One document matched: draft-polk-reason-header-for-preemption-00.txt
Internet Engineering Task Force James M. Polk
Internet Draft Cisco Systems
Expiration: April 8th, 2004
File: draft-polk-reason-header-for-preemption-00.txt
Extending the Session Initiation Protocol
Reason Header to account for Preemption Events
October 8th, 2003
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 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
2. Access Preemption Events . . . . . . . . . . . . . . . . . . 3
2.1 Effects of Preemption at the User Agent . . . . . . . . 5
2.2 Reason Header Requirements for
Access Preemption Events . . . . . . . . 5
3. Network Preemption Events . . . . . . . . . . . . . . . . . 5
3.1 Reason Header Requirements for
Network Preemption Events . . . . . . . . 8
4. Proposed Reason Header Values and Descriptions . . . . . . . 8
4.1 Access Preemption Event Reason Value . . . . . . . . . . 9
4.1.1 Access Preemption Event Call Flow . . . . . . . . . . 10
4.2 Network Preemption Events Reason Value . . . . . . . . . 11
4.2.1 Network Preemption Event Call Flow . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Author Information . . . . . . . . . . . . . . . . . . . . . 14
10. Full Copyright Statement . . . . . . . . . . . . . . . . . . 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 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 will be proposed here is extending
SIP to synchronize SIP elements as to why a preemption event
occurred and which type of 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. No other means exists today to give
this feedback as to why a session was torn down for 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, the Reason
Header [1] was created to be included in the BYE Request. This was
not the only reason for this new Header; [1] also discusses the
CANCEL Method Request.
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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 value for
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 [5].
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.
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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"?
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).
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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 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 [6]: ResvErr messages - which travel
downstream towards appropriate receivers. The ResvErr message has
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the ability to carry within it a code why a reservation is being
torn down. The ResvErr does not travel upstream to the other UA.
This document here proposes that 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.
The following 2 diagrams (Figure 2 and 3) illustrate the network
preemption scenario:
UA1 UA2
\ /
\ /
+--------+ +--------+
| | | |
| RTR1 | | RTR2 |
| Int7-------Int5 |
| | | |
+--------+ +--------+
/ \
/ \
UA3 UA4
Figure 2. Network Diagram Scenario A
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). 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, Router 2
will (in this example) preempt lower priority BW to ensure this
higher priority session is completed. The congestion point is in
Router 2, interface 5 (in figure 2). 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 will not know
anything other than its reservation went away. If there is 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.
In Figure 3 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
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reasons. The signaling between UA3 and UA4 is also not included.
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"?
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.
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 Proposed Reason Header Values and Descriptions
RFC 3326 [1] defines the format of the header as follows:
Reason = "Reason" HCOLON reason-value *(COMMA reason-value)
reason-value = protocol *(SEMI reason-params)
protocol = "SIP" / "Q.850" / token
reason-params = protocol-cause / reason-text
/ reason-extension
protocol-cause = "cause" EQUAL cause
cause = 1*DIGIT
reason-text = "text" EQUAL quoted-string
reason-extension = generic-param
The following are currently the only defined values in the protocol
field in [1]:
SIP: The cause parameter contains a SIP status code.
Q.850: The cause parameter contains an ITU-T Q.850 cause value
in decimal representation.
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4.1 Access Preemption Event Reason Value
The following Reason Header value is proposed for cases of Access
Preemption Events:
Reason = "Reason" HCOLON reason-value *(COMMA reason-value)
reason-value = protocol *(SEMI reason-params)
protocol = "SIP" / "Q.850" / token
token = preempted_UA
with the following "token" value being proposed in this document for
addition to the IANA Registry under possible Reason Values:
preempted_UA : a user agent in a session has 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)
This token was chosen because there is no SIP (or other Standards
Body recognized) cause code that replicates the meaning of a
existing session intentionally preempted at the user agent.
An example usage of this header value would be:
Reason: preempted_UA ;text="user agent preemption"
This effort is open to the idea of registering a 400 or 500 level
error code just for this, but it seems excessive at this time.
A 37X level warning code can also be created - but it is not
apparent that this is necessary. Comments regarding this document
will determine if creating new a new code number is a better course
of action than the above token name.
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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 : preempted_UA)| |
|<--------------------------| |
| | 200 OK |
| |------------------------>|
| 200 OK | |
|-------------------------->| |
| | ACK |
| |<------------------------|
| | RTP |
| |<=======================>|
| | |
Figure 4. Access Preemption with Reason : Preempted_UA
UA1 invites UA2 to a session with the Resource Priority level of 3
(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 token name
"preempted_UA" is unique to what is proposed in the Network
Preemption Event.
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4.2 Network Preemption Events Reason Value
The following Reason Header value is proposed for cases of Network
Preemption Events:
Reason = "Reason" HCOLON reason-value *(COMMA reason-value)
reason-value = protocol *(SEMI reason-params)
protocol = "SIP" / "Q.850" / token
token = preempted_network
with the following "token" value being proposed in this documents
for addition to the IANA Registry under possible Reason Values:
preempted_network : 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 the network a layer
3/4 device (router) has preempted this session
This token was chosen because there is no SIP (or other Standards
Body recognized) cause code that replicates the meaning of a
existing session as a RSVP flow intentionally preempted by an
intermediate router within the network between UAs.
An example usage of this header value would be:
Reason: preempted_network ;text="session preempted within network"
This effort is open to the idea of registering a 400 or 500 level
error code just for this, but it seems excessive at this time.
A 37X level warning code can also be created - but it is not
apparent that this is necessary. Comments regarding this document
will determine if creating new a new code number is a better course
of action than the above token name.
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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 : preempted_Network) |
|------------------------------------------------->|
| 200 OK |
|<-------------------------------------------------|
| |
Figure 5. Network Preemption with obscure Reason
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 token name is unique to that proposed in
section 4.1 for Access Preemption Events.
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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
This document defines a new SIP header field values for the "Reason
Header" from RFC 3326 [1]. Two new token values are to be IANA
registered based on this document.
Token value Defined in section of this document:
----------- -----------------------------------
preempted_UA section 4.1
preempted_network section 4.2
There is no protocol name or cause value associated with these token
Values.
7.0 Acknowledgements
Your name here
8.0 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
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[5] Bradner, S., "Key words for use in RFCs to indicate requirement
levels," BCP 14, RFC 2119, March 1997.
[6] R. Braden, Ed., L. Zhang, S. Berson, S. Herzog, S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997
9.0 Author Information
James M. Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, Texas 75082 USA
jmpolk@cisco.com
10. Full Copyright Statement
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The Expiration date for this Internet Draft is:
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