Internet DRAFT - draft-jesske-dispatch-forking-answer-correlation
draft-jesske-dispatch-forking-answer-correlation
dispatch R.J. Jesske
Internet-Draft Deutsche Telekom
Intended status: Informational 4 March 2024
Expires: 5 September 2024
Correlation of multiple responses of forked INVITES in Back to Back User
Agents
draft-jesske-dispatch-forking-answer-correlation-04
Abstract
This document describe how a correlation of multiple responses of a
forked INVITE in Back to Back User Agents can apply.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 5 September 2024.
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
2. Consideration of RFC's on SIP signalling procedures under
consideration for forking use cases . . . . . . . . . . . 5
2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. RFC3261 Session Initiation Protocol . . . . . . . . . . . 6
2.3. RFC3960 Early Media and Ringing Tone Generation in the
Session Initiation Protocol (SIP) . . . . . . . . . . . 7
2.4. RFC3262 Reliability of provisional responses . . . . . . 8
2.5. RFC3312 Integration of Resource Management and Session
Initiation Protocol (SIP) . . . . . . . . . . . . . . . 8
2.6. RFC3841 Caller Preferences for the Session Initiation
Protocol (SIP) . . . . . . . . . . . . . . . . . . . . . 9
2.7. RFC5393 Addressing an Amplification Vulnerability in
Session Initiation Protocol (SIP) Forking Proxies . . . 9
2.8. RFC6228 Session Initiation Protocol (SIP) Response Code for
Indication of Terminated Dialog . . . . . . . . . . . . 10
2.9. RFC3326 The Reason Header Field for the Session Initiation
Protocol (SIP) . . . . . . . . . . . . . . . . . . . . . 10
2.10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 10
3. Requirements on forking in SIP networks interconnecting with
other SIP networks . . . . . . . . . . . . . . . . . . . 11
4. Forking use cases . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Normal Forking use case . . . . . . . . . . . . . . . . . 11
4.2. Multiples provisional responses without SDP . . . . . . . 13
4.3. Forking use case with provisional responses with SDP using
100rel . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.4. Forking use case with provisional responses with SDP using
100rel and preconditions . . . . . . . . . . . . . . . . 18
4.5. Forking use case with early media played . . . . . . . . 22
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
8. Normative References . . . . . . . . . . . . . . . . . . . . 24
9. Informative References . . . . . . . . . . . . . . . . . . . 26
Appendix A. Appendix . . . . . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Problem Statement
Within RFC3261 [RFC3261] the handling of multiples early responses
received from different UAS for one sent INVITE is described as basis
feature. Such behavior results usually in case of forking or also
when an INVITE is forwarded to another target.
The forking feature described in RFC3261 [RFC3261] allows to spread
the INVITE request towards multiples registered UA's for one identity
but is present at different locations.
A forwarding as described in RFC3960 [RFC3960]of a SIP Dialog may
result in multiples early dialogs(181 and 180 with different to
tags). A 181 response may also initiate an early dialog.
There are scenarios within the scope of this document where multiple
responses received should be multiplexed to a single dialog to avoid
complexity or miss behavior for the originating network/UAC:
1. When a SIP entity (B2BUA) provides a service within the SIP path
where multiples early responses received must be multiplexed into a
single dialog.
2. Providing interconnecting between different networks with
different behavior where a the originating network implementations do
not wish, have restrictions or may not able handle multiples
responses while the destination still deliver multiples responses or
early dialogs.
3. Providing interconnection where the terminating service provider
will have or must have control what kind of early dialog information
should be provided to the originating network based on bilateral
agreement, service provider policy or regulatory policy.
4. Service provider providing the forking feature or interconnecting
to networks providing multiples early dialogs to INVITES would like
to increase the reliability of the network in increasing successful
calls even if UAC do not really support the correct handling of
multiples early dialogs. Please Note that this scenario is assuming
that old or faulty implementations are existing. This is real life
and service provider are already using healing functions within B2BUA
but it is clear that the correct way is to replace or correct faulty
implementations of SIP.
Scenario 1:
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A service example may be a directory service providing a early
dialog/session with a Interactive Voice Response (IVR) system for
requesting a destination number where the user will be automatically
forwarded to a identity where a couple of UAS are registered within
the SIP network. Thus the INVITE will be forked and result in
multiples responses to the INVITE sent by the SIP entity providing
the service. It could be also a normal call forwarding scenario
providing a short announcement to the UAC indicating the forwarding.
Due to the fact that a early session was already provided by the
service it needs an mapping of the multiples responses towards the
UAC to provide the ringing state, tone or provide further
announcements.
Scenario 2:
Due to the fact that the world of SIP networks is steadily growing
and SIP networks based on IETF RFC 3261 are existing with different
flavors like based on pure SIP, 3GPP IMS or ETSI TISPAN NGN and many
others. Now connecting these networks which may be operated by
different service provides may result in complexity due to signaling,
charging or even worse in faulty cases. Providing voice services via
SIP over restricted access capabilities may need restrictions with
regard to numbers of early sessions provided in backward direction or
signaling load.
Other service providers do only allow single dialog over
interconnection to avoid charging complexity with too many early
sessions or early dialogs.
Also equipment and UAC not understanding or misbehaving when
receiving multiples responses may be in focus when restricting the
interconnection use case to a single early dialog.
Scenario 3:
Within this scenario the terminating service provider apply specific
policy. This may be either in guaranteeing his customers to provide
forking to for all entities registered for a specific identity, or
allow only specific announcements (e.g in a specific language) passed
through or to ensure the bilateral agreements with other operators
where only single dialog is mandated. Also in forwarding scenarios
some originating networks do not wish to have a forwarding indication
(i.e. 181) or early session in backward direction.
