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Network Working Group J. Polk
Request for Comments: 4411 Cisco Systems
Category: Standards Track February 2006
Extending the Session Initiation Protocol (SIP)
Reason Header for Preemption Events
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document proposes an IANA Registration extension to the Session
Initiation Protocol (SIP) Reason Header to be included in a BYE
Method Request as a result of a session preemption event, either at a
user agent (UA), or somewhere in the network involving a
reservation-based protocol such as the Resource ReSerVation Protocol
(RSVP) or Next Steps in Signaling (NSIS). This document does not
attempt to address routers failing in the packet path; instead, it
addresses a deliberate tear down of a flow between UAs, and informs
the terminated UA(s) with an indication of what occurred.
Polk Standards Track [Page 1]
RFC 4411 SIP Reason Header for Preemption Events February 2006
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................4
2. Access Preemption Events ........................................4
2.1. Effects of Preemption at the User Agent ....................6
2.2. Reason Header Requirements for Access Preemption Events ....6
3. Network Preemption Events .......................................7
3.1. Reason Header Requirements for Network Preemption Events ..10
4. Including a Hybrid Infrastructure ..............................10
4.1. Hybrid Infrastructure Requirements ........................11
5. Preemption Reason Header Cause Codes and Semantics .............11
5.1. Access Preemption Event Reason Code .......................12
5.1.1. Access Preemption Event Call Flow ..................12
5.2. Network Preemption Events Reason Code .....................14
5.2.1. Network Preemption Event Call Flow .................15
5.3. Generic Preemption Event Reason Code ......................16
5.4. Non-IP Preemption Event Reason Code .......................16
5.4.1. Non-IP Preemption Event Call Flow ..................17
6. Security Considerations ........................................17
7. IANA Considerations ............................................17
7.1. "Preemption" Namespace Registry ...........................18
7.2. Default Reason-Text IANA Registry for the SIP
Reason Header .............................................20
8. Contributions ..................................................20
9. Acknowledgements ...............................................20
10. References ....................................................21
10.1. Normative References .....................................21
10.2. Informative References ...................................21
1. Introduction
With the introduction of the SIP Resource-Priority (R-P) header [4],
there became the possibility of sessions being torn down for (scarce)
resource reasons, meaning there weren't enough resources for a
particular session to continue. Certain domains will implement this
mechanism where resources may become constrained either at the user
agent (UA) or at congested router interfaces where more important
sessions are to be completed at the expense of less important
sessions. Which sessions are more or less important than others will
not be discussed here. What is proposed here is a SIP [2] extension
to synchronize SIP elements as to why a preemption event occurred and
which type of preemption event occurred, as viewed by the element
that performed the preemption of a session.
The SIP Reason Header is an application layer feedback mechanism to
synchronize SIP elements of events; the particular event explained
here deals with preemption of a session. Q.850 [5] provides an
Polk Standards Track [Page 2]
RFC 4411 SIP Reason Header for Preemption Events February 2006
indication for preemption (cause=8) and for preemption "circuit
reserved for reuse" (cause=9). Q.850 Cause=9 does not apply to IP,
as IP has no concept of circuits. Some domains wish to differentiate
appropriate IP reasons for preemption of sessions and to indicate
topologically where the preemption event occurred. No other means
exists today to give feedback as to why a session was torn down on
preemption grounds.
In the event that a session is terminated for a specific reason that
can (or should) be shared with SIP Servers and UAs sharing dialog,
the Reason Header [1] was created to be included in the BYE Request.
This was not the only Method for this new Header; [1] also discusses
the CANCEL Method usage.
This document will define two use cases in which new preemption
Reason values are necessary:
Access Preemption Event - This is when a UA receives a new SIP
session request message with a valid R-P value that is
higher than the one associated with the currently active
session at that UA. The UA must discontinue the existing
session in order to accept the new one (according to local
policy of some domains).
