Network Working Group H. Schulzrinne
Internet-Draft Columbia U.
Expires: August 15, 2004 J. Polk
Cisco
February 15, 2004
Communications Resource Priority for the Session Initiation Protocol
(SIP)
draft-ietf-sip-resource-priority-02
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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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines two new SIP header fields for communications
resource priority, namely "Resource-Priority" and
"Accept-Resource-Priority". The "Resource-Priority" header field can
influence the behavior of SIP UAs, such as GSTN gateways, and SIP
proxies. It does not directly influence the forwarding behavior of
IP routers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 6
3. The Resource-Priority and Accept-Resource-Priority SIP
Header Fields . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 The Resource-Priority Header Field . . . . . . . . . . . . . 7
3.2 The Accept-Resource-Priority Header Field . . . . . . . . . 8
3.3 Usage of the Resource-Priority and
Accept-Resource-Priority Header Fields . . . . . . . . . . . 8
3.4 The Resource-Priority Option Tag . . . . . . . . . . . . . . 8
4. Behavior of SIP Elements that Receive Prioritized
Requests . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1 General Rules . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Error Conditions . . . . . . . . . . . . . . . . . . . . . . 10
4.2.1 Known Namespace and Priority Value . . . . . . . . . . . . . 10
4.2.2 Handling Unknown Namespaces and Priority Values . . . . . . 11
4.3 User Agent Client Behavior . . . . . . . . . . . . . . . . . 12
4.4 User Agent Server Behavior . . . . . . . . . . . . . . . . . 12
4.5 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 13
5. Third-Party Authentication . . . . . . . . . . . . . . . . . 14
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . 15
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1 Simple Call . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2 Receiver Does Not Understand Namespace . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . 20
8.1 Authentication and Authorization . . . . . . . . . . . . . . 20
8.2 Confidentiality and Integrity . . . . . . . . . . . . . . . 21
8.3 Anonymity . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.4 Denial-of-Service Attacks . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 23
9.1 IANA Registration of 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields . . . . . . . . . . 23
9.2 IANA Registration for Option Tag resource-priority . . . . . 23
9.3 IANA Registration for Response Code 417 . . . . . . . . . . 23
9.4 IANA Namespace and Priority Registrations . . . . . . . . . 24
9.5 Initial Namespace Registrations . . . . . . . . . . . . . . 25
9.5.1 Namespace dsn . . . . . . . . . . . . . . . . . . . . . . . 25
9.5.2 Namespace q735 . . . . . . . . . . . . . . . . . . . . . . . 25
9.5.3 Namespace DRSN . . . . . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 27
Normative References . . . . . . . . . . . . . . . . . . . . 28
Informative References . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . 31
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1. Introduction
During emergencies, communications resources including telephone
circuits, IP bandwidth and gateways between the circuit-switched and
IP networks may become congested. Congestion can occur due to heavy
usage, loss of resources caused by the natural or man-made disaster
and attacks on the network during man-made emergencies. This
congestion may make it difficult for persons charged with emergency
assistance, recovery or law enforcement to coordinate their efforts.
As IP networks become part of converged or hybrid networks along with
public and private circuit-switched (telephone) networks, it becomes
necessary to ensure that these networks can assist during such
emergencies.
Also, users may want to interrupt their lower-priority communications
activities and dedicate their end system resources to the
high-priority communications attempt if a high-priority
communications request arrives at their end system.
There are many IP-based services that can assist during emergencies.
This memo only covers real-time communications applications involving
SIP, including voice-over-IP, multimedia conferencing and instant
messaging/presence.
Session Initiation Protocol (SIP) [RFC3261] applications may involve
at least five different resources that may become scarce and
congested during emergencies. These resources include gateway
resources, circuit-switched network resources, IP network resources,
receiving end system resources and SIP proxy resources. IP network
resources are beyond the scope of SIP signaling and are therefore not
considered here.
In order to improve emergency response, it may become necessary to
prioritize access to SIP-signaled resources during periods of
emergency-induced resource scarcity. We call this "resource
prioritization".
