nominations
and keep-alives) between the ICE agents until all the candidates
are released or ICE-restart is triggered.
ICE Agent, Agent: An ICE agent (sometimes simply referred to as an
agent) is the protocol implementation involved in the ICE
candidate exchange. There are two agents involved in a typical
candidate exchange.
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Initiating Peer, Initiating Agent, Initiator: An initiating agent is
an ICE agent that initiates the ICE candidate exchange process.
Responding Peer, Responding Agent, Responder: A receiving agent is
an ICE agent that receives and responds to the candidate exchange
process initiated by the initiating agent.
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.
Peer: From the perspective of one of the ICE 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.
Data, Data Stream, Data Session: When ICE is used to setup data
sessions, the data is transported using some protocol. Media is
usually transported over RTP, composed of a stream of RTP packets.
Data session refers to data packets that are exchanged between the
peer on the path created and tested with ICE.
Candidate, Candidate Information: A transport address that is a
potential point of contact for receipt of data. Candidates also
have properties -- their type (server reflexive, relayed, or
host), priority, foundation, and base.
Component: A component is a piece of a data stream. A data stream
may require multiple components, each of which has to work in
order for the data stream as a whole to work. For RTP/RTCP data
streams, unless RTP and RTCP are multiplexed in the same port,
there are two components per data stream -- one for RTP, and one
for RTCP. A component has a candidate pair, which cannot be used
by other components
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).
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Server Reflexive Candidate: A candidate whose IP address and port
are a binding allocated by a NAT for an ICE agent when it sent a
packet through the NAT to a server, such as a STUN server.
Peer Reflexive Candidate: A candidate whose IP address and port are
a binding allocated by a NAT for an ICE agent when it sent a
packet through the NAT to its peer.
Relayed Candidate: A candidate obtained from a relay server, such as
a TURN server.
Base: The transport address that an ICE 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 used in the freezing algorithm to
group similar candidates. 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.
Local Candidate: A candidate that an ICE agent has obtained and may
send to its peer.
Remote Candidate: A candidate that an ICE agent received from its
peer.
Default Destination/Candidate: The default destination for a
component of a data stream is the transport address that would be
used by an ICE 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 pair of a local candidate and a remote candidate.
Check, Connectivity Check, STUN Check: A STUN Binding request for
the purposes of verifying connectivity. A check is sent from the
base of the local candidate to the remote candidate of a candidate
pair.
Check List: An ordered set of candidate pairs that an ICE agent will
use to generate checks.
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Ordinary Check: A connectivity check generated by an ICE 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 data stream that
have been validated by a successful STUN transaction.
Check List Set: The ordered list of all check lists. The order is
determined by each ICE usage.
Full Implementation: An ICE implementation that performs the
complete set of functionality defined by this specification.
Lite Implementation: 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 nominates a candidate pair.
In any session, one agent is always controlling. The other is the
controlled agent.
Controlled Agent: The ICE agent that waits for the controlling agent
to nominate a candidate pair.
Nomination, Regular Nomination: The process of the controlling agent
indicating to the controlled agent which candidate pair the ICE
agents should use for sending and receiving data.
Nominated, Nominated Flag: Once the nomination of a candidate pair
has succeeded, the candidate pair has become nominated, and the
value of its nominated flag is set to true.
Selected Pair, Selected Candidate Pair: The candidate pair used for
sending and receiving data for a component of a data stream is
referred to as the selected pair. Before selected pairs have been
produced for a data stream, any valid candidate pair associated
with a component of a data stream can be used for sending and
receiving data for the component. Once there are nominated pairs
for each component of a data stream, the nominated pairs become
the selected pairs for the data stream. The candidates associated
with the selected pairs are referred to as selected candidates.
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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.
5. 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 the encoding mechanism and the
semantics of candidate information exchange is out of scope of this
specification.
However at a higher level, the diagram below shows how the ICE agents
(initiator and responder) exchange 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) redundant
candidate elimination, (4) (possibly) default candidate selection,
and (5) sending of the candidates to the peer. All but the last of
these five steps differ for full and lite implementations.
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5.1. Full Implementation
5.1.1. Gathering Candidates
An ICE 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 has 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 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 of the
application associated with the ICE session. Such gathering MAY
begin when an agent starts.
5.1.1.1. Host Candidates
Host candidates are obtained by binding to ports on an IP address
attached to an interface (physical or virtual, including VPN
interfaces) on the host.
For each component of each data stream the ICE agent wishes to use,
the agent SHOULD obtain a candidate 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/RTCP data 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
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checks on the RTCP candidate. 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.
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.
For other than RTP/RTCP 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 address
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.
o Host candidates corresponding to IPv6 link-local addresses MUST
NOT be gathered.
The IPv6 default address selection specification [RFC6724] specifies
that temporary addresses [RFC4941] are to be preferred over permanent
addresses.
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5.1.1.2. Server Reflexive and Relayed Candidates
An ICE agent SHOULD gather server reflexive and relayed candidates.
These requirements are at SHOULD strength to allow for provider
variation. Use of STUN and TURN servers may be unnecessary in
certain networks and use of TURN servers may be expensive, so some
deployments may elect not to use them. 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.
The agent pairs each host candidate with the STUN or TURN servers
with which it is configured or has discovered by some means. 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.
When multiple STUN or TURN servers are available (or when they are
learned through DNS records and multiple results are returned), the
agent MAY gather candidates for all of them and SHOULD gather
candidates for at least one of them (one STUN server and one TURN
server). It does so by pairing host candidates with STUN or TURN
servers and, for each pair, the agent sends a Binding or Allocate
request to the server from the 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.
The gathering process is controlled using a timer, Ta. Every time Ta
expires, 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 time Ta expires. See Section 15 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
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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
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 also gather 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.
5.1.1.3. Computing Foundations
The ICE agent assigns each candidate a foundation. Two candidates
MUST have the same foundation when all of the following are true:
o They have 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).
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.
5.1.1.4. 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 8.3. 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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Host candidates do not time out, but the candidate addresses may
change or disappear for a number of reasons. An ICE agent SHOULD
monitor the interfaces it uses, invalidate candidates whose base has
gone away, and acquire new candidates as appropriate when new
interfaces appear.
