Internet DRAFT - draft-jennings-sipping-certs
draft-jennings-sipping-certs
SIP C. Jennings
Internet-Draft Cisco Systems
Expires: January 16, 2005 J. Peterson
NeuStar, Inc.
July 18, 2004
Certificate Management Service for SIP
draft-jennings-sipping-certs-04
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This draft defines a Credential Service that uses a SIP subscribe/
notify mechanism to discover other users' certificates and
credentials and be notified about changes to these certificates.
Other user agents that want to contact that AOR can retrieve these
certificates from the server. The result is that widespread
deployment of S/MIME in SIP is possible, because no extra expense or
effort is required of the end user.
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This work is being discussed on the sipping@ietf.org mailing list.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. UA Discovering Certificates . . . . . . . . . . . . . . . . 4
5. UA Discovering and Publishing Credentials . . . . . . . . . 5
6. Credential Server Behavior . . . . . . . . . . . . . . . . . 5
7. Negotiation of Secure Session . . . . . . . . . . . . . . . 6
8. Encrypting Bodies of SIP messages . . . . . . . . . . . . . 7
9. Signing Bodies of SIP message . . . . . . . . . . . . . . . 8
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1 Encrypted Page Mode IM Message . . . . . . . . . . . . . 8
10.2 SRTP Phone Call . . . . . . . . . . . . . . . . . . . . 9
10.3 Setting and Retrieving UA Credentials . . . . . . . . . 10
11. Security Considerations . . . . . . . . . . . . . . . . . . 11
11.1 Trusting the Identity of a Certificate . . . . . . . . . 11
11.2 Conformity to the SACRED Framework . . . . . . . . . . . 12
12. IANA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1 Certificate Event Package . . . . . . . . . . . . . . . 12
12.2 Credential Event Package . . . . . . . . . . . . . . . . 13
12.3 PKCS #8 . . . . . . . . . . . . . . . . . . . . . . . . 13
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
13.1 Normative References . . . . . . . . . . . . . . . . . . . 14
13.2 Informational References . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . 17
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1. Introduction
SIP provides a mechanism for end to end encryption and integrity
using S/MIME, and several security properties of SIP depend on S/
MIME. S/MIME has not been widely implemented or deployed due to the
complexity of providing a reasonable key management infrastructure.
This document proposes a way to address certificate discovery,
retrieval, and management for SIP deployments. It follows the Sacred
Framework RFC 3760 [7] for management of the credentials. Combined
with the Identity [2] work, this work allows users to have
certificates that are not signed by any well known certificate
authority while still strongly binding the user's identity to the
certificate. This mechanism allows UAs such as IP phones to enroll
and get their credentials without any more configuration information
than they commonly have today, without any extra effort or key clicks
by the end user, and without any extra expense for the end user.
This mechanism also lets the UA discover and retrieve the public
certificate for any other user and find out about certificate
revocations.
The general approach is to provide a new SIP service referred to as a
Credential Server that allows UAs to subscribe to some other user's
certificate. The certificate is delivered in a SIP NOTIFY to the UA
that subscribes. The identity of the certificate can be vouched for
using Identity [2] work. The Credential Service can manage public
certificates as well as credentials that include the user's private
key. The user can install new credentials to the Credential Server
using a SIP PUBLISH. The Credential Server authenticates UAs that
are changing credentials or requesting private keys using a shared
secret that both the UA and the Server know. Typically this will be
the same shared secret that is used in Register with the Registrar
for the domain.
The mechanism described in this document works for both self signed
certificates and certificates signed by a well known certificate
authority; however, it is imagined that most UAs using this would
only use self signed certificates and would use an Authentication
Service as described in [2] to provide strong identity binding to the
certificates.
Previous versions of this draft (00 to 02) used HTTP instead of SIP
for communicating with the Credential Server. The key difference
with using SIP is that a certificate can be revoked by sending a new
NOTIFY; in the HTTP based scheme, the certificates were cached for a
predefined period of time, typically one day, so that a revocation
could only take effect after the cache expired. The earlier version
also did not deal with the SACRED problem and allowed several devices
with the same AOR to all have different private keys. This resulted
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in very large SIP message and was looking fairly unwieldy; so now,
the UAs for one AOR share private keying material and use the SACRED
framework to move it between devices.
This basic approach of this work is independent of the details of
body modification [13] and identity discussions. However, the
choices made there will affect the mechanisms used to implement the
approach described here.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].