Currently, SIP does not include a mechanism that allows a request
originator to indicate to a SIP element that it wishes the request to
invoke such resource prioritization. To address this need, this
document adds a SIP protocol element that labels certain SIP
requests.
This document defines (Section 3) a new SIP [RFC3261] header field
for communications resource priority, called 'Resource-Priority' This
header field MAY be used by SIP user agents, including GSTN gateways
and terminals, and SIP proxy servers to influence their treatment of
SIP requests, including the priority afforded to GSTN calls. For
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GSTN gateways, the behavior translates into analogous schemes in the
GSTN, for example the ITU Recommendation Q.735.3 [Q.735.3]
prioritization mechanism, in both the GSTN-to-IP and IP-to-GSTN
directions.
The 'Resource-Priority' header field may be used in several
situations. A SIP request with such an indication can be treated
differently in these situations:
1. The request can be given elevated priority for access to GSTN
gateway resources such as trunk circuits.
2. The request can interrupt lower-priority requests at a user
terminal, such as an IP phone.
3. The request can carry information from one multi-level priority
domain in the telephone network, e.g., using the facilities of
Q.735.3 [Q.735.3], to another, without the SIP proxies themselves
inspecting or modifying the header field.
4. In SIP proxies and back-to-back user agents, requests of higher
priorities may displace existing signaling requests or bypass
GSTN gateway capacity limits in effect for lower priorities.
This header field is related to, but differs in semantics from, the
'Priority' header field (RFC 3261 [RFC3261], Section 20.26). The
'Priority' header field describes the importance that the SIP request
should have to the receiving human or its agent. For example, that
header may be factored into decisions about call routing to mobile
devices and assistants and call acceptance when the call destination
is busy. The 'Priority' header field does not affect the usage of
PSTN gateway or proxy resources, for example. In addition, any UAC
can assert any 'Priority' value, while access to resource priority
values is subject to authorization.
While the 'Resource-Priority' header does not directly influence the
forwarding behavior of IP routers or the use of communications
resources such as packet forwarding priority, procedures for using
this header to cause such influence may be defined in other
documents.
Existing implementations of RFC 3261 that do not participate in the
resource priority mechanism follow the normal rules of RFC 3261,
Section 8.2.2: "If a UAS does not understand a header field in a
request (that is, the header field is not defined in this
specification or in any supported extension), the server MUST ignore
that header field and continue processing the message." Thus, the use
of this mechanism is wholly invisible to existing implementations.
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The mechanism described here can be used for emergency preparedness
in emergency telecommunications systems, but is only a small part of
an emergency preparedness network and is not restricted to such use.
The mechanism is structured so that it works in all SIP/RTP
transparent networks defined in [RFC3487], i.e., all network elements
and SIP proxies let valid SIP requests pass through unchanged. This
is important since it is likely that this mechanism will often be
deployed in networks where the edge networks are unaware of the
resource priority mechanism and provide no special privileges to such
requests. The request then reaches a PSTN gateway or set of SIP
elements that are aware of the mechanism.
For conciseness, we refer to SIP proxies and user agents that act on
the 'Resource-Priority' header field as RP actors.
We define the header field syntax in Section 3 and then describe the
behavior of user agents and proxies in Section 4.3 through Section
4.5. Section 6 briefly describes how this feature affects existing
systems that do not support it. Third-party authentication is
discussed in Section 5, while general security issues are enumerated
in Section 8. This specification does not propose any new SIP
security mechanisms. Examples can be found in Section Section 7.
The mechanism aims to satisfy the requirements in [RFC3487].
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2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant
implementations.
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3. The Resource-Priority and Accept-Resource-Priority SIP Header Fields
This document defines the 'Resource-Priority' and
'Accept-Resource-Priority' SIP header fields.
The SIP element behavior is described for UACs in Section Section
4.3, for UAS in Section Section 4.4, for proxies in Section Section
4.5.
3.1 The Resource-Priority Header Field
The 'Resource-Priority' header field marks a SIP request as desiring
prioritized resource access, as described in the introduction. In
responses, the 'Resource-Priority' header fields indicates the actual
resource priority that was granted to the request. While it is
usually the same value contained in the request, implementations MAY
insert a different value based on local policy.
