| Rfc | 3487 |
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| Title | Requirements for Resource Priority Mechanisms for the Session
Initiation Protocol (SIP) |
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| Author | H. Schulzrinne |
|---|
| Date | February 2003 |
|---|
| Format: | TXT, HTML |
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| Status: | INFORMATIONAL |
|---|
|
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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