Network Working Group H. Schulzrinne
Internet-Draft Columbia U.
Expires: February 27, 2005 J. Polk
Cisco
August 29, 2004
Communications Resource Priority for the Session Initiation Protocol
(SIP)
draft-ietf-sip-resource-priority-04
Status of this Memo
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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 user agents, such as telephone gateways
and IP telephones, and Session Initiation Protocol (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 . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The Resource-Priority and Accept-Resource-Priority SIP
Header Fields . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 The 'Resource-Priority' Header Field . . . . . . . . . . . 6
3.2 The 'Accept-Resource-Priority' Header Field . . . . . . . 7
3.3 Usage of the 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields . . . . . . . . . 7
3.4 The 'resource-priority' Option Tag . . . . . . . . . . . . 8
4. Behavior of SIP Elements that Receive Prioritized Requests . . 8
4.1 General Rules . . . . . . . . . . . . . . . . . . . . . . 8
4.2 Error Conditions . . . . . . . . . . . . . . . . . . . . . 9
4.2.1 Known Namespace and Priority Value . . . . . . . . . . 9
4.2.2 Handling Unknown Namespaces and Priority Values . . . 9
4.3 User Agent Client Behavior . . . . . . . . . . . . . . . . 10
4.4 User Agent Server Behavior . . . . . . . . . . . . . . . . 11
4.5 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 11
5. Third-Party Authentication . . . . . . . . . . . . . . . . . . 12
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 12
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1 Simple Call . . . . . . . . . . . . . . . . . . . . . . . 13
7.2 Receiver Does Not Understand Namespace . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8.1 Authentication and Authorization . . . . . . . . . . . . . 17
8.2 Confidentiality and Integrity . . . . . . . . . . . . . . 18
8.3 Anonymity . . . . . . . . . . . . . . . . . . . . . . . . 18
8.4 Denial-of-Service Attacks . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1 IANA Registration of 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields . . . . . . . . . 19
9.2 IANA Registration for Option Tag resource-priority . . . . 19
9.3 IANA Registration for Response Code 417 . . . . . . . . . 19
9.4 IANA Namespace and Priority Registrations . . . . . . . . 20
9.5 Initial Namespace Registrations . . . . . . . . . . . . . 21
9.5.1 Namespace drsn . . . . . . . . . . . . . . . . . . . . 21
9.5.2 Namespace dsn . . . . . . . . . . . . . . . . . . . . 21
9.5.3 Namespace ets . . . . . . . . . . . . . . . . . . . . 21
9.5.4 Namespace q735 . . . . . . . . . . . . . . . . . . . . 22
9.5.5 Namespace wps . . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 23
11.2 Informative References . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . 26
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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
the Session Initiation Protocol (SIP) [RFC3261], including
voice-over-IP, multimedia conferencing, instant messaging and
presence.
SIP 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".
The mechanism itself may well be in place at all times, but only
materially affect call handling during times of resource scarcity.
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
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header field MAY be used by SIP user agents, including General
Switched Telephone Network (GSTN) gateways and terminals, and SIP
proxy servers to influence their treatment of SIP requests, including
the priority afforded to GSTN calls. For 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. ITU Recommendation I.255.3
[I.255.3] is another example.
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
GSTN gateway or proxy resources, for example. In addition, any User
Agent Client (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
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of this mechanism is wholly invisible to existing implementations
unless the request includes the Require header field with the
resource-priority option tag.
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 aims to satisfy the requirements in [RFC3487]. It is
structured so that it works in all SIP and Real-Time Transport
Protocol (RTP) [RFC3550] transparent networks defined in [RFC3487].
In such networks, 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 GSTN gateway or set of SIP elements that are aware of the
mechanism.
For conciseness, we refer to SIP proxies and user agents (UAs) 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 7.
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.
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 user agent clients (UACs)
in Section 4.3, for UAS in Section 4.4 and for SIP proxy servers in
Section 4.5.
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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
likely that responses contain the same 'Resource-Priority' header
field value as the requests, local policy MAY call for the UAS to
insert no header field or a different value.
There is no requirement that all requests within a SIP dialog or
session use the 'Resource-Priority' header field. Again, local
policy dictates the appropriate behavior; thus, implementations MUST
be configurable accordingly.
The syntax of the 'Resource-Priority' header field is described
below. The "token-nodot" production is copied from [RFC3265].
Resource-Priority = "Resource-Priority" HCOLON
r-value *(COMMA r-value)
r-value = namespace "." r-priority
namespace = token-nodot
r-priority = token-nodot
token-nodot = 1*( 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-insensitive ASCII. Each
namespace has at least one priority value. Namespaces and priority
values within each namespace MUST be registered with IANA (Section
9). Initial namespace registrations are described in Section 9.5.
There may be multiple resource values or, equivalently, multiple
'Resource-Priority' header field instances.
