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This memo captures protocol requirements to enable session peering of voice, presence, instant messaging and other types of multimedia traffic. It is based on the use cases that have been described in the SPEERMINT working group. This informational document is intended to link the session peering use cases to protocol solutions.
3. General Requirements
3.2. Border Elements
3.3. Session Establishment Data
3.3.1. User Identities and SIP URIs
3.3.2. URI Reachability
4. Requirements for Session Peering of Presence and Instant Messaging
5. Security Considerations
5.1. Security Properties for the Acquisition of Session Establishment Data
5.2. Security Properties for the SIP signaling exchanges
5.3. End-to-End Media Security
7. IANA Considerations
8.1. Normative References
8.2. Informative References
Appendix A. Policy Parameters for Session Peering
A.1. Categories of Parameters for VoIP Session Peering and Justifications
A.2. Summary of Parameters for Consideration in Session Peering Policies
§ Author's Address
§ Intellectual Property and Copyright Statements
Peering at the session level represents an agreement between parties to exchange multimedia traffic. It is assumed that these sessions use the Session Initiation Protocol (SIP) protocol to enable peering between two or more actors. These actors are called SIP Service Providers (SSPs) and they are typically represented by users, user groups such as enterprises, real-time collaboration service communities, or other service providers offering voice or multimedia services using SIP.
A reference architecture for SIP session peering is described in [I‑D.ietf‑speermint‑architecture] (Uzelac, A., Penno, R., Hammer, M., Malas, D., Khan, S., Kaplan, H., Livingood, J., Schwartz, D., and R. Shockey, “SPEERMINT Peering Architecture,” March 2010.). A number of use cases describe how session peering has been or could be deployed based on the reference architecture ([I‑D.ietf‑speermint‑voip‑consolidated‑usecases] (Uzelac, A. and Y. Lee, “VoIP SIP Peering Use Cases,” April 2010.) and [I‑D.ietf‑speermint‑consolidated‑presence‑im‑usecases] (Houri, A., “Presence & Instant Messaging Peering Use Cases,” July 2008.)).
Peering at the session layer can be achieved on a bilateral basis (direct peering established directly between two SSPs), or on an indirect basis via a session intermediary (indirect peering via a third-party SSP that has a trust relationship with the SSPs) - see the terminology document for more details.
This document first describes general requirements. The use cases are then analyzed in the spirit of extracting relevant protocol requirements that must be met to accomplish the use cases. These requirements are intended to be independent of the type of media exchanged such as Voice over IP (VoIP), video telephony, and instant messaging. Requirements specific to presence and instant messaging are defined in Section 4 (Requirements for Session Peering of Presence and Instant Messaging).
It is not the goal of this document to mandate any particular use of IETF protocols by SIP Service Providers in order to establish session peering. Instead, the document highlights what requirements should be met and what protocols may be used to define the solution space.
Finally, we conclude with a list of parameters for the definition of a session peering policy, provided in an informative appendix. It should be considered as an example of the information SIP Service Providers may have to discuss or agree on to exchange SIP traffic.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
This document also reuses the terminology defined in [I‑D.ietf‑speermint‑terminology] (Malas, D. and D. Meyer, “SPEERMINT Terminology,” November 2008.). It is assumed that the reader is familiar with the Session Description Protocol (SDP) [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) and the Session Initiation Protocol (SIP) [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.). Finally, when used with capital letters, the terms 'Authentication Service' are to be understood as defined by SIP Identity [RFC4474] (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.).
The following sub-sections contain general requirements applicable to multiple use cases for multimedia session peering.
The primary focus of this document is on the requirements applicable to the boundaries of Layer 5 SIP networks: SIP entities, Signaling path Border Elements (SBEs), and the associated protocol requirements for the look-up and location routing of the session establishment data. The requirements applicable to SIP UAs or related to the provisioning of the session data are considered out of scope.
SSPs desiring to establish session peering relationships have to reach an agreement on numerous points.
This document highlights only certain aspects of a session peering agreement, mostly the requirements relevant to protocols: the declaration, advertisement and management of ingress and egress border elements for session signaling and media, information related to the Session Establishment Data (SED), and the security properties that may be desirable for secure session exchanges.
