Internet Engineering Task Force (IETF) P. Saint-Andre

Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 7572 &yet
Category: Standards Track A. Houri
ISSN: 2070-1721 IBM
J. Hildebrand
Cisco Systems, Inc.
June 2015

Interworking between the Session Initiation Protocol (SIP) and the
Extensible Messaging and Presence Protocol (XMPP): Instant Messaging

Abstract

This document defines a bidirectional protocol mapping for the
exchange of single instant messages between the Session Initiation
Protocol (SIP) and the Extensible Messaging and Presence Protocol
(XMPP).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7572.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Intended Audience . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Message Size . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Content Types . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Internationalization Considerations . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13

1. Introduction

In order to help ensure interworking between instant messaging (IM)
systems that conform to the instant messaging / presence requirements
[RFC2779], it is important to clearly define protocol mappings
between such systems. Within the IETF, work has proceeded on two
instant messaging technologies:

o Various extensions to the Session Initiation Protocol ([RFC3261])
for instant messaging, in particular the MESSAGE method extension
[RFC3428]; collectively the capabilities of SIP with these
extensions are commonly called SIP for Instant Messaging and
Presence Leveraging Extensions (SIMPLE).

o The Extensible Messaging and Presence Protocol (XMPP), which
consists of a formalization of the core XML streaming protocols
developed originally by the Jabber open-source community; the
relevant specifications are [RFC6120] for the XML streaming layer
and [RFC6121] for basic presence and instant messaging extensions.

One approach to helping ensure interworking between these protocols
is to map each protocol to the abstract semantics described in
[RFC3860]; that is the approach taken by [SIMPLE-CPIM] and [RFC3922].
In contrast, the approach taken in this document is to directly map
semantics from one protocol to another (i.e., from SIP / SIMPLE to
XMPP and vice versa), since that is how existing systems solve the
interworking problem.

Both XMPP systems and IM-capable SIP systems enable entities to
exchange "instant messages". The term "instant message" usually
refers to a message sent between two entities for delivery in close

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to real time (rather than a message that is stored and forwarded to
the intended recipient upon request). This document specifies
mappings only for single messages (sometimes called "pager-mode"
messaging), since they form the lowest common denominator for IM.
Separate documents cover "session-mode" instant messaging in the form
of one-to-one chat sessions [RFC7573] and multi-party chat sessions
[GROUPCHAT]. In particular, session-mode instant messaging supports
several features that are not part of pager-mode instant messaging,
such as a higher level of assurance regarding end-to-end message
delivery. As with all of these documents, the architectural
assumptions underlying such direct mappings are provided in
[RFC7247], including mapping of addresses and error conditions.

2. Intended Audience

The documents in this series are intended for use by software
developers who have an existing system based on one of these
technologies (e.g., SIP) and who would like to enable communication
from that existing system to systems based on the other technology
(e.g., XMPP). We assume that readers are familiar with the core
specifications for both SIP [RFC3261] and XMPP [RFC6120], with the
base document for this series [RFC7247], and with the following
IM-related specifications:

o "Session Initiation Protocol (SIP) Extension for Instant
Messaging" [RFC3428]

o "Extensible Messaging and Presence Protocol (XMPP): Instant
Messaging and Presence" [RFC6121]

Note well that not all protocol-compliant messages are shown (such as
SIP 100 TRYING messages), in order to focus the reader on the
essential aspects of the protocol flows.

3. Terminology

A number of terms used here are explained in [RFC3261], [RFC3428],
[RFC6120], and [RFC6121].

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].

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4. XMPP to SIP

As described in [RFC6121], a single instant message is an XML
<message/> stanza of type "normal" sent over an XML stream (since
"normal" is the default for the 'type' attribute of the <message/>
stanza, the attribute is often omitted).

