One document matched: draft-ietf-sip-callerprefs-03.txt
Differences from draft-ietf-sip-callerprefs-02.txt
Internet Engineering Task Force SIP WG
Internet Draft Schulzrinne/Rosenberg
draft-ietf-sip-callerprefs-03.txt Columbia U./dynamicsoft
November 24, 2000
Expires: May 2001
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.
These preferences include the ability to select which URIs a request
gets routed to, and to specify certain request handling directives in
proxies and redirect servers. It does so by defining three new
request headers, Accept-Contact, Reject-Contact and Request-
Disposition, which specify the callers preferences. The extension
also defines new parameters for the Contact header that describe the
characterstics of a UA.
1 Introduction
When a SIP [1] server receives a request, there are a number of
decisions it can make regarding processing of the request. These
include
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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;
The server can base these decisions on any local policy. This policy
can be statically configured, or can be based on programmtic
execution or database access.
However, the administrator of the server is the not the only entity
with an interest in call processing. There are at least three parties
which have an interest: (1) the administrator of the server, (2) the
requestor, and (3) the user to whom the request is directed. The
directives of the administrator are embedded in the policy of the
server. The preferences of the user to whom the request is directed
(referred to as callee, even though the request may not be INVITE)
can be expressed most easily through a script written in the call
processing language (CPL) [2]. However, no mechanism exists to
incorporate the preferences of the requestor (also referred to as the
caller, even though the request may not be INVITE). This extension
fills that gap by specifying mechanisms by which a caller can provide
preferences on processing of a request. These preferences include the
ability to select which URIs a request gets proxied or redirected to,
and to specify certain request handling directives in proxies and
redirect servers. It does so by defining three new request headers,
Accept-Contact, Reject-Contact and Request-Disposition, which specify
the callers preferences. The extension also defines new parameters
for the Contact header that describe attributes of a UA at a
specified URI.
2 Overview of Operation
This extension defines a set of additional parameters to the Contact
header. These parameters specify attributes that define the
characteristics of the UA at the URI in the header. For example,
there is a mobility parameter which indicates whether the UA is fixed
or mobile. When a UA registers, it places these parameters in the
Contact headers to characterize the URIs it is registering. This
allows the proxy to have information about the contact addresses for
a user. The parameters are also mirrored in the Contact header in a
REGISTER response.
Certain requests, such as the INVITE message, and its response, also
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contain Contact headers used to route subsequent messaging. This
extension allows these headers to contain extension parameters to
provide additional information about the type of user agent being
used. For example, by including the feature parameter with value
"voicemail" in the 200 OK to an INVITE, the UAS can indicate to the
UAC that it is a voicemail server. This information is useful for
user interfaces, as well as automated call handling.
When a caller sends a request (INVITE or otherwise; caller
preferences apply to all requests), it can optionally include new
headers which request certain handling at a server. These preferences
fall into two categories. The first category, carried in the
Request-Disposition header, describe desired server behavior. This
includes whether the caller wishes the server to proxy or redirect,
and whether sequential or parallel search is desired. These
preferences can be applied at every proxy or redirect server on the
call signaling path.
The second category of preferences are carried in the Accept-Contact
and Reject-Contact headers. These preferences contain rules that
describe the set of desired URIs that the caller would like the
request to be routed to. These rules are processed in proxies
(generally ones that access a registration database, although not
necessarily), and in user agents. In proxies, these rules are matched
against the Contact headers sent in a registration (or through some
other location service). If a rule in a Reject-Contact header matches
a Contact header, that address is not proxied or redirected to. If a
rule in a Accept-Contact header matches a Contact header, the q
values in the rule are combined with the q values in the Contact
header, resulting in a "merged" q value. This merged q value is then
used by the proxy to determine the ordering of addresses to proxy or
redirect to.
In user agents, only Accept-Contact headers marked with the "only"
attribute are processed. The request is rejected by the user agent if
none of its registered Contacts matches a rule in the Accept-Contact
headers marked with this attribute.
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.
Practically speaking, however, a callee cannot install logic at each
server in the network. It can only do so (using the CPL, for
example), at those 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.
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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 request-by-request basis. Callers can thus place
their preferences for this particular request 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 request, the callee takes a
passive role in waiting for requests. 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 [3]. 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 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 [4] and
indicate requirement levels for compliant SIP caller preferences
implementations.