Numerous other considerations of session peering arrangements are critical to reach a successful agreement but they are considered out of scope of the SPEERMINT working group. They include information about SIP protocol support (e.g. SIP extensions and field conventions), media (e.g., type of media traffic to be exchanged, compatible media codecs and transport protocols, mechanisms to ensure differentiated quality of service for media), layer-3 IP connectivity between the Signaling and Data path Border Elements, accounting and traffic capacity control (e.g. the maximum number of SIP sessions at each ingress point, or the maximum number of concurrent IM or VoIP sessions).
The informative Appendix A (Policy Parameters for Session Peering) lists parameters that may be considered when discussing the technical parameters of SIP session peering. The purpose of this list is to capture the parameters that are considered outside the scope of the protocol requirements.
For border elements to be operationally manageable, maximum flexibility should be given for how they are declared or dynamically advertised. Indeed, in any session peering environment, there is a need for a SIP Service Provider to declare or dynamically advertise the SIP entities that will face the peer's network. The data path border elements are typically signaled dynamically in the session description.
The use cases defined in [I‑D.ietf‑speermint‑voip‑consolidated‑usecases] (Uzelac, A. and Y. Lee, “VoIP SIP Peering Use Cases,” April 2010.) catalog the various border elements between SIP Service Providers; they include Signaling path Border Elements (SBEs) and SIP proxies (or any SIP entity at the boundary of the Layer 5 network).
If the SSP also provides media streams to its users as shown in the use cases for "Originating" and "Terminating" SSPs, a mechanism must exist to allow SSPs to advertise their egress and ingress data path border elements (DBEs), if applicable. While some SSPs may have open policies and accept media traffic from anywhere outside their network to anywhere inside their network, some SSPs may want to optimize media delivery and identify media paths between peers prior to traffic being sent (layer 5 to layer 3 QoS mapping).
In some SIP networks, SSPs operate the same border elements for all peers. In other SIP networks, it is common for SSPs to advertise specific SBEs and DBEs to certain peers: the advertisement of SBEs and DBEs may be peer-dependent.
In the use cases provided as part of direct and indirect peering scenarios, an SSP deals with multiple SIP entities and multiple SBEs in its own domain. There is often a many-to-many relationship between the SIP Proxies considered inside the trusted network boundary of the SSP and its Signaling path Border Elements at the network boundaries.
It should be possible for an SSP to define which egress SBE a SIP entity must use based on a given peer destination.
For example, in the case of an indirect peering scenario (section 5. of [I‑D.ietf‑speermint‑voip‑consolidated‑usecases] (Uzelac, A. and Y. Lee, “VoIP SIP Peering Use Cases,” April 2010.), Figure 5), it should be possible for the SIP proxy in the originating network (O-Proxy) to select the appropriate egress SBE (O-SBE) to reach the SIP target based on the information the proxy receives from the Lookup Function (O-LUF) and/or Location Routing Function (O-LRF) - message response labeled (2). Note that this example also applies to the case of Direct Peering when a service provider has multiple service areas and each service area involves multiple SIP Proxies and a few SBEs.
It is desirable for an SSP to be able to communicate how authentication of a peer's SBEs will occur (see the security requirements for more details).
The Session Establishment Data (SED) is defined in [I‑D.ietf‑speermint‑terminology] (Malas, D. and D. Meyer, “SPEERMINT Terminology,” November 2008.) as the data used to route a call to the next hop associated with the called domain's ingress point. The following paragraphs capture some general requirements on the SED data.
User identities used between peers can be represented in many different formats. Session Establishment Data should rely on URIs (Uniform Resource Identifiers, [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.)) and SIP URIs should be preferred over tel URIs ([RFC3966] (Schulzrinne, H., “The tel URI for Telephone Numbers,” December 2004.)) for session peering of VoIP traffic.
The use of DNS domain names and hostnames is recommended in SIP URIs and they should be resolvable on the public Internet. As for the user part of the SIP URIs, the mechanisms for session peering should not require an SSP to be aware of which individual user identities are valid within its peer's domain.
Based on a well-known URI type (for e.g. sip:, pres:, or im: URIs), it must be possible to determine whether the SSP domain servicing the URI allows for session peering, and if it does, it should be possible to locate and retrieve the domain's policy and SBE entities.
