SIP Working Group W. Marshall Internet Draft AT&T Document: <draft-ietf-sip-call-auth-05.txt> F. Andreasen Category: Informational Cisco D. Evans D. R. Evans Consulting May, 2002 SIP Extensions for Media Authorization Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026 [1]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document describes the need for QoS and media authorization and defines a SIP extension that can be used to integrate QoS admission control with call signaling and help guard against denial of service attacks. The use of this extension is only applicable in administrative domains, or among federations of administrative domains with previously agreed-upon policies, where both the SIP proxy authorizing the QoS, and the policy control of the underlying network providing the QoS belong to that administrative domain or federation of domains. Table of Contents 1. Scope of Applicability...............................................2 2. Conventions used in this document....................................2 3. Background and Motivation............................................2 4. Overview.............................................................3 5. Changes to SIP to Support Media Authorization........................4 5.1 SIP Header Extension.............................................4 5.2 SIP Procedures...................................................4 5.2.1 User Agent Client (UAC)......................................5 5.2.2 User Agent Server (UAS)......................................5 SIP Working Group Informational - Expires 11/02/02 1 SIP Extensions for Media Authorization May 2002 5.2.3 Originating Proxy (OP).......................................5 5.2.4 Destination Proxy (DP).......................................6 6. Examples.............................................................6 6.1 Requesting Bandwidth via RSVP messaging..........................6 6.1.1 User Agent Client Side.......................................6 6.1.2 User Agent Server Side.......................................8 7. Advantages of the Proposed Approach.................................10 8. Security Considerations.............................................10 9. IANA Considerations.................................................11 10. Notice Regarding Intellectual Property Rights......................11 11. Normative References...............................................11 12. Informative References.............................................11 13. Contributors.......................................................11 14. Acknowledgments....................................................12 15. Authors' Addresses.................................................12 1. Scope of Applicability This document defines a SIP extension that can be used to integrate QoS admission control with call signaling and help guard against denial of service attacks. The use of this extension is only applicable in administrative domains, or among federations of administrative domains with previously agreed-upon policies, where both the SIP proxy authorizing the QoS, and the policy control of the underlying network providing the QoS belong to that administrative domain or federation of domains. Furthermore, the mechanism is generally incompatible with end- to-end encryption of message bodies that describe media sessions. This is in contrast with general Internet principles which separate data transport from applications, and the solution described in this document is thus not applicable to the Internet at large. Despite these limitations, there are sufficiently useful specialized deployments that meet the assumptions described above, and can accept the limitations that result, to warrant publication of this mechanism. An example deployment would be a closed network which emulates a traditional circuit switched telephone network. 2. Conventions used in this document 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 RFC 2119 [2]. 3. Background and Motivation Current IP telephony systems assume a perfect world in which there is either unlimited amount of bandwidth or network layer Quality of Service (QoS) is provided without any kind of policy control. However the reality is that end-to-end bandwidth is not unlimited and uncontrolled access to QoS in general is unlikely. The primary reason for this is that QoS provides preferential treatment of one flow at the expense of another. Consequently, it is important to have policy control over whether a given flow should have access to QoS. This will not only enable fairness in general, but can also prevent denial of service attacks. SIP Working Group Informational - Expires 11/02/02 2 SIP Extensions for Media Authorization May 2002 In this document, we are concerned with providing QoS for media streams established via the Session Initiation Protocol (SIP) [3]. We assume an architecture that integrates call signaling with media authorization as illustrated in the Figure below. The solid lines (A and B) show interfaces whereas the dotted line (C) illustrates the QoS enabled media flow: +---------+ | Proxy | +--------->| | | +---------+ | ^ A)| B) | | { } | | | v v +------+ +------+ C) | Edge | | UA |........