Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)
Ericsson
Hirsalantie 11
02420
Jorvas
Finland
christer.holmberg@ericsson.com
Google
747 6th St W
98033
Kirkland
WA
United States of America
justin@uberti.name
ICE
PAC
Candidate
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.
Introduction
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
and 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.
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
when, and only
when, they appear in all capitals, as shown here.
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.
No Candidates from Peer
Per , 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
strictly will result in immediate
ICE failure, since the checklist at the peer agent will be
empty.
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.
Immediate Candidate Pair Failure
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.
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 , 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
)
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 .
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 .
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 . 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.
Update to RFC 8838
Trickle ICE
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
, 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.
Security Considerations
The security considerations for ICE are defined in .
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.
IANA Considerations
This document has no IANA actions.
Normative References
Trickle ICE: Incremental Provisioning of Candidates for the
Interactive Connectivity Establishment (ICE) Protocol
Acknowledgements
, , and provided lots of useful input and
comments.