IETF SIP Working Group INTERNET-DRAFT Ravideep Bhatia Category: Information Mike Coulas Expires: May 10, 2007 Motorola Inc. November 10, 2006 SIP Proxy Discovery using Anycast Address draft-rbhatia-anycast-sip-proxy-discovery-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 To view the list of Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 01, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract SIP is an application-layer control protocol that can establish, modify, and terminate multimedia sessions. SIP makes use of elements called proxy servers to help route requests to and from the user's current location. Before a SIP request can be sent by a SIP client, Bhatia, et al. Expires - May 10, 2007 [Page 1] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 an outbound SIP proxy (first hop proxy) has to be discovered to which the SIP request can be forwarded. This draft proposes a new method for discovering the address of first hop outbound SIP proxy server based on the use of anycast addressing and the SIP OPTIONS request. This new method can be used with either IPv4 or IPv6, however the description and examples given in this draft are for IPv6 only. TABLE OF CONTENTS 1. TERMINOLOGY....................................................3 2. INTRODUCTION...................................................3 3. DEFINITIONS....................................................4 4. OVERVIEW.......................................................4 4.1 Background.................................................4 4.1.1 DHCP..................................................4 4.1.2 Static or DM provisioning.............................5 4.1.3 IP-CAN specific (3GPP)................................5 4.2 Proposed method............................................5 4.2.1 Using source IP address...............................6 4.2.2 Using SIP OPTIONS request.............................6 5. DETAILS........................................................6 5.1 Description of Mechanism based on using source address.....6 5.1.1 Sending REGISTER request..............................6 5.1.2 Processing at SIP proxy server........................6 5.1.3 Processing at SIP client..............................7 5.2 Description of Mechanism based on using OPTIONS request....7 5.2.1 Sending OPTIONS request...............................7 5.2.2 Processing at SIP proxy server........................8 5.2.3 Processing at SIP client..............................8 5.3 Lost connectivity and Proxy Server re-selection............8 6. EXAMPLES.......................................................8 6.1 Example of OPTIONS request sent by SIP client..............8 6.2 Example of OPTIONS response returned by outbound SIP proxy server.........................................................9 7. SECURITY CONSIDERATIONS........................................9 8. IANA CONSIDERATIONS...........................................10 9. REFERENCES....................................................11 9.1 NORMATIVE REFERENCES......................................11 9.2 INFORMATIVE REFERENCES....................................11 10. AUTHORS' ADDRESSES...........................................12 11. IPR STATEMENTS...............................................12 12. COPYRIGHT NOTICE.............................................13 13. DISCLAIMER OF VALIDITY.......................................13 14. ACKNOWLEDGEMENTS.............................................13 APPENDIX A: Anycast Addresses....................................14 Constructing IPv6 anycast address.............................14 APPENDIX B: Discovering prefix of the SIP proxy server network...15 Bhatia, et al. Expires - May 10, 2007 [Page 2] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 Using assigned prefix.........................................15 Using DNS query...............................................15 Using manual configuration....................................15 1. TERMINOLOGY In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 [2] and indicate requirement levels for compliant SIP implementations. 2. INTRODUCTION SIP is an application-layer control protocol that can establish, modify, and terminate multimedia sessions. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide application services to users. Any SIP request that is originated by a SIP client needs to be routed through an outbound SIP proxy server (first hop proxy). The outbound SIP proxy server is typically an edge SIP proxy server of the SIP network being directly accessed by the SIP client. A SIP client has to discover the outbound SIP proxy server before sending any SIP requests. This draft proposes a new method for discovering the address of first hop outbound SIP proxy server based on the use of anycast addressing and the SIP OPTIONS request. This new method is intended to supplement but not replace existing outbound SIP proxy discovery methods. The advantages offered by our proposed method over existing methods are: - The proposed method does not require nor use DHCP services for locating SIP servers in the network. - This method does not require a DHCP client in the terminal. - This method is considerably simpler to implement than the DHCP- based method. - This method provides more flexibility than static or DM provisioning of the outbound proxy in the terminal which can impose restrictions on the SIP networks that the SIP client can access thus restricting mobility. Bhatia, et al. Expires - May 10, 2007 [Page 3] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 - Immediate IP connectivity to the outbound SIP proxy server is established upon successful completion of this method. 