Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo (Editor)
Ericsson
W. Marshall (Editor)
AT&T
Jonathan Rosenberg
dynamicsoft
draft-ietf-sip-manyfolks-resource-04.txt
February 25, 2002
Expires: August, 2002
Integration of Resource Management and SIP
STATUS OF THIS MEMO
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all provisions of Section 10 of RFC2026.
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Abstract
This document discusses how network quality of service can be made a
precondition to establishment of sessions initiated by the Session
Initiation Protocol (SIP). These preconditions require that the
participant reserve network resources before continuing with the
session. We do not define new quality of service reservation
mechanisms; these preconditions simply require a participant to use
existing resource reservation mechanisms before beginning the
session.
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1 Introduction
Some architectures require that at session establishment time, once
the callee has been alerted, the chances of a session establishment
failure are minimum. One source of failure is the inability to
reserve network resources for a session. In order to minimize "ghost
rings", it is necessary to reserve network resources for the session
before the callee is alerted. However, the reservation of network
resources frequently requires learning the IP address, port, and
session parameters from the callee. This information is obtained as a
result of the initial offer/answer exchange carried in SIP. This
exchange normally causes the "phone to ring", thus introducing a
chicken-and-egg problem: resources cannot be reserved without
performing an initial offer/answer exchange, and the initial
offer/answer exchange can't be done without performing resource
reservation.
The solution is to introduce the concept of a precondition. A
precondition is a set of constraints about the session which are
introduced in the offer. The recipient of the offer generates an
answer, but does not alert the user or otherwise proceed with session
establishment. That only occurs when the preconditions are met. This
can be known through a local event (such as a confirmation of a
resource reservation), or through a new offer sent by the caller.
This document deals with sessions that use SIP [1] as signalling
protocol and SDP [2] to describe the parameters of the session.
We have chosen to include the quality of service preconditions in the
SDP description rather than in the SIP header because preconditions
are stream specific.
2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
3 Overview
In order to ensure that session establishment does not take place
until certain preconditions are met we distinguish between two
different state variables that affect a particular media stream:
current status and desired status. This document defines quality of
service status.
The desired status consists of a threshold for the current status.
Session establishment stops until the current status reaches or
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surpasses this threshold. Once this threshold is reached or
surpassed, session establishment resumes.
For example, the following values for current and desired status
would not allow session establishment to resume:
current status = resources reserved in the send direction
desired status = resources reserved in both (sendrecv) directions
On the other hand, the values of the example below would make session
establishment resume:
current status = resources reserved in both (sendrecv) directions
desired status = resources reserved in the send direction
These two state variables define a certain piece of state of a media
stream the same way as the direction attribute or the codecs in use,
define other pieces of state. Consequently, we treat these two new
variables in the same way as other SDP media attributes are treated
in the offer/answer model used by SIP [4]: they are exchanged between
two user agents using an offer and an answer in order to have a
shared view of the status of the session.
Figure 1 shows a typical message exchange between two SIP user agents
using preconditions. A includes quality of service preconditions in
the SDP of the initial INVITE. A does not want B to be alerted until
there is network resources reserved in both directions (sendrecv)
end-to-end. B agrees to reserve network resources for this session
before alerting the callee. B will handle resource reservation in the
B->A direction, but wants A to handle the A->B direction. To indicate
so, B returns a 183 response to A asking A to start resource
reservation and to warn B as soon as the A->B direction is ready for
the session. A and B both start resource reservation. B finishes
reserving resources in the B->A direction, but does not alert the
user yet, because network resources in both directions are needed.
When A finishes reserving resources in the A->B direction, it sends
an UPDATE to B. B returns a 200 (OK) response for the UPDATE
indicating that all the preconditions for the session have been met.
At this point of time, B starts alerting the user, and session
establishment completes normally.
4 SDP parameters
We define the following media level SDP attributes:
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current-status = "a=curr:" precondition-type
= SP status-type SP direction-tag
desired-status = "a=des:" precondition-type
= SP strength-tag SP status-type
= SP direction-tag
confirm-status = "a=conf:" precondition-type
= SP status-type SP direction-tag
precondition-type = "qos"
strength-tag = ("mandatory" | "optional" | "none"
= | "failure")
status-type = ("e2e" | "local" | "remote")
direction-tag = ("none" | "send" | "recv" | "sendrecv")
Current status: The current status attribute carries the current
status of network resources for a particular media stream.
