SIP WG J. Peterson
Internet-Draft NeuStar
Expires: April 28, 2003 October 28, 2002
Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP)
draft-ietf-sip-identity-00
Status of this Memo
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all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The existing mechanisms for expressing identity in the Session
Initiation Protocol oftentimes do not permit an administrative domain
to verify securely the identity of the originator of a request. This
document recommends practices and conventions for authenticating end
users, and proposes a way to distribute cryptographically secure
authenticated identities within SIP messages.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Using an Authentication Service . . . . . . . . . . . . . . . 5
4. How to Share Verified Identities . . . . . . . . . . . . . . . 5
4.1 Body Added by Authentication Service . . . . . . . . . . . . . 6
4.2 Body Added by Client . . . . . . . . . . . . . . . . . . . . . 7
4.3 Using Content Indirection . . . . . . . . . . . . . . . . . . 8
5. Identity in Responses . . . . . . . . . . . . . . . . . . . . 9
6. Receiving an Authentication Token . . . . . . . . . . . . . . 9
6.1 Authentication Service Handling of Authentication Tokens . . . 9
7. Selective Sharing of Identity . . . . . . . . . . . . . . . . 10
7.1 Requesting Privacy . . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
Normative References . . . . . . . . . . . . . . . . . . . . . 12
Informative References . . . . . . . . . . . . . . . . . . . . 13
B. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP) [1]. An identity, for the purposes of this document, is
defined as a canonical SIP URI employed to reach a user (such as
'sip:alice@atlanta.com').
RFC3261 enumerates a number of places within a SIP request that a
user can express an identity for themselves, notably the From header
field. However, the recipient of a SIP request has no way to verify
that the From header field has been populated appropriately without
some sort of cryptographic authentication mechanism.
Today, RFC3261 specifies a number of security mechanisms that can be
used by SIP UAs, including Digest, TLS and S/MIME (and
implementations may support other security schemes as well).
However, few SIP user agents today can support the end-user
certificates necessary to authenticate themselves via TLS or S/MIME,
and Digest authentication is limited by the fact that the originator
and destination must share a secret. It is desirable for SIP user
agents to be able to send requests to destinations with they have no
previous association - just as in the telephone network today, one
can receive a call from someone with whom one has no previous
association, and still have a reasonable assurance that their
displayed Caller-ID is accurate.
Many SIP user agents today support a means of authenticating
themselves to a SIP registrar - commonly with a shared secret
(Digest, which MUST be supported by SIP user agents, is typically
used for this purpose). Registration allows a user agent to express
that it is the proper place to which requests should be sent for a
particular address-of-record SIP URI. However, the credentials with
which a user agent proves to a registrar that they are, for example,
an authorized recipient of requests for 'sip:alice@atlanta.com'
cannot be shared with a server in another domain - these credentials
are currently only useful for local registration.
Coincidentally, the address-of-record URI of a SIP user is also the
URI with which a SIP UA populates the From header of requests from
that user - in other words, the address-of-record is an identity. So
it turns out that users already have a means of providing their
identity, if only to a registrar; in fact, since the contents of a
From header field are essentially a 'return address' for SIP
requests, being able to prove that you are eligible to receive
requests for that 'return address' is identical to proving that you
are authorized to assert this identity. In other words, the best way
for a SIP user to prove that they can legitimately claim an identity
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is to provide the same credentials they would need to provide in
order to register to receive requests for that identity; the two have
the same security properties. Since the operator of the registrar
controls the namespace of local SIP users (assigning the user portion
of SIP URIs in the domain), it is the ideal arbiter for identity in
SIP.
In the absence of end-user certificates in user agents, it is
possible to implement a mediated authentication architecture for SIP
in which requests are sent to a server in the user's local domain
which authenticates them (using the same practices by which the
domain would authenticate REGISTER requests). Once a request has
been authenticated, the local domain then needs some way to
communicate to remote domains that it has sanctioned the request.
This draft addresses how that identity can could be securely shared.
RFC3261 already describes an architecture very similar to this in
Section 26.3.2.2, in which a user agent authenticates itself to a
local proxy server which in turn authenticates itself to a remote
proxy server via mutual TLS, creating a two-link chain of transitive
authentication between the originator and the remote domain. While
this works well in some architectures, there are a few respects in
which this is impractical. For one, it is possible for SIP requests
to cross multiple intermediaries in separate administrative domains,
in which case transitive trust becomes far less compelling. It also
requires intermediaries to act as proxies, rather than redirecting
requests to their destinations (redirection lightens loads on SIP
intermediaries). Both of these limitations result from the fact that
authentication takes place outside the application, at the transport
layer, rather than within SIP itself.