3. DEFINITIONS Anycast Address An identifier for a set of interfaces, typically belonging to different nodes. A packet sent to an anycast address is delivered to one of the interfaces identified by that address, i.e., the "nearest" one, according to the routing protocols' measure of distance. SIP An application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. Outbound SIP Proxy A SIP proxy that receives SIP requests from a client and routes them towards their destination. A stateful outbound SIP proxy also maintains the SIP transactions between itself and the SIP client. 4. OVERVIEW 4.1 Background Any SIP request that is originated by a SIP client needs to be routed through an outbound SIP proxy server (first hop proxy). Thus, before sending a SIP request, a SIP client has to discover the address of an outbound proxy server. Current methods for discovering the outbound SIP proxy server are: 4.1.1 DHCP A DHCP option (option 120 for DHCPv4 and options 21/22 for DHCPv6) is defined for returning SIP proxy servers. With these options, SIP clients can either request IP address(es) or domain name(s) of outbound SIP proxy server(s). In case of domain name(s) being returned, subsequent NAPTR, SRV and A/AAAA DNS queries are required Bhatia, et al. Expires - May 10, 2007 [Page 4] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 to resolve the domain name(s) to an IP address(s). Refer to [5] and [6]. 4.1.2 Static or DM provisioning The outbound SIP proxy address(es) (IP address(es) or FQDN(s)) may be statically provisioned in the SIP client. Alternatively, provisioning may be accomplished using some Device Management mechanism. If FQDN(s) is provisioned, then DNS queries are required to resolve the FQDN(s) to an IP address(s). 4.1.3 IP-CAN specific (3GPP) Sometimes, discovery of the outbound proxy server address(es) may be integrated into the IP-CAN specific procedures used to establish an IP connection enabling access to the SIP core metwork. For example, the 3GPP GPRS network supports a P-CSCF discovery option in PDP context activations. The device requests and receives the address of the IMS outbound proxy server (i.e., the P-CSCF) as a result of a successful PDP context activation. Refer to [7] and [8]. In the absence of a DHCP service for obtaining SIP proxy servers and without a provisioned outbound SIP proxy server, the SIP client will not have any means of obtaining an outbound SIP proxy server for routing its SIP requests. Consequently, this draft proposes a new method of discovering the outbound SIP proxy server that may be used as an alternative to the methods described above. This new method can be used with either IPv4 or IPv6, however the description and examples given in this draft are for IPv6 only. 4.2 Proposed method The proposed method uses an anycast address (for a description of anycast addresses, refer to Appendix A and also to [4]), the SIP proxy server anycast address, to identify SIP proxy servers that can act as outbound SIP proxy servers to SIP clients. Every such SIP proxy server SHOULD configure this anycast address on the IP interface that it uses to communicate with SIP clients. In a network there can be multiple SIP proxy servers that are configured to act as outbound proxy servers and all of these servers SHOULD configure this anycast address on the appropriate IP interface. An SIP proxy server with an IP interface configured with the SIP proxy server anycast address MUST be able to accept packets addressed to that address. We are proposing two different mechanisms for discovering the address of an outbound SIP proxy (i.e., IP address or FQDN) using Bhatia, et al. Expires - May 10, 2007 [Page 5] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 the anycast address for outbound SIP proxies. 4.2.1 Using source IP address The first mechanism uses the source IP address of IP packets returned by the selected outbound SIP proxy server. When the first SIP response to a SIP request sent to the SIP proxy server anycast address is received from an outbound SIP proxy server, the SIP client takes the source IP address of the IP packet in which SIP response is received and then uses it as the address of the outbound SIP proxy server for all future communication. 4.2.2 Using SIP OPTIONS request The second mechanism for obtaining the address of an outbound SIP proxy server is based on the use of the OPTIONS SIP Request (defined in [2]). Specifically, a SIP OPTIONS request is sent to the SIP proxy server anycast address and the unicast address of the discovered outbound SIP proxy server is returned in the Contact header of the response. This is the preferred and RECOMMENDED mechanism. 