SIPPING K. Ono
Internet-Draft S. Tachimoto
Expires: August 23, 2005 NTT Corporation
February 19, 2005
End-to-middle Security in the Session Initiation Protocol (SIP)
draft-ono-sipping-end2middle-security-04
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Some services provided by intermediaries depend on the ability to
inspect the message bodies in the Session Initiation Protocol (SIP).
When sensitive information is included in these bodies, a SIP User
Agent (UA) needs to protect it from intermediaries except those that
the UA agreed to disclose it to. This document proposes a mechanism
for securing information passed between an end user and
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intermediaries using S/MIME. It also proposes mechanisms for an
intermediary to notify the UA about its need to inspect an
S/MIME-secured message body, and to notify the need for the message
body to be transmitted securely.
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. Generating S/MIME-secured message body . . . . . . . . . . . 3
2.1 Generating S/MIME CMS EnvelopedData . . . . . . . . . . . . 3
2.2 Generating S/MIME CMS SignedData . . . . . . . . . . . . . . 4
3. Indicating the Target Content . . . . . . . . . . . . . . . 5
4. Notification of the Proxy Server's Policies . . . . . . . . 5
5. Behavior of UAs and Proxy Servers . . . . . . . . . . . . . 6
5.1 UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2 UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 8
6. Proxy-Required-Body Header Field Use . . . . . . . . . . . . 9
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1 Example of Request for End-to-Middle Confidentiality . . . . 10
7.2 Example of Request for End-to-Middle Integrity . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
10. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1 Normative References . . . . . . . . . . . . . . . . . . . . 14
12.2 Informative References . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . 16
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1. Introduction
When a SIP [2] UA requires services provided by intermediaries that
depend on the message bodies in request/response messages, end-to-end
confidentiality currently has to be disabled. This problem is
pointed out in Section 23 of [2]. Since such intermediaries are not
always adjacent to the UA, this situation requires security between
the UA and the intermediary for the message bodies. We call this
"end-to-middle security", where by "end" we mean a UA and by "middle"
we mean a specific intermediary, typically a proxy server.
This document describes proposed mechanisms for providing data
confidentiality and integrity for end-to-middle security to meet the
requirements discussed in [3]. Since the major requirement is to
have little impact on the standardized end-to-end security
mechanisms, the proposed mechanisms are based on S/MIME [4]. The
mechanisms consist of generating S/MIME-secured message body and
indicating the target message body for a proxy server selected by the
UA. In addition, this document includes mechanisms for an
intermediary to notify the UA about its need to inspect an
S/MIME-secured message body, and to notify that the message body
needs to be transmitted securely.
2. Generating S/MIME-secured message body
2.1 Generating S/MIME CMS EnvelopedData
For end-to-middle confidentiality, a UA MUST generate S/MIME CMS [5]
EnvelopedData. Before generating it, a UA needs to identify the
target proxy servers. The UA MUST obtain their credentials, such as
their public key certificates or shared secrets. The structure of
the S/MIME CMS EnvelopedData contains encrypted data specified in the
"encryptedContentInfo" field and its recipient list specified in the
"recipientInfos" field. The encrypted data is encrypted with a
content-encryption-key (CEK) and the recipient list contains the CEKs
encrypted with different key-encryption-keys (KEKs), one for each
recipient. The KEKs are the public keys of each recipient or the
shared keys between the UA and each recipient.
If the data is encrypted for a selected proxy server, the recipient
list contains only the selected proxy server. If the data is
encrypted and destined for multiple proxy servers, the recipient list
contains the multiple proxy servers. If there is encrypted data
destined for each proxy server, the recipient list of each piece of
encrypted data contains the targeted proxy server. In order to
concatenate multiple pieces of encrypted data, the UAC MUST generate
a multipart MIME body.
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Since proxy servers are prohibited from deleting any body, the
encrypted data for the proxy server is transmitted to the user agent
server (UAS) but the UAS is not able to decrypt it. In order to
avoid causing unnecessary error conditions in the UAS, the user agent
client (UAC) MUST set the value "optional" in the handling parameter
of the "Content-Disposition" MIME header for the message body.
If the multipart MIME body consists of encrypted MIME bodies with the
value "optional", the "Content-Disposition" MIME header of the
multipart MIME body MUST also contain the value "optional" in the
handling parameter. The UAS SHOULD NOT try to decrypt encrypted data
that has the value "optional".
If the multipart MIME body contains a body with the value "required"
and another body with the value "optional", the multipart MIME body
SHOULD have the value "required" in the handling parameter of the
"Content-Disposition" MIME header.