Certificate: An X.509 style certificate containing a public key and a
list of identities in the SubjectAltName that are bound to this key.
The certificates discussed in this draft are generally self signed
and use the mechanisms in the Identity work [2] to vouch for their
validity.
Credential: For this document, this means the combination of a
certificate and the associated private key.
3. Goals
o Allow negotiation of E2E encrypted sessions.
o Allow end to end encryption and integrity of SIP bodies that may
be delivered in SIP signaling, such as page mode MESSAGEs or
NOTIFY bodies in presence.
o Work for users with multiple UA devices.
o Provide a certificate revocation mechanism.
4. UA Discovering Certificates
UAs discover certificates by sending a SUBSCRIBE with an event type
of pkix-cert to the AOR for which a certificate is desired. This
could be a SIP or tel URL. The resulting NOTIFY will contain an
application/pkix-cert body which contains the certificates. The UA
MUST follow the procedures in Section 11.1 to decide if the received
certificate can be used. The UA needs to cache this certificate for
future use. The certificate MUST be removed from the cache if it has
expired, if it is updated by a subsequent NOTIFY, or if the
subscription has been terminated. The NOTIFY containing a
certificate must be signed by an Authentication Service as described
in Identity. If the identity asserted by the Authentication Service
does not match the identity requests, the certificates in the NOTIFY
are discarded and MUST NOT be used.
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5. UA Discovering and Publishing Credentials
UAs discover credentials by subscribing to their AOR with an event
type of credential, which will result in a message containing both an
application/pkix-cert body and an application/pkcs8 body that has the
associated private key information for the certificate. The UA can
change the user's certificate and private key by sending the server a
PUBLISH[3] with an event type of credential that contains both an
application/pkix-cert and an application/pkcs8 body.
The UA needs to authenticate to the Credential Server for these
operations. The UA MUST use TLS to connect to the server. The UA
may be configured with a specific name for the Credential Server;
otherwise it defaults to the name of the domain in the User's AOR.
The TLS connection MUST present a certificate that matches the
expected name for the credential server, so that the UA knows it is
talking to the correct server. If the certificate presented by the
server does not match the expected server, the UA MUST terminate the
connection and notify the user. If the UA does not do so, it may end
up publishing its private key information to an attacker. The
Credential Server will authenticate the UA using the usual SIP Digest
mechanism, so the UA can expect to receive a SIP challenge to the
SUBSCRIBE or PUBLISH messages.
The application/pkix-cert body is a DER encoded X.509 certificate
[10]. The application/pkcs8 bodies contains a DER encoded PKCS #8
object that contains the private key. The PKCS #8 objects MUST be of
type PrivateKeyInfo. The integrity and confidentiality of the PKCS
#8 objects is provided by the TLS transport. The transport encoding
of all the MIME bodies is binary.
6. Credential Server Behavior
The Credential Server stores credentials for users and can provide
the credentials or certificates to other user agents. The
credentials are indexed by an URI that corresponds to the AOR of the
user. When a UA requests a public certificate with a SUBSCRIBE, the
server sends it in a NOTIFY and sends a subsequent NOTIFY any time it
changes. When a credential is requested, the Server digest
challenges the requesting UA to authenticate it so that the Server
can verify that the UA is authorized to receive the requested
credentials.
When the Credential Server receives a SUBSCRIBE for a certificate, it
first checks to see if it has credentials for the requested URI. If
it does not it returns a response indicating the user was not found.
Otherwise it sets up a subscription and forms a NOTIFY with the
certificate in the body and the From header field value set to the
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request URI of the SUBSCRIBE. It MUST send this NOTIFY through an
Authentication Service (as described in Identity [2]) or implement an
Authentication Service itself. The Server is encouraged to keep the
subscriptions active for AORs that are communicating frequently but
MAY unsubscribe at any point of time. Any time the credentials for
this URI change, the Server MUST send a new NOTIFY to any active
subscriptions.
When a Credential Server receives a SUBSCRIBE for a credential, the
Server has to authenticate and authorize the UA and validate that
adequate transport security is being used. The Server MUST digest
challenge the UA to authenticate the UA and then decide if it is
authorized to receive the credentials.
Once the UA has authenticated with the Server, the Server can set up
a subscription and send a Notify message that MUST contain the
credentials. This NOTIFY message is sent thought an Authorization
Service in the same way as the certificate subscriptions. If the
credential changes, the Server MUST terminate any current
subscriptions and force the UA to re-authenticate. This is so that
if a secret for retrieving the credentials gets compromised, the
rogue UA will not continue to receive credentials after the
compromised secret has been changed.