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5 Header Field Definitions
Table 5 specifies an extension of Table 5 in RFC 2543 [1] for the
three new headers defined here.
where enc e-e ACK BYE CAN INV OPT REG
_________________________________________________________
Accept-Contact R n h - o o o o -
Reject-Contact R n h - o o o o -
Request-Disposition R n h - o 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.
5.1 Contact, Accept-Contact and Reject-Contact Parameters
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. These parameters apply to a
single URI. When used in a Contact header, they specify
characteristics of the URI in the header. When used in the Accept-
Contact or Reject-Contact headers, they specify rules to apply for
matching URIs.
cp-params = class-param | duplex-param |
feature-param | language-param | media-param |
mobility-param | other-param
class-param = "class" "=" <"> [<!>] 1#class-value <">
duplex-param = "duplex" "=" <"> [<!>] 1#duplex-value <">
feature-param = "feature" "=" <"> [<!>] 1#feature-value <">
language-param = "language" "=" <"> [<!>] 1#language-tag <">
media-param = "media" "=" <"> [<!>] 1#media-value <">
mobility-param = "mobility" "=" <"> [<!>] 1#mobility-value <">
other-param = other-name "=" <"> [<!>] 1#other-value <">
mobility-value = "fixed" | "mobile" | other-value
class-value = "personal" | "business" | other-value
duplex-value = "full" | "half" | "receive-only" |
"send-only" | other-value
media-value = ( "*/*" | (type "/" "*") |
(type "/" subtype) )
feature-value = "voice-mail" | "attendant" | other-value
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other-name = UTF8-TOKEN
other-value = UTF8-TOKEN
UTF8-TOKEN = <any UTF-8 character encoding
except separator, CTL, and LWS>
The BNF and semantics of the language-tag are defined in Section 3.10
of RFC 2616 [5]. Note, however, that in their usage here they are
case sensitive, and MUST appear as all lowercase. Also note that
there MUST NOT be any linear white space between the tokens and
quoted strings of the media-value. This is to align with HTTP 1.1
[5].
The exclamation mark in the parameter value MUST NOT be included if
the cp-params are included in a Contact header. Most importantly,
there MUST NOT be more than one class-value, duplex-value, or
mobility-value when cp-params is included in a Contact header. These
parameters refer to attributes which are mutually exclusive. As a
result, a URI can only have one as a characteristic, whereas a rule
in the Accept-Contact or Reject-Contact can specify more than one.
The parameters and their values have the following meanings:
class: The class parameter indicates whether the UA is found in
a residential or business setting. (A caller may defer a
personal call if only a business line is available, for
example.)
duplex: The duplex parameter lists whether the UA can
simultaneously send and receive media ("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 UA over a half-duplex UA
and these over receive-only or send-only UAs.
features: The feature parameter enumerates additional features
of the UA. It is assumed that these features are
orthogonal, and could occur in any combination. "voice-
mail" means that an automated system exists at this UA,
which is capable of recording messages. "attendant" means
that a human operator is available to take messages.
language: The language parameter lists the languages spoken by
user or system behind the UA. This parameter may, for
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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
[5] header fields, which describe the languages for headers
and content acceptable in responses (in the former case)
and used in the request (in the latter case). This is
opposed to the language of the media exchanged for actual
communications, defined by this parameter.
media: The media parameter lists the media types supported by
the UA. In this context, supported means that the media
type is acceptable as part of the media session established
by SIP (and usually described by SDP [6]). It does not
refer to the media types which can be supported within the
bodies of SIP messages. Media types can be the standard
Internet media types ("audio", "video", "text",
"application"), optionally followed by a subtype (e.g.,
"text/html").
mobility: The mobility parameter indicates if the UA is fixed or
mobile. In some locales, this may affect audio quality or
charges.
In addition, the Contact header field MAY contain the description-
param, methods-param and priority-param parameters.
The description parameter further describes, as text, the terminal.
The description parameter MUST NOT be used in the matching operation
described in Section 6.4.1.
The priority parameter indicates the minimum priority level this UA
is to be used for. It can be used for automatically restricting the
choice of terminals available to the caller. The priority parameter
is not used in the matching operation described in Section 6.4.1. Its
application is described in the procedure in Section 6.4.2.