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3.2 The 'Accept-Resource-Priority' Header Field
The 'Accept-Resource-Priority' response header field enumerates the
resource values a SIP user agent server is willing to accept. The
syntax of the 'Accept-Resource-Priority' header field is as follows:
Accept-Resource-Priority = "Accept-Resource-Priority" HCOLON
[r-value *(COMMA r-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
The following table extends the values in Table 2 of RFC3261
[RFC3261]. (The PRACK method, labeled as PRA, is defined in
[RFC3262], the SUBSCRIBE (labeled SUB) and NOTIFY (labeled NOT)
methods in [RFC3265], the UPDATE (UPD) method in [RFC3311], the
MESSAGE (MSG) method in [RFC3428], the REFER (REF) method in
[RFC3515], the INFO (INF) method in [RFC2976], and the PUBLISH (PUB)
method in [I-D.ietf-sip-publish].)
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
Resource-Priority 200 - o - o o o o o
Accept-Resource-Priority 200 - o o o o o o o
Accept-Resource-Priority 417 - m - m m m m m
Accept-Resource-Priority 420 - o - o o o o o
Header field where proxy SUB NOT UPD MSG REF INF PUB
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
Resource-Priority 200 - o o o o o o o
Accept-Resource-Priority 200 - o o o o o o o
Accept-Resource-Priority 417 - m m m m m m m
Accept-Resource-Priority 420 - o o o o 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
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value contained in the request.
While all methods listed above allow the optional use of the
'Resource-Priority' header fields, only request methods that start a
dialog or deliver content, such as MESSAGE, are likely to benefit
from this mechanism and other methods SHOULD NOT use them. This
consideration also applies to methods not listed above.
3.4 The 'resource-priority' Option Tag
This document also defines the "resource-priority" option tag. The
behavior is described in Section 4.2.2 and the IANA registration is
in Section 9.2.
4. Behavior of SIP Elements that Receive Prioritized Requests
4.1 General Rules
All SIP 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, ACK, PRACK [RFC3262],
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 and priority values,
the request is treated according to the highest priority value which
the recipient supports and authorizes. The total ordering of
priorities between different namespaces is defined by local policy.
An OPTIONS request can be used to determine if an element supports
the mechanism. A compliant implementation MUST return a
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'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.) Note
that according to RFC 3261, proxies reached with a Max-Forwards value
of zero answer the OPTIONS request, allowing a UAC to discover the
capabilities of both proxy and user agent servers.
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.
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 namespaces or priority values,
elements can operate in two modes, "strict" and "loose", identified
by the presence or absence of a 'Require' header field with the
'Resource-Priority' option tag. If the request includes a 'Require'
header field with the 'Resource-Priority' option tag, a UAS MUST
follow the strict-mode rules, otherwise UAS and proxies MUST operate
in loose mode. Both are described in detail below.
The use of the 'resource-priority' option tag in 'Proxy-Require'
header field is NOT RECOMMENDED.
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4.2.2.1 Strict Mode
Following standard SIP behavior (Section 8.2.2.3 of [RFC3261]), a UAS
operating in strict mode MUST reject the request with response code
420 (Bad Extension) if it does not support the resource priority
option tag.
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.
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). Implementations MUST,
as a configurable option, support returning a
'Accept-Resource-Priority' header field enumerating all the resource
values that the server is willing to process, but network operators
MAY disable returning this information. Note that the user may not
be authorized to use all of these resource values.
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.
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4.4 User Agent Server Behavior
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
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 existing 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, MUST describe
the desired behavioral properties of user agents and proxy servers.
4.5 Proxy Behavior
SIP proxies MAY ignore or inspect the 'Resource-Priority' header
field. SIP proxies MAY reject any unauthenticated request bearing
the header field.
If there are multiple namespace or priority choices available to the
user agent client, a proxy MAY return the request with an appropriate
'Accept-Resource-Priority' header field. Details are a matter of
local policy.
If the header fields are protected via S/MIME encapsulation in a SIP
message fragment [RFC3420], the 'Resource-Priority' value cannot be
changed or added by proxies.
If no S/MIME is used, SIP proxies MAY downgrade or upgrade the
'Resource-Priority' of a request or insert a new 'Resource-Priority'
header if allowed by 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
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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 treatment 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 retarget to a SIP node or SIP URI 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).
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-authid-body] 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.
6. Backwards Compatibility
The resource priority mechanism described in this document is fully
backwards compatible with SIP systems following [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 backwards
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compatibility, as systems that do not support these mechanism are no
better off 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.
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.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserA@client.atlanta.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
F2 180 Ringing User B -> User A
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SIP/2.0 180 Ringing
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserB@client.biloxi.example.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.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserB@client.biloxi.example.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.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Resource-Priority: dsn.flash
Contact: <sip:UserA@client.atlanta.example.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.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.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.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: 0
F3 ACK User A -> User B
ACK sip:UserB@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bd5
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 ACK
Content-Length: 0
F4 INVITE User A -> User B
INVITE sip:UserB@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: BigGuy <sip:UserA@atlanta.example.com>;tag=9fxced76sl
To: LittleGuy <sip:UserB@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Resource-Priority: q735.3
Contact: <sip:UserA@client.atlanta.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: ...
...
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.