If the encrypted data is meant to be shared with the UAS and selected
proxy servers, the recipient list SHOULD be addressed to the UAS and
the selected proxy servers. The UAC SHOULD set the value "required"
in the handling parameter of the "Content-Disposition" MIME header
for the message body. The UAS MUST try to decrypt the encrypted data
that has the value "required" in the handling parameter. If the
handling parameter is not set, the default behavior is the same as
for setting the value "required", as specified in [2].
If a piece of encrypted data is destined for a selected proxy server
and another piece of encrypted data for the UAS, the recipient of
each piece of encrypted data is each entity respectively. In this
case, the UAC MUST generate a multipart MIME body to concatenate the
two.
For example, a UA uses this mechanism when keying materials, such
as keys used for Secure RTP (SRTP), are included in the SDP[9].
One CMS EnvelopedData contains SDP that includes keying materials
of an SRTP stream only for the UA. The other CMS EnvelopedData
contains an SDP that does not include the keying materials for a
selected proxy server that needs to view SDP (i.e., for a firewall
traversal service).
2.2 Generating S/MIME CMS SignedData
For end-to-end data integrity, a UA generates S/MIME CMS SignedData
that can be verified by any entity that knows the public key of the
UA. For end-to-middle data integrity, a UA MUST generate S/MIME CMS
SignedData in the same way as end-to-end data integrity.
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3. Indicating the Target Content
A UA needs a way to indicate content that they expect to be viewed by
a proxy server, in order for the proxy server to easily determine
whether to process MIME bodies and if so, which one. To meet this
requirement, the UAs SHOULD set a label to indicate a selected proxy
server and the target content with a new SIP header,
"Proxy-Required-Body". This header contains one or more "content-id"
parameter(s) for setting the "Content-ID" MIME header into the target
body.
Note: There were three other options to label a body: a new SIP
parameter to an existing SIP header, a new MIME header, or a new
parameter to an existing MIME header.
1) Using a new parameter to Route header. Since a proxy server
views this header when forwarding a request message, it seems to
be a reasonable option. However, it cannot work with strict
routing.
2) Using a new MIME header, "Content-Target", as proposed in a
previous version of this draft. Since this option is not
necessary as a generic mechanism of MIME, it is not preferred.
3) Using a new MIME parameter to "Content-Disposition". The same
reason as above.
When a UA labels the encrypted data, it SHOULD set the
"Proxy-Required-Body" SIP header that contains the address of the
server and "content-id" parameter indicating the S/MIME CMS
EnvelopedData. When a UA labels the data with signature, it SHOULD
set the "Proxy-Required-Body" SIP header that contains the address of
the server and "content-id" parameter indicating the S/MIME CMS
SignedData.
4. Notification of the Proxy Server's Policies
A notification mechanism for the proxy server's policies is needed
when a UA does not know the policies of the proxy server in a
signaling path and the proxy server has its own policies for
providing some services which needs the UA's compliance. There are
two ways in which a UA can learn the policies of the proxy server.
The UA MAY learn them by getting a policy package from a policy
server.[10] When a proxy server needs to inspect the message body
contained in the response, it needs to learn the policies from a
policy server before sending the response.
Alternatively, the UA MAY learn them by receiving a response with an
error code. When the proxy server receives a request in which it
cannot view the message body that has to be read in order to proceed,
the proxy server MUST send a response with an error code. If the
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request contains encrypted data, the error code SHOULD be 493
(Undecipherable), accompanied with a proxy's public key certificate
and required Content-Type for viewing. The proxy public key
certificate is set as an "application/pkix-cert" body. The required
Content-Type is set in the Warning header.
When the proxy server receives a request that can not be accepted due
to its condition, the proxy server MUST also send a response with an
error code. If a digital signature is not attached to the request
and it required for an integrity check, the error code SHOULD be 495
(Signature Required) accompanied with a required Content-Type for the
integrity check. The required Content-Type is set in the Warning
header.
Note: 495 (Signature Required) response is not only generated by a
proxy server, but also by the UAS. Using this error code is more
explicit than using an existing error code 403, that was described in
the last version of this draft.
Open Issues: How should the error message indicate the
Content-Type to which a signature needs to be attached? Can these
Content-Types be nested such as "Content-Type: multipart/mixed"
for "Content-Type: application/sdp" and "Content-Type:
message/sipfrag"?
When proxy servers require both disclosure and an integrity check,
how should it be described?
When the UAC receives one of the above error codes, it needs to
authenticate the proxy server. Therefore, the error code SHOULD
contain the digital signature of the proxy server.