There is no requirement that all requests within a SIP dialog or call
use the 'Resource-Priority' header field.
The syntax of the 'Resource-Priority' header field is as follows:
Resource-Priority = "Resource-Priority" HCOLON
Resource-value (*COMMA Resource-value)
Resource-value = namespace "." r-priority
namespace = alphanum / "-"
r-priority = alphanum / "-"
An example 'Resource-Priority' header field is shown below:
Resource-Priority: q735.1, dsn.flash
The 'Resource-value' parameter in the 'Resource-Priority' header
indicates the resource priority desired by the request originator.
Since a request may traverse multiple administrative domains with
multiple different namespaces, it is necessary to be able to
enumerate several different namespaces. However, each namespace MUST
NOT appear more than once in a SIP message.
Each resource value is formatted as 'namespace' '.' 'priority value'.
The value is drawn from the namespace identified by the 'namespace'
token. Namespaces and priorities are case-independent ASCII. Each
namespace has at least one priority value. Namespaces and priority
values within each namespace are registered with IANA (Section
Section 9); some initial namespaces are described in Section Section
9.5.
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There may be multiple resource values or, equivalently, multiple
'Resource-Priority' header field instances.
3.2 The Accept-Resource-Priority Header Field
The 'Accept-Resource-Priority' response header field enumerates the
resource values a SIP user agent server implements. The syntax of
the 'Accept-Resource-Priority' header field is as follows:
Accept-Resource-Priority = "Accept-Resource-Priority" HCOLON
[Resource-value (*COMMA Resource-value)]
An example is given below:
Accept-Resource-Priority: dsn.flash-override,
dsn.flash, dsn.immediate, dsn.priority, dsn.routine
3.3 Usage of the Resource-Priority and Accept-Resource-Priority Header
Fields
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amd o o o o o o o
Resource-Priority 200 - o - - - - - -
Accept-Resource-Priority 200 - o - - - - - -
Accept-Resource-Priority 417 - o - - - - - -
Accept-Resource-Priority 420 - o - - - - - -
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority R amd o o o o o o o
Resource-Priority 200 - o o - o - - -
Accept-Resource-Priority 200 - - - - - - - -
Accept-Resource-Priority 417 - m m - m - - -
Accept-Resource-Priority 420 - o o - o - - -
Other request methods MAY define their own handling rules; unless
otherwise specified, recipients MAY ignore these header fields.
'Accept-Resource-Priority' MUST be returned in 420 (Not Supported)
responses marked as 'o' in table above if the element implements the
resource priority mechanism with some other namespaces or priority
values, but does not implement the particular namespace or priority
value contained in the request.
3.4 The Resource-Priority Option Tag
This document also defines the "resource-priority" option tag. The
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behavior is described in Section 4.2.2 and the IANA registration is
in Section 9.2.
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4. Behavior of SIP Elements that Receive Prioritized Requests
4.1 General Rules
All user agent servers and proxy servers that receive SIP requests
share certain common behavior, which we describe below. Behavior
that is specific to user agent servers is covered in Section 4.4,
while Section 4.5 deals with proxy behavior.
A SIP element supporting this specification MUST be able to interpret
the 'Resource-Priority' header field in INVITE, MESSAGE [RFC3428],
UPDATE [RFC3311], SUBSCRIBE [RFC3265] and NOTIFY [RFC3265] requests,
if it supports a particular request. (This does not imply that all
elements supporting this specification need to support all of these
request methods.) In all such requests, the priority MAY influence
the order in which requests are handled and MUST influence the
resources, such as circuits, bandwidth or memory, allocated based on
the request. For example, for SUBSCRIBE, a higher-priority request
may get preferential treatment if storage or bandwidth for
notifications are scarce, possibly displacing a lower-priority
subscription. (As always, the precise behavior is defined by a
namespace definition, or, if left unspecified, by an implementation
or configuration.)
A SIP element MAY ignore the header field in other requests unless
the request definition defines behavior for the particular method.