The methods parameter indicates the methods this UA understands. It
MUST NOT be used in any request excepting REGISTER. The methods
parameter is not used in the matching operation described in Section
6.4.1. Its application is described in the procedure in Section
6.4.2. The methods parameter is extremely useful for breaking a UA
into seperate pieces of software, each of which handles specific
requests. For example, one piece might handle instant messages (the
MESSAGE method [7]), while other handles phone calls (the INVITE and
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BYE methods)
priority-param = "priority" "=" <"> priority-value <">
description-param = "description" "=" quoted-string
methods-param = "methods" "=" <"> 1#methods-value <">
methods-value = ( "INVITE" | "OPTIONS" | "BYE" | "REGISTER"
| token)
Note that priority-value is defined in section 6.25 of [1].
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.
5.2 Accept-Contact
The syntax for the Accept-Contact header is defined below:
Accept-Contact = ("Accept-Contact" | "a") ":" 1# rule
rule = ( name-addr | addr-spec | "*")
[ *( ";" (cp-params | q-param | scheme-param |
strength)) ]
q-param = "q" "=" qvalue
scheme-param = "scheme" "=" <"> [<!>] 1#scheme <">
strength = "only=true"
Note that a compact form, with the letter a, can be used with this
header.
The BNF for qvalue is specified in RFC2616 [5], and the BNF for
scheme is defined in RFC2396 [8].
The header field specifies contact addresses which are acceptable to
the UAC. If a "*" is specified instead of a name-addr or addr-spec,
it means the UAC doesn't care about the URI of the user eventually
reached. Only the parameters of the Contact header are important. If
the name-addr or addr-spec is present, and the userinfo field of the
SIP URL is not present, it means the UAC doesn't care about the
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username of the user eventually reached. If the host portion of the
SIP URL is a hostname, and has the value "x", it means the UAC
doesn't care about the host portion of the URI eventually reached. If
the name-addr or addr-spec is present, and contains URI parameters,
if means the UAC wishes to be connected to an address that has been
registered with these parameters.
We use "x" as the wildcard domain because of the URI
formatting constraints. The domain must be present in a SIP
URL, and cannot be the "*" character. The "x" character is
allowed and looks kind of similar.
The scheme parameter describes the set of URI schemes which the UAC
is willing to accept redirects to or communicate with. The BNF for
scheme is given in RFC 2396 [8], and can be any valid URI scheme.
The strength parameter is used to indicate that the UAC wishes for
this request to *only* be processed by a UA matching the rule
(absence of this parameter implies that the UAC has preferences, but
still will communicate with UAs not matching the rule).
In the following example, the UAC would prefer not to talk to
sales@acme.com later. They have 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
In the next example, the caller would prefer to speak to someone from
sales.org that supports video:
Accept-Contact: sip:sales.org;media="video"
5.3 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:
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Reject-Contact = ("Reject-Contact" | "j") ":"
1# ( ( name-addr | addr-spec | "*")
[ *( ";" cp-params | scheme-param ) ] )
Note that a compact form, using the letter j (for reJect), is
defined.
If name-addr or addr-spec is not present (the "*" is present), it
means the UAC does not care about the particular user or domain the
request is routed to. The cp-params are used to filter out contact
addresses based on their parameters alone. This process is described
in Section 6.4.1. If either name-addr or addr-spec is present, and
the URI does not contain a userinfo field, it means the UAC does not
have a preference regarding the user name and/or password of the UA
eventually reached. If domain of the URI is equal to "x", it means
the UAC does not have a preference regarding the domain of the UA
eventually reached.
The scheme parameter describes the set of URI schemes which the UAC
is not willing to accept redirects to or communicate with. The BNF
for scheme is given in RFC 2396 [8], and can be any valid URI scheme.