5.1.2. Prioritizing Candidates
The prioritization process results in the assignment of a priority to
each candidate. Each candidate for a data 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 ICE agent SHOULD compute this priority using the formula in
Section 5.1.2.1 and choose its parameters using the guidelines in
Section 5.1.2.2. If an agent elects to use a different formula, ICE
may take longer to converge since the agents will not be coordinated
in their checks.
The process for prioritizing candidates is common across the
initiating and the responding agent.
5.1.2.1. Recommended Formula
The recommended formula combines a preference for the candidate type
(server reflexive, peer reflexive, relayed, and host), a preference
for IP address for which the candidate was obtained, and component ID
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 (lowest preference) to
126 (highest preference) inclusive and 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. Setting the
value to 0 means that candidates of this type will only be used as a
last resort. Note that candidates gathered based on the procedures
of Section 5.1.1 will never be peer reflexive candidates; candidates
of these type are learned from the connectivity checks performed by
ICE.
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The local preference MUST be an integer from 0 (lowest preference) to
65535 (highest preference) inclusive. When there is only a single IP
address, this value SHOULD be set to 65535. If there are multiple
candidates for a particular component for a particular data stream
that have the same type, the local preference MUST be unique for each
one. If an ICE agent 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 MUST be an integer between 1 and 256 inclusive.
5.1.2.2. Guidelines for Choosing Type and Local Preferences
The RECOMMENDED values for type preferences are 126 for host
candidates, 110 for peer reflexive candidates, 100 for server
reflexive candidates, and 0 for relayed candidates.
If an ICE agent is multihomed and has multiple IP addresses, the
recommendations 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.
When choosing type preferences, agents may take into account factors
such as latency, packet loss, cost, network topology, security,
privacy, and others.
5.1.3. Eliminating Redundant Candidates
Next, the ICE agents (initiating and responding) eliminate redundant
candidates. 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. A candidate is
redundant if and only if its transport address and base equal those
of another candidate. The agent SHOULD eliminate the redundant
candidate with the lower priority.
5.2. Lite Implementation Procedures
Lite implementations only utilize host candidates. A lite
implementation MUST, for each component of each data 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 data stream, if an ICE agent has multiple IPv4
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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/RTCP data streams, unless RTCP is multiplexed in the same
port with RTP, the RTP itself has a component ID of 1, and RTCP a
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 data
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
data stream. If a host is IPv4-only, there would only be one
candidate for each component of each data 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.
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5.3. Exchanging Candidate Information
ICE agents (initiating and responding) need the following information
about candidates to be exchanged. Each ICE usage MUST define how the
information is exchanged with the using protocol. This section
describes the information that needs to be exchanged.
Candidates: One or more candidates. For each candidate:
Address: The IP address and transport protocol port of the
candidate.
Transport: The transport protocol of the candidate. This MAY be
omitted if the using protocol only runs over a single transport
protocol.
Foundation: A sequence of up to 32 characters.
Component ID: The component ID of the candidate. This MAY be
omitted if the using protocol does not use the concept of
components.
Priority: The 32-bit priority of the candidate.
Type: The type of the candidate.
Related Address and Port: The related IP address and port of the
candidate. 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 or Full: Whether the agent is a lite agent or full agent.
Connectivity check pacing value: The pacing value for connectivity
checks that the agent wishes to use. This MAY be omitted if the
agent wishes to use a defined default value.
Username Fragment and Password: Values used to perform connectivity
checks. The username fragment MUST contain at least 24 bits of
randomness, and the password MUST contain at least 128 bits of
randomness.
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Extensions: New media-stream or session-level attributes (ice-
options).
If the using protocol is vulnerable to, and able to detect, ICE
mismatch (Section 5.4), a way is needed for the detecting agent to
convey this information to its peer. It is a boolean flag.
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.
Once an agent has sent its candidate information, it MUST be prepared
to receive both STUN and data packets on each candidate. As
discussed in Section 12.1, data packets can be sent to a candidate
prior to its appearance as the default destination for data.
5.4. ICE Mismatch
Certain middleboxes, such as ALGs, can alter signaling information in
ways that break ICE. This is referred to as ICE mismatch. If the
using protocol is vulnerable to ICE mismatch, the responding agent
needs to be able to detect it and inform the peer ICE agent about the
ICE mismatch.
Each using protocol needs to define whether the using protocol is
vulnerable to ICE mismatch, how ICE mismatch is detected, and whether
specific actions need to be taken when ICE mismatch is detected.
6. ICE Candidate Processing
Once an ICE agent has gathered its candidates and exchanged
candidates with its peer (Section 5), it will determine its own role.
In addition, full implementations will form check lists, and begin
performing connectivity checks with the peer.
6.1. Procedures for Full Implementation
6.1.1. Determining Role
For each session, each ICE 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
data stream, and for updating the peer with the ICE's selection, when
needed. The controlled agent is told which candidate pairs to use
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for each data 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 8.1 to
nominate pairs that will become (if the connectivity checks
associated with the nominations succeed) the selected pairs, and
then both agents end ICE as described in Section 8.1.2.
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 8.1 to nominate pairs that will become (if the
connectivity checks associated with the nominations succeed) the
selected pairs, and use the logic in Section 8.1.2 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 8.2. For the lite implementation, the state
of ICE processing for each data 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 8 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 data stream is considered to be Running,
and the state of ICE overall is Running.
Once the roles are determined for a session, they persist throughout
the lifetime of the session. The roles can be re-determined as part
of an ICE restart (Section 9), 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:
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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 7.3.1.1.
NOTE: There are certain 3PCC scenarios where an ICE restart might
cause a role conflict.
NOTE: The 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 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.
6.1.2. Forming the Check Lists
There is one check list for each data stream. To form a check list,
an ICE agent (initiating and responding) forms candidate pairs,
computes pair priorities, orders pairs by priority, prunes pairs,
removes lower-priority pairs, and sets check list states. If
candidates are added to a check list (e.g, due to detection of peer
reflexive candidates), the agent will re-perform these steps for the
updated check list.