If a request contains multiple valid namespace/priority values, the
request is treated according to the highest supported and authorized
value. The total ordering of priorities between different namespaces
is defined by local policy.
4.2 Error Conditions
4.2.1 Known Namespace and Priority Value
Two error conditions can occur if a request reaches an element that
supports the namespace and resource priority. Elements receiving
requests with namespaces or priority values that they do not
understand act according to the rules in the next section.
Insufficient authorization: If the element receives a request with a
namespace and priority value it recognizes, but the originator is
not authorized for that level of service, the element MUST return
a 403 (Forbidden) response.
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Insufficient resources: If there are insufficient resources at an
element for a given priority, a request might be delayed or
refused, depending on local policy or the definition of the
namespace. If it is refused, the element returns a 503 (Service
Unavailable) response. The response MAY also include a 'Warning'
header with warning code 370 (Insufficient Bandwidth) if the
request failed due to insufficient capacity for the media streams,
rather than insufficient signaling capacity.
The 503 (Service Unavailable) response provides sufficient
indication to the originator to re-attempt with a higher
appropriate resource priority or to add a resource priority
indication, if authorized.
4.2.2 Handling Unknown Namespaces and Priority Values
When handling requests with unknown namespsaces or priority values,
elements can operate in two modes, "strict" and "loose". If the
request includes a 'Require' header field with the
'Resource-Priority' option tag, a UAS MUST follow the strict rules,
otherwise UAS and proxies may choose either mode according to local
policy.
Following standard behavior (Section 8.2.2.3 of [RFC3261]), a UAS
MUST then reject the request with response code 420 (Bad Extension)
if it does not understand the resource priority mechanism.
For example, a gateway that is unaware of a resource priority
namespace might accept a request at non-elevated priority, but
then the request could later be preempted by other requests.
Also, use of the 'Require' restriction ensures that in parallel
forking, only branches that support the resource priority
mechanism succeed.
The use of the 'Resource-Priority' option tag with 'Proxy-Require' is
NOT RECOMMENDED.
4.2.2.1 Strict Mode
In strict mode, an element that receives a request with a
'Resource-Priority' header field containing one or more namespace or
priority values that it does not implement rejects the request with
status code 417 (Unknown Resource-Priority) and includes a
'Accept-Resource-Priority' header field enumerating all the resource
values that the server is willing to process. Note that the user may
not be authorized to use all of these resource values.
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Strict mode is particularly useful for operational testing of
systems supporting resource priority, as otherwise it might be
difficult to detect under non-overload conditions whether an
element supports the functionality or not.
4.2.2.2 Loose Mode
In loose mode, unknown priority values or namespaces are ignored; the
request is treated as if these values were not included. If there
are no valid priority values or namespaces, the request is treated as
if it had no 'Resource-Priority' header field. Thus, no 417 (Unknown
Resource-Priority) is generated.
4.3 User Agent Client Behavior
SIP UACs supporting this specification MUST be able to generate the
'Resource-Priority' header field for requests that require elevated
resource access priority.
If the request is returned with 417 (Unknown Resource-Priority), the
UAC MAY retry the request with a different set of namespace/priority
combinations, drawing from the values returned by the
'Accept-Resource-Priority' header field in the response.
4.4 User Agent Server Behavior
The OPTIONS request can be used to determine if a UAS supports the
mechanism. A compliant implementation MUST return a
'Accept-Resource-Priority' header field in OPTIONS responses
enumerating all valid resource values. An implementation MAY reveal
this capability only to authorized UACs. (Note that an overloaded UAS
may not be able to provide this information at all times.)
If the UAS understands the resource value, but refuses to honor the
request with elevated priority for this particular user, it returns
the 403 (Forbidden) response code. It MAY include the list of
resource values that the user is allowed to use in the
'Accept-Resource-Priority' response header field.
The lookup of the authorized values may take significant resources
since it may involve an AAA interaction. Thus, it seems imprudent
to require that the list is customized to the user. In general,
legitimate users know their highest resource value that they are
entitled to.