|router|...... +------+ +------+ Figure 1 - Basic Architecture In this architecture, we assume an untrusted SIP UA connected to a QoS enabled network with an edge router acting as a Policy Enforcement Point (PEP) [6]. We furthermore assume that a SIP UA, that wishes to obtain QoS, initiates sessions through a proxy which can interface with the QoS policy control for the data network being used. We will refer to such a proxy as a QoS enabled proxy. We assume that the SIP UA needs to present an authorization token to the network in order to obtain Quality of Service (C). The SIP UA obtains this authorization token via SIP (A) from the QoS enabled proxy by means of an extension SIP header defined in this document. The proxy in turn communicates either directly with the edge router or with a Policy Decision Point (PDP - not shown) [6] in order to obtain a suitable authorization token for the UA. Examples of access data networks where such a QoS enabled proxy could be used include DOCSIS based cable networks and 3rd generation (3G) wireless networks. 4. Overview A session, that needs to obtain QoS for the media streams in accordance with our basic architecture described above is performed as follows. The SIP UA sends an INVITE to the QoS enabled proxy which for each resulting dialog includes a media authorization token in all unreliable provisional responses (except 100), the first reliable 1xx or 2xx response, and all retransmissions of that reliable response for the SIP Working Group Informational - Expires 11/02/02 3 SIP Extensions for Media Authorization May 2002 dialog. When the UA requests QoS, it includes the media authorization token with the resource reservation. A SIP UA may also receive an INVITE from its QoS enabled proxy which includes a media authorization token. In that case, when the UA requests QoS, it includes the media authorization token with the resource reservation. 5. Changes to SIP to Support Media Authorization This document defines a private SIP header extension to support a media authorization scheme. In this architecture a QoS enabled SIP proxy supplies the UA with an authorization token which is to be used in QoS requests. The extension defined allows network QoS resources to be authorized by the QoS enabled SIP proxy. 5.1 SIP Header Extension A new P-Media-Authorization general header field is defined. The Media- Auth header field contains a media authorization token which is to be included in subsequent resource reservations for the media flows associated with the session. The media authorization token is used for authorizing QoS for the media stream(s). The P-Media-Authorization header field is described by the following ABNF [4]: P-Media-Authorization = "P-Media-Authorization" HCOLON P-Media-Authorization-Token *(COMMA P-Media-Authorization-Token) P-Media-Authorization-Token = 1*HEXDIG Table 1 below is an extension of tables 2 and 3 in [3] for the new header field defined here: Where proxy ACK BYE CAN INV OPT REG P-Media-Authorization ad - - - o - - Table 1: Summary of header fields. The P-Media-Authorization header field can be used with the INVITE method as well as any response to it. Extension methods MAY define usage of the header field as well. 5.2 SIP Procedures This section defines the SIP [3] procedures for usage in media authorization compatible systems from the point of view of authorizing QoS. SIP Working Group Informational - Expires 11/02/02 4 SIP Extensions for Media Authorization May 2002 5.2.1 User Agent Client (UAC) The initial SIP INVITE message, mid-call messages that result in network QoS resource changes, and mid-call changes in call destination should be authorized. These SIP messages are sent through the QoS enabled proxies to receive this authorization. In order to authorize QoS, the QoS enabled SIP proxy MAY need to inspect message bodies that describe the media streams, e.g. SDP, and hence such message bodies SHOULD NOT be encrypted end-to-end. The P-Media-Authorization-Token, which is contained in the P-Media- Authorization header, is included for each dialog in all unreliable provisional responses (except 100), the first reliable 1xx or 2xx response, and all retransmissions of that reliable response for the dialog sent by the QoS enabled SIP proxy to the UAC. The UAC SHOULD use all the P-Media-Authorization-Token(s) from the most recent request/response that contained the P-Media-Authorization header when requesting QoS for the associated media stream(s). This applies both to the initial and subsequent refresh reservation messages. The UAC converts the string of hex digits into binary, and utilizes the result as a Policy-Element as defined in RFC 2750 [5] (including Length and P- Type). This Policy-Element would typically contain the authorizing entity and credentials, and be used in an RSVP request for media data stream QoS resources. 