One document matched: draft-ietf-sip-callerprefs-00.txt
SIP Caller Preferences and Callee Capabilities
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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Abstract
This document describes a set of extensions to SIP which allow a
caller to express preferences about request handling in servers. It
also extends the SIP Contact header to allow users to describe their
communications capabilities and characteristics.
1 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant SIP caller preferences
implementations.
2 Introduction
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When a SIP [2] server receives a request, there are a number of
decisions it can make regarding processing of the request. These
include
o whether to proxy or redirect the request;
o which URIs to proxy or redirect to;
o whether to fork or not;
o whether to search recursively or not;
o whether to search in parallel or sequentially;
o whether to return just the first 200-class response, or all
2xx responses.
The server can base these decisions on a large number of factors,
such as time of day, caller or callee identity, call urgency, caller
preferences, network status, and the content of external databases.
There are three parties which have an interest in influencing the
call processing logic at the server: (1) the administrator of the
server, (2) the callee, and (3) the caller. The directives of the
administrator are usually embedded in the configuration of the
server, and can be expressed, for example, in the form of SIP CGI
scripts [3]. The preferences of the callee can be expressed most
easily through a script written in the call processing language (CPL)
[4]. This document specifies SIP extensions which express caller
preferences and describe callee capabilities by adding three new
header fields (Request-Disposition, Accept-Contact, Reject-Contact)
and extending the parameter list for the Contact header field.
If the client wants to be sure that the server understands the
headers described in this specification, it MUST include a Require
option of org.ietf.rfc.XXX
Since the Accept-Contact and Request-Disposition header are advisory
only, a client can save an extra round-trip time if it only includes
the Require option if the Reject-Contact header is present.
3 Design Alternatives
There are a number of alternatives for expressing caller preferences.
Ideally, caller preferences, callee preferences, and administrator
prefernces "meet" at each server which makes processing decisions. In
practicality, a callee cannot install logic at each server in the
network. It can only do so (using the CPL, for example), at those
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servers with which it has some kind of established trust
relationship. These servers are those whose main goal is to provide
services for the callee.
One might try to place caller logic at these "callee servers" in much
the same way the callee places logic there - through the CPL or some
other programmtic directives. However, this is also infeasible. A
caller cannot apriori install logic in every server for every
individual he might call.
As another alternative, one could embed a script in the request, to
be executed by proxy or redirect servers when making forwarding
decisions. This would be an application-layer version of active
networks. However, the generality of a script does not seem to be
needed. It also makes combining caller and callee preferences a
rather difficult problem and raises possible performance and security
issues. Unlike the callee script, which needs to handle unknown
callers, with a wide range of call properties, at unknown times in
the future, a caller knows all but the set of communications
capabilities of the callee. The caller can present the servers with
its preferences on a call-by-call basis. Callers can thus place their
preferences for this particular call in the request message. We
propose a simple ordered list of preferences to make it possible to
reconcile caller and callee preferences algorithmically.
In summary, there is a strong asymmetry in how preferences for
callers and callees can be presented to the network. While a caller
takes an active role by initiating the call, the callee takes a
passive role in waiting for calls. This motivates the use of callee-
supplied scripts and caller preferences included in the call request.
This asymmetry is also reflected in the appropriate relationship
between caller and callee preferences. A server for a callee SHOULD
respect the wishes of the caller to avoid certain locations, while
the preferences among locations has to be the callee's choice, as it
determines where, for example, the phone rings and whether the callee
incurs mobile telephone charges for incoming calls.
The problem of feature negotation has also been approached in a more
general way by [5]. However, that proposal is far more complicated
than appears to be needed here, with syntax that does not fit into
the current SIP syntax structure.
4 Protocol Semantics
The extension specified in this document consists of three new header
fields: Reject-Contact, Accept-Contact, and Request-Disposition. The
first two express preferences about which URIs the client would like
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the request to reach. The Reject-Contact header field (Section 8.3)
contains a list of URIs the user does not wish the request to be
proxied (or redirected) to. The Accept-Contact header field (Section
8.2) is a list of addresses that the caller would like to be proxied
(or redirected) to, with the strength of preferences expressed
through a q parameter, ranging in value from zero to one, with one
indicating the highest preference. The Request-Disposition header
field (Section 8.4) contains a set of tags which request particular
processing for this request, such as whether or not the caller
prefers proxy or redirection.
4.1 Applying the Preferences
The Accept-Contact header field indicates preferences for certain
destinations among those chosen by a proxy server. The Reject-Contact
header field describes those locations that the proxy server should
not direct the request to.
