16 Proxy Behavior
17.1 Client Transaction
17.1.1 INVITE Client Transaction
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17.1.1.1 Overview ofINVITETransaction TheINVITEtransaction consists of a three-way handshake.
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The client transaction sends anINVITE, the server transaction sends responses, and the client transaction
3258
sends an ACK. For unreliable transports (such as UDP), the client transaction retransmits requests at an
3259
interval that starts at T1 seconds and doubles after every retransmission. T1 is an estimate of the
round-3260
trip time (RTT), and it defaults to 500 ms. Nearly all of the transaction timers described here scale with
3261
T1, and changing T1 adjusts their values. The request is not retransmitted over reliable transports. After
3262
receiving a 1xx response, any retransmissions cease altogether, and the client waits for further responses.
3263
The server transaction can send additional 1xx responses, which are not transmitted reliably by the server
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transaction. Eventually, the server transaction decides to send a final response. For unreliable transports,
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that response is retransmitted periodically, and for reliable transports, it is sent once. For each final response
3266
that is received at the client transaction, the client transaction sends an ACK, the purpose of which is to
3267
quench retransmissions of the response.
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17.1.1.2 Formal Description The state machine for theINVITEclient transaction is shown in Figure 5.
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The initial state, “calling”,MUSTbe entered when the TU initiates a new client transaction with anINVITE
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request. The client transactionMUSTpass the request to the transport layer for transmission (see Section 18).
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If an unreliable transport is being used, the client transaction MUSTstart timer A with a value of T1. If a
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reliable transport is being used, the client transactionSHOULD NOTstart timer A (Timer A controls request
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retransmissions). For any transport, the client transactionMUSTstart timer B with a value of 64*T1 seconds
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(Timer B controls transaction timeouts).
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When timer A fires, the client transaction MUST retransmit the request by passing it to the transport
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layer, andMUSTreset the timer with a value of 2*T1. The formal definition of retransmit within the context
3277
of the transaction layer is to take the message previously sent to the transport layer and pass it to the transport
3278
layer once more.
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When timer A fires 2*T1 seconds later, the request MUSTbe retransmitted again (assuming the client
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transaction is still in this state). This processMUSTcontinue so that the request is retransmitted with intervals
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that double after each transmission. These retransmissionsSHOULDonly be done while the client transaction
3282
is in the “calling” state.
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The default value for T1 is 500 ms. T1 is an estimate of the RTT between the client and server
trans-3284
actions. ElementsMAY(though it isNOT RECOMMENDED) use smaller values of T1 within closed, private
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networks that do not permit general Internet connection. T1 MAY be chosen larger, and this is RECOM
-3286
MENDED if it is known in advance (such as on high latency access links) that the RTT is larger. Whatever
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the value of T1, the exponential backoffs on retransmissions described in this sectionMUSTbe used.
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|INVITE from TU Timer A fires |INVITE sent Reset A, V Timer B fires INVITE sent +−−−−−−−−−−−+ or Transport Err.
+−−−−−−−−−| |−−−−−−−−−−−−−−−+inform TU | | Calling | | +−−−−−−−−>| |−−−−−−−−−−−−−−>|
+−−−−−−−−−−−+ 2xx | | | 2xx to TU | | |1xx | 300−699 +−−−−−−−−−−−−−−−+ |1xx to TU | ACK sent | | | resp. to TU | 1xx V | | 1xx to TU −−−−−−−−−−−+ | | +−−−−−−−−−| | | | | |Proceeding |−−−−−−−−−−−−−−>|
| +−−−−−−−−>| | 2xx | | +−−−−−−−−−−−+ 2xx to TU | | 300−699 | | | ACK sent, | | | resp. to TU| | | | | NOTE:
| 300−699 V | | ACK sent +−−−−−−−−−−−+Transport Err. | transitions | +−−−−−−−−−| |Inform TU | labeled with | | | Completed |−−−−−−−−−−−−−−>| the event | +−−−−−−−−>| | | over the action | +−−−−−−−−−−−+ | to take | ^ | | | | | Timer D fires | +−−−−−−−−−−−−−−+ | − | | | V | +−−−−−−−−−−−+ | | | | | Terminated|<−−−−−−−−−−−−−−+
| | +−−−−−−−−−−−+
Figure 5: INVITEclient transaction
If the client transaction is still in the “calling” state when timer B fires, the client transaction SHOULD
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inform the TU that a timeout has occurred. The client transactionMUST NOTgenerate anACK. The value of
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64*T1 is equal to the amount of time required to send seven requests in the case of an unreliable transport.
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If the client transaction receives a provisional response while in the ”Calling” state, it transitions to the
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“proceeding” state. In the “proceeding” state, the client transactionSHOULD NOTretransmit the request any
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longer. Furthermore, the provisional responseMUSTbe passed to the TU. Any further provisional responses
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MUSTbe passed up to the TU while in the “proceeding” state.
