[Docs] [txt|pdf|xml] [Tracker] [Email] [Nits]

Versions: 00 01 02 03

SIP WG                                                         J. Elwell
Internet-Draft                         Siemens Enterprise Communications
Intended status:  Informational                             GmbH & Co KG
Expires:  January 8, 2009                                   July 7, 2008


 End-to-End Identity Important in the Session Initiation Protocol (SIP)
             draft-elwell-sip-e2e-identity-important-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.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on January 8, 2009.

Abstract

   This document surveys existing mechanisms in the Session Initiation
   Protocol (SIP) for identifying and authenticating the source of a SIP
   request (or caller identification).  It describes how identification
   and authentication are not always end-to-end and the problems that
   this can lead to, particularly since media security based on
   techniques such as DTLS-SRTP is dependent on end-to-end authenticated
   identification of parties.

   This work is being discussed on the sip@ietf.org mailing list.






Elwell                   Expires January 8, 2009                [Page 1]


Internet-Draft        End-to-End Identity Important            July 2008


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of existing mechanisms and their shortcomings . . . .  3
     2.1.  The From header field URI  . . . . . . . . . . . . . . . .  4
     2.2.  The P-Asserted-Identity (PAI) header field . . . . . . . .  4
     2.3.  Authenticated Identity Body (AIB)  . . . . . . . . . . . .  5
     2.4.  SIP Identity . . . . . . . . . . . . . . . . . . . . . . .  5
     2.5.  Problems with SIP URIs based on E.164 numbers  . . . . . .  6
     2.6.  Other causes of URI change at intermediate domains . . . .  7
     2.7.  Problems with PSTN interworking  . . . . . . . . . . . . .  7
   3.  Why end-to-end identification is important . . . . . . . . . .  8
     3.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Example 4  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Example 5  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.6.  Example 6  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.7.  Example 7  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.8.  Example 8  . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.9.  Example 9  . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.10. Example 10 . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.11. Example 11 . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.12. Example 12 . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.13. Summary of examples  . . . . . . . . . . . . . . . . . . . 12
   4.  Why end-to-end authentication of identity is important . . . . 13
   5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17


















Elwell                   Expires January 8, 2009                [Page 2]


Internet-Draft        End-to-End Identity Important            July 2008


1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] provides two basic
   mechanisms for identifying users involved in a session or call:  the
   From header field URI [RFC3261] and the P-Asserted-Identity header
   field [RFC3325].  Used alone, these are vulnerable to misuse, but two
   mechanisms exist for providing authentication of the From header
   field URI:  Authenticated Identity Body [RFC3893] and the Identity
   and Identity-Info header fields (SIP Identity, [RFC4474]).  These
   various mechanisms provide to the recipient of a SIP request (the UAS
   and its user) identification (with or without authentication) of the
   source of a SIP request (the UAC's user).

   Further, by binding an end of a secure bidirectional medium using
   SRTP [RFC3711] to a SIP request whose source has been identified, the
   recipient of that SIP request can know the identity of the user who
   is the source and sink of that medium.  This is the principle behind
   DTLS-SRTP [I-D.ietf-sip-dtls-srtp-framework], which uses certificates
   in the endpoints to agree a security context for SRTP.  DTLS-SRTP
   also exchanges fingerprints of those certificates in SIP messages,
   thereby binding the media to those SIP messages.  If a SIP message
   carrying such a certificate fingerprint also includes the
   authenticated identity of the user on behalf of which the SIP message
   has been sent, the secure media are bound to that user.  DTLS-SRTP
   currently relies on the From header field URI and SIP Identity to
   achieve this.

   This is the theory, but there are a number of practical
   considerations that make this very difficult to deploy in many
   situations, particularly when there are intermediaries that change
   identity information or break signatures.  This has led to a number
   of proposed work-arounds, but has also has led to a questioning of
   the need for end-to-end authenticated identification.  This document
   explains why end-to-end identification is important.

   Although the primary function of SIP is to initiate sessions (Session
   Initiation Protocol), it also includes some methods for use outside
   the context of a session, e.g., MESSAGE, SUBSCRIBE, NOTIFY, PUBLISH.
   Although the main focus of this document is on identifying users
   involved in sessions, many of the considerations apply equally to
   other uses of SIP.