5.2.2 User Agent Server (UAS) The User Agent Server receives the P-Media-Authorization-Token in an INVITE (or other) message from the QoS enabled SIP proxy. If the response contains a message body that describes media streams for which the UA desires QoS, said message body SHOULD NOT be end-to-end encrypted. The UAS SHOULD use all the P-Media-Authorization-Token(s) from the most recent request/response that contained the P-Media-Authorization header when requesting QoS for the associated media stream(s). This applies both to the initial and subsequent refresh reservation messages. The UAS converts the string of hex digits into binary, and utilizes the result as a Policy-Element as defined in RFC 2750 [5] (including Length and P- Type). This Policy-Element would typically contain the authorizing entity and credentials, and be used in an RSVP request for media data stream resources. 5.2.3 Originating Proxy (OP) When the originating QoS enabled proxy (OP) receives an INVITE (or other) message from the UAC, the proxy authenticates the caller, and verifies that the caller is authorized to receive QoS. In cooperation with an originating Policy Decision Point (PDP-o), the OP obtains and/or generates a media authorization token that contains sufficient information for the UAC to get the authorized QoS for the media streams. The media authorization token is formatted as a Policy- Element, as defined in RFC 2750 [5], and then converted to a string of SIP Working Group Informational - Expires 11/02/02 5 SIP Extensions for Media Authorization May 2002 hex digits to form a P-Media-Authorization-Token. The proxy MAY inspect message bodies that describe the media streams, e.g. SDP, in both requests and responses in order to decide what QoS to authorize. For each dialog that results from the INVITE (or other) message received from the UAC, the originating proxy MUST add a P-Media-Authorization header with the P-Media-Authorization-Token in all unreliable provisional responses (except 100), the first reliable 1xx or 2xx response, and all retransmissions of that reliable response the proxy sends to the UAC, if that response may result in network QoS changes. A response with an SDP may result in such changes. 5.2.4 Destination Proxy (DP) The Destination QoS Enabled Proxy (DP) verifies that the called party is authorized to receive QoS. In cooperation with a terminating Policy Decision Point (PDP-t), the DP obtains and/or generates a media authorization token that contains sufficient information for the UAS to get the authorized QoS for the media streams. The media authorization token is formatted as a Policy- Element, as defined in RFC 2750 [5], and then converted to a string of hex digits to form a P-Media-Authorization-Token. The proxy MAY inspect message bodies that describe the media streams, e.g. SDP, in both requests and responses in order to decide what QoS to authorize. The Destination Proxy MUST add the P-Media-Authorization header with the P-Media-Authorization-Token in the INVITE (or other) request that it sends to the UAS, if that message may result in network QoS changes. A message with an SDP body may result in such changes. 6. Examples 6.1 Requesting Bandwidth via RSVP messaging Below we provide an example for how the P-Media-Authorization header field can be used in conjunction with the Resource Reservation Protocol (RSVP) [7]. The example assumes that resource management is integrated with SIP signaling and therefore utilizes the 183 Session Progress provisional response, which contains an SDP description of the media flow. 6.1.1 User Agent Client Side Figure 2 presents a high-level overview of a basic call flow with media authorization from the viewpoint of the UAC. Some policy interactions have been omitted for brevity. When a user goes off-hook and dials a telephone number, the UAC collects the dialed digits and sends the initial (1)INVITE message to the originating SIP proxy. SIP Working Group Informational - Expires 11/02/02 6 SIP Extensions for Media Authorization May 2002 The originating SIP proxy (OP) authenticates the user/UAC and forwards the (2)INVITE message to the proper SIP proxy. Assuming the call is not forwarded, the terminating end-point sends a (3)183 response to the initial INVITE via OP. Included in this response is an indication of the negotiated bandwidth requirement for the connection (in the form of an SDP description [8]). When OP receives the (3)183, it has sufficient information regarding the end-points, bandwidth and characteristics of the media exchange. It initiates a Policy-Setup message to PDP-o, (4)AuthProfile. The PDP-o stores the authorized media description in its local store, generates an authorization token that points to this description, and returns the authorization token to the OP, (5)AuthToken. UAC ER-o PDP-o OP |(1)Invite | | | Client Authentication |------------------------------------------->| and Call Authorization | | | | (2)Invite | | | |--------------> | | | | (3)180/3 | | |(4)AuthProfile |<-------------- | | |<--------------| | | |(5)AuthToken | | | |-------------->| Auth. Token put into | | |(6)180/3 | P-Media-Authorization |<-------------------------------------------| header extension. |Copies the RSVP policy object | |from the P-Media-Authorization | |(7)RSVP-PATH | | |----------->| (8)REQ | | | |-------------->| Using the Auth-Token and Authorized | | (9)DEC | Profile that is set by the SIP Proxy | |<--------------| the PDP makes the decision | | | |(10)RSVP-PATH | |------------------------------------------------> | | | |(11)RSVP-PATH |<-------------------------------------------------------------- |Copies the RSVP policy