Desired status: The desired status attribute carries the
preconditions for a particular media stream. When the
current status value has the same or a better value than
the desired status value, the preconditions are considered
to be met.
Confirmation status: The desired status attribute carries
threshold conditions for a media stream. When the status of
network resources reach these conditions, the peer user
agent will send an update of the session description
containing an updated current status attribute for this
particular media stream.
Precondition type: This document defines quality of service
preconditions. Extensions may define other types of
preconditions.
Strength tag: The strength tag indicates whether or not the
callee can be alerted in case the network fails to meet the
preconditions.
Status type: We define two types of status: end-to-end and
segmented. The end-to-end status reflects the status of the
end-to-end reservation of resources. The segmented status
reflects the status of the access network reservations of
both user agents. The end-to-end status corresponds to the
tag "e2e" defined above and the segmented status to the
tags "local" and "remote". End-to-end status is useful when
end-to-end resource reservation mechanisms. The segmented
status is useful when one or both UAs perform resource
reservations on their respective access networks.
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A B
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------(2) 183 Session Progress SDP 2-------|
| *** *** |
|--*R*-----------(3) PRACK-------------*R*-->|
| *E* *E* |
|<-*S*-------(4) 200 OK (PRACK)--------*S*---|
| *E* *E* |
| *R* *R* |
| *V* *V* |
| *A* *A* |
| *T* *T* |
| *I* *I* |
| *O* *O* |
| *N* *N* |
| *** *** |
| *** |
| *** |
|-------------(5) UPDATE SDP3--------------->|
| |
|<--------(6) 200 OK (UPDATE) SDP4-----------|
| |
|<-------------(7) 180 Ringing---------------|
| |
|-----------------(8) PRACK----------------->|
| |
|<------------(9) 200 OK (PRACK)-------------|
| |
| |
| |
|<-----------(10) 200 OK (INVITE)------------|
| |
|------------------(11) ACK----------------->|
| |
| |
Figure 1: Basic session establishment using preconditions
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Direction tag: The direction tag indicates the direction a
concrete attribute is applicable to.
The values of the tags "send", "recv", "local" and "remote" represent
the point of view of the entity generating the SDP description. In an
offer, "send" is the direction offerer->answerer and "local" is the
offerer's access network. In an answer, "send" is the direction
answerer->offerer and "local" is the answerer's access network.
The following example shows these new SDP attributes in two media
lines of a session description:
m=audio 20000 RTP/AVP 0
a=curr:qos e2e send
a=des:qos optional e2e send
a=des:qos mandatory e2e recv
m=audio 20002 RTP/AVP 0
a=curr:qos local sendrecv
a=curr:qos remote none
a=des:qos optional local sendrecv
a=des:qos mandatory remote sendrecv
5 Usage of the new SDP status parameters in the SIP offer/answer model
Parameter negotiation in SIP is carried out using the offer answer
model described in [4]. The idea behind this model is to provide a
shared view of the session parameters for both user agents once the
answer has been received by the offerer. This section describes which
values can our new SDP attributes take in an answer depending on
their value in the offer.
To achieve a shared view of the status of a media stream, we define a
model that consists of three tables: both user agents implement a
local status table, and each offer/answer exchange has a transaction
status table associated to it. The offerer generates a transaction
status table identical to its local status table and sends it to the
answerer in the offer. The anwerer uses the information of this
transaction status table to update its local status table. The
answerer also updates the transaction status table fields that were
out of date and returns this table to the offerer in the answer. The
offerer can then update its local status table with the information
received in the answer. After this offer/answer exchange, the local
status tables of both user agents are synchronised. They now have a
common view of the status of the media stream. Sessions that involve
several media streams implement these tables per media stream. Note,
however, that this is a model of user agent behavior, not of
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software. An implementation is free to take any approach that
replicates the external behavior this model defines.
5.1 Generating an offer
Both user agents MUST implement a table, referred to as "local status
table", reflecting the status of the resource reservation. Tables 1
and 2 show the format of this tables for both the end-to-end and the
segmented status types. For the end-to-end status type, the table
contains two rows; one for each direction (i.e., send and recv). A
value of "yes" in the "Current" field indicates that resource has
been successfully reserved in the corresponding direction. "No"
indicates that resources have not been reserved yet. The "Desired
Strength" field indicates the strength of the preconditions in the
corresponding direction. The table for the segmented status type
contains four rows: both directions in the local access network and
in the peer's access network. The meaning of the fields is the same
as in the end-to-end case.
