Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)
Why ICE should wait a little longer before declaring failure, and how the PAC timer does it.
Holmberg, C. and J. Uberti, "Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)", RFC 8863, DOI 10.17487/RFC8863, January 2021, <https://www.rfc-editor.org/info/rfc8863>.
@misc{rfc8863,
series = {Request for Comments},
number = 8863,
howpublished = {RFC 8863},
publisher = {RFC Editor},
doi = {10.17487/RFC8863},
url = {https://www.rfc-editor.org/info/rfc8863},
author = {C. Holmberg and J. Uberti},
title = {{Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)}},
pagetotal = 6,
year = 2021,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Holmberg
Request for Comments: 8863 Ericsson
Updates: 8445, 8838 J. Uberti
Category: Standards Track Google
ISSN: 2070-1721 January 2021
Interactive Connectivity Establishment Patiently Awaiting Connectivity
(ICE PAC)
Abstract
During the process of establishing peer-to-peer connectivity,
Interactive Connectivity Establishment (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.
This document updates RFCs 8445 and 8838 by requiring that an ICE
agent wait a minimum amount of time before declaring ICE failure,
even if there are no candidate pairs left to check.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8863.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Conventions
3. Relevant Scenarios
3.1. No Candidates from Peer
3.2. All Candidates Discarded
3.3. Immediate Candidate Pair Failure
4. Update to RFC 8445
5. Update to RFC 8838
6. Security Considerations
7. IANA Considerations
8. Normative References
Acknowledgements
Authors' Addresses
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;
once discovered, these candidates will be used to form new candidate
pairs, which will be tested like any other. However, there is an
inherent problem 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
problem can occur in many common situations.
This specification updates [RFC8445] and [RFC8838] 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 all candidate pairs have failed. This delay provides enough time
for the discovery 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 have 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
Per [RFC8838], an ICE agent can 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 that may not be usable for some other reason.
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" (Patiently Awaiting
Connectivity), to ensure that 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 PAC timer is equal to the agent's
connectivity check transaction timeout, including all
retransmissions. When using default values for retransmission
timeout (RTO) and Rc, this amounts to 39.5 seconds, as explained in
[RFC5389], Section 7.2.1. 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.
While the timer is still running, the ICE agent MUST NOT update a
checklist state from Running to Failed, even if there are no pairs
left in the checklist 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 period eventually elapses, the ICE agent MUST resume
typical ICE processing, including setting the state of any checklists
to Failed if they have no pairs left to check 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 -- it will 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 quickly 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 PAC timer period elapsing.
5. Update to RFC 8838
Trickle ICE [RFC8838] 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 [RFC8838], 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.
Note also that sending an end-of-candidates indication is only a
SHOULD-strength requirement, which means that ICE agents will need to
implement a 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
period of time before they declare ICE failure; it does not introduce
new security considerations.
7. IANA Considerations
This document has no IANA actions.
8. 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>.
[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>.
[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>.
[RFC8838] Ivov, E., Uberti, J., and P. Saint-Andre, "Trickle ICE:
Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol", RFC 8838,
DOI 10.17487/RFC8838, January 2021,
<https://www.rfc-editor.org/info/rfc8838>.
Acknowledgements
Roman Shpount, Nils Ohlmeier, and Peter Thatcher provided lots of
useful input and comments.
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland
Email: christer.holmberg@ericsson.com
Justin Uberti
Google
747 6th St W
Kirkland, WA 98033
United States of America
Email: justin@uberti.name
AI ELI5
On this page
§1The Premature Failure Problem
ICE (Interactive Connectivity Establishment, RFC 8445) is a protocol for NAT traversal for UDP-based communication. Endpoints exchange candidates, form a list of candidate pairs, and test each pair. If all pairs fail, the overall ICE process is usually considered failed.
The catch is that successful inbound checks from the remote peer can create new candidates, called peer-reflexive candidates. These form new pairs that are tested like any other. If an endpoint runs out of pairs before learning of such candidates, it declares failure too soon and ends ICE processing.
This RFC simply requires an ICE agent to wait a minimum amount of time before declaring failure, even when no pairs are left to check or all have failed.
Sometimes a new clue shows up after you have already run out of places to look. The fix is to wait a bit before giving up.
§2When the Problem Occurs
In most ICE scenarios, the long timeouts for connectivity check transactions, typically tens of seconds, hide the problem. Three specific cases make it frequent.
- No candidates from the peer. Under RFC 8838 an agent may provide zero candidates, for example when it knows the other side is directly reachable. Following RFC 8445 strictly, the peer's checklist is empty and ICE fails at once.
- All candidates discarded. The peer may discard candidates it cannot use, such as a host candidate with an IPv6 address in a NAT64 scenario. The checklist is empty again.
- Immediate candidate pair failure. Some pairs fail well before the transaction timeout, for example a check to a non-routable address that gets an immediate ICMP error. The checklist may hold only failed pairs.
It happens when you were given no places to look, when the places you were given are no good, or when every place you try says no right away.
- No candidates from peerThe peer sent zero candidates, so the checklist is empty.
- All candidates discardedThe peer cannot use the candidates, for example an unsupported address family.
- Immediate pair failurePairs fail at once, for example on an ICMP error.
§3The PAC Timer
When a full ICE implementation begins ICE processing, it MUST set a PAC timer (Patiently Awaiting Connectivity) so ICE runs for a minimum time before determining failure. The agent starts it once it believes connectivity checks are starting: it has sent the values needed for checks (such as the username fragment and password) and has some sign that the remote side is ready. It starts the timer even if it has sent or received no candidates.
The RECOMMENDED duration equals the agent's connectivity check transaction timeout including all retransmissions. With default retransmission timeout (RTO) and Rc values this is 39.5 seconds, as explained in RFC 5389. The value is described as an educated guess, since the agent does not know how many pairs or what pacing the remote side uses. The application is best placed to decide what is reasonable.
While the timer runs, the agent MUST NOT change a checklist from Running to Failed, even if no pairs are left, so it will not remove data streams or set the ICE session to Failed. When the timer expires, the agent MUST resume normal processing, including setting checklists with no pairs left to Failed.
A consequence is that where ICE should fail, such as when both sides give candidates with unsupported address families, it now fails only when the PAC timer expires. The RFC expects minimal application impact.
The program sets a timer, like an egg timer. Until it rings, the program is not allowed to say it failed, even if it has nothing left to try.
§4Nomination and Trickle ICE
The PAC timer also bears on nomination, the choice of the final candidate pair (RFC 8445, Section 8.1.1). That RFC sets no minimum time before nominating, but to pick the best pair, ICE needs enough time to find and check peer-reflexive candidates. The controlling agent SHOULD wait a sufficient time before nominating pairs and MAY use the PAC timer for this. It keeps full discretion and MAY nominate earlier.
Trickle ICE (RFC 8838) waits for an end-of-candidates indication before declaring failure. But peer-reflexive candidates can still appear after that indication, so the PAC timer MUST still be used with Trickle ICE. Since sending end-of-candidates is only a SHOULD, agents need a backup such as a timer, and they MAY use the PAC timer as that fallback.
The timer also helps the program pick the best path, since better paths may show up a little later. It can also stand in when the other side forgets to say it is done sending clues.
§5Security and IANA
The security considerations for ICE are in RFC 8445. This specification only recommends that agents wait for a period before they declare ICE failure, and it adds no new security considerations. It has no IANA actions.
Waiting a little longer does not open any new doors for trouble, so there is nothing new to worry about.