When the Credential Server receives a PUBLISH to update credentials,
it MUST authenticate and authorize this the same way it does the
subscriptions for credentials. If this succeeds, the Server updates
the credential for this URI and processes all the active
subscriptions to this URI as described above.
7. Negotiation of Secure Session
SIP uses an offer/answer negotiation mechanism[16] that describes
sessions using SDP that may contain keying material, described in
[14], for media protocols such as SRTP [15]. This keying material
needs to be protected, and SIP does this by encrypting the SDP bodies
using S/MIME.
If a UA receives both an unencrypted and an encrypted SDP offer in an
multipart/alternative body, it interprets these as it would a normal
multipart alternative as defined in RFC 2046 [17], which means it
picks the last alternative that it can support. Any bodies that
cannot be decrypted are treated as unsupportable. The sending UA
should generally put encrypted offers after unencrypted ones, since
encrypted ones are preferred. The UA constructs the answer to the
offer as it normally would and may include both encrypted and
unencrypted versions of the answer using multipart/alternative. The
only wrinkle here is that if the UA sent multiple bodies with an
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offer, it needs to be able to match the answer (or answeres) to the
offer that was chosen.
The UA that made the offer can uniquely identify the various MIME
bodies using a MIME Content-ID header. However, the UA sending the
answers needs to provide the label of the Content-ID in the response.
Solutions were considered that put the Content-ID identifier in a SIP
Header, a MIME header, or an SDP attribute. Since the issue here is
fundamentally about providing information that is all at the MIME
level about the relation between one set of multipart/alternatives
and the other MIME body that is being sent, the best solution seems
to involve passing this tag at the MIME level. A new MIME header
called "Content-Related-To" updates RFC 2045 with:
rid := "Content-Related-To" ":" msg-id
and adds "[rid CRLF]" to the entity-headers.
The identifier supplied in the Content-Related-To header must be a
valid Content-ID from a previous MIME message that this body is
related to.
The UA looks at the multipart/alternatives and selects the best one
it can use. It MUST include a Content-Related-To in the MIME for the
answer that copies the tag from the related Content-ID header of the
offer body it has chosen to use.
In a typical call from Alice to Bob, Alice would first subscribe to
Bob's certificate. If this worked, then Alice would send an Invite
to Bob that contained an RTP session in unencrypted SDP and an SRTP
session in encrypted SDP. Bob would select the SRTP session and send
an answer with encrypted SDP selecting the SRTP session. Both
Alice's and Bob's UAs would indicate to the user that a secure call
had been negotiated. Alice and Bob could note that the call was
secure and adjust their conversation accordingly.
8. Encrypting Bodies of SIP messages
Applications such as presence and 911 location information result in
information with significant privacy requirements being sent in SIP.
Particular MIME types may define special meanings when both an
encrypted and unencrypted body are received, but, unless otherwise
specified, the UA SHOULD use the encrypted version if it can decrypt
it, and ignore the unencrypted version. There is no requirement for
the two versions to have the same information. For example, a page
mode message could have an unencrypted version that said "I'm in the
Middle East visiting people" while the encrypted version had much
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more sensitive information like "I'm over at Osama's house at
21.25'24"N 39.49'24"E". Depending whether the receiving device can
decrypt this or not, a different message gets displayed to the
receiving user.
9. Signing Bodies of SIP message
In general, signing messages with self-signed certificates is not
that useful unless some other means is used to vouch that the
certificate has some meaning. If the Authentication Service is used
to do this, then the Authentication Service is providing integrity
across all the bodies and binding them with an identity. In this
case, the additional signature becomes redundant. Because of this,
it is recommended that signing bodies SHOULD NOT be used if the
certificate is a self signed certificate.
10. Examples
In all these examples, large parts of the message are omitted to
highlight what is relevant to this draft. The lines in the examples
that are prefixed by $ represent encrypted blocks of data.
10.1 Encrypted Page Mode IM Message
In this example, Alice sends Bob an encrypted page mode instant
message. If Alice does not already have Bob's public key from
previous communications, she fetches Bob's public key from Bob's
credential server:
SUBSCRIBE sip:bob@biloxi.example.com SIP/2.0
...
Event: certificate
The credential server responds with the certificate in a NOTIFY.