When the XMPP user Juliet with a Jabber Identifier (JID) of
<[email protected]> wants to send an instant message to Romeo, she
interacts with her XMPP client, which generates an XMPP <message/>
stanza. The syntax of the <message/> stanza, including required and
optional elements and attributes, is defined in [RFC6121] (for single
instant messages, Section 5.1 of [RFC6121] recommends that the value
of the 'to' address be a "bare JID" of the form
"localpart@domainpart"). The following is an example of such a
stanza:

Example 1: XMPP User Sends Message

| <message from='[email protected]/yn0cl4bnw0yr3vym'
| to='[email protected]'>
| <body>Art thou not Romeo, and a Montague?</body>
| </message>

Upon receiving such a message stanza, the XMPP server needs to
determine the identity of the domainpart in the 'to' address, which
it does by following the procedures explained in Section 5 of
[RFC7247]. If the domain is a SIP domain, the XMPP server will hand
off the message stanza to an XMPP-to-SIP gateway or connection
manager that natively communicates with IM-aware SIP servers.

The XMPP-to-SIP gateway is then responsible for translating the XMPP
message stanza into a SIP MESSAGE request from the XMPP user to the
SIP user:

Example 2: XMPP User Sends Message (SIP Transformation)

| MESSAGE sip:[email protected] SIP/2.0
| Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bK776sgdkse
| Max-Forwards: 70
| To: sip:[email protected]
| From: <sip:[email protected];gr=yn0cl4bnw0yr3vym>;tag=12345
| Call-ID: D9AA95FD-2BD5-46E2-AF0F-6CFAA96BDDFA
| CSeq: 1 MESSAGE
| Content-Type: text/plain
| Content-Length: 35
|
| Art thou not Romeo, and a Montague?

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The destination SIP server is responsible for delivering the message
to the intended recipient, and the recipient is responsible for
generating a response (e.g., 200 OK).

Example 3: SIP User Agent Indicates Receipt of Message

| SIP/2.0 200 OK
| Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bK776sgdkse
| From: sip:[email protected];tag=12345
| To: sip:[email protected];tag=vwxyz
| Call-ID: D9AA95FD-2BD5-46E2-AF0F-6CFAA96BDDFA
| CSeq: 1 MESSAGE
| Content-Length: 0

As described in [RFC3428], a downstream proxy could fork a MESSAGE
request, but it would return only one 200 OK to the gateway.

Note: This document does not specify handling of the 200 OK by the
XMPP-to-SIP gateway (e.g., to enable message acknowledgements).
See [RFC7573] for a mapping of message acknowledgements in the
context of one-to-one chat sessions.

The mapping of XMPP syntax to SIP syntax MUST be as shown in the
following table.

Table 1: Message Syntax Mapping from XMPP to SIP

+-----------------------------+--------------------------+
| XMPP Element or Attribute | SIP Header or Contents |
+-----------------------------+--------------------------+
| <body/> | body of MESSAGE |
| <subject/> | Subject |
| <thread/> | Call-ID |
| from | From (1) |
| id | transaction identifier |
| to | To or Request-URI |
| type | (no mapping) (2) |
| xml:lang | Content-Language |
+-----------------------------+--------------------------+

1. As shown in the foregoing example and described in [RFC7247], the
XMPP-to-SIP gateway MUST map the bare JID
("localpart@domainpart") of the XMPP sender to the SIP From
header and include the resourcepart of the "full JID"
("localpart@domainpart/resourcepart") as the Globally Routable
User Agent URI (GRUU) portion [RFC5627] of the SIP URI.

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2. Because there is no SIP header field that matches the meaning of
the XMPP message 'type' values ("normal", "chat", "groupchat",
"headline", "error"), no general mapping is possible here.

5. SIP to XMPP

As described in [RFC3428], a single instant message is a SIP MESSAGE
request sent from a SIP user agent to an intended recipient who is
most generally referenced by an Instant Messaging (IM) URI [RFC3861]
of the form <im:user@domain> but who might be referenced by a SIP or
SIPS URI of the form <sip:user@domain> or <sips:user@domain>.

When a SIP user Romeo with a SIP URI of <sip:[email protected]> wants
to send an instant message to Juliet, he interacts with his SIP user
agent, which generates a SIP MESSAGE request. The syntax of the
MESSAGE request is defined in [RFC3428]. The following is an example
of such a request:

Example 4: SIP User Sends Message

| MESSAGE sip:[email protected] SIP/2.0
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKeskdgs677
| Max-Forwards: 70
| To: sip:[email protected]
| From: sip:[email protected];tag=vwxyz
| Call-ID: 9E97FB43-85F4-4A00-8751-1124FD4C7B2E
| CSeq: 1 MESSAGE
| Content-Type: text/plain
| Content-Length: 44
|
| Neither, fair saint, if either thee dislike.