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When in either the ”Calling” or “Proceeding” states, reception of a response with status code from
3296
300-699MUSTcause the client transaction to transition to “Completed”. The client transaction MUSTpass
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the received response up to the TU, and the client transaction MUSTgenerate anACK request, even if the
3298
transport is reliable (guidelines for constructing theACKfrom the response are given in Section 17.1.1.3)
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and then pass theACKto the transport layer for transmission. TheACKMUSTbe sent to the same address,
3300
port, and transport to which the original request was sent. The client transaction SHOULD start timer D
3301
when it enters the “Completed” state, with a value of at least 32 seconds for unreliable transports, and a
3302
value of zero seconds for reliable transports. Timer D reflects the amount of time that the server transaction
3303
can remain in the “Completed” state when unreliable transports are used. This is equal to Timer H in the
3304
INVITEserver transaction, whose default is 64*T1. However, the client transaction does not know the value
3305
of T1 in use by the server transaction, so an absolute minimum of 32s is used instead of basing Timer D on
3306
T1.
3307
Any retransmissions of the final response that are received while in the “Completed” stateMUSTcause
3308
theACK to be re-passed to the transport layer for retransmission, but the newly received response MUST
3309
NOTbe passed up to the TU. A retransmission of the response is defined as any response which would match
3310
the same client transaction based on the rules of Section 17.1.3.
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If timer D fires while the client transaction is in the “Completed” state, the client transaction MUSTmove
3312
to the terminated state, and itMUSTinform the TU of the timeout.
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When in either the “Calling” or “Proceeding” states, reception of a 2xx response MUSTcause the client
3314
transaction to enter the ”Terminated” state, and the responseMUSTbe passed up to the TU. The handling of
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this response depends on whether the TU is a proxy core or a UAC core. A UAC core will handle generation
3316
of theACKfor this response, while a proxy core will always forward the 200 (OK) upstream. The differing
3317
treatment of 200 (OK) between proxy and UAC is the reason that handling of it does not take place in the
3318
transaction layer.
3319
The client transaction MUST be destroyed the instant it enters the ”Terminated” state. This is actually
3320
necessary to guarantee correct operation. The reason is that 2xx responses to anINVITEare treated
differ-3321
ently; each one is forwarded by proxies, and theACKhandling in a UAC is different. Thus, each 2xx needs
3322
to be passed to a proxy core (so that it can be forwarded) and to a UAC core (so it can be acknowledged). No
3323
transaction layer processing takes place. Whenever a response is received by the transport, if the transport
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layer finds no matching client transaction (using the rules of Section 17.1.3), the response is passed directly
3325
to the core. Since the matching client transaction is destroyed by the first 2xx, subsequent 2xx will find no
3326
match and therefore be passed to the core.
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17.1.1.3 Construction of theACK Request This section specifies the construction ofACK requests
3328
sent within the client transaction. A UAC core that generates anACKfor 2xxMUSTinstead follow the rules
3329
described in Section 13.
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TheACKrequest constructed by the client transactionMUSTcontain values for theCall-ID,From, and
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Request-URIthat are equal to the values of those header fields in the request passed to the transport by
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the client transaction (call this the “original request”). TheToheader field in theACKMUSTequal theTo
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header field in the response being acknowledged, and therefore will usually differ from theToheader field
3334
in the original request by the addition of the tag parameter. The ACKMUST contain a singleViaheader
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field, and thisMUSTbe equal to the topViaheader field of the original request. TheCSeqheader field in
3336
theACK MUSTcontain the same value for the sequence number as was present in the original request, but
3337
the method parameterMUSTbe equal to “ACK”.
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If the INVITErequest whose response is being acknowledged had Routeheader fields, those header
3339
fields MUSTappear in theACK. This is to ensure that theACKcan be routed properly through any
down-3340
stream stateless proxies.
3341
Although any request MAYcontain a body, a body in an ACK is special since the request cannot be
3342
rejected if the body is not understood. Therefore, placement of bodies inACKfor non-2xx isNOT RECOM
-3343
MENDED, but if done, the body types are restricted to any that appeared in theINVITE, assuming that the
3344
response to theINVITEwas not 415. If it was, the body in theACKMAYbe any type listed in theAccept
3345
header field in the 415.
3346
For example, consider the following request:
3347
INVITE sip:[email protected] SIP/2.0
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Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
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To: Bob <sip:[email protected]>
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From: Alice <sip:[email protected]>;tag=88sja8x
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Max-Forwards: 70
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Call-ID: 987asjd97y7atg
3353
CSeq: 986759 INVITE
3354
TheACKrequest for a non-2xx final response to this request would look like this:
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ACK sip:[email protected] SIP/2.0
3356
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKkjshdyff
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To: Bob <sip:[email protected]>;tag=99sa0xk
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From: Alice <sip:[email protected]>;tag=88sja8x
3359
Max-Forwards: 70
3360
Call-ID: 987asjd97y7atg
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CSeq: 986759 ACK
3362