Network Working Group                                     H. Schulzrinne
Request for Comments: 3487                           Columbia University
Category: Informational                                    February 2003

         Requirements for Resource Priority Mechanisms for the
                   Session Initiation Protocol (SIP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document summarizes requirements for prioritizing access to
   circuit-switched network, end system and proxy resources for
   emergency preparedness communications using the Session Initiation
   Protocol (SIP).

Table of Contents

   1.  Introduction ................................................  2
   2.  Terminology .................................................  3
   3.  Resources ...................................................  4
   4.  Network Topologies ..........................................  5
   5.  Network Models ..............................................  6
   6.  Relationship to Emergency Call Services .....................  7
   7.  SIP Call Routing ............................................  8
   8.  Policy and Mechanism ........................................  8
   9.  Requirements ................................................  9
   10. Security Requirements ....................................... 12
       10.1 Authentication and Authorization ....................... 12
       10.2 Confidentiality and Integrity .......................... 13
       10.3 Anonymity .............................................. 14
       10.4 Denial-of-Service Attacks .............................. 14
   11. Security Considerations ..................................... 15
   12. Acknowledgements ............................................ 15
   13. Normative References ........................................ 15
   14. Informative References ...................................... 15
   15. Author's Address ............................................ 16
   16. Full Copyright Statement .................................... 17

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.

   There are many IP-based services that can assist during emergencies.
   This memo only covers requirements for real-time communications
   applications involving the Session Initiation Protocol (SIP) [1],
   including voice-over-IP, multimedia conferencing and instant
   messaging/presence.

   This document takes no position as to which mode of communication is
   preferred during an emergency, as such discussion appears to be of
   little practical value.  Based on past experience, real-time
   communications is likely to be an important component of any overall
   suite of applications, particularly for coordination of emergency-
   related efforts.

   As we will describe in detail below, such Session Initiation Protocol
   (SIP) [1] applications involve at least five different resources that
   may become scarce and congested during emergencies.  In order to
   improve emergency response, it may become necessary to prioritize
   access to such resources during periods of emergency-induced resource
   scarcity.  We call this "resource prioritization".

   This document describes requirements rather than possible existing or
   new protocol features.  Although it is scoped to deal with SIP-based
   applications, this should not be taken to imply that mechanisms have
   to be SIP protocol features such as header fields, methods or URI
   parameters.

   The document is organized as follows.  In Section 2, we explain core
   technical terms and acronyms that are used throughout the document.
   Section 3 describes the five types of resources that may be subject
   to resource prioritization.  Section 4 enumerates four network
   hybrids that determine which of these resources are relevant.  Since
   the design choices may be constrained by the assumptions placed on

   the IP network, Section 5 attempts to classify networks into
   categories according to the restrictions placed on modifications and
   traffic classes.

   Since this is a major source of confusion due to similar names,
   Section 6 attempts to distinguish emergency call services placed by
   civilians from the topic of this document.

   Request routing is a core component of SIP, covered in Section 7.

   Providing resource priority entails complex implementation choices,
   so that a single priority scheme leads to a set of algorithms that
   manage queues, resource consumption and resource usage of existing
   calls.  Even within a single administrative domain, the combination
   of mechanisms is likely to vary.  Since it will also depend on the
   interaction of different policies, it appears inappropriate to have
   SIP applications specify the precise mechanisms.  Section 8 discusses
   the call-by-value (specification of mechanisms) and call-by-reference
   (invoke labeled policy) distinction.

   Based on these discussions, Section 9 summarizes some general
   requirements that try to achieve generality and feature-transparency
   across hybrid networks.

   The most challenging component of resource prioritization is likely
   to be security (Section 10).  Without adequate security mechanisms,
   resource priority may cause more harm than good, so that the section
   attempts to enumerate some of the specific threats present when
   resource prioritization is being employed.

2.  Terminology

   CSN: Circuit-switched network, encompassing both private
      (closed) networks and the public switched telephone network
      (PSTN).

   ETS: Emergency telecommunications service, identifying a
      communications service to be used during large-scale emergencies
      that allows authorized individuals to communicate.  Such
      communication may reach end points either within a closed network
      or any endpoint on the CSN or the Internet.  The communication
      service may use voice, video, text or other multimedia streams.

   Request: In this document, we define "request" as any SIP
      request.  This includes call setup requests, instant message
      requests and event notification requests.