5. DETAILS 5.1 Description of Mechanism based on using source address 5.1.1 Sending REGISTER request When the SIP client sends the first SIP request (normally REGISTER, but could be any other SIP request) to the SIP core network, it MUST set the destination address of the IP packet that contains the SIP request to the SIP proxy anycast address proposed in this document (refer to Appendix B for information on discovering the IPv6 prefix of the sip proxy server network). When this packet is received by the IP router of the network, it will find a single outbound SIP proxy server that is “nearest” to this user. After selecting a SIP proxy server, the router will forward the packet to that server. Since the proxy server is listening on the anycast address it will accept the packet and pass it to the SIP layer for processing. 5.1.2 Processing at SIP proxy server When a SIP proxy server gets the first SIP request (e.g., REGISTER request) from a SIP client, it processes the request and forwards it towards the destination addressed in the Request URI (e.g., could be a SIP UA or SIP registrar server or SIP application server). When Bhatia, et al. Expires - May 10, 2007 [Page 6] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 the first non-100 trying response is received from the destination, the SIP proxy server forwards it to the SIP client that originated the SIP request. While forwarding the request, the SIP proxy server MUST make sure that the source address of the IP packet, in which SIP request is carried, is set to the unicast address (i.e., not the anycast address) of the interface on which the proxy server wants to receive SIP requests from the SIP clients. Note that in order for this method to work, the outbound SIP proxy server must be a stateful proxy. 5.1.3 Processing at SIP client When SIP client receives a non-100 Trying response from the SIP proxy server, it MUST save the unicast address from the source address of the IP packet in which the response was received (the “100 Trying” response is normally only returned for INVITE transactions). This unicast address MUST be used for all subsequent communication with the outbound SIP proxy server. For example, this address may be converted to SIP URI format for inclusion in an initial route set. 5.2 Description of Mechanism based on using OPTIONS request 5.2.1 Sending OPTIONS request The SIP OPTIONS method allows a SIP User Agent (UA) to query a SIP UA or server as to its capabilities. This enables a client to discover information about the addressed SIP entity, in this case an outbound SIP proxy server. A SIP client using this method to discover the address of an outbound SIP proxy server MUST construct an OPTIONS request with the Request-URI set to the SIP proxy server anycast address and the default SIP port. In addition, a Max-Forwards header with a value of 0 is used to ensure that the proxy server does not forward the request. For example, SIP::5060 After constructing the OPTIONS request, the SIP client sends the request to the anycast address and port specified in the Request URI, i.e., as a result of resolving the address in the Request-URI. When this packet is received by the router of the network it will find a single outbound SIP proxy server that is “nearest” to this user and route it to that server. Since the proxy server is listening on the anycast address it will accept the packet and pass it to the SIP layer for processing. Bhatia, et al. Expires - May 10, 2007 [Page 7] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 5.2.2 Processing at SIP proxy server When a SIP proxy server receives an OPTIONS request from a SIP client addressed to the SIP proxy server anycast address and with Max-Forwards set to 0, it knows that it has to process the request and return a response to the SIP client. After processing the OPTIONS requests, the proxy server returns a 200(OK) response to the SIP client listing the capabilities of the server, as specified in [2]. This response MUST include one or more Contact headers containing address(es) of the proxy server. These addresses can either be unicast IP address(es) or FQDN(s). While sending the response, the SIP proxy server SHOULD set the source address in the IP packet to the unicast source address of the interface on which the proxy server wants to receive SIP requests from the SIP clients. 5.2.3 Processing at SIP client When the SIP client receives a 200 (OK) response for the OPTIONS request from the selected SIP proxy server, it MUST parse the Contact header(s) and store at least one of the contact addresses to be used in the initial route set of subsequent SIP requests. Outbound SIP proxy server address is typically placed in the topmost Route header of outbound SIP requests. 5.3 Lost connectivity and Proxy Server re-selection If, at any time, connectivity with a discovered outbound SIP proxy server is lost, the SIP client can simply re-use one the aforementioned mechanisms to discover and establish connectivity with a new outbound SIP proxy server. It may be desirable for a mobile SIP client to monitor outbound SIP proxy servers by periodically executing the outbound SIP proxy discovery mechanism based on using the OPTIONS request. The IMS client could then have the option to re-connect to a “nearer” outbound SIP proxy server if one is discovered. 