5.4 Contact Header
The cp-params parameter is allowed as an extension attribute to the
Contact header, along with the priority-param, methods-param and
description-param. This effectively means that the BNF for
extension-attribute, defined in Section 6.13 of RFC 2543 [1] can be
redefined as:
extension-attribute = (cp-params |
priority-param | methods-param |
description-param |
(extension-name [ "=" extension-value]))
The 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"
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;media="audio,video,application/chat"
;duplex="full"
;priority="urgent"
As another example, an INVITE message is sent with a Contact header
that includes some of the parameters defined here:
INVITE sip:user@example.com SIP/2.0
Via: SIP/2.0/UDP host.example.com
From: sip:caller@university.edu
To: sip:user@example.com
Call-ID: 9sdnasdbasd@1.2.3.4
CSeq: 3 INVITE
Contact: Joe Caller <sip:caller@university.edu>;mobility="mobile"
In this case, Joe is indicating he is calling from a mobile host.
5.5 Request-Disposition
The Request-Disposition header field specifies caller preferences for
how a proxy or redirect server should process a request. It is not
processed by user agents. 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" | "d" ) ":"
1# (proxy-feature | cancel-feature |
fork-feature | recurse-feature |
parallel-feature | queue-feature)
proxy-feature = "proxy" | "redirect"
cancel-feature = "cancel" | "no-cancel"
fork-feature = "fork" | "no-fork"
recurse-feature = "recurse" | "no-recurse"
parallel-feature = "parallel" | "sequential"
queue-feature = "queue" | "no-queue"
extension-feature = token
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Note that a compact form, using the letter d, has been defined.
proxy-feature: This feature indicates whether the caller would
like each server to proxy or redirect. If the server is
incapable of performing the requested feature, it SHOULD
ignore the feature request.
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 (generally the one with the
highest q value). 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.
Example:
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Request-Disposition: proxy, recurse, parallel
6 Protocol Semantics
6.1 UA Behavior for Registering
User agent servers MAY include cp-params, priority-param, methods-
param or description-param parameters as part of each Contact URI
they register. These parameters can be set through configuration,
user input, or any means the implementor seeks to use. They SHOULD
reflect actual characteristics of the URLs being registered.
Furthermore, the REGISTER request MAY contain a Require header with
the option tag "pref" if the client wants to be sure that the
registration server honors caller preferences.
6.2 UAS Behavior
When a UAS receives a request with the Accept-Contact, Reject-Contact
and Request-Disposition, it SHOULD process just the Accept-Contact
header values marked with the "only=true" parameter. If no values
contain this parameter, request processing at the UAS continues
normally, as if there were no caller preferences expressed in the
request.
Assuming at least one Accept-Contact header is marked with this
parameter, the UAS creates a contact list. The contact list is equal
to the list of unexpired contacts registered by the UA at all
registrars. It also creates a set of rules, obtained from those
Accept-Contact header values in the request which contain the
"only=true" parameter. Then, the matching process in Section 6.4.1 is
applied. If none of the contacts in the contact list match any of the
rules, the UAS SHOULD reject the request with a 404 Not Found.
Note that if the UAS is planning on redirecting the request, the UAS
SHOULD follow the rules described in Section 6.4 for a redirect
server.
If the request contains a Contact header with the additional
parameters described here, those parameters MAY be used as an input
to any local processing at the UAS. For example, they can be used to
guide an automated screening service, or they may be rendered to the
user.
If the UAS receives a request whose request URI has been registered
with any of the new Contact parameters defined here, it is
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RECOMMENDED that the Contact in a 200 response to that request (given
that a response to this request method would normally contain a
Contact header) include those parameters.
6.3 UAC Behavior
A UAC wishing to express preferences for a request includes the
Accept-Contact, Reject-Contact, or Request-Disposition headers in the
request, depending on its particular preferences. No additional
behavior is required after the request is sent.
If the client wants to be sure that servers understand the headers
described in this specification, it MAY include a Proxy-Require and
Require option tag of "pref". However, this is NOT RECOMMENDED, as it
leads to interoperability problems. In any case, client preferences
can only be considered as preferences - there is no guarantee that
the requested service or capability is executed. As such, inclusion
of Proxy-Require and Require does not mean the preferences will be
executed.
In requests that contain Contact headers for the purposes of routing
subsequent requests (such as INVITE), it is RECOMMENDED that the
Contact header contain any of the Contact extension parameters
defined here which describe the properties of the UA in the Contact
URI.