3.  Resources

   Prioritized access to at least five resource types may be useful:

   Gateway resources: The number of channels (trunks) on a CSN
      gateway is finite.  Resource prioritization may prioritize access
      to these channels, by priority queuing or preemption.

   CSN resources: Resources in the CSN itself, away from the access
      gateway, may be congested.  This is the domain of traditional
      resource prioritization mechanisms such as MLPP and GETS, where
      circuits are granted to ETS communications based on queuing
      priority or preemption (if allowed by local telecommunication
      regulatory policy and local administrative procedures).  A gateway
      may also use alternate routing (Section 8) to increase the
      probability of call completion.

      Specifying CSN behavior is beyond the scope of this document, but
      as noted below, a central requirement is to be able to invoke all
      such behaviors from an IP endpoint.

   IP network resources: SIP may initiate voice and multimedia
      sessions.  In many cases, audio and video streams are inelastic
      and have tight delay and loss requirements.  Under conditions of
      IP network overload, emergency services applications may not be
      able to obtain sufficient bandwidth in any network.  When there
      are insufficient network resources for all users and it is not
      practical to simply add more resources, quality of service
      management is necessary to solve this problem.  This is orthogonal
      to SIP, out of the scope for SIP, and as such these requirements
      will be discussed in another document.

      Bandwidth used for SIP signaling itself may be subject to
      prioritization.

   Receiving end system resources: End systems may include
      automatic call distribution systems (ACDs) or media servers as
      well as traditional telephone-like devices.  Gateways are also end
      systems, but have been discussed earlier.

      Since the receiving end system can only manage a finite number of
      sessions, a prioritized call may need to preempt an existing call
      or indicate to the callee that a high-priority call is waiting.
      (The precise user agent behavior is beyond the scope of this
      document and considered a matter of policy and implementation.)

      Such terminating services may be needed to avoid overloading, say,
      an emergency coordination center. However, other approaches beyond
      prioritization, e.g., random request dropping by geographic
      origin, need to be employed if the number of prioritized calls
      exceeds the terminating capacity.  Such approaches are beyond the
      scope of this memo.

   SIP proxy resources: While SIP proxies often have large request
      handling capacities, their capacity is likely to be smaller than
      their access network bandwidth.  (This is true in particular since
      different SIP requests consume vastly different amounts of proxy
      computational resources, depending on whether they invoke external
      services, sip-cgi [2] and CPL [3] scripts, etc.  Thus, avoiding
      proxy overload by restricting access bandwidth is likely to lead
      to inefficient utilization of the proxy.)  Therefore, some types
      of proxies may need to silently drop selected SIP requests under
      overload, reject requests, with overload indication or provide
      multiple queues with different drop and scheduling priorities for
      different types of SIP requests.  However, this is strictly an
      implementation issue and does not appear to influence the protocol
      requirements nor the on-the-wire protocol.  Thus, it is out of
      scope for the protocol requirements discussion pursued here.

      Responses should naturally receive the same treatment as the
      corresponding request.  Responses already have to be securely
      mapped to requests, so this requirement does not pose a
      significant burden.  Since proxies often do not maintain call
      state, it is not generally feasible to assign elevated priority to
      requests originating from a lower-privileged callee back to the
      higher-privileged caller.

   There is no requirement that a single mechanism be used for all five
   resources.

4.  Network Topologies

   We consider four types of combinations of IP and circuit-switched
   networks.

   IP end-to-end: Both request originator and destination are on an
      IP network, without intervening CSN-IP gateways.  Here, any SIP
      request could be subject to prioritization.

   IP-to-CSN (IP at the start): The request originator is in the IP
      network, while the callee is in the CSN.  Clearly, this model only
      applies to SIP-originated phone calls, not generic SIP requests
      such as those supporting instant messaging services.

   CSN-to-IP (IP at the end): A call originates in the CSN and
      terminates, via an Internet telephony gateway, in the IP network.

   CSN-IP-CSN (IP bridging): This is a concatenation of the two
      previous ones.  It is worth calling out specifically to note that
      the two CSN sides may use different signaling protocols.  Also,
      the originating CSN endpoint and the gateway to the IP network may
      not know the nature of the terminating CSN.  Thus, encapsulation
      of originating CSN information is insufficient.

   The bridging model (IP-CSN-IP) can be treated as the concatenation of
   the IP-to-CSN and CSN-to-IP cases.

   It is worth emphasizing that CSN-to-IP gateways are unlikely to know
   whether the final destination is in the IP network, the CSN or, via
   SIP forking, in both.