6. EXAMPLES 6.1 Example of OPTIONS request sent by SIP client Request-URI of OPTIONS request is addressed to the outbound SIP proxy anycast address. OPTIONS sip:[5555::FDFF:FFFF:FFFF:FFFD]:5060 SIP/2.0 Bhatia, et al. Expires - May 10, 2007 [Page 8] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 Via: SIP/2.0/UDP [5555::a:b:c:d]:1357; comp=sigcomp;branch=z9hG4bKnashds7 Max-Forwards: 0 From: ;tag=4fa3 To: Contact: Call-ID: apb03a0s09dkjdfglkj49111 CSeq: 1 OPTIONS Supported: path, sec-agree Content-Length: 0 6.2 Example of OPTIONS response returned by outbound SIP proxy server Contact addresses (unicast IP address or FQDN) of selected SIP outbound proxy are returned in Contact headers of 200 (OK) response to OPTIONS request: SIP/2.0 200 Ok Via: SIP/2.0/UDP [5555::a:b:c:d]:1357; comp=sigcomp;branch=z9hG4bKnashds7 From: ;tag=4fa3 To: ;tag=5ef4 Call-ID: apb03a0s09dkjdfglkj49111 Contact: Contact: CSeq: 1 OPTIONS Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, MESSAGE, NOTIFY, PRACK, REFER, UPDATE Accept-Language: en Supported: path, sec-agree Content-Length: 0 7. SECURITY CONSIDERATIONS A discussion of the security issues impacting anycast addressing is given in [11]. In it, three fundamental security requirements are identified: 1. Access to anycast group membership must be controlled. Routers must advertise routes of legitimate anycast servers only. 2. An anycast client has no way of knowing whether the source address of a response to an anycast requests is that of a Bhatia, et al. Expires - May 10, 2007 [Page 9] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 legitimate anycast server or not. Therefore, responses to anycast requests may need to be authenticated. 3. Secure communication between anycast clients and servers may be required. Anycast communications may need to be confidential, authenticated and protected from replay attacks. As the purpose of the method described in this draft is to discover an outbound SIP proxy that will be used for subsequent communication with a SIP core, the first two requirements are of particular importance. Once the outbound proxy has been discovered, standard mechanisms can be used to provide secure communication between the client and the outbound proxy as well as the SIP core being accessed. A good example of this are the security mechanisms specified for accessing the 3GPP IMS core network [9][10]. Also, a secure IP connection is often used to access the subnet of the outbound SIP proxy server (e.g., IPsec VPN tunnel or 3GPP PDP context). To address the first security requirement, [11] proposes that an anycast server registration mechanism be used during which access control functions can be performed. Access control functions could be based on usage of Access Control Lists or alternatively, authorization certificates. To address the second security requirement, [11] proposes the use of digital signature with authorization certificates proving that the response to an anycast request was generated by a server belong to that anycast group. In the case of requests sent to the SIP proxy server anycast address, the certificate would be used to prove that the SIP response was generated by a legitimate outbound SIP proxy server. Use of anycast addressing may make SIP proxy servers more universally accessible for denial of service attacks. However, SIP proxy servers discovered using other methods, such as DHCP with SIP server option for example, are no less susceptible to denial of service attacks than are proxies discovered using anycast addressing. 8. IANA CONSIDERATIONS This section is to provide the Internet Assigned Numbers Authority (IANA) with guidelines to allow assignment of identifier related to the anycast address. Bhatia, et al. Expires - May 10, 2007 [Page 10] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 The method described in this document requires a new anycast identifier to be assigned by the IANA for constructing a new anycast address. This new anycast address will be assigned to all nodes that can act as outbound SIP proxy server for a SIP client. For discovering the SIP proxy servers, a SIP client will address the first SIP request to the SIP proxy server anycast address and the request will be sent with the destination in the IP packet set to this anycast address. 9. REFERENCES 9.1 NORMATIVE REFERENCES [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] 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. [3] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [4] Hinden, R., Deering S., "IP Version 6 Addressing Architecture", RFC 4291, February 2006. 