SIP Working Group J. Polk
Internet-Draft Cisco
Expires: August 21st, 2005 H. Schulzrinne
Columbia U.
February 21st, 2005
Communications Resource Priority for the Session Initiation Protocol
(SIP)
draft-ietf-sip-resource-priority-06
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
This document defines two new SIP header fields for communicating
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 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 . . . . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . . . . . 9
4.2 Usage of Require Header with Resource-Priority . . . . . . . 9
4.3 OPTIONS Request with Resource-Priority . . . . . . . . . . . 9
4.4 Alternatives for Preferential Treatment of Requests . . . . 10
4.4.1 Preemption Policy . . . . . . . . . . . . . . . . . . . 10
4.4.2 Priority Queuing Policy . . . . . . . . . . . . . . . . 10
4.5 Rejection Messages . . . . . . . . . . . . . . . . . . . . 11
4.6 Error Conditions . . . . . . . . . . . . . . . . . . . . . 12
4.6.1 Known Namespace and Priority Value . . . . . . . . . . 12
4.6.2 Handling Unknown Namespaces and Priority Values . . . 12
4.7 User Agent Client Behavior . . . . . . . . . . . . . . . . 13
4.7.1 User Agent Client Behavior with a Preemption Policy . . 13
4.7.2 User Agent Client Behavior with a Priority Queue Policy 13
4.8 User Agent Server Behavior . . . . . . . . . . . . . . . . 13
4.8.1 User Agent Servers and Preemption Policy . . . . . . . . 14
4.8.1 User Agent Servers and Priority-Queue Policy . . . . . . 14
4.9 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 14
5. Third-Party Authentication . . . . . . . . . . . . . . . . . . 15
6. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 15
7. Multiple Namespaces in a Message . . . . . . . . . . . . . . 16
8. Namespace Definitions . . . . . . . . . . . . . . . . . . . . 18
8.1 Namespace Descriptions . . . . . . . . . . . . . . . . . . . 18
8.1.1 The "DSN" Namespace . . . . . . . . . . . . . . . . . . 18
8.1.2 The "DRSN" Namespace . . . . . . . . . . . . . . . . . . 19
8.1.3 The "Q735" Namespace . . . . . . . . . . . . . . . . . . 20
8.1.4 The "ETS" Namespace . . . . . . . . . . . . . . . . . . 20
8.1.5 The "WPS" Namespace . . . . . . . . . . . . . . . . . . 22
8.2 Future Namespace Considerations . . . . . . . . . . . . . . 24
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1 Simple Call . . . . . . . . . . . . . . . . . . . . . . . 25
9.2 Receiver Does Not Understand Namespace . . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10.1 Authentication and Authorization . . . . . . . . . . . . . 29
10.2 Confidentiality and Integrity . . . . . . . . . . . . . . 30
10.3 Anonymity . . . . . . . . . . . . . . . . . . . . . . . . 31
10.4 Denial-of-Service Attacks . . . . . . . . . . . . . . . . 31
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
11.1 IANA Registration of 'Resource-Priority' and
'Accept-Resource-Priority' Header Fields . . . . . . . . . 32
11.2 IANA Registration for Option Tag resource-priority . . . . 32
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11.3 IANA Registration for Response Code 417 . . . . . . . . . 32
11.4 IANA Namespace Registrations . . . . . . . . . . . . . . . 32
11.5 IANA Priority-Value Registrations . . . . . . . . . . . . 33
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
13.1 Normative References . . . . . . . . . . . . . . . . . . . 34
13.2 Informative References . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 36
Intellectual Property and Copyright Statements . . . . . . . . 36
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
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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 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 delay, reject or error existing signaling requests
of 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 are 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
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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
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.
The remainder of this document is structured as follows. After
defining terminology in Section 2, we define the syntax for the two
new SIP header fields in Section 3 and then describe protocol
behavior in Section 4. The two principal mechanisms for
differentiated treatment of SIP requests, namely preemption and
queuing, are described in Section 4.4. Rejection messages are
covered in Section 4.5, error conditions in Section 4.6. Section 4.7
through Section 4.9 detail the behavior of specific SIP elements.
Third-party authentication is briefly summarized in Section 5.
Section 6 describes how this feature affects existing systems that
do not support it. Sections 7 and 8 delve into namespace issue,
namely their general properties, how to deal with multiple
namespaces on the same SIP element and in the same SIP message,
as well as enumerating five namespaces that are registered through
this document. Protocol examples are given in Section 9. Security
issues are considered in Section 10, but this document does not
define new security mechanisms.
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
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implementations.
3. The Resource-Priority and Accept-Resource-Priority SIP Header Fields
This document defines the 'Resource-Priority' and
'Accept-Resource-Priority' SIP header field syntax. Behavior is
described in Section 4..
The SIP element behavior is described for user agent clients (UACs)
in Section 4.7, for UAS in Section 4.8 and for SIP proxy servers in
Section 4.9.
3.1 The 'Resource-Priority' Header Field
The 'Resource-Priority' header field marks a SIP request as desiring
prioritized access to resource, 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 those 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 protocol requirement that all requests within a SIP
dialog or session use the 'Resource-Priority' header field. Local
administrative policy MAY mandate the inclusion of the 'Resource-
Priority' header field in all requests. Implementations of this
specification MUST allow inclusion to be either by explicit user
request or automatic.