NOTIFY alice@atlanta.example.com SIP/2.0
Subscription-State: active; expires=7200
....
From: <sip:bob@biloxi.example.com>;tag=1234
Identity: "12dsfsdk2389403823cbed"
Identity-Info: sips:billoxi.example.com
....
Event: certificate
Content-Type: application/pkix-cert
< certificate data >
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Next Alice sends a SIP MESSAGE message to Bob:
MESSAGE sip:bob@biloxi.example.com SIP/2.0
...
Content-Type: application/pkcs7-mime
$ Content-Type: text/plain
$
$ < encrypted version of "Hello" >
10.2 SRTP Phone Call
In this example, Alice calls Bob and offers both an RTP and an SRTP
session. The SDP for the SRTP session contains the SRTP keying
material and is encrypted with S/MIME. If Alice does not already
have Bob's public key from previous communications, she fetches Bob's
public key from Bob's credential server in the same way as shown in
the previous example.
Alice sends an INVITE to Bob that offers two alternative SDP bodies,
one of which is encrypted and contains the SRTP keying information.
The
INVITE sip:bob@biloxi.example.com SIP/2.0
...
Content-Type: multipart/alternative;boundary=boundary
--boundary
Content-ID: 123
Content-Type: application/sdp
Content-Disposition: session
< SDP offer for ordinary RTP only >
--boundary
Content-ID: 456
Content-Type: application/pkcs7-mime
Content-Disposition: session
$ Content-Type: application/sdp
$
$ < encrypted SDP with key for SRTP >
--boundary
If Bob's UA does not have Alice's public key, Bob's UA would fetch it
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as shown in the previous example. Assuming that Bob's UA supported
encryption, it would select the second alternative offer and
construct an appropriate answer. The 200 includes the MIME
Content-Related-To header that indicates which alternative MIME body
was chosen.
200 OK
...
Content-ID: 789
Content-Related-To: 456
Content-Type: application/pkcs7-mime
Content-Disposition: session
$ Content-Type: application/sdp
$
$ < encrypted SDP with key for SRTP >
10.3 Setting and Retrieving UA Credentials
When Alice's UA wishes to publish Alice's public and private keys to
the Credential Server, it sends a PUBLISH message like the one below.
This must be sent over a TLS connection in which the other end of the
connection presents a certificate that matches the Credential Server
for Alice and digest challenges the message to authenticate her.
PUBLISH sip:alice@atlanta.example.com SIP/2.0
...
Content-Type: multipart/mixed;boundary=boundary
--boundary
Content-ID: 123
Content-Type: application/pkix-cert
Content-Disposition: session
< Public certificate for Alice >
--boundary
Content-ID: 456
Content-Type: application/pkcs8
Content-Disposition: session
< Private Key for Alice >
--boundary
If one of Alice's UAs subscribes to the credential event, the UA will
be digest challenged, and the NOTIFY will include a body similar to
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the one in the PUBLISH section above.
11. Security Considerations
This whole scheme is highly dependent on trusting the operators of
the Credential Server and trusting that the Credential Server will
not be compromised. The security of all the users will be completely
compromised if the Credential Server is compromised.
This work requires the TLS session to be used for communications to
the Credential Server. Failing to use TLS or selecting a poor cipher
suite (such as NULL encryption) will result in credentials being sent
unencrypted over the network and will render the whole system
useless. Implementation really must use TLS or there is no point in
implementing any of this. In addition, the correct checking of
chained certificates as specified in TLS [11] is critical for the
client to authenticate the server.
If a particular credential needs to be revoked, the new credential is
simply published to the Credential Server. Every device keeping this
current in its cache will have a subscription to the credential and
will rapidly (order of seconds) be notified and replace its cache.
Clients that are not subscribed will subscribe and get the new
certificate, so they will not end up using the old invalid
certificate.
11.1 Trusting the Identity of a Certificate
When a UA wishes to discover the certificate for
sip:alice@example.com, the UA subscribes to the certificate for
alice@example.com and receives a certificate in the body of a SIP
Notify message. The term original URI is used to describe the
original URI that was subscribed to.