Scenario 4:
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As already stated this is a real life scenario but seen from
implementation point of view the wrong approach. The real solution
is to replace or correct faulty implementations of SIP.
But nevertheless the problem existing is that feature forking is not
really supported by all UAC in that way that a connection between UAC
and one of the UAS will succeed in a successful communication even
one of the UAS sends a 200 OK for the INVITE. This apply also to
networks having B2BUA's (e.g. PSTN interworking Gateways or Session
Border Controller) in the path which should understand multiples
responses and handle it correctly.
Providing solutions for scenarios 1-3 will also provide a possibility
for service provider to support scenario 4.
This document evaluates the existing forking mechanism described in
RFC3261 [RFC3261] and further RFC_s extending SIP for early dialog
handling, resource handling and reliability of provisional responses.
Also directives and SIP extensions will be investigated where forking
or fraud based on forking may be avoided. It will result in
different use cases and solutions for the above mentioned scenarios
where multiples responses are received by B2BUA and may be
multiplexed to a single dialog.
This document describes how a correlation/multiplexing for multiples
early dialogs and other received Responses can done within a B2BUA.
The possible roles of a B2BUA is described in the taxonomy document
RFC7029 [RFC7029] The role of the B2BUA is a Signaling/Media Plane
B2BUA Role. Thus many possible use cases which will be possible
looking on the used features are considered in this document.
It is NOT within the scope of this document to deploy new SIP
procedures but it is within the scope to use existing SIP procedures
to describe the proper multiplexing.
2. Consideration of RFC's on SIP signalling procedures under
consideration for forking use cases
2.1. Overview
The following sections shows the documentation for forking in
different RFCs and what is missing to apply a correlation of
multiples early dialogs. Also what procedures should apply for a
correct handling of multiples early dialogs.
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2.2. RFC3261 Session Initiation Protocol
SIP defined in RFC3261 [RFC3261]describes how forking should apply.
In Section 10 of RFC3261 [RFC3261] it is defined that registration
creates bindings in a location service for a particular domain that
associates an address-of-record URI with one or more contact
addresses. Thus when receiving an INVITE with a Request-URI that
matches the address-of-record, the proxy will forward the request to
the contact addresses registered to that address-of-record. Proxy
behavior in RFC3261 [RFC3261] section 16.6 of describes that a
stateful proxy MAY choose to "fork" a request, routing it to multiple
destinations. Any request that is forwarded to more than one
location MUST be handled statefully. Forking apply serial or
parallel. Thus the forking proxy is either waiting for a final
response or a timeout before sending the INVITE to the next contact
or sent the INVITE parallel to the contacts registered for one
address of record. Considering the forking proxy when the INVITE was
sent one or multiple provisional responses may arrive before one or
more final responses are received. As RFC3261 [RFC3261] describes
these provisional responses may create "early dialogs". and will be
forwarded to the UAC.
This concludes that receiving provisional responses may either create
early dialogs or will be ignored by the UAC.
The UAC procedures allow receiving one or more provisional responses.
The description how to be have exactly is not given by RFC3261
[RFC3261]. Under Section 12.1 "Creation of a Dialog" describes that
early dialogs will be established by non final 101-199 responses. No
more hints given within this section (e.g. 12.2.1.2 Processing the
Responses) how to behave when receiving multiples provisional
responses. The termination of early dialogs is clear described that
if the dialog is terminated (with BYE) all early dialogs are also
terminated. The termination of early dialogs will also be processed
when receiving a final non 200 OK response.
For session creation the description of response handling to an sent
INVITE is more precise. Within section 13.1 it is stated that a UA
will open multiples dialogs when receiving multiples 200OK to an
forked INVITE which are then within the same call. Within section
13.2.2.1 1xx Responses it is stated that Provisional responses for an
INVITE request can create "early dialogs". Early dialogs are only
needed for sending requests to its peer within the dialog before the
initial INVITE transaction completes. This imply that if the UAC
does not support the creation of "early dialogs" that provisional
responses will be ignored by the UAC. Provisional responses may
contain the same exact SDP answer sent prior to the answer within the
final 2xx response. Editors Note: It is not stated in RFC3261 what
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happen in cases of early dialogs arriving with different SDP and also
receiving RTP for that dialogs. i.e. more information will deliver
RFC3960 "Early Media and Ringing Tone Generation in the Session
Initiation Protocol (SIP)"
Note that it is not clearly stated what happen if the SDP is
different between provisional response and final answer. It is
assumed that such information will be ignored by the UA.
The conclusion is that rules for UAC on responses in general and
merged requests are defined. For the normal RFC3261 [RFC3261]
forking use case only final Responses (200 OK) will be considered to
create a dialog/session. This also apply when provisional responses
due to other services (e.g. 181 in case of forwarding) will arrive at
the UAC.
A couple of rules with regard to responses when forwarding it to the
UAC apply to the forking proxy. 1xx and 2xx response will be
forwarded immediately. For 6xx Responses the proxy SHOULD cancel all
client pending transactions. The Response-Context as defined in
RFC3261 [RFC3261] Section 16.7 Response Proceeding" does describe how
the forking proxy should behave when receiving responses from
different branches. According to the rules in this section the proxy
will hold the received non 2xx final responses until for all INVITE
transactions a final response is received. The Forking proxy has to
choose a final response. One free chosen out of of the lowest
response class stored. Preference to that response giving additional
information affecting resubmission (such as 401, 407, 415, 420, and
484 if the 4xx class is chosen) A 503 should be changed to 500.