Before generating an offer, the offerer MUST build a transaction
status table with the current and the desired status for each media
stream. The different values of the strength tag for the desired
status attribute have the following semantics:
o None: no resource reservation is needed.
o Optional: the user agents SHOULD try to provide resource
reservation, but the session can continue regardless of
whether this provision is possible or not.
o Mandatory: the user agents MUST provide resource reservation.
Otherwise, session establishment MUST NOT continue.
The offerer then decides whether it is going to use the end-to-end
status type or the segmented status type. If the status type of the
media line will be end-to-end, the user agent generates records with
the desired status and the current status for each direction (send
and recv) independently, as shown in table 1:
Direction Current Desired Strength
____________________________________
send no mandatory
recv no mandatory
Table 1: Table for the end-to-end status type
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If the status type of the media line will be segmented, the user
agent generates records with the desired status and the current
status for each direction (send and recv) and each segment (local and
remote) independently, as shown in table 2:
Direction Current Desired Strength
______________________________________
local send no none
local recv no none
remote send no optional
remote recv no none
Table 2: Table for the segmented status type
At the time of sending the offer, the offerer's local status table
and the transaction status table contain the same values.
With the transaction status table, the user agent generates the
current-status and the desired status lines following the syntax of
Section 4 and the rules described below in Section 5.1.1.
5.1.1 SDP encoding
For the end-to-end status type, the user agent MUST generate one
current status line with the tag "e2e" for the media stream. If the
strength tags for both directions are equal (e.g., both mandatory) in
the transaction status table, the user agent MUST add one desired
status line with the tag "sendrecv". If both tags are different, the
user agent MUST include two desired status lines, one with the tag
"send" and the other with the tag "recv".
The semantics of two lines with the same strength tag, one
with a "send" tag and the other with a "recv" tag, is the
same as one "sendrecv" line. However, in order to achieve a
more compact encoding, we have chosen to make mandatory the
latter format.
For the segmented status type, the user agent MUST generate two
current status lines: one with the tag "local" and the other with the
tag "remote". The user agent MUST add one or two desired status lines
per segment (i.e., local and remote). If for a particular segment
(local or remote) the tags for both directions in the transaction
status table are equal (e.g., both mandatory), the user agent MUST
add one desired status line with the tag "sendrecv". If both tags are
different, the user agent MUST include two desired status lines, one
with the tag "send" and the other with the tag "recv".
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Note that the rules above apply to the desired strength tag "none" as
well. This way, a user agent that supports quality of service but
does not intend to use them, adds desired status lines with the
strength tag "none". Since this tag can be upgraded in the answer, as
described in Section 5.2, the answerer can request quality of service
reservation without a need of another offer/answer exchange.
The example below shows the SDP corresponding to tables 1 and 2.
m=audio 20000 RTP/AVP 0
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
m=audio 20002 RTP/AVP 0
a=curr:qos local none
a=curr:qos remote none
a=des:qos optional remote send
a=des:qos optional local none
5.2 Generating an Answer
When the answerer receives the offer, it recreates the transaction
status table using the SDP attributes contained in the offer. The
answerer updates both its local status and the transaction status
table following the rules below:
Desired Strength: We define an absolute ordering for the
strength tags: none, optional and mandatory. Mandatory is
the tag with highest grade and none the tag with lowest
grade. An answerer MAY upgrade the desired strength in any
entry of the transaction status table, but it MUST NOT
downgrade it. Therefore, it is OK to upgrade a row from
none to optional, from none to mandatory or from optional
to mandatory, but not the other way around.
Current Status: For every row, the value of the "Current" field
in the transaction status table and in the local status
table of the answerer have to be compared. Table 3 shows
the four possible combinations. If both fields have the
same value (two first rows of table 3, nothing needs to be
updated. If the "Current" field of the transaction status
table is "Yes" and the field of the local status table is
"No" (third row of table 3), the latter MUST be set to
"Yes". If the "Current" field of the transaction status
table is "No" and the field of the local status table is
"Yes" (forth row of table 3), the answerer needs to check
if it has local information (e.g., a confirmation of a
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resource reservation has been received) about that
particular current status. If it does, the "Current" field
of the transaction status table is set to "Yes". If the
answerer does not have local information about that current
status, the "Current" field of the local status table MUST
be set to "No".
Transaction status table Local status table
____________________________________________
no no
yes yes
yes no
no yes
Table 3: Possible values for the "Current" fields
Once both tables have been updated an answer is generated following
the rules described in Section 5.1.1 and taking into account that
"send", "recv", "local" and "remote" tags have to be inverted in the
answer, as shown in table 4.
