SIPPING K. Ono
Internet-Draft S. Tachimoto
Expires: August 16, 2004 NTT Corporation
Feb 16, 2004
End-to-middle Security in the Session Initiation Protocol(SIP)
draft-ono-sipping-end2middle-security-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
A Session Initiation Protocol (SIP) User Agent (UA) does not always
trust all proxy servers in a request path enough to allow them
inspect the message bodies and/or headers contained in a message. The
UA might want to protect the message bodies and/or headers from all
proxy servers except the particular proxy that provides services that
depend on the ability to inspect them. In this situation, SIP needs a
mechanism for securing information passed between the UA and an
intermediary proxy, also called "end-to-middle security", which can
work with end-to-end security. This document proposes mechanisms to
achieve end-to-middle security, mainly data confidentiality for
end-to-middle communication.
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Conventions used in this document
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 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Proposed Mechanisms . . . . . . . . . . . . . . . 4
2.1 Creation of S/MIME CMS Bodies for UAs and Proxy servers . . . 4
2.2 Indicating the Target Content . . . . . . . . . . . . . . . . 5
2.3 Discovery of Proxy Server's Policies . . . . . . . . . . . . . 5
3. Behavior of UAs and Proxy Servers . . . . . . . . . . . . . . 7
3.1 UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Summary of Content-Disposition Header Field Use . . . . . . . 10
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1 Request Example . . . . . . . . . . . . . . . . . . . . . . . 11
5.2 Response Example . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 19
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1. Introduction
The Session Initiation Protocol (SIP) [2] supports hop-by-hop
security using TLS [3] and end-to-end security using S/MIME [4].
However, a UA sometimes wants to protect the message bodies and/or
headers from all proxy servers except a selected proxy server, which
provides some sort of service based on their content. Such a proxy is
not always adjacent to the UA. These situations require security
between the UA and the intermediary proxy server for the message
bodies and/or message headers. We call this "end-to-middle security",
where by "end" we mean a UA and by "middle" we mean a specific
proxy.
End-to-middle security is useful in a network where trusted and
partially trusted proxy servers both exist in a message path. The
partially trusted proxy servers are trusted only to view headers
related to routing. The trusted proxy servers are trusted to view the
message bodies and/or headers to provide services based on their
content. For a UA requiring such services from intermediaries,
end-to-end confidentiality will currently have to be disabled to take
advantage of them. This problem is pointed out in Section 23 of [2].
Some examples of services that a proxy provides using the content of
message bodies and/or headers follow. One example is firewall
traversal. A midcom agent co-located with a proxy server controls a
firewall entity. The agent needs to view certain Session Description
Protocol (SDP) attributes in a message body or the same kind of data
in a SIP header. Another example is the archiving of instant
messaging traffic, where the archiving function co-located with a
proxy server logs the message bodies in the MESSAGE method. A similar
example is the archiving of all SIP headers and bodies traffic after
being checked by the proxy server. These services might be deployed
for financial or health care applications, where archiving
communications is required by policies, as well as other
applications.
This document describes proposed mechanisms to achieve data
confidentiality and the integrity of end-to-middle security to meet
the requirements discussed in [9]. The major requirement is to be
compatible with the end-to-end encryption that the S/MIME mechanism
provides. Therefore, the proposed mechanisms are based on S/MIME. The
mechanisms consists of the creation of encrypted data that is not
readable by other proxy servers and the indication by the UA of the
data that the UA requests the proxy server to read. In addition, it
also includes a mechanism for the discovery of intermediate proxy
servers.
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2. Overview of Proposed Mechanisms
First, UAs MUST support the creation of CMS EnvelopedData body for
multiple recipients for end-to-middle confidentiality. For
compatibility with end-to-end security, the data needs to be
encrypted for UAs and selected proxy servers. Second, UAs SHOULD
support an indication mechanism to specify content in S/MIME that
needs to be disclosed to a selected proxy server. Proxy servers MUST
support to inspect the indicated content in S/MIME CMS bodies. Third,
UAs MAY support a discovery mechanism to find which proxy in a
signaling path needs to inspect and/or validate what data. In some
cases, UAs will be statically configured with the intermediary
proxy's policies and so they would not need to use this discovery
mechanism. Proxy servers SHOULD provide the discovery mechanism to
notify their policies of UAs.