9.2 INFORMATIVE REFERENCES [5] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP- for-IPv4) Option for Session Initiation Protocol (SIP) Servers", RFC 3361, August 2002. [6] Schulzrinne, H., Volz, B, "Dynamic Host Configuration Protocol (DHCPv6) Option for Session Initiation Protocol (SIP) Servers", RFC 3319, July 2003. [7] 3GPP TS 24.008, v6.1.10, “Mobile radio interface layer 3 specification; Core Network Protocols” [8] 3GPP TS 24.228, V5.13.0, “Signaling flows for the IP Multimedia call control based on SIP and SDP” [9] 3GPP TS 24.229, V7.2.0, “IP multimedia call control protocol based on SIP and SDP” [10] 3GPP TS 33.203, V7.0.0, “Access security for IP-based services” Bhatia, et al. Expires - May 10, 2007 [Page 11] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 [11] Dondeti, L., Hardjono, T., Haberman, B., “Security Requirements of IPv6 Anycast”, draft-dondeti-ipv6-anycast-security-00.txt, June 2001. 10. AUTHORS' ADDRESSES Ravideep Bhatia 600 North US Hwy 45 Motorola Inc. Libertyville, IL 60048, USA Phone: +1 847-523-0246 Email: ravideep.bhatia@motorola.com Michael F. Coulas Motorola Inc. 600 North US Hwy 45 Libertyville, IL 60048, USA Phone: +1 847-523-1827 Email: mcoulas1@motorola.com 11. IPR STATEMENTS The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Bhatia, et al. Expires - May 10, 2007 [Page 12] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 12. COPYRIGHT NOTICE Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 13. DISCLAIMER OF VALIDITY This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, (THE IETF TRUST) AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. 14. ACKNOWLEDGEMENTS Funding for the RFC Editor function is currently provided by the Internet Society. Bhatia, et al. Expires - May 10, 2007 [Page 13] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 APPENDIX A: Anycast Addresses The idea behind anycast addressing is that a client would like to send packets to a server offering a particular service or application, but it is not important which server is chosen. An anycast address identifies a set of interfaces, but only one of the interfaces, the nearest, receives the packet. The routing infrastructure forwards packets that are addressed to an anycast address to the nearest interface to which the anycast address is assigned. To facilitate delivery, the routing infrastructure must track the interfaces that have been assigned anycast addresses and their distance in terms of routing metrics. Anycast addresses are allocated from the unicast address space and do not look any different from ordinary unicast addresses. By being assigned to more than one interface, a unicast address becomes an anycast address. Constructing IPv6 anycast address The global unicast addresses are required to have 64-bit interface identifiers in EUI-64 format. The interface identifier field is formed from a 7-bit anycast identifier with the remaining highest order bits filled with all ones, except for the universal/local bit which is set to 0 (local) to indicate that the interface identifier in the address is not globally unique. The reserved subnet anycast address is constructed as follows: -------------------------------------------------------------- | 64 bits | 57 bits | 7 bits | | prefix | 111111011…111111 |anycast ID| --------------------------------------------------------------- The current IPv6 reserved subnet anycast identifiers are listed below. Value Description ----- ----------- 0x7F Reserved 0x7E Mobile IPv6 Home Agents anycast 0x00 Reserved Bhatia, et al. Expires - May 10, 2007 [Page 14] Internet-Draft SIP Proxy Discovery using Anycast Address November 2006 APPENDIX B: Discovering prefix of the SIP proxy server network Before using the IPv6 anycast address defined in this draft, a SIP client needs to determine the IPv6 prefix that should be used for constructing it. This prefix should belong to the SIP core network that the SIP client wishes to access, i.e., the IP network where the SIP proxy servers are available. For example, if the SIP client is in mobile device that is roaming in a visited network, then the prefix assigned to the mobile for accessing that network belongs to the visited network. Whether or not that prefix can be used by the SIP client to construct a SIP proxy server anycast address depends on: . Whether SIP proxy servers are available in the visited network. . Even if SIP proxy servers are available, policy in the visited network may not allow the visiting client to use the SIP proxy servers of the network. Therefore, a method is required whereby a SIP client can discover an appropriate IPv6 prefix that can be used to construct the SIP proxy server anycast address. We are suggesting following methods to discover the prefix: Using assigned prefix SIP client uses the same prefix as the IP address assigned to it by the network. This will work if the SIP client has an IP connection to same subnet as the outbound SIP proxy server that it wishes to access. This will normally be the case for many SIP access scenarios including IMS access. Using DNS query We are suggesting that a fixed name, like “sip-proxy-anycast@”, could be defined for each domain. This name would resolve to the prefix that would be used by SIP clients to construct the anycast address for the requested domain. Using manual configuration Through manual configuration (or static provisioning), a SIP client can be indicated to use a particular prefix (e.g. prefix of home network) irrespective of the location. Bhatia, et al. 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