Section 5 of [RFC3428] stipulates that a SIP user agent presented
with an im: URI should resolve it to a sip: or sips: URI. Therefore,
we assume that the Request-URI of a request received by an IM-capable
SIP-to-XMPP gateway will contain a sip: or sips: URI. Upon receiving
the MESSAGE, the SIP server needs to determine the identity of the
domain portion of the Request-URI or To header, which it does by
following the procedures explained in Section 5 of [RFC7247]. If the
domain is an XMPP domain, the SIP server will hand off the MESSAGE to
an associated SIP-to-XMPP gateway or connection manager that natively
communicates with XMPP servers.

The SIP-to-XMPP gateway is then responsible for translating the
request into an XMPP message stanza from the SIP user to the XMPP
user and returning a SIP 200 OK message to the sender:

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Example 5: SIP User Sends Message (XMPP Transformation)

| <message from='[email protected]/dr4hcr0st3lup4c'
| to='[email protected]'>
| <body>Neither, fair saint, if either thee dislike.</body>
| </message>

Note that the stanza-handling rules specified in [RFC6121] allow the
receiving XMPP server to deliver a message stanza whose 'to' address
is a bare JID ("localpart@domainpart") to multiple connected devices.
This is similar to the "forking" of messages in SIP.

The mapping of SIP syntax to XMPP syntax MUST be as shown in the
following table.

Table 2: Message Syntax Mapping from SIP to XMPP

+--------------------------+-----------------------------+
| SIP Header or Contents | XMPP Element or Attribute |
+--------------------------+-----------------------------+
| Call-ID | <thread/> |
| Content-Language | xml:lang |
| CSeq | (no mapping) |
| From | from (1) |
| Subject | <subject/> |
| Request-URI or To | to |
| body of MESSAGE | <body/> |
| transaction identifier | id |
+--------------------------+-----------------------------+

1. As shown in the foregoing example and described in [RFC7247], if
the IM-capable SIP-to-XMPP gateway has information about the GRUU
[RFC5627] of the particular endpoint that sent the SIP message,
then it MUST map the sender's address to a full JID
("localpart@domainpart/resourcepart") in the 'from' attribute of
the XMPP stanza and include the GRUU as the resourcepart.

When transforming SIP pager-mode messages, an IM-capable SIP-to-XMPP
gateway MUST specify no XMPP 'type' attribute or, equivalently, a
'type' attribute whose value is "normal" [RFC6121].

See Section 7 of this document about the handling of SIP message
bodies that contain content types other than plain text.

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6. Message Size

[RFC3428] specifies that (outside of a media session) the size of a
MESSAGE request is not allowed to exceed 1300 bytes. Although, in
practice, XMPP instant messages do not often exceed that size,
neither [RFC6120] nor [RFC6121] sets an upper limit on the size of
XMPP stanzas. However, XMPP server deployments usually do limit the
size of stanzas in order to help prevent denial-of-service attacks,
and [RFC6120] states that if a server sets a maximum stanza size,
then the limit is not allowed to be less than 10,000 bytes. Because
of this mismatch, an XMPP-to-SIP gateway SHOULD return a <policy-
violation/> stanza error if an XMPP user attempts to send an XMPP
message stanza that would result in a SIP MESSAGE greater than 1300
bytes. Although such a gateway might decide to "upgrade" from page
mode to session mode using the Message Session Relay Protocol (MSRP)
-- thus treating the instant message as part of a chat session as
described in [RFC7573] -- such behavior is application-specific and
this document provides no guidelines for how to complete such an
upgrade.

7. Content Types

SIP requests of type "MESSAGE" are allowed to contain essentially any
content type. The recommended procedures for SIP-to-XMPP gateways to
use in handling these content types are as follows.

An IM-aware SIP-to-XMPP gateway MUST process SIP messages that
contain message bodies of type "text/plain" and MUST encapsulate such
message bodies as the XML character data of the XMPP <body/> element.