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: dsn.flash
The 'Resource-value' parameter in the 'Resource-Priority' header
indicates the resource priority desired by the request originator.
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 tokens that do not contain periods. Thus, ædsn.flashÆ and
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æDSN.FlashÆ, for example, are equivalent. Each namespace has at
least one priority value. Namespaces and priority values within
each namespace MUST be registered with IANA (Section 11). Initial
namespace registrations are described in Section 11.4.
Since a request may traverse multiple administrative domains with
multiple different namespaces, it is necessary to be able to
enumerate several different namespaces within the same message.
However, a particular namespace MUST NOT appear more than once
in the same SIP message. These may be expressed equivalently as
either comma-separated lists within a single header field, as
multiple header fields or some combination. The ordering of r-values
within the header field has no significance. Thus, for example, the
following three header snippets are equivalent:
Resource-Priority: dsn.flash, wps.3
Resource-Priority: wps.3, dsn.flash
Resource-Priority: wps.3
Resource-Priority: dsn.flash
3.2 The 'Accept-Resource-Priority' Header Field
The 'Accept-Resource-Priority' response header field enumerates the
resource values (r-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
Some administrative domains MAY choose to disable the use of the
Accept-Resource-Priority header as revealing too much information
about that domain in responses. However, this behavior is NOT
RECOMMENDED, as this header field aids in troubleshooting.
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
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[RFC3515], the INFO (INF) method in [RFC2976], and the PUBLISH (PUB)
method is in [RFC3903].)
Header field where proxy INV ACK CAN BYE REG OPT PRA
----------------------------------------------------------------
Resource-Priority R amdr o o o o o o o
Resource-Priority r amdr o o o o o o o
Resource-Priority 200 amdr o - o o o o o
Accept-Resource-Priority 200 amdr o - o o o o o
Accept-Resource-Priority 417 amdr o - o o o o o
Accept-Resource-Priority 420 amdr 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 r amdr o o o o o o o
Resource-Priority 200 amdr o o o o o o o
Accept-Resource-Priority 200 amdr o o o o o o o
Accept-Resource-Priority 417 amdr o o o o o o o
Accept-Resource-Priority 420 amdr 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.
The 'Accept-Resource-Priority' SHOULD be returned in 420 (Bad
Extension) 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 this current request.
3.4 The 'resource-priority' Option Tag
This document also defines the "resource-priority" option tag. The
behavior is described in Section 4.2. and the IANA registration is
in Section 11.2.
4. Behavior of SIP Elements that Receive Prioritized Requests
All SIP user agents and proxy servers that support this
specification share certain common behavior, which we describe
below in Section 4.1. The behavior when encountering a 'resource-
priority' option tag in a 'Require' header field is describe in
Section 4.2. Section 4.3 describes the treatment of OPTIONS
requests. The two fundamental resource contention resolution
mechanisms, preemption and queuing, are described in Section 4.4.
Rejection Behavior specific to user agent clients is covered in
section 4.7, for user agent servers in Section 4.8, while Section
4.9 deals with proxy behavior.
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4.1 General Rules
The Resource-Priority header field is potentially applicable to all
SIP request messages. At a minimum, implementations of the following
request types MUST support the Resource-Priority header to be in
compliance with this specification:
1) INVITE [RFC3261]
2) ACK [RFC3261]
3) PRACK [RFC3262]
4) UPDATE [RFC3311]
5) REFER [RFC3515]
and SHOULD support the Resource-Priority header in the following
request types (if the SIP element does) to be in compliance with
this specification:
6) MESSAGE [RFC3428]
7) SUBSCRIBE [RFC3265]
8) NOTIFY [RFC3265]
Note that this does not imply all implementations to support every
request method listed.
If a SIP element receives the Resource-Priority header in a Request
message other than what is listed above, the header MAY be ignored,
according to the foundational rules of [RFC3261].
4.2 Usage of Require Header with Resource-Priority
Following standard SIP behavior, if a SIP request contains the
Require header field with the æresource-priorityÆ option tag, a SIP
element MUST respond with a 420 (Bad Extension) if it does not
support the SIP extensions described in this document. It then lists
æresource-priorityÆ in the æUnsupportedÆ header field included in
the response.
The use of the 'resource-priority' option tag in 'Proxy-Require'
header field is NOT RECOMMENDED.
4.3 OPTIONS Request with Resource-Priority
An OPTIONS request can be used to determine if an element supports
the mechanism. A compliant implementation MUST return a
'Accept-Resource-Priority' header field in OPTIONS responses
enumerating all valid resource values. An RP actor MAY be
configured not to return such values or only return them to
authorized requestors.
Note that according to RFC 3261, proxies reached with a Max-Forwards
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value of zero answer the OPTIONS request, allowing a UAC to discover
the capabilities of both proxy and user agent servers.
4.4 Alternatives for Preferential Treatment of Requests
SIP elements may use the resource priority mechanism to modify a
variety of behavior such as routing requests, authentication
requirements or logging. the request itself or of the session
created by the request. We use the terms session and call
interchangeably in this document, both implying a continuous data
stream between two or more parties. Sessions are established by SIP
dialogs.
