One document matched: draft-ietf-sip-callerprefs-01.txt
Differences from draft-ietf-sip-callerprefs-00.txt
Internet Engineering Task Force SIP WG
Internet Draft Schulzrinne/Rosenberg
draft-ietf-sip-callerprefs-01.txt Columbia U./dynamicsoft
March 9, 2000
Expires: September 2000
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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http://www.ietf.org/ietf/1id-abstracts.txt
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http://www.ietf.org/shadow.html.
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 call
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. These
extra parameters are present in the Contact header in REGISTER
requests, and are used to associated attributes with particular
addresses.
1 Introduction
When a SIP [1] server receives a request, there are a number of
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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;
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
callee, and (3) the caller. The directives of the administrator are
embedded in the policy of the server. The preferences of the callee
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 caller. This extension fills that
gap by specifying mechanisms by which a caller can provide
preferences on processing of a call. These preferences include the
ability to select which URIs a call 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. These extra parameters are present in the
Contact header in REGISTER requests, and are used to associated
attributes with particular addresses.
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 address 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.
When a caller sends an INVITE, it can optionally include new headers
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which request certain handling at a proxy. 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 both the Accept-
Contact and Reject-Contact headers. These preferences contain rules
that describe the set of desired URIs that the caller would like the
server to proxy or redirect to. These rules are matched against the
Contact headers sent in a registration (or through some other
configuration means). 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.
Note that this second category of preferences can only be applied at
a proxy which accesses a registration database.
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
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
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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 [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.
5 Header Field Definitions
Table 5 specifies an extension of Table 5 in RFC 2543 [1] for the
three new headers defined here.
Since all three headers specify call routing logic, they can apply to
any request which can normally be proxied or redirected.
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
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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 -
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.
single URI. When used in a Contact header, they specify
characteristics of that URI. When used in the Accept-Contact or
Reject-Contact headers, they specify rules to apply for matching
URIs.
cp-params = class-param | duplex-param |
features-param | language-param | media-param |
mobility-param | priority-param | scheme-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 <">
scheme-param = "scheme" "=" <"> [<!>] 1#scheme <">
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
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
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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. The scheme-param
parameter MUST NOT be present if cp-params is 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
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.
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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.
scheme: The scheme parameter describes the set of URI schemes
which the caller is willing to accept redirects to or
communicate with. The BNF for scheme is given in RFC 2396
[7], and can be any valid URI scheme. The scheme parameter
MUST NOT appear as a Contact parameter. It is only
permitted as an Accept-Contact or Reject-Contact parameter.
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 UAC 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 in Section 6.3.1.
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 6.3.1. Its
application is described in the procedure in Section 6.3.2.
priority-param = "priority" "=" <"> priority-value <">
description-param = "description" "=" quoted-string
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
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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" ":" 1# rule
rule = ( name-addr | addr-spec | "*")
[ *( ";" (cp-params | q-param) ) ]
q-param = "q" "=" qvalue
The header field specifies contact addresses which are acceptable to
the caller. 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 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.
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
In the next example, the caller would prefer to speak to someone from
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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:
Reject-Contact = "Reject-Contact" ":"
1# ( ( name-addr | addr-spec | "*")
[ *( ";" cp-params ) ] )
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 6.3.1. 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.
5.4 Contact Header
The cp-params parameter is allowed as an extension attribute to the
Contact header, along with the priority-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 | 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.
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Contact: Carol <sip:carol@example.com> ;language="en,es,de"
;media="audio,video,application/chat"
;duplex="full"
;priority="urgent"
5.5 Request-Disposition
The Request-Disposition header field specifies caller preferences for
how a 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"
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 (generally the one with the
highest q value). The feature is ignored if "redirect" has
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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.
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 cause calls to
be accepted without alerting the called party.
Example:
Request-Disposition: proxy, recurse, parallel
6 Protocol Semantics
6.1 UAS Behavior
User agent servers MAY include cp-params, priority-param or
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description-param parameters as part of each Contact addresses 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.
When a UAS receives a request with the Accept-Contact, Reject-Contact
and Request-Disposition, it MAY ignore these headers so long as it
does not redirect the request. If the request is redirected, the UAS
SHOULD follow the rules described in Section 6.3 for a proxy/redirect
server.
6.2 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.
6.3 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.
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.
6.3.1 Rule Matching Procedures
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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
rule, like a contact entry, consists of a URI (or the "*" character),
and a set of parameters. To determine if a rule matches a contact
entry, the following steps are taken:
o If the rule does not contain a URI, only parameters are
compared. If the rule contains a URI, both the URI and
parameters must match.
o If the URIs in the rule and contact entry are not SIP URLs,
matching is based on the URL matching rules for the particular
scheme. If they are both SIP URLs, the following procedure is
followed to determine a match:
- 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. Note that if the rule contains a userinfo field,
but the URI in the contact address does not, the rule does
not match the contact entry. If the rule does not contain a
userinfo field, no comparison is made on this portion of the
URI.
- If the rule contains a domain not equal to "x", the domain
must match the domain the URI in the contact entry. Matching
is based on case insensitive string comparison. If the rule
has a domain equal to "x", no comparison is made on this
portion of the URI.
- If the rule contains URI parameters (port is considered a
URI parameter for purposes of this discussion), each
parameter 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 rule contains a parameter with a default
value, this matches a contact entry which does not contain
this parameter. Similarly, a rule without a parameter
matches a contact entry which contains a default value for
that parameter.
o The parameters in the rule must match the parameters in the
contact entry. Parameter names are matched by case-sensitive
comparison. Parameter values are compared by set-comparisons.