In the worst scenario, this notification 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 an increase in the delay before session establishment, it
is recommended that a UA learns in advance the policies of as many
proxy servers as they can.
5. Behavior of UAs and Proxy Servers
We describe here an example of the behavior of UAs and proxy servers
in a model in which a proxy server that provides a logging service
for instant messages exists in a message path as shown in Figure 1.
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+-----+ +-----+ +-----+ +-----+
| C |-----| C |-----| [C] |-----| C |
+-----+ +-----+ +-----+ +-----+
UA #1 Proxy #1 Proxy #2 UA #2
w/Logging function
C : Content that UA #1 allows the entity to inspect
[C]: Content that UA #1 prevents the entity from inspecting
Figure 1: Deployment example
5.1 UAC Behavior
When a UAC sends a MESSAGE [11] request including encrypted message
content for end-to-end and end-to-middle confidentiality, it MUST use
S/MIME CMS EnvelopedData to encrypt them. In this example, UA #1 is
assumed to know the services and the public key of Proxy #1. UA #1
MUST use S/MIME CMS EnvelopedData body for UA #2 and Proxy #1. UA #1
SHOULD specify the hostname of Proxy #1 and Content-ID of the S/MIME
CMS EnvelopedData to be decrypted in the "Proxy-Required-Body" SIP
header.
When the UAC sends a request and needs both end-to-end and
end-to-middle integrity for the message body, it MUST use S/MIME CMS
SignedData to attach a digital signature. In this example, UA #1
MUST use the CMS SignedData body of the contents. UA #1 SHOULD
specify the hostname of Proxy#1 and Content-ID of the CMS SignedData
to be validated in the "Proxy-Required-Body" SIP header.
When the UAC sends multiple requests to the same UAS, the CEK reuse
mechanism is beneficial in letting UAs efficiently encrypt/decrypt
data. The CEK reuse mechanism is described in [6][7]. The UAC
SHOULD use the "unprotectedAttrs" field to stipulate reuse of the CEK
and indicate its identifier. When the UAC reuses the CEK in the
previous request as the KEK, it generates CMS EnvelopedData with the
type "KEKRecipientInfo" of "RecipientInfo" attribute.
5.2 UAS Behavior
When a UAS sends a response to the request with this mechanism, the
use of the same type of S/MIME CMS data is recommended. For example,
if the UAS receives an INVITE request in which the SDP is encrypted
by using CMS EnvelopedData body, the response is RECOMMENDED to be a
"200 OK" containing the encrypted SDP based on the CMS EnvelopedData
body. In the above example, however, the response of the MESSAGE
request does not need to use the same type of S/MIME CMS data since
the response does not contain the message content.
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In particular, when the CMS EnvelopedData body of the request
contains the "unprotectedAttrs" attribute specifying reuse of the
CEK, the UAS SHOULD keep the CEK with the identifier specified in the
"unprotectedAttrs" attribute.
When the UAS receives a request that uses S/MIME, it decrypts and/or
validates the S/MIME bodies as usual.
Even when the UAS receives a request without this mechanism, the UAS
MAY need end-to-end and end-to-middle confidentiality of the message
bodies and/or headers in the response. In this case, the UAS MUST
use CMS EnvelopedData to encrypt them. When the UAS sends a response
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 no different from how the UAC normally operates
with this mechanism.
5.3 Proxy Behavior
When a proxy server supporting this mechanism receives a message, it
MUST inspect the "Proxy-Required-Body" header. If the header
includes the processing server's own hostname, the proxy server MUST
inspect the specified body in the Content-ID. When the specified
body is CMS EnvelopedData, the proxy server MUST inspect it and try
to decrypt the "recipientInfos" field.
Even if there is the header does not include the server's own name,
nor the header exists, the proxy server MAY view a message body. If
the UA encrypted data for the proxy, the proxy server will succeed in
decryption using the "recipientInfos" field.
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, otherwise any existing dialog MAY be terminated.
If the proxy server succeeds in this decryption, it MUST inspect the
"unprotectedAttrs" field of the CMS EnvelopedData body. If the
attribute gives the key's identifier, the proxy server MUST keep the
CEK with its identifier until the lifetime of the CEK has expired.
If it receives subsequent messages within the lifetime, it MUST try
to decrypt the type "KEKRecipientInfo" of the "RecipientInfo"
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 fails, the proxy server SHOULD reject the subsequent
procedure and respond with a 495 (Signature Required) response if the
message is a request, otherwise any existing dialog MAY be
terminated.