6.4 Proxy Behavior
The behavior described here assumes a server (proxy or redirect) has
received a valid request with either the Accept-Contact or Reject-
Contact headers, and that this proxy has a list of Contact headers
obtained from looking up the Request-URI in the location service. The
location service may have obtained this data through registrations,
as described in Section 6.1, but other means may exist. Note that the
presence of Route headers in a request eliminates the need for
processing of the caller preferences headers; as per RFC 2543, the
proxy forwards the request to the URI in the top Route header.
The processing depends heavily on a rule matching operation. This
operation takes a rule (defined as a single element from the comma
separate list of elements in the Accept-Contact or Reject-Contact
headers), and matches it against the contact list obtained from the
location service.
6.4.1 Rule Matching Procedures
The contact list is composed of a set of contact entries. Each
contact entry consists of a URI along with a set of parameters. A
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rule, like a contact entry, consists of a URI (or the "*" character),
and a set of parameters. If the rule does not contain a URI (just the
"*" character), the rule matches the contact entry if and only if the
parameters in the rule and the parameters in the contact entry match.
If the rule contains a URI, both the URI and parameters must match
for the rule to match the contact entry.
The URI in the rule and the URI in the contact entry match depending
on the scheme. For non-SIP URIs, matching is based on the URI
equivalency rules for that scheme. For SIP URLs, the userinfo, host,
and URI parameters must match, where matching is defined as follows.
Note that these matching rules are not the same as the general URI
matching rules in SIP [1].
If the rule contains a userinfo field, that userinfo field must match
the userinfo field in the URI in the contact entry. Matching is based
on case sensitive string comparison. If the rule contains a userinfo
field, but the URI in the contact address does not, the userinfo in
the rule does not match the userinfo in the contact entry. If the
rule does not contain a userinfo field, the userinfo component
matches.
If the rule contains a host not equal to "x", the host in the URI of
the rule must match the host of the URI in the contact entry.
Matching is based on case insensitive string comparison. If the rule
has a host equal to "x", it matches any value of the host in the URI
in the contact entry.
If the URI in the rule contains URI parameters (port is considered a
URI parameter for purposes of this discussion), each parameter in the
URI in the rule must match a parameter in the URI in the contact
entry. Matching is based on case sensitive string comparison of both
parameter names and values. Note, however, if the URI in the rule
contains a parameter with a default value, this matches a contact
entry with a URI that does not contain this parameter. If the URI in
the rule contains a URI parameter that is not the default value, this
does not match a contact entry whose URI does not contain this
parameter. If there are no URI parameters in the rule, this is
considered a match to any set of URI parameters in the contact entry.
To determine if the parameters in the rule match the parameters in
the contact entry, the following process is followed.
The parameters match if and only if each parameter in the rule
matches the contact entry. A single parameter in the rule matches the
contact entry if that parameter is present in the contact entry, and
their values match. If a parameter exists in the rule, and there is
no parameter of the same name in the contact entry, whether or not
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this is a match is context dependent. If the rule is present in the
Accept-Contact header, it is considered a match. If the rule is
present in the Reject-Contact header, it is not considered a match.
Parameter names are matched by case-sensitive comparison. Parameter
values are matched by set-comparisons. Parameter values in quoted
strings are interpreted as sets, with elements separated by commas.
Two elements in the set match if they are equal based on a case
sensitive string comparison. 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 of the rule set with the contact set is non-empty. Note
that this process does not apply to the priority-param, methods-
param, description-param or scheme-param.
Case sensitive comparisons are necessary because of
internationalization. Case insensitive matching in UTF-8
depends on regional rules, and overly complicates the
procedure.
If there is a scheme-param in the rule, and the quoted-string
parameter value in the rule starts with an exclamation mark, the
scheme of the URI in the contact entry must not match any of the
schemes listed in the rule. If the quoted-string parameter value in
the scheme-param doesn't start with an exclamation mark, the scheme
of the URI in the contact entry must match one of the schemes listed
in the rule. Matching of schemes is done by case insensitive string
comparison [8].
The C code below describes the matching procedure between a rule and
a contact entry. The function MATCH performs the desired operation.