Network Preemption Event - This is when a network element - such
as a router - reaches capacity on a particular interface and
has the ability to statefully choose which session(s) will
remain active when a new session/reservation is signaled for
under the parameters outlined in SIP Preconditions per [3]
that would otherwise overload that interface (perhaps
adversely affecting all sessions). In this case, the router
must terminate one or more reservations of lower priority in
order to allow this higher priority reservation access to
the requested amount of bandwidth (according to local policy
of some domains).
This document will cover the semantics for these two cases and
request IANA registration of the new protocol value "Preemption" for
the Reason Header field, with 4 cause values for the above preemption
conditions. Additionally, this document will create a new IANA
Registry for reason-text strings that are not currently defined
through existing SIP Response codes or Q.850 cause codes. This new
Registry will be useful for future protocols used by the SIP Reason
header.
This document will emphasize an existing SIP RFC [3] as the starting
point for network preemption events. RFC 3312 set rules surrounding
SIP interaction using a reservation protocol for QoS preconditions,
Polk Standards Track [Page 3]
RFC 4411 SIP Reason Header for Preemption Events February 2006
using RSVP as the example protocol. That effort did not preclude
other preconditions or future protocol work from becoming a means of
preconditions. NSIS is a new reservation protocol effort that
specifies a preemption operation similar to RSVP's ResvErr message
involving the NSIS NOTIFY message in [8] with a Transient error code
0x04000005 (Resources Pre-empted).
Note that SIP itself does not cause RSVP or NSIS reservation
signaling to start or end. That operation is part of a separate API
within each UA.
1.1. Conventions Used in This Document
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 [6].
2. Access Preemption Events
As mentioned previously, Access Preemption Events (APE) occur at the
user agent. It does not matter which UA in a unicast or multicast
session this happens to (the UAC or UAS of a session). If local
policy dictates in a particular domain rules regarding the
functionality of a UA, there must be a means by which that UA (not
the user) informs the other UA(s) why a session was just torn down
prematurely. The appropriate mechanism is the BYE Method. The user
of the other far side UA will not understand why that session "just
went away" without there being a means of informing the UA of what
occurred (if this event was purposeful). Through this type of
indication to the preempted UA, it can indicate to the user of that
device appropriately.
The rules within a domain surrounding the UA to be informed can be
different from the rules for informing the user. Local policy should
determine if the user should be informed of the specific reason.
This indication in SIP will provide a means for the UA to react in a
locally determined way, if appropriate (play a certain tone or tone
sequence, point towards a special announcement uri, cause the UA's
visual display to do something, etc.).
Figure 1 illustrates the scenario. UA1 invites UA2 to a session with
the Resource Priority level of 3 (levels 1 and 2 are higher is this
domain, and the namespace element is not necessary for this
discussion).
Polk Standards Track [Page 4]
RFC 4411 SIP Reason Header for Preemption Events February 2006
UA1 UA2 UA3
| | |
| INVITE (R-P:3) | |
|----------------------->| |
| 200 OK | |
|<-----------------------| |
| ACK | |
|----------------------->| |
| RTP | |
|<======================>| |
| | INVITE (R-P:2) |
| |<------------------------|
| BYE (Reason : ? ) | |
|<-----------------------| |
| | 200 OK |
| |------------------------>|
| 200 OK | |
|----------------------->| |
| | ACK |
| |<------------------------|
| | RTP |
| |<=======================>|
| | |
Figure 1. Access Preemption with obscure Reason
After the session between UA1 and UA2 is established, UA3 invites UA2
to a new session with an R-P of 2 (a higher priority than the current
session between UA1 and UA2). Local policy within this domain
dictates that UA2 must preempt all existing calls of lower priority
in order to accept a higher priority call.
What Reason value could be inserted above to mean "preemption" at a
UA? There are several choices: 410 "Gone", 480 "Temporarily
Unavailable", 486 "Busy Here", and 503 "Service Unavailable". The
use of any of these here is questionable because the session is
already established. It is further complicated if there needs to be
a difference in the Reason value for an Access versus a Network
Preemption Event (which is a requirement here). The limits of Q.850
[5] have been stated previously in this document.
