Network Working Group
Request for Comments: 3924
Category: Informational
F. Baker
B. Foster
C. Sharp
Cisco Systems
October 2004

Cisco Architecture for Lawful Intercept in IP Networks

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 © The Internet Society (2004).


This RFC is not a candidate for any level of Internet Standard. The IETF disclaims any knowledge of the fitness of this RFC for any purpose, and in particular notes that the decision to publish is not based on IETF review for such things as security, congestion control or inappropriate interaction with deployed protocols. The RFC Editor has chosen to publish this document at its discretion. Readers of this document should exercise caution in evaluating its value for implementation and deployment.


For the purposes of this document, lawful intercept is the lawfully authorized interception and monitoring of communications. Service providers are being asked to meet legal and regulatory requirements for the interception of voice as well as data communications in IP networks in a variety of countries worldwide. Although requirements vary from country to country, some requirements remain common even though details such as delivery formats may differ. This document describes Cisco's Architecture for supporting lawful intercept in IP networks. It provides a general solution that has a minimum set of common interfaces. This document does not attempt to address any of the specific legal requirements or obligations that may exist in a particular country.

Table of Contents

   1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . .  2
      1.1. Requirements Motivating the Architecture . . . . . . . . .  3
      1.2. Document Organization. . . . . . . . . . . . . . . . . . .  4
   2. Reference Model . . . . . . . . . . . . . . . . . . . . . . . .  5
      2.1. Reference Model Components . . . . . . . . . . . . . . . .  6
      2.2. Operational Considerations . . . . . . . . . . . . . . . .  7
   3. Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . .  9
      3.1. Content Intercept Request Interface. . . . . . . . . . . .  9
      3.2. Intercept Content Interface (f). . . . . . . . . . . . . . 10
   4. Applying the Reference Model. . . . . . . . . . . . . . . . . . 11
      4.1. Voice over IP networks . . . . . . . . . . . . . . . . . . 11
           4.1.1. Interception of Voice over IP Services. . . . . . . 11
           4.1.2. Local Voice Services. . . . . . . . . . . . . . . . 12
      4.2. Data Services. . . . . . . . . . . . . . . . . . . . . . . 13
   5. Security Considerations . . . . . . . . . . . . . . . . . . . . 13
      5.1. Content Request Interface (d) - SNMPv3 Control . . . . . . 14
   6. Informative References. . . . . . . . . . . . . . . . . . . . . 14
   7. Acronyms. . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   8. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . 17
   9. Full Copyright Statement. . . . . . . . . . . . . . . . . . . . 18

1. Introduction

For the purposes of this document, lawful intercept is the lawfully authorized interception and monitoring of communications of an intercept subject. The term "intercept subject", "subject", "target subscriber" or "target" in this document refers to the subscriber of a telecommunications service whose communications and/or intercept related information (IRI) has been lawfully authorized to be intercepted and delivered to some agency. Note that although the term "Law Enforcement Agency" (LEA) is used throughout this document, this may refer to any agency that is able to request lawfully authorized interception.

By intercept related information (IRI) we mean information related to the IP traffic of interest. There is currently no standardized definition for IRI for IP traffic. IRI has been defined for a few services that might run over IP (e.g., Voice over IP) or that IP runs on top of (e.g., GPRS). For example, IRI for voice over IP could be the called and calling phone numbers. The definition of IRI from [14] is shown below:

Intercept Related Information: collection of information or data associated with telecommunication services involving the target identity, specifically communication associated information or data (e.g., unsuccessful communication attempts), service associated
information or data and location

Service providers are being asked to meet legal and regulatory requirements for the interception of voice as well as data communications in IP networks in a variety of countries worldwide. Although requirements vary from country to country, some requirements remain common even though details such as delivery formats may differ. This document describes Cisco's Architecture for supporting lawful intercept in IP networks. It provides a general solution that has a minimum set of common interfaces. This document does not deal with legal requirements or obligations.

This document describes one method for supporting lawful intercept. Other methods may be available.

The IESG wishes to draw the reader's attention to RFC 2804 [15] for a description of why architectures such as these are vendor-specific, rather than a topic of standardization for the IETF.

1.1. Requirements Motivating the Architecture

The purpose of the following list of requirements is to provide an understanding of the motivation behind the architecture and some of the requirements imposed on components and interfaces that are described in the later sections of the document. This does not imply any legal requirements on service providers or equipment vendors although such requirements may coincide.