Below, we define two common policies, namely preemption and priority
queuing. Preemption applies only to sessions created by SIP
requests, while both sessions and request handling can be subject to
priority queuing. Both policies can sometimes be combined in the
same element, although none of the namespaces described in this
document do this. Policies can be defined for each namespace or, in
some cases, can be specific to an administrative domain.
Naturally, only SIP elements that understand this mechanism and the
namespace and resource value perform these policies. Section 4.8
discusses what happens if an RP actor does not understand namespaces
or priority values contained in a request.
4.4.1 Preemption Policy
An RP actor following a preemption policy may disrupt an existing
session to make room for a higher-priority incoming session. Since
sessions may require different amounts of bandwidth or number of
circuits, a single higher priority session may displace more than
one lower-priority session. As noted above, the processing of SIP
requests itself is not preempted. Thus, since proxies do not manage
sessions, they do not perform preemption.
[] for more details
and examples of this behavior.
UAS behavior for preemption is discussed in Section 4.8.
4.4.2 Priority Queuing Policy
In a priority queuing policy, requests that find no available
resources are queued on a first-come, first-served basis to the
queue assigned to the priority value. Each priority value may have
its own queue or several priority values may share a single queue.
If a resource becomes available, a request from the queue with the
highest priority is served. Each queue can hold a finite number of
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pending requests. If the queue for a newly arriving request is full,
the request is rejected immediately. In addition, a priority queuing
policy MAY impose a residence time limit. If a request exceeds a
specified waiting time, the RP actor then rejects the request and
removes it from the queue.
In addition, an RP actor MAY impose a global queue size limit summed
across all queues and drop waiting lower-priority requests. This
does not imply preemption since the session has not been established
yet.
Often, normal, non-prioritized requests will not be queued, i.e.,
the queue size for such requests is zero.
4.5 Rejection Messages
The following is a list of rejections to SIP messages that contain a
Resource-Priority header specifically because of the contents of the
header.
If a UA is currently occupied with another session and receives a
dialog generating message containing a valid Resource-Priority
header of equal or lower relative priority value, the response is
the same as stated in section 13.3.1.3 of [RFC3261]:
- a 486 (Busy Here) is returned if the UAS knows it cannot or will
not accept the request,
- a 600 (Busy Everywhere) is returned if the UAS knows there are no
other SIP elements that can accept the INVITE, and
- a 488 (Not Acceptable Here) if the UAS is rejecting the INVITE.
[RFC3261] advises that a 488 response SHOULD include a warning
header with a reason for the rejection.
If the message from the UAC contains a known namespace, and the
priority-value is higher than is authorized, this error condition is
addressed in the next subsection (4.6).
In the case in which a UAS is currently occupied with another
session and receives an INVITE containing a valid Resource-Priority
header of higher relative priority than that of the current dialog,
this does not create an error condition. The UAS will act according
to the behaviors defined for that namespace within that
administrative domain. Section 8.1.1 through 8.1.5 define the rules
for the 5 namespaces that are created with this document.
It can also be stated that if a Proxy Server is currently
experiencing processing difficulties (perhaps due to overload
conditions), this is an error condition that will be addressed in
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section 4.6.1.
4.6 Error Conditions
4.6.1 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 if no Resource-Priority header
was present in the original request (which may be local policy
not to include one for normal level calls), to add a resource
priority indication, if authorized.
4.6.2 Handling Unknown Namespaces and Priority Values
When handling requests with unknown namespaces or priority values,
elements will operate according to local policy. Some
administrative domains will openly reject with a 417 (Unknown
Resource-Priority) Response message regardless of whether the
namespace or header field (or both) are not known, giving no further
details to the requestor. Other administrative domains will return
417 (Unknown Resource-Priority) Response with an Accept-Resource-
Priority header indicating which valid namespace(s) and priority-
value(s) are "good" within that domain. As previously stated, a
single SIP message can have more than one Resource-Priority header.
If one header is understood, the remaining Resource-Priority headers
SHOULD NOT be modified, deleted or cause a rejection of the message.
The reason for this is that there will be cases where multiple
namespaces will be necessary for end-to-end communications.
Allowing multiple instances of the header in the same message allows
the message to traverse multiple administrative domains - without
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each having to know about the policy of the other(s) - all while
providing a valid resource-Priority per administrative domain. An
example of this is the case where the ETS namespace will be enabled
by the US Government networks (used by very few individuals within
that administrative domain) that also support the DSN and/or DRSN
namespaces for most (if not all) individuals in those domains.
These namespaces will be defined later in this document.
Some SIP elements MAY allow process messages with unknown Resource-
Priority headers without returning a 417. This is a matter of local
policy.
4.7 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. As stated previously, the UAC SHOULD be
able to generate more than one resource value in a single SIP request.
If a 417 (Unknown Resource-Priority) is received, the
UAC MAY attempt a subsequent request with the same combination
of namespace/priority-value, or the 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, if included, and if authorized for that user.
4.7.1 User Agent Client Behavior with a Preemption Policy
A UAC in an administrative domain that employs preemption MUST be
prepared to receive a BYE Request with a Reason header explaining
why the session was terminated.
[] discusses this.
