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Differences from draft-sparks-sip-noninvite-00.txt
Network Working Group R. Sparks
Internet-Draft dynamicsoft
Expires: April 16, 2004 October 17, 2003
Considerations for the Session Initiation Protocol's non-INVITE
Transaction
draft-sparks-sip-noninvite-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on April 16, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This draft explores several issues with the Session Initiation
Protocol's non-INVITE transaction. It focuses on the use of
provisional responses and on problems related to transaction
timeouts. It proposes two alternative improvements to the existing
situation.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problems under the current specifications . . . . . . . . . 3
2.1 NITs must complete immediately or risk losing a race . . . . 3
2.2 Provisional responses can delay recovery from lost final
responses . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Delayed responses will temporarily blacklist an element . . 6
2.4 408 for non-INVITE is not useful . . . . . . . . . . . . . . 7
2.5 Non-INVITE timeouts doom forking proxies . . . . . . . . . . 8
2.6 Mismatched timer values make winning the race harder . . . . 8
3. Alternative A: Improving the situation with a fixed NIT
duration . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1 Improving the situation when responses are only delayed . . 9
3.1.1 Proposal 1: Make the best use of provisional responses . . . 9
3.1.2 Proposal 2: Remove the useless late-response storm . . . . . 10
3.1.3 Proposal 3: Improve a UAS's knowledge of how much time
it has to respond . . . . . . . . . . . . . . . . . . . . . 10
3.2 Improving the situation when an element is not going to
respond . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.1 Proposal 4: Strengthen specification of caching success
and failures in RFC 3263 . . . . . . . . . . . . . . . . . . 12
3.3 When an application needs more time . . . . . . . . . . . . 13
3.3.1 Strawman Proposal 5: Specify try again later behavior . . . 13
4. Alternative B: Allowing NITs to pend . . . . . . . . . . . . 14
4.1 Proposal 6: Allow the non-INVITE transaction to pend
indefinitely . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . 17
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1. Introduction
This draft explores several issues with the non-INVITE transaction.
It proposes two alternative paths towards improving the existing
situation. Alternative A works within the existing fixed transaction
length. Alternative B allows transactions to pend. We can choose one
of these alternatives, or choose to pursue alternative A in the short
term, and B with a longer term focus.
Alternative A contains several proposals. These proposals stand on
their own and may be accepted or rejected independently. Some of
Alternative A's proposals are reused in Alternative B, where they
also stand independent of each other.
2. Problems under the current specifications
There are a number of unpleasant edge conditions created by the SIP
non-INVITE transaction model's fixed duration. The negative aspects
of some of these are exacerbated by the effect provisional responses
have on the non-INVITE transaction state machines as currently
defined.
2.1 NITs must complete immediately or risk losing a race
The non-INVITE transaction is designed to have a fixed and finite
duration (dependent on T1). A consequence of this design is that
participants must strive to complete the transaction as quickly as
possible. Consider the race condition shown in Figure 1.
UAC UAS
| request |
--- |---. |
^ | `---. |
| | `-->| ---
| | | ^
| | | |
64*T1 | | |
| | | |
| | | 64*T1
| | | |
| | | |
v | | |
timeout <=== --- | 200 OK | |
| .---| v
| .---' | ---
|<--' |
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Figure 1: NI Race Condition
The UAS in this figure believes it has responded to the request in
time, and that the request succeeded. The UAC, on the other hand,
believes the request has timed-out, hence failed. No longer having a
matching client transaction, the UAC core will ignore what it
believes to be a spurious response. As far as the UAC is concerned,
it received no response at all to its request. The ultimate result is
the UAS and UAC have conflicting views of the outcome of the
transaction.
Therefore, a UAS cannot wait until the last possible moment to send a
final response within a NIT. It must, instead, send its response so
that it will arrive at the UAC before that UAC times out.
Unfortunately, the UAS has no way to accurately measure the
propagation time of the request or predict the propagation time of
the response. The uncertainty it faces is compounded by each proxy
that participates in the transaction. Thus, the UAS's only choice is
to send its final response as soon as it possibly can and hope for
the best.