It should be possible to configure UAs receiving a preemption
indication to indicate to the user that no particular type of
preemption occurred. There are some domains that might prefer their
users to remain unaware of the specifics of network behavior. This
should not ever prevent a known preemption indication from being sent
in a BYE from a UA.
Polk Standards Track [Page 5]
RFC 4411 SIP Reason Header for Preemption Events February 2006
2.1. Effects of Preemption at the User Agent
If 2 UAs are in a session and one UA must preempt that session to
accept another session, a BYE Method message is the appropriate
mechanism to perform this task. However, taking this a step further,
if a UA is the common point of a 3-way (or more) ad hoc conference
and must preempt all sessions in that conference due to receipt of a
higher-priority session request (that this UA must accept), then a
BYE message must be sent to all UAs in that ad hoc conference.
2.2. Reason Header Requirements for Access Preemption Events
The following is a list of requirements for adding an appropriate
Reason value for an Access Preemption Event (APE) as described above
and shown in Figure 1:
APE_REQ#1 - create a means by which one UA can inform another UA
(within the same active session) that the active
session between the two devices is being purposely
preempted at one UA for a higher-priority session
request from another UA.
APE_REQ#2 - create a means by which all relevant SIP elements can
be informed of this Access Preemption Event to a
specific session.
For example: perhaps SIP Servers that have incorporated a Record-
Route header into that session set up need to be informed of this
occurrence.
APE_REQ#3 - create a means of informing all participants in an ad
hoc conference that the primary UA (the mixer) has
preempted the conference by accepting a higher-
priority session request.
APE_REQ#4 - create a separate indication for the access preemption
event than the one used for a Network Preemption Event
(described in the next section) in the session BYE
message.
APE_REQ#5 - create a means to generate a specific indication of a
preemption event at the user agent to inform all
relevant SIP entities, yet have the ability to
generalize this indication (based on local policy) to
the receiving UA such that this UA cannot display more
information than the domain wants the user to see.
Polk Standards Track [Page 6]
RFC 4411 SIP Reason Header for Preemption Events February 2006
3. Network Preemption Events
Network Preemption Events (NPE) are instances in which an
intermediate router between SIP user agents preempts one or more
sessions at one of its interfaces to place a higher-priority session
through that interface. Within RSVP, there exists a means to execute
this functionality per [7]: ResvErr messages, which travel downstream
towards appropriate receivers. The ResvErr message has the ability
to carry within it a code indicating why a reservation is being torn
down. The ResvErr does not travel upstream to the other UA. This
document proposes that a SIP message be generated to synchronize all
relevant SIP elements to this preemption event, including the
upstream UA. Creating another Reason value describing that a network
element preempted the session is necessary in certain domains.
Figures 2 and 3 illustrate a network preemption scenario with RSVP.
NSIS, not shown in examples here, can be imagined from [8] with a
NOTIFY error message indicating that a reservation has been preempted
with the Transient ERROR_SPEC 0x04000005. SIP behavior will be
identical using either reservation protocol.
UA1 invites UA2 to a session with the Resource Priority level of 3
(levels 1 and 2 are higher in this domain) and is accepted. This SIP
signaling translated the Resource Priority value to an appropriate
RSVP priority level for that flow. The link between Router 1 and
Router 2 became saturated with this session reservation between UA1
and UA2 (in this example).