Thus with regard to non 2xx and 1xx final responses the forking proxy
has to aggregate and act as central element
Conclusion is that RFC3261 [RFC3261] describes the possibility of
forking a INVITE, the forking proxy behavior and the UAC behavior to
either ignore all provisional responses or open multiples early
dialogs and accept one or more final responses.
2.3. RFC3960 Early Media and Ringing Tone Generation in the Session
Initiation Protocol (SIP)
As for early media and Forking RFC3960 [RFC3960] describes two models
for providing early media. There are the gateway model and the
Application Server model described. Both have their constrains with
forking. The gateway model will present all early media streams to
the user in cases where the UAC will not choose one media stream it
will confuse the human hearing this mix of media streams. Also
bandwidth restrictions will end in bad quality.
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The application Server model is based on an early-session disposition
type which will not be supported by every UAC.
This RFC does not describe the possibility of multiplexing multiples
early media streams to one. Or a choosing mechanism for UAC which of
the early dialog should be used to play the announcement.
Since this RFC does not consider B2BUA with media awareness as
defined in RFC7029 [RFC7029] some possible functionality is missing
where media can be correlated.
2.4. RFC3262 Reliability of provisional responses
The RFC describing the reliability of provisional Responses RFC3262
[RFC3262] does not describe directly interactions with forking. It
defines the PRACK method and describes the procedures to make a
provisional response reliable. Thus the SDP of a 200OK is not longer
needed.
The important statement is that user agents that support this 100rel
MUST support all offer/answer exchanges that are possible based on
the rules in Section 13.2 of RFC3261 [RFC3261], based on the
existence of INVITE and PRACK as requests, and 2xx and reliable 1xx
as non-failure reliable responses. Thus with forking the UAC has to
make each received provisional Response reliable. Specific
procedures for a forking proxy does not exist since it has only to
pass the responses.
2.5. RFC3312 Integration of Resource Management and Session Initiation
Protocol (SIP)
RFC3313 [RFC3312] describing the precondition mechanism is not
mentioning any interactions with forking relevant issues.
Since RFC3262 [RFC3262] is a precondition to apply to the procedures
of RFC3313 [RFC3312]. Following that the UAC has to make the
resourcereservation with each forking branch where the provisional
response is stating 100rel required. The UAC will have to implement
complex procedures to make the ressource reservation to all received
SDP. And it could lead to the same problems as stated for the early
media use case. Also network procedures may be influences. I.e
B2BUA with media awareness as defined in RFC7029 [RFC7029] have to
reserve resources and have to proceed correctly like in releasing
brances where capacity restirctions will aply within the media
reservation functionality.
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2.6. RFC3841 Caller Preferences for the Session Initiation Protocol
(SIP)
RFC3841 [RFC3841] describes extensions to the Session Initiation
Protocol (SIP) which allow a caller (UAC) to express preferences
about request handling in servers.
One of the caller preferences defined in RFC3841 [RFC3841]. is a
method to signal the "fork-directive" to indicate that the UAC does
not wish that SIP proxy will fork the INVITE request. This directive
is a optional SIP feature the forking proxy is not forced to apply to
the directive sent by the UAC. Also not each network element
providing forrking has implemented this directive.
The parallel-directive does indicate how a SIP proxy may fork the
request. Either "parallel" or "sequential"
A B2BUA with media awareness as defined in RFC7029 [RFC7029] may use
this directive to avoid multiples provisional responses sent back due
to forking. But as said it will not give security for avoiding
forking. Thia may a tool for interconnection between service
providers.
2.7. RFC5393 Addressing an Amplification Vulnerability in Session
Initiation Protocol (SIP) Forking Proxies
To avoid too many forkings (possible early Dialogs) RFC5393 [RFC5393]
defines the Max-Breadth header to avoid to many forked Requests
caused by froking proxies. But there is no effect on correlating the
responses. This helps to reduce a cascading of multiples forkings in
the forward path. The number in the header gives the maximum
branches (parallel possible early dialogs) of a forked request.
Exceeding the maximum will result in error responses 440
A Max-Breadth of 1 restricts a request to pure serial forking rather
than restricting it from being forked at all.
RFC5393 [RFC5393] specifies normative changes to the SIP protocol.
And it mandates if a SIP proxy receives a request with no Max-Breadth
header field value, it MUST add one, with a value that is RECOMMENDED
to be 60. Seent from theory a parallel forking could be avoinded but
nevertheless not all SIP networks will have implemented this RFC and
will understnd these extensions.
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2.8. RFC6228 Session Initiation Protocol (SIP) Response Code for
Indication of Terminated Dialog
RFC6228 [RFC6228]defines a new response code to close early dialogs
proper. In case where a forking proxy realizes that a 200 OK has
been processed the proxy can sent 199 responses to the other open
dialogs. This helps in case of correlation when early dialogs has
been sent till the end user.
In consideration with the rules defined in RFC3261 for forking proxy
a received non 2xx final to an initial dialog initiation request that
it recognizes as terminating one or more early dialogs associated
with the request. The forking proxy generates normally (e.g. non
100rel, no final response sent) 199 response upstream for each of the
terminated early dialogs.
2.9. RFC3326 The Reason Header Field for the Session Initiation
Protocol (SIP)
RFC3326 [RFC3326] defines the Reason header and describes one use
where the INVITE is forked and results in a rejection, the error
response may never be forwarded to the client unless all the other
branches also reject the request. This problem is known as the
"Heterogeneous Error Response Forking Problem", or HERFP. It is
foreseen that a solution to this problem may involve encapsulating
the final error response in a provisional response. The Reason
header field is a candidate to be used for such encapsulation. In
this case the forking proxy will release the dialogs.
