Internet Engineering Task Force (IETF) J. Polk
Request for Comments: 7135 Cisco Systems
Category: Informational May 2014
ISSN: 2070-1721
Registering a SIP Resource Priority Header Field Namespace for
Local Emergency Communications
Abstract
This document creates the new Session Initiation Protocol (SIP)
Resource Priority header field namespace 'esnet' and registers this
namespace with IANA. The new header field namespace allows for local
emergency session establishment to a public safety answering point
(PSAP), between PSAPs, and between a PSAP and first responders and
their organizations.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7135.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Rules of Usage of the Resource Priority Header Field . . . . . 4
3. "esnet" Namespace Definition . . . . . . . . . . . . . . . . . 6
3.1. Namespace Definition Rules and Guidelines . . . . . . . . 6
3.2. The 'esnet' Namespace . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. IANA Resource-Priority Namespace Registration . . . . . . 7
4.2. IANA Priority-Value Registrations . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This document creates the new Session Initiation Protocol (SIP)
Resource Priority header (RPH) field namespace 'esnet' for local
emergency usage and registers this namespace with IANA. The SIP
Resource-Priority header field is defined in RFC 4412 [RFC4412]. The
new 'esnet' namespace is to be used for inbound calls towards a
public safety answering point (PSAP), between PSAPs, and between a
PSAP and first responders or their organizations within managed IP
networks. This namespace is not for use on the open public Internet
because it can be trivially forged.
Adding an RPH with the 'esnet' namespace can be differentiated from
the marking of an emergency call using a service URN as defined in
[RFC5031] in that the RPH specifically requests preferential
treatment in networks that honor it, while the marking merely
identifies an emergency call without necessarily affecting resources
allocated to it. It is appropriate to use both where applicable.
RPH with 'esnet' may also be used within public safety networks for
SIP sessions that are not emergency calls and thus not marked per RFC
5031.
This new namespace is included in SIP requests to provide an explicit
priority indication within controlled environments, such as an IP
Multimedia Subsystem (IMS) infrastructure or Emergency Services
network (ESInet) where misuse can be reduced to an acceptable level
because these types of networks have controls in place. The function
facilitates differing treatment of emergency SIP requests according
to local policy, or more likely, a contractual agreement between the
network organizations. This indication is used solely to
differentiate certain SIP requests, transactions, or dialogs from
other SIP requests, transactions, or dialogs that do not have the
need for priority treatment. If there are differing, yet still
understandable and valid Resource-Priority header values in separate
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SIP requests, then this indication can be used by local policy to
determine which SIP request, transaction, or dialog receives which
treatment (likely better or worse than another).
Application Service Providers (ASPs) that are securely connected to
an ESInet may have sufficient controls policing the header, and a
trust relationship with the entities inside the ESInet. SIP requests
from such ASPs could make use of this 'esnet' namespace for
appropriate treatment when requests are passed from the ASP to the
ESInet.
The 'esnet' namespace may also be used on calls from a PSAP or other
public safety agency on an ESInet towards a private or public
network, ASP or User Agent ("call back") when priority is needed.
Again, the request for priority is not for use on the public Internet
due to the ease of forging the header.
This document merely creates the namespace, per the rules within
[RFC4412] as updated by [RFC7134], which necessitates that new RPH
namespaces and their relative priority-value order be IETF reviewed
before being registered with IANA.
There is the possibility that within emergency services networks,
Multilevel Precedence and Preemption (MLPP)-like behavior can be
achieved (likely without the 'preemption' part), provided the local
policy supports enabling this function. For example, calls placed
between law enforcement agents could be marked similarly to MLPP
systems used by military networks, and some of those calls could be
handled with higher priority than an emergency call from an ordinary
user. Therefore, the 'esnet' namespace is given five priority-levels
instead of just one. This document does not define MLPP-like SIP
signaling for emergency calls like those using emergency service
numbers (such as 911, 112, and 999), but it is not prevented either.
Within the ESInet, there will be emergency calls requiring different
treatments, according to the type of call. Does a citizen's call to
a PSAP require the same, a higher, or a lower relative priority than
a PSAP's call to a police department or the police chief? What about
either relative to a call from within the ESInet to a national
government's department responsible for public safety, disaster
relief, national security/defense, etc.? For these additional
reasons, the 'esnet' namespace has multiple priority levels.