   These models differ in the type of controllable resources, identified
   as gateway, CSN, IP network resources, proxy and receiver.  Items
   marked as (x) are beyond the scope of this document.

   Topology       Gateway  CSN  IP   proxy  receiver
   _________________________________________________
   IP-end-to-end                (x)  (x)    x
   IP-to-CSN      x        x    (x)  (x)    (x)
   CSN-to-IP      x        x    (x)  (x)    x
   CSN-IP-CSN     x        x    (x)  (x)    (x)

5.  Network Models

   There are at least four IP network models that influence the
   requirements for resource priority.  Each model inherits the
   restrictions of the model above it.

   Pre-configured for ETS: In a pre-configured network, an ETS
      application can use any protocol carried in IP packets and modify
      the behavior of existing protocols.  As an example, if an ETS
      agency owns the IP network, it can add traffic shaping, scheduling
      or support for a resource reservation protocol to routers.

   Transparent: In a transparent network, an ETS application can
      rely on the network to forward all valid IP packets, however, the
      ETS application cannot modify network elements.  Commercial ISP
      offer transparent networks as long as they do not filter certain
      types of packets.  Networks employing firewalls, NATs and
      "transparent" proxies are not transparent.  Sometimes, these types
      of networks are also called common-carrier networks since they
      carry IP packets without concern as to their content.

   SIP/RTP transparent: Networks that are SIP/RTP transparent allow
      users to place and receive SIP calls.  The network allows ingress
      and egress for all valid SIP messages, possibly subject to
      authentication.  Similarly, it allows RTP media streams in both
      directions.  However, it may block, in either inbound or outbound
      direction, other protocols such as RSVP or it may disallow non-
      zero DSCPs.  There are many degrees of SIP/RTP transparency, e.g.,
      depending on whether firewalls require inspection of SDP content,
      thus precluding end-to-end encryption of certain SIP message
      bodies, or whether only outbound calls are allowed.  Many
      firewalled corporate networks and semi-public access networks such
      as in hotels are likely to fall into this category.

   Restricted SIP networks: In restricted SIP networks, users may
      be restricted to particular SIP applications and cannot add SIP
      protocol elements such as header fields or use SIP methods beyond
      a prescribed set.  It appears likely that 3GPP/3GPP2 networks will
      fall into this category, at least initially.

      A separate and distinct problem are SIP networks that
      administratively prohibit or fail to configure access to special
      access numbers, e.g., the 710 area code used by GETS.  Such
      operational failures are beyond the reach of a protocol
      specification.

   It appears desirable that ETS users can employ the broadest possible
   set of networks during an emergency.  Thus, it appears preferable
   that protocol enhancements work at least in SIP/RTP transparent
   networks and are added explicitly to restricted SIP networks.

   The existing GETS system relies on a transparent network, allowing
   use from most unmodified telephones, while MLPP systems are typically
   pre-configured.

6.  Relationship to Emergency Call Services

   The resource priority mechanisms are used to have selected
   individuals place calls with elevated priority during times when the
   network is suffering from a shortage of resources.  Generally, calls
   for emergency help placed by non-officials (e.g., "911" and "112"
   calls) do not need resource priority under normal circumstances.  If
   such emergency calls are placed during emergency-induced network
   resource shortages, the call identifier itself is sufficient to
   identify the emergency nature of the call.  Adding an indication of
   resource priority may be less appropriate, as this would require that
   all such calls carry this indicator.  Also, it opens another attack

   mechanism, where non-emergency calls are marked as emergency calls.
   (If network elements can recognize the request URI as an emergency
   call, they would not need the resource priority mechanism.)

7.  SIP Call Routing

   The routing of a SIP request, i.e., the proxies it visits and the UAs
   it ends up at, may depend on the fact that the SIP request is an ETS
   request.  The set of destinations may be larger or smaller, depending
   on the SIP request routing policies implemented by proxies.  For
   example, certain gateways may be reserved for ETS use and thus only
   be reached by labeled SIP requests.

8.  Policy and Mechanism

   Most priority mechanisms can be roughly categorized by whether they:

   o  use a priority queue for resource attempts,

   o  make additional resources available (e.g., via alternate routing
      (ACR)), or

   o  preempt existing resource users (e.g., calls.)

   For example, in GETS, alternate routing attempts to use alternate
   GETS-enabled interexchange carriers (IXC) if it cannot be completed
   through the first-choice carrier.

   Priority mechanisms may also exempt certain calls from network
   management traffic controls.