This will help to analyse problems with forking but will
2.10. Conclusions
All above mentioned RFCs and procedures describes a piece of the
whole picture how forking apply and what procedures are useabel for
such cases. It is also fact that UA's and B2BUA's (e.g. PSTN GW)
are existing that will not support multiples early dialogs. Also the
support of caller preferences is not secured or implemented by UAC
and also SIP forking proxies. Service providers will provide forking
independent what the source will support or not. Thus the only
solution seen is to describe procedures which apply in B2BUA to
support an correlation of multiples early dialogs and other received
18x Responses.
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3. Requirements on forking in SIP networks interconnecting with other
SIP networks
To improve interoperability with devices which do not support
forking, a service provider shall have the possibility to use a B2BUA
to multiplex multiple downstream dialogs into a single dialog toward
the caller."
The B2BUA providing such possibility must have to understand where
the SIP dialog request is coming from and if this originating network
or entity or UA can or cannot support multiples responses. This
could be also assumed by Service Level Agreements (SLA) between the
interconnection partners.
The main requirement is to have an entity that can receive multiples
responses based on forked INVITES which now can be correlated to one
single dialog towards the originating entity.
To act proactive the B2BUA should also be possible to include and
handle preferences (e.g. non forking wished) based on the originating
and terminating network.
4. Forking use cases
4.1. Normal Forking use case
SIP defined in RFC3261 [RFC3261]describe how forking should apply.
Also rules for UA for responses and merged requests are defined.
Since the provisional response is not final there should be no
influence on the call stated due to media. An SDP sent in a 18x must
be the same as for the final response. The numbers in brackets shows
the INVITES/early dialogs created.
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UAC Proxy Forking Proxy UAS_2 UAS_3 UAS_4
| | | | | |
|-- INVITE -->| | | | |
| |-- INVITE -->|-- INVITE (2) ->| | |
| | |-- INVITE (3) --------->| |
| | |-- INVITE (4) ----------------->|
| | |<-- 18x (2) ----| | |
|<- 18x (2) --|<- 18x (2) --| | | |
| | |<-- 18x (3) ------------| |
|<- 18x (3) --|<- 18x (3) --| | | |
| | |<-- 18x (4) --------------------|
|<- 18x (4) --|<- 18x (4) --| | | |
| | | | | |
| | |<-- 200 (4) --------------------|
| |<- 200 (4) --| | | |
|<- 200 (4) --| | | | |
|-- ACK (4) ->| | | | |
| |--- ACK (4) >| | | |
| | |--- ACK (4) ------------------->|
| | | | | |
| | |--CANCEL (2)--->| | |
| | |<-- 200 (2) ----| | |
| | |<-- 487 (2) ----| | |
| | |--- ACK (2)---->| | |
| | | | | |
| | |--- CANCEL(3) --------->| |
| | |<-- 200 (3) ------------| |
| | |<-- 487 (3) ------------| |
| | |--- ACK (3)------------>| |
| | | | | |
Figure 1: Example Call Flow Forking
Figure 1
As you can see, this figure shows the normal forking case where each
UA sends a 18x either with or without SDP. UAS_4 sends a final
response and the forking proxy has to cancel all other open
provisional responses. The 487 is sent back to the forking proxy on
each early dialog (2) and (3) created in the UAS.
Further possible scenarios are that two or three UAS will answer the
call with 200 in time so that the UAC has now three open sessions
which is not really the goal when a user would like to communicate
with only one person.
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Figure 1 shows an normal example of forking. With receiving 18x the
UAC has to open transaction. So only UAA_4 answers the dialog
correctly. It is task of the forking proxy to cancel the remaining
open early dialogs
4.2. Multiples provisional responses without SDP
This section describes the use case where multiples 18x responses are
sent back which doesn't contain any SDP. This use case appear when
the UAS instances where the INVITE is forked without any specific
requirements and support of specific extensions like 100rel
In this specific case the B2BUA has not to anchor the media and acts
only as signalling B2BUA and has to maintain the signalling legs
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UAC B2BUA Forking Proxy UAS_2 UAS_3 UAS_4
| | | | | |
|-INVITE(1)F1->| | | | |
| |- INVITE F2 -->|- INVITE F3(2)->| | |
| | |- INVITE F4(3)------->| |
| | |- INVITE F5(4)------------->|
| | |<-- 18x (2) F6--| | |
|<- 18x (1) F8-|<- 18x (2) F7 -| | | |
| | |<-- 18x (3) F9--------| |
| |<- 18x (3) F10-| | | |
| | | | | |
| | |<-- 18x (4) F11-------------|
| |<- 18x (4) F12-| | | |
| | | | | |
| | | | | |
| | |<-- 200 SDP (4)F13----------|
| |<- 200 (4) F14-| | | |
|<- 200(1) F15-| | | | |
| | |- CANCEL F16 -->| | |
| | |<- 200 (2) F17 -| | |
| | |<- 487 (2) F18 -| | |
| | |-- ACK (2) F19->| | |
| | | | | |
| | |- CANCEL (3) F20 ---->| |
| | |<----200 (3) F21 -----| |
| | |<--- 487 (3) F22 -----| |
| | |---- ACK (3) F23 ---->| |
| | | | | |
|- ACK(1) F24->| | | | |
| |- ACK (4) F26->| | | |
| | |--- ACK (4) F27------------>|
Figure 2: Example Call Flow
Figure 2
The B2BUA will maintain a Dialog between UAC and the incoming part of
the B2BUA (UAS) And acts as UAC towards the terminating network
(UAS_2, UAS_3 and UAS_4). The INVITE F1 will have another Call-Id as
INVITE F2, also the to-tags are different.