object | |from the P-Media-Authorization | |(12)RSVP-RESV | | |----------->| (13)REQ | | | |-------------->| Using the Auth-Token and Authorized | | (14)DEC | Profile that is set by the SIP Proxy | |<--------------| the PDP makes the decision | | | |(15)RSVP-RESV | |---------------------------------------------------> | | | |(16)RSVP-RESV |<---------------------------------------------------------------- | | | | Figure 2 - Media Authorization with RSVP (UAC) SIP Working Group Informational - Expires 11/02/02 7 SIP Extensions for Media Authorization May 2002 The OP includes the authorization token in the P-Media-Authorization header extension of the (6)183 message. Upon receipt of the (6)183 message, the UAC stores the media authorization token from the P-Media-Authorization header. Before sending any media, the UAC requests QoS by sending an (7)RSVP-PATH message which includes the previously stored P-Media-Authorization-Token as a Policy-Element. ER-o, upon reception of the (7)RSVP-PATH message checks the authorization through a PDP-o COPS message exchange, (8)REQ. PDP-o checks the authorization using the stored authorized media description that was linked to the authorization token it returned to OP. If authorization is successful, PDP-o returns an "install" Decision, (9)DEC. ER-o checks the admissibility for the request and if admission succeeds, it forwards the (10)RSVP-PATH message. Once UAC receives the (11)RSVP-PATH message from UAS it sends the (12)RSVP-RESV message to reserve the network resources. ER-o, upon receiving the (12)RSVP-RESV message checks the authorization through a PDP-o COPS message exchange, (13)REQ. PDP-o checks the authorization using the stored authorized media description that was linked to the authorization token it returned to OP. If authorization is successful PDP-o returns an "install" Decision, (14)DEC. ER-o checks the admissibility for the request and if admission succeeds, it forwards the (15)RSVP-RESV message. Upon receiving the (16)RSVP-RESV message, network resources have been reserved in both directions. 6.1.2 User Agent Server Side Figure 3 presents a high-level overview of a call flow with media authorization from the viewpoint of the UAS. Some policy interactions have been omitted for brevity. Since the destination SIP proxy (DP) has sufficient information regarding the endpoints, bandwidth and characteristics of the media exchange, it initiates a Policy-Setup message to the terminating Policy Decision Point (PDP-t) on receipt of the (1)INVITE. SIP Working Group Informational - Expires 11/02/02 8 SIP Extensions for Media Authorization May 2002 UAS ER-t PDP-t DP | | | | (1)Invite | | | |<-------------- | | | | Proxy Authentication | | | (2)AuthProfile| and Call Authorization | | |<--------------| | | | (3)AuthToken | | | |-------------->| Auth. Token put into | | | (4)Invite | P-Media-Authorization |<------------------------------------------| header extension | (5)180/3 | | | |------------------------------------------>| (6)180/3 |Copies the RSVP policy object |--------------> |from the P-Media-Authorization | |(7)RSVP-PATH | | |---------->| (8)REQ | | | |-------------->| Using the Auth-Token and Authorized | | (9)DEC | Profile that is set by the SIP Proxy | |<--------------| the PDP makes the decision | | | |(10)RSVP-PATH | |--------------------------------------------------> | | | |(11)RSVP-PATH |<-------------------------------------------------------------- |Copies the RSVP policy object | |from the P-Media-Authorization | | (12)RSVP-RESV | | |---------->| | | | | (13)REQ | | | |-------------->| Using the Auth-Token and Authorized | | (14)DEC | Profile that is set by the SIP Proxy | |<--------------| the PDP makes the decision | | | |(15)RSVP-RESV | |---------------------------------------------------> | | | |(16)RSVP-RESV |<--------------------------------------------------------------- | | | | Figure 3 - Media Authorization with RSVP (UAS) PDP-t stores the authorized media description in its local store, generates an authorization token that points to this description, and returns the authorization token to DP. The token is placed in the (4)INVITE message and forwarded to the UAS. Assuming that the call is not forwarded, the UAS sends a (5)183 response to the initial INVITE message, which is forwarded back to UAC. At the same time, the UAS sends an (7)RSVP-PATH message which includes the previously stored P-Media-Authorization-Token as a Policy-Element. ER-t, upon receiving the (7)RSVP-PATH message checks the authorization through a PDP-t COPS message exchange. PDP-t checks the authorization using the stored authorized media description that was linked to the SIP Working Group Informational - Expires 11/02/02 9 SIP Extensions for Media Authorization May 2002 authorization token it returned to DP. If authorization is successful PDP-t returns an "install" Decision, (9)DEC. ER-t checks the admissibility for the call and if admission succeeds, it forwards the (10)RSVP-PATH message. Once the UAS receives the (11)RSVP-PATH message, it sends (12)RSVP-RESV message to reserve the network resources. ER-t, upon reception of the (12)RSVP-RESV message, checks the authorization through a PDP-t COPS message exchange. PDP-t checks the authorization using the stored authorized media description that was linked to the authorization token that it returned to DP. If authorization is successful PDP-t returns an "install" Decision, (14)DEC. ER-t checks the admissibility for the call and if admission succeeds, it forwards the (15)RSVP-RESV message. Upon receiving the (16)RSVP-RESV message, network resources have been reserved in both directions. 