An IM-aware SIP-to-XMPP gateway SHOULD process SIP messages that
contain message bodies of type "text/html"; if so, a gateway MUST
transform the "text/html" content into XHTML content that conforms to
the XHTML-IM Integration Set specified in [XEP-0071].

Although an IM-aware SIP-to-XMPP gateway MAY process SIP messages
that contain message bodies of types other than "text/plain" and
"text/html", the handling of such content types is a matter of
implementation.

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8. Internationalization Considerations

Both XMPP and SIP support the UTF-8 encoding [RFC3629] of Unicode
characters [UNICODE] within messages, along with tagging of the
language for a particular message (in XMPP via the 'xml:lang'
attribute and in SIP via the Content-Language header). Gateways MUST
map these language tagging mechanisms if they are present in the
original message. Several examples follow, using the "XML Notation"
[RFC3987] for Unicode characters outside the ASCII range.

Example 6: SIP User Sends Message

| MESSAGE sip:[email protected] SIP/2.0
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKeskdgs677
| Max-Forwards: 70
| To: sip:[email protected]
| From: sip:[email protected];tag=vwxyz
| Call-ID: 5A37A65D-304B-470A-B718-3F3E6770ACAF
| CSeq: 1 MESSAGE
| Content-Type: text/plain
| Content-Length: 45
| Content-Language: cs
|
| Nic z ob쎩ho, m쎡 d쒛vo spanil쎡,
| nenavid쎭얡-li jedno nebo druh쎩.

Example 7: SIP User Sends Message (XMPP Transformation)

| <message from='[email protected]'
| to='[email protected]'
| xml:lang='cs'>
| <body>
| Nic z ob쎩ho, m쎡 d쒛vo spanil쎡,
| nenavid쎭얡-li jedno nebo druh쎩.
| </body>
| </message>

9. Security Considerations

Detailed security considerations are given in the following
documents:

o For instant messaging protocols in general, see [RFC2779]

o For SIP-based instant messaging, see [RFC3428] and also [RFC3261]

o For XMPP-based instant messaging, see [RFC6121] and also [RFC6120]

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o For SIP-XMPP interworking in general, see [RFC7247]

This document specifies methods for exchanging "pager-mode" instant
messages through a gateway that translates between SIP and XMPP, and
[RFC7573] specifies such methods for "session-mode" instant messaging
between MSRP and XMPP. Such a gateway MUST be compliant with the
minimum security requirements of the textual chat protocols for which
it translates (i.e., SIP or MSRP and XMPP).

The addition of gateways to the security model of instant messaging
specified in [RFC2779] introduces some new risks. In particular,
end-to-end security properties (especially confidentiality and
integrity) between instant messaging clients that interface through a
gateway can be provided only if common formats are supported.
Specification of those common formats is out of scope for this
document. For instant messages, it is possible to use the methods
described in [RFC3862] and [RFC3923], but those methods are not
widely implemented. A more widely implemented, albeit
nonstandardized, method for interoperable end-to-end encryption would
be Off-the-Record Messaging [OTR].

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M.,
and E. Schooler, "SIP: Session Initiation Protocol",
RFC 3261, DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.

[RFC3428] Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
Huitema, C., and D. Gurle, "Session Initiation Protocol
(SIP) Extension for Instant Messaging", RFC 3428,
DOI 10.17487/RFC3428, December 2002,
<http://www.rfc-editor.org/info/rfc3428>.

[RFC3861] Peterson, J., "Address Resolution for Instant Messaging
and Presence", RFC 3861, DOI 10.17487/RFC3861, August
2004, <http://www.rfc-editor.org/info/rfc3861>.

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[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable
User Agent URIs (GRUUs) in the Session Initiation
Protocol (SIP)", RFC 5627, DOI 10.17487/RFC5627,
October 2009, <http://www.rfc-editor.org/info/rfc5627>.

[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <http://www.rfc-editor.org/info/rfc6120>.

[RFC6121] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 6121, DOI 10.17487/RFC6121, March 2011,
<http://www.rfc-editor.org/info/rfc6121>.