2.1 Creation of S/MIME CMS Bodies for UAs and Proxy servers
Since end-to-middle security needs to be compatible with end-to-end
security, a creation mechanism for S/MIME CMS body is required. For
end-to-end data integrity, UAs use S/MIME CMS SignedData body that
can be validated by any entity. Therefore no new CMS SignedData
creation mechanism is required.
For data confidentiality, UAs use S/MIME CMS EnvelopedData body,
whose recipients are specified. There are two ways of creating this
data. The first way is to create an S/MIME CMS EnvelopedData body
that contains encrypted content that is addressed to multiple
recipients, such as a UA and a selected proxy server. UA MUST create
an CMS EnvelopedData body can contain multiple recipients for
encrypted data as specified in [7]. The structure contains data
encrypted with a content-encryption-key (CEK) and the CEK is then
encrypted with different key-encryption-keys (KEKs), that are
actually the public keys for each recipient. For example, one would
be for the recipient UA and another would be for the selected proxy
server in the end-to-middle case.
The second way is to create multiple S/MIME CMS EnvelopedData bodies,
one for each recipient. For example, one for UA and one for a
selected proxy server, and make them part of a multipart MIME body.
UAs SHOULD use this method when keying materials, such keys for use
by Secure RTP (SRTP)[14], are included in the SDP. One CMS
EnvelopedData body contains SDP that includes keying materials of an
SRTP stream only for the UA, and the other EnvelopedData body
contains an SDP that does not include the keying materials of an SRTP
stream that are for the UA and a selected proxy server that needs to
view SDP (i.e.: for a firewall control service).
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2.2 Indicating the Target Content
When proxy servers receive a message, the proxy server MUST inspect
the "Content-Disposition" MIME headers to determine whether to
process the S/MIME bodies and if so, which one. UA MUST support a new
parameter called "required-entity" in the "Content-Disposition" MIME
header that indicates required proxy servers to view the content of
the MIME body. There is less of an impact on proxy servers that do
not support end-to-middle security because these proxy servers do not
inspect the "Content-Disposition" MIME header anyway.
Note: There is an altenative option that use a new SIP header. The
proposed mechanism requires more cost on proxy servers to
determine the necessity of MIME body handling than using a new SIP
header. However, the proxy can view the indicated MIME body more
effectively than using a new SIP header. In addition, using a new
SIP header could be negative impact on intermediary proxy servers
that do not support end-to-middle security, causing unnecessary
processing load. We feel that this MIME header parameter mechanism
is not as simple, but it is equally effective.
2.3 Discovery of Proxy Server's Policies
A discovery mechanism for proxy server's policies is needed when UAs
do not know the policies of the proxy server in a signaling path and
the proxy server has its own policy for providing some services.
When the proxy server receives a request in which it cannot view some
data that must be read in order to proceed or the proxy server
receives a request whose sending policy cannot be accepted, the proxy
MUST send a response with an error code. If the request is in plain
text, the error code SHOULD be 403 (Forbidden) accompanied with a
required Content-Type, such as "application/sdp". If the request is
in plain text and the digital signature of it is required for an
integrity check, the error code SHOULD be 403 (Forbidden) accompanied
with a required Content-Type that is "multipart/signed". If the
request contains encrypted data, the error code SHOULD be 493
(Undecipherable), accompanied with a proxy's public key certificate
and required Content-Type.
Open Issues: Which header is the appropriate one to use to set the
required Content-Types in a response? When nested Content-Types
are required such as "Content-Type: multipart/signed" for
"Content-Type: application/pkcs7-mime;smime-type=enveloped-data",
how will it be described?