The first 18x (F7) will be passed towards the UAC. The tags (to-tag,
from-tag), call-id etc will be stored by the B2BUA. The 18x sent
towards the UAC will contain the to-tag, Call-Id of INVITE F1 and the
from-tag is generated by the B2BUA. With arriving further 18x (F10,
F12) the B2BUA has to store the status of the to-tags etc and will
not forward the 180 to the UA.
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With arriving arriving the 200OK (F14) at the B2BUA with the SDP sent
back form UAS_4 the B2BUA will sent a 200 OK towards the UAC. In
this case the B2BUA re-written the to-tag, from-tan and call-id as
already done or response 18x F7.
As for normal forking the forking proxy is responsible for canceling
the open dialogs with the CANCEL F16 and F20. The CANCEL will be
initiated with receiving/sending the final 200 OK response And ACK
F24 finalizes the call initiation.
The B2BUA has normally not to anchor signalling plane i.e as
signalling B2BUA. Such option may be applied when sending the INVITE
F2 towards the UAS. In cases where the received INVITE has no tags
included which indicate the possibility of 100 rel, preconditions or
early media the B2BUA may handle this specific call as a stateless
proxy.
4.3. Forking use case with provisional responses with SDP using 100rel
This section describes use case where 100rel will be used. This
apply in networks where announcements will be played reliability.
The mechanism for making the provisional responses reliable is
described in RFC3262 [RFC3262]. A B2BUA doing correlation in between
allows only one early dialog sent back to the UAC. The B2BUA has to
anchor the SIP Dialogs as well as the media reservation streams.
This use case apply where multiples 18x responses are sent back with
different SDP content. This use case appear when the UAS instances
where the INVITE is forked to will use different codecs. E.G one UAS
is a video phone answering with a video codec the other one a mobile
phone and a further one using a DECT entity.
And one or more UAS will require reliability of provisional
responses. Please Note that such a behavior could be also caused by
an application server playing specific announcements which acts on
behalf of the UAS
There is the possibility based on the request if the 100rel mechanism
will only be used between B2BUA and UAS or really end to end. In
case where the 100rel is stated as supported it is not mandatory to
use it. When the UAS want to have the 18x reliable it will set the
100rel into the require header field. In that case where a 18x is
sent back with a 100rel required then the B2BUA may play the role of
anchoring the media and apply the 100rel between B2BUA and UAS and
let the UAC to B2BUA connection as unreliable.
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Please note that this is only needed in cases where the media
anchoring has to do some manipulation of the media e.g. transcoding
of codecs.
Where the B2BUA decides to pass the required 100rel header field the
UAC will send then the PRACK and waits for the 200 OK. In case there
are further 18x with equal other type of SDP arriving at the B2BUA .
B2BUA has to keep handle the 100rel and sent the PRACK to the UAS.
Preconditions: INVITE 100rel supported is set and in 18x a SDP with
100rel required is sent back
UAC B2BUA Forking Proxy UAS_2 UAS_3 UAS_4
| | | | | |
|- INVITE F1-->| | | | |
| |- INVITE F2 -->|- INVITE F3(2)->| | |
| | |- INVITE F4(3)------->| |
| | |- INVITE F5(4)------------->|
| | |<-18x SDP(2)F6--| | |
| |<-18x SDP(2)F7-| | | |
|<-18xSDP(1)F8-| | | | |
|- PRACK(1)F9->| | | | |
| |- PRACK(2)F10->| | | |
| | |- PRACK(2) F10->| | |
| | |<-- 200 F11(2)-| | |
| |<- 200 (2)F12--| | | |
|<- 200(1) F13-| | | | |
| | |<-- 18x (3) F14-------| |
| |<- 18x (3)F15--| | | |
| |- PRACK(2)F16->| | | |
| | |- PRACK (3) F17 ----->| |
| | |<---200 (3) F18 ------| |
| |<- 200 (3)F19--| | | |
| | | | | |
| | | | | |
| | |<-- 18x (4) F20-------------|
| |<- 18x (4)F21 -| | | |
| |- PRACK(4)F22->| | | |
| | |- PRACK(4) F23 ------------>|
| | |<-- 200(4) F24 -------------|
| |<- 200 (4)F25 -| | | |
| | | | | |
| | | | | |
| | |<-- 200 SDP (4)F26----------|
| |<-200SDP(4)F27-| | | |
|<UPDATE(1) F28| | | | |
|- 200 (1)F29->| | | | |
|<- 200(1) F30-| | | | |
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| | |-CANCEL(2) F31->| | |
| | |<--200 (2) F32--| | |
| | |<--487 (2) F33--| | |
| | |---ACK (2) F34->| | |
| | | | | |
| | |- CANCEL (3) F35 ---->| |
| | |<----200 (3) F46 -----| |
| | |<----487 (3) F37 -----| |
| | |<----ACK (3) F38 -----| |
| | | | | |
| | | | | |
|- ACK(2) F39->| | | | |
| |- ACK (4) F40->| | | |
| | |--- ACK (4) F41------------>|
Figure 3: Example Call Flow with 100rel
Figure 3
F1: INVITE SDP offer A1 (Codec A, Codec B)Dialog 1 F3: INVITE SDP
offer A2 (Codec A, Codec B)Dialog 2 F4: INVITE SDP offer A2 (Codec A,
Codec B)Dialog 3 F5: INVITE SDP offer A2 (Codec A, Codec B)Dialog 4
F7: 18x SDP answer A2 (Codec A) Dialog 2 F8: 18x SDP answer A1 (Codec
A) Dialog 1 F9: PRACK no SDP Dialog 1 F10: PRACK no SDP Dialog 2 F12:
200 OK(PRA) Dialog 2 F13: 200 OK(PRA) Dialog 1 F15: 18x SDP answer A3
(Codec B) Dialog 3 F16: PRACK no SDP Dialog 3 F19: 200 OK(PRA) Dialog
3 F21: 18x SDP answer A4 (Codec A,Codec B) Dialog 4 F22: PRACK no SDP
Dialog 4 F25: 200 OK(PRA) Dialog 4 F27: 200 OK(INV) Dialog 4 F28:
UPDATE SDP offer A4 (Codec A,Codec B) Dialog 1 F29: 200 OK(UPD) SDP
answer A4 (Codec A,Codec B) Dialog 1 F30: 200 OK(INV) SDP Answer A4
(Codec A,Codec B) Dialog 1
Call will be forked to 3 end devices. UAS_2 answers with 18x (F6-F7)
containing a SDP and 100rel required. 18x (F8) is passed to the UAC
and made reliable with PRACK (F9-F13) Further 18x (F15/F21) arrive at
the B2BUA. The B2BUA stores the to tag for the call context. The
B2BUA will answer the 18x (F15/F21) and made it reliable with PRACK
(F16-F19/F22-F24). And also update the SDP negotiated between B2BUA
and UAC with further UPDATE messages. No further activity is done
towards the UAC. The B2BUA does not need any media awareness for
this procedures as long there is no need for transcoding or other
manipulation of media.