Listing only URIs is of limited use, since the caller often will not
know where the callee is located. For this reason, this specification
proposes a number of URI parameters which describe characteristics of
the user. These parameters include whether the location is a home or
work address, whether it is fixed or mobile, and what media types are
available. A server may know about a number of URIs for a user, along
with parameters describing each one, for example, through user
registration (REGISTER request). The combination of a URI along with
a set of parameters is called a contact entry. A set of contact
entries is called the contact list. When a request arrives with
either Reject-Contact or Accept-Contact header field, the server
performs a matching operation, described below, to create an ordered
list of contact addresses that reflect the joint caller's and
callee's preferences.
There is some overlap between the indication of receiver
capabilities in the session description message body and
the Accept-Contact and Reject-Contact header fields.
However, current session description formats cannot express
the preferences described here. Also, the capabilities
described here are fundamental to call-routing and thus
should not depend on the particulars of the various session
description formats that might be used.
The server matches rules in the Accept-Contact and Reject-Contact
request headers to contact entries in the contact list according to
the following rules.
o If rule does not contain URI, only parameters are compared. If
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the request header rule contains a URI, both the URI and
parameters must match.
o A URI in a rule matches a SIP URI in a contact entry if the
user and host parts are equal, where equality is based on
string equivalence. Either part may be omitted from the rule,
and in this case it matches any value. For example, the rule "
@acme.com " matches " foo@acme.com " or " bar@acme.com ",
while the rule " joe@ " matches " joe@acme.com " and "
joe@example.com ". Matches are case-insensitive. Other URIs
are matched according to the equality rules for that URI
scheme.
o The parameters in a rule match the parameters in a contact
entry if all parameters in the rule either have a matching
value in the contact entry, or the parameter is not present in
the contact entry.
Ignoring the parameter if it does not exist in the
contact list avoids that a parameter that the server
does not know about causes the match to fail.
The pseudo-code below describes the matching procedure between the
rule in the request header and the contact entry. For simplicity, we
assume that the list of parameters in each rule is stored as an
associative array, so that rule.para[duplex] yields the value of the
attribute duplex, and is undefined if duplex is not specified.
struct {
uri_t URI; /* URI */
parameter_t para[]; /* list of parameters */
} rule, entry;
boolean MATCH(rule r, entry e) {
boolean match;
match = TRUE;
if (r.URI != "") {
if (r.URI.scheme == e.URI.scheme) {
if (r.URI.scheme == "sip") {
match = (r.URI.host == "" || r.URI.host == e.URI.host) &&
(r.URI.user == "" || r.URI.user == e.URI.user)
} else {
match = scheme-appropriate comparison;
}
} else {
return FALSE;
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}
}
if(match == FALSE) return FALSE;
/* compare parameters */
foreach parameter p in r.para[] {
if (isdefined(e.para[p])) {
if (r.para[p][0] == "!")
if (r.para[p] intersect e.para[p] != "") {
return FALSE;
}
} else {
if (r.para[p] is not subset of e.para[p]) {
return FALSE;
}
}
}
}
return TRUE;
}
Parameter names are matched by case-insensitive string comparison.
Parameter values are compared by set-comparisons. Parameter values in
quoted strings are interpreted as sets, with elements separated by
commas. The names of elements are case-insensitive. Parameter values
that are tokens are interpreted as sets with one element. There are
two cases: if the quoted-string parameter value in a rule starts with
an exclamation mark (!), the rule matches if the intersection of the
set in the rule and in the contact entry is empty. Otherwise, the
rule matches if the intersection is the rule set itself, i.e., if the
rule set is a subset of the contact entry parameter.
The syntax for empty intersection is ugly. Using operators
instead of equal may be preferable, but breaks the basic
SIP parser model.
For example, the rule
;language="!en,de"
matches the contact entry containing
;language="es,nl"
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but not any of
;language="en"
;language="de,en"
;language="en,es,fi"
As another example, the rule
;duplex="full,half"
matches the contact entry
;duplex="full"
but not
;duplex="send-only"
A server SHOULD NOT be aware of the particular semantics of any of
the parameters. This allows for the definition of new parameters and
values without explicitly programming them into the servers.
4.2 Contact List Processing - no CPL
It is assumed that the server has a contact list for the callee. In
this case, the server MAY elect to follow the procedure below for
merging caller preferences and callee preferences. When callee logic
(such as a CPL) is present, the procedure for combining caller and
callee preferences is different. General guidelines for the process
are given in Section 5.
The server first removes any contact entry from the contact list that
matches a rule in the Reject-Contact header field.