Note that there are a variety of requirements that have been defined for lawfully authorized intercept throughout the world. Some of these have been defined by standards bodies (e.g., [13]), while others are country specific. The following itemized list is a distillation of some of these, although a given item may or may not apply to a specific country:

  • Lawful Intercept (LI) should be undetectable by the intercept subject.
  • Mechanisms should be in place to limit unauthorized personnel from performing or knowing about lawfully authorized intercepts.
  • There is often a requirement (especially for telecommunications services) to provide intercept related information (IRI) separately from the actual Internet Protocol (IP) traffic (or content) of interest (Note: some authorizations may be restricted to IRI).
  • If IRI is delivered separately from content, there should be some means to correlate the IRI and the content with each other.
  • If the information being intercepted is encrypted by the service provider and the service provider has access to the keys, then the information should be decrypted before delivery to the Law Enforcement Agency (LEA) or the encryption keys should be passed to the Law Enforcement Agency to allow them to decrypt the information.
  • If the information being intercepted is encrypted by the intercept subject and its associate and the service provider has access to the keys, then the service provider may deliver the keys to the LEA.
  • There is often a requirement for a service provider to be able to do multiple simultaneous intercepts on a single subject. The fact that there are multiple intercepts should be transparent to the LEAs.
  • There is often a requirement that the service provider should not deliver any unauthorized information to the LEA.

The architecture and interfaces described in this document attempts to address these requirements.

1.2. Document Organization

Section 1 of this document lists requirements motivating the architecture. Section 2 of this document describes a reference model along with some operation considerations. Section 3 provides more detailed requirements on the interfaces related to content interception. Section 4 applies the reference model to voice over IP and data intercepts and Section 5 examines security considerations.

2. Reference Model

This section describes a generic reference model (Figure 1) for lawful intercept.

                          +--------------------+               +-----+
                          |  LI Administration |     HI1(a)    |     |
                          |      Function      |<--------------|     |
                          +--------------------+               |     |
                                 |                             |     |
                                 | MD Provisioning             |     |
                                 | Interface(b)                | LEA |
                                 v                             |     |
   +-----------+           +--------------------+              |     |
   |           |<---(c)----|                    |              |     |
   |  IRI IAP  |--IRI(e)-->|      Mediation     |----HI2(g)--->|     |
   |           |           |      Device (MD)   |              |     |
   +-----------+           |                    |----HI3(h)--->|     |
                           +--------------------+              +-----+
                                |         ^
                      Intercept |         | Intercepted
                     Request(d) |         | Content(f)
                                |         |
                                v         |
                        User  |       Content      |  User
                      ------->|         IAP        |-------->
                      Content +--------------------+  Content

Figure 1: Intercept Architecture

A brief description of the interfaces is included in table 1 below. For a more detailed description of the interfaces refer to section 3. For a description of the components refer to section 2.1.

Table 1 LI Interfaces

     Interface               Description
   ---------------------  -------------------------------------------
   (a) HI1                   Handover Interface 1 - Administration
                             Interface: The LEA provides intercept
                             information to the service provider
                             administration function.
   (b) MD Provisioning       Mediation Device provisioning interface.
                             Parameters include: target identifier,
                             duration of intercept, type of intercept,
   (c) IRI IAP Provisioning  Specifies Target identifier, duration,
                             etc. for provisioning of delivery of
                             Intercept Related Information (IRI).
   (d) Content Intercept     Provisioning of the Content IAP.
   (e) IRI to MD             Internal interface between IRI Intercept
                             Access Point (IAP) and Mediation device
                             (MD) for delivery of IRI.
   (f) Content to MD         Internal interface between content
                             IAP and MD for delivery of Content.
   (g) HI2                   Handover Interface 2: Interface between
                             the MD and LEA for delivering IRI.  This
                             interface may vary from country to
   (h) HI3                   Handover Interface 3: Interface between
                             the MD and LEA for delivering Content.
                             This interface may vary from country to

2.1. Reference Model Components

A brief description of the key components in the reference model is as follows:

Lawful Intercept (LI) Administration Function:

This function provides the (typically manual) provisioning interface for the intercept as a result of a court order or warrant delivered by the Law Enforcement Agency (LEA). It could involve separate provisioning interfaces for several components, but more typically is a single interface to the Mediation Device (MD), which then takes care of provisioning of other components in the network. Because of the requirement in some laws to limit accessibility to authorized personnel, the provisioning interface has to be strictly controlled. In many cases, the identity of the subject received from the LEA has to be translated into an identity that can be used by the network to enable the intercept.