2.   Overview of existing mechanisms and their shortcomings







Elwell                   Expires January 8, 2009                [Page 3]


Internet-Draft        End-to-End Identity Important            July 2008


2.1.  The From header field URI

   Although a UAC should place its Address of Record (AoR) in the From
   header field of a SIP request, it is a well known fact that in
   practice a UAC is free to place any value there.  SIP proxies are not
   allowed to change the value, but a SIP proxy could demand that the
   UAC authenticate itself (using SIP digest authentication) and reject
   a request if the From URI does not match the authenticated user.  A
   B2BUA could also do this, or could rectify the From URI and forward
   the request, as an alternative to rejecting the request.

   However, a user is likely to have a SIP digest authentication shared
   secret only with a SIP entity (proxy or B2BUA) in the same domain,
   and any downstream SIP entities (in other domains) will not be in a
   position to challenge for digest authentication.  Those SIP entities
   will have no means of knowing whether the request has been validated
   by an entity in the source user's domain, and therefore no means of
   trusting the From URI.

2.2.  The P-Asserted-Identity (PAI) header field

   This was introduced to counter some of the problems with the From
   URI.  A SIP entity that has validated the source of a SIP request can
   include a PAI header field containing the validated URI, which may
   differ from the From URI.  A downstream entity in the same trust
   domain will place some trust in this value.  Entities within the same
   trust domain must exchange SIP messages over a secure transport
   (e.g., TLS), so that the upstream entity is authenticated.  That
   upstream entity is then trusted to provide a correct identity in the
   PAI header field.

   This mechanism was introduced for use in closed environments where a
   trust domain could be established, rather than for use on the
   Internet.  However, it has seen very considerable deployment.

   The problem lies in its notion of transitive trust, i.e., A asserts
   an identity and sends it over a secure transport to B. B trusts the
   assertion, and passes the assertion on over a secure transport to C.
   C trusts B, and passes the assertion on over a secure transport to D,
   and so on.  D trusts C, and has to rely on C's trust of any upstream
   entities (in this case B).  C has to rely on B's trust of any
   upstream entities (in this case A).  The problem is, a downstream
   entity does not know the entire upstream path of trust, so in
   trusting its neighbour it does not know who else it is being forced
   to trust.  As SIP continues to grow, eventually a bad actor or
   malicious site will be trusted by another party many hops away.

   Furthermore, when an entity receives a request from outside its trust



Elwell                   Expires January 8, 2009                [Page 4]


Internet-Draft        End-to-End Identity Important            July 2008


   domain it can place a default value in the PAI header field when
   forwarding the request.  For example, when a service provider
   receives a request from an enterprise, if it does not trust the PAI
   received from the enterprise it is common practice to insert the
   default number for the enterprise, e.g., that of an attendant or
   reception desk.  This can be misleading, particularly if the request
   originated outside the enterprise and has been forwarded by the
   enterprise to the service provider.  Arguably it also violates
   [RFC3325], since the default number is being placed into PAI without
   having authenticated that number as the source of the SIP request.
   This practice can also cause the PAI URI to deviate from the From URI
   (typically they are the same in many simple situations), causing a
   dilemma for the UAS - which one to present to the user (or a dilemma
   for the user if both are presented).

2.3.  Authenticated Identity Body (AIB)

   With AIB, the UAC copies the From URI and some other header fields
   into a body of the SIP request and signs it using S/MIME [RFC3851].
   The ability to include S/MIME in [RFC3261] (and likewise PGP
   [RFC2015] in the original version of SIP [RFC2543]) demonstrates that
   end-to-end security has always been considered important in SIP, and
   AIB binds the From URI to the end-to-end authentication that S/MIME
   provides.  AIB has not been deployed because S/MIME has not been
   deployed, and that is turn can probably be blamed on the need for
   each SIP UA to have its own certificate and private key and the
   infrastructure needed to manage that.  However, the mechanism is in
   theory capable of true end-to-end authenticated identity.

