ICE Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 8445 (if approved) J. Uberti
Intended status: Standards Track Google
Expires: January 25, 2020 July 24, 2019
Interactive Connectivity Establishment Patiently Awaiting Connectivity
(ICE PAC)
draft-ietf-ice-pac-02
Abstract
During the process of establishing peer-to-peer connectivity, ICE
agents can encounter situations where they have no candidate pairs to
check, and, as a result, conclude that ICE processing has failed.
However, because additional candidate pairs can be discovered during
ICE processing, declaring failure at this point may be premature.
This document discusses when these situations can occur and proposes
a way to avoid premature failure.
Status of This Memo
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This Internet-Draft will expire on January 25, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Relevant Scenarios . . . . . . . . . . . . . . . . . . . . . 3
3.1. No Candidates From Peer . . . . . . . . . . . . . . . . . 3
3.2. All Candidates Discarded . . . . . . . . . . . . . . . . 3
3.3. Immediate Candidate Pair Failure . . . . . . . . . . . . 4
4. Update to RFC 8445 . . . . . . . . . . . . . . . . . . . . . 4
5. Update to RFC XXXX . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
9. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
[RFC8445] describes a protocol, Interactive Connectivity
Establishment (ICE), for Network Address Translator (NAT) traversal
for UDP-based communication.
When using ICE, endpoints will typically exchange ICE candidates,
form a list of candidate pairs, and then test each candidate pair to
see if connectivity can be established. If the test for a given pair
fails, it is marked accordingly, and if all pairs have failed, the
overall ICE process typically is considered to have failed.
During the process of connectivity checks, additional candidates may
be created as a result of successful inbound checks from the remote
peer. Such candidates are referred to as peer-reflexive candidates,
and once discovered, will be used to form new candidate pairs which
will be tested like any other. However, there is an inherent race
condition here; if, before learning about any peer-reflexive
candidates, an endpoint runs out of candidate pairs to check, either
because it has none, or it considers them all to have failed, it will
prematurely declare failure and terminate ICE processing. This race
condition can occur in many common situations.
This specification updates [RFC8445], by simply requiring that an ICE
agent wait a minimum amount of time before declaring ICE failure,
even if there are no candidate pairs to check, or if all candidate
pairs have failed. This delay provides enough time for the discovery
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of peer-reflexive candidates, which may eventually lead to ICE
processing completing successfully.
2. Conventions
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Relevant Scenarios
As noted above, the core problem this specification attempts to
address is the situation where even after local gathering and remote
candidate signaling has completed, the ICE agent immediately ends up
with no valid pairs and no candidate pairs left to check, resulting
in a premature ICE failure. This failure is premature because not
enough time has elapsed to allow for discovery of peer-reflexive
candidates from inbound connectivity checks; if discovered, these
candidates are very likely to result in a valid pair.
In most ICE scenarios, the lengthy timeouts for connectivity check
transactions, typically tens of seconds, will prevent this problem
from occurring. However, there are certain specific cases where this
problem will frequently occur.
3.1. No Candidates From Peer
It is entirely legal for an ICE agent to provide zero candidates of
its own. If the agent somehow knows that the remote endpoint is
directly reachable, gathering local candidates is unnecessary and
will only cause delays; the peer agent can discover the appropriate
local candidate via connectivity checks.
However, following the procedures from [RFC8445] strictly will result
in immediate ICE failure, since the checklist at the peer agent will
be empty.
3.2. All Candidates Discarded
Even if the ICE agent provides candidates, they may be discarded by
the peer agent if it does not know what to do with them. For
example, candidates may use an address family that the peer agent
does not support, (e.g., a host candidate with an IPv6 address in a
NAT64 scenario), or may not be usable for some other reason.
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In these scenarios, when the candidates are discarded, the checklist
at the peer agent will once again be empty, leading to immediate ICE
failure.
3.3. Immediate Candidate Pair Failure
Section 7.2.5.2 of [RFC8445] describes several situations in which a
candidate pair will be considered to have failed, well before the
connectivity check transaction timeout.
As a result, even if the ICE agent provides usable candidates, the
pairs created by the peer agent may fail immediately when checked,
e.g., a check to a non-routable address that receives an immediate
ICMP error.
In this situation, the checklist at the peer agent may contain only
failed pairs, resulting in immediate ICE failure.