7. Advantages of the Proposed Approach The use of media authorization makes it possible to control the usage of network resources. This in turn makes IP Telephony more robust against denial of service attacks and various kinds of service frauds. By using the authorization capability, the number of flows, and the amount of network resources reserved can be controlled thereby making the IP Telephony system dependable in the presence of scarce resources. 8. Security Considerations In order to control access to QoS, a QoS enabled proxy may want to authenticate the UA before providing it with a media authorization token. Both the method and policy associated with such authentication is outside the scope of this document, however it could for example be done by using standard SIP authentication mechanisms as described in [3]. Media authorization tokens sent in the P-Media-Authorization header from a QoS enabled proxy to a UA MUST be protected from eavesdropping and tampering. This can for example be done through a mechanism such as IPSec or TLS. However, this will only provide hop-by-hop security. If there is one or more intermediaries, e.g. proxies, between the UA and the QoS enabled proxy, these intermediaries will have access to the P-Media- Authorization header field value thereby compromising confidentiality and integrity. This will enable both theft-of-service and denial-of-service attacks against the UA. Consequently, the P-Media-Authorization header field MUST NOT be available to any untrusted intermediary in clear or without integrity protection. There is currently no mechanism defined in SIP that would satisfy these requirements. Until such a mechanism exists, proxies MUST NOT send P-Media-Authorization headers through untrusted intermediaries. SIP Working Group Informational - Expires 11/02/02 10 SIP Extensions for Media Authorization May 2002 QoS enabled proxies may need to inspect message bodies describing media streams, e.g. SDP. Consequently, such message bodies SHOULD NOT be encrypted. This will in turn prevent end-to-end confidentiality of said message bodies which lowers the overall security possible. 9. IANA Considerations This document defines a new private SIP header for media authorization which is "P-Media-Authorization". 10. Notice Regarding Intellectual Property Rights The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights. 11. Normative References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, February 2002. [4] Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium and Demon Internet Ltd., November 1997. [5] Herzog, S., RSVP Extensions for Policy Control, RFC 2750, January 2000. 12. Informative References [6] Yavatkar, R., Pendarakis, D., Guerin, R., "A Framework for Policy-based Admission Control", RFC 2753, January 2000. [7] Braden, R., Zhang, L., Berson, S., Herzog, S., Jamin, S., "Resource Reservation Protocol (RSVP) -- Version 1 Functional Specification ", RFC 2205, September 1997. [8] Handley, M., Jacobson, V., "SDP: Session Description Protocol", RFC 2327, April 1998. 13. Contributors The following people contributed significantly and were co-authors on earlier versions of this spec: K. K. Ramakrishnan (TeraOptic Networks), Ed Miller (Terayon), Glenn Russell (CableLabs), Burcak Beser (Pacific Broadband SIP Working Group Informational - Expires 11/02/02 11 SIP Extensions for Media Authorization May 2002 Communications), Mike Mannette (3Com), Kurt Steinbrenner (3Com), Dave Oran (Cisco), John Pickens (Com21), Poornima Lalwaney (Nokia), Jon Fellows (Copper Mountain Networks), and Keith Kelly (NetSpeak). 14. Acknowledgments The Distributed Call Signaling work in the PacketCable project is the work of a large number of people, representing many different companies. The authors would like to recognize and thank the following for their assistance: John Wheeler, Motorola; David Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jon Fellows, Jay Strater, Jeff Ollis, Clive Holborow, Motorola; Doug Newlin, Guido Schuster, Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay Networks; Farzi Khazai, Nortel; John Chapman, Bill Guckel, Michael Ramalho, Cisco; Chuck Kalmanek, Doug Nortz, John Lawser, James Cheng, Tung- Hai Hsiao, Partho Mishra, AT&T; Telcordia Technologies; and Lucent Cable Communications. Dean Willis provided valuable feedback as well. 15. Authors' Addresses Bill Marshall AT&T Florham Park, NJ 07932 Email: wtm@research.att.com Flemming Andreasen Cisco Edison, NJ 08837 Email: fandreas@cisco.com Doc Evans D. R. Evans Consulting Boulder, CO 80303 Email: n7dr@arrl.net SIP Working Group Informational - Expires 11/02/02 12 SIP Extensions for Media Authorization May 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. SIP Working Group Informational - Expires 11/02/02 13
Datatracker
draft-ietf-sip-call-auth-05
Internet-Draft