This result constrains the set of problems that can be solved with a
single NIT. Any delay introduced during processing of a request
increases the probability of losing the race. If the timing
characteristics of that processing are not predictable and
controllable, a single NIT is an inappropriate model for handling the
request. One viable alternative is to accept the request with a 202
and send the ultimate results in a new request in the reciprocal
direction.
In specialized networks, a UAS might have some reliable knowledge of
inter-hop latency and could use that knowledge to determine if it has
time to delay its final response in order to perform some processing
such as a database lookup while mitigating its risk of losing the
race in Figure 1. Establishing this knowledge across arbitrary
networks (perhaps using resource reservation techniques and
deterministic transports) is not currently feasible.
2.2 Provisional responses can delay recovery from lost final responses
The non-INVITE client transaction state machine provides reliability
for NITs over unreliable transports (UDP) through retransmission of
the request message. Timer E is set to T1 when a request is initially
transmitted. As long as the machine remains in the Trying state, each
time Timer E fires, it will be reset to twice its previous value
(capping at T2) and the request is retransmitted.
If the non-INVITE client transaction state machine sees a provisional
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response, it transitions to the Proceeding state, where
retransmission continues, but the algorithm for resetting Timer E is
simply to use T2 instead of doubling at each firing. (Note that Timer
E is not altered during the transition to Proceeding).
Making the transition to the Proceeding state before Timer E is reset
to T2 can cause recovery from a lost final response to take extra
time. Figure 2 shows recovery from a lost final response with and
without a provisional message during this window. Recovery occurs
within 2*T1 in the case without the provisional. With the
provisional, recovery is delayed until T2, which by default is 8*T1.
In practical terms, a provisional response to a NIT in currently
deployed networks can delay transaction completion by up to 3.5
seconds.
UAC UAS UAC UAS
| | | |
--- |----. | --- |----. |
^ | `-->| ^ | `--->|
E = T1 | | E = T1 | .-----|(provisional)
v | | v |<--' |
--- |----. | --- |----. |
^ | `-->| ^ | `--->|
| | X<----|(lost final) | | X<-----|(lost final)
| | | | | |
E = 2*T1 | | | | |
| | | | | |
| | | | | |
v | | | | |
--- |----. | | | |
| `-->| | | |
| .-----|(final) | | |
|<-' | | | |
| | | | |
\/\ /\/ /\/ /\/ /\/
E = T2
\/\ /\/ /\/ /\/ /\/
| | | | |
| | v | |
| | --- |----. |
| | | `--->|
| | | .-----|(final)
| | |<--' |
| | | |
Figure 2: Provisionals can harm recovery
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No additional delay is introduced if the first provisional response
is received after Timer E has reached its maximum reset interval of
T2.
2.3 Delayed responses will temporarily blacklist an element
A SIP element's use of SRV is specified in RFC 3263 [2]. That
specification discusses how SIP assures high availability by having
upstream elements detect failure of downstream elements. It proceeds
to define several types of failure detection and instructions for
failover. Two of the behaviors it describes are important to this
document:
o Within a transaction, transport failure is detected either through
an explicit report from the transport layer or through timeout.
Note specifically that timeout will indicates transport failure
regardless of the transport in use. When transport failure is
detected, the request is retried at the next element from the
sorted results of the SRV query.
o Between transactions, locations reporting temporary failure
(through 503/Retry-After for example) are not used until their
requested black-out period expires.
The specification notes the benefit of caching locations that are
successfully contacted, but does not discuss how such a cache is
maintained. It is unclear whether an element should stop using
(temporarily blacklist) a location returned in the SRV query that
results in a transport error. If it does, when should such a location
be removed from the blacklist?