A contact entry may contain a priority parameter. This means that a
request should not be proxied or redirected to that contact entry
unless the request is of equal or higher priority. The priority value
of the request is derived from the Priority header field. If the
request does not contain a Priority header field, the request
priority is set to "non-urgent". Priorities are ordered from "non-
urgent" (lowest), "normal", "urgent" to "emergency" (highest).
Priority values not known to the server are mapped to "non-urgent".
The server then removes any contact entry whose priority value is
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higher than that of the request.
Each rule in the Accept-Contact header field is then processed. If
the rule matches a contact entry, the q value of that entry is
updated, in order to incorporate the caller's preferences. If the
rule does not match a contact entry, nothing is done. This document
does not prescribe a certain algorithm for updating. Among many
possibilities, a server MAY set the q value to the average of the
original value specified by the callee, and the average q value of
the caller's rules that match the contact entry. This gives equal
weight to caller and callee preferences. If a rule or contact entry
does not have a q value, it is taken to be one (this is in agreement
with the HTTP defaults).
Note that this preference computation only determines the
ordering of call attempts, so that the properties of the
preference computation are of secondary importance. The q-
value ordering provides only limited flexibility to
indicate, for example, that a particular parameter is more
important than another one or that combinations of two
parameters should be weighed heavily.
If the server proxies, the contact list is then sorted according to
the q value. The server first attempts to contact those with the
highest q value in parallel. If these contact entries do not respond
with a 2xx or 6xx response, the server tries the entries with the
next-highest q value.
Due to round-off errors and the computation of joint
preferences, there may be an excessive bias here towards
serialization rather than parallel attempts. Maybe a server
should group all q values within, say, 0.1 into a single
parallel-search group.
If the server receives a redirection, and elects to recurse
(depending on its configuration and the preference specified in the
Request-Disposition header field), the contact addresses are added to
the contact list and the algorithm continues. Note that this may
cause addresses that were added by redirection to be tried before
contact entries in the original contact list.
5 Contact List Processing - CPL
When a CPL is present, the processing rules in the previous section
do not apply. These rules are effectively a default CPL, which can
and should be overriden when the user supplies a CPL.
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In order to control processing of caller preferences and callee
capabilities within the CPL, the CPL must have access to the Accept-
Contact and Reject-Contact headers from the request. It must also
have access to the registrations uploaded by the callee, and be able
to examine their preferences. The exact way in which this might be
done is TBD.
6 IANA Registration
New URI parameters and values can be defined at any time and
registered with IANA. When registering new parameters and values, the
following information MUST be provided:
Contact: Name, organization, email address, and phone number of
person registering the attributes.
Attributes: A list of the new attributes being registered. For
each, the meaning of the attribute must be described, in
sufficient detail so that a user agent would be able to
ascertain whether the parameter applies to it, and if so,
which value to use. The attributes MUST also be associated
with a finite set of values, each of which is a valid
unicode string. For each value, a description of the value
must be provided.
7 Use with REGISTER
A user agent normally registers with one or more servers, providing
each with a list of Contact addresses. The user agent MAY add the
parameters cp-param described in Section 8.2 to the Contact header
field.
Furthermore, the REGISTER request MAY contain a Require header with
the name of this extension if the client wants to be sure that the
server honors callee preferences.
8 Header Field Definitions
The table below specifies which requests can contain which headers.
Since all three headers specify call routing logic, they can apply to
any request which can normally be proxied.
8.1 Contact, Accept-Contact and Reject-Contact Parameters
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type ACK BYE INV OPT REG CAN
__________________________________________________
Accept-Contact R - o o o o -
Reject-Contact R - o o o o -
Request-Disposition R - o o o o -
Table 1: Summary of header fields. "o": optional "-": not applicable,
"R': request header, "r": response header, "g": general header, "*":
needed if message body is not empty. A numeric value in the "type"
column indicates the status code the header field is used with.
This specification adds the following extension parameters to the
Contact header field and defines the same parameters for the Accept-
Contact and Reject-Contact header fields.
cp-params = class-param | duplex-param |
features-param | language-param | media-param |
mobility-param | priority-param | service-param
class-param = "class" "=" <"> [<!>] class-tag <">
duplex-param = "duplex" "=" <"> [<!>] duplex-tag <">
feature-param = "feature" "=" <"> [<!>] 1#feature-tag <">
language-param = "language" "=" <"> [<!>] 1#language-tag <">
media-param = "media" "=" <"> [<!>] 1#media-tag <">
mobility-param = "mobility" "=" <"> [<!>] mobility-tag <">
service-param = "service" "=" <"> [<!>] service-tag <">
language-tag = primary-tag *( "-" subtag )
primary-tag = 1*8ALPHA
subtag = 1*8ALPHA
mobility-tag = "fixed" | "mobile"
class-tag = "personal" | "business"
duplex-tag = "full" | "half" | "receive-only" | "send-only"
service-tag = "fax" | "IP" | "PSTN" | "ISDN" |
"text" | "numeric"
media-tag = ( "*/*" | (type "/" "*") |
(type "/" subtype) )
feature-tag = "voice-mail" | "attendant"
The exclamation mark in the parameter value MUST NOT be included if
the cp-params are included in a Contact header.
class: The class parameter indicates whether this terminal is
found in a residential or business setting. (A caller may
defer a personal call if only a business line is available,
for example.)