[RFC7247] Saint-Andre, P., Houri, A., and J. Hildebrand,
"Interworking between the Session Initiation Protocol
(SIP) and the Extensible Messaging and Presence
Protocol (XMPP): Architecture, Addresses, and Error
Handling", RFC 7247, DOI 10.17487/RFC7247, May 2014,
<http://www.rfc-editor.org/info/rfc7247>.

[XEP-0071] Saint-Andre, P., "XHTML-IM", XSF XEP 0071, November
2012.

10.2. Informative References

[GROUPCHAT] Saint-Andre, P., Corretge, S., and S. Loreto,
"Interworking between the Session Initiation Protocol
(SIP) and the Extensible Messaging and Presence
Protocol (XMPP): Groupchat", Work in Progress,
draft-ietf-stox-groupchat-11, March 2015.

[OTR] Goldberg, I., "Off-the-Record Messaging",
<https://otr.cypherpunks.ca/>.

[RFC2779] Day, M., Aggarwal, S., Mohr, G., and J. Vincent,
"Instant Messaging / Presence Protocol Requirements",
RFC 2779, DOI 10.17487/RFC2779, February 2000,
<http://www.rfc-editor.org/info/rfc2779>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
November 2003,
<http://www.rfc-editor.org/info/rfc3629>.

[RFC3860] Peterson, J., "Common Profile for Instant Messaging
(CPIM)", RFC 3860, DOI 10.17487/RFC3860, August 2004,
<http://www.rfc-editor.org/info/rfc3860>.

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[RFC3862] Klyne, G. and D. Atkins, "Common Presence and Instant
Messaging (CPIM): Message Format", RFC 3862,
DOI 10.17487/RFC3862, August 2004,
<http://www.rfc-editor.org/info/rfc3862>.

[RFC3922] Saint-Andre, P., "Mapping the Extensible Messaging and
Presence Protocol (XMPP) to Common Presence and Instant
Messaging (CPIM)", RFC 3922, DOI 10.17487/RFC3922,
October 2004, <http://www.rfc-editor.org/info/rfc3922>.

[RFC3923] Saint-Andre, P., "End-to-End Signing and Object
Encryption for the Extensible Messaging and Presence
Protocol (XMPP)", RFC 3923, DOI 10.17487/RFC3923,
October 2004, <http://www.rfc-editor.org/info/rfc3923>.

[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <http://www.rfc-editor.org/info/rfc3987>.

[RFC7573] Saint-Andre, P. and S. Loreto, "Interworking between
the Session Initiation Protocol (SIP) and the
Extensible Messaging and Presence Protocol (XMPP):
One-to-One Text Chat Sessions", RFC 7573,
DOI 10.17487/RFC7573, June 2015,
<http://www.rfc-editor.org/info/rfc7573>.

[SIMPLE-CPIM] Campbell, B. and J. Rosenberg, "CPIM Mapping of SIMPLE
Presence and Instant Messaging", Work in Progress,
draft-ietf-simple-cpim-mapping-01, June 2002.

[UNICODE] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>.

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Acknowledgements

The authors wish to thank the following individuals for their
feedback: Mary Barnes, Dave Cridland, Dave Crocker, Adrian Georgescu,
Christer Holmberg, Saul Ibarra Corretge, Olle Johansson, Paul
Kyzivat, Salvatore Loreto, Daniel-Constantin Mierla, and Tory Patnoe.

Special thanks to Ben Campbell for his detailed and insightful
reviews.

Francis Dupont reviewed the document on behalf of the General Area
Review Team.

Spencer Dawkins, Stephen Farrell, and Barry Leiba provided helpful
input during IESG review.

The authors gratefully acknowledge the assistance of Markus Isomaki
and Yana Stamcheva as the working group chairs and Gonzalo Camarillo
and Alissa Cooper as the sponsoring Area Directors.

Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier draft versions of this
document.

Authors' Addresses

Peter Saint-Andre
&yet
EMail: [email protected]
URI: https://andyet.com/

Avshalom Houri
IBM
Rorberg Building, Pekris 3
Rehovot 76123
Israel
EMail: [email protected]

Joe Hildebrand
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
United States
EMail: [email protected]

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