UA1 UA2
\ /
\ /
+--------+ +--------+
| | | |
| RTR1 | | RTR2 |
| Int7-------Int5 |
| | | |
+--------+ +--------+
/ \
/ \
UA3 UA4
Figure 2. Network Diagram Scenario A
After the session between UA1 and UA2 is established, UA3 invites UA4
to a new session with a Resource Priority level of 2 (a higher
priority than the current reservation between UA1 and UA2). Again,
the priority value within the Resource-Priority header of this INVITE
is translated into an appropriate RSVP priority (that is also higher
Polk Standards Track [Page 7]
RFC 4411 SIP Reason Header for Preemption Events February 2006
in relative priority to the UA1_UA2 session/RSVP flow). When this
second, higher-priority session is signaled, one Path message goes
from UA3 to UA4, resulting in the RESV message going from UA4 back to
UA3. Because this link between the two routers is at capacity (at
Int7 in Figure 5), Router 1 will (in this example) make the decision
or will communicate with another network entity that will make the
decision to preempt lower-priority BW to ensure that this higher-
priority session reservation is completed. A ResvErr message is sent
to UA2. The result is that UA2 will know that there has been a
preemption event in a router (because the ResvErr message has a error
code within it, stating "preemption"). At this point, UA1 will not
know anything of this preemption. If there are any SIP Proxies
between UAs 1 and 2 (perhaps that inserted a Record-Route Header),
each will also need to be informed as to why this reservation was
torn down.
Figure 3 shows the call flow with Router 2 from Figure 2 included at
the RSVP layer sending the ResvErr message. A complete call flow
including all UAs and Routers is not shown here for diagram
complexity reasons. The complete signaling between UA3 and UA4 is
also not included.
Polk Standards Track [Page 8]
RFC 4411 SIP Reason Header for Preemption Events February 2006
UA1 Rtr2 UA2
| | |
| INVITE with QoS Preconditions (R-P:3) |
|------------------------------------------------->|
| ******************************************** |
| * - QoS Preconditions established UA1-UA2 * |
| * - SIP signaling continues... * |
| ******************************************** |
| 200 OK |
|<-------------------------------------------------|
| ACK |
|------------------------------------------------->|
| RTP |
|<================================================>|
| ******************************************** |
| * -UA3 sends INV with QoS Preconditions * |
| * to UA4 w/ RP:2; * |
| * -Reservation set-up occurs between UA3 * |
| * and UA4 * |
| * -Router 2 in Figure 2 must preempt * |
| * reservation between UA1 & UA2 * |
| * ****************************************** |
| |
| | ResvErr |
| |------------------------>|
| | |
| |
| BYE (Reason : ? ) |
|<-------------------------------------------------|
| 200 OK |
|------------------------------------------------->|
| |
Figure 3. Network Preemption with obscure Reason
What Reason value could be inserted above to mean "preemption at a
router interface"? There are several choices: 410 "Gone", 480
"Temporarily Unavailable", 486 "Busy Here", and 503 "Service
Unavailable". The use of any of these here is questionable because
the session is already established. It is further complicated if
there needs to be a difference between the Reason value for an Access
Preemption Event versus a Network Preemption Event. The limits of
Q.850 [5] have already been stated previously, showing there is
nothing in that spec to indicate a problem in an IP network.
To state that all preemptions are equal is possible, but will not
provide adequate information. Therefore, another Reason Header value
is necessary to differentiate the APE from the NPE.
Polk Standards Track [Page 9]
RFC 4411 SIP Reason Header for Preemption Events February 2006
3.1. Reason Header Requirements for Network Preemption Events
The following are the requirements for the appropriate SIP signaling
in reaction to a Network Preemption Event (NPE):
NPE_REQ#1 - create a means of informing the far-end UA that a
Network Preemption Event has occurred in an
intermediate router.
NPE_REQ#2 - create a means by which all relevant SIP elements can
be informed of a Network Preemption Event to a
specific session.
For example: perhaps SIP Servers have incorporated a Record-Route
header into that session set up.
NPE_REQ#3 - create a means of informing all participants in an ad
hoc conference that the primary UA (the mixer) has
been preempted by a Network Preemption Event.
NPE_REQ#4 - create a separate description of the Network
Preemption Event relative to an Access Preemption
Event in SIP.