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duplex: The duplex parameter lists whether the terminal can
simultaneously send and receive ("full"), alternate between
sending and receiving ("half"), can only receive
("receive-only") or only send ("send-only"). Typically, a
caller will prefer a full-duplex terminal over a half-
duplex terminal and these over receive-only or send-only
terminals.
features: The feature list enumerates additional features. It is
assumed that these features are orthogonal, and could occur
in any combination. "Voice-mail" includes the recording of
any multimedia stream, as appropriate.
language: The language parameter lists the languages spoken by
the caller or callee. This feature may, for example, be
used to have a caller automatically be directed to the
appropriate attendant or customer service representative.
Note that this parameter has a different functionality than
the Accept-Language and Content-Language header fields,
which describe the acceptable languages and languages used
in the request and the media description, not the actual
communications.
media: The media tag lists the media types supported by the
terminal. Media types can be the standard Internet media
types ("audio", "video", "text", "application"), optionally
followed by a subtype (e.g., "text/html"). In addition, the
type "application/email" is defined.
mobility: The mobility parameter indicates if the terminal is
fixed or mobile. In some locales, this may affect audio
quality or charges.
service: The service tag describes what service is being
provided by the terminal. The "text" service refers to a
device that can send or receive unformatted ASCII text,
such as a pager or a TTY. The "IP" tag indicates a device,
such as a personal computer, with Internet connectivity.
The "fax" tag indicates a fax machine. The "PSTN" tag
indicates a telephone connected to the public switched
telephone network. The "ISDN" tag indicates a telephone
connected to the Integrated Services Digital Network. The
"numeric" tag indicates a device that supports numeric text
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only, such as a simple pager.
The service tags were chosen to maximize the
orthogonality of the mobility and service parameters.
In addition, the Contact header field may contain the description-
param and priority-param parameters. The description parameter
further describes, as text, the terminal. The user agent MAY present
this text when it is contained in a Contact header field in a 3xx
response. The description parameter MUST NOT be used in the matching
operation described above. The priority parameter indicates the
minimum priority level this terminal is to be used for. It can be
used for automatically restricting the choice of terminals available
to the user, as described in the procedure above.
priority-param = "priority" "=" <"> priority-tag <">
description-param = "description" "=" quoted-string
priority-tag = "emergency" | "urgent" | "normal" | "non-urgent"
The first example below describes a SIP terminal whose owner speaks
English, Spanish and German. The terminal is capable of sending and
receiving audio and video and can participate in a chat session.
However, the owner only wants callers to use the terminal if the call
is of priority "urgent" or higher. This Contact header would normally
be included in a REGISTER message.
Contact: Carol <sip:carol@example.com> ;language="en,es,de"
;media="audio,video,application/chat"
;duplex="full"
;priority="urgent"
8.2 Accept-Contact
The syntax for the Accept-Contact header is defined below:
Accept-Contact = "Accept-Contact" ":" 1# rule
rule = ( name-addr | addr-spec )
[ *( ";" (cp-params | extension-param | q-param) ) ]
q-param = "q" "=" qvalue
extension-param = extension-name "=" extension-value
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extension-name = token
extension-value = token | quoted-string
The header field specifies contact addresses which are acceptable to
the caller.
In the following example, the caller would prefer not to talk to
sales@acme.com later. She has a slight preference for fixed as
opposed to mobile phones.
Accept-Contact: sip:sales@acme.com ;q=0,
;media="!video" ;q=0.1,
;mobility="fixed" ;q=0.6,
;mobility="!fixed" ;q=0.4
8.3 Reject-Contact
The Reject-Contact header field specifies a list of URIs that the
caller does not wish to communicate with. The BNF for the header is:
Reject-Contact = "Reject-Contact" ":"
1# ( ( name-addr | addr-spec )
[ *( ";" new-params ) ] )
8.4 Request-Disposition
The Request-Disposition header field specifies caller preferences for
how a proxy or user agent server should process a request. Its value
is a list of tokens, each of which specifies a particular feature.