Intercept Access Point (IAP):

An IAP is a device within the network that is used for intercepting lawfully authorized intercept information. It may be an existing device that has intercept capability or it could be a special device that is provided for that purpose. Two types of IAP's are discussed here: IAP's that provide content; and IAP's that provide intercept related information (IRI).

Content IAP:

A content IAP is an IAP that is used to intercept the IP traffic of interest.

IRI IAP: This is an IAP that is used to provide intercept related

information (IRI).

Law Enforcement Agency (LEA):

This is the agency that has requested the intercept and to which the service provider delivers the information.

Mediation Device (MD):

The MD requests intercepts from IAPs through interfaces (c) and (d) in Figure 1. The Mediation Device receives the data from the IAP, packages it in the correct format (which may vary from country to country) and delivers it to the LEA. In the case where multiple law enforcement agencies are intercepting the same subject, the mediation device may replicate the information multiple times. The assumption is that the service provider operates the MD (via specially authorized personnel) and that the LEA only has access to interfaces (a), (g) and (h) in Figure 1.

2.2. Operational Considerations

In a typical operation, a lawfully authorized surveillance request arrives for a specified intercept subject. Authorized personnel provision the intercept using interface (b) in Figure 1, which may be for content only, IRI only or both. Once the intercept is provisioned, the IAP's send the IRI and/or content to the MD, which formats the information into the appropriate format for delivery to the LEA. Some operational issues that need to be considered:

  • Location and Address Information for Content Intercepts: In some cases where the location and/or addressing information for the intercept is not known until the subject registers (or makes a call in the case of voice), the IRI may provide needed information in order to do the content tap (e.g., the IP address and port for the content streams).
  • Content Encryption: If the intercept content is encrypted and the service provider has access to the encryption keys (e.g., receives keys in Session Description Protocol for Voice over IP), then the keys can be sent via IRI. It is, however, possible for end-users to exchange keys by some other means without any knowledge of the service provider in which case the service provider will not be able to provide the keys. Content transformations could make decryption at the LEA impossible. This is why the original packets are provided on interface (f) rather than attempting to convert them to some other format.
  • Detection by the Intercept Subject: One requirement is to ensure that the intercept subject is unable to detect that they are being intercepted. This document assumes a sophisticated subject:
  • Able to check IP addresses, use traceroute, etc.
  • Able to check if any unusual signaling is occurring on their customer premises equipment (CPE).
  • Able to detect degradation or interruptions in service.

This is why the intercept mechanism described here does not involve special requests to the CPE, re-routing of packets or end-to-end changes in IP addresses. Instead, content intercept is done on a device along the normal content path (i.e., no re- routing has occurred) that is within the service provider's network. A convenient content IAP is a router or switch at the edge of the service provider's network to which the intercept subject connects. This is illustrated in Figure 2.

        Customer Premises  | Service Provider's Network
            +-----+             |       |
            | CPE |-------------| Router|----------
            +-----+             | (IAP) |
                                |       |
              Figure 2  Content IAP - Router

Another possibility of course is to provide a special device along the path to provide the content IAP capabilities.

Note that in the case where there is multi-homing (two or more routers connected to provide access for the CPE), intercept taps may have to be installed on more than one access router. If the CPE is multi-homed to multiple service providers, then the intercept will have to be installed on each service provider separately and the LEA will have to correlate the data.

  • Unauthorized Creation and Detection: Another concern is the prevention of unauthorized creation and detection of intercepts. This is particularly important when a network element such as a router is used as a content IAP. Those routers that have the capability should be carefully controlled with access to intercept capability and information only via authorized personnel. In one approach using the reference model in Figure 1, the MD is in a controlled environment and the MD does the intercept request to the content IAP over an encrypted link. Logging and auditing are used to detect unauthorized attempts to access the intercept capability.
   *  Capacity:  Support for lawful intercept on a network element
      supporting customers consumes resources on that equipment.
      Therefore, support for lawful intercept requires capacity planning
      and engineering to ensure that revenue-producing services are not
      adversely affected.