4. Including a Hybrid Infrastructure
If User 1 is in a non-IP portion of infrastructure (using a TDM
phone) in a session with a UA through a SIP gateway, and if the TDM
portion had the ability to preempt the session and indicate to the
SIP gateway when it did such a preemption, the SIP GW would need to
be able to convey this preemption event into the SIP portion of this
session just as if User 1 were a UA in the session. Below is a
diagram of this:
**************************
* TDM network *
* +---------+
* User 1 | |
* O ==========>| SIP GW1 |================> UA2
* /|\ ^ | | |
* / \ | +---------+ |
* | * |
**********|*************** | |
| | Preemption |
Preemption ---------> |--------------------->|
Event Indication
Figure 4. TDM/IP Preemption Event
Polk Standards Track [Page 10]
RFC 4411 SIP Reason Header for Preemption Events February 2006
4.1. Hybrid Infrastructure Requirements
The following are the requirements unique to the topology involving
both IP infrastructure and TDM (or non-IP) infrastructure.
HYB_REQ#1 - create a means of informing the far-end UA in a dialog
through a SIP gateway with a non-IP phone that the TDM
portion of the session indicated to the SIP gateway
that a preemption event terminated the session.
HYB_REQ#2 - create a means of identifying this preemption event
uniquely with respect to an access preemption and
network preemption event.
5. Preemption Reason Header Cause Codes and Semantics
This document defines the following new protocol value for the
protocol field of the Reason header field in RFC 3326 [1]:
Preemption: The cause parameter contains a preemption cause code.
We define the following preemption cause codes:
Value Default Text Description
1 UA Preemption The session has been preempted by a UA.
2 Reserved Resources The session preemption has been
Preempted initiated within the network via a
purposeful RSVP preemption occurrence,
and not a link error.
3 Generic Preemption This is a limited-use preemption
indication to be used on the final leg
to the preempted UA to generalize the
event.
4 Non-IP Preemption The session preemption has occurred in
a non-IP portion of the infrastructure,
and this is the Reason cause code given
by the SIP Gateway.
Example syntax for the above preemption types are as follows:
Reason: preemption ;cause=1 ;text="UA Preemption"
Reason: preemption ;cause=2 ;text="Reserved Resources Preempted"
Reason: preemption ;cause=3 ;text="Generic Preemption"
Reason: preemption ;cause=4 ;text="Non-IP Preemption"
Polk Standards Track [Page 11]
RFC 4411 SIP Reason Header for Preemption Events February 2006
Sections 5.1, 5.2, 5.3, and 5.4 provide use cases and extended
definitions for the above four cause codes with message flow
diagrams.
5.1. Access Preemption Event Reason Code
A more elaborate description of the Access Preemption Event cause=1
is as follows:
A user agent in a session has purposely preempted a session and is
informing the far-end user agent, or user agents (if part of a
conference), and SIP Proxies (if stateful of the session's
transactions)
An example usage of this header value would be:
Reason: preemption ;cause=1 ;text="UA Preemption"
5.1.1. Access Preemption Event Call Flow
Figure 5 replicates the call flow from Figure 1, but with an
appropriate Reason value indication that was proposed in Section 4.1,
above:
Polk Standards Track [Page 12]
RFC 4411 SIP Reason Header for Preemption Events February 2006
UA1 UA2 UA3
| | |
| INVITE (R-P:3) | |
|---------------------------------->| |
| 200 OK | |
|<----------------------------------| |
| ACK | |
|---------------------------------->| |
| RTP | |
|<=================================>| |
| | INVITE (R-P:2) |
| |<-------------------|
| BYE (Reason: Preemption ; | |
| cause=1 ;text="UA Preemption") | |
|<----------------------------------| |
| | 200 OK |
| |------------------->|
| 200 OK | |
|---------------------------------->| |
| | ACK |
| |<-------------------|
| | RTP |
| |<==================>|
| | |
Figure 5. Access Preemption with Reason: UA Preemption
UA1 invites UA2 to a session with the Resource Priority level of 3
(levels 1 and 2 are higher in this domain). After the session
between UA1 and UA2 is established, UA3 invites UA2 to a new session
with an R-P of 2 (a higher priority than the current session to UA1).