4.7.2 User Agent Client Behavior with a Priority Queue Policy
By standard SIP protocol rules, a UAC MUST be prepared to receive a
182 (Queued) response from an RP actor that is currently at
capacity, but has put the original request into a queue. This UAC
SHOULD indicate this queued status to the user by some audio or
visual indication to prevent the user from interpreting the call as
having failed.
4.8 User Agent Server Behavior
The precise effect of the 'Resource-Priority' indication depends on
the type of UAS, the namespace and local policy.
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4.8.1 User Agent Servers and Preemption Policy
A UAS compliant with this specification MUST terminate a session
established with a valid namespace and lower priority value in favor
of a new session set-up with a valid namespace and higher relative
priority-value, unless local policy has some form of call-waiting
capability enabled. If a session is terminated, the BYE method is
used with a Reason header indicating why/where the preemption took
place.
[] provides additional
information in the case of purposeful or administrative termination
of a session by including the Reason header in the BYE message that
states why the BYE was sent (in this case, a preemption event).
That document offers several reasons for where the termination
occurred ('at the UA', 'in the network', 'at a IP/PSTN gateway'),
and includes call flow examples of each reason.
4.8.2 User Agent Servers and Priority-Queue Policy
A UAS compliant with this specification SHOULD generate a 182
(Queued) Response if that element's resources are busy, until it is
able to handle the request and provide a final response.
Local policy will determine additional behaviors of a queue-based
SIP element. However, the frequency of provisional messages
indicated in [RFC3261] still need to apply to keep each session set-
up active until successful or rejected.
4.9 Proxy Behavior
SIP proxies MAY ignore or inspect the 'Resource-Priority' header
field. SIP proxies MAY reject any unauthenticated request bearing
the header field.
A compliant SIP Server experiencing a processing queue due to
excessive load MUST process messages containing higher relative
priority-values up to the queue the header indicates on a FIFO basis
based on the arrival time of the message. It is conceivable
that priority-levels within a proxy are grouped to include more than
one level per queue. This is a matter of local policy.
If a Proxy is expecting a message to have a 'Resource-Priority'
header and the header is protected via S/MIME encapsulation in a SIP
message fragment [RFC3420], the Proxy MUST return an error response.
Therefore, the use of SIPFRAG in administrative domains with this
type of policy is not recommended.
If no S/MIME is used, SIP proxies MAY downgrade or upgrade the
'Resource-Priority' of a request or insert a new 'Resource-Priority'
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Internet-Draft SIP Resource Priority February 21st, 2005
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.
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.
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.
When the 'Require' header is included in a message, it ensures that
in parallel forking, only branches that support the resource-
priority mechanism succeed. There are not additional special
considerations for proxies when forking requests that contain a
resource priority indication.
Otherwise, the proxy behavior is the same as for user agent servers
Section 4.7).
5. Third-Party Authentication
In some cases, 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
[RFC3893] 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
compatibility, as systems that do not support these mechanism are no
better off rejecting the request due to feature failure. Since the
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Internet-Draft SIP Resource Priority February 21st, 2005
intent of resource priority indications is to increase the
probability of call completion, adding failure modes appears
counterproductive.
7. Multiple Namespaces in a Message
There are two cases where multiple namespaces might occur. Namely,
a single RP actor may understand more than one namespace and a SIP
request may contain more than one resource value from different
namespaces. As noted earlier, the order of resource values in a
SIP request is immaterial.
o If a SIP element understands multiple namespaces, it must create
a total ordering across all resource values from these
namespaces, maintaining the relative ordering within each
namespace. It is RECOMMENDED that the same ordering is used
across an administrative domain.
o If a 'Resource-Priority' header contains multiple namespaces, the
RP actor MUST select one resource value, typically the one with
the highest ranking.
o If a SIP element supports this specification and the request
includes a 'Require' header field with the 'Resource-Priority'
option tag, it MUST be possible to configure a UAS to copy the
Resource-Priority header value or values from the Request to the
Response without changing any field values (this means the
Offerer is in control of this header's value within an
administrative domain).
This rule MUST be followed even if multiple instances of the
Resource-Priority header exist in a Request, unless the UAS lacks
the authorization to copy unknown header fields. User authorization
of priority-values within a namespace is outside the scope of this
document.
Here is a series of examples (both good and bad) of a SIP element
that supports two namespaces (foo and bar). Foo's priority-values
are 3 (highest), then 2 and then 1 (lowest ), and bar's priority-
values are C (highest), then B and then A (lowest).
The following are 3 lists of acceptable priority orders the SIP
element MAY process are:
Foo.3 Foo.3 Bar.C (Highest Priority)
Foo.2 Bar.C Foo.3
Foo.1 or Foo.2 or Foo.2
Bar.C Bar.B Foo.1
Bar.B Foo.1 Bar.B
Bar.A Bar.A Bar.A (Lowest Priority)
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Bar.C (Highest Priority)
Foo.3 Bar.B <= both are equivalently processed FIFO)
or Foo.2 Bar.A <= both are equivalently processed FIFO)
Foo.1 (Lowest Priority)
Bar.C (Highest Priority)
Foo.3
or Foo.2
Foo.1 (Lowest Priority)
Where Bar.A and Bar.B are ignored within this example administrative
Domain;
Based on the stated priority order of each namespace above, the
following (non-inclusive list of) combinations are NOT acceptable
and MUST NOT be configurable:
Example 1 Example 2 Example 3
--------- --------- ---------
Foo.3 Foo.3 Bar.C
Foo.2 Bar.A Foo.1
Foo.1 or Foo.2 or Foo.3
Bar.C Bar.B Foo.2
Bar.A Foo.1 Bar.A
Bar.B Bar.C Bar.B
Example 4
---------
Bar.C
Foo.1 Bar.B
or Foo.3 Bar.A
Foo.2
In Example 1 above, Bar.A is ordered higher than Bar.B - which is
not consistent within the order of the namespace of this section.