3. Interfaces

This section provides a brief description of the interfaces in the reference model (Figure 1). A list of these interfaces is included in Table 1 in Section 2.

One of the objectives in defining these interfaces is to keep the internal interfaces (b to f) the same regardless of country-specific requirements. The MD then formats the IRI and the content to meet the country specific requirements for interfaces (g) and (h).

3.1. Content Intercept Request Interface

This section describes some of the requirements for the content intercept request interface (d) in Figure 1. It makes use of a common request protocol (SNMPv3) regardless of the type of application (e.g., voice, data) and suggests the usage of a TAP-MIB, which is defined in a separate document [1]. Some of the considerations that lead to the use of SNMPv3 and to the definition of the specific Management Information Base (MIB) defined in [1] are provided here.

In order to provide a generic interface for intercepting, replicating, encapsulating and transporting content packets to the MD, the content intercept interface ((d) in Figure 1) should specify:

  • A Filter specification for classifying the packets to be intercepted.
  • The destination address of the MD (where to send the packets).
  • Encapsulation and Transport parameters.

In addition, a timeout value for the intercept should also be specified. This defines a limited lifetime for the intercept so that failures will not result in intercepts remaining beyond their authorized lifetime. If a failure of the MD occurs such that it is not able to supply the refresh to the timeout, then the intercept will cease to exist after the timeout expires. Similarly, if the IAP re-boots, then the intercept will not survive the re-boot unless the IAP is capable of ascertaining that the intercept lifetime requirements will continue to be met.

In order for this to work, it must be possible for the mediation device to realize that there is a failure in the IAP such that it must re-establish the intercept. This may be in the form of an audit (from the MD to the IAP), or in the form of a heartbeat mechanism in the content stream, or both.

3.2. Intercept Content Interface (f)

The encapsulation method should retain all of the information in the original packets (source and destination addresses as well as payload) and provide an identifier for correlating the packets with the IRI. One encapsulation that meets those requirements is described in Section 4 of [2]. For non-voice intercepts, the "Intercepted Information" field in Table 1 of [2] contains the original intercepted IP packet.

   Note, however, that the interface defined in [2] is based on UDP
   which is an unreliable and unordered transport protocol (i.e.,
   provides neither retransmission on detection of errors nor ordering
   of data).  If this transport is used, the underlying network (Layers
   1 -    - 3) should be engineered to meet the overall reliability
   requirements for delivery of content.

If a more reliable transport protocol is required, then a mechanism that provides timely delivery as well as limits the burden (both processing and buffering) on the Content IAP should be used. One mechanism that meets these requirements is a NACK-oriented retransmission scheme based on [12].

If [12] is used, the call content channel identifier may be placed in the SSRC field of the encapsulating RTP packet. The payload type may be used to identify the type of packet encapsulated in RTP (e.g., IP, PPP, Ethernet MAC). Note that usage of [12] is still under investigation and may need further specification. Usage of [12] in the content IAP places more processing burden on the content IAP than a UDP-based intercept and can affect the capacity of the content IAP.

4. Applying the Reference Model

This section applies the reference model to some example applications.

4.1. Voice over IP networks

This section will look at some of the issues surrounding interception of voice over IP calls, taking local voice services as a specific service example. The reference model from Figure 1 will be applied with the use of a common set of interfaces that are independent of the call signaling protocols in use.

4.1.1. Interception of Voice over IP Services

There are a variety of architectures in use for voice over IP (e.g., centralized versus distributed) as well as various protocols (SIP [6], H.323 [9], MGCP [7], H.248 [8]). There are also a variety of services that may be offered:

  • Local Voice Services (i.e., service to a user that has an IP phone or a phone connected to a gateway)
  • Transit services
  • Long distance access services (e.g., calling/debit card).

This document does not address any obligations that a service provider might or might not have to support intercepts. It simply describes how intercept might be done using the reference model in Figure 1.

Note that in the case of services where the intercept subject accesses the network via a non-IP endpoint (e.g., TDM), the detectability issue is less acute (e.g., re-routing of packets to intercept them in a special device is a possible option), since the intercept subject does not have access to the IP addresses or to traceroute.