Without such a blacklist (or equivalent mechanism), the intended
availability mechanism fails miserably. Consider traffic between two
domains. Proxy pA in domain A needs to forward a sequence of
non-INVITE requests to domain B. Through DNS SRV, pA discovers pB1
and pB2, and the ordering rules of [2] and [3] indicate it should use
pB1 first. The first request to pB1 times out. Since pA is a proxy
and a NIT has a fixed duration, pA has no opportunity to retry the
request at pB2. If pA does not remember pB1's failure, the second
request (and all subsequent non-INVITE requests until pB1 recovers)
are doomed to the same failure. Caching would allow the subsequent
requests to be tried at pB2.
Since miserable failure is not acceptable in deployed networks, we
should anticipate that elements will, in fact, cache timeout failures
between transactions. Then the race in Figure 1 becomes important. If
an element fails to respond "soon enough", it has effectively not
responded at all, and will be blacklisted at its peer for some period
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of time.
(Note that even with caching, the first request timeout results in a
timeout failure all the way back to the original submitter. The
failover mechanisms in [2] work well to increase the resiliency of a
given INVITE transaction, but do nothing for a given non-INVITE
transaction.)
2.4 408 for non-INVITE is not useful
Consider the race condition in Figure 1 when the final response is
408 instead of 200. Under the current specification, the race is
guaranteed to be lost. Most existing endpoints will emit a 408 for a
non-INVITE request 64*T1 after receiving the request if they haven't
emitted an earlier final response. Such a 408 is guaranteed to arrive
at the next upstream element too late to be useful. In fact, in the
presence of proxies, these messages are even harmful. When the 408
arrives, each proxy will have already terminated its associated
client transaction due to timeout. So, each proxy must forward the
408 upstream statelessly. This, in turn, is guaranteed to arrive too
late. As Figure 3 shows, this can ultimately result in bombarding
the original requester with spurious 408s. (Note that the proxy's
client transaction state machine never enters the Completed state, so
Timer K does not enter into play).
UAC P1 P2 P3 UAS
| | | | |
--- ===---. | | | |
^ | `-->===---. | | |
| | | `-->===---. | |
| | | | `-->===---. |
64*T1 | | | | `-->===
| | | | | |
| | | | | |
v | | | | |
(timeout) --- === | | | |
| .-408=== | | |
|<--' | .-408=== | |
| .-408-|<--' | .-408=== |
|<--' | .-408-|<--' | .-408===
| .-408-|<--' | .-408-|<--' |
|<--' | .-408-|<--' | |
| .-408-|<--' | | |
|<--' | | | |
| | | | |
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Figure 3: late 408s to non-INVITEs
This response bombardment is not limited to the 408 response, though
it only exists when participating client transaction state machines
are timing out. Figure 4 generalizes Figure 1 to include multiple
hops. Note that even though the UAS responds "in time" to P3, the
response is too late for P2, P1 and the UAC.
UAC P1 P2 P3 UAS
| | | | |
--- ===---. | | | |
^ | `-->===---. | | |
| | | `-->===---. | |
| | | | `-->===---. |
64*T1 | | | | `-->===
| | | | | |
| | | | | |
v | | | | |
(timeout) --- === | | | |
| .-408=== | | .-200-|
|<--' | .-408=== .-200-|<--' |
| .-408-|<--'.-200-|<--' === |
|<--'.-200-|<--' | | ===
|<--' | | | |
| | | | |
Figure 4: Additional timeout related error
2.5 Non-INVITE timeouts doom forking proxies
A single branch with a delayed or missing final response will
dominate the processing at proxy that receives no 2xx responses to a
forked non-INVITE request. Since this proxy is required to allow all
of its client transactions to terminate before choosing a "best
response". This forces the proxy's server transaction to lose the
race in Figure 1. Any response it ultimately forwards (a 401 for
example) will arrive at the upstream elements too late to be used.
Thus, if no element among the branches would return a 2xx response,
failure of a single element (or its transport) dooms the proxy to
failure.