6.1.2.1. Check List State
Each check list has a state, which captures the state of ICE checks
for the data stream associated with the check list. The states are:
Running: The check list is neither Completed nor Failed yet. Check
lists are initially set to the Running state.
Completed: The check list contains a nominated pair for each
component of the data stream.
Failed: The check list does not have a valid candidate pair for each
component of the data stream and all of the candidate pairs in the
check list are in either the Failed or Succeeded state. In other
words, at least one component of the check list has candidate
pairs that are all in the Failed state, which means the component
has failed, which means the check list has failed.
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6.1.2.2. Forming Candidate Pairs
The ICE agent pairs each local candidate with each remote candidate
for the same component of the same data stream with the same IP
address family. 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 data stream. If this happens, the number of components for
that data 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 data 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 data if both agents had not been ICE aware.
Figure 6 shows the properties of and relationships between transport
addresses, candidates, candidate pairs, and check lists.
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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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6.1.2.3. Computing Pair Priority and Ordering Pairs
The ICE agent computes a priority for each candidate pair. 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 follows, where G>D?1:0
is an expression whose value is 1 if G is greater than D, and 0
otherwise.
pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)
The agent sorts each check list in decreasing order of candidate pair
priority. If two pairs have identical priority, the ordering amongst
them is arbitrary.
6.1.2.4. 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 ICE 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.
The agent prunes each check list. This is done by removing a
candidate pair if it is redundant with a higher priority candidate
pair in the same check list. Two candidate pairs are redundant if
their local candidates have the same base and their remote candidates
are identical. The result is a sequence of ordered candidate pairs,
called the check list for that data stream.
6.1.2.5. Removing lower-priority Pairs
In order to limit the attacks described in Section 17.4.1, an ICE
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.
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6.1.2.6. Computing Candidate Pair States
Each candidate pair in the check list has a foundation (the
combination of the foundations of the local and remote candidates in
the pair) and one of the following states:
Waiting: A check has not been sent for this pair, but the pair is
not Frozen.
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.
Pairs 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
1. The initial states for each pair in a check list are computed by
performing the following sequence of steps:
2. The check lists are placed in an ordered list (the order is
determined by each ICE usage), called the check list set.
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3. The ICE agent initially places all candidate pairs in the Frozen
state.
4. The agent sets all of the check lists in the check list set to
the Running state.
5. For each foundation, the agent sets the state of exactly one
candidate pair to the Waiting state (unfreezing it). The
candidate pair to unfreeze is chosen by finding the first
candidate pair (ordered by lowest component ID and then highest
priority if component IDs are equal) in the first check list
(according to the usage-defined check list set order) that has
that foundation.
NOTE: The procedures above are different from RFC5245, where only
candidate pairs in the first check list of were initially placed in
the Waiting state. Now it applies to candidate pairs in the the
first check list which have that foundation, even if the first check
list to have that foundation is not the first check list in the check
list set.
The table in Figure 8 illustrates an example.
Table legend:
Each row (m1, m2,...) represents a check list associated with a data
stream. m1 represents the first check list in the check list set.
Each column (f1, f2,...) represents a foundation. Every candidate 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 the check list set to the Frozen
state.
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
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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 in the check list set.
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 in the check list set has been placed in the Waiting state.
Figure 8: Initial Pair State
6.1.3. ICE State
The ICE agent has a state determined by the state of the check lists.
The state is Completed if all check lists are Completed, Failed if
all check lists are Failed, and Running otherwise.
6.1.4. Scheduling Checks
6.1.4.1. Triggered Check Queue
Once the ICE agent has computed the check lists and created the check
list set, as described in Section 6.1.2, the agent will begin
performing connectivity checks (ordinary and triggered). For
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triggered connectivity checks, the agent maintains a FIFO queue for
each check list, referred to as the triggered check queue, which
contains candidate pairs for which checks are to be sent at the next
available opportunity. The triggered check queue is initially empty.
6.1.4.2. Performing Connectivity Checks
The generation of ordinary and triggered connectivity checks is
governed by timer Ta. As soon as the initial states for the
candidate pairs in the check list set have been set, a check is
performed for a candidate pair within the first check list in the
Running state, following the procedures in Section 7. After that,
whenever Ta fires the next check list in the Running state in the
check list set is picked, and a check is performed for a candidate
within that check list. After the last check list in the Running
state in the check list set has been processed, the first check list
is picked again. Etc.
Whenever Ta fires, the ICE agent will perform a check for a candidate
pair within the picked check list by performing the following steps:
1. If the triggered check queue associated with the check list
contains one or more candidate pairs, the agent removes the top
pair from the queue, performs a connectivity check on that pair,
puts the candidate pair state to In-Progress, and aborts the
subsequent steps.
2. If there is no candidate pair in the Waiting state, and if there
are one or more pairs in the Frozen state, for each pair in the
Frozen state the agent checks the foundation associated with the
pair. For a given foundation, if there is no pair (in any check
list in the check list set) in the Waiting or In-Progress state,
the agent puts the candidate pair state to Waiting and continues
with the next step.
3. If there are one or more candidate pairs in the Waiting state,
the agent picks the highest-priority candidate pair (if there are
multiple pairs with the same priority, the pair with the lowest
component ID is picked) in the Waiting state, performs a
connectivity check on that pair, puts the candidate pair par
state to In-Progress, and abort the subsequent steps.
4. If this step is reached, no check could be performed for the
picked check list. So, without waiting for timer Ta to expire
again, select the next check list in the Running state and return
to step #1. If this happens for every single check list in the
Running state, meaning there are no remaining candidate pairs to
perform connectivity checks for, abort these steps.
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Once the agent has picked a candidate pair, for which a connectivity
check is to be performed, the agent performs the check by sending a
STUN request from the base associated with the local candidate of the
pair to the remote candidate of the pair, as described in
Section 7.2.4.
Based on local policy, an agent MAY choose to terminate performing
the connectivity checks for one or more checks lists in the check
list set at any time. However, only the controlling agent is allowed
to conclude ICE (Section 8).
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 check
lists. On the other hand the responding agent either performs the
triggered or ordinary checks as described above.
6.2. Lite Implementation Procedures
Lite implementations skips most of the steps in Section 6 except for
verifying the peer's ICE support and determining its role in the ICE
processing.