However, in the case of local services, this is a much more difficult problem. The intercept for a call originating and terminating on-net (i.e., a call that is voice over IP end-to-end) has to be intercepted along its normal route in order to be undetectable. In addition, the call-forwarding feature that is often provided as a local service feature makes interception even more difficult: If call forwarding is invoked, a call that was intended to terminate on the intercept subject may be forwarded anywhere in the network resulting in the media stream bypassing the original content IAP (since in voice over

IP, the media stream goes directly from end-to-end). Also, since call forwarding can often be set up on a call-by-call basis, the location of the content IAP will often not be known until the call is set up.

4.1.2. Local Voice Services

This sub-section will look at the specific case in which the intercept subject under surveillance is being provided with a local voice service by the same provider that also provides the network access (e.g., controls the edge router or switch). This is an important assumption, since in VoIP the entity providing call control (e.g., SIP server) can be totally separate from the entity providing network access (e.g., operates edge routers).

Suppose that a subscriber that subscribes to a local (e.g., residential) voice service is a target for a lawfully authorized surveillance. Part of the system providing these services is a subscriber database that includes addressing information about the subscriber as well information on what features are in effect (e.g., call forwarding). Some call control entity (CCE) accesses that database in order to provide local services. For example, if the subject has call forwarding invoked, that fact (and where to forward the call) is indicated in the subscriber database. A call arriving at the CCE that "owns" that subscriber can then take the appropriate action (e.g., forward the call).

The CCE that "owns" the target subscriber (which could be an H.323 gatekeeper, a SIP proxy or a Media Gateway Controller) is provisioned with the intercept parameters (e.g., subject identification information such as the telephone number and where to deliver the IRI). The provisioning of this CCE could be through interface (c) in Figure 1. The CCE in question is the IRI IAP and once provisioned, it passes the IRI to the MD. In the scenario being discussed, the CCE typically remains in the signaling path throughout the call, even in the call-forwarding case. Part of the IRI it passes to the MD is the media signaling information (i.e., SDP [11] or H.245 [10]), which includes endpoint IP address and port information for the media (content) streams. Armed with this media address information, the MD can determine the content IAP (e.g., [5]) and make the request via interface (d). The request identifies the voice stream to be intercepted based on information received in the call signaling (i.e., IP addresses and UDP port numbers).

Note that the content IAP in the case of voice over IP could be an edge router or a PSTN gateway (e.g., a call from the PSTN forwarded to the PSTN). SIP, H.323, MGCP or H.248 call signaling protocols could be used. However, the protocol (SNMPv3 [1]) used for interface

(d), is not dependent on the type of call signaling protocol used; nor is the encapsulation format and transport protocol (interface "f"). The same reference model (Figure 1) with the same interfaces can be used for lawfully authorized surveillance, regardless of the signaling protocol and regardless of the type of service being provided (Note: even though a local voice service was used in this example, other voice services could use the same model and interfaces).

4.2. Data Services

The same model (Figure 1) can also be used for data services. In this case the IRI IAP could be a server that acts as registration, authentication and authorization point for the data service (e.g., a RADIUS server). If a potential IRI IAP does not have the available interfaces (c) and (e), the MD may have to do a content tap on registration signaling in order to obtain the IRI.

The IRI in the case of a data service could include:

   *  The time that the user registered or de-registered for the
   *  Addressing information (i.e., given the user identity, what IP
      address or other information is available that could be used in
      interface (d) to do the content tap).

Once suitable addressing information is available in order to do content tapping the MD can invoke the tap via interface (d).

Clearly the IRI interfaces (c, e, g) are different for data than they are for voice services. However, the content IAP is typically the same (an edge router). Interfaces (d, f, and h) may also be the same.

5. Security Considerations

Given the sensitive nature of lawful intercept (LI) -- both from the standpoint of the need to protect sensitive data, as well as conceal the identities of the intercept subjects, the LI solution should have the ability to provide stringent security measures to combat threats such as impersonation of MD's, privacy and confidentiality breaches, as well as message forgery and replay attacks.

While this document doesn't discuss issues of physical security, operating system, or application hardening within the principals of the LI solution, they are clearly important. In particular, the MD server would be considered a prime target for attacks.