2.6 Mismatched timer values make winning the race harder
There are many failure scenarios due to misconfiguration or
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misbehavior that the SIP specification does not discuss. One is
placing two elements with different configured values for T1 and T2
on the same network. Review of Figure 1 illustrates that the race
failure is only made more likely in this misconfigured state (it may
appear that shortening T1 at the element behaving as a UAS improves
this particular situation, but remember that these elements may trade
roles on the next request). Since the protocol provides no mechanism
for discovering/negotiating a peer's timer values, exceptional care
must be taken when deploying systems with non-defaults to ensure they
will _never_ directly communicate with elements with default values.
3. Alternative A: Improving the situation with a fixed NIT duration
3.1 Improving the situation when responses are only delayed
There are two goals to achieve when we constrain the problem to those
cases where all elements are ultimately responsive and networks
ultimately deliver messages:
o Reduce the probability of losing the race, preferably to the point
that it is negligible
o Reduce or eliminate useless messaging
3.1.1 Proposal 1: Make the best use of provisional responses
o Disallow non-100 provisionals to non-INVITE requests
o Disallow 100 Trying to non-INVITE requests before Timer E reaches
T2 (for UDP hops)
o Allow 100 Trying after Timer E reaches T2 (for UDP hops)
o Allow 100 Trying for hops over reliable transports
Since Non-INVITE transactions must complete rapidly (Section 2.1),
any information beyond "I'm here" (which can be provided by a 100
Trying) can be just as usefully delayed to the final response.
Sending non-100 provisionals wastes bandwidth.
As shown in Section 2.2, sending any provisional response inside a
NIT before Timer E reaches T2 damages recovery from failure of an
unreliable transport.
Without a provisional, a late final response is the same as no
response at all and will likely result in blacklisting the late
responding element (Section 2.3). If an element is delaying its final
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response at all, sending a 100 Trying after Timer E reaches T2
prevents this blacklisting without damaging recovery from unreliable
transport failure.
Blacklisting on a late response occurs even over reliable transports.
Thus, if an element processing a request received over a reliable
transport is delaying its final response at all, sending a 100 Trying
well in advance of the timeout will prevent blacklisting. Sending a
100 Trying immediately will not harm the transaction as it would over
UDP, but a policy of always sending such a message results in
unneccessary traffic. A policy of sending a 100 Trying after the
period of time in which Timer E reaches T2 had this been a UDP hop is
one reasonable compromise.
3.1.2 Proposal 2: Remove the useless late-response storm
o Disallow 408 to non-INVITE requests
o Absorb late non-INVITE responses at proxies
A 408 to non-INVITE will always arrive too late to be useful (Section
2.4). The client already has full knowledge of the timeout. The only
information this message would convey is whether or not the server
believed the transaction timed out. However, with the current design
of the NIT, a client can't do anything with this knowledge. Thus the
408 simply wasting network resources and contributes to the response
bombardment illustrated in Figure 3.
If a proxy were able to identify a response as a useless late
non-INVITE response, it could absorb the message and not abuse
upstream elements with it. A simple change to the non-INVITE client
state machine will allow a proxy to identify these responses. Modify
the machine to continue to live after Timer F fires to absorb the
useless responses. This is similar to what is already provided by
Timer K for absorbing retransmitted responses, but the absorption
behavior must exist even for reliable transports. (Perhaps it would
be sufficient to move the Timer F transition to the Completed state
and always set Timer K regardless of transport). This approach
suppresses late final responses, such as the 200 in Figure 4, at the
element where it first becomes useless.
3.1.3 Proposal 3: Improve a UAS's knowledge of how much time it has to
respond
Consider the race lost in Figure 4. The UAS could win this race if it
responded soon enough for its 200 to reach the UAC before the UAC
timed out. Unfortunately, there is no way, given the current
specifications, for the UAC to know how much time it really has left.
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It might make a rough guess at the propagation time due to network
transmission by counting Via header field values and assuming each
hop took at most T1, but it has no idea at all what the propagation
delay through each of the proxies was.