Local policy within this domain dictates that UA2 must preempt all
existing calls of lower priority in order to accept a higher-priority
call.
UA2 sends a BYE Request message with a Reason header with a value of
UA Preemption. This will inform the far-end UA (UA1) and all
relevant SIP elements (for example, SIP Proxies). The cause code is
unique to what is proposed in the RSVP Preemption Event for
differentiation purposes.
Polk Standards Track [Page 13]
RFC 4411 SIP Reason Header for Preemption Events February 2006
5.2. Network Preemption Events Reason Code
A more elaborate description of the Reserved Resources Preempted
Event cause=2 is as follows:
A router has preempted a reservation flow and generated a
reservation error message: a ResvErr traveling downstream in RSVP,
and a NOTIFY in NSIS. The UA receiving the preemption error
message generates a BYE request towards the far-side UA with a
Reason Header with this value indicating that somewhere between
two or more UAs, a router has administratively preempted this
session.
An example usage of this header value would be:
Reason: Preemption :cause=2 ;text="Reserved Resources Preempted"
Polk Standards Track [Page 14]
RFC 4411 SIP Reason Header for Preemption Events February 2006
5.2.1. Network Preemption Event Call Flow
Figure 6 replicates the call flow from Figure 5, but with an
appropriate Reason value indication that was proposed in Section 4.2,
above.
UA1 Rtr2 UA2
| | |
| INVITE with QoS Preconditions (R-P:3) |
|---------------------------------------------------->|
| ******************************************** |
| * - QoS Preconditions established UA1-UA2 * |
| * - SIP signaling continues... * |
| ******************************************** |
| 200 OK |
|<----------------------------------------------------|
| ACK |
|---------------------------------------------------->|
| RTP |
|<===================================================>|
| ******************************************** |
| * -UA3 sends INV with QoS Preconditions * |
| * to UA4 w/ RP:2; * |
| * -Reservation set-up occurs between UA3 * |
| * and UA4 * |
| * -Router 2 in Figure 2 must preempt * |
| * reservation between UA1 & UA2 * |
| * ********************************************* |
| |
| | ResvErr |
| |------------------------>|
| | |
| |
| BYE (Reason : Preemption ;cause=2 ; |
| text="Reserved Resources Preempted") |
|<----------------------------------------------------|
| 200 OK |
|---------------------------------------------------->|
| |
Figure 6. Network Preemption with "Reserved Resources Preempted"
Above is the call flow with Router 2 from Figure 2 included at the
RSVP layer sending the Resv messages. A complete call flow including
all UAs and Routers is not included for diagram complexity reasons.
The signaling between UA3 and UA4 is also not included.
Polk Standards Track [Page 15]
RFC 4411 SIP Reason Header for Preemption Events February 2006
Upon receipt of the ResvErr message with the preemption error code,
UA2 can now appropriately inform UA1 why this event occurred. This
BYE message will also inform all relevant SIP elements, synchronizing
them. The cause value is unique to that proposed in Section 4.1 for
Access Preemption Events for differentiation purposes.
5.3. Generic Preemption Event Reason Code
A more elaborate description of the Generic Preemption Event cause=3
is as follows:
This cause code is for infrastructures that do not wish to provide
the preempted UA with a more precise reason than just
"preemption". It is possible that UAs will have code that will
indicate the type of preemption event that is contained in the
Reason header, and certain domains have expressed this as not
being optimal, and wanted to generalize the indication. This MUST
NOT be the initial indication within these domains, as valuable
traffic analysis and other NM applications will be generalized as
well. If this cause value is to be implemented, it SHOULD only be
done at the final SIP Proxy in such a way that the cause value
indicating which type of preemption event actually occurred is
changed to this generalized preemption indication to be received
by the preempted UA.