In general, all interfaces should have the capability of providing strong cryptographic authentication to establish the identity of the principals, and be able to correlate the identity of the principal with the action they are attempting to perform. All interfaces should be capable of performing some sort of cryptographic message integrity checking such as, for example, HMAC-MD5. Message integrity checking can also be used to counter replay attacks. Privacy and confidentiality considerations, may also require the use of encryption.

The content and IRI IAPs also should also provide protection of the identity of the intercept subject and the existence of an intercept.

5.1. Content Request Interface (d) - SNMPv3 Control

For interface (d,) native SNMPv3 security module mechanism is used. The additional requirement is that the IAP should support the ability to protect the TAP MIB's [1] from disclosure or control by unauthorized USM [3] users. VACM [4] provides the necessary tools to limit the views to particular USM users, but there are also special considerations:

  • The ability to limit access to the appropriate TAP MIB's by only those SNMPv3 USM users which have keys established and the proper VACM views defined.
  • Segregation of the TAP MIB such that only operators of sufficient privilege level can create VACM views that include the TAP MIB [1].

6. Informative References

   [1]  Baker, F., "Cisco Lawful Intercept Control MIB", Work in
        Progress, April 2004.
   [2]  PacketCable(TM) Electronic Surveillance Specification, PKT-SP-
   [3]  Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
        for version 3 of the Simple Network Management Protocol
        (SNMPv3)", STD 62, RFC 3414, December 2002.
   [4]  Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Access
        Control Model (VACM) for the Simple Network Management Protocol
        (SNMP)", STD 62, RFC 3415, December 2002.
   [5]  Warnicke, E., "A Suggested Scheme for DNS Resolution of Networks
        and Gateways", Work in Progress.
   [6]  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.
   [7]  Andreasen, F. and B. Foster, "Media Gateway Control Protocol
        (MGCP) Version 1.0", RFC 3435, January 2003.
   [8]  ITU-T Recommendation H.248.1, Gateway Control Protocol: Version
        2, May 2002.
   [9]  ITU-T Recommendation H.323, Packet-based Multimedia
        Communications Systems, July 2003.
   [10] ITU-T Recommendation H.245, Control Protocol for Multimedia
        Communications, July 2003.
   [11] Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.
   [12] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenber,
        "RTP Retransmission Payload Format", Work in Progress.
   [13] ETSI TS 101 331, Telecommunications security; Lawful
        Interception (LI); Requirements of law enforcement agencies.
   [14] ETSI TS 33.108 v6.7.0, 3rd Generation Partnership Project;
        Technical Specification Group Services and System Aspects; 3G
        Security; Handover Interface for Lawful Interception (Release

[15] IAB and IESG, "IETF Policy on Wiretapping", RFC 2804, May 2000.

7. Acronyms

   CCE            Call Control Entity
   CMTS           Cable Modem Termination System
   CPE            Customer Premises Equipment
   ETSI           European Telecommunications Standards Institute
   GPRS           Generalized Packet Radio Service
   HMAC-MD5       Hash-based Message Authentication Code -
                   Message Digest 5
   IAP            Intercept Access Point
   IETF           Internet Engineering Task Force
   IRI            Intercept Related Information
   ITU-T          International Telecommunications Union -
                   Telecommunications Sector
   LEA            Law Enforcement Agency
   LI             Lawful Intercept
   MGCP           Media Gateway Control Protocol
   MD             Mediation Device
   MIB            Management Information Base
   NACK           Negative Acknowledgement
   PSTN           Public Switched Telecommunications Network
   RFC            Request for Comment
   RTP            Real-time Transport Protocol
   SDP            Session Description Protocol
   SIP            Session Initiation Protocol
   SSRC           Synchronization Source
   TDM            Time Division Multiplex
   UDP            User Datagram Protocol
   USM            User Service Model
   VACM           View-based Access Control Model
   VoIP           Voice over IP

8. Authors' Addresses

   Fred Baker
   Cisco Systems
   1121 Via Del Rey
   Santa Barbara, CA  93117
   Phone: +1-408-526-4257
   Fax:   +1-413-473-2403

Bill Foster
Cisco Systems
Suite 2150
1050 West Pender St.
Vancouver, BC, V6E 3S7

   Phone: +1-604-647-2315
   Chip Sharp
   Cisco Systems
   7025 Kit Creek Road
   RTP, NC 27709  USA

9. Full Copyright Statement

Copyright © The Internet Society (2004).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and at, and except as set forth therein, the authors retain all their rights.


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