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
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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,
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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
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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.
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- 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
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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
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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
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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:<SIP proxy server anycast address>: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.
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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
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Via: SIP/2.0/UDP [5555::a:b:c:d]:1357;
comp=sigcomp;branch=z9hG4bKnashds7
Max-Forwards: 0
From: <sip:user1_public1@home1.net>;tag=4fa3
To: <sip:[5555::FDFF:FFFF:FFFF:FFFD]>
Contact: <sip:[5555::a:b:c:d]:1357;comp=sigcomp>
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: <sip:user1_public1@home1.net>;tag=4fa3
To: <sip:[5555::FDFF:FFFF:FFFF:FFFD]>;tag=5ef4
Call-ID: apb03a0s09dkjdfglkj49111
Contact: <sip:proxy1.visited1.net:5060:comp=sigcomp>
Contact: <sip:[5555::c:d:e:f]:5060;comp=sigcomp>
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
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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.
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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”
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[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.
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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.
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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
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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@<home-
domain-name>”, 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.
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