Internet Engineering Task Force                                   SIP WG
Internet Draft                                               Schulzrinne
draft-schulzrinne-sip-register-01.txt                        Columbia U.
April 16, 2001
Expires: October 2001

                            SIP Registration


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   SIP registration provides personal, pre-call terminal and service
   mobility. We describe the registration process in detail, considering
   different options for roaming users.

1 Introduction

   The SIP [1] REGISTER request is the core mechanism for supporting
   personal, service and pre-call terminal mobility.Here, we define
   personal mobility as the ability for a user to be reachable under the
   same identifier while using different terminals, possibly several at
   the same time. Service mobility refers to the ability to obtain the
   same services regardless of where a user may be roaming.For VoIP
   services, service mobility may include the ability to use the same
   speed dial functionality, preprogrammed user interface elements and
   possibly even the user interface itself even when using a terminal

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   owned by a third party, e.g., an Internet "payphone" or kiosk. A
   related aspect of service mobility is the ability to maintain the
   same set of services when changing providers or proxies.  This is
   supported, for example, by uploading CPL [2] or cgi [3] scripts to
   the local proxy server, either via HTTP or as REGISTER bodies [4].
   This aspect is beyond the scope of this document.

   Pre-call terminal mobility describes the ability of a terminal to
   dynamically acquire IP addresses, but remain reachable under the same
   application-layer identifier.

   This document does not add functionality to SIP [1]. Rather, it
   spells out in more detail possible implementations and suggests where
   additional functionality is needed.

2 Assumptions

   We assume that each terminal is configured with a user address, a SIP
   URI, such as This identifier may be embedded by
   configuration into the communications device (e.g., for an Ethernet
   phone, personal laptop or workstation), established via local login
   into a shared computer or associated temporarily with a device by
   some token carried by the user. Examples of such tokens include SIMs,
   smartcards, iButtons, PDAs or magnetic swipe cards.

   This address also implies a home registrar , where the home registrar
   is derived by the DNS SRV [5] lookup of the host portion of the SIP
   URI, in this example.

   In addition, each device has at least a temporary network address
   which can be used to identify it during a session. This address is
   provided in the Contact header. However, the temporary address may
   not be directly reachable by everybody, due to firewalls and network
   address translators.

   Networks are identified by their domain name, independent of whether
   they belong to the same autonomous system, multicast scope or link-
   layer local area network. The same physical network may share several
   such domains. For example, while and are
   part of the same autonomous system and organization, but they are
   different domains. would be visiting the domain as soon as he obtains a Columbia, rather than
   Computer Science, IP address. A user's local domain is defined by the
   domain name option configured via DHCP. Some domains do not have a
   DHCP server, such as the addresses administered by virtual SIP
   domains [TBD:  need better terminology - this refers to domains such
   as or that offer the equivalent of web-based
   email, without any association to a physical network.]

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   We define a traveling user or visitor as a SIP end point that is
   visiting a domain other than the domain indicated in its SIP URI.
   This could be a mobile device with an embedded identifier visiting a
   foreign network or a local device that is personalized via a token of
   a visitor, as described above. Thus, a SIP payphone would always be a

   The outbound proxy and registrar server in the visited network are
   called the local proxy and local registrar , respectively. That
   network is referred to as the visited network , while the user's
   domain is called the home network , which has home proxy and home

   In any network, a SIP end system needs to establish two SIP-related
   configuration parameters, namely the local registrar and whether
   there is an outbound proxy. There are many possible ways this
   information can be configured, but manual configuration is ill-
   advised. It is RECOMMENDED that the end system obtain local proxy
   information via the SIP server DHCP option [6]. In this approach, the
   local proxy is assumed to know where the local registrar is located,
   if it is not co-located with the proxy.

   In the absence of DHCP or manual configuration, a SIP end system has
   to assume that there is no outbound proxy.

3 Registration in Visited Network

   In the examples, we let visit the network

        Home registration only: In this model, the visiting user simply
             acquires a local IP address in the visited network and
             sends a registration with a Contact header indicating that

             REGISTER SIP/2.0
             To: <>
             From: <>
             Contact: sip:alice@

             It makes no difference here whether the visited network
             provides SIP services or not. An outbound proxy can be
             used, but it simply forwards the REGISTER request based on
             its request URI.

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             This approach works only if the visited network does not
             use a firewall.  It also means that every location update
             has to go back to the home network. (This is likely to
             matter only if IP address changes are frequent.)

             The proxy in the visited network can still provide
             localized services such as emergency calling [7] by
             remapping these addresses.

        Outbound proxy intercept: Here, the outbound proxy intercepts
             the registration request and any other outbound requests
             and changes the Contact address to its own address. It also
             has to forward the request to the local registrar. It has
             to create a new temporary user identifier that allows it to
             identify incoming requests for that visiting user. This
             could be a random identifier or the concatenation of the
             visitor's address and the proxy's domain, such as
   , where the %40 is the
             URL-escaped "@" symbol. We call the latter the canonical
             visitor name. (The proxy cannot just replace the host part
             and keep the user identifier as there may be several users,
             local and visiting, by that name.)