   The choice between these mechanisms depends on the operational needs
   and characteristics of the network, e.g., on the number of active
   requests in the system and the fraction of prioritized calls.
   Generally, if the number of prioritized calls is small compared to
   the system capacity and the system capacity is large, it is likely
   that another call will naturally terminate in short order when a
   higher-priority call arrives.  Thus, it is conceivable that the
   priority indication can cause preemption in some network entities,
   while elsewhere it just influences whether requests are queued
   instead of discarded and what queueing policy is being applied.

   Some namespaces may inherently imply a preemption policy, while
   others may be silent on whether preemption is to be used or not,
   leaving this to local entity policy.

   Similarly, the precise relationships between labels, e.g., what
   fraction of capacity is set aside for each priority level, is also a
   matter of local policy.  This is similar to how differentiated
   services labels are handled.

9.  Requirements

   In the PSTN and certain private circuit-switched networks, such as
   those run by military organizations, calls are marked in various ways
   to indicate priorities.  We call this a "priority scheme".

   Below are some requirements for providing a similar feature in a SIP
   environment; security requirements are discussed in Section 10.  We
   will refer to the feature as a "SIP indication" and to requests
   carrying such an indication as "labelled requests".

   Note:  Not all the following requirements are possible to meet at
   once.  They may represent in some case tradeoffs that must be
   considered by the designer.

   REQ-1: Not specific to one scheme or country: The SIP indication
      should support existing and future priority schemes.  For example,
      there are currently at least four priority schemes in widespread
      use: Q.735, also implemented by the U.S.  defense telephone
      network ("DSN" or "Autovon") and NATO, has five levels, the United
      States GETS (Government Emergency Telecommunications Systems)
      scheme with implied higher priority and the British Government
      Telephone Preference Scheme (GTPS) system, which provides three
      priority levels for receipt of dial tone.

      The SIP indication may support these existing CSN priority schemes
      through the use of different namespaces.

      Private-use namespaces may also be useful for certain
      applications.

   REQ-2: Independent of particular network architecture: The SIP
      indication should work in the widest variety of SIP-based systems.
      It should not be restricted to particular operators or types of
      networks, such as wireless networks or protocol profiles and
      dialects in certain types of networks.  The originator of a SIP
      request cannot be expected to know what kind of circuit-switched
      technology is used by the destination gateway.

   REQ-3: Invisible to network (IP) layer: The SIP indication must
      be usable in IP networks that are unaware of the enhancement and
      in SIP/RTP-transparent networks.

      This requirement can be translated to mean that the request has to
      be a valid SIP request and that out-of-band signaling is not
      acceptable.

   REQ-4: Mapping of existing schemes: Existing CSN schemes must be
      translatable to SIP-based systems.

   REQ-5: No loss of information: For the CSN-IP-CSN case, there
      should be no loss of signaling information caused by translation
      from CSN signaling SIP and back from SIP to CSN signaling if both
      circuit-switched networks use the same priority scheme.  Loss of
      information may be unavoidable if the destination CSN uses a
      different priority scheme from the origin.

      One cannot assume that both CSNs are using the same signaling
      protocol or protocol version, such as ISUP, so that transporting
      ISUP objects in MIME [4,5] is unlikely to be sufficient.

   REQ-6: Extensibility: Any naming scheme specified as part of the
      SIP indication should allow for future expansion.  Expanded naming
      schemes may be needed as resource priority is applied in
      additional private networks, or if VoIP-specific priority schemes
      are defined.

   REQ-7: Separation of policy and mechanism: The SIP indication
      should not describe a particular detailed treatment, as it is
      likely that this depends on the nature of the resource and local
      policy.  Instead, it should invoke a particular named policy.  As
      an example, instead of specifying that a certain SIP request
      should be granted queueing priority, not cause preemption, but be
      restricted to three-minute sessions, the request invokes a certain
      named policy that may well have those properties in a particular
      implementation.  An IP-to-CSN gateway may need to be aware of the
      specific actions required for the policy, but the protocol
      indication itself should not.

      Even in the CSN, the same MLPP indication may result in different
      behavior for different networks.

   REQ-8: Method-neutral: The SIP indication chosen should work for
      any SIP method, not just, say, INVITE.

   REQ-9: Default behavior: Network terminals configured to use a
      priority scheme may occasionally end up making calls in a network
      that does not support such a scheme.  In those cases, the protocol
      must support a sensible default behavior that treats the call no
      worse than a call that did not invoke the priority scheme.  Some
      networks may choose to disallow calls unless they have a suitable

      priority marking and appropriate authentication.  This is a matter
      of local policy.