In Example 2 above, Bar.A is ordered higher than Bar.B and Bar.C -
which is not consistent within the order of the namespace of this
section.
In Example 3 above, Foo.1 is ordered higher than Foo.2 and Foo.3 -
which is not consistent within the order of the namespace of this
section.
In Example 4 above, Foo.1 is ordered higher than Foo.3 and Foo.2 is
ignored - which is not consistent within the order of the namespace
of this section.
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8. Namespace Definitions
This specification defines five unique namespaces below: dsn, drsn,
q735, ets and wps, constituting their registration with IANA. Each
IANA registration contains:
1) The namespace label, a unique namespace label within the IANA
registry for the Resource-Priority header;
2) Number of Priority levels - the number of relative priority
levels the namespace has defined;
3) The "Intended Operation" - identifying whether the namespace is
to be used with priority queuing or preemption;
4) Any new Rejection codes or behaviors unique to the namespace
being defined
5) Any new Error codes specific to the namespace being defined
6) The IETF Reference document specifying the parameters and
specifications of the namespace, the listing of the priority-
values in relative order, any new rejection messages, and any new
error codes or error behaviors.
8.1 Namespace Descriptions
8.1.1 The "DSN" Namespace
The DSN namespace comes from the name of a US Government network
called "The Defense Switched Network".
The DSN namespace has a finite list of relative priority-values
listed here in the order of lowest priority to highest priority:
(lowest) dsn.routine
dsn.priority
dsn.immediate
dsn.flash
(highest) dsn.flash-override
In compliant administrative domains utilizing the dsn namespaces,
the UAS MUST copy the Resource-Priority header priority-values from
the Request to the Response for every header in the message unless
the UAS lacks local authorization to copy certain
namespace.priority-value combinations.
The dsn namespace introduces no new error behaviors.
The IANA Considerations section will have the following entry for
the DSN namespace:
Polk & Schulzrinne [page 18]
Internet-Draft SIP Resource Priority February 21st, 2005
Intended New New Error
Namespace Levels Operation Rej. code code Reference
--------- ------ ---------------- --------- ---- ---------
dsn 5 preemption-based no no [This RFC]
8.1.2 The "DRSN" Namespace
The DRSN namespace comes from the name of a US Government network
called "The Defense RED Switched Network".
The DRSN namespace has a finite list of relative priority-values
listed here in the order of lowest priority to highest priority:
(lowest) drsn.routine
drsn.priority
drsn.immediate
drsn.flash
drsn.flash-override
(highest) drsn.flash-override-override
One unique facet regarding the DRSN namespace relative to the DSN
and Q735 namespaces is the behavior of SIP elements with regard to
the 6th priority-value:
drsn.flash-override-override
This is the highest relative priority-value within the DRSN
namespace, but it is only to be used at this higher level in message
processing in SIP Proxies, and dialog establishment in SIP UAs
(including gateways). In any UAS, once the session is established
at the drsn.flash-override-override priority level, it defends that
dialog with a priority level of drsn.flash-override only; thus
allowing a new drsn.flash-override-override to preempt any existing
session. This behavior MUST be followed to be compliant with this
specification.
In compliant administrative domains utilizing the drsn namespaces,
the UAS MUST copy the Resource-Priority header priority-values from
the Request to the Response for every header in the message unless
the UAS lacks local authorization to copy certain (perhaps
restricted) namespace.priority-value combinations.
The drsn namespace operates on the preemption policy.
The drsn namespace introduces no new error behavior.
The IANA Considerations section will have the following entry for
the DRSN namespace:
Polk & Schulzrinne [page 19]
Internet-Draft SIP Resource Priority February 21st, 2005
Intended New New Error
Namespace Levels Operation Rej. code code Reference
--------- ------ ---------------- --------- ---- ---------
drsn 6 preemption-based no no [This RFC]
8.1.3 The "Q735" Namespace
The Q735 namespace is defined here as Q.735.3 was defined, namely as
operationally equivalent to the DSN specification for MLPP. Q.735.3
was created to be a commercial version of the same specification.
The Q735 namespace has a finite list of relative priority-values
listed here in the order of lowest priority to highest priority:
(lowest) q735.4
q735.3
q735.2
q735.1
(highest) q735.0
In compliant administrative domains utilizing the q735 namespaces,
the UAS MUST copy the Resource-Priority header priority-values from
the Request to the Response for every header in the message unless
the UAS lacks local authorization to copy certain (perhaps
restricted) namespace.priority-value combinations.
The q735 namespace operates according to the preemption policy.