For example, an originating service provider must be able to determine whether a SIP URI is open for direct interconnection without requiring an SBE to initiate a SIP request. Furthermore, since each call setup implies the execution of any proposed algorithm, the establishment of a SIP session via peering should incur minimal overhead and delay, and employ caching wherever possible to avoid extra protocol round trips.
This section describes requirements for presence and instant messaging session peering. Several use cases for presence and instant messaging peering are described in [I‑D.ietf‑speermint‑consolidated‑presence‑im‑usecases] (Houri, A., “Presence & Instant Messaging Peering Use Cases,” July 2008.), a document authored by A. Houri, E. Aoki and S. Parameswar. Credits for this section must go to A. Houri, E. Aoki and S. Parameswar.
The following requirements for presence and instant messaging session peering are derived from [I‑D.ietf‑speermint‑consolidated‑presence‑im‑usecases] (Houri, A., “Presence & Instant Messaging Peering Use Cases,” July 2008.) and an initial set of related requirements published by A. Houri, E. Aoki and S. Parameswar:
This section describes the security properties that are desirable for the protocol exchanges in scope of session peering. Three types of information flows are described in the architecture and use case documents: the acquisition of the Session Establishment Data (SED) based on a destination target via the Lookup and Location Routing Functions (LUF and LRF), the SIP signaling between SIP Service Providers, and the associated media exchanges.
This section is focused on three security services, authentication, data confidentiality and data integrity as summarized in [RFC3365] (Schiller, J., “Strong Security Requirements for Internet Engineering Task Force Standard Protocols,” August 2002.). However, this text does not specify the mandatory-to-implement security mechanisms as required by [RFC3365] (Schiller, J., “Strong Security Requirements for Internet Engineering Task Force Standard Protocols,” August 2002.); this is left for future protocol solutions that meet the requirements.
A security threat analysis provides additional guidance for session peering ([I‑D.niccolini‑speermint‑voipthreats] (Niccolini, S., Chen, E., Seedorf, J., and H. Scholz, “SPEERMINT Security Threats and Suggested Countermeasures,” October 2008.)).
The Look-Up Function (LUF) and Location Routing Function (LRF) are defined in [I‑D.ietf‑speermint‑terminology] (Malas, D. and D. Meyer, “SPEERMINT Terminology,” November 2008.). They provide mechanisms for determining the SIP target address and domain the request should be sent to, and the associated SED to route the request to that domain.
The SIP signaling exchanges are out of scope of this document. This section describes some of the security properties that are desirable in the context of SIP interconnects between SSPs without formulating any normative requirements.
In general, the security properties desirable for the SIP exchanges in an inter-domain context apply to session peering. These include:
The fundamental mechanisms for securing SIP between proxy servers intra- and inter-domain are applicable to session peering; refer to Section 26.2 of [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) for transport-layer security of SIP messages using TLS, [I‑D.ietf‑sip‑connect‑reuse] (Gurbani, V., Mahy, R., and B. Tate, “Connection Reuse in the Session Initiation Protocol (SIP),” August 2009.) for establishing TLS connections between proxies, [RFC4474] (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.) for the protocol mechanisms to verify the identity of the senders of SIP requests in an inter-domain context, and [RFC4916] (Elwell, J., “Connected Identity in the Session Initiation Protocol (SIP),” June 2007.) for verifying the identity of the sender of SIP responses).
Media security is critical to guarantee end-to-end confidentiality of the communication between the end-users' devices, independently of how many direct or indirect peers are present along the signaling path. A number of desirable security properties emerge from this goal.
The establishment of media security may be achieved along the media path and not over the signaling path given the indirect peering use cases.
For example, media carried over the Real-Time Protocol (RTP) can be secured using secure RTP (SRTP [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.)). A framework for establishing SRTP security using Datagram TLS [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) is described in [I‑D.ietf‑sip‑dtls‑srtp‑framework] (Fischl, J., Tschofenig, H., and E. Rescorla, “Framework for Establishing an SRTP Security Context using DTLS,” March 2009.): it allows for end-to-end media security establishment using extensions to DTLS ([I‑D.ietf‑avt‑dtls‑srtp] (McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” February 2009.)).