One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP
header. A mechanism for doing so (with the P-Asserted-Identity
header) is given in [6]. However, this solution allows only hop-by-
hop trust between intermediaries, not end-to-end cryptographic
authentication, and it assumes a managed network of nodes with strict
mutual trust relationships, an assumption that is incompatible with
widespread Internet deployment.
The desired mediated authentication architecture has quite a bit in
common with the problem space of Kerberos [5]. Ideally, there should
be a way for a user to authenticate themselves to the local domain,
and receive some kind of token that they can share with recipients of
requests that lets them know that the user has been authenticated by
the local domain. However, Kerberos support in SIP user agents is
not widespread, and moreover SIP uses other means (such as Digest) to
perform key authentication functions already. An ideal solution
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would adapt existing SIP security mechanisms to address this problem.
Therefore, this document defines a new logical role for SIP network
intermediaries (typically proxy servers) called an 'authentication
service'. Once an authentication service has verified the identity
of the originator of a request, as describe above, it creates a
cryptographic token that contains the authenticated identity of the
user, and which has some reference integrity with the request itself.
This token can then be added to a SIP request and inspected by
recipients of the request who would like a cryptographic guarantee of
the identity of the user.
One possible format for such tokens is the Authenticated Identity
Body (AIB) described in [4]. Other plausible token formats are a
matter for further investigation. Throughout this document, the use
of the AIB format as a token is considered exclusively.
2. 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 RFC2119 [2] and indicate requirement levels for
compliant SIP implementations.
3. Using an Authentication Service
A SIP user agents sends requests to an authentication service in
order to receive an authentication token for the request. How
exactly the association with an authentication service is configured
is an implementation-specific matter for the user agent - it might be
implemented with a pre-loaded Route header. The guidelines given in
RFC3261 Sections 26.3.2.1 and 26.3.2.2 should be used when connecting
to an authentication service; ideally, an authentication service
should be one hop away from a user agent, it should use a lower-layer
security protocol such as TLS or IPSec to authenticate the
authentication service before providing credentials (especially
shared secrets).
This document places no requirements on how an authentication
services authenticates requests. Since Digest authentication MUST be
supported by all SIP entities, the use of Digest for this purpose is
likely.
4. How to Share Verified Identities
When an authentication service has authenticated the user, it must
construct an identity for that user that will be contained in the
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token. It is RECOMMENDED that these identities take the form of
addresses-of-record, as they are defined in Section 10 of RFC3261; in
other words, URIs of the form 'sip:alice@atlanta.com'.
This identity must be expressed in the authentication token that will
be signed by the authentication service. For example, if the
Authentication Identity Body (AIB) format described in [4] is used,
this identity would be stored in the From header field within a
'message/sip' or 'message/sipfrag' [7] body that will be signed by
the authentication service.
In some cases, an authentication service will hold a certificate
corresponding to each user in its administrative domain (in other
words, a certificate whose subjectAltName contains a URI equivalent
to the address-of-record URI of the user). The appropriate
certificate for the authenticated user will be used to sign the
authentication token. However, an authentication service MAY instead
use a common certificate for its administrative domain. The
subjectAltName of this certificate MUST correspond with the host
portion of the From header field of the identity in the
authentication token (if the identity were 'sip:alice@atlanta.com',
the subjectAltName of the certificate would be 'atlanta.com'); this
should be the same certificate that the authentication service
provides when proving its own identity (via TLS or some similar
protocol). Maintaining individual certificates for each user is
RECOMMENDED, since the name subordination rules involved with the use
of a common certificate for the domain can become complicated.
After the authentication token has been signed, the authentication
token MUST be added to any existing MIME bodies in the request, if
necessary by transitioning the outermost MIME body to a 'multipart/
mixed' format. Two options are considered for ways that an
authentication token could be added to a SIP message: one in which
the authentication service adds the token itself, and one in which it
pushes the token back to the client, essentially asking the client to
include the token in a retry of the request. Another possibility,
using content indirection, is mentioned as a direction for future
work. Authentication services MUST support the mechanism in Section
4.2 and MAY support the mechanism in Section 4.1.
4.1 Body Added by Authentication Service
The first possibility is that the authentication service could add
the body to the request itself before forwarding the request.