If the lite implementation is the controlling agent (which will only
happen if the peer ICE agent is also a lite implementation), it
selects 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 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 data stream, and
no further candidate updates are needed to signal this information.
7. Performing Connectivity Checks
This section describes how connectivity checks are performed.
An ICE agent MUST be compliant to [RFC5389]. A full implementation
acts both as a STUN client and a STUN server, while a lite
implementation only acts as a STUN server (as it does not generate
connectivity checks).
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7.1. STUN Extensions
ICE extends STUN by defining new attributes: PRIORITY, USE-CANDIDATE,
ICE-CONTROLLED, and ICE-CONTROLLING. The new attributes are formally
defined in Section 18.1. This section describes the usage of the new
attributes.
The new attributes are only applicable to ICE connectivity checks.
7.1.1. PRIORITY
The priority attribute MUST be included in a Binding request and be
set to the value computed by the algorithm in Section 5.1.2 for the
local candidate, but with the candidate type preference of peer
reflexive candidates.
7.1.2. USE-CANDIDATE
The controlling agent MUST include the USE-CANDIDATE attribute in
order to nominate a candidate pair Section 8.1.1. The controlled
agent MUST NOT include the USE-CANDIDATE attribute in a Binding
request.
7.1.3. ICE-CONTROLLED and ICE-CONTROLLING
The controlling agent MUST include the ICE-CONTROLLING attribute in a
Binding request. The controlled agent MUST include the ICE-
CONTROLLED attribute in a Binding request.
The content of either attribute are used as tie-breaker values when
an ICE role conflict occurs Section 7.3.1.1.
7.2. STUN Client Procedures
7.2.1. 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 ICE 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.
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7.2.2. Forming Credentials
A connectivity check Binding request 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 ICE 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 ICE agent L is the Initiating
agent and ICE 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 passwords as the requests
(note that the USERNAME attribute is not present in the response).
7.2.3. DiffServ Treatment
If an ICE agent is using Diffserv Codepoint markings [RFC2475] in its
data packets, the agent SHOULD apply those same markings to its
connectivity checks.
If multiple DSCP markings are used on the data packets, the agent
SHOULD choose one of them for use with the connectivity check.
7.2.4. Sending the Request
A connectivity check is generated by sending a Binding request from
the base associated with a local candidate to a remote candidate.
[RFC5389] describes how Binding requests are constructed and
generated.
Support for backwards compatibility with RFC 3489 MUST NOT be assumed
when performing connectivity checks. The FINGERPRINT mechanism MUST
be used for connectivity checks.
7.2.5. Processing the Response
This section defines additional procedures for processing Binding
responses specific to ICE connectivity checks.
When a Binding response is received, it is correlated to the
corresponding Binding request using the transaction ID [RFC5389],
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which then associates the response with the candidate pair for which
the Binding request was sent. After that, the response is processed
according to the procedures for a role conflict, a failure, or a
success, according to the procedures below.
7.2.5.1. Role Conflict
If the Binding request generates a 487 (Role Conflict) error
response, and if the ICE agent included an ICE-CONTROLLED attribute
in the request, the agent MUST switch to the controlling role. If
the agent included an ICE-CONTROLLING attribute in the request, the
agent MUST switch to the controlled role.
Once the agent has switched its role, the agent MUST add the
candidate pair whose check generated the 487 error response to the
triggered check queue associated with the check list to which the
pair belongs, and set the candidate pair state to Waiting. When the
triggered connectivity check is later performed, the ICE-CONTROLLING/
ICE-CONTROLLED attribute of the Binding request will indicate the
agent's new role. The agent MAY change the tie-breaker value.
NOTE: A role switch requires an agent to recompute pair priorities
(Section 6.1.2.3), since the priority values depend on the role.
NOTE: A role switch will also impact whether the agent is responsible
for nominating candidate pairs, and whether the agent is responsible
for initiating the exchange of the updated candidate information with
the peer once ICE is concluded.
7.2.5.2. Failure
This section describes cases when the candidate pair state is set to
Failed.
NOTE: When the ICE agent sets the candidate pair state to Failed as a
result of a connectivity check error, the agent does not change the
states of other candidate pairs with the same foundation.
7.2.5.2.1. Non-Symmetric Transport Addresses
The ICE agent MUST check that the source and destination transport
addresses in the Binding request and response are symmetric. I.e.,
the source IP address and port of the response MUST be equal the
destination IP address and port to which the Binding request was
sent, and that the destination IP address and port of the response
MUST be equal to the source IP address and port from which the
Binding request was sent. If the addresses are not symmetric, the
agent MUST set the candidate pair state to Failed.
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7.2.5.2.2. ICMP Error
An ICE agent MAY support processing of ICMP errors for connectivity
checks. If the agent supports processing of ICMP errors, and if a
Binging request generates an ICMP error, the agent SHOULD set the
state of the candidate pair to Failed.
7.2.5.2.3. Timeout
If the Binding request times out, the ICE agent SHOULD set the
candidate pair state to Failed.
7.2.5.2.4. Unrecoverable STUN Response
If the Binding request generates a STUN error response that is
unrecoverable [RFC5389] the ICE agent SHOULD set the candidate pair
state to Failed.
7.2.5.3. Success
A connectivity check is considered a success if each of the following
criteria is true:
o The Binding request generated a success response; and
o The source and destination transport addresses in the Binding
request and response are symmetric.
If a check is considered a success, the ICE agent performs (in order)
the actions described in the following sections.
7.2.5.3.1. Discovering Peer Reflexive Candidates
The ICE agent MUST check 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, a peer
reflexive candidate has a type, base, priority, and foundation. They
are computed as follows:
o The type is peer reflexive.
o The base is local candidate of the candidate pair from which the
Binding request was sent.
o The priority is the value of the PRIORITY attribute in the Binding
request.
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o The foundation is described in Section 5.1.1.3.
The peer reflexive candidate is then added to the list of local
candidates for the data stream. The username fragment and password
are the same as for all other local candidates for that data stream.