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When the proxy server forwards the request, it modifies the routing
headers normally, but does not modify the message body. The proxy
server MAY delete the "Proxy-Required-Body" header that contains own
hostname.
When a provider operating the proxy server does not allow any
information related to its security policies to be revealed to the
proxy serving the recipient UA, the proxy server deletes the
"Proxy-Required-Body" header. However, when a request message is
transmitted to the proxy server via a proxy server operated by
another provider, there is no way to conceal the header from the
other proxy server.
If a proxy does not support this mechanism and receives a message
with the "Proxy-Required-Body" header, the proxy MUST ignore the
header and operate as usual.
6. Proxy-Required-Body Header Field Use
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [8]. The new header
"Proxy-Required-Body" is defined as a SIP header.
Proxy-Required-Body = "Proxy-Required-Body" HCOLON required-proxy
SEMI target-body
required-proxy = host
target-body = cid-param *(COMMA cid-param)
cid-param = "cid" EQUAL content-id
content-id = LDQUOT dot-atom "@" (dot-atom / host) RDQUOT
dot-atom = atom *( "." atom )
atom = 1*( alphanum / "-" / "!" / "%" / "*" /
"_" / "+" / "'" / "`" / "~" )
Information about the use of headers in relation to SIP methods and
proxy processing is summarized in Table 1.
Header field where proxy ACK BYE CAN INV OPT REG
--------------------------------------------------------------
Proxy-Required-Body R adr - o - o o o
Proxy-Required-Body 200-699 adr - o - o o o
Proxy-Required-Body 1xx adr - - - o - -
Header field where proxy SUB NOT PRK IFO UPD MSG
--------------------------------------------------------------
Proxy-Required-Body R adr o o - o o o
Proxy-Required-Body 200-699 adr o o - o o o
Table 1: Summary of header field use
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The "where" column gives the request and response types in which
the header field can be used. The values in the "where" column
are as follows:
* R: The header field may appear in requests
* 200-699: A numeral range indicates response codes with which
the header field can be used.
The "proxy" column gives the operations a proxy may perform on the
header field:
* a: A proxy can add or concatenate the header field if it is not
present.
* r: A proxy must be able to read the header field, so it cannot
be encrypted.
* d: A proxy can delete a header field value.
The next six columns relate to the presence of a header field in a
method:
* o: The header field is optional.
* -: The header field is not applicable.
7. Examples
The following examples illustrate the use of the mechanism defined in
the previous section.
7.1 Example of Request for End-to-Middle Confidentiality
In the following example, a UA needs the message content in a MESSAGE
request to be confidential and it allows a selected proxy server to
view the message content. It also needs to protect the label of the
target content. In addition, it needs to reuse the CEK in the
subsequent request messages. In the example encrypted message below,
the text with the box of asterisks ("*") is encrypted:
MESSAGE alice@atlanta.example.com --> ss1.atlanta.example.com
MESSAGE 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 MESSAGE
Date: Fri, 20 June 2003 13:02:03 GMT
Proxy-Required-Body: ss1.atlanta.example.com;
cid=1234@atlanta.example.com
Content-Type: multipart/signed;protocol="application/pkcs7-signature";
micalg=sha1;boundary=boundary1
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Content-Length: ...
--boundary1
Content-Type: application/pkcs7-mime;smime-type=enveloped-data;
name=smime.p7m
Content-Transfer-Encoding: binary
Content-ID: 1234@atlanta.example.com
Content-Disposition: attachment;filename=smime.p7m;handling=required
Content-Length: ...
******************************************************************
* (encryptedContentInfo) *
* Content-Type: text/plain *
* Content-Length: ... *
* *
* Hello. *
* This is confidential. *
* *
* (recipientInfos) *
* RecipientInfo[0] for ss1.atlanta.example.com public key *
* RecipientInfo[1] for Bob's public key *
* *
* (unprotectedAttrs) *
* CEKReference *
******************************************************************
--boundary1--
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: binary
Content-Disposition: attachment;
filename=smime.p7s;handling=required
[binary data]
--boundary1--
7.2 Example of Request for End-to-Middle Integrity
In the following example, a UA needs the integrity of the message
content in a MESSAGE request to be validated by a selected proxy
server before it views the message content. It also needs to protect
the label of the target content.
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MESSAGE alice@atlanta.example.com --> ss1.atlanta.example.com
MESSAGE 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 MESSAGE
Date: Fri, 20 June 2003 13:02:03 GMT
Proxy-Required-Body: ss1.atlanta.example.com;
cid=1234@atlanta.example.com
Content-Type: multipart/signed;protocol="application/pkcs7-signature";
micalg=sha1;boundary=boundary1
Content-Length: ...