When the UA receives one of the above error codes, the UA needs to
authenticate the proxy server. Therefore, the error code SHOULD
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contain the digital signature of the proxy server.
In the worst case, this discovery mechanism requires two messages for
each proxy server in the signaling path to establish a session
between the UAs. In addition, it requires validation procedures using
the digital signatures for all proxy servers. Since this causes a
increase in the delay before session establishment, it is recommended
that UAs learn in advance the policies of as many proxy servers as
they can.
Open Issue: How does this mechanism apply in the case when a proxy
server needs to inspect the message body contained in the request
in order to provide a service for a UAS? This might be happen
where there a firewall in the network on the UAS's side.
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3. Behavior of UAs and Proxy Servers
We describe here some examples of the behavior of UAs and proxy
servers in a model in which trusted and partially trusted proxy
servers are mixed along a message path. In the following example,
User #1 does not know the services or security policies of Proxy #1.
User#1 sends an INVITE request including encrypted SDP for end-to-end
security as shown in Figure 1. Proxy #1 may reject the request
because its inability to offer a firewall traversal service. Or Proxy
#1 may delete the encrypted data from the body based on a security
policy that prevents it from sending unknown data.
Home network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+
User #1------| | C |-----| * |-----| * |-----| C |-- User #2
| +-----+ +-----+ | +-----+ +-----+
| UA #1 Proxy #1 | Proxy #2 UA #2
+---------------------+
C: Content that UA #1 allows the entity to inspect
*: Content that UA #1 prevents the entity from inspecting
Figure 1: Deployment example
3.1 UAC Behavior
When a UAC sends a request and requires end-to-end and end-to-middle
confidentiality of the message bodies and/or headers, it uses S/MIME
to encrypt them. In the above examples, UA #1 MUST use CMS
EnvelopedData body for UA #2 and Proxy #1. At the SIP layer, UA #1
MUST require Proxy #1 to decrypt selected content and to view the
content by using the "required-entity" parameter in the
Content-Disposition header. Proxy #1 then provides some services
based on the decrypted content.
When the UAC needs Proxy #1 to inspect the message bodies and/or
headers in the response, it SHOULD request the UAS to encrypt the
response by using the same CEK as for the request. The UAC uses the
"unprotectedAttrs" attribute to stipulate reuse of the CEK and
indicate its identifier as described in [10] [11].
When the UAC sends a request and needs end-to-end and end-to-middle
integrity for the message bodies and/or headers, it uses S/MIME to
attach a digital signature. In the above examples, it uses the CMS
SignedData body of the contents. At the SIP layer, UA #1 requires
Proxy #1 to validate the integrity of the selected content by
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employing the "required-entity" parameter.
When the UAC receives a response that uses S/MIME, it decrypts and/or
validates the S/MIME bodies as usual. If it receives a response that
uses CMS EnvelopedData body with the "KEKRecipientInfo" type of
"RecepientInfo" attribute, it should decrypt the "RecipientInfo" by
using the same CEK as for the sending request.
3.2 UAS Behavior
When a UAS sends a response for the request with this mechanism,
using the same type of S/MIME CMS body is recommended. For example,
if the UAS receives an INVITE request in which the SDP is encrypted
by using CMS EnvelopedData body, the recommended response would be a
"200 OK" containing the encrypted SDP based on CMS EnvelopedData
body.
In particular, when the CMS EnvelopedData body of the request
contains the "unprotectedAttrs" attribute specifying reuse of the
CEK, the UAS SHOULD encrypt a CEK with the CEK that was used in the
request, instead of the public key of the UAC. The UAS SHOULD use the
CMS EnvelopedData body to contain the encrypted SDP in the "200 OK"
response. In addition, the UAS SHOULD set the same proxy server in
the "required-entity" parameter of the "Content-Dispositon" MIME
header in the request.
If the UAS encrypted the SDP with a CEK that was itself encrypted
with the CEK in the request, the proxy server selected by the UAC can
view the SDP in the "200 OK" response.