However, the authentication service role is usually played by
entities that act as proxy servers for most requests, and proxy
servers cannot modify message bodies (see RFC3261 Section 16.6). In
order to add an authentication token, the authentication service
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needs to act as a transparent back-to-back user agent, effectively
terminating the request and re-originating it with a new body
appended to any existing MIME bodies (again, transposing to various
MIME multipart forms as necessary).
This mechanism has some potential advantages over push the
authentication token back to the originating user agent. For one, it
saves on additional round-trip times for signaling that result from
passing the body back to the user agent. It also requires no new SIP
mechanisms, whereas any method of asking a user agent to include a
body in a resubmission to the current request would introduce new
protocol requirements.
However, there are proposed SIP integrity mechanisms that place a
signature over the entire message body in a SIP message header. Were
a server to modify the body of a message that was protected by such
signature, that would be perceived as an integrity violation by
downstream recipients of the message. Presumably, a back-to-back
user agent function would have to sacrifice this end-to-end
integrity.
4.2 Body Added by Client
Alternatively, the authentication service could in some fashion
return the authentication token to the originating user agent,
prompting the user agent to retry the request with the authentication
token attached. No existing SIP mechanism performs this function.
Therefore, this document defines a 428 "Use Authentication Token"
response code.
An authentication service sends a 428 with a MIME body in order to
request that a user agent add the enclosed MIME body to their request
and retry the request. A 428 MUST have at most a single MIME body.
This MIME body MUST be signed by the authentication service.
The use of 428 without any MIME body is also defined in this
document. It can be sent by any server to reject a request because
the request does not contain an authentication token. A user agent
receiving this rejection SHOULD retry their request through an
authentication service.
In order to signal to the authentication services that the
originating user agent supports the receipt of the 428 response code,
a new option-tag has been defined, the 'auth-id' option-tag. User
agents SHOULD supply the 'auth-id' option-tag in a Supported header
whenever they provide credentials to a server (for example, in Digest
authentication, whenever a Proxy-Authorization header is added to a
request).
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Using the 428 response code may introduce extra round-trip times for
messages, delaying the setup of requests (one RTT for the 407,
another for the 428). However, there are some circumstances under
which extra RTTs may not impede performance. If the originating user
agent possesses a non-stale nonce (assuming Digest authentication)
from the authentication service, it can pre-emptively include a
Proxy-Authorization header, eliminating one RTT. With regard to the
second RTT, note that the request needn't necessarily go through the
authentication service again once the authentication token has been
added - it could go directly to its destination, which reduce the
impact of the second RTT.
There are two reasons why the originating user agent should be the
party responsible for adding the authentication token to the request.
Firstly, because this gives the client the opportunity to inspect the
body itself (perhaps only to see whether or not it is encrypted; see
[4]) in order to verify that the authenticated identity corresponds
with the provided credentials and the user's preferences. Secondly,
the client can provide a signature over the entire body of the
message (either with S/MIME or some header-based mechanism) so that
the final recipient of messages can be assured that all information
in the body is there at the originator's behest.
4.3 Using Content Indirection
Work [8] is currently underway in the SIP WG to define a content
indirection mechanism for SIP, a mechanism by which a MIME body in a
SIP request can refer, with a URL, to a document that it hosted
somewhere in the network. This raises another interesting
possibility for authentication token management.
A SIP user agent could create a content indirection MIME body (using
the RFC2017 [9] URL MIME External-Body Access-Type) that contains a
URL that identities a resource controlled by the authentication
service, anticipating that the authentication service will make the
authentication token available at that URL. Authentication services
could define a standard way to anticipate URIs for a particular
request (for example, an HTTP URL could be structured with a hostname
corresponding to the authentication service, and a path corresponding
to a unique identifier selected by the user agent for this request:
http://auth-serv.biz/12345678901234567890). Once an authentication
service has validated the request, it simply makes the authentication
token available at the anticipated URL; recipients of the message
would then dereference the URL in order to inspect the token.
This approach could allow user agents to have full control over the
integrity of SIP requests, while still not requiring the extra RTT
caused by the use of the 428 response code. It also has numerous
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advantages over other ways of handling authentication tokens issued
for SIP response messages (see Section 5).