The precise effect of the 'Resource-Priority' indication depends on
the type of UAS, the namespace and local policy. For example, a
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circuit-switched telephony gateway might move requests with elevated
priority to the front of the queue of requests waiting for outbound
lines, it may utilize additional resources or it may preempt existing
calls. For a terminal, such as a SIP phone, requests with elevated
priority might trigger a special alert tone or preempt other,
lower-priority ongoing calls. The generic protocol mechanism
described here does not mandate the particular element behavior, but
namespace definitions, such as the ones in Section 9.5, need to spell
out the desired behavioral properties of user agents and proxy
servers.
4.5 Proxy Behavior
SIP proxies MAY ignore, inspect, insert and modify the
'Resource-Priority' header field. SIP proxies MAY downgrade the
'Resource-Priority' of a request or reject unauthenticated requests.
If there are multiple namespace or priority choices available to the
user agent, a proxy MAY return the request with an appropriate
'Accept-Resource-Priority' header field. Details are a matter of
local policy.
This behavior is similar to that for any header field, as a UA can
decide to reject a request for the presence, absence or value of
any information in the request. The session policy mechanism does
not fit well, as user agents may not have a choice in the
namespace or priority available to them, there are no privacy
concerns and the resource priority mechanism does not involve
message bodies or session descriptions.
If a stateful proxy has authorized a particular resource priority
level and if it offers differentiated treatement to responses
containing resource priority levels, the proxy SHOULD ignore any
higher value contained in responses, to avoid that colluding user
agents artificially raise the priority level.
It is unlikely that the resource priority value in responses will
have any influence on response handling.
A SIP proxy MAY use the 'Resource-Priority' indication in its
routing decisions, e.g., to find a next hop that is reserved for a
particular resource priority.
There do not appear to be any special considerations when forking
requests containing a resource priority indication.
Otherwise, the proxy behavior is the same as for user agent
servers Section 4.4).
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5. Third-Party Authentication
In some case, the RP actor may not be able to authenticate the
requestor or determine whether an authenticated user is authorized to
make such a request. In these circumstances, the SIP entity may
avail itself of general SIP mechanisms that are not specific to this
application. The authenticated identity management mechanism
[I-D.ietf-sip-identity] allows a third party to verify the identity
of the requestor and certify this towards an RP actor. In networks
with mutual trust, the SIP asserted identity mechanism [RFC3325] can
help the RP actor determine the identity of the requestor.
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6. Backwards Compatibility
The resource priority mechanism described in this document is fully
backwards compatible with SIP systems following RFC 3261 [RFC3261].
Systems that do not understand the mechanism can only deliver
standard, not elevated, service priority. User agent servers and
proxies can ignore any 'Resource-Priority' header field just like any
other unknown header field and then treat the request like any other
request. Naturally, the request may still succeed.
Introducing 'Require' or 'Proxy-Require' would not help, as systems
that do not support the mechanism will not improve by rejecting the
request due to feature failure. Since the intent of resource
priority indications is to increase the probability of call
completion, adding failure modes appears counterproductive.
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7. Examples
The SDP message body and the BYE and ACK exchanges are the same as in
RFC 3665 [RFC3665] and omitted for brevity.
7.1 Simple Call
User A User B
| |
| INVITE F1 |
|----------------------->|
| 180 Ringing F2 |
|<-----------------------|
| |
| 200 OK F3 |
|<-----------------------|
| ACK F4 |
|----------------------->|
| Both Way RTP Media |
|<======================>|
| |
In this scenario, User A completes a call to User B directly. The
call from A to B is marked with a resource priority indication.
F1 INVITE User A -> User B
INVITE sip:UserB@biloxi.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.com>
Call-ID: 3848276298220188511@atlanta.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserA@client.atlanta.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
F2 180 Ringing User B -> User A
SIP/2.0 180 Ringing
Via: SIP/2.0/TCP client.atlanta.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.com>;tag=8321234356
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Call-ID: 3848276298220188511@atlanta.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserB@client.biloxi.com;transport=tcp>
Content-Length: 0
F3 200 OK User B -> User A
SIP/2.0 200 OK
Via: SIP/2.0/TCP client.atlanta.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserB@client.biloxi.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
7.2 Receiver Does Not Understand Namespace
In this example, the receiving UA does not understand the "dsn"
namespace and thus returns a 417 (Unknown Resource-Priority) status
code. We omit the message details for messages F5 through F7 since
they are essentially the same as in the first example.