The ICE agent does not need to pair the peer reflexive candidate with
remote candidates, as a valid candidate pair will be created due to
the procedures in Section 7.2.5.3.2. If an agent wishes to pair the
peer reflexive candidate with remote candidates other than the one in
the valid pair that will be generated, the agent MAY provide updated
candidate information to the peer that includes the peer reflexive
candidate. This will cause the peer reflexive candidate to be paired
with all other remote candidates.
7.2.5.3.2. Constructing a Valid Pair
The ICE 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.
The valid pair may equal the pair that generated the connectivity
check, or it may equal a different pair in a check list (sometimes in
a different check list than the one to which the pair that generated
the connectivity checks), or it may be a pair not currently in any
check list.
The agent maintains a separate list, referred to as the valid list.
There is a valid list for each check list in the check list set. The
valid list will contain valid pairs. Initially each valid list is
empty.
Each valid pair within the valid list has a flag, called the
nominated flag. When a valid pair is added to a valid list, the flag
value is set to 'false'.
The valid pair will be added to a valid list as follows:
1. If the valid pair equals the pair that generated the check, the
pair is added to the valid list associated with the check list to
which the pair belongs; or
2. If the valid pair equals another pair in a check list, that pair
is added to the valid list associated with the check list of that
pair. The pair that generated the check is not added to a valid
list; or
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3. If the valid pair is not in any check list, the agent computes
the priority for the pair based on the priority of each
candidate, using the algorithm in Section 6.1.2. The priority of
the local candidate depends on its type. Unless the type is peer
reflexive, the priority is equal to the priority signaled for
that candidate in the candidate exchange. If the type 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.
NOTE: It will be very common that the valid pair will not be in 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 Binding
request 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.
7.2.5.3.3. Updating Candidate Pair States
The ICE agent sets the states of both the candidate pair that
generated the check and the constructed valid pair (which may be
different) to Succeeded.
The agent MUST set the states for all other Frozen candidate pairs in
all check lists with the same foundation to Waiting.
NOTE: Within a given check list, candidate pairs with the same
foundations will typically have different component ID values.
7.2.5.3.4. Updating the Nominated Flag
If the controlling agent sends a Binding request with the USE-
CANDIDATE attribute set, and if the ICE agent receives a successful
response to the request, the agent sets the nominated flag of the
pair to true. If the request fails Section 7.2.5.2, the agent MUST
remove the candidate pair from the valid list, set the candidate pair
state to Failed and set the check list state to Failed.
If the controlled agent receives a successful response to a Binding
request sent by the agent, and that Binding request was triggered by
a received Binding request with the USE-CANDIDATE attribute set
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Section 7.3.1.4, the agent sets the nominated flag of the pair to
true. If the triggered request fails, the agent MUST remove the
candidate pair from the valid list, set the candidate pair state to
Failed and set the check list state to Failed.
Once the nominated flag is set for a component of a data stream, it
concludes the ICE processing for that component. See Section 8.
7.2.5.4. Check List State Updates
Regardless of whether a connectivity check was successful or failed,
the completion of the check may require updating of check list
states. For each check list in the check list set, if all of the
candidate pairs are in either Failed or Succeeded state, and if there
is not a valid pair in the valid list for each component of the data
stream associated with the check list, the state of the check list is
set to Failed. If there is a valid pair for each component in the
valid list, the state of the check list is set to Succeeded.
7.3. STUN Server Procedures
An ICE agent (lite or full) MUST be prepared to receive Binding
requests on the base of each candidate it included in its most recent
candidate exchange.
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
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 7.3.1.2). 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 7.3.1.3,
Section 7.3.1.4, and Section 7.3.1.5 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.
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If the agent is using Diffserv Codepoint markings [RFC2475] in its
data packets, it SHOULD apply the same markings to Binding responses.
The same would apply to any layer 2 markings the endpoint might be
applying to data packets.
7.3.1. Additional Procedures for Full Implementations
This subsection defines the additional server procedures applicable
to full implementations, when the full implementation accepts the
Binding request.
7.3.1.1. Detecting and Repairing Role Conflicts
In certain usages of ICE (such as third party call control), both ICE
agents may end up choosing the same role, resulting in a role
conflict. The section describes a mechanism for detecting and
repairing role conflicts. 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
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.
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* 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 6.1.2.3), since those priorities are a function of role.
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 7.3.1 are followed if the agent
generated a successful response to the Binding request, even if the
agent changed roles.
7.3.1.2. Computing Mapped Address
For requests 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
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.
7.3.1.3. 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 type is peer reflexive.
o The priority is the value of the PRIORITY attribute in the Binding
request.
o The foundation is an arbitrary value, different from the
foundations of all other remote candidates. If any subsequent
candidate exchanges contain this peer reflexive candidate, it will
signal the actual foundation for the candidate.
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o The component ID is the component ID of the local candidate to
which the request was sent.
This candidate is added to the list of remote candidates. However,
the ICE agent does not pair this candidate with any local candidates.
7.3.1.4. Triggered Checks
Next, the agent constructs a pair whose local candidate has the
transport address (as seen by the agent) 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.
* 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.
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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 7.2.4. 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 picked.
7.3.1.5. Updating the Nominated Flag
If the controlled agent receives a Binding request with the USE-
CANDIDATE attribute set, and if the ICE agent accepts the request,
the following action is based on the state of the pair computed in
Section 7.3.1.4:
o If the state of this pair is Succeeded, it means that the check
previously sent by this pair produced a successful response, and
generated a valid pair (Section 7.2.5.3.2). The agent sets the
nominated flag value of the pair to true.
o If the received Binding request triggered a new check to be enqued
in the triggered check queue (Section 7.3.1.4), once the check is
sent and if it generates a successful response, and generates a
valid pair, the agent sets the nominated flag of the pair to true.
If the request fails Section 7.2.5.2, the agent MUST remove the
candidate pair from the valid list, set the candidate pair state
to Failed and set the check list state to Failed.
If the controlled agent does not accept the request from the
controlling agent, the controlled agent MUST reject the nomination
request with an appropriate error code response (e.g., 400)
[RFC5389].
Once the nominated flag is set for a component of a data stream, it
concludes the ICE processing for that component. See Section 8.