This document does not define any of these behaviors, outside of
reminding readers that the rules of RFC 4412 apply - though examples
of usage are included for completeness. This document registers the
'esnet' RPH namespace with IANA for use within any emergency services
networks, not just of those from citizens to PSAPs.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Rules of Usage of the Resource Priority Header field
This document retains the behaviors of the SIP Resource Priority
header field, defined in [RFC4412], when choosing between the
treatment options surrounding this new 'esnet' namespace. Given the
environment this is to be used within (i.e., within an ESInet), the
usage of the 'esnet' namespace does not have a 'normal' or routine
call level; that is left for local jurisdictions to define within
their respective parts of the ESInet, which could be islands of local
administration.
The 'esnet' namespace MUST only be used where at least one end of the
signaling, setting aside the placement of B2BUAs (Back-to-Back User
Agents), is within a local emergency organization. In other words,
if either the originating human caller's User Agent (UA) or the
destination human callee's UA is part of the local emergency
organization, this is a valid use of 'esnet'.
The 'esnet' namespace has 5 priority-values, in a specified relative
priority order, and is registered as a queue-based namespace in
compliance with [RFC4412]. SIP entities that support preemption
treatment (see Section 5 of [RFC4412]) can be configured according to
local policy. Display names for the 'esnet' values displayed can
likewise be set according to local policy.
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The following network diagram provides one example of local policy
choices when using the 'esnet' namespace:
|<-'esnet' namespace->|
| is used |
'esnet' namespace | ,-------.
usage out of scope | ,' `.
|<------------>|<---'esnet' namespace ---->| / \
+----+ | can be used +-----+ | ESInet |
| UA |--- | --------------------|Proxy|-+ ------ |
+----+ \ | / +-----+ | |
\ ,-------+ ,-------. | | +------+ |
+----+ ,' `. ,' `. | | |PSAP-1| |
| UA |--- / User \ / Application \ | | +------+ |
+----+ ( Network +---+ Service )| | |
\ / \ Provider / | | +------+ |
+----+ /`. ,' `. .+-----+ | |PSAP-2| |
| UA |---- '-------' '-------' |Proxy|-+ +------+ |
+----+ | +-----+ | |
| | | |
+----+ | +-----+ | +------+ |
| UA |--- | --------------------|Proxy|-+ |PSAP-3| |
+----+ \ | / +-----+ | +------+ |
\ ,-------+ ,-------. | | |
+----+ ,' `. ,' `. | | |
| UA |--- / User \ / Application \ | | +------+ |
+----+ ( Network +---+ Service )| | |PSAP-4| |
\ / \ Provider / | | +------+ |
+----+ /`. ,' `. .+-----+ | |
| UA |---- '-------' '-------' |Proxy|-+ ANY can |
+----+ | +-----+ | xfer/call |
| | \ | | | /
`. | | | ,'
'-|-|-|-'
| | |
Police <--------------+ | |
Fire <----------+ |
National Agency <-------+
A Possible Network Architecture Using the 'esnet' Namespace
In the figure, the 'esnet' namespace is used within the ESInet on the
right side of the diagram. How it is specifically utilized is out of
scope for this document and is left to local jurisdictions to define.
Whether preemption is implemented in the ESInet and the values
displayed to the ESInet users is likewise out of scope. Adjacent
ASPs to the ESInet may have a trust relationship that includes
allowing this/these neighboring ASP(s) to use the 'esnet' namespace
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to differentiate SIP requests and dialogs within the ASP's network.
The exact mapping between the internal and external sides of the edge
proxy at the ESInet boundaries is out of the scope of this document.
3. "esnet" Namespace Definition
The 'esnet' namespace is not generic for all emergencies because
there are a lot of different kinds of emergencies, some on a military
scale ([RFC4412] defines 3 of these), some on a national scale
([RFC4412] defines 2 of these), and some on an international scale.
Each type of emergency can also have its own namespace(s); although
there are many defined for other uses, more are possible -- so using
the public emergency service number (such as 911, 112, or 999) to
call for police officers, firefighters, or emergency medical
technicians (etc.) does not have a monopoly on the word "emergency".
The namespace 'esnet' has been chosen, roughly to stand for
"Emergency Services NETwork", for a citizen's call for help from a
public authority type of organization. This namespace will also be
used for communications between emergency authorities, and it MAY be
used by emergency authorities to call public citizens. An example of
the latter is a PSAP operator calling back someone who previously
called an emergency service number (such as 911, 112, or 999) and the
communication was terminated before it -- in the PSAP operator's
judgment -- should have been.