It takes, as an argument, the rule to match, and a contact entry to
match it against (note that the contact entry is of the same type,
rule_t, as the rule, even though it is not a rule), and a context,
which indicates whether the matching is for Accept-Contact or
Reject-Contact. The function intersect() takes two arrays of strings,
and returns true if there are any values common to both arrays, false
otherwise. The function getparameterbyname() takes a rule and a
string defining a parameter name. It returns a parameter from the
rule with that name. However, if the parameter name is "scheme", the
function returns a "scheme" parameter. This parameter structure has
the name set to "scheme", the exclamation set to false, and the
values array with a single value, containing the name of the scheme
in the URI of the rule.
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/* context values */
#define ACCEPT_CONTACT 0
#define REJECT_CONTACT 1
typedef int boolean;
typedef struct uri_parameters_s {
char *name;
char *value;
} uri_parameters_t;
typedef struct uri_s {
char *scheme;
char *userinfo;
char *host;
uri_parameters_t **params;
} uri_t;
typedef struct parameter_s {
char *name; /* parameter name */
boolean exclamation; /* whether ! was present in value */
char **values; /* list of elements in the value */
} parameter_t;
typedef struct rule_s {
uri_t *URI; /* URI */
parameter_t **para; /* list of parameters */
} rule_t;
/* little helper function to look up a parameter by its
name within an entry */
parameter_t *getparameterbyname(rule_t *r, char *name) {
int i;
parameter_t *p;
if(strcmp(name,"scheme") == 0) {
p = calloc(1, sizeof(parameter_t));
p->name = "scheme";
p->values = calloc(2, sizeof(char *));
p->values[0] = malloc(sizeof(char) * (strlen(r->URI->scheme) + 1));
strcpy(p->values[0], r->URI->scheme);
return(p);
}
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for(i=0; r->para[i] != NULL; i++) {
if(strcmp(r->para[i]->name, name) == 0)
return(r->para[i]);
}
return(NULL);
}
/* check if two sets of strings share at least one
common value */
boolean intersect(char *a[], char *b[]) {
int i,j;
for(i = 0; a[i] != NULL; i++) {
for(j = 0; b[j] != NULL; j++) {
if(strcmp(a[i], b[j]) == 0)
return(TRUE);
}
}
return(FALSE);
}
/* returns the default value of a URI parameter */
char *defaultvalue(char *name) {
if(strcmp(name, "transport") == 0)
return("udp");
return("some-value-which-matches-no parameter");
}
boolean matchuriparameters(uri_parameters_t **r, uri_parameters_t **e) {
int i,j;
boolean match;
/* for each rule */
for(i=0; r[i] != NULL; i++) {
match = FALSE;
for(j=0; e[j] != NULL; j++) {
/* found the matching URI parameter in the entry */
if(strcmp(r[i]->name, e[j]->name) == 0) {
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/* if they're not equal, return FALSE. Otherwise, set match
to TRUE, indicating that we found a matching parameter*/
if(strcmp(r[i]->value, e[j]->value) != 0)
return(FALSE);
else
match = TRUE;
}
}
/* parameter in rule not in entry */
if(match == FALSE) {
/* check if rule contains default value */
if(strcmp(defaultvalue(r[i]->name), r[i]->value) != 0)
return(FALSE);
}
}
return(TRUE);
}
boolean MATCH(rule_t *r, rule_t *e, int context) {
boolean match;
int i;
parameter_t *p, *q;
match = TRUE;
/* We represent a rule with a * as the match for URIs, as
a URI with a scheme of * */
if (strcmp(r->URI->scheme, "*") != 0) {
/* the schemes must match */
if (strcasecmp(r->URI->scheme, e->URI->scheme) == 0) {
/* for sip, perform our SIP rules */
if (strcasecmp(r->URI->scheme, "sip") == 0) {
/* check for match of user and host */
match=(((strcasecmp(r->URI->host, "x") == 0) ||
(strcasecmp(r->URI->host, e->URI->host) == 0)) &&
((r->URI->userinfo == NULL) ||
((e->URI->userinfo != NULL) &&
(strcmp(r->URI->userinfo, e->URI->userinfo) == 0))));
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if(match == FALSE) return(FALSE);
/* match URI parameters */
match = matchuriparameters(r->URI->params, e->URI->params);
if(match == FALSE) return(FALSE);
} else {
/* match = scheme-appropriate comparison; */
if(match == FALSE) return(FALSE);
}
} else {
/* schemes don't match */
return FALSE;
}
}
/* compare parameters */
for(i = 0; r->para[i] != NULL; i++) {
p = r->para[i];
/* is this parameter defined in the contact entry */
if ((q =getparameterbyname(e, p->name)) != NULL) {
/* is this an empty set match */
if (p->exclamation == TRUE) {
if (intersect(p->values, q->values) == TRUE) {
return FALSE;
}
} else {
/* not an empty set case */
if (intersect(p->values, q->values) == FALSE) {
return FALSE;
}
}
} else {
/* this parameter is not present in the entry.