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All user agents and proxy servers which support this extension MUST
implement SIP over TLS [RFC3546] and the sips: URI scheme as
described in Section 26.2 of RFC 3261, and Digest Authentication
[RFC2617] as described in Section 22 of RFC 3261. In addition, user
agents which support this extension SHOULD also implement S/MIME
[RFC2633] as described in Section 23 of RFC 3261 to allow for signing
and verification of signatures over requests which use this
extension.
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, receipt of asserted identity [RFC3325]
from a trusted entity may be sufficient authorization. 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. With
trait-based authorization, a network element can directly determine,
by inspecting the certificate, that a request is authorized to obtain
a particular type of service, without having to consult a mapping
mechanism that converts user identities to authorizations.
Authorization may be based on factors beyond the identity of the
caller, such as the requested destination. Namespaces MAY also
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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' and 'Subject' 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]. To be considered valid
authentication for the purposes of this specification, S/MIME signed
SIP messages or fragments MUST contain, at a minimum, the Date, To,
From, Call-ID, and Resource-Priority header fields. Encapsulation in
S/MIME body parts allows the user to protect this header field
against inspection or modification by proxies. 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
elements carrying the request where they desire resource priority
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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 mechanism described in [RFC3323].
8.4 Denial-of-Service Attacks
As noted, systems described here are likely to be subject to
deliberate denial-of-service (DoS) 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.
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
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RFC number: XXXX
Response code: 417
Default reason phrase: Unknown Resource-Priority
9.4 IANA Namespace and Priority Registrations
Additional namespaces and priority values MUST be 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 defining 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 ahead 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. If the namespace does not define a particular
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policy, the term 'implementation-defined' should be used.
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 drsn
Namespace: drsn
Description: United States Defense Red Switched Network.
Organization: United States Department of Defense, Defense
Information Systems Agency (DISA).
Policy: Preemption with rejection.
Priority values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override", "flash-override-override"
9.5.2 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).
Policy: Preemption with rejection.
Priority values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override"
9.5.3 Namespace ets
Namespace: ets
Description: Emergency Telecommunications Service (ETS) specifies
Signaling System No. 7 (SS7) and Bearer Independent Call Control
(BICC) protocols for (Government) Emergency Telecommunications
Service (ETS) for authorized users with national security and
emergency preparedness responsibilities. The namespace "ets"
supports interworking with ATIS.ETS (or equivalent) PSTN,
including ISDN, entities. This interworking allows, for example,
carrying signaling information between ATIS.ETS entities without
loss of information. One or both of the SIP endpoints might be
PSTN gateways. The namespace "ets" allows, based on policy, to
provide priority treatment in processing and allocating resources
to call and session requests marked with this namespace.
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Documentation: ATIS.ETS
Organization: Alliance for Telecommunications Industry Solutions
(ATIS)
Policy: Implementation-defined.
Priority values (least to greatest): "4", "3", "2", "1", "0"
9.5.4 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
Policy: Precedence.
Priority values (least to greatest): "4", "3", "2", "1", "0"
9.5.5 Namespace wps
Namespace: wps
Description: Description: Wireless Priority Service (WPS) extends
(Government) Emergency Telecommunications Service (ETS)-like
priority treatment to authorized users with national security and
emergency preparedness responsibilities who use mobile terminals
to place voice or voice-band data calls. The namespace "wps"
supports interworking with ATIS.TR.WPS (or equivalent) PSTN,
including ISDN, entities. This interworking allows, for example,
carrying signaling information between ATIS.TR.ETS entities
without loss of information. One or both of the SIP endpoints
might be PSTN gateways. The namespace "wps" allows, based on
policy, to provide priority treatment in processing and allocating
resources to call or session requests, including air interface,
labeled with this namespace.
Documentation: ATIS.TR.ETS
Organization: Alliance for Telecommunications Industry Solutions
(ATIS)
Policy: Implementation-defined.
Priority values (least to greatest): "4", "3", "2", "1", "0"
10. Acknowledgments
Ben Campbell, Janet Gunn, Paul Kyzivat, Rohan Mahy, Mike Pierce and
Samir Srivastava provided helpful comments.
Martin Dolly, An Nguyen and Niranjan Sandesara assisted with the
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sections on the NCS namespaces.
11. References
11.1 Normative References
[I.255.3] International Telecommunications Union, "Integrated
Services Digital Network (ISDN) - General Structure and
Service Capabilities - Multi-Level Precedence and
Preemption", Recommendation I.255.3, July 1990.
[Q.735.3] International Telecommunications Union, "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.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, 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.
11.2 Informative References
[I-D.ietf-ieprep-framework]
Carlberg, K., Brown, I. and C. Beard, "Framework for
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Supporting ETS in IP Telephony",
draft-ietf-ieprep-framework-09 (work in progress), April
2004.
[I-D.ietf-sip-authid-body]
Peterson, J., "SIP Authenticated Identity Body (AIB)
Format", draft-ietf-sip-authid-body-03 (work in progress),
May 2004.
[I-D.ietf-sip-publish]
Niemi, A., "An Event State Publication Extension to the
Session Initiation Protocol (SIP)",
draft-ietf-sip-publish-04 (work in progress), May 2004.
[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.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A. and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2633] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999.
[RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976, October
2000.
[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.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
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[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 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.
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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