With arriving of a 200 OK (F27) at the B2BUA from UAS_4 the B2BUA has
to construct first an UPDATE (F28) in the case that the SDP differs
from the last 18x (F28) sent to the UAC. The 200 OK (F26) received
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by the B2BUA and triggers the final response to the UAC (F27).
Editors NOTE: Clarification needed if the UPDATE or 200OK INVITE must
be sent first. Problem could be that media clipping may appear. and
if so, what needs the 200 OK to contain in the SDP.
Also all other open Call legs are canceled (F31-44) by normal forking
proxy procedures as described in RFC3261 [RFC3261].
4.4. Forking use case with provisional responses with SDP using 100rel
and preconditions
This section describes use case where preconditions will be used.
This apply in mobile networks where resource reservation is needed.
Also normal networks may use preconditions when access bandwith is
problematic . The recondition mechanism in RFC3312 [RFC3312] shows
the needed additional procedures. A B2BUA doing correlation in
between to allows only one early dialog sent back to the UAC. The
B2BUA has to anchor the SIP Dialogs as well as the media reservation
streams.
This use case apply where multiples 18x responses are sent back with
different SDP content. This use case appear when the UAS instances
where the INVITE is forked to will use different codecs. E.G one UAS
is a video phone answering with a video codec the other one a mobile
phone and a further one using a DECT entity.
This section describes the use case where the UAC sends a dialog
request with 100rel and preconditions supported/required and the
UAS_2 apply to the requested mechanisms. The B2BUA has to have media
awareness and also 3PCC capabilities.
With applying preconditions the resource reservation needs to be
finalized before 200 OK is sent. In this scenario where the SIP call
is forked and the first UAS answers with a 183 containing 100rel and
preconditions requires an the correct SDP answer UAC, and UAS starts
their resource reservation mechanism (F5-F18). The B2BUA acts as
signalling and media-aware functionality between the Forking Server
and the UAC.
When the UAS_3 will send back also an 183 containing SDP and their
required header is set to 100rel and preconditions the B2BUA has to
reserve the resources towards the UAS (F30-F37). This use case
assumes that the leg between the UAC and B2BUA will not be updated to
avoid to many codec renegotiation and resource reservation. Thus the
B2BUA will renegotiate when the final 200 OK will arrive at the
B2BUA.
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UAC B2BUA Forking Proxy UAS_2 UAS_3 UAS_4
| | | | | |
|- INVITE F1-->| | | | |
| |- INVITE F2 -->|- INVITE F3(2)->| | |
| | |- INVITE F4(3)------->| |
| | |<-- 183 (2) F5--| | |
| |<- 183 (2) F6 -| | | |
|<- 183 (1) F7-| | | | |
|- PRACK(1)F8->| | | | |
| |- PRACK(2)F9-->| | | |
| | |- PRACK(2) F10->| | |
| | |<-- 200 F11(2)-| | |
| |<- 200 (2) F12-| | | |
|<- 200(1) F13-| | | | |
|UPDATE(1)F13->| | | | |
| | UPDATE(2)F14->| | | |
| | |- UPDATE(2)F15->| | |
| | |<-- 200 F16(2)-| | |
| |<- 200 (2) F17-| | | |
|<- 200(1) F18-| | | | |
| | |<-- 180 (2) F19-| | |
| |<- 180(2) F20 -| | | |
|<- 180(1) F21-| | | | |
|-PRACK(1)F22->| | | | |
| |- PRACK(2)F23->| | | |
| | |- PRACK(2) F24->| | |
| | |<-- 200 F25(2)-| | |
| |<- 200 (2) F26-| | | |
|<- 200(1) F27-| | | | |
| | |<-- 183 (3) F28-------| |
| |<- 183 (3) F29-| | | |
| |- PRACK(3)F30->| | | |
| | |- PRACK F4(3)F31----->| |
| | |<----200 (3) F32 -----| |
| |<- 200 (3) F33-| | | |
| | | | | |
| |- UPDATE F34->| | | |
| | |- UPDATE F35 ------->| |
| | |<----200 (3) F36 -----| |
| |<- 200 (3) F37-| | | |
| | | | | |
| | |<-- 200 SDP (3)F40----| |
| |<- 200 (4) F41-| | | |
|<- UPDATE F42-| | | | |
|-- 200 F43 ->| | | | |
|<- UPDATE F44-| | | | |
|-- 200 F45 ->| | | | |
|<-- 200 F46 -| | | | |
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| | |- CANCEL F47 -->| | |
| | |<-- 200 F48(2)-| | |
| | |<--487 (3) F49 -| | |
| | |<--ACK (3) F50 -| | |
|- ACK(1) F51->| | | | |
| |- ACK (3) F52->| | | |
| | |--- ACK (3) F53------>|
Figure 4: Example Call Flow with preconditions
Figure 4
F1: INVITE SDP offer A1 (Codec A, Codec B)
F7: 183 SDP answer A1 (Codec A)
F13: UPDATE SDP offer A2 (Codec A) UAC resources reserve
F28: 200 OK SDP Answer A2 (Codec A) UAS resources reserved
F45: UPDATE SDP offer A3 (Codec B)
F46: 200 OK SDP Answer A3 (Codec B)
F47: UPDATE SDP offer A4 (Codec B) 2BUA resources reserved
F48: 200 OK SDP Answer A4 (Codec B) UAS resources reserved
F49: 200 OK Final Response
Figure 5
Where the B2BUA decides to pass the required 100rel the UAC will send
then the PRACK and waits for the 200 OK. In case there are further
18x with other type of SDP arriving at the B2BUA the UAC needs to be
informed about change of codec. The UAC has again to sent the PRACK.