             This approach has the advantage that it forces incoming
             requests to use the proxy server and thus solves the
             firewall problem.

             If the registrar and proxy are not co-located, the REGISTER
             request forwarded to the registrar has to use the "real",
             local Contact address and the REGISTER request forwarded to
             the visitor's home address contains the address of the
             visited proxy.

             A rogue user can easily override the registration of the
             visiting user, although the proxy can provide some security
             by discarding any registrations where the registration
             fails in the visiting user's home network. Thus, the
             visited registrar MUST only act on the registration after a
             200 (OK) response has been returned by the home registrar.
             This approach is vulnerable to response spoofing, unless
             the response is also authenticated by Digest authentication
             or cryptographic signatures.

             The visiting user could also provide a random basic
             password when first registering and then be forced to re-
             use this secret on subsequent registrations. This would
             limit registration spoofing to those intruders that can
             snoop the initial registration. A Diffie-Hellman generated

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             key may also be useful, as long as the intruder cannot
             insert itself into the middle of the registration
             exchanges. It is probably safest if the local proxy has
             access to the local AAA mechanism, as that mechanism has
             verified the visiting user and knows which IP address has
             been assigned to it.

             As a simple precaution, proxies in visited networks can
             simply disallow changes of IP addresses for visiting users;
             however, that then only allows a single instance of a
             visiting user per visited network.

        User-initiated proxy registration: This is a variation of the
             previous approach. The visitor recognizes that it is in a
             foreign network by comparing its URI domain to the domain
             returned by DHCP in the domain name (Option 15, Section
             3.17 of [8]) or the SIP server option [6]. If they differ,
             it uses the address of the SIP server returned by the DHCP
             SIP server option as its Contact address. This assumes that
             this address is externally reachable, but even if the
             domain has it own local DNS and address space, only the
             name has to be the same, as it will be resolved by DNS SRV
             records. In most cases, this entry will simply be the
             domain name.

             The outbound proxy server intercepts the REGISTER request
             and updates its internal registration.

             User-initiated proxy registration has the advantage that it
             does not interfere with cryptographically signing
             registration requests. However, it does require minor
             adjustments in SIP UAs and additional functionality in SIP

             To avoid adding numerous configuration options, this only
             works if all outbound proxy servers can handle such
             registrations without prior configuration of the user
             identifier. This method has the same spoofing vulnerability
             as the previous one.

        Dual registration: In dual registration mode, the visiting UA
             sends two REGISTER requests, one to the local registrar,
             e.g., via multicast or the DHCP-configured outbound proxy,
             and another to the home registrar. The registration to the
             local registrar uses the canonical visitor name to avoid
             collisions, while the registration at home follows the same
             rules as the "user-initiated proxy registration" case,
             except that the proxy server can simply proxy the REGISTER

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             request not addressed to it, rather than having to also
             interpret it.

             This approach has the advantage that error handling is
             simplified, as each registration operation can fail

             However, the visitor generally has no credentials to
             authenticate the local registration, unless the registrar
             and UA somehow "borrow" credentials from some AAA
             mechanism, e.g., a CHAP secret. This is not likely to work
             across network types. (For example, it does not work in the
             common case where visitors are allowed to plug in laptops
             in a local area network while visiting a university or
             research lab.)

             This approach has the disadvantage that it requires two
             messages between UA and local registrar, which is
             undesirable particularly for bandwidth-constrained
             environments. It also requires changes in current SIP UAs.

        Third-party registration: The home registrar registers the
             visitor in the visited network, supplying its own
             credentials. The home registrar uses the domain name
             supplied in the Contact header of the visitor. This can
             obviously only work if the UA supplies a domain name rather
             than a numeric IP address.

             This approach has the fundamental architectural flaw that
             the home registrar is now acting as a UA.

   Note that while the first INVITE in a session uses the outbound
   proxies, the regular Route mechanism ([1], Section 6.38) takes over
   for subsequent requests.

4 Aliases

   Often, SIP UAs have several names, such as a SIP URI derived from the
   user's email address (e.g.,, a name reflecting
   a telephone extension (e.g., to ease dialing on
   IP phones equipped only with a numeric keypad and possibly an E.164
   address (e.g.,

   UAs may not always know their domain name, so that configurations
   derived from user logins may produce identifiers such as However, for registration in the
   visited network, the proxy or registrar in the visited network has no
   way of knowing whether these two identifiers are indeed the same

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   user, so that these two identities cannot be mapped to the same

   Particularly with telephone extensions, some care needs to be taken,
   since extensions have traditionally referred to physical lines, not
   users. Thus, the extension may be associated with a particular device
   or line.

   Rather than making aliases visible at the protocol level, it may be
   preferable to have the SIP UA simply register the same Contact for
   each of these aliases. The registrar then uses the user profile or
   rewriting rules to associate several different To values with the
   same internal registration record.

   Similarly, the location server MAY also, without registration,
   translate the request URI in incoming requests from various alias
   forms into a canonical user identifier. If the location server can
   perform this translation, it removes the need for multiple
   registrations. (TBD: are there cases where this is not the case?)