It should also be noted that media can be carried in numerous protocols other than RTP such as SIP (SIP MESSAGE method), MSRP, XMPP, etc., over TCP ([RFC4571] (Lazzaro, J., “Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection-Oriented Transport,” July 2006.)), and that it can be encrypted over secure connection-oriented transport sessions over TLS ([RFC4572] (Lennox, J., “Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP),” July 2006.)).
A desirable security property for session peering is for SIP entities to be transparent to the end-to-end media security negotiations: SIP entities should not intervene in the Session Description Protocol (SDP) exchanges for end-to-end media security.
This document is based on the input and contributions made by a large number of people in the SPEERMINT working group, including: Edwin Aoki, Scott Brim, John Elwell, Mike Hammer, Avshalom Houri, Richard Shocky, Henry Sinnreich, Richard Stastny, Patrik Faltstrom, Otmar Lendl, Daryl Malas, Dave Meyer, Sriram Parameswar, Jon Peterson, Jason Livingood, Bob Natale, Benny Rodrig, Brian Rosen, Eric Rosenfeld, Adam Uzelac, and David Schwartz.
Specials thanks go to Rohan Mahy, Brian Rosen, John Elwell for their initial drafts describing guidelines or best current practices in various environments, to Avshalom Houri, Edwin Aoki and Sriram Parameswar for authoring the presence and instant messaging requirements and to Dan Wing for providing detailed feedback on the security consideration sections.
This document does not register any values in IANA registries.
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|[I-D.ietf-avt-dtls-srtp]||McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” draft-ietf-avt-dtls-srtp-07 (work in progress), February 2009 (TXT).|
|[I-D.ietf-pmol-sip-perf-metrics]||Malas, D. and A. Morton, “SIP End-to-End Performance Metrics,” draft-ietf-pmol-sip-perf-metrics-04 (work in progress), September 2009 (TXT).|
|[I-D.ietf-sip-connect-reuse]||Gurbani, V., Mahy, R., and B. Tate, “Connection Reuse in the Session Initiation Protocol (SIP),” draft-ietf-sip-connect-reuse-14 (work in progress), August 2009 (TXT).|
|[I-D.ietf-sip-dtls-srtp-framework]||Fischl, J., Tschofenig, H., and E. Rescorla, “Framework for Establishing an SRTP Security Context using DTLS,” draft-ietf-sip-dtls-srtp-framework-07 (work in progress), March 2009 (TXT).|
|[I-D.ietf-sip-hitchhikers-guide]||Rosenberg, J., “A Hitchhiker's Guide to the Session Initiation Protocol (SIP),” draft-ietf-sip-hitchhikers-guide-06 (work in progress), November 2008 (TXT).|
|[I-D.ietf-speermint-architecture]||Uzelac, A., Penno, R., Hammer, M., Malas, D., Khan, S., Kaplan, H., Livingood, J., Schwartz, D., and R. Shockey, “SPEERMINT Peering Architecture,” draft-ietf-speermint-architecture-10 (work in progress), March 2010 (TXT).|
|[I-D.ietf-speermint-consolidated-presence-im-usecases]||Houri, A., “Presence & Instant Messaging Peering Use Cases,” draft-ietf-speermint-consolidated-presence-im-usecases-05 (work in progress), July 2008 (TXT).|
|[I-D.ietf-speermint-terminology]||Malas, D. and D. Meyer, “SPEERMINT Terminology,” draft-ietf-speermint-terminology-17 (work in progress), November 2008 (TXT).|
|[I-D.ietf-speermint-voip-consolidated-usecases]||Uzelac, A. and Y. Lee, “VoIP SIP Peering Use Cases,” draft-ietf-speermint-voip-consolidated-usecases-18 (work in progress), April 2010 (TXT).|
|[I-D.niccolini-speermint-voipthreats]||Niccolini, S., Chen, E., Seedorf, J., and H. Scholz, “SPEERMINT Security Threats and Suggested Countermeasures,” draft-niccolini-speermint-voipthreats-05 (work in progress), October 2008 (TXT).|
|[RFC2198]||Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, “RTP Payload for Redundant Audio Data,” RFC 2198, September 1997 (TXT, HTML, XML).|
|[RFC3261]||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 (TXT).|
|[RFC3263]||Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” RFC 3263, June 2002 (TXT).|
|[RFC3365]||Schiller, J., “Strong Security Requirements for Internet Engineering Task Force Standard Protocols,” BCP 61, RFC 3365, August 2002 (TXT).|
|[RFC3455]||Garcia-Martin, M., Henrikson, E., and D. Mills, “Private Header (P-Header) Extensions to the Session Initiation Protocol (SIP) for the 3rd-Generation Partnership Project (3GPP),” RFC 3455, January 2003 (TXT).|