2.4.  SIP Identity

   SIP Identity addresses the impracticalities of AIB by having a SIP
   entity that has validated the source of a SIP request (e.g., using
   SIP digest authentication) place a signature over the From header
   field URI and other parts of the message to assert the correctness of
   the From URI and provide integrity protection over the signed parts.
   The signature is placed in the Identity header field and information
   needed for validating the signature is placed in the Identity-Info
   header field.  This provides authenticated identification between the
   source domain and the UAS, or between the source domain and a
   verifying entity in the destination domain.  Therefore it can be
   considered to provide end-domain-to-end-domain authentication.  In
   the context of a session of call, the SIP Identity in the INVITE
   request can authenticate the calling user and SIP Identity in the
   reverse direction [RFC4916] can authenticate the connected user.
   DTLS-SRTP relies on SIP Identity to bind SRTP media to a calling or
   connected user.




Elwell                   Expires January 8, 2009                [Page 5]


Internet-Draft        End-to-End Identity Important            July 2008


   However, SIP Identity has seen little (if any) deployment, and that
   is partly due to lack of a perceived need (many regard PAI as
   sufficient) and partly because it has been shown not to work in many
   common situations.

   One difficulty is with SIP URIs based on E.164 number (see
   Section 2.5), which can result in B2BUAs changing the From header
   field URI, thereby breaking the original signature.  Re-signing by
   the domain that modifies the From URI destroys the end-domain-to-end-
   domain principle and re-introduces the transitive trust problem that
   PAI suffers from.

   Another difficult is with B2BUAs (e.g., Session Border Controller,
   SBC) that modify other parts of the signed information, in particular
   the SDP body.  This tends to happen every time media is handed off to
   another service provider.  Although this could perhaps be solved for
   an SBC in the end domain (or even for an SBC in the next domain, if
   it has a commercial arrangement with the end domain, for example
   allowing it to use the end domain's private key), it can't be solved
   in general if there are intermediate hand-offs of media with SBCs
   behaving in this way.  SBC behaviour in this respect, and the reasons
   for it, are documented in [I-D.kaplan-sip-uris-change].

2.5.  Problems with SIP URIs based on E.164 numbers

   If a user receives a caller or connected identity in the form of a
   SIP URI containing a global E.164 number (e.g.,
   sip:+123456789@example.com;user=phone), and if this information is
   made available to the user, how would the user interpret it?  The
   user might recognise the telephone number and ignore the domain part.
   The user might treat the domain part as significant and disregard the
   number (particularly if she fails to recognise the number).  Or the
   user might take account of both items of information.

   Problems arise when the user attaches importance to the domain part,
   because there is no defined meaning for the domain part (other than
   that by routing a request to that URI to that domain, that domain
   should be able to route it onwards towards the user of the telephone
   number).  In practice, the domain part is often changed by
   intermediate domains (typically to reflect their own domain), so a
   request starting out with sip:+123456789@mybank.com;user=phone in the
   From or PAI header field could end up with
   sip:+123456789@serviceprovider.net;user=phone in that header field
   when delivered to the UAS, where serviceprovider.net is the last
   domain it passed through.  The recipient would not see that the
   request really originated in mybank.com, and this information may
   have been important to the recipient.




Elwell                   Expires January 8, 2009                [Page 6]


Internet-Draft        End-to-End Identity Important            July 2008


   Moreover, any such change of From URI breaks the SIP Identity
   signature, as described earlier.

   Clearly these problems do not exist with tel URIs [RFC3966] since
   there is no domain part and therefore no scope for change.  Therefore
   they have the advantage of not providing a false or misleading domain
   part, but the disadvantage of not providing a domain part at all for
   users who would benefit from this information.  Also tel URIs cannot
   be used with SIP Identity.

   The E.164 problem is described in more detail in
   [I-D.elwell-sip-e164-problem-statement].

2.6.  Other causes of URI change at intermediate domains

   As described in Section 2.5, intermediate domains can change a URI
   based on an E.164 number, such that the recipient does not receive
   the original identifier.  This is not the sole circumstance in which
   intermediate domains are known to change an identifier identifying
   the source of a SIP request.  Another circumstance is where a domain
   does not accept a received identifier as a valid source and
   substitutes a default value.  This often occurs when an enterprise
   submits an identifier to a service provider, the identifier not being
   within the range recognised by the service provider as belonging to
   that enterprise.  There are legitimate reasons why an enterprise
   might submit an identify outside the recognised range, as highlighted
   by some of the examples in Section 3.  When delivered to the UAS, the
   new identifier might be misleading.