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7.3.2. Additional Procedures for Lite Implementations
If the controlled agent receives a Binding request with the USE-
CANDIDATE attribute set, and if the ICE agent accepts the request,
the agent constructs a candidate pair whose local candidate has 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 the valid list of the associated check
list. The agent sets the nominated flag for that pair to true.
Once the nominated flag is set for a component of a data stream, it
concludes the ICE processing for that component. See Section 8.
8. Concluding ICE Processing
This section describes how an ICE agent completes ICE.
8.1. Procedures for Full Implementations
Concluding ICE involves nominating pairs by the controlling agent and
updating of state machinery.
8.1.1. Nominating Pairs
Prior to nominating, the controlling agent let connectivity checks
continue until some stopping criterion is met. After that, based on
an evaluation criterion, the controlling agent picks a pair among the
valid pairs in the valid list for nomination.
Once the controlling agent has picked a valid pair for nomination, it
repeats the connectivity 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 Section 7.3.1.5.
Eventually, if the nominations succeed, both the controlling and
controlled agents will have a single nominated pair in the valid list
for each component of the data stream. Once an ICE agent sets the
state of the check list to Completed (when there is a nominated pair
for each component of the data stream), that pair becomes the
selected pair for that agent, and is used for sending and receiving
data for that component of the data stream.
If an agent is not able to produce selected pairs for a data stream,
the agent MUST take proper actions for informing the other agent, and
e.g., removing the stream. The exact actions are outside the scope
of this specification.
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The criterion details for stopping the connectivity checks and for
selecting a pair for nomination, are outside the scope of this
specification. They are a matter of local optimization. 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 set.
If more than one candidate pair is nominated by the controlling
agent, and if the controlled agent accepts multiple nominations
requests, the agents MUST produce the selected pairs using the pairs
with the highest priority.
NOTE: A controlling agent that does not support this specification
(i.e. it is implemented according to RFC 5245) might nominate more
than one candidate pair. This was referred to as aggressive
nomination in RFC 5245. The usage of the 'ice2' ice option
Section 10 by endpoints supporting this specification should prevent
such controlling agents from using aggressive nomination.
8.1.2. 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 data
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
data stream and the state of the check list is Running:
* The ICE 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 data 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.
o Once there is at least one nominated pair in the valid list for
every component of at least one data stream and the state of the
check list is Running:
* The agent MUST change the state of processing for its check
list for that data stream to Completed.
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* The agent MUST continue to respond to any checks it may still
receive for that data stream, and MUST perform triggered checks
if required by the processing of Section 7.3.
* The agent MUST continue retransmitting any In-Progress checks
for that check list.
* The agent MAY begin transmitting data for this data stream as
described in Section 12.1.
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 data stream. The correct behavior depends on
the state of the check lists for other data 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 data streams is
Completed, the controlling agent SHOULD remove the failed data
stream from the session while sending updated candidate list to
its peer.
* If none of the check lists for other data streams are
Completed, but at least one is Running, the agent SHOULD let
ICE continue.
8.2. Procedures for Lite Implementations
When ICE concludes, a lite ICE agent can free host candidates that
were not used by ICE, as described in Section 8.3.
If the peer is a full agent, once the lite agent accepts a nomination
request for a candidate pair, the lite agent considers the pair
nominated. Once there are nominated pairs for each component of a
data stream, the pairs become the selected pairs for the components
of the data stream. Once the lite agent has produced selected pairs
for all components of all data streams, the ICE session state is set
to Completed.
If the peer is a lite agent, the agent pairs local candidates with
remote candidates that are for the same data stream and have the same
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component, transport protocol, and IP address family. For each
component of each data stream, if there is only one candidate pair,
that pair is added to the valid list. If there is more than one
pair, it is RECOMMENDED that an agent follow the procedures of RFC
6724 [RFC6724] to select a pair and add it to the valid list.
If all of the components for all data streams had one pair, the state
of ICE processing is Completed. Otherwise, the controlling agent
MUST send an updated candidate list to reconcile different agents
selecting different candidate pairs. ICE processing is complete
after and only after the updated candidate exchange is complete.
8.3. Freeing Candidates
8.3.1. Full Implementation Procedures
The procedures in Section 8 require that an ICE agent continue to
listen for STUN requests and continue to generate triggered checks
for a data 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 did not become
a selected candidate (is not associated with a selected pair), and
then free the candidate.
Once a check list has reached the Completed state, the agent SHOULD
wait an additional three seconds, and then it can cease responding to
checks or generating triggered checks on all local candidates other
than the ones that became selected candidates. Once all ICE sessions
have ceased using a given local candidate (a candidate may be used by
multiple ICE sessions, e.g. in forking scenarios), the agent can free
that candidate. The three-second delay handles cases when aggressive
nomination is used, and the selected pairs can quickly change after
ICE has completed.
Freeing of server reflexive candidates is never explicit; it happens
by lack of a keepalive.
8.3.2. Lite Implementation Procedures
A lite implementation can free candidates that did not become
selected candidates as soon as ICE processing has reached the
Completed state for all ICE sessions using those candidates.
9. ICE Restarts
An ICE agent MAY restart ICE for existing data streams. An ICE
restart causes all previous state of the data streams, excluding the
roles of the agents to be flushed. The only difference between an
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ICE restart and a brand new data session is that during the restart,
data can continue to be sent using existing data sessions, and that a
new data session always requires the roles to be determined.
The following actions can be accomplished only using an ICE restart
(the agent MUST use ICE restarts to do so):
o Change the destinations of data streams.
o Change from a lite implementation to a full implementation.
o Change from a full implementation to a lite implementation.
To restart ICE, an agent MUST change both the password and the
username fragment for the data stream(s) being restarted. The new
candidate set MAY include some, none, or all of the previous
candidates.
As described in Section 6.1.1, agents MUST NOT re-determine the roles
as part as an ICE restart, unless certain criteria that require the
roles to be re-determined are fulfilled.
10. ICE Option
This section defines a new ICE option, 'ice2'. The ICE option
indicates that the ICE agent that includes it in a candidate exchange
is compliant to this specification. For example, the agent will not
use the aggressive nomination procedure defined in [RFC5245].