If the certificate is signed by a trusted CA, and one of the names in
the SubjectAltName matches the original URI, then this certificate
MAY be used but only for exactly the Original URI and not for other
identities found in the SubjectAltName. Otherwise, there are several
steps the UA MUST perform before using this certificate.
o The From header in the NOTIFY message MUST match the original URI.
o The UA MUST check the Identity header as described in the Identity
[2] work to validate that bodies have not been tampered with and
that an Authentication Service has validated this From header.
o The UA MUST check the validity time of the certificate and stop
using the certificate once it is invalid.
o The certificate MAY have several names in the SubjectAltName but
the UA MUST only use this certificate when it needs the
certificate for the identity in the Original URI. This means that
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the certificate should only be indexed in the certificate cache by
the value of the original URI, not by the value of all the
identities found in the SubjectAltName.
These steps result in a chain of bindings that result in a trusted
binding between the original URI and a public key. The Original URI
is forced to match the From. The Authentication Service validates
that this message did come from the identity claimed in the From and
that the bodies and From have not been tampered with. The
certificate in the body contains the public key for the identity.
Only the UA that can authenticate as this user can tamper with this
body, so the owner of the identity can provide a false public key but
other users cannot. This chain of assertion from original URI, to
From, to body, to public key is critical to the security of the
mechanism described in this document. If any of the steps above are
not followed, this chain of security will be broken and the system
will not work.
11.2 Conformity to the SACRED Framework
This work uses the security design outlined in the SACRED Framework
[7]. Specifically, it follows the cTLS architecture described in
section 4.2.2 of RFC 3760. The client authenticates the server using
the server's TLS certificate. The server authenticates the client
using a SIP digest transaction inside the TLS session. The TLS
sessions form a strong session key that is used to protect the
credentials being exchanged.
Credential Servers SHOULD implement the server name indication
extensions in RFC 3546 [8] and they MUST support a TLS profile of
TLS_RSA_WITH_AES_128_CBC_SHA as described in RFC 3268 [9] and a
profile of TLS_RSA_WITH_3DES_CBC_SHA.
12. IANA
The MIME Content-Related-To header does not require any IANA actions.
12.1 Certificate Event Package
To: ietf-sip-events@iana.org
Subject: Registration of new SIP event package
Package Name: certificate
Is this registration for a Template Package: No
Published Specification(s): draft-jennings-sipping-certs
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Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
12.2 Credential Event Package
To: ietf-sip-events@iana.org
Subject: Registration of new SIP event package
Package Name: credential
Is this registration for a Template Package: No
Published Specification(s): draft-jennings-sipping-certs
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
12.3 PKCS #8
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To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs8
MIME media type name: application
MIME subtype name: pkcs8
Required parameters: None
Optional parameters: None
Encoding considerations: will be binary for 8-bit transports
Security considerations: Carries a cryptographic private key
Interoperability considerations: None
Published specification: draft-jennings-sipping-certs
Applications which use this media type: Any MIME-complaint transport
Additional information:
Magic number(s): None
File extension(s): .p8
Macintosh File Type Code(s): none
Person & email address to contact for further information:
Cullen Jennings <fluffy@cisco.com>
Intended usage: COMMON
Author/Change controller:
Cullen Jennings <fluffy@cisco.com>
13. References
13.1 Normative References
[1] RSA Laboratories, "Private-Key Information Syntax Standard,
Version 1.2", PKCS 8, November 1993.
[2] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-02 (work in progress), May 2004.
[3] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", draft-ietf-sip-publish-04 (work in
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progress), May 2004.
[4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] 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.
[7] Gustafson, D., Just, M. and M. Nystrom, "Securely Available
Credentials (SACRED) - Credential Server Framework", RFC 3760,
April 2004.
[8] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and
T. Wright, "Transport Layer Security (TLS) Extensions", RFC
3546, June 2003.
[9] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[10] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC 2585,
May 1999.
[11] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
13.2 Informational References
[12] Gutmann, P., "Internet X.509 Public Key Infrastructure
Operational Protocols: Certificate Store Access via HTTP",
draft-ietf-pkix-certstore-http-07 (work in progress), May 2004.
[13] Mahy, R., "Pros and Cons of allowing SIP Intermediaries to add
MIME bodies", draft-mahy-sipping-body-add-00 (work in
progress), July 2004.
[14] Andreasen, F., Baugher, M. and D. Wing, "Session Description
Protocol Security Descriptions for Media Streams",
draft-ietf-mmusic-sdescriptions-06 (work in progress), July
2004.
[15] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
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[16] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[17] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
Authors' Addresses
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 902-3341
EMail: fluffy@cisco.com
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/
Jennings & Peterson Expires January 16, 2005 [Page 16]
Internet-Draft SIP Certificates July 2004
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