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User A User B
| |
| INVITE F1 |
|----------------------->|
| 417 R-P failed F2 |
|<-----------------------|
| ACK F3 |
|----------------------->|
| |
| INVITE F4 |
|----------------------->|
| 180 Ringing F5 |
|<-----------------------|
| 200 OK F6 |
|<-----------------------|
| ACK F7 |
|----------------------->|
| |
| Both Way RTP Media |
|<======================>|
F1 INVITE User A -> User B
INVITE sip:UserB@biloxi.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.com>
Call-ID: 3848276298220188511@atlanta.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserA@client.atlanta.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
F2 417 Resource-Priority failed User B -> User A
SIP/2.0 417 Resource-Priority failed
Via: SIP/2.0/TCP client.atlanta.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.com
CSeq: 1 INVITE
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Accept-Resource-Priority: q735.0, q735.1, q735.2, q735.3, q735.4
Contact: <sip:UserB@client.biloxi.com;transport=tcp>
Content-Type: application/sdp
Content-Length: 0
F3 ACK User A -> User B
ACK sip:UserB@biloxi.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.com:5060;branch=z9hG4bK74bd5
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.com
CSeq: 1 ACK
Content-Length: 0
F4 INVITE User A -> User B
INVITE sip:UserB@biloxi.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.com>
Call-ID: 3848276298220188511@atlanta.com
CSeq: 2 INVITE
Resource-Priority: q735.3
Contact: <sip:UserA@client.atlanta.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
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8. Security Considerations
Any resource priority mechanism can be abused to obtain resources and
thus deny service to other users. An adversary may be able to take
over a particular gateway, cause additional congestion during PSTN
during emergencies or deny service to legitimate users.
While the indication itself does not have to provide separate
authentication, any SIP request carrying such information has higher
authentication requirements than regular requests. Below, we
describe authentication and authorization aspects, confidentiality
and privacy requirements, protection against denial of service
attacks and anonymity requirements. Naturally, the general
discussion in RFC 3261 [RFC3261] applies.
8.1 Authentication and Authorization
Prioritized access to network and end system resources imposes
particularly stringent requirements on authentication and
authorization mechanisms since access to prioritized resources may
impact overall system stability and performance, not just result in
theft of, say, a single phone call.
Under certain emergency conditions, the network infrastructure,
including its authentication and authorization mechanism, may be
under attack.
Given the urgency during emergency events, normal statistical fraud
detection may be less effective, thus placing a premium on reliable
authentication.
Common requirements for authentication mechanisms apply, such as
resistance to replay, cut-and-paste and bid-down attacks.
Authentication MAY be SIP-based or use other mechanisms. Use of
Digest authentication and/or S/MIME is RECOMMENDED for UAS
authentication. Digest authentication requires that the parties
share a common secret, thus limiting its use across administrative
domains. SIP systems employing resource priority SHOULD implement S/
MIME at least for integrity, as described in Section 23 of [RFC3261].
However, in some environments, asserted identity [RFC3325] and
transitive trust may be used to build a sufficiently robust system.
Section 5 describes third-party authentication.
Trait-based authorization [I-D.ietf-sipping-trait-authz] "entails an
assertion by a authorization service of attributes associated with an
identity" and may be appropriate for this application as it avoids
that all network elements need to maintain or consult a mapping from
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user identifiers to authorizations.
Authorization may be based on factors beyond the identity of the
caller, such as the requested destination. Namespaces MAY also
impose particular authentication or authorization consideration that
are stricter than the baseline described here.
8.2 Confidentiality and Integrity
Calls which use elevated resource priority levels provided by the
'Resource-Priority' header field are likely to be sensitive and often
need to be protected from intercept and alteration. In particular,
requirements for protecting the confidentiality of communications
relationships may be higher than for normal commercial service. For
SIP, the 'To', 'From', 'Organization', 'Subject' and 'Via' header
fields are examples of particularly sensitive information. Systems
MUST implement encryption at the transport level using TLS and MAY
implement other transport-layer or network-layer security mechanisms.
