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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


   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

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   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

   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

   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",
   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

   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
         (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

   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

   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

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

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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

   Phone: +1-817-271-3552
   EMail: jmpolk@cisco.com

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