The q735 namespace introduces no new error behaviors.
The IANA Considerations section will have the following entry for
the Q735 namespace:
Intended New New Error
Namespace Levels Operation Rej. code code Reference
--------- ------ ---------------- --------- ---- ---------
q735 5 preemption-based no no [This RFC]
8.1.4 The "ETS" Namespace
The ETS namespace derives its name indirectly from the name of the
US Government Telecommunications Service called "Government
Emergency Telecommunications Service" (or GETS), though the
organization responsible for the GETS service chose the acronym
"ETS" for its GETS over IP service, which stands for "Emergency
Telecommunications Service".
The ETS namespace has a finite list of relative priority-values
listed here in the order of lowest priority to highest priority:
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(lowest) ets.4
ets.3
ets.2
ets.1
(highest) ets.0
The ets namespace operates according to the priority queuing policy.
ETS-based administrative domains will not queue normal, non-
prioritized requests.
ETS-based administrative domains will not queue normal, non-
prioritized messages. If a queue depth exists, the messages will be
rejected according to section 4.5.
A UAS within this type of administrative domain SHOULD be
configurable to limit the number of requests in a queue. One of two
different ways of limiting the queue depth is RECOMMENDED:
1) having a maximum number within a queue (perhaps with a different
number queued per priority-level), and
2) consider all requests above the default level to be in a single
queue on a FIFO basis (for example, based on the reception
timestamp of the request)
A UAS within this type of administrative domain SHOULD be
configurable to limit the time a request sits in a queue. One of
two different ways of limiting the time in queue is RECOMMENDED:
1) having a maximum time within a queue (perhaps with a different
time per priority-level) before a/each message moves into a
rejection condition discussed in section 4.5., and
2) considering all requests above the default level in a single
queue on a FIFO basis (for example, based on the reception
timestamp of the request), all with the same maximum time in
queue before a/each message moves into a rejection condition
discussed in section 4.5.
Local policy will determine additional behaviors of a queue-based
SIP element. However, the frequency of provisional messages
indicated in [RFC3261] still need to apply to keep each session set-
up active until successful or rejected.
A compliant SIP Server experiencing a processing queue due to
excessive load MUST move the messages containing higher relative
priority-values up to the queue the header indicates on a FIFO basis
based on the arrival time of the message (as there may be more than
one message at the same level already in queue). It is conceivable
that priority-levels within a proxy are grouped to include more than
one level per queue. This is a matter of local policy.
Polk & Schulzrinne [page 21]
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In compliant administrative domains utilizing the ets namespaces,
the UAS MUST copy the Resource-Priority header priority-values from
the Request to the Response for every header in the message unless
the UAS lacks local authorization to copy certain (perhaps
restricted) namespace.priority-value combinations.
Additional ETS policy is left to domain administrators, and to
future efforts.
The ets namespace does not define new error behaviors.
The IANA Considerations section will have the following entry for
the ETS namespace:
Intended New New Error
Namespace Levels Operation Rej. code code Reference
--------- ------ ---------------- --------- ---- ---------
ets 5 queue-based no no [This RFC]
8.1.5 The "WPS" Namespace
The WPS namespace derives its name from the "Wireless Priority
Service" defined in GSM and other wireless technologies.
Each namespace has a finite list of relative priority-values. Each
is listed here in the order of lowest priority to highest priority:
(lowest) wps.4
wps.3
wps.2
wps.1
(highest) wps.0
The wps namespace operates with the intended preferential treatment
of a queue-based policy (as defined in section 4), where higher
relative priority requests are queued ahead of lower relative
priority requests for processing at any point of scarce resource
constraints in an administrative domain.
WPS-based administrative domains will not queue normal, non-
prioritized messages. If a queue depth exists, the messages will be
rejected according to section 4.5.
A UAS within this type of administrative domain SHOULD be
configurable to limit the number of requests in a queue. One of two
different ways of limiting the queue depth is RECOMMENDED:
1) having a maximum number within a queue (perhaps with a different
number queued per priority-level), and
Polk & Schulzrinne [page 22]
Internet-Draft SIP Resource Priority February 21st, 2005
2) consider all requests above the default level to be in a single
queue on a FIFO basis (for example, based on the reception
timestamp of the request)
A UAS within this type of administrative domain SHOULD be
configurable to reject a queued lower priority request in lieu of
the newly arrived higher priority level request if the maximum
aggregate queue depth has been reached for that element. This
behavior is not considered a "preemption event" because the session
had not been previously established. This bumping will result in
the lower level request being rejected (discussed in section 4.5)
A UAS within this type of administrative domain SHOULD be
configurable to limit the time a request sits in a queue. One of
two different ways of limiting the time in queue is RECOMMENDED:
1) having a maximum time within a queue (perhaps with a different
time per priority-level) before a/each message moves into a
rejection condition discussed in section 4.5., and
2) considering all requests above the default level in a single
queue on a FIFO basis (for example, based on the reception
timestamp of the request), all with the same maximum time in
queue before a/each message moves into a rejection condition
discussed in section 4.5.
Local policy will determine additional behaviors of a queue-based
SIP element. However, the frequency of provisional messages
indicated in [RFC3261] still need to apply to keep each session set-
up active until successful or rejected.