Note: In the case when the response does not contain a message
body, even if the request contains a message body and was
encrypted by using CMS EnvelopedData body, the UAS does not need
to use the CMS EnvelopedData body.
When the UAS receives a request that uses S/MIME, it decrypts and/or
validates the S/MIME bodies as usual.
When the UAS sends a response for the request without this mechanism
and needs end-to-end and end-to-middle confidentiality of the message
bodies and/or headers , it MUST use CMS EnvelopedData to encrypt
them. When the UAC sends a request and needs end-to-end and
end-to-middle integrity of the message bodies and/or headers, it MUST
use CMS SignedData to attach a digital signature. This is the same
way the UAC normally performs with this mechanism.
3.3 Proxy Behavior
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When a proxy supporting this mechanism receives a message, the proxy
server MUST inspect the "Content-Disposition" MIME header and the
"required-entity" parameter of that. If the MIME header includes the
processing server's own name, the proxy server MUST inspect the
specified content.
When the specified content is CMS EnvelopedData body, the proxy
server MUST inspect it and try to decrypt the "recipientInfo"
attribute. If the proxy server fails to decrypt that, it SHOULD
cancel the subsequent procedure and respond with a 493
(Undecipherable) response if it is a request, or any existing dialog
MAY be terminated. If the proxy server succeeds in this decryption,
it MUST inspect the "unprotectedAttrs" data of the CMS EnvelopedData
body. If the attribute gives the key's identifier, the proxy server
MUST keep the CEK with its identifier during the dialog. When it
receives subsequent messages in the dialog, it MUST try to decrypt
the "KEKRecipientInfo" type of "recepientInfo" attribute by using
this CEK.
When the specified content is CMS SignedData body, the proxy server
MUST inspect it and validate the digital signature. If the
verification is failed, the proxy server SHOULD reject the subsequent
procedure and respond with a 403 (Forbidden) response if the message
is a request, or any existing dialog MAY be terminated.
When the proxy server forwards the request, it modifies the routing
headers normally. It does not need to modify the S/MIME body.
If a proxy does not support this mechanism and receives a message
with the "required-parameter" parameter in the "Content-Disposition"
header, the proxy MUST ignore the header and perform as usual.
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4. Summary of Content-Disposition Header Field Use
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [13]. The new parameter
"required-entity" is defined in "required-param" as one of
"disp-param".
Content-Disposition = "Content-Disposition" HCOLON
disp-type *( SEMI disp-param )
disp-type = "render" / "session" / "icon" / "alert"
/ disp-extension-token
disp-param = handling-param / required-param / generic-param
handling-param = "handling" EQUAL
( "optional" / "required" / other-handling )
other-handling = token
disp-extension-token = token
required-param = "required-entity" EQUAL
proxy-uri *(COMMA proxy-uri)
proxy-uri = ( name-addr / addr-spec )
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5. Examples
The following examples illustrate the use of the mechanism defined in
the previous section.
5.1 Request Example
In the following example, a UA needs SDP in an INVITE message to be
confidential and the UA allows a proxy server to view the SDP in an
INVITE request. In addition, the UA needs to protect the policy of
the proxy server. In the example encrypted message below, the text
with the box of asterisks ("*") is encrypted:
INVITE alice@atlanta.example.com --> ss1.atlanta.example.com
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
Route: <sip:ss1.atlanta.example.com;lr>
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Date: Fri, 20 June 2003 13:02:03 GMT
Content-Type: multipart/signed;protocol="application/pkcs7-signature";
micalg=sha1;boundary=boundary1
Content-Length: ...
--boundary1
Content-Type: application/pkcs7-mime;smime-type=enveloped-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: session;filename=smime.p7m;handling=required;
required-entity=ss1.atlanta.example.com
Content-Length: ...