The UAS's situation could be dramatically improved if the next
upstream element explicitly indicated how much time was left. Each
element would assume a network delay for any message of T1, and
estimate the sum of its own internal propagation delay for both the
request and the final response, resulting in the messaging shown in
Figure 5 (which for compactness assumes T1=500ms at each hop). Assume
the internal delay introduced by P1, P2, and P3 is 1.5s, 3s, and 0.5s
respectively. P1 advertises a timeleft of 32 - 1.5 - 2*T1 = 29.5. P2
advertises a timeleft of 29.5 - 3 - 2*T1 = 25.5. P3 advertises 25.5 -
0.5 - 2*T1 = 24
UAC P1 P2 P3 UAS
| NI-Timeleft: 32 | | |
--- ===---. | NI-Timeleft: 29.5 | |
^ | `-->===---. | NI-Timeleft: 25.5 |
| | | `-->===---. | NI-Timeleft: 24
| | | | `-->===---. |
| | | | | `-->===
| | | | | |
| | | | | |
32s | | | | |
| | | | | .-200-|
| | | | .-200-|<--' ===
| | | .-200-|<--' === |
| | .-200-|<--' | | |
| |<--' | === | |
v | === | | |
(timeout) --- === | | | |
Figure 5: Explicitly indicating timeleft
Note that each element determines how much time was and will be lost
to network propagation delay over the first upstream hop in
incorporates that into its calculation. The UAS will need to do this
as well, so in our example above, it knows that it only has 23
seconds to respond.
The estimate of timeleft can be improved if an element has better
knowledge of the real network propagation delay. The element can
measure its internal propagation delay for the request, but will have
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to estimate the propagation delay for the response.
To improve behavior in the presence of existing elements that will
not supply a timeleft indication, an element that receives a
non-INVITE request without the indication could behave as if it had
received value of
64*T1 - (2*T1 + IPD)*(n_Via-1)
where
IPD = estimate of internal processing delay of a
request and a response (strawman: 1s)
n_Via = number of Via header field values in the request
3.2 Improving the situation when an element is not going to respond
When we expand the scope of the problem to also deal with element or
network failure, we have more goals to achieve:
o Identifying when an element is non-responsive
o Minimizing or eliminating falsely identifying responsive elements
as non-responsive
o Avoiding non-responsive elements with future requests
Accepting Proposal 1 will dramatically improve an elements ability to
distinguish between failure and delayed response from the next
downstream element. With this proposal, some response, either
provisional or final, is almost certainly going to be received before
the transaction times out. So, an element can more safely assume that
no response at all indicates the peer is not available and follow the
existing requirements in [1] and [2] for that case.
Accepting Proposal 3 provides a similar, but not as strong,
improvement in differentiating delayed responses from failure.
Proposals 1 and 3 taken together provide the best improvement.
Proposal 3 also addresses the proxy doom problem (Section 2.5).
As Section 2.3 discusses, behavior once an element is identified as
non-responsive is currently underspecified. [2] speaks only
non-normatively about caching the addresses of servers that have
successfully been communicated with for an unspecified period of
time.
3.2.1 Proposal 4: Strengthen specification of caching success and
failures in RFC 3263
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o Make the caching recommendation normative for servers successfully
reached (SHOULD)
o Add failures due to non-responsiveness to that cache (also SHOULD)
o Recommend a expiration for cache members (strawman: 5 minutes)
This cache could also be used to remember servers that have issued a
503 (with or without a Retry-After.
3.3 When an application needs more time
Application designers are faced with significant challenges when the
semantics of processing a request require more time (human
intervention for example) than the non-INVITE transaction allows. SIP
Events ([4]) deals with this by spreading the semantics of processing
a new subscription request across two or more non-INVITE requests - a
SUBSCRIBE and subsequent NOTIFYs. For example, if a server receives a
request for a subscription that cannot be granted or refused until a
human provides input, the SUBSCRIBE request will be accepted with a
202 Accepted. A subsequent NOTIFY will convey whether or not the
subscription has been allowed or denied.
An alternate approach is to allow a server to tell a client "I can't
do this right now, but try again in a little while".