An agent compliant to this specification MUST inform the peer about
the compliance using the 'ice2' 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].
11. Keepalives
All endpoints MUST send keepalives for each data session. These
keepalives serve the purpose of keeping NAT bindings alive for the
data 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
ICE agent is a full ICE implementation and is communicating with a
peer that supports ICE (lite or full).
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For each candidate pair that an agent is using to send data, if no
packet has been sent on that pair in the last Tr seconds, an agent
MUST send a keepalive on that pair. Agents SHOULD use a Tr value of
15 seconds. Agents MAY use a bigger value, but MUST NOT use a value
smaller than 15 seconds.
Once selected pairs have been produced for a data stream, keepalives
are only sent on those pairs.
An agent MUST stop sending keepalives on a data stream if the data
stream is removed. If the ICE session is terminated, an agent MUST
stop sending keepalives on all data streams.
An agent MAY use another value for Tr, 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.
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 data. 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.
Agents MUST by default use STUN keepalives. Individual ICE usages
and ICE extensions MAY specify usage/extension-specific keepalives.
12. Data Handling
12.1. Sending Data
An ICE agent MAY send data on any valid candidate pair before
selected pairs have been produced for the data stream.
Once selected pairs have been produced for a data stream, an agent
MUST send data on those pairs.
An agent sends data from the base of the local candidate to the
remote candidate. In the case of a local relayed candidate, data is
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forwarded through 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 data stream is:
o empty if the state of the check list for that data 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 data
stream if the state of the check list for that data 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 each component
associated with a data stream, the agent MUST NOT continue sending
data for any component associated with that data stream.
12.2. Procedures for Lite Implementations
A lite implementation MUST NOT send data until it has a valid list
that contains a candidate pair for each component of that data
stream. Once that happens, the ICE agent MAY begin sending data
packets. To do that, it sends data 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 candidate pair used for sending data. In case of a relayed
candidate, data is sent from the agent and forwarded through the base
(located in the TURN server), using the procedures defined in
[RFC5766].
12.3. 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, ICE agents
are encouraged to re-adjust jitter buffers when there are changes in
source or destination address of data 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 data from one
candidate pair to another.
13. Receiving Data
Even though ICE agents are only allowed to send data using valid
candidate pairs (and, once selected pairs have been produced, only on
the selected pairs) ICE implementations SHOULD by default be prepared
to receive data on any of the candidates provided in the most recent
candidate exchange with the peer. ICE usages MAY define rules that
differs from this, e.g., by defining that data must not be sent until
selected pairs have been produced for a data stream.
It is RECOMMENDED that, when an agent receives an RTP packet with a
new source or destination IP address for a particular RTP/RTCP data
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 data streams switch between
candidates. An agent will be able to determine that a data stream is
from the same peer as a consequence of the STUN exchange that
proceeds media data transmission. Thus, if there is a change in
source transport address, but the media data packets come from the
same peer agent, this MUST NOT be treated as an SSRC collision.
14. Extensibility Considerations
This specification makes very specific choices about how both ICE
agents in a session coordinate to arrive at the set of candidate
pairs that are selected for data. 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.
One of the complications in achieving interoperability is that ICE
relies on a distributed algorithm running on both agents to converge
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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 8 eliminates some of the tight coordination by delegating the
selection algorithm completely to the controlling agent.
Consequently, when a controlling agent is communicating with a peer
that supports options it doesn't know about, the agent MUST run 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 data streams beyond RTP, and for
transport protocols beyond UDP. Extensions to ICE for non-RTP data
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.
15. Setting Ta and RTO
15.1. General
During the ICE gathering phase (Section 5.1.1) and while ICE is
performing connectivity checks (Section 7), an ICE 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 data 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
packet loss. The loss of the first single packet for any
connectivity check is likely to cause that pair to take a long time
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to be validated, and instead, a lower-priority check (but one for
which there was no packet loss) is much more likely to complete
first. This results in ICE performing sub-optimally, choosing lower-
priority pairs over higher-priority pairs. Implementors should be
aware of this consequence, but still should utilize the timer values
described here.
15.2. Ta
ICE agents SHOULD use the default Ta value, 50 ms, but MAY use
another value based on the characteristics of the associated data.
If an agent wants to use another Ta value than the default value, the
agent MUST indicate the proposed value to its peer during the
establishment of the ICE session. 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 agent, the combination of
all transactions from all agents (if a given implementation runs
several concurrent agents) MUST NOT be sent more often than once
every 5ms (as though there were one global Ta value for pacing all
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 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 equivalent to having a global Ta value.
NOTE: Appendix C shows examples of required bandwidth, using
different Ta values.
15.3. 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, 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.
Agents MAY calculate the RTO value using other mechanisms than those
described above. Agents MUST NOT use a RTO value smaller than 500
ms.
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16. Example
The example is based on the simplified topology of Figure 9.
+-------+
|STUN |
|Server |
+-------+
|
+---------------------+
| |
| Internet |
| |
+---------------------+
| |
| |
+---------+ |
| NAT | |
+---------+ |
| |
| |
+-----+ +-----+
| L | | R |
+-----+ +-----+
Figure 9: Example Topology
Two ICE agents, L and R, are using ICE. Both are full ICE
implementations. 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.
Finally, seq-no is a sequence number that is different for each
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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 focus
on a single data stream between two full implementations.
L NAT STUN R
|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| |
|------------------------------------------->|
| | | | STUN
| | | | alloc.
| | |(6) STUN Req |
| | |S=$R-PUB-1 |
| | |D=$STUN-PUB-1 |
| | |<-------------|
| | |(7) STUN Res |
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| | |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 | | |
|------------->| | |
| |(11) Bind Req | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |---------------------------->|
| |(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 | | |
|<-------------| | |
|Data flows | | |
| |(14) Bind Req | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |<----------------------------|
|(15) Bind Req | | |
|S=$R-PUB-1 | | |
|D=$L-PRIV-1 | | |
|<-------------| | |
|(16) Bind Res | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|MA=$R-PUB-1 | | |
|------------->| | |
| |(17) Bind Res | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |MA=$R-PUB-1 | |
| |---------------------------->|
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| | | |Data 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.