2.7.  Problems with PSTN interworking

   A PSTN gateway will generally deliver a number received from PSTN as
   the From or PAI URI.  The gateway has no means of validating that
   number and has either to trust the PSTN or disregard the number
   (placing its own identity or an anonymous value in the From URI).
   There are known means of a false caller number in PSTN (depending on
   country), and therefore trusting a number from PSTN can be dangerous.

   Furthermore, from a DTLS-SRTP perspective, it can be dangerous to
   assume that media are secured all the way to a PSTN user.  First, the
   PSTN has known vulnerabilities in terms of interception of calls for
   legal or other reasons.  Second, there is no way of detecting whether
   the PSTN user is attached to the PSTN via an unsecured IP network.
   Therefore, at best, a call can be considered secure only as far as
   the gateway and true end-to-end (or end-domain-to-end-domain)
   security is not achievable.  Solutions are required to the problem of
   misleading the user concerning the end-to-end security status of a
   call to/from PSTN, but this issue is not discussed further in this



Elwell                   Expires January 8, 2009                [Page 7]


Internet-Draft        End-to-End Identity Important            July 2008


   document.


3.  Why end-to-end identification is important

   In Section 2.5 and Section 2.6 it was shown how the identifier
   representing the source of SIP request can be modified by SIP
   intermediaries before being delivered to the UAS.  Furthermore,
   Section 2.4 mentioned how an intermediate domain could change the
   From URI in order to "fix" a broken RFC 4474 signature.  In these
   cases, identification delivery is not end-to-end and often fails to
   deliver information needed by the recipient.  In this section a
   number of example use cases are given, only some of which deliver
   end-to-end identification.

   In the figures associated with the examples below, caller
   identification is shown in the From header field URI, but a similar
   problem can arise with PAI.

3.1.  Example 1

   Consider a call from an employee Bob at bank.com to Alice, who
   obtains a SIP service from service provider sp.net.  Alice would be
   prepared to accept a call from her bank.  Bob's identifier is
   sip:bob@bank.com.  In this case, hopefully Alice would receive this
   identifier unchanged.  She might not know Bob, but at least she knows
   the call is from her bank and can accept the call on that basis.

     bank.com                    sp.net                        Alice
         From:sip:bob@bank.com          From:sip:bob@bank.com
         ----------------------->      ------------------------>

   This example delivers end-to-end identification, but in practice it
   is likely that any RFC 4474 signature provided by the originating
   domain will be broken because an intermediate B2BUA modifies signed
   information.

3.2.  Example 2

   Suppose the service provider removes Bob's identifier and substitutes
   the default for the bank, based on the bank's default telephone
   number +123456000 and the bank's domain name.  Alice would receive
   sip:+123456000@bank.com;user=phone.

     bank.com               sp.net                                Alice
       From:sip:bob@bank.com   From:sip:+123456000@bank.com;user=phone
       -------------------->     ---------------------------------->




Elwell                   Expires January 8, 2009                [Page 8]


Internet-Draft        End-to-End Identity Important            July 2008


   This example does not deliver end-to-end identification.  In this
   case Alice still knows the call is from her bank but there is no
   indication of who at the bank is calling.  Furthermore, if she were
   to make a return call the bank, it would arrive at a default user
   (e.g., attendant, receptionist) and would not reach Bob. This may be
   what the bank desires (in which case it would not disclose Bob's
   identifier to the service provider), but in many cases it may not be
   what the bank desires.

3.3.  Example 3

   Suppose the service provider removes Bob's identifier and substitutes
   the default for the bank, based on the bank's default telephone
   number +123456000 and the service provider's domain name.  Alice
   would receive sip:+123456000@sp.net;user=phone.

     bank.com               sp.net                                Alice
       From:sip:bob@bank.com     From:sip:+123456000@sp.net;user=phone
       -------------------->     ---------------------------------->

   This example does not deliver end-to-end identification.  In this
   case Alice cannot tell from the received identifier that the call is
   from her bank, unless she happens to recognise the telephone number.
   This is no worse than PSTN (or no worse than if a tel:  URI were used
   in SIP), but SIP has the potential to be better than PSTN.  As for
   example 2, there is also a problem with return calls.