A compliant SIP Server experiencing a processing queue due to
excessive load MUST move the messages containing higher relative
priority-values up to the queue the header indicates on a FIFO basis
based on the arrival time of the message (as there may be more than
one message at the same level already in queue). It is conceivable
that priority-levels within a proxy are grouped to include more than
one level per queue. This is a matter of local policy.
In compliant administrative domains utilizing the wps namespaces,
the UAS MUST copy the Resource-Priority header priority-values from
the Request to the Response for every header in the message unless
the UAS lacks local authorization to copy certain (perhaps
restricted) namespace.priority-value combinations.
Additional WPS policy is left to domain administrators, and to
future efforts.
The wps namespace operates according to the priority queuing policy
and will not queue normal, non-prioritized messages.
The wps namespace defines no new error behaviors.
Polk & Schulzrinne [page 23]
Internet-Draft SIP Resource Priority February 21st, 2005
The IANA Considerations section will have the following entry for
the WPS namespace:
Intended New New Error
Namespace Levels Operation Rej. code code Reference
--------- ------ ---------------- --------- ---- ---------
wps 5 queue-based no no [This RFC]
8.2 Future Namespace Considerations
Organizations considering the use of the Resource-Priority header
should investigate if an existing combination of namespace and
priority-values meets the needs. For example, emergency first
responders around the world are discussing utilizing this mechanism
for preferential treatment in future networks. There should not be
a unique namespace for different jurisdictions. This will greatly
increase interoperability and reduce development time, and probably
reduce future confusion if there is ever a need to map one namespace
to another in an interworking function.
In the specification of any future namespace, several facets need to
be done or included:
1) New namespace/priority-value combinations proposed in the future
MUST be defined in a Standards Track RFC and MUST include an
augmentation to following table offered to IANA for Registration:
Intended New New Error
Namespace Levels Operation Rej. code code Reference
--------- ------ ---------------- --------- ---- ---------
<new <# of <preemption <new error <new Rej <which
namespace> Levels> or queue> code> code> IETF doc>
- as well as list the finite set of priority values in relative
priority order (with no wildcards) for IANA Registration.
2) New priority-values MUST NOT be added to any previously (IANA)
registered list associated with a particular namespace. This
will cause interoperability problems.
3) If there is a new "intended behavior" (other than preemption-
based or priority queue-based), the parameters for that behavior
must be provided and how said behavior affects different types of
RP actors.
4) Any new Response Codes unique to this new namespace need to be
explained.
Polk & Schulzrinne [page 24]
Internet-Draft SIP Resource Priority February 21st, 2005
5) Any new Rejection Codes or changes to existing rejections codes
as a result of the creation of the namespace need to be offered
and explained.
9. Examples
The SDP message body and the BYE and ACK exchanges are the same as in
RFC 3665 [RFC3665] and omitted for brevity.
9.1 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
SIP/2.0 180 Ringing
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
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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: ...
...
9.2 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.
User A User B
| |
| INVITE F1 |
|----------------------->|
| 417 R-P failed F2 |
|<-----------------------|
| ACK F3 |
|----------------------->|
| |
| INVITE F4 |
|----------------------->|
| 180 Ringing F5 |
|<-----------------------|
| 200 OK F6 |
|<-----------------------|
| ACK F7 |
|----------------------->|
| |
Polk & Schulzrinne [page 26]
Internet-Draft SIP Resource Priority February 21st, 2005
| 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
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>
Polk & Schulzrinne [page 27]
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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: ...
...
10. 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
emergencies or deny service to legitimate users. In the Internet,
or any IP domain, this mechanism can cause certain messages or
sessions (calls) to be given a higher relative priority of
processing within a SIP element (move to the head of the line
scenario), to the point of prematurely terminating an existing
session in favor of a newer one. In some administrative domains,
this will be the expected behavior for authenticated and authorized
users (see section 8). Unauthorized users MUST NOT be given this
opportunity to abuse network/element resources.
While the indication itself does not have to provide separate
authentication, any SIP request containing this header has higher
authentication requirements than regular requests.
A poor implementation of authentication and authorization will
likely cause illegitimate higher priority messages to process
without being successfully challenged for the privilege to do so.
While this will not likely have a great affect on the performance of
SIP Servers, this could have some (to a great) impact on the
premature termination of existing sessions. Those domains wishing
to utilize a namespace with an operation of preemption of lower
relative priority sessions should use caution to ensure only the
proper usage of this header is granted.
Care should be taken on 3 fronts:
1) To the domain that enables usage of the Resource-Priority header
such that adequate control exists to prevent unwanted
preferential message treatment from internal users.
Without an authentication and authorization mechanism to validate
each user request, unwanted usage (and potentially user hacked
settings) can have undesired affects on any internal network.
2) To the domain that enables usage of the Resource-Priority header
such that inadequate control exists to prevent unwanted
preferential message treatment from SIP messages from outside the
Polk & Schulzrinne [page 28]
Internet-Draft SIP Resource Priority February 21st, 2005
domain coming into the domain (and outside the area of direct AAA
control).
3) In the choosing of a namespace itself, to make sure the desired
behavior of SIP elements have equivalent behaviors defined in this
document to ensure interoperability and no surprises.