   REQ-10: Address-neutral: Any address or URI scheme may be a
      valid destination and must be usable with the priority scheme.
      The SIP indication cannot rely on identifying a set of destination
      addresses or URI schemes for special treatment.  This requirement
      is motivated by existing ETS systems.  For example, in GETS and
      similar systems, the caller can reach any PSTN destination with
      increased probability of call completion, not just a limited set.
      (This does not preclude local policy that allows or disallows,
      say, calls to international numbers for certain users.)

      Some schemes may have an open set of destinations, such as any
      valid E.164 number or any valid domestic telephone number, while
      others may only reach a limited set of destinations.

   REQ-11: Identity-independent: The user identity, such as the
      From header field in SIP, is insufficient to identify the priority
      level of the request.  The same identity can issue non-prioritized
      requests as well as prioritized ones, with the range of priorities
      determined by the job function of the caller.  The choice of the
      priority is made based on human judgement, following a set of
      general rules that are likely to be applied by analogy rather than
      precise mapping of each condition.  For example, a particular
      circumstance may be considered similarly grave compared to one
      which is listed explicitly.

   REQ-12: Independent of network location: While some existing CSN
      schemes restrict the set of priorities based on the line identity,
      it is recognized that future IP-based schemes should be flexible
      enough to avoid such reliance.  Instead, a combination of
      authenticated user identity, user choice and policy determines the
      request treatment.

   REQ-13: Multiple simultaneous schemes: Some user agents will
      need to support multiple different priority schemes, as several
      will remain in use in networks run by different agencies and
      operators.  (Not all user agents will have the means of
      authorizing callers using different schemes, and thus may be
      configured at run-time to only recognize certain namespaces.)

   REQ-14: Discovery: A terminal should be able to discover which,
      if any, priority namespaces are supported by a network element.
      Discovery may be explicit, where a user agent requests a list of
      the supported namespaces or it may be implicit, where it attempts
      to use a particular namespace and is then told that this namespace
      is not supported.  This does not imply that every element has to

      support the priority scheme.  However, entities should be able
      discover whether a network element supports it or not.

   REQ-15: Testing: It must be possible to test the system outside
      of emergency conditions, to increase the chances that all elements
      work during an actual emergency.  In particular, critical elements
      such as indication, authentication, authorization and call routing
      must be testable.  Testing under load is desirable.  Thus, it is
      desirable that the SIP indication is available continuously, not
      just during emergencies.

   REQ-16: 3PCC: The system has to work with SIP third-party call
      control.

   REQ-17: Proxy-visible: Proxies may want to use the indication to
      influence request routing (see Section 7) or impose additional
      authentication requirements.

10.  Security Requirements

   Any resource priority mechanism can be abused to obtain resources and
   thus deny service to other users.  While the indication itself does
   not have to provide separate authentication, any SIP request carrying
   such information has more rigorous authentication requirements than
   regular requests.  Below, we describe authentication and
   authorization aspects, confidentiality and privacy requirements,
   protection against denial of service attacks and anonymity
   requirements.  Additional discussion can be found in [6].

10.1 Authentication and Authorization

   SEC-1: More rigorous: Prioritized access to network and end
      system resources enumerated in Section 3 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.

      The authentication and authorization requirements for ETS calls
      are, in particular, much stronger than for emergency calls (112,
      911), where wide access is the design objective, sacrificing
      caller identification if necessary.

   SEC-2: Attack protection: Under certain emergency conditions,
      the network infrastructure, including its authentication and
      authorization mechanism, may be under attack.  Thus,
      authentication and authorization must be able to survive such
      attacks and defend the resources against these attacks.

      Mechanisms to delegate authentication and to authenticate as early
      as possible are required.  In particular, the number of packets
      and the amount of other resources such as computation or storage
      that an unauthorized user can consume needs to be minimized.

      Unauthorized users must not be able to block CSN resources, as
      they are likely to be more scarce than packet resources. This
      implies that authentication and authorization must take place on
      the IP network side rather than using, say, a CSN circuit to
      authenticate the caller via a DTMF sequence.

      Given the urgency during emergency events, normal statistical
      fraud detection may be less effective, thus placing a premium on
      reliable authentication.

      SIP nodes should be able to independently verify the authorization
      of requests to receive prioritized service and not rely on
      transitive trust within the network.