5. Identity in Responses
Many of the practices described in the preceding sections can be
applied to responses as well as requests, with some important
differences. Primarily, the distinction is that a response cannot be
challenged or resubmitted in the same manner as a request. However,
when a user agent registers under a particular identity, and thereby
becomes eligible to receive requests and send responses associated
with that identity, it provides credentials that prove its identity,
and thus the registrar is in a reasonable position to act as an
authentication service for responses.
Note that the identity in an authentication token in a response
almost certainly will not correspond with identity asserted in the
From header field of the response (which is copied from the Request);
the identity in the authentication token represents a different
entity. In many network implementations, the identity in the
authentication token of a response will correspond to the To header
field of the request, but there are numerous legitimate architectures
(which contain redirections) in which this will not be the case.
An authentication service that acts as a registrar can add to a
response an authentication token that corresponds to the identity of
the originator of that response in roughly the same manner described
in Section 4.1 - the authentication service adds the authentication
token to a response before it forwards the response towards the
originator of the request. There is no way for an authentication
service to perform a function for responses comparable to the
mechanism described in Section 4.2; however, content indirection (see
Section 4.3 could provide an alternative that would allow the client
to retain end-to-end integrity properties on responses.
6. Receiving an Authentication Token
The manner in which an authentication token is handled is dependent
on the nature of the token itself; for rules for handling the
Authentication Identity Body (AIB) format, see [4].
6.1 Authentication Service Handling of Authentication Tokens
SIP intermediaries generally should not attempt to inspect MIME
bodies; following the rules of RFC3261 Section 16.6, MIME bodies may
be encrypted end-to-end or have other properties that make them
unsuitable for consumption by intermediaries. However,
intermediaries that implement the authentication service logical role
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MAY inspect MIME bodies in order to find one with a Content-
Disposition of 'auth-id'.
For the most part, the actual value of an authenticated identity is
not likely to be of interest to a proxy server, though it MAY refuse
to process a request that does not contain a valid authentication
token (using the 428 request, as described in Section 4.2). However,
proxy servers MAY additionally maintain lists of known problem users
that are banned from making requests to their administrative domain,
for example, and subsequently reject some requests after comparing
their authenticated identities to such access control lists.
7. Selective Sharing of Identity
Most of the time, there is no need to restrict the propagation of
verified identities in the network. User agents and intermediaries
benefit from receiving verified identities. However, in some cases
intermediaries might want to restrict the distribution of identity
information, for example if
o the authenticated identity body contains an identity that is only
meaningful as an internal identifier within a particular service
provider's network, or,
o the originating user agent has requested privacy, and the
unrestricted distribution of the authenticated identity body would
violate that request.
If it is not appropriate to share an authenticated identity, an
authenticated identity body SHOULD NOT be created and distributed.
However, in some cases there may be other entities in the
administrative domain of the authentication service that are
consumers of the authenticated identity. If, for example, each of
these servers needed to challenge the user individually for identity,
it might significantly delay the processing of the request. For that
reason, it may be appropriate to circulate authenticated identity
bodies among a controlled set of entities. For that purpose, an
encryption mechanism for authenticated identities is required.
7.1 Requesting Privacy
When users authenticate themselves to an authentication service, they
MAY use SIP to explicitly notify the service that they do not wish
their authenticated identity to be circulated. Usually, the user in
question would also be taking other steps to preserve their privacy
(perhaps by including an anonymous From header in the SIP request,
and following other standard privacy practices).
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Authentication services MUST support the privacy mechanism described
in [3]. Users requesting privacy should also support the mechanisms
described in that document.
In particular, this document uses an identity-specific priv-value
that can appear in the Privacy header, a value of 'id', which was
registered by [6]. This Privacy value requests that the results of
authentication should not be shared by the authenticating server. An
authentication service SHOULD NOT create an authentication token for
a request when 'id' privacy has been requested. If such a token is
created, it MUST be encrypted or rendered confidential in the manner
most appropriate to the token. Guidelines for encrypting AIBs are
given in [4], and these mechanisms MUST be supported by any
authentication service that uses AIBs.
8. Security Considerations
Users SHOULD NOT provide credentials to an authentication service to
which they cannot initiate a direct connection, preferably one
secured by a network or transport layer security protocol such as
TLS. If a user does not receive a certificate from the
authentication service over this lower-layer protocol that
corresponds to the expected domain (especially when they receive a
challenge via a mechanism such as Digest), then it is possible that a
rogue server is attempting to pose as a authentication service for a
domain that it does not control, possibly in an attempt to collect
shared secrets for that domain. If a user cannot connect directly to
the desired authentication service, the user SHOULD at least use a
SIPS URI to ensure that mutual TLS authentication will be used to
reach the remote server.