When the caller specifies a feature, the server SHOULD treat it as a
hint, not as a requirement and MAY ignore the feature request.
The header field has the following syntax:
Request-Disposition = "Request-Disposition" ":"
1# (proxy-feature | cancel-feature |
fork-feature | recurse-feature |
parallel-feature | queue-feature |
ring-feature)
proxy-feature = "proxy" | "redirect"
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cancel-feature = "cancel" | "no-cancel"
fork-feature = "fork" | "no-fork"
recurse-feature = "recurse" | "no-recurse"
parallel-feature = "parallel" | "sequential"
queue-feature = "queue" | "no-queue"
ring-feature = "ring" | "no-ring"
proxy-feature: This feature indicates whether the caller would
like each server to proxy or redirect.
cancel-feature: This feature indicates whether the caller would
like each proxy server to send a CANCEL request downstream
in response to a 200 OK from the downstream server, or
whether this function should be left to the caller.
fork-feature: This feature indicates whether a proxy should fork
a request, or proxy to only a single address. If the server
is requested not to fork, the server should proxy the
request to the "best" address. The feature is ignored if
"redirect" has been requested.
recurse-feature: This feature indicates whether a proxy server
receiving a 300-class response should send requests to the
addresses listed in the response (i.e., recurse), or
forward the list of addresses upstream towards the caller.
The feature is ignored if "redirect" has been requested.
parallel-feature: For a forking proxy server, this feature
indicates whether the caller would like the proxy server to
proxy the request to all known addresses at once, or go
through them sequentially, contacting the next address only
after it has received a non-200 or non-600 final response
for the previous one. The feature is ignored if "redirect"
has been requested.
queue-feature: If the called party is temporarily unreachable,
e.g., because it is in another call, the caller can
indicate that it wants to have its call queued rather than
rejected immediately. If the call is queued, the server
returns "182 Queued". A pending call be terminated by a
SIP CANCEL or BYE request.
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ring-feature: In certain cases, the caller is an administrator
who wishes to convey an important announcement. In this
case, the UAS should not alert the user and should
automatically accept the call. If the UAC does not wish the
user to be alerted, the "no-ring" token is included.
Requests containing this token SHOULD be authenticated, and
only priviledged users should be allowed to send requests
without alerting the called party.
The normal Proxy-Require/Require/Unsupported mechanism is used to
indicate to the caller and/or downstream proxies that a particular
service is required to complete the request. Otherwise, the service
indication is to be taken as a hint.
Example:
Request-Disposition: proxy, recurse, parallel
9 Changes since -00
o Addition of a no-ring preference, which can be used to support
operator barge in and mid-call announcements.
o Changed option tag in Require header to org.ietf.rfc.XXX, with
RFC number to be determined.
o Added numeric service tag
o Added some discussion of CPL
10 To be Done
o More details on interactions with CPL
11 Acknowledgements
Parameters of the terminal negotiation mechanism in Section 8.1 were
influenced by Scott Petrack's CMA design. Jonathan Lennox and John
Hearty provided helpful comments.
12 Bibliography
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments (Best Current Practice) 2119, Internet
Engineering Task Force, Mar. 1997.
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[2] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments (Proposed
Standard) 2543, Internet Engineering Task Force, Mar. 1999.
[3] J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
interface for SIP," Internet Draft, Internet Engineering Task Force,
May 1999. Work in progress.
[4] J. Lennox and H. Schulzrinne, "Call processing language framework
and requirements," Internet Draft, Internet Engineering Task Force,
July 1999. Work in progress.
[5] G. Klyne, "A syntax for describing media feature sets," Internet
Draft, Internet Engineering Task Force, Dec. 1998. Work in progress.
Full Copyright Statement
Copyright (c) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Table of Contents
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1 Terminology ......................................... 1
2 Introduction ........................................ 1
3 Design Alternatives ................................. 2
4 Protocol Semantics .................................. 3
4.1 Applying the Preferences ............................ 4
4.2 Contact List Processing - no CPL .................... 7
5 Contact List Processing - CPL ....................... 8
6 IANA Registration ................................... 9
7 Use with REGISTER ................................... 9
8 Header Field Definitions ............................ 9
8.1 Contact, Accept-Contact and Reject-Contact
Parameters ..................................................... 9
8.2 Accept-Contact ...................................... 12
8.3 Reject-Contact ...................................... 13
8.4 Request-Disposition ................................. 13
9 Changes since -00 ................................... 15
10 To be Done .......................................... 15
11 Acknowledgements .................................... 15
12 Bibliography ........................................ 15
Schulzrinne/Rosenberg [Page 17]
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