--boundary1
Content-Type: text/plain
Content-Length: ...
Hello.
This is protected with the signature.
--boundary1--
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: binary
Content-ID:1234@atlanta.example.com
Content-Disposition: attachment;
filename=smime.p7s;handling=required
[binary data]
--boundary1--
8. Security Considerations
This document describes a mechanism to encrypt data for multiple
recipients, such as multiple proxy servers, or the recipient UA and
proxy servers. This means that the encrypted data is decipherable at
a previous hop and it may be modified by an entity at the previous
hop. Therefore, the UA SHOULD protect the data integrity before
encryption, when the encrypted data is meant to be shared with
multiple proxy servers, or to be shared with the UAS and selected
proxy servers. The UA SHOULD generate S/MIME CMS SignedData and then
SHOULD generate the EnvelopedData to encrypt attached data with a
digital signature. The recipient entity SHOULD verify the signature
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to see if the encrypted data has been modified after decryption at
another entity on the recipient list.
This document also describes a new SIP header for labeling a message
body for a proxy server. If a malicious user or proxy server
modified/added/deleted the label, the specified message body will not
be inspected by the specified proxy server, and then some service
using its content can not be served. Or a proxy server will do
unnecessary processing on message bodies such as unpacking MIME
structure, and/or signature verification. This causes a
Denial-of-Services attack to a proxy server.
To prevent such attacks, protection of the label integrity is needed.
UAs and proxy servers SHOULD use TLS mechanism to communicate each
other. A proxy server trusted to provide SIP routing is basically
trusted to process SIP headers other than those related to routing.
Therefore, hop-by-hop security is reasonable for the protection. Of
course, UAs MAY generate a "message/sipfrag" body and attach a
digital signature for the whole body in order to protect the label
integrity.
9. IANA Considerations
This document requires a new "Proxy-Required-Body" SIP header.
10. Open Issues
o How should the error message indicate the Content-Type to which a
signature needs to be attached? Can these Content-Types be nested
such as "Content-Type: multipart/mixed" for
"Content-Type:application/sdp" and "Content-Type:
message/sipfrag"?
o When proxy servers require both disclosure and an integrity check,
how should it be described?
o If the label of the target body has a wrong value, what should we
do? 403 (Forbidden) response?
o Do we need to add text about CMS AuthenticationData for the
integrity protection?
o Do we need to add text about the reason we don't have to define a
new option-tag?
11. Acknowledgments
Thanks to Rohan Mahy and Cullen Jennings for their initial support of
this concept and to many people for useful comments, especially Jon
Peterson, Jonathan Rosenberg, Eric Burger.
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12. References
12.1 Normative 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] Ono, K. and S. Tachimoto, "Requirements for end-to-middle
security in the Session Initiation Protocol (SIP)",
Internet-Draft draft-ietf-sipping-e2m-sec-reqs-04, October 2004.
[4] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Certificate Handling", RFC 3850, July 2004.
[5] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
1999.
[6] Farrell, S. and S. Turner, "Reuse of CMS Content Encryption
Keys", RFC 3185, October 2001.
[7] Ono, K. and S. Tachimoto, "Key reuse in S/MIME for SIP",
Internet-Draft draft-ono-sipping-keyreuse-smime-00, February
2004.
[8] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
12.2 Informative References
[9] Andreasen, F., Baugher, M. and D. Wing, "Session Description
Protocol Security Descriptions for Media Streams",
Internet-Draft draft-ietf-mmusic-sdescriptions-09, February
2005.
[10] Hilt, V., Camarillo, G. and J. Rosenberg, "Profile Data for
Session Initiation Protocol (SIP) Policies", September 2003.
[11] Campbell, Ed., B., Rosenberg, J., Schulzrinne, H., Huitema, C.
and D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[12] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
Ono & Tachimoto Expires August 23, 2005 [Page 14]
Internet-Draft End-to-middle security in SIP February 2005
Authors' Addresses
Kumiko Ono
Network Service Systems Laboratories, NTT Corporation
Musashino-shi, Tokyo 180-8585
Japan
Email: ono.kumiko@lab.ntt.co.jp
Shinya Tachimoto
Network Service Systems Laboratories, NTT Corporation
Musashino-shi, Tokyo 180-8585
Japan
Email: tachimoto.shinya@lab.ntt.co.jp
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Internet-Draft End-to-middle security in SIP February 2005
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