whether its a match or not is dependent on
context */
if(context == REJECT_CONTACT)
return(FALSE);
}
}
return TRUE;
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}
For example, the rule:
sip:example.com;language="!en,de"
matches the contact entry:
sip:joe@example.com;language="es,nl"
but not any of:
sip:joe@example.com;language="en"
sip:bob@example.com;language="de,en"
sip:alice@example.com;language="en,es,fi"
As another example, the rule
*;duplex="full,half"
matches the contact entry
sip:user@host;duplex="full"
but not
sip:user@host;duplex="send-only"
The rule
*;scheme="http"
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matches the contact entry
http://www.example.com
A server need 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.
6.4.2 Contact List Processing
Given the matching rule above, the formal processing rules at the
server proceed as follows. The server begins with a contact list for
the user in the request URI, and a set of rules in the Accept-Contact
and Reject-Contact headers.
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 from the contact list whose
priority value is higher than that of the request.
A contact entry may contain a methods parameter. This means that a
request should not be proxied or redirected to that contact entry
unless the method of the request is listed among those in the methods
parameter of the contact entry. The server removes any contact entry
from the contact list whose list of methods in the methods parameter
does not include the method of the SIP request. Note that this
operation is performed only when there is a methods parameter in the
contact entry. For example, if the Contact list is:
sip:pager@service;methods="MESSAGE"
sip:user@host
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and a request arrives at the proxy with method INVITE, the first
contact entry (pager@service) is removed. The second one remains,
since it has no methods parameter.
Each rule in the Accept-Contact header field is then processed. Note
that any "only=true" parameters are ignored by proxies and redirect
servers.
Since a proxy or redirect server cannot determine if a
contact entry is a UA or another proxy, it cannot guarantee
that a contact won't lead to a UA matching one of the
rules.
If the rule matches a contact entry (according to the matching rule
in section 6.4.1, the q value of that entry is updated (for this
transaction only), 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 specific algorithm for updating the q
value. 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). The only requirement for the
updating process is that if a contact entry has a q value of q1, and
the q values among the matching rules are q2,q3,..qn, the merged q
value, qm, must satisfy:
MIN(q1,q2,q3,..qn) <= qm <= MAX(q1,q2,q3,..,qn)
For those contact entries which did not match any rule in the
Accept-Contact header, their final q value is set to zero.
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
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the q value. Processing from this point depends on the configuration
and policy of the server. If the server elects to do a sequential
proxy, it SHOULD try the highest q value contact entry first, trying
addresses with decreasing q values as each attempt fails. If the
server elects to do a forking proxy, it SHOULD group contact entries
with "close" q values together, and try the group with the highest q
value first, then the group with the next lowest q values, and so on.
The precise method of the grouping is left to the implementor. A
reasonable choice is to round each q value to the nearest tenth, and
group those with the same rounded value.
If a proxy server is recursing, it SHOULD apply the caller
preferences to the Contact headers returned in the redirect
responses. Any contact entries remaining after the application of
caller preferences should be added to the list of untried addresses.
This list is then resorted based on q values. The server uses this
list for subsequent proxy operations.
If the server is redirecting, it SHOULD return all entries in the
contact list, including those with a zero q value.
If the server is executing any other type of policy, as a general
guideline, it SHOULD prefer contact entries with higher q values than
those with lower q values.
6.4.3 Request-Disposition Processing
If the request contains a Request-Disposition header, the server
SHOULD execute the behaviors described by the tokens, unless it has
local policy configured to direct it otherwise.