Offer Answer
______________
send recv
recv send
local remote
remote local
Table 4: Values of tags in offer and answers
At the time the answer is sent, the transaction status table and the
answerer's local status table contain the same values. Therefore,
this answer contains the shared view of the status of the media line
in the current-status attribute and the negotiated strength and
direction tags in the desired-status attribute.
If the resource reservation mechanism used requires participation of
both user agents, the answerer SHOULD start resource reservation
after having sent the answer and the offerer SHOULD start resource
reservation as soon as the answer is received. If participation of
the peer user agent is not needed (e.g., segmented status type), the
offerer MAY start resource reservation before sending the offer and
the answerer MAY start it before sending the answer.
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The status of the resource reservation of a media line can change
between two consecutive offer/answer exchanges. Therefore, both user
agents MUST keep their local status tables up to date using local
information through the duration of the session.
6 Suspending and Resuming Session Establishment
A user agent server that receives an offer with preconditions
SHOULDNOT alert the user until all the mandatory preconditions are
met; session establishment is suspended until that moment.
A user agent server may receive an INVITE request with no offer in
it. In this case, following normal SIP procedures, the user agent
server will provide an offer in a reliable response (1xx or 2xx). The
user agent client will send the answer in another SIP request. If the
offer and the answer contain preconditions, the user agent client
SHOULD NOT alert the user until all the mandatory preconditions in
the answer are met.
While session establishment is suspended, user agents SHOULD not send
any data over any media stream. In the case of RTP [5], neither RTP
nor RTCP packets are sent.
A user agent server knows that all the preconditions are met for a
media line when its local status table has a value of "yes" in all
the rows whose strength tag is "mandatory". When the preconditions of
all the media lines of the session are met, session establishment
SHOULD resume.
For an initial INVITE suspending and resuming session establishment
is very intuitive. The callee will not be alerted until all the
mandatory preconditions are met. However, offers containing
preconditions sent in the middle of an ongoing session need further
explanation. Both user agents SHOULD continue using the old session
parameters until all the mandatory preconditions are met. At that
moment, the user agents can begin using the new session parameters.
Section 10 contains an example of this situation.
7 Status Confirmation
The qos-confirm-status attribute MAY be used in both offers and
answers. This attribute represents a threshold for the resource
reservation. When this threshold is reached or surpassed, the user
agent MUST send an offer to the peer user agent reflecting the new
current status of the media line. If this threshold is crossed again
(e.g., the network stops providing resources for the media stream),
the user agent MUST send a new offer as well.
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If a peer has requested confirmation on a particular stream, an agent
MUST mark that stream with a flag in its local status table. When all
the rows with this flag have a value of "yes", the user agent MUST
send a new offer to the peer. This offer will contain the current
status of resource reservation in the qos-current-status attributes.
If later any of the rows with this flag transition to "No", a new
offer MUST be sent as well.
Confirmation attributes are not negotiated. The answerer uses the
value of the qos-confirm-status attribute in the offer and the
offerer uses the value of this attribute in the answer.
For example, if a user agent receives an SDP description with the
following attributes:
m=audio 20002 RTP/AVP 0
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=conf:qos remote sendrecv
It will send an offer as soon as it reserves resources in its access
network ("remote" tag in the received message) for both directions
(sendrecv).
8 Refusing an offer
We define a new SIP status code:
Server-Error = "580" ;Precondition Failure
When an answerer cannot or is not willing to meet the preconditions
in the offer it SHOULD reject the offer by returning a 580
(Precondition-Failure) response. This response SHOULD contain an SDP
description indicating which desired status triggered the failure.
The corresponding desired status line MUST a value of the strength
tag of "failure", as shown in the example below:
m=audio 20000 RTP/AVP 0
a=des:qos failure e2e send
SDP description indicating this type of failure MUST follow the
format for describing media capabilities defined in the SIP
offer/answer model [4].
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Using the 580 (Precondition Failure) status code to refuse an offer
is useful when the offer came in an INVITE or in an UPDATE request.
However, SIP does not provide a means to refuse offers that contained
in a response (1xx or 2xx) to an INVITE.
If a UAC generates an initial INVITE without an offer and receives an
offer in a 1xx or 2xx response which is not acceptable, it SHOULD
respond to this offer with a correctly formed answer and immediately
after that send a CANCEL or a BYE.