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.
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
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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). 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. Agent L can now send
data 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 data
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 data.
17. Security Considerations
The process of probing for candidates reveals the source addresses of
the client and its peer to any on-network listening attacker, and the
process of exchanging candidates reveals the addresses to any
attacker that is able to see the negotiation. Some addresses, such
as the server reflexive addresses gathered through the local
interface of VPN users, may be sensitive information. If these
potential attacks can not be mitigated, the implementation may want
to institute controls for which addresses are revealed to the
negotiation and/or probing process. Such controls need to be
specified as part of the ICE usage.
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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17.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks.
Ultimately, all of these attacks fool an ICE 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
data.
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 data 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 data path is secured
(e.g., using SRTP [RFC3711]), the attacker will not be able to
process the data packets, but will only be able to discard them,
effectively disabling the data stream. 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 data stream, it's much easier to just disrupt it with the same
mechanism, rather than attack ICE.
17.2. 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 establishment of the ICE session.
For this candidate to actually be used for data, 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 ICE agent in the session, and is prevented by SRTP
if it identifies the attacker itself.
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 data. 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.
17.3. 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.
17.4. 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.
17.4.1. 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 data 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. ICE 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.
18. STUN Extensions
18.1. New Attributes
This specification defines four new STUN 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 data.
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 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 agent MUST use the same number for all Binding
requests, for all streams, within an ICE session. The agent MAY
change the number when an ICE restart occurs.
18.2. 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.
19. Operational Considerations
This section discusses issues relevant to network operators looking
to deploy ICE.
19.1. 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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19.2. Bandwidth Requirements
Deployment of ICE can have several interactions with available
network capacity that operators should take into consideration.
19.2.1. 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 data 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 data. The amount of calls requiring
TURN for data 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.
19.2.2. 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 data traffic itself. This was done to ensure that,
if a network is designed to support communication traffic of a
certain type (voice, video, or just text), it will have sufficient
capacity to support the ICE checks for that data. 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.
19.2.3. Keepalives
STUN keepalives (in the form of STUN Binding Indications) are sent in
the middle of a data session. However, they are sent only in the
absence of actual data 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.
19.3. 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.
19.4. 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.
Signaling servers, 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 signaling is performed exactly for the
purposes of educating network equipment (such as a diagnostic tool
attached to a signaling) about the results of ICE processing.
As a consequence, through the logs generated by a signaling server, a
network operator can observe what types of candidates are being used
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for each call, and what address were selected by ICE. This is the
primary information that helps evaluate how ICE is performing.
19.5. 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.
20. 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.
20.1. STUN Attributes
IANA has registered four STUN attributes:
0x0024 PRIORITY
0x0025 USE-CANDIDATE
0x8029 ICE-CONTROLLED
0x802A ICE-CONTROLLING
20.2. 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.
20.3. 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
21. IAB Considerations
The IAB has studied the problem of "Unilateral Self-Address Fixing",
which is the general process by which an ICE 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.
21.1. 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.
21.2. 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 picks 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.
21.3. 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 ICE 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.
21.4. 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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21.5. 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.
22. 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.
23. 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, Suhas Nandakumar, Taylor Brandstetter, Peter Saint-Andre,
Harald Alvestrand and Roman Shpount.
24. References
24.1. 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, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008, <https://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, <https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc6724>.
24.2. Informative References
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003, <https://www.rfc-
editor.org/info/rfc3605>.
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[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, <https://www.rfc-
editor.org/info/rfc3261>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002, <https://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, <https://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, <https://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,
<https://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, <https://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, <https://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, <https://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, <https://www.rfc-editor.org/info/rfc3550>.
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[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,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC3879] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, DOI 10.17487/RFC3879, September
2004, <https://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,
<https://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, <https://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,
<https://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, <https://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, <https://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,
<https://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,
<https://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, <https://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, <https://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,
<https://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, <https://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,
<https://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,
<https://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, <https://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, <https://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, <https://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,
<https://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,
"Session Description Protocol (SDP) Offer/Answer
procedures for Interactive Connectivity Establishment
(ICE)", draft-ietf-mmusic-ice-sip-sdp-14 (work in
progress), October 2017.
[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,
<https://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 ICE 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 17 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
ICE 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 data 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 data 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 5.1.3 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 ICE 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 ICE 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 data traffic. They can then interact, through policy servers,
with access routers in the network, to establish guaranteed QoS for
the data 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 data, 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 ICE 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 ICE agent, a lower-priority candidate is never used
until all higher-priority candidates have been tried.
B.6. Why Are Keepalives Needed?
Once data 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 data stream packets themselves (e.g.,
RTP) meet this objective. However, several cases merit further
discussion. Firstly, in some RTP usages, such as SIP, the data
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
data in these cases. However, doing so may cause NAT bindings to
timeout, and data 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 data transmission to cease sufficiently long for NAT
bindings to time out.
For these reasons, the data 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 5.1.2 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 17. It is much
easier for an attacker to cause an ICE 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?
Data keepalives are described in Section 11. 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 data
begins flowing, network elements will assume that the data stream has
a fairly regular structure, making use of periodic packets at fixed
intervals, with the possibility of jitter. If an ICE agent is
sending data packets, and then receives a Binding request, it would
need to generate a response packet along with its data packets. This
will increase the actual bandwidth requirements for the 5-tuple
carrying the data 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
data.
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.
B.9. Selecting Candidate Type Preference
One criterion for selection of the type and local preference values
is the use of a data intermediary, such as a TURN server, a tunnel
service such as VPN server, or NAT. With a data intermediary, if
data is sent to that candidate, it will first transit the data
intermediary before being received. Relayed candidates are one type
of candidate that involves a data intermediary. Another are host
candidates obtained from a VPN interface. When data is transited
through a data 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
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hops that may be taken. It may increase the cost of providing
service, since data will be routed in and right back out of a data
intermediary run by a provider. If these concerns are important, the
type preference for relayed candidates must be carefully chosen.
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 excessive delays in the
connectivity check phase if broken paths exist.
Another criterion for selecting preferences is topological awareness.
This is most useful for candidates that make use of intermediaries.
In those cases, if an ICE 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.
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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