Relying on an authentication token generated by a remote
administrative domain assumes that the domain uses some trustworthy
practice to authenticate its users. However, it is possible that
some domains will implement policies that effectively make users
unaccountable (such as accepting unauthenticated registrations from
arbitrary users). Therefore, it is RECOMMENDED that authentication
tokens contain some indication of the specific practice (for example,
Digest) that was used to authenticate this request as a rough
indicator of credential strength.
If a common certificate is used by an authentication service (rather
than individual certificates for each identity), certain problems can
arise with name subordination. For example, if an authentication
service holds a common certificate for the hostname
'sip.atlanta.com', can it legitimately sign a token containing an
identity of 'sip:alice@atlanta.com'? It is difficult for the
recipient of a request to ascertain whether or not 'sip.atlanta.com'
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is authoritative for the 'atlanta.com' domain unless the recipient
has some foreknowledge of the administration of 'atlanta.com'.
Therefore, it is RECOMMEND that user agent recipients of
authentication tokens notify end users if there is ANY discrepancy
between the subjectAltName of the signers certificate and the
identity within the authentication token.
Authentication tokens MUST have some form of replay protection. The
best protection is to copy the SIP request in its entirety (via the
'message/sip' MIME type) into the authentication token - in that way,
it will be clear that this token has been issued for this request,
since collectively the headers of a SIP request provide a unique
identifier. However, SIP requests can be large, and it is reasonable
to include only a subset of the SIP headers in a request (using the
'message/sipfrag' MIME type) as long as certain critical headers are
provided. For further discussion of this issue, including guidelines
for including particular headers in a sipfrag, see [4].
Because the common certificates that can be used by authentication
services need to assert only the hostname of the authentication
service, existing certificate authorities can provide adequate
certificates for this mechanism. However, not all proxy servers and
user agents will be able support the root certificates of all
certificate authorities, and moreover there are some significant
differences in the policies by which certificate authorities issue
their certificates. This document makes no recommendations for the
usage of particular certificate authorities, nor does it describe any
particular policies that certificate authorities should follow, but
it is anticipated that operational experience will create de facto
standards for the purposes of authentication services. Some
federations of service providers, for example, might only trust
certificates that have been provided by a certificate authority
operated by the federation.
9. IANA Considerations
This document defines a new SIP status code, '428 Use Authentication
Token'. The use of this status code is further described in Section
4.2.
Normative References
[1] 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.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
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[3] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", draft-ietf-sip-privacy-general-02 (work in
progress), June 2002.
[4] Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
draft-ietf-sip-authid-body-00 (work in progress), October 2002.
Informative References
[5] Kohl, J. and C. Neumann, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[6] Jennings, C., Peterson, J. and M. Watson, "Private Extensions to
the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", draft-ietf-sip-asserted-identity-02
(work in progress), June 2002.
[7] Sparks, R., "Internet Media Type message/sipfrag", draft-ietf-
sip-sipfrag-00 (work in progress), September 2002.
[8] Olson, S., "A Mechanism for Content Indirection in SIP
Messages", draft-ietf-sip-content-indirect-mech-01 (work in
progress), August 2002.
[9] Freed, N., "Definition of the URL MIME External-Body Access-
Type", RFC 2017, November 1996.
Author's Address
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/
Appendix A. Acknowledgments
The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
Sparks, Jonathan Rosenberg, Mark Watson and Patrik Faltstrom for
their comments. Cullen Jennings assisted greatly in the development
of the content indirection mechanism considered in Section 4.3.
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Appendix B. Changelog
Changes from draft-peterson-sip-identity-01:
- Split off child draft-ietf-sip-authid-body-00 for defining of
the AIB
- Clarified scope in introduction
- Removed a lot of text that was redundant with RFC3261
(especially about authentication practices)
- Added mention of content indirection mechanism for adding token
to requests and responses
- Improved Security Considerations (added piece about credential
strength)
Changes from draft-peterson-sip-identity-00:
- Added a section on authenticated identities in responses
- Removed hostname convention for authentication services
- Added text about using 'message/sip' or 'message/sipfrag' in
authenticated identity bodies, also RECOMMENDED a few more headers
in sipfrags to increase reference integrity
- Various other editorial corrections
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Peterson Expires April 28, 2003 [Page 15]