Editor's Note: Question is if the above described mechanism would be
a proper mechanism since the later renegotiation + resource
reservation could cause media clipping. Figure 5 shows the other
possibility.
UAC B2BUA Forking Proxy UAS_2 UAS_3
| | | | |
|- INVITE F1-->| | | |
| |- INVITE F2 -->|- INVITE F3(2)->| |
| | |- INVITE F4(3)------->|
| | |<-- 183 (2) F5--| |
| |<- 183 (2) F6 -| | |
|<- 183 (1) F7-| | | |
|- PRACK(1)F8->| | | |
| |- PRACK(2)F9-->| | |
| | |- PRACK(2) F10->| |
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| | |<-- 200 F11(2)-| |
| |<- 200 (2) F12-| | |
|<- 200(1) F13-| | | |
|UPDATE(1)F13->| | | |
| | UPDATE(2)F14->| | |
| | |- UPDATE(2)F15->| |
| | |<-- 200 F16(2)-| |
| |<- 200 (2) F17-| | |
|<- 200(1) F18-| | | |
| | |<-- 180 (2) F19-| |
| |<- 180(2) F20 -| | |
|<- 180(1) F21-| | | |
|-PRACK(1)F22->| | | |
| |- PRACK(2)F23->| | |
| | |- PRACK(2) F24->| |
| | |<-- 200 F25(2)-| |
| |<- 200 (2) F26-| | |
|<- 200(1) F27-| | | |
| | |<-- 183 (3) F28-------|
| |<- 183 (3) F29-| | |
| |- PRACK(3)F30->| | |
| | |- PRACK F4(3)F31----->|
| | |<----200 (3) F32 -----|
| |<- 200 (3) F33-| | |
| | | | |
| |- UPDATE F34->| | |
| | |- UPDATE F35 ------->|
| | |<----200 (3) F36 -----|
| |<- 200 (3) F37-| | |
|<- UPDATE F38-| | | |
|-- 200 F39 ->| | | |
|<- UPDATE F40-| | | |
|-- 200 F41 ->| | | |
|<- UPDATE F42-| | | |
|<-- 200 F43 -| | | |
| | |<-- 200 SDP (3)F44----|
| |<- 200 (4) F45-| | |
|<-- 200 F46 -| | | |
| | |- CANCEL F47 -->| |
| | |<-- 200 F48(2)-| |
|- ACK(2) F49->| | | |
| |- ACK (4) F50->| | |
| | |--- ACK (4) F51------>|
Figure 5: Example Call Flow with preconditions
Figure 6
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F1: INVITE SDP offer A1 (Codec A, Codec B)
F7: 183 SDP answer A1 (Codec A)
F13: UPDATE SDP offer A2 (Codec A) UAC resources reserved
F28: 200 OK SDP Answer A2 (Codec A) UAS resources reserved
F38: UPDATE SDP offer A3 (Codec B)
F39: 200 OK SDP Answer A3 (Codec B)
F40: UPDATE SDP offer A4 (Codec B) 2BUA resources reserved
F44-46 200 OK Final Response
Figure 7
4.5. Forking use case with early media played
This use case describes the case where early media is played due to
application server actions. e.g playing a ring tone or a specific
announcement sent back.
Note: More work is needed to describe the use case.