An example of this is choosing a namespace throughout a domain and
configuring it for preferential treatment with no preemption, even
though a neighbor domain uses it as it is IANA defined (with
preemption as one expected behavior), resulting in poor performance
of fist domain's calls into the second administrative domain's
network.
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.
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.
10.1 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
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Internet-Draft SIP Resource Priority February 21st, 2005
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
impose particular authentication or authorization consideration that
are stricter than the baseline described here.
10.2 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
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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).
10.3 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
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].
10.4 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.
11. IANA Considerations
This section defines two new SIP headers (11.1), one SIP OPTION tag
(11.2), one new 4XX error code (11.3), a new registry within the
sip-parameters section of IANA for Resource-Priority namespaces
(11.4) and a new registry within the sip-parameters section of IANA
for Resource-Priority and priority-values (11.5).
Additional namespaces and priority values MUST be registered with
IANA. Within each namespace, the registration MUST indicate the
relative priority levels, expressed as an ordered list. New
priority-values MUST NOT be added to existing namespace registries.
The registration MUST describe, in the registration itself, how SIP
elements should treat requests from that namespace in 'operation',
e.g., whether preemption or only preferential queuing are allowed.
The SIP Change Process [RFC 3427] establishes a policy for the
registration of new SIP extension headers. Resource priority
namespaces and priority values have similar interoperability
requirements to those of SIP extension headers. Consequently,
registration of new resource priority namespaces and priority values
Polk & Schulzrinne [page 31]
Internet-Draft SIP Resource Priority February 21st, 2005
requires documentation in an RFC using the extension header approval
process specified in RFC 3427.
11.1 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
11.2 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.
11.3 IANA Registration for Response Code 417
RFC number: XXXX
Response code: 417
Default reason phrase: Unknown Resource-Priority
11.4 IANA Namespace and Priority-Value Registrations
A new registry ("Resource-Priority Namespaces") in the
sip-parameters section of IANA is to be created taking a form
similar to this table below:
Intended New New Error
Namespace Levels Operation Rej. code code Reference
--------- ------ ---------------- --------- ---- ---------
dsn 5 preemption no no [This RFC]
drsn 6 preemption no no [This RFC]
Polk & Schulzrinne [page 32]
Internet-Draft SIP Resource Priority February 21st, 2005
q735 5 preemption no no [This RFC]
ets 5 priority-queue no no [This RFC]
wps 5 priority-queue no no [This RFC]
Legend
------
Namespace = the unique string of the namespace
Levels = the number of priority-values within the namespace
Operation = Intended (or expected) operational behavior of SIP
elements encountering this namespace
New Rej. Code = New Rejection Codes introduced for this namespace
New Error Code = New Error Codes introduced for this namespace
Reference = IETF Document reference for this namespace
11.5 IANA Priority-Value Registrations
A new registry ("Resource-Priority Priority-values") in the
sip-parameters section of IANA is to be created taking a form
similar to this table below (Reference [XXXX] is this RFC):
Namespace: drsn
Reference: [XXXX]
Priority-Values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override", "flash-override-override"
Namespace: dsn
Reference: [XXXX]
Priority-Values (least to greatest): "routine", "priority",
"immediate", "flash", "flash-override",
Namespace: q735
Reference: [XXXX]
Priority values (least to greatest): "4", "3", "2", "1", "0"
Namespace: ets
Reference: [XXXX]
Priority values (least to greatest): "4", "3", "2", "1", "0"
Namespace: wps
Reference: [XXXX]
Priority values (least to greatest): "4", "3", "2", "1", "0"
12. Acknowledgments
Ben Campbell, Paul Kyzivat, Rohan Mahy, Mike Pierce, Samir
Srivastava and Allison Mankin provided helpful comments.
Polk & Schulzrinne [page 33]
Internet-Draft SIP Resource Priority February 21st, 2005
Dean Willis provided much help with this effort.
Martin Dolly, An Nguyen and Niranjan Sandesara assisted with the
ets and wps namespaces.
Janet Gunn helped improve the queue-based policy text.
13. References
13.1 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 Signaling 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.
[]
Polk, J., "Reason Header for Preemption within the Session
Polk & Schulzrinne [page 34]
Internet-Draft SIP Resource Priority February 21st, 2005
Initiation Protocol (SIP)",
draft-ietf-sipping-reason-header-for-preemption-02 (work
in progress), August 2004
13.2 Informative References
[I-D.ietf-ieprep-framework]
Carlberg, K., Brown, I. and C. Beard, "Framework for
Supporting ETS in IP Telephony",
draft-ietf-ieprep-framework-09 (work in progress), April
2004.
[RFC3893] Peterson, J., "SIP Authenticated Identity Body (AIB)
Format", RFC 3893, May 2004.
[RFC3903] Niemi, A., "An Event State Publication Extension to the
Session Initiation Protocol (SIP)", RFC3903, 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-01 (work in progress),
February 2005.
[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
Polk & Schulzrinne [page 35]
Internet-Draft SIP Resource Priority February 21st, 2005
Method", RFC 3515, April 2003.
[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
James Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, TX 75082
USA
EMail: jmpolk@cisco.com
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
USA
Phone: +1 212 939 7042
EMail: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
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Polk & Schulzrinne [page 37]