UACs SHOULD use the "sips" URI to request a secure transport
association to the destination.
The 'Resource-Priority' header field can be carried in the SIP
message header or can be encapsulated in a message fragment carried
in the SIP message body [RFC3420]. Encapsulation allows to protect
this header field against inspection or modification by proxies,
using S/MIME. However, in many cases, proxies will need to
authenticate and authorize the request, so that encapsulation is
undesirable.
Removal of a Resource-Priority header field or downgrading its
priority value affords no additional opportunities to an adversary
since that man-in-the-middle could simply drop or otherwise
invalidate the SIP request and thus prevent call completion.
Only SIP elements within the same administrative trust domain
employing a secure channel between their SIP elements will trust a
Resource-Priority header field that is not appropriately signed.
Others will need to authenticate the request independently. Thus,
insertion of a Resource-Priority header field or upgrading the
priority value has no further security implications except causing a
request to fail (see discussion in the previous paragraph).
8.3 Anonymity
Some users may wish to remain anonymous to the request destination.
Anonymity for requests with resource priority is no different than
for any other authenticated SIP request. For the reasons noted
earlier, users have to authenticate themselves towards the SIP
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elements carrying the request where they desire resource priority
treatment. The authentication may be based on capabilities and noms,
not necessarily their civil name. Clearly, they may remain anonymous
towards the request destination, using the network-asserted identity
and general privacy mechanisms [RFC3323][RFC3325].
8.4 Denial-of-Service Attacks
As noted, systems described here are likely to be subject to
deliberate denial- of-service attacks during certain types of
emergencies. DOS attacks may be launched on the network itself as
well as its authentication and authorization mechanism. As noted,
systems should minimize the amount of state, computation and network
resources that an unauthorized user can command. The system must not
amplify attacks by causing the transmission of more than one packet
to a network address whose reachability has not been verified.
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9. IANA Considerations
9.1 IANA Registration of 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields
[NOTE TO RFC EDITOR: Replace RFC XXXX with RFC number of this
document.]
The following is the registration for the 'Resource-Priority' header
field:
RFC number: XXXX
Header name: 'Resource-Priority'
Compact form: none
The following is the registration for the 'Accept-Resource-Priority'
header field:
RFC number: XXXX
Header name: Accept-Resource-Priority
Compact form: none
9.2 IANA Registration for Option Tag resource-priority
RFC number: XXXX
Name of option tag: 'resource-priority'
Descriptive text: Indicates or requests support for the resource
priority mechanism.
9.3 IANA Registration for Response Code 417
RFC number: XXXX
Response code: 417
Default reason phrase: Unknown Resource-Priority
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9.4 IANA Namespace and Priority Registrations
Additional namespaces and priority values are registered with IANA.
Within each namespace, the registration may indicate the relative
precedence levels, expressed as an ordered list. New labels should
not be added to existing namespaces. The registration MUST describe,
in the registration itself or by reference, how SIP elements should
treat requests from that namespace, e.g., whether preemption or only
preferential queueing are allowed. A reference to a stable external
document, e.g., by the International Telecommunication Union, other
SDOs or national regulatory bodies, suffices. An expert review, by
an expert designated by the Transport Area Director or designate, is
required.
Namespaces do not describe how they relate to other existing
namespaces, as each namespace is independent of other registrations.
Below is a template for the registration of a new namespace:
Namespace: Designation of the namespace, according to the BNF
'namespace' in Section 3.
Description: Description of the use and application of this
particular namespace.
Documentation: If applicable, reference to a document describing the
namespace in more detail.
Organization: If applicable, organization definining this namespace.
(For example, an IETF standards-track RFC could also define a
namespace, not just an external organization.)
Policy: Either if not defined normatively elsewhere or for
informative purposes, this element describes how a SIP element
handles requests containing priority values with this namespace.