3.4.  Example 4

   Bob's identifier is sip:+123456789@bank.com;user=phone.  If the
   service provider delivers this to Alice she will see it is from her
   bank.  She may or may not recognise the telephone number as belonging
   to Bob or to the bank.

     bank.com               sp.net                     Alice
        From:sip:+123456789       From:sip:+123456789
        @bank.com;user=phone      @bank.com;user=phone
       -------------------->      ---------------------->

   This example delivers end-to-end identification, but in practice it
   is likely that any RFC 4474 signature provided by the originating
   domain will be broken because an intermediate B2BUA modifies signed
   information.

3.5.  Example 5

   Suppose the service provider substitutes its own domain name for the
   bank's domain name.  Alice would receive



Elwell                   Expires January 8, 2009                [Page 9]


Internet-Draft        End-to-End Identity Important            July 2008


   sip:+123456789@sp.net;user=phone.

     bank.com               sp.net                     Alice
        From:sip:+123456789       From:sip:+123456789
        @bank.com;user=phone      @sp.net;user=phone
       -------------------->      ---------------------->

   This example does not deliver end-to-end identification.  In this
   case Alice cannot see that the call is from her bank, unless she
   happens to recognise the telephone number.  However, the number is
   delivered end-to-end, which may be sufficient for some purposes.

3.6.  Example 6

   Suppose the service provider substitutes its own domain name for the
   bank's domain name, and also substitutes the default telephone number
   for the bank.  Alice would receive sip:+123456000@sp.net;user=phone.

     bank.com               sp.net                     Alice
        From:sip:+123456789       From:sip:+123456000
        @bank.com;user=phone      @sp.net;user=phone
       -------------------->      ---------------------->

   This example does not deliver end-to-end identification.  Alice
   receives the same identifier as in example 3, and the same
   considerations apply.

3.7.  Example 7

   Consider a call from Carol at client.org to Dave at example.com.
   Dave is working at home and has arranged for calls to be forwarded to
   him via his SIP service provider sp.net.  Suppose Carol's identifier
   is carol@client.org and this identifier reaches example.com, where it
   is forwarded, with the INVITE request, to sp.net.  If sp.net delivers
   this unchanged to Dave at home, Dave will see that the call is from
   Carol at his client and can accept the call on that basis.  Also he
   can make a return call, e.g., if he is unable to answer at the time
   and Carol's identifier is stored in his missed call log.

     client.org       example.com            sp.net               Alice
        From:sip:carol          From:sip:carol     From:sip:carol
        @client.org             @client.org        @client.org
        ------------->          -------------->    --------------->

   This example delivers end-to-end identification, but in practice it
   is likely that any RFC 4474 signature provided by the originating
   domain will be broken because an intermediate B2BUA modifies signed
   information.



Elwell                   Expires January 8, 2009               [Page 10]


Internet-Draft        End-to-End Identity Important            July 2008


3.8.  Example 8

   Suppose the service provider does not accept sip:carol@client.org as
   an identifier received from example.com and substitutes the default
   identifier for example.com, based on its default number and its
   domain name (sip:+123456000@example.com;user=phone).

     client.org       example.com          sp.net                 Alice
        From:sip:carol        From:sip:carol     From:sip:+123456000
        @client.org           @client.org        @example.com
                                                 ;user=phone
        ------------->        -------------->    ------------------>

   This example does not deliver end-to-end identification.  Dave will
   now see that the call comes from his own company, and will not have a
   clue that it comes from his client.  Similarly if the service
   provider's domain name is used (sip:+123456000@sp.net;user=phone),
   Dave would presumably recognise his company's own default telephone
   number but would not see that the call is from his client.  Also any
   attempted return call would just go to his company's default
   answering point.

3.9.  Example 9

   Suppose Carol's identifier is E.164-based:
   sip:+123498765@client.org;user=phone.  If this is delivered to Dave,
   he will see the calling telephone number, which he may recognise (or
   software in his phone may match it with an existing contact) and he
   will also see that it is from client.org.

     client.org         example.com           sp.net             Alice
       From:sip:+123498765   From:sip:+123498765   From:sip:+123498765
       @client.org           @client.org           @client.org
       ;user=phone           ;user=phone           ;user=phone
       ------------------>   ------------------>   ------------------>

   This example delivers end-to-end identification, but in practice it
   is likely that any RFC 4474 signature provided by the originating
   domain will be broken because an intermediate B2BUA modifies signed
   information.