An example usage of this header value would be:
Reason: preemption ;cause=3 ;text="Generic Preemption"
5.4. Non-IP Preemption Event Reason Code
A more elaborate description of the Non-IP Preemption Event cause=4
is as follows:
A session exists in a hybrid IP/non-IP infrastructure and the
preemption event occurs in the non-IP portion, and was indicated
by that portion that this call termination was due to preemption.
This is the indication that would be generated by a SIP Gateway
towards the SIP UA that is being preempted, traversing whichever
SIP Proxies are involved in session signaling (a question of
server state).
An example usage of this header value would be:
Reason: preemption ;cause=4 ;text="Non-IP Preemption"
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RFC 4411 SIP Reason Header for Preemption Events February 2006
5.4.1. Non-IP Preemption Event Call Flow
Figure 7 is a simple call flow diagram of the Non-IP Preemption
Event.
............
UA1 SIP GW1 . User3 .
| | . .
| INVITE (R-P:1) | . .
|-------------------------------------->| . Non-IP .
| 200 OK | . .
|<--------------------------------------| . Network .
| ACK | . .
|-------------------------------------->| . .
| RTP | . .
|<=====================================>| . .
| | . .
| BYE (Reason: Preemption ; |<==Preemption Indication
| cause=4 ;text="Non-IP Preemption") | . .
|<--------------------------------------| . .
| | ............
Figure 7. Non-IP Preemption Flow
In this case, UA1 signals User3 to a session. Once established,
there is a preemption event in the non-IP portion of the
session/call, and the TDM portion has the ability to inform the SIP
GW of this type of event. This non-IP signal can be translated into
SIP signaling (into the BYE session termination message). Within
this BYE, there should be a Reason header indicating such an event to
synchronize all SIP elements.
6. Security Considerations
Eavesdropping on this header field should not prevent proper
operation of the SIP protocol, although some domains utilizing this
mechanism for notifying and synchronizing SIP elements will likely
want the integrity to be assured. It is therefore RECOMMENDED that
integrity protection be applied when using this header to prevent
unwanted changes to the field and snooping of the messages. The
accepted choices for providing integrity protection in SIP are TLS
and S/MIME.
7. IANA Considerations
This document adds to one existing IANA Registry and creates one new
Registry. The existing IANA Registry for the SIP Reason Header is as
follows:
Polk Standards Track [Page 17]
RFC 4411 SIP Reason Header for Preemption Events February 2006
Protocol Value Protocol Cause Reference
-------------- -------------- ---------
SIP Status code RFC 3261
Q.850 Cause value in decimal ITU-T Q.850
This document adds to that Registry with the following entry
(including the '*' comment):
Protocol Value Protocol Cause Reference
-------------- -------------- ---------
Preemption Cause value in decimal* RFC 4411
* See the separate "Preemption" Registry for default reason-text
strings.
The cause values created by the Preemption Protocol namespace in this
document are defined in Section 7.1. Each cause value has a Reason-
text string as a general description of what the cause value is for.
This is shown for the existing Reason header in Section 2 of RFC
3326. Before this document, the Reason-text was taken from the SIP
Response code string from all SIP Response codes, or the default
description from Q.850 cause codes. Currently, there is no place to
register new reason-text strings other than from those two sources.
Because this document defines a new Reason header protocol namespace,
a new IANA Registry is created in Section 7.2 just for this and
future Reason header protocol namespaces (other than SIP Response
codes or Q.850 cause values) to register their respective general
descriptive text strings. These text strings are non-binding and
merely the default for human understanding, but they are deemed
important enough to have their own Registry.
7.1. "Preemption" Namespace Registry
RFC 4411 creates the new SIP "Reason Header" [1] protocol namespace:
"Preemption", with 4 defined cause codes:
In instances where this namespace is used to indicate preemption
at a UA, the following syntax shall be used (the reason-text is a
default string; it is not mandatory, and may be different):
Reason: preemption ;cause=1 ;text="UA Preemption"
Section 5.1 of this document describes in detail the semantics
of this cause code.