There are many possible behaviors that cannot be exhaustively
anticipated. Three common behaviors are preemption, precedence
and threshold-exemption. Preemption means that a request with
greater priority can displace an existing request with lower
priority that is already in progress. Precedence means that a
higher-priority request assumes a position in the queue aheadd of
a lower-priority request, but any in-progress request is not
affected by its arrival. In addition, systems with preemption MAY
specify whether requests that cannot obtain resources immediately
are queued or rejected immediately. Threshold-exemption allows
higher-priority calls to access resources, such as circuits, that
are unavailable to lower-priority calls, e.g., because they are
held in reserve.
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Behavior: Either a reference to a stable document describing the
behavior of SIP elements when receiving priority indications from
this namespace, or the description itself. There are many detailed
Additional behavior could describe whether a particular call is,
for example, exempt from capacity restrictions.
Priority values (least to greatest): A list of priority values,
ordered from least to highest priority.
9.5 Initial Namespace Registrations
9.5.1 Namespace dsn
Namespace: dsn
Description: United States Defense Switched Network. The values are
adopted from RFC 791 [RFC0791], omitting the levels "critic-ecp",
"network control" and "internetwork control", as these are
inappropriate here.
Documentation: ANSI T1.619, Section B1
Organization: United States Department of Defense, Defense
Information Systems Agency (DISA).
Preemption or precedence: Preemption with rejection.
Priority values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override"
9.5.2 Namespace q735
Namespace: q735
Description: ITU Q.735.3 describes multi-level precedence and
preemption in SS7. The namespace "q735" supports interworking
with Q.735.3 (or equivalent) GSTN (ISDN) entities; this allows,
for example, carrying information between Q.735.3 entities without
loss of information. One or both of the SIP endpoints might be
PSTN gateways.
Documentation: Q.735.3 [Q.735.3]
Organization: ITU-T
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Preemption or precedence: Precedence.
Priority values (least to greatest): "4", "3", "2", "1", "0"
9.5.3 Namespace DRSN
Namespace: drsn
Description: United States Defense Red Switched Network.
Organization: United States Department of Defense, Defense
Information Systems Agency (DISA).
Preemption or precedence: Preemption with rejection.
Priority values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override", "flash-override-override"
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10. Acknowledgments
Ben Campbell, Paul Kyzivat, Rohan Mahy, and Mike Pierce provided
helpful comments.
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Normative References
[Q.735.3] , ITU., "Stage 3 description for community of interest
supplementary services using Signalling System No. 7:
Multi-level precedence and preemption", Recommendation
Q.735.3, March 1993.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] 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.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC3420] Sparks, R., "Internet Media Type message/sipfrag", RFC
3420, November 2002.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.
and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.
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Informative References
[I-D.ietf-ieprep-framework]
Carlberg, K., Brown, I. and C. Beard, "Framework for
Supporting IEPS in IP Telephony",
draft-ietf-ieprep-framework-08 (work in progress),
February 2004.
[I-D.ietf-ieprep-sip-reqs]
Schulzrinne, H., "Requirements for Resource Priority
Mechanisms for the Session Initiation Protocol",
draft-ietf-ieprep-sip-reqs-00 (work in progress), August
2002.
[I-D.ietf-sip-identity]
Peterson, J., "Enhancements for Authenticated Identity
Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-01 (work in progress), March 2003.
[I-D.ietf-sipping-trait-authz]
Peterson, J., "Trait-based Authorization Requirements for
the Session Initiation Protocol (SIP)",
draft-ietf-sipping-trait-authz-00 (work in progress),
February 2004.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, November 2002.
[RFC3325] Jennings, C., Peterson, J. and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC3487] Schulzrinne, H., "Requirements for Resource Priority
Mechanisms for the Session Initiation Protocol (SIP)", RFC
3487, February 2003.
[RFC3665] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and
K. Summers, "Session Initiation Protocol (SIP) Basic Call
Flow Examples", BCP 75, RFC 3665, December 2003.
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Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7042
EMail: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
James Polk
Cisco
2200 East President George Bush Turnpike
Richardson, TX 75082
US
EMail: jmpolk@cisco.com
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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