4. Update to RFC 8445
In order to avoid the problem raised by this document, the ICE agent
needs to wait enough time to allow peer-reflexive candidates to be
discovered. Accordingly, when a full ICE implementation begins its
ICE processing, as described in [RFC8445], Section 6.1, it MUST set a
timer, henceforth known as the PAC timer, to ensure ICE will run for
a minimum amount of time before determining failure.
Specifically, the ICE agent will start its timer once it believes ICE
connectivity checks are starting. This occurs when the agent has
sent the values needed to perform connectivity checks (e.g., the
Username Fragment and Password denoted in [RFC8445], Section 5.3) and
has received some indication that the remote side is ready to start
connectivity checks, typically via receipt of the values mentioned
above. Note that the agent will start the timer even if it has not
sent or received any ICE candidates.
The RECOMMENDED duration for the timer is equal to the agent's
connectivity check transaction timeout, including all
retransmissions. This timeout value is chosen to roughly coincide
with the maximum possible duration of ICE connectivity checks from
the remote peer, which, if successful, could create peer-reflexive
candidates. Because the ICE agent doesn't know the exact number of
candidate pairs and pacing interval in use by the remote side, this
timeout value is simply a guess, albeit an educated one. Regardless,
for this particular problem, the desired benefits will be realized as
long as the agent waits some reasonable amount of time, and, as
usual, the application is in the best position to determine what is
reasonable for its scenario.
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While the timer is running, the ICE agent MUST NOT set the state of a
checklist to Failed, even if the checklist has no pairs left to
check. As a result, the ICE agent will not remove any data streams
or set the state of the ICE session to Failed as long as the timer is
running.
When the timer eventually elapses, the ICE agent MUST resume typical
ICE processing, including setting any checklists containing only
Failed pairs to the Failed state, as usual, and handling any
consequences as indicated in [RFC8445], Section 8.1.2. Naturally, if
there are no such checklists, no action is necessary.
One consequence of this behavior is that in cases where ICE should
fail, e.g., where both sides provide candidates with unsupported
address families, ICE will no longer fail immediately, and only fail
when the PAC timer expires. However, because most ICE scenarios
require an extended period of time to determine failure, the fact
that some specific scenarios no longer fail fast should have minimal
application impact, if any.
Note also that the PAC timer is potentially relevant to the ICE
nomination procedure described in [RFC8445], Section 8.1.1. That
specification does not define a minimum duration for ICE processing
prior to nomination of a candidate pair, but in order to select the
best candidate pair, ICE needs to run for enough time in order to
allow peer-reflexive candidates to be discovered and checked, as
noted above. Accordingly, the controlling ICE agent SHOULD wait a
sufficient amount of time before nominating candidate pairs, and it
MAY use the PAC timer to do so. As always, the controlling ICE agent
retains full discretion, and MAY decide, based on its own criteria,
to nominate pairs prior to the timer elapsing.
5. Update to RFC XXXX
[RFC EDITOR NOTE: Please replace RFC XXXX with the RFC number of
draft-ietf-ice-trickle once it has been published.]
Trickle ICE [I-D.ietf-ice-trickle] considers a similar problem,
namely whether an ICE agent should allow a checklist to enter the
Failed state if more candidates might still be provided by the remote
peer. The solution, specified in [I-D.ietf-ice-trickle], Section 8,
is to wait until an end-of-candidates indication has been received
before determining ICE failure.
However, for the same reasons described above, the ICE agent may
discover peer-reflexive candidates after it has received the end-of-
candidates indication, and so the solution proposed by this document
MUST still be used even when the ICE agent is using Trickle ICE.
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Note also that sending an end-of-candidates indication is only a
SHOULD-strength requirement, which means that ICE agents will need to
implement an backup mechanism to decide when all candidates have been
received, typically a timer. Accordingly, ICE agents MAY use the PAC
timer to also serve as an end-of-candidates fallback.
6. Security Considerations
The security considerations for ICE are defined in [RFC8445]. This
specification only recommends that ICE agents wait for a certain time
of period before they declare ICE failure, and does not introduce new
security considerations.
7. IANA considerations
This specification makes no requests to IANA.
8. Acknowledgements
9. 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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018, <https://www.rfc-
editor.org/info/rfc8445>.
[I-D.ietf-ice-trickle]
Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
"Trickle ICE: Incremental Provisioning of Candidates for
the Interactive Connectivity Establishment (ICE)
Protocol", draft-ietf-ice-trickle-21 (work in progress),
April 2018.
Authors' Addresses
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Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Justin Uberti
Google
747 6th St W
Kirkland 98033
USA
Email: justin@uberti.name
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