UAC B2BUA Forking Proxy UAS_2 UAS_3 UAS_4
| | | | | |
|- INVITE F1-->| | | | |
| |- INVITE F2 -->|- INVITE F3(2)->| | |
| | |- INVITE F4(3)------->| |
| | |- INVITE F5(4)------------->|
| | |<-18x SDP(2)F6--| | |
| |<-18x SDP(2)F7-| | | |
|<-18xSDP(1)F8-| | | | |
|- PRACK(1)F9->| | | | |
| |- PRACK(2)F10->| | | |
| | |- PRACK(2) F10->| | |
| | |<-- 200 F11(2)-| | |
| |<- 200 (2)F12--| | | |
|<- 200(1) F13-| | | | |
| | (Announcement)| | | |
|<==============================================| | |
| | | | | |
| | | | | |
| | |<-- 18x (3) F14-------| |
| |<- 18x (3)F15--| | | |
| |- PRACK(2)F16->| | | |
| | |- PRACK (3) F17 ----->| |
| | |<---200 (3) F18 ------| |
| |<- 200 (3)F19--| | | |
| | | | | |
| | (Announcement)| | | |
| |<=====================================| |
| | | | | |
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| | | | | |
| | |<-- 18x (4) F20-------------|
| |<- 18x (4)F21 -| | | |
| |- PRACK(4)F22->| | | |
| | |- PRACK(4) F23 ------------>|
| | |<-- 200(4) F24 -------------|
| |<- 200 (4)F25 -| | | |
| | | | | |
| | (Announcement)| | | |
| |<===========================================|
| | | | | |
| | | | | |
| | |<-- 200 SDP (4)F26----------|
| |<-200SDP(4)F27-| | | |
|<UPDATE(1) F28| | | | |
|- 200 (1)F29->| | | | |
|<- 200(1) F30-| | | | |
| | |-CANCEL(2) F31->| | |
| | |<--200 (2) F32--| | |
| | |<--487 (2) F33--| | |
| | |---ACK (2) F34->| | |
| | | | | |
| | |- CANCEL (3) F35 ---->| |
| | |<----200 (3) F46 -----| |
| | |<----487 (3) F37 -----| |
| | |<----ACK (3) F38 -----| |
| | | | | |
| | | | | |
|- ACK(2) F39->| | | | |
| |- ACK (4) F40->| | | |
| | |--- ACK (4) F41------------>|
Figure 6: Example Call Flow with announcement
Figure 8
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F1: INVITE SDP offer A1 (Codec A, Codec B)Dialog 1
F3: INVITE SDP offer A2 (Codec A, Codec B)Dialog 2
F4: INVITE SDP offer A2 (Codec A, Codec B)Dialog 3
F5: INVITE SDP offer A2 (Codec A, Codec B)Dialog 4
F7: 18x SDP answer A2 (Codec A) Dialog 2
F8: 18x SDP answer A1 (Codec A) Dialog 1
F9: PRACK no SDP Dialog 1
F10: PRACK no SDP Dialog 2
F12: 200 OK(PRA) Dialog 2
F13: 200 OK(PRA) Dialog 1
F15: 18x SDP answer A3 (Codec B) Dialog 3
F16: PRACK no SDP Dialog 3
F19: 200 OK(PRA) Dialog 3
F21: 18x SDP answer A4 (Codec A,Codec B) Dialog 4
F22: PRACK no SDP Dialog 4
F25: 200 OK(PRA) Dialog 4
F27: 200 OK(INV) Dialog 4
F28: UPDATE SDP offer A4 (Codec A,Codec B) Dialog 1
F29: 200 OK(UPD) SDP answer A4 (Codec A,Codec B) Dialog 1
F30: 200 OK(INV) SDP Answer A4 (Codec A,Codec B) Dialog 1
Figure 9
5. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
6. Security Considerations
Currently no further security considerations are needed beyond
considerations made in the referred RFC's for SIP RFC3261 [RFC3261],
reliability of provisional responses RFC3262 [RFC3262] and resource
management RFC3312 [RFC3312].
7. Acknowledgments
The author like to thank Paul Kyzivat for his extensive review and
comments on the first draft version.
8. Normative References
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[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>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,
<https://www.rfc-editor.org/info/rfc3262>.
[RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October
2002, <https://www.rfc-editor.org/info/rfc3312>.
[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, DOI 10.17487/RFC3326, December 2002,
<https://www.rfc-editor.org/info/rfc3326>.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, DOI 10.17487/RFC3841, August 2004,
<https://www.rfc-editor.org/info/rfc3841>.
[RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
Tone Generation in the Session Initiation Protocol (SIP)",
RFC 3960, DOI 10.17487/RFC3960, December 2004,
<https://www.rfc-editor.org/info/rfc3960>.
[RFC5393] Sparks, R., Ed., Lawrence, S., Hawrylyshen, A., and B.
Campen, "Addressing an Amplification Vulnerability in
Session Initiation Protocol (SIP) Forking Proxies",
RFC 5393, DOI 10.17487/RFC5393, December 2008,
<https://www.rfc-editor.org/info/rfc5393>.
[RFC6026] Sparks, R. and T. Zourzouvillys, "Correct Transaction
Handling for 2xx Responses to Session Initiation Protocol
(SIP) INVITE Requests", RFC 6026, DOI 10.17487/RFC6026,
September 2010, <https://www.rfc-editor.org/info/rfc6026>.
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[RFC6141] Camarillo, G., Ed., Holmberg, C., and Y. Gao, "Re-INVITE
and Target-Refresh Request Handling in the Session
Initiation Protocol (SIP)", RFC 6141,
DOI 10.17487/RFC6141, March 2011,
<https://www.rfc-editor.org/info/rfc6141>.
[RFC6228] Holmberg, C., "Session Initiation Protocol (SIP) Response
Code for Indication of Terminated Dialog", RFC 6228,
DOI 10.17487/RFC6228, May 2011,
<https://www.rfc-editor.org/info/rfc6228>.
[RFC7029] Hartman, S., Wasserman, M., and D. Zhang, "Extensible
Authentication Protocol (EAP) Mutual Cryptographic
Binding", RFC 7029, DOI 10.17487/RFC7029, October 2013,
<https://www.rfc-editor.org/info/rfc7029>.
9. Informative References
[TS24.229] 3GPP, "IP multimedia call control protocol based on
Session Initiation Protocol (SIP) and Session Description
Protocol (SDP); Stage 3", 17 March 2014.
Appendix A. Appendix
Author's Address
Roland Jesske
Deutsche Telekom
Heinrich-Hertz-Strasse 3-7
64307 Darmstadt
Germany
Phone: +4961515812766
Email: r.jesske@telekom.de
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