******************************************************************
* (encryptedContentInfo) *
* Content-Type: application/sdp *
* Content-Length: ... *
* *
* v=0 *
* o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com*
* s=- *
* c=IN IP4 192.0.2.101 *
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* t=0 0 *
* m=audio 49172 RTP/AVP 0 *
* a=rtpmap:0 PCMU/8000 *
* *
* (recipientInfos) *
* RecepientInfo[0] for ss1.atlanta.example.com public key *
* RecepientInfo[1] for bob's public key *
* *
* (unprotectedAttr) *
* CEKReference *
******************************************************************
--boundary1--
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename=smime.p7s;handling=required
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary1--
5.2 Response Example
In the following example, a UA sends a response with this mechanism.
In the example encrypted message below, the text boxed in asterisks
("*") is encrypted:
200 OK bob@biloxi.example.com --> ss2.biloxi.example.com
SIP/2.0 200 OK
Via: SIP/2.0/TCP
ss2.biloxi.example.com:5060;branch=z9hG4bK721e418c4.1
;received=192.0.2.222
Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
;received=192.0.2.111
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
Record-Route: <sip:ss2.biloxi.example.com;lr>,
<sip:ss1.atlanta.example.com;lr>
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=314159
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Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
Content-Type:multipart/signed;protocol="application/pkcs7-signature";
micalg=sha1;boundary=boundary41
Content-Length: ...
--boundary41
Content-Type: application/pkcs7-mime;
smime-type=enveloped-data;name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: session;
filename=smime.p7m;handling=required;
Content-Length: ...
******************************************************************
* (encryptedContentInfo) *
* Content-Type: application/sdp *
* Content-Length: ... *
* *
* v=0 *
* o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com *
* s=- *
* c=IN IP4 192.0.2.201 *
* t=0 0 *
* m=audio 3456 RTP/AVP 0 *
* a=rtpmap:0 PCMU/8000 *
* *
* (recipientInfos) *
* RecepientInfo[0] for KEKidentifier *
******************************************************************
--boundary41--
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s;handling=required
hhhHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary41--
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6. Security Considerations
This specification is about applying S/MIME-secured messages for use
in end-to-middle security. It is also about applying the CEK reuse
method defined in [10]. This requires the same security consideration
as those of [4] and [10].
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7. IANA Considerations
This document requires a new parameter in "Content-Disposition"
header fields, namely "required-entity".
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8. Acknowledgments
Thanks to Rohan Mahy and Cullen Jennings for their initial support of
this concept, and to Jon Peterson for his helpful comments.
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[3] Allen, C. and T. Dierks, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[4] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1992.
[5] Srisuresh, P., Kuthan, J., Rosenberg, J., Brim, S., Molitor, A.
and A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[6] Campbell, Ed., B., Rosenberg, J., Schulzrinne, H., Huitema, C.
and D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[7] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
1999.
[8] Rosenberg, J., "Requirements for Session Policy for the Session
Initiation Protocol (SIP)",
draft-rosenberg-sipping-session-policy-req-00 (work in
progress), December 2002.
[9] Ono, K. and S. Tachimoto, "Requirements for end-to-middle
security in the Session Initiation Protocol (SIP)",
draft-ietf-sipping-e2m-sec-reqs-01 (work in progress),
February 2004.
[10] Farrell, S. and S. Turner, "Reuse of CMS Content Encryption
Keys", RFC 3185, October 2001.
[11] Ono, K. and S. Tachimoto, "Key reuse in S/MIME for SIP",
draft-ono-sipping-keyreuse-smime-00 (work in progress),
February 2004.
[12] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
[13] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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[14] Andreasen, F., Baugher, M. and D. Wing, "Session Description
Protocol Security Descriptions for Media Streams",
draft-ietf-mmusic-sdescriptions-03.txt (work in progress),
February 2004.
Authors' Addresses
Kumiko Ono
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-shi, Tokyo 180-8585
Japan
EMail: ono.kumiko@lab.ntt.co.jp
Shinya Tachimoto
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-shi, Tokyo 180-8585
Japan
EMail: tachimoto.shinya@lab.ntt.co.jp
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