7 Interactions with CPL
When the called party has a Call Processing Language (CPL) [9] script
present, feature interactions are introduced. CPL addresses this by
allowing the CPL script to control whether caller preferences are
applied to the location list or not. CPL also allows the called party
to discard certain rules from the caller preferences before their
application. For more information, see [9].
8 IANA Registration Procedures
8.1 cp-params
New cp-params 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:
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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
UTF8-TOKEN. For each value, a description of the value must
be provided. The registration MUST indicate whether the
parameter values are mutually exclusive or not; that is,
whether only one, or more than one, can appear in the
Contact header.
9 Changes since -02
o Removed IANA registration for request-disposition tokens
o Generalized text so that caller preferences processing is
method independent
o Added requirement for UAs to process the Accept-Contact
headers.
o Added "only" parameter to Accept-Contact
o Indicated that Route headers override caller preferences
handling in proxies.
o Added compact forms.
10 Security Considerations
The presence of caller preferences in a request has a significant
effect on the ways in which the request is handled at a server. As a
result, is is especially important that requests with caller
preferences be authenticated. The same holds true for registrations
with contact parameters.
Processing of caller preferences requires set operations and searches
which can require some amount of computation. This enables a DOS
attack whereby a user can send requests with substantial numbers of
caller preferences, in the hopes of overloading the server. To
counter this, servers SHOULD reject requests with too many rules. A
reasonable number is around 20.
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11 Acknowledgements
Parameters of the terminal negotiation mechanism in Section 5.1 were
influenced by Scott Petrack's CMA design. Jonathan Lennox and John
Hearty provided helpful comments.
12 Author's Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
Henning Schulzrinne
Columbia University
M/S 0401
1214 Amsterdam Ave.
New York, NY 10027-7003
email: schulzrinne@cs.columbia.edu
13 Bibliography
[1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments 2543, Internet
Engineering Task Force, Mar. 1999.
[2] J. Lennox and H. Schulzrinne, "Call processing language framework
and requirements," Request for Comments 2824, Internet Engineering
Task Force, May 2000.
[3] G. Klyne, "A syntax for describing media feature sets," Request
for Comments 2533, Internet Engineering Task Force, Mar. 1999.
[4] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments 2119, Internet Engineering Task Force,
Mar. 1997.
[5] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
Leach, and T. Berners-Lee, "Hypertext transfer protocol -- HTTP/1.1,"
Request for Comments 2616, Internet Engineering Task Force, June
1999.
Schulzrinne/Rosenberg [Page 26]
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[6] M. Handley and V. Jacobson, "SDP: session description protocol,"
Request for Comments 2327, Internet Engineering Task Force, Apr.
1998.
[7] J. Rosenberg et al. , "SIP extensions for instant messaging,"
Internet Draft, Internet Engineering Task Force, June 2000. Work in
progress.
[8] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
identifiers (URI): generic syntax," Request for Comments 2396,
Internet Engineering Task Force, Aug. 1998.
[9] J. Lennox and H. Schulzrinne, "CPL: a language for user control
of internet telephony services," Internet Draft, Internet Engineering
Task Force, July 2000. Work in progress.
Table of Contents
1 Introduction ........................................ 1
2 Overview of Operation ............................... 2
3 Design Alternatives ................................. 3
4 Terminology ......................................... 4
5 Header Field Definitions ............................ 5
5.1 Contact, Accept-Contact and Reject-Contact
Parameters ..................................................... 5
5.2 Accept-Contact ...................................... 8
5.3 Reject-Contact ...................................... 9
5.4 Contact Header ...................................... 10
5.5 Request-Disposition ................................. 11
6 Protocol Semantics .................................. 13
6.1 UA Behavior for Registering ......................... 13
6.2 UAS Behavior ........................................ 13
6.3 UAC Behavior ........................................ 14
6.4 Proxy Behavior ...................................... 14
6.4.1 Rule Matching Procedures ............................ 14
6.4.2 Contact List Processing ............................. 22
6.4.3 Request-Disposition Processing ...................... 24
7 Interactions with CPL ............................... 24
8 IANA Registration Procedures ........................ 24
8.1 cp-params ........................................... 24
9 Changes since -02 ................................... 25
10 Security Considerations ............................. 25
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11 Acknowledgements .................................... 26
12 Author's Addresses .................................. 26
13 Bibliography ........................................ 26
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