   SEC-3: Independent of mechanism: Any indication of the resource
      priority must be independent of the authentication mechanism,
      since end systems will impose different constraints on the
      applicable authentication mechanisms. For example, some end
      systems may only allow user input via a 12-digit keypad, while
      others may have the ability to acquire biometrics or read
      smartcards.

   SEC-4: Non-trusted end systems: Since ETS users may use devices
      that are not their own, systems should support authentication
      mechanisms that do not require the user to reveal her secret, such
      as a PIN or password, to the device.

   SEC-5: Replay: The authentication mechanisms must be resistant
      to replay attacks.

   SEC-6: Cut-and-paste: The authentication mechanisms must be
      resistant to cut-and-paste attacks.

   SEC-7: Bid-down: The authentication mechanisms must be resistant
      to bid down attacks.

10.2 Confidentiality and Integrity

   SEC-8: Confidentiality: All aspects of ETS are likely to be
      sensitive and should be protected from unlawful intercept and
      alteration.  In particular, requirements for protecting the
      confidentiality of communications relationships may be higher than
      for normal commercial service.  For SIP, the To, From,

      Organization, Subject, Priority and Via header fields are examples
      of particularly sensitive information.  Callers may be willing to
      sacrifice confidentiality if the only alternative is abandoning
      the call attempt.

      Unauthorized users must not be able to discern that a particular
      request is using a resource priority mechanism, as that may reveal
      sensitive information about the nature of the request to the
      attacker.  Information not required for request routing should be
      protected end-to-end from intermediate SIP nodes.

      The act of authentication, e.g., by contacting a particular
      server, itself may reveal that a user is requesting prioritized
      service.

      SIP allows protection of header fields not used for request
      routing via S/MIME, while hop-by-hop channel confidentiality can
      be provided by TLS or IPsec.

10.3 Anonymity

   SEC-9: Anonymity: Some users may wish to remain anonymous to the
      request destination.  For the reasons noted earlier, users have to
      authenticate themselves towards the network carrying the request.
      The authentication may be based on capabilities and noms, not
      necessarily their civil name.

      Clearly, they may remain anonymous towards the request
      destination, using the network-asserted identity and general
      privacy mechanisms [7,8].

10.4 Denial-of-Service Attacks

   SEC-10: Denial-of-service: ETS systems are likely to be subject
        to deliberate denial-of-service attacks during certain
        types of emergencies.  DOS attacks may be launched on the
        network itself as well as its authentication and
        authorization mechanism.

   SEC-11: Minimize resource use by unauthorized users: Systems
        should minimize the amount of state, computation and
        network resources that an unauthorized user can command.

   SEC-12: Avoid amplification: The system must not amplify attacks
        by causing the transmission of more than one packet or SIP
        request to a network address whose reachability has not
        been verified.

11.  Security Considerations

   Section 10 discusses the security issues related to priority
   indication for SIP in detail and derives requirements for the SIP
   indicator.  As discussed in Section 6, identification of priority
   service should avoid multiple concurrent mechanisms, to avoid
   allowing attackers to exploit inconsistent labeling.

12.  Acknowledgements

   Ran Atkinson, Fred Baker, Scott Bradner, Ian Brown, Ken Carlberg,
   Janet Gunn, Kimberly King, Rohan Mahy and James Polk provided helpful
   comments.

13.  Normative References

   [1]  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.

14.  Informative References

   [2]  Lennox, J., Schulzrinne, H. and J. Rosenberg, "Common Gateway
        Interface for SIP", RFC 3050, January 2001.

   [3]  Lennox J. and H. Schulzrinne, "CPL: A language for user control
        of internet telephony services", Work in Progress.

   [4]  Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet, F.,
        Watson, M. and M. Zonoun, "MIME media types for ISUP and QSIG
        objects", RFC 3204, December 2001.

   [5]  Vemuri, A. and J. Peterson, "Session Initiation Protocol for
        Telephones (SIP-T): (SIP-T)", BCP 63, RFC 3372, September 2002.

   [6]  Brown, I., "A security framework for emergency communications",
        Work in Progress.

   [7]  Peterson, J., "A Privacy Mechanism for the Session Initiation
        Protocol (SIP)", RFC 3323, November 2002.

   [8]  Watson, M., "Short Term Requirements for Network Asserted
        Identity", RFC 3324, November 2002.

15.  Author's Address

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   USA

   EMail: schulzrinne@cs.columbia.edu

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