3.10.  Example 10

   Suppose the identifier in the last example is not accepted by the
   service provider, not only because of the domain part (client.org
   rather than example.com) but also because the telephone number does
   not fall within the range assigned to example.com.  As in example 8
   it might substitute a default identifier.



Elwell                   Expires January 8, 2009               [Page 11]


Internet-Draft        End-to-End Identity Important            July 2008


     client.org         example.com           sp.net             Alice
       From:sip:+123498765   From:sip:+123498765   From:sip:+123498000
       @client.org           @client.org           @sp.net
       ;user=phone           ;user=phone           ;user=phone
       ------------------>   ------------------>   ------------------>

   This example does not deliver end-to-end identification.
   Consequences are similar to those in example 8.

3.11.  Example 11

   Eve in the US office of enterprise e.com
   (sip:+123456789@e.com;user=phone) makes a call to Fred, who has a UK
   telephone number (+44...) and is served by UK service provider
   uksp.net.  The US proxy in e.com forwards the request to the UK proxy
   of e.com, where the call "breaks out" to uksp.net.  The service
   provider does not accept a non-UK identifier and substitutes a
   default value for the enterprise (sip:+445678000@e.com;user=phone).

     e.com                      uksp.net                        Fred
         From:sip:+123456789             From:sip:+445678000
         @e.com;user=phone               @e.com;user=phone
        -------------------->           ------------------------>

   This example does not deliver end-to-end identification.  In this
   case Fred still knows the call is from e.com, but is not aware that
   Eve is calling or that that the caller is in the US.

3.12.  Example 12

   Suppose the service provider uses its own domain name in the modified
   SIP URI.

     e.com                      uksp.net                        Fred
         From:sip:+123456789             From:sip:+445678000
         @e.com;user=phone               @uksp.net;user=phone
        -------------------->           ------------------------>

   This example does not deliver end-to-end identification.  In this
   case Fred does not know that the call is from e.com (unless he
   happens to recognise the UK telephone number).  Also Fred is not
   aware that Eve is calling or that that the caller is in the US.

3.13.  Summary of examples

   Examples 1, 4, 7 and 9 are fine, because identification of the caller
   is end-to-end (although, as pointed out, any RFC 4474 signature might
   be broken).  In the remaining examples, identification is not end-to-



Elwell                   Expires January 8, 2009               [Page 12]


Internet-Draft        End-to-End Identity Important            July 2008


   end, leading to problems.

   More complex examples can be derived with more domains involved.
   Clearly the more domains involved, the more there is scope for
   failure to deliver an identifier end-to-end, and the greater the
   consequences for the recipient, both in terms of recognising the
   source of the call and being able to make a return call.  These
   examples illustrate the importance of delivering an identifier end-
   to-end, without changing it at intermediate domains.


4.  Why end-to-end authentication of identity is important

   Assuming an identifier is delivered end-to-end, where authenticated
   identity is required it is important that the assertion of
   authenticity is provided at source, or at least in the originating
   domain.  This is what SIP Identity aims to achieve.  However, because
   of the difficulties with SIP Identity, as described in Section 2.4,
   some have asked why hop-by-hop assertions are insufficient.  PAI is
   one solution to hop-by-hop assertions.  Another possibility would be
   for each domain to provide its own cryptographic signature.  Note
   that SIP Identity does not allow this, because the signer has to have
   the same domain name as that in the From URI, so only the originating
   domain can sign (unless the identifier is also changed, which would
   mean that requirements for end-to-end identification would not be
   met).

   With end-to-end authentication, the relying party has to trust the
   originating domain, which also means trusting the certificate chain
   up to the top level certification authority.  This is similar to
   other applications using PKI-based security, such as secure web
   pages.  In many cases there will just be the signing domain's
   certificate and a single CA certificate.  The relying party can see
   the whole chain and make its own judgements.

   With hop-by-hop authentication based on PAI, the relying party knows
   only that the upstream neighbour domain is asserting that domain.  It
   does not know how many further upstream domains there are, what those
   domains are, and how far the trust domain extends.  Just because the
   relying party trusts its own domain and perhaps its upstream
   neighbour domain, does not mean that it would trust further domains
   that its upstream neighbour domain trusts.