Parameter values in quoted strings are interpreted as sets,
with elements separated by commas. Two elements 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
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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, 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.
o If there is a scheme-param in the rule, and the rule starts
with an exclamation mark, the scheme in the URI in the contact
entry must not match any of the schemes listed in the rule. If
the rule in the scheme-param doesn't start with an exclamation
mark, the scheme in 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 [7].
The pseudo-code below describes the matching procedure between a rule
and a contact entry. 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.
struct uri_t {
userinfo_t userinfo;
host_t host;
parameters_t params;
}
typedef struct {
char *name; /* parameter name */
boolean exclamation /* whether ! was present in value */
char *values[]; /* list of elements in the value */
} parameter_t;
struct {
uri_t URI; /* URI */
parameter_t para[]; /* list of parameters */
} rule, entry;
boolean MATCH(rule r, entry e) {
boolean match;
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parameter_t p, q;
match = TRUE;
if (r.URI != "*") {
if (r.URI.scheme == e.URI.scheme) {
if (r.URI.scheme == "sip") {
match=(r.URI.host == "x" || (strcmp(r.URI.host,e.URI.host) == 0))&&
(r.URI.user == "" || (strcmp(r.URI.user, e.URI.user) == 0))
} else {
match = scheme-appropriate comparison;
}
} else {
return FALSE;
}
}
if(match == FALSE) 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;
}
}
}
}
return TRUE;
}
For example, the rule:
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sip:dynamicsoft.com;language="!en,de"
matches the contact entry:
sip:jdrosen@dynamicsoft.com;language="es,nl"
but not any of:
sip:jdrosen@dynamicsoft.com;language="en"
sip:phuang@dynamicsoft.com;language="de,en"
sip:islepchin@dynamicsoft.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:jtoto@dynamicsoft.com;duplex="send-only"
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.
6.3.2 Contact List Processing
Given the matching rule above, the formal processing rules at the
proxy proceeds as follows. The server begins with a contact list for
the callee, 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
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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 from the contact list
whose priority value is 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 specific 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). 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
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
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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 the 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 proxy is redirecting, it SHOULD return all entries in the
contact list, including those with a zero q value.
If the proxy 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.
6.3.3 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) [8] 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 [8].
8 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
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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.
9 Changes since -01
o Discussion of procedures for redirects
o Addition of scheme-parameter for redirects
o Changed option tag in Require header to "pref"
o Added extension capabilities to BNF for cp-params and
internationalized them
o Consistent use of terms "parameter", "tag", and "value"
o Changed case matching rules from insensitive to sensitive in
order to enable internationalization
o Aligned URI matching rules with rfc2543bis
o Clarified that media parameter refers to acceptable media
types for the RTP sessions, not to the media parameters
acceptable in SIP bodies.
o Removed the service parameter. It was not clear how this could
really be used. It contained values like ISDN and PSTN;
however, SIP is an IP protocol. The notion it was trying to
convey - of connecting to fax machines or phones through
gateways, can be supported more easily through the scheme
parameter and media type parameters.
o Updated discussion of CPL
o Updated code with more detail
o Reorganized for clarity
10 Security Considerations
The presence of caller preferences in a request has a significant way
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
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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.
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
200 Executive Drive
Suite 120
West Orange, NJ 07052
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 (Proposed
Standard) 2543, Internet Engineering Task Force, Mar. 1999.
[2] J. Lennox and H. Schulzrinne, "Call processing language framework
and requirements," Internet Draft, Internet Engineering Task Force,
July 1999. Work in progress.
[3] G. Klyne, "A syntax for describing media feature sets," Request
for Comments (Proposed Standard) 2533, Internet Engineering Task
Force, Mar. 1999.
Schulzrinne/Rosenberg [Page 20]
Internet Draft SIP Caller Prefs March 9, 2000
[4] 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.
[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 (Draft Standard) 2616, Internet Engineering Task
Force, June 1999.
[6] M. Handley and V. Jacobson, "SDP: session description protocol,"
Request for Comments (Proposed Standard) 2327, Internet Engineering
Task Force, Apr. 1998.
[7] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform resource
identifiers (URI): generic syntax," Request for Comments (Draft
Standard) 2396, Internet Engineering Task Force, Aug. 1998.
[8] J. Lennox and H. Schulzrinne, "CPL: a language for user control
of internet telephony services," Internet Draft, Internet Engineering
Task Force, Mar. 1999. Work in progress.
Full Copyright Statement
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and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Schulzrinne/Rosenberg [Page 21]
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Table of Contents
1 Introduction ........................................ 1
2 Overview of Operation ............................... 2
3 Design Alternatives ................................. 3
4 Terminology ......................................... 4
5 Header Field Definitions ............................ 4
5.1 Contact, Accept-Contact and Reject-Contact
Parameters ..................................................... 4
5.2 Accept-Contact ...................................... 8
5.3 Reject-Contact ...................................... 9
5.4 Contact Header ...................................... 9
5.5 Request-Disposition ................................. 10
6 Protocol Semantics .................................. 11
6.1 UAS Behavior ........................................ 11
6.2 UAC Behavior ........................................ 12
6.3 Proxy Behavior ...................................... 12
6.3.1 Rule Matching Procedures ............................ 12
6.3.2 Contact List Processing ............................. 16
6.3.3 Request-Disposition Processing ...................... 18
7 Interactions with CPL ............................... 18
8 IANA Registration ................................... 18
9 Changes since -01 ................................... 19
10 Security Considerations ............................. 19
11 Acknowledgements .................................... 20
12 Author's Addresses .................................. 20
13 Bibliography ........................................ 20
Schulzrinne/Rosenberg [Page 22]
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