Internet Engineering Task Force James M. Polk
Internet Draft Cisco Systems
Expiration: July 8th, 2004
File: draft-ietf-sipping-reason-header-for-preemption-00.txt
Extending the Session Initiation Protocol
Reason Header for Preemption Events
January 8th, 2004
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
This document proposes an IANA Registration extension to the Session
Initiation Protocol (SIP) Reason Header [1] to include in a BYE
Method Request [2] as a result of a session preemption event either
at a user agent (UA), or somewhere in the network using RSVP [3].
This document does not attempt to address routers failing in the
packet path; but a deliberate event of tearing down a flow between
UAs.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Conventions . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Changes from the Individual submission Version -00 . . . . 3
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 . . . . . . . . . . . . . . . . . 6
3.1 Reason Header Requirements for
Network Preemption Events . . . . . . . . 9
4. Preemption Reason Header Cause Codes and Semantics . . . . . 9
4.1 Access Preemption Event Reason Code . . . . . . . . . . . 10
4.1.1 Access Preemption Event Call Flow . . . . . . . . . . . 10
4.2 Network Preemption Events Reason Code . . . . . . . . . . 11
4.2.1 Network Preemption Event Call Flow . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
7. Contributions . . . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 13
9. Normative References . . . . . . . . . . . . . . . . . . . . 14
10. Author Information . . . . . . . . . . . . . . . . . . . . . 14
1.0 Introduction
With the introduction of the 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 for 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
extending SIP 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 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
indication for preemption (cause=8) and for preemption "circuit
reserved for reuse" (cause=9). Q.850 Cause=9 does not apply to IP
because IP has no concept of circuits. Some domains wish to
differentiate appropriate IP reasons for preemption of sessions and
topologically where the preemption event occurred. No other means
exists today to give this feedback as to why a session was torn down
for preemption grounds.
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In the event that a session is terminated for a specific reason that
can (or should) be shared with SIP Servers and UAs, 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 (based on 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 sessions
will remain active when a new session/reservation is
signaled for under the parameters of RSVP in [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 (based on local policy
of some domains).
This document will cover the semantics for these two cases, and
request IANA registration two new Reason Header values for the above
preemption conditions.
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].
1.2 Changes from the Individual Submission Version -00
The following is a list of changes that have been made to this
document from the Individual Submission Version -00:
- Changed the proposed indication from a new token (as defined by
RFC 3326) to proposing a new namespace ("preemption") with cause
values for each type of occurrence;
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- Changed the notion of the "preempted_network" value - which could
mean anything not at the endpoint to "RSVP_Preemption" because
this more accurately reflects the signaling behavior of the type
of event explained here;
- Added the contributions and acknowledgements sections for those
who have provided much help with this effort
- Added an IPR section to this document;
2.0 Access Preemption Events
As mentioned previously, Access Preemption Events (APE) occur at
the user agent. It doesn't 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 to utilize 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 what occurred (if this event was purposeful), and
why it occurred. 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 informing of UA can be
different than the rules of 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, etc).
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The following diagram (Figure 1) illustrates the scenario here. UA1
invites UA2 to a session with the Resource Priority level of 3
(levels 1 and 2 are higher is this domain).
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 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.
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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) adhoc
conference participants and must preempt all sessions in that
conference due to a higher priority session request received (that
this UA must accept), then a BYE message MUST be sent to all UAs in
that adhoc 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.
APE_REQ#3 - create a means of informing all participants in a
adhoc 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 any used for a Network
Preemption Event (described in the next section) in
the session BYE message.
3.0 Network Preemption Events
Network Preemption Events (NPE) are those instances in which a
intermediate router between SIP elements preempts one or more
session(s) at one of its interfaces to place a higher priority
session through that interface. Within RSVP, there exists a means to
execute this functionality in [7]: ResvErr messages - which travel
downstream towards appropriate receivers. The ResvErr message has
the ability to carry within it a code why a reservation is being
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torn down. The ResvErr does not travel upstream to the other UA.
This document here proposes a SIP message be generated to
synchronize all relevant SIP elements to this preemption event.
Creating another Reason value describing that a network element
preempted the session is necessary in certain domains.
The following 2 diagrams (Figure 2 and 3) illustrate the network
preemption scenario:
UA1 invites UA2 to a session with the Resource Priority level of 3
(levels 1 and 2 are higher is this domain) and is accepted. The
link between Router 1 and Router 2 became saturated with this
session 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 an Resource Priority level of 2 (a higher
priority than the current session between UA1 and UA2). When this
second (higher priority session) is signaled, the 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 congested (at
Interface 5 in Figure 2), Router 2 will (in this example) preempt
lower priority BW to ensure this higher priority session is
completed. A ResvErr message is sent to UA1. The result is that
UA1 will know that there has been a preemption event in a router
(because the ResvErr message has a error code within it stating
"preemption"), UA2 at this point will not know anything other than
its reservation went away. If there are any SIP Proxies in between
the 2 UAs (perhaps that inserted a Record-Route Header), each will
need to be informed also 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 signaling between UA3 and UA4 is also not
included.