5 Home Services while in Visited Network

   For some applications, the user would like to employ services of the
   home network while generating outbound requests in the visited
   network.  The visiting UA needs to detect that it is in a foreign
   network and insert a Route header pointing to its home proxy server.
   The UA has to include the address of callee in the Route URI and the
   network address of the home server in the maddr parameter. For
   example, if Alice calls Bob, she would include the following in her
   outgoing requests:

     From:  <>
     Route: <;>

   The home proxy can either be configured statically, based on the
   user's From domain, as in the example above, or could be obtained via
   some configuration information. The details remain to be worked out.

6 SIP Naming

   It is RECOMMENDED that a user have a single identifier for email, SIP
   and as a network access identifier (NAI) [9]. Thus, every SIP URI
   SHOULD also be usable as an email address. Note that this implies
   that the algorithm for resolving aliases in proxy servers and SMTP
   servers SHOULD be the same.

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7 Registration Proxying

8 Registrar Redundancy

9 Stale Registrations

   It can occur that a device does not have the opportunity to remove a
   registration for a particular IP address before being powered down or
   otherwise being unable to communicate. Registrations will expire
   automatically, but the expiration time can be sufficiently long that
   such "orphan" registrations can cause requests to be directed to a
   network address that has in the mean time been reassigned to another

   Recipients of misdirected requests SHOULD respond with 404 (Not
   Found), which then allows the proxy to remove the registration.

   Also, since registrations are additive, a UA that could not remove a
   registration at a previous network, will just add the new
   registration, causing requests to be forked to both the new and the
   "stale" registration. The UA will obtain all current registrations,
   but if a single user has multiple devices, it is not easy for the UA
   to detect stale registrations and remove them.

   One possible solution is to add a unique "tag" parameter to the
   Contact header of REGISTER requests for those Contact fields where
   the UA is the authoritative source. The tag value is selected to be
   independent of the UAs current IP address and only depend on its
   device identity. Thus, tags are selected such that it never makes
   sense to have two registrations with the same tag value. The
   registrar keeps track of the tags associated with a registration and
   then replaces rather than adds registrations that duplicate existing
   Contact header tag values.

        Using the tag parameter in the To header field was
        considered, but since a registration may contain many
        Contact headers, it is not clear whether it should apply to
        all of them. This UAC-initiated use of the tag parameter
        also violates the UAS-initiated basic usage in other

10 Security Considerations

   It is RECOMMENDED that the user name in Basic and Digest is the same
   as the To header field, rather than a different user name, to
   simplify the use of global user databases in multi-domain SIP

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   Digest authentication does not protect the Contact header against
   alteration by an adversary. This allows the adversary to redirect
   calls to another location if it can alter requests. The
   Authentication-Info header field contains a response digest, but it
   only protects the response entity body, not header fields. It may be
   feasible to create a new qop value, "auth-header", that includes all
   headers of the request except those marked with "c", "a" or "m" in
   Tables 4 and 5 of [1].  (TBD: this is not particularly easy to
   implement since it's not clear what to do with unknown headers. Do
   the kludge that only headers before Authorization are included?)

        A2 _  Method ":" digest-uri-value ":" H(entity-body) ":" H(e2e-headers)

11 Changes Since Version 00

        o Clarification on local proxy configuration.

12 Acknowledgements

   This draft is based on discussions with Jonathan Lennox, Jonathan
   Rosenberg and Stinson Mathai.

13 Author's Address

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   electronic mail:

14 Bibliography

   [1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   Session initiation protocol," Internet Draft, Internet Engineering
   Task Force, Aug. 2000.  Work in progress.

   [2] J. Lennox and H. Schulzrinne, "CPL: a language for user control
   of internet telephony services," Internet Draft, Internet Engineering
   Task Force, Mar.  1999.  Work in progress.

   [3] J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
   interface for SIP," Internet Draft, Internet Engineering Task Force,
   May 1999.  Work in progress.

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   [4] J. Lennox and H. Schulzrinne, "Transporting user control
   information in SIP REGISTER payloads," Internet Draft, Internet
   Engineering Task Force, Mar. 1999.  Work in progress.

   [5] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying
   the location of services (DNS SRV)," Request for Comments 2782,
   Internet Engineering Task Force, Feb. 2000.

   [6] G. Nair and H. Schulzrinne, "DHCP option for SIP servers,"
   Internet Draft, Internet Engineering Task Force, Apr. 2000.  Work in

   [7] H. Schulzrinne, "Providing emergency call services for sip-based
   internet telephony," Internet Draft, Internet Engineering Task Force,
   July 2000.  Work in progress.

   [8] S. Alexander and R. Droms, "DHCP options and BOOTP vendor
   extensions," Request for Comments 2132, Internet Engineering Task
   Force, Mar. 1997.

   [9] B. Aboba and M. Beadles, "The network access identifier," Request
   for Comments 2486, Internet Engineering Task Force, Jan. 1999.

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