|[RFC3550]||Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” STD 64, RFC 3550, July 2003 (TXT, PS, PDF).|
|[RFC3603]||Marshall, W. and F. Andreasen, “Private Session Initiation Protocol (SIP) Proxy-to-Proxy Extensions for Supporting the PacketCable Distributed Call Signaling Architecture,” RFC 3603, October 2003 (TXT).|
|[RFC3611]||Friedman, T., Caceres, R., and A. Clark, “RTP Control Protocol Extended Reports (RTCP XR),” RFC 3611, November 2003 (TXT).|
|[RFC3702]||Loughney, J. and G. Camarillo, “Authentication, Authorization, and Accounting Requirements for the Session Initiation Protocol (SIP),” RFC 3702, February 2004 (TXT).|
|[RFC3711]||Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” RFC 3711, March 2004 (TXT).|
|[RFC3824]||Peterson, J., Liu, H., Yu, J., and B. Campbell, “Using E.164 numbers with the Session Initiation Protocol (SIP),” RFC 3824, June 2004 (TXT).|
|[RFC3966]||Schulzrinne, H., “The tel URI for Telephone Numbers,” RFC 3966, December 2004 (TXT).|
|[RFC3986]||Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).|
|[RFC4347]||Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” RFC 4347, April 2006 (TXT).|
|[RFC4474]||Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” RFC 4474, August 2006 (TXT).|
|[RFC4566]||Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” RFC 4566, July 2006 (TXT).|
|[RFC4571]||Lazzaro, J., “Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection-Oriented Transport,” RFC 4571, July 2006 (TXT).|
|[RFC4572]||Lennox, J., “Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP),” RFC 4572, July 2006 (TXT).|
|[RFC4916]||Elwell, J., “Connected Identity in the Session Initiation Protocol (SIP),” RFC 4916, June 2007 (TXT).|
This informative section lists various types of parameters that should be considered by implementers when deciding what configuration variables to expose to system administrators or management stations, as well as SSPs or federations of SSPs when discussing the technical part of a session peering policy.
In the context of session peering, a policy can be defined as the set of parameters and other information needed by an SSP to exchange traffic with another peer. Some of the session policy parameters may be statically exchanged and set throughout the lifetime of the peering relationship. Others parameters may be discovered and updated dynamically using by some explicit protocol mechanisms. These dynamic parameters may be session-dependent, or the may apply over multiple sessions or peers.
Various types of policy information may need to be discovered or exchanged in order to establish session peering. At a minimum, a policy should specify information related to session establishment data in order to avoid session establishment failures. A policy may also include information related to QoS, billing and accounting, layer-3 related interconnect requirements which are out of the scope of this document.
Some aspects of session peering policies must be agreed to and manually implemented; they are static and are typically documented as part of a business contract, technical document or agreement between parties. For some parameters linked to protocol support and capabilities, standard ways of expressing those policy parameters may be defined among SSP and exchanged dynamically. For e.g., templates could be created in various document formats so that it could be possible to dynamically discover some of the domain policy. Such templates could be initiated by implementers (for each software/hardware release, a list of supported RFCs, RFC parameters is provided in a standard format) and then adapted by each SSP based on its service description, server or device configurations and variable based on peer relationships.
The following list should be considered as an initial list of "discussion topics" to be addressed by peers when initiating a VoIP peering relationship.
The following is a summary of the parameters mentioned in the previous section. They may be part of a session peering policy and appear with a level of requirement (mandatory, recommended, supported, ...).
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