The default text above is part of a new IANA Registry for
default text strings for any new protocol namespace cause code.
See Section 7.2 for details.
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RFC 4411 SIP Reason Header for Preemption Events February 2006
In instances where this namespace is used to indicate preemption
because an RSVP ResvErr message was received at a SIP UA, the
following syntax shall be used (the reason-text is a default
string; it is not mandatory, and may be different):
Reason: preemption ;cause=2 ;text="Reserved Resources Preempted"
Section 5.2 of this document describes in detail the semantics
of this cause code.
The default text above is part of a new IANA Registry for
default text strings for any new protocol namespace cause code.
See section 7.2 for details.
In instances where this namespace is used to indicate a
generalized preemption event to the destination UA from a Proxy
that modifies the Reason value only during this last SIP hop, the
following syntax shall be used (the reason-text is a default
string; it is not mandatory, and may be different):
Reason: preemption ;cause=3 ;text="Generic Preemption"
Section 5.3 of this document describes in detail the semantics
of this cause code.
The default text above is part of a new IANA Registry for
default text strings for any new protocol namespace cause code.
See Section 7.2 for details.
In instances where this namespace is used to indicate preemption
from a non-IP portion of a call leg, a SIP Gateway shall use the
following syntax to inform the SIP infrastructure of this event
(the reason-text is a default string; it is not mandatory, and may
be different):
Reason: preemption ;cause=4 ;text=" Non-IP Preemption"
Section 5.4 of this document describes in detail the semantics
of this cause code.
The default text above is part of a new IANA Registry for
default text strings for any new protocol namespace cause code.
See Section 7.2 for details.
Additional definitions of the preemption namespace and its cause
codes MUST be defined in Standards Track documents.
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RFC 4411 SIP Reason Header for Preemption Events February 2006
7.2. Default Reason-Text IANA Registry for the SIP Reason Header
Below is a new IANA Registry for SIP Reason Header reason-text
strings, associated with their respective protocol type and Reason-
param cause values. Per RFC 3326, the Reason-text string is a quoted
default string with only human understandability meant. These
strings can be changed by local policy.
Reason-
Protocol param Reason-Text Reference
-------- ------- ------------ ---------
Preemption Cause=1 UA Preemption RFC 4411
Preemption Cause=2 Reserved Resources RFC 4411
Preempted
Preemption Cause=3 Generic Preemption RFC 4411
Preemption Cause=4 Non-IP Preemption RFC 4411
8. Contributions
The following individuals contributed to this effort:
Subhasri Dhesikan
Gonzalo Camarillo
Dave Oran
The author thanks these individuals greatly for their aid in this
effort.
9. Acknowledgements
To Haluk Keskiner for providing a valued sanity check. To Dean
Willis, Rohan Mahy, and Allison Mankin for their belief in and
backing of this effort. To Adam Roach and Arun Kumar for helpful
comments to this document.
Thanks to Mike Pierce for helpful comments and catching a flaw in
this spec late in the process (before it was too late).
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RFC 4411 SIP Reason Header for Preemption Events February 2006
10. References
10.1. Normative References
[1] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason Header
Field for the Session Initiation Protocol (SIP)", RFC 3326,
December 2002.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[3] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
[4] Schulzrinne, H. and J. Polk, "Communications Resource-Priority
Header in the Session Initiation Protocol (SIP)", RFC 4412,
February 2006.
[5] ITU-T Recommendation Q.850 (1993)
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[7] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
10.2. Informative References
[8] J. Manner, G. Karagiannis, A. McDonald, S. Van den Bosch, "NSLP
for Quality-of-Service signalling", Work in Progress, September
2005.
Author Information
James M. Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, Texas 75082 USA
EMail: jmpolk@cisco.com
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RFC 4411 SIP Reason Header for Preemption Events February 2006
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