   For example, consider a call from Alice in enterprise1.biz
   (sip:alice@enterprise.biz), via service provider sp1.net, via second
   service provider sp2.org, and terminating at Bob in enterprise2.com
   (sip:bob@enterprise2.com).  The call is routed that way because
   enterprise1.biz routes all external calls through sp1.net, and



Elwell                   Expires January 8, 2009               [Page 13]


Internet-Draft        End-to-End Identity Important            July 2008


   enterprise2.com only accepts external calls that have arrived via
   sp2.org.  Bob is happy to accept a secure call from enterprise1.biz.
   With hop-by-hop authentication, Bob would have to rely on an
   assertion by enterprise2.com, which in turn would rely on an
   assertion by sp2.org, and so on.  Bob has no visibility of the
   upstream entities, although he would probably be aware of his
   enterprise's own service provider (sp2.org).  He would be unlikely to
   be aware of sp1.net, and even if he were aware, he may not have heard
   of sp1.net and may not wish to trust such an assertion.  It could be
   that sp1.net is located in a country where practices are not of the
   standard expected in Bob's country.

   Suppose also that DTLS-SRTP is to be used to secure media between
   Alice and Bob. If authentication is hop-by hop, Bob can be sure that
   media is secured as far as sp2.org, but cannot be sure that there is
   no man-in-the-middle between sp2.org and enterpise1.biz.  End-to-end
   authentication is required to give Bob the assurance he needs.

   Referring back to the examples in Section 4, those that deliver end-
   to-end identification have the potential to deliver end-to-end
   authentication, but in practice, SIP Identity as specified in
   [RFC4474] is often broken by the actions of B2BUAs.  The remaining
   examples, because they do not deliver end-to-end identification,
   cannot deliver end-to-end authentication.


5.  Conclusions

   This documented has demonstrated the importance of end-to-end
   identifiers (or at least end-domain-to-end-domain identifiers) and
   authentication of those identifiers in SIP.  Although in many simple
   cases hop-by-hop identification or hop-by-hop assertions can be shown
   to be adequate, there are many cases where this is simply not the
   case.

   SIP Identity as a solution to end-to-end authenticated identifiers is
   known to have some shortcomings, and these need to be fixed, either
   by enhancement to SIP Identity or by provision of an alternative
   mechanism.


6.  IANA considerations

   This document requires no IANA actions.







Elwell                   Expires January 8, 2009               [Page 14]


Internet-Draft        End-to-End Identity Important            July 2008


7.  Security considerations

   Authentication of parties involved in a call is an essential part of
   this document and is fully discussed in the preceding sections.
   There are no other security considerations.


8.  Acknowledgements

   The author received valuable comments from Kai Fischer, Hadriel
   Kaplan and Dan Wing during drafting.


9.  Informative References

   [RFC2015]  Elkins, M., "MIME Security with Pretty Good Privacy
              (PGP)", RFC 2015, October 1996.

   [RFC2543]  Handley, M., Schulzrinne, H., Schooler, E., and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              March 1999.

   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC3893]  Peterson, J., "Session Initiation Protocol (SIP)
              Authenticated Identity Body (AIB) Format", RFC 3893,
              September 2004.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for



Elwell                   Expires January 8, 2009               [Page 15]


Internet-Draft        End-to-End Identity Important            July 2008


              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.

   [I-D.elwell-sip-e164-problem-statement]
              Elwell, J., "SIP E.164 Problem Statement",
              draft-elwell-sip-e164-problem-statement-00 (work in
              progress), February 2008.

   [I-D.kaplan-sip-uris-change]
              Kaplan, H., "Why URIs Are Changed Crossing Domains",
              draft-kaplan-sip-uris-change-00 (work in progress),
              February 2008.

   [I-D.ietf-sip-dtls-srtp-framework]
              Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing an SRTP Security Context using DTLS",
              draft-ietf-sip-dtls-srtp-framework-01 (work in progress),
              February 2008.


Author's Address

   John Elwell
   Siemens Enterprise Communications GmbH & Co KG
   Hofmannstrasse 51
   D-81379 Munich
   Germany

   Phone:  +44 115 943 4989
   Email:  john.elwell@siemens.com


















Elwell                   Expires January 8, 2009               [Page 16]


Internet-Draft        End-to-End Identity Important            July 2008


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   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.

   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.


Intellectual Property

   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.











Elwell                   Expires January 8, 2009               [Page 17]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/