3.3.1 Strawman Proposal 5: Specify try again later behavior
When a server discovers it needs more time than the current
non-INVITE transaction will allow to finish the work needed to
process the request, it could return a 302 response with:
o A contact pointing to itself with NO expiration time so that this
value cannot be cached.
o A Retry-After header indicating when the client should try the
request again
A client receiving this response SHOULD retry the request at the
indicated time. A server MUST NOT apply the results of the request
until the client successfully retries the request. (This limits the
set of problems this tool can be used with to those whose side
effects can be undone.) A client can effectively CANCEL a request by
not coming back.
There are several issues that would need to be resolved if this
approach is pursued:
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o [1] forbids emitting a 302 with a contact equal to the
Request-URI, so the "contact point to self" above would have to
change each time (with respect to URI equality) such that the
request still arrived at the same agent (requiring a GRUU).
o Emitting and handling 300-class responses for requests inside a
dialog is not well-specified in [1]. It is unlikely that existing
implementations would exhibit interoperable behavior if they
encountered them.
o Proxies would need to know to not recurse on this kind of 302
response. This might require an explicitly signaled extension, or
indicate that a 4xx or 5xx class response is more appropriate.
4. Alternative B: Allowing NITs to pend
The root causes of the problems this document attempts to address are
the fixed-length NIT (which causes the race condition of Figure 1)
and the extra mechanics for providing reliability over unreliable
transports.
4.1 Proposal 6: Allow the non-INVITE transaction to pend indefinitely
We can change the definition of the non-INVITE transaction to allow
it to pend indefinitely by removing Timer F. By doing so,
o the race condition goes away
o the 408 response would become meaningful once again
o the late response blacklisting problem disappears
o the 408 bombardment problem disappears
o the proxy doom problem is eliminated
Clients would use CANCEL to pending non-INVITEs to stimulate a final
response when they are through waiting, similar to INVITE. Proxies
will be spared the doom described in Section 2.5 since they can force
branches to complete with CANCEL before sending a final response.
Responsibility for reliability over UDP would remain with the
requester. This means that provisional responses will still not
squelch request retransmission. A long pending non-INVITE request
would be retransmitted once 4 seconds (for the default value of T2)
once timer E reaches T2, but only over UDP. This might be mitigated
by replacing T2 with another, larger, configurable value for use with
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the non-INVITE transaction.
The primary disadvantage of this approach is that it raises the
expense for handling non-INVITE transactions at proxies to the same
level as INVITE transactions. Proxies will have to maintain state for
NITs longer than they currently do. Proxies will need a way to end
the transaction. We can give them this by duplicating INVITE
behavior: create a timer analogous to Timer C. When it fires, send
CANCELs down any outstanding branches and once they complete, send a
408 (assuming no branch returned a better final response) to the
requester.
This change is backwards-safe, if not completely backwards
compatible:
o Existing client, proposed server: The client's experience is
unchanged. It will still abandon the transaction after Timer F
fires. The failure scenarios are exactly those we currently have.
The server will need to protect itself against never receiving a
CANCEL (with an analog to Timer C).
o Proposed client, existing server: The behavior here is an
improvement over the existing client-server behavior. The 408
emitted by an existing server would become meaningful to the
proposed client. New methods that take advantage of the pending
property will be rejected by the existing server with a 501.
Existing servers might not be expecting CANCEL to non-INVITEs, but
are not compliant to the existing specification if such a CANCEL
induces incorrect behavior. We would need to add a constraint,
similar to that already on the INVITE transaction, binding clients
that receive no response within a short time to abandon the
transaction instead of pending indefinitely to account for server
failure.
If Alternative B is pursued, Proposals 1 (best use of provisionals)
and 4 (3263 caching) from Alternative A should also be considered.
5. Acknowledgments
This document attempts to capture many conversations about non-INVITE
issues. Significant contributers include Ben Campbell, Gonzalo
Camarillo, Steve Donovan, Rohan Mahy, Dan Petrie, Adam Roach,
Jonathan Rosenberg, and Dean Willis.
References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
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Session Initiation Protocol", RFC 3261, June 2002.
[2] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[3] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
Author's Address
Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: rsparks@dynamicsoft.com
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