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UA1 Rtr2 UA3
| | |
| INVITE (R-P:3) |
|------------------------------------------------->|
| 200 OK |
|<-------------------------------------------------|
| ACK |
|------------------------------------------------->|
| RTP |
|<================================================>|
| ******************************************** |
| * -UA3 sends INV to UA4 w/ RP:2; * |
| * -Reservation set-up occurs between UA3 * |
| * and UA4 * |
| * -Router 2 must preempt 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 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. 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 generically 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.
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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_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 that have incorporated a Record-
Route header into that session set-up.
NPE_REQ#3 - create a means of informing all participants in a
adhoc 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.
4.0 Preemption Reason Header Cause Codes and Semantics
We propose 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 RSVP_Preemption The session preemption has been
initiated within the network via a
purposeful RSVP preemption occurrence,
and not a link error
Example syntax is as follows:
Reason: preemption ;cause=1 ;text="UA_Preemption"
Reason: preemption ;cause=2 ;text="RSVP_Preemption"
Sections 4.1 and 4.2 provide use cases for the above two cause
codes with message flow diagrams.
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4.1 Access Preemption Event Reason Code
The 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"
4.1.1 Access Preemption Event Call Flow
The following diagram (Figure 4) replicates the call flow from
Figure 1 - but with an appropriate Reason value indication that was
proposed in section 4.1 above:
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 4. Access Preemption with Reason : Preemption
UA1 invites UA2 to a session with the Resource Priority level of 3
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(levels 1 and 2 are higher is 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:
preempted_UA. 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.
4.2 Network Preemption Events Reason Code
The more elaborate description of the RSVP Preemption Event
cause=2 is as follows:
A router has preempted a reservation flow and generated a ResvErr
(downstream). The (downstream) UA receiving the ResvErr message
generates a BYE request towards the far side UA with a Reason
Header with this value indicating that somewhere in between two
or more UAs, a layer 3/4 device (router) has administratively
preempted this session
An example usage of this header value would be:
Reason: Preemption :cause=2 ;text="RSVP_Preemption"
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4.2.1 Network Preemption Event Call Flow
The following diagram (Figure 5) replicates the call flow from
Figure 3 - but with an appropriate Reason value indication that was
proposed in section 4.2 above.
UA1 Rtr2 UA3
| | |
| INVITE (R-P:3) |
|---------------------------------------------------->|
| 200 OK |
|<----------------------------------------------------|
| ACK |
|---------------------------------------------------->|
| RTP |
|<===================================================>|
| |
| *********************************************** |
| * -UA3 sends INV to UA4 w/ RP:2; * |
| * -Reservation set-up occurs between UA3 * |
| * and UA4 * |
| * -Router 2 must preempt UA1-UA2 * |
| * ********************************************* |
| |
| ResvErr | |
|<--------------------------| |
| | |
| |
| BYE (Reason : Preemption ;cause=1 ; |
| text="RSVP_Preemption") |
|---------------------------------------------------->|
| 200 OK |
|<----------------------------------------------------|
| |
Figure 5. Network Preemption with "RSVP_Preemption"
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 here for diagram complexity
reasons. The signaling between UA3 and UA4 is also not included.
Upon receipt of the ResvErr message with the preemption error code,
UA1 can now appropriately inform UA2 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.
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5.0 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.
6.0 IANA Considerations
RFC [XXXX} (this document) proposes the new SIP "Reason Header" [1]
protocol namespace: "Preemption", with 2 defined cause codes:
In instances where this namespace is used to indicate preemption
at a UA, the following syntax shall be used:
Reason: preemption ;cause=1 ;text="UA_Preemption"
Section 4.1 of this document describes in detail the semantics
of this cause code.
In instances where this namespace is used to indicate preemption
based on receipt of an RSVP ResvErr message at a SIP UA, the
following syntax shall be used:
Reason: preemption ;cause=2 ;text="RSVP_Preemption"
Section 4.2 of this document describes in detail the semantics
of this cause code.
Additional definitions of the preemption namespace cause codes shall
be defined in Standards Track documents.
7.0 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.
8.0 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.
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9.0 Normative References
[1] H. Schulzrinne, D. Oran, G. Camarillo, "The Reason Header Field
for the Session Initiation Protocol (SIP)", RFC 3326 Reason
Header, 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] G. Camarillo, Ed., W. Marshall, Ed., J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312 Preconditions, October 2002
[4] H. Schulzrinne, J. Polk, "Communications Resource-Priority Header
in SIP", Internet Draft, work in progress, July 2003
[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] R. Braden, Ed., L. Zhang, S. Berson, S. Herzog, S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997
10.0 Author Information
James M. Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, Texas 75082 USA
jmpolk@cisco.com
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