Below is an example of a Resource-Priority header field using the
'esnet' namespace:
Resource-Priority: esnet.0
3.1. Namespace Definition Rules and Guidelines
This specification defines one unique namespace for emergency calling
scenarios, 'esnet' and registers this namespace with IANA. This IANA
registration contains the facets defined in Section 9 of [RFC4412].
3.2. The 'esnet' Namespace
Per the rules of [RFC4412], each namespace has a finite set of
relative priority-value(s), listed (below) from lowest priority to
highest priority. In an attempt to not limit this namespace's use in
the future, more than one priority-value is assigned to the 'esnet'
namespace. This document does not recommend which Priority-value is
used where in which situation or scenario. That is for another
document to specify. To be effective, the choice within a national
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jurisdiction needs to be coordinated by all sub-jurisdictions to
maintain uniform SIP behavior throughout an emergency calling system
of that nation.
The relative priority order for the 'esnet' namespace is as follows:
(lowest) esnet.0
esnet.1
esnet.2
esnet.3
(highest) esnet.4
The 'esnet' namespace will have priority queuing registrations for
these levels per Section 4.5.2 of [RFC4412]. Although no preemption
is specified in this document for any levels of 'esnet', local
jurisdiction(s) MAY configure their SIP infrastructure to use this
namespace with preemption, as defined in RFC 4412.
The remaining rules that originated in RFC 4412 apply with regard to
an RP actor who understands more than one namespace, and must
maintain its locally significant relative priority order.
4. IANA Considerations
4.1. IANA Resource-Priority Namespace Registration
The following entry has been added to the "Resource-Priority
Namespaces" registry of the sip-parameters section of IANA (created
by [RFC4412]):
Intended New New resp.
Namespace Levels Algorithm Code warn-code Reference
--------- ------ ----------- --------- --------- ---------
esnet 5 queue no no RFC 7135
4.2. IANA Priority-Value Registrations
The following entry has been added to the "Resource-Priority
Priority-values" registry of the sip-parameters section of IANA:
Namespace: esnet
Reference: (this document)
Priority-Values (least to greatest): "0", "1","2", "3", "4"
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5. Security Considerations
The Security considerations that apply to RFC 4412 [RFC4412] apply
here.
For networks that act on the SIP Resource-Priority header field,
incorrect use of namespaces can result in traffic that should have
been given preferential treatment not receiving it, and vice versa.
This document does not define a use case where an endpoint outside
the ESInet marks its call for preferential treatment. Precautions
need to be taken to prevent granting preferential treatment to
unauthorized users not calling for emergency help even if they are in
the ESInet, as well as to prevent misuse by callers outside the
ESInet.
A simple means of preventing this usage is to not allow traffic
marked 'esnet' to get preferential treatment unless the destination
is towards the local/regional ESInet. This is not a consideration
for internetwork traffic within the ESInet, or generated out of the
ESInet. Calling an emergency service number (such as 911, 112, or
999) is fairly local in nature, with a finite number of URIs that are
likely to be considered valid within a portion of a network receiving
SIP signaling.
This namespace is not intended for use on the Internet because of the
difficulty in detecting abuse; specifically, it can trivially be
forged and used on a non-emergency session to obtain resource
priority. Some networks may determine that it can reasonably prevent
abuse and/or that the consequences of undetected abuse is not
significant. In such cases, use of 'esnet' on the Internet MAY be
allowed.
6. Acknowledgements
Thanks to Ken Carlberg, Janet Gunn, Fred Baker, and Keith Drage for
help and encouragement with this effort. Thanks to Henning
Schulzrinne, Ted Hardie, Hannes Tschofenig, and Marc Linsner for
constructive comments. A big thanks to Robert Sparks for being
patient with the author and Brian Rosen for completing the final
edits.
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7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource
Priority for the Session Initiation Protocol (SIP)", RFC
4412, February 2006.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", RFC 5031,
January 2008.
[RFC7134] Rosen, B., "The Management Policy of the Resource Priority
Header (RPH) Registry Changed to "IETF Review"", RFC 7134,
March 2014.
Author's Address
James Polk
Cisco Systems
3913 Treemont Circle
Colleyville, TX 76034
USA
Phone: +1-817-271-3552
EMail: jmpolk@cisco.com
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