If the offer comes in a 1xx or 2xx response to a re-INVITE, A would
not have a way to reject it without terminating the session at the
same time. The same recommendation given in Section 14.2 of [1]
applies here:
"The UAS MUST ensure that the session description overlaps
with its previous session description in media formats,
transports, other parameters that require support from the
peer. This is to avoid the need for the peer to reject the
session description. If, however, it is unacceptable to A,
A SHOULD generate an answer with a valid session
description, and then send a BYE to terminate the session."
9 Option tag for Require and Supported header fields
We define the option tag "precondition" for use in the Require and
Supported header fields. An offerer MUST include this tag if the
offer contains one or more strength tags with the value "mandatory".
If all the strength tags in the description are "optional" or "none"
the offerer MUST include this tag either in a Supported header field
or in a Require header field. It is, however, RECOMMENDED, that the
Supported header field is used in this case. The lack of
preconditions in the answer would indicate that the answerer did not
support this extension.
The mapping of offers and answers to SIP requests and responses is
performed following the rules given in . Therefore, a user agent
including preconditions in the SDP MUST include both "100rel" [6] and
"update" tags in the Require header field.
10 Examples
The following examples cover both status types; end-to-end and
segmented.
10.1 End-to-end Status Type
The call flow of figure 2 shows a basic session establishment using
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the end-to-end status type. The SDP descriptions of this example are
shown below:
SDP1: B includes end-to-end quality of service preconditions in the
initial offer.
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
SDP2: Since B uses RSVP, it can know when resources in its "send"
direction are available, because it will receive RESV messages from
the network. However, it does not know the status of the reservations
in the other direction. B requests confirmation for resource
reservations in its "recv" direction to the peer user agent A in its
answer.
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
a=conf:qos e2e recv
After having sent the answer B starts reserving network resources for
the media stream. When A receives this answer (2) it starts
performing resource reservation as well. Both UAs use RSVP, so A
sends PATH messages towards B and B sends PATH messages towards A.
As time passes by, B receives RESV messages confirming the
reservation. However, B waits until resources in the other direction
as reserved as well since it did not receive any confirmation and the
preconditions still have not been met.
SDP3: When A receives RESV messages it sends an updated offer (5) to
B:
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos e2e send
a=des:qos mandatory e2e sendrecv
SDP4: B responds with an answer (6) which contains the current status
of the resource reservation (i.e., sendrecv):
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m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv
At this point of time, session establishment resumes and B returns a
180 (Ringing) response (7).
Note that now the media stream has been already established, and A
has received a 180 (Ringing) response. Since the direction of the
stream is "sendrecv", A will not generate local ringback, since it
assumes that it will receive early media over this stream.
However, if B wants A to generate local ringback, it can put the
media stream on hold in SDP4. In this case, B would put the media
stream off hold by sending an offer in the 200 OK for the INVITE (8).
The contents of the messages for this alternative flow is shown
below:
SDP4 (on hold):
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=recvonly
a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv
SDP5 in the 200 OK (10):
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=sendrecv
a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv
SDP6 in the ACK (11):
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=sendrecv
a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv
Let's assume that in the middle of the session A wishes to change the
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A B
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------(2) 183 Session Progress SDP 2-------|
| *** *** |
|--*R*-----------(3) PRACK-------------*R*-->|
| *E* *E* |
|<-*S*-------(4) 200 OK (PRACK)--------*S*---|
| *E* *E* |
| *R* *R* |
| *V* *V* |
| *A* *A* |
| *T* *T* |
| *I* *I* |
| *O* *O* |
| *N* *N* |
| *** *** |
| *** |
| *** |
|-------------(5) UPDATE SDP3--------------->|
| |
|<--------(6) 200 OK (UPDATE) SDP4-----------|
| |
|<-------------(7) 180 Ringing---------------|
| |
|-----------------(8) PRACK----------------->|
| |
|<------------(9) 200 OK (PRACK)-------------|
| |
| |
| |
|<-----------(10) 200 OK (INVITE)------------|
| |
|------------------(11) ACK----------------->|
| |
| |
Figure 2: Example using the end-to-end status type
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IP address where it is receiving media. Figure 3 shows this scenario.
A B
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------(2) 183 Session Progress SDP 2-------|
| *** *** |
|--*R*-----------(3) PRACK-------------*R*-->|
| *E* *E* |
|<-*S*-------(4) 200 OK (PRACK)--------*S*---|
| *E* *E* |
| *R* *R* |
| *V* *V* |
| *A* *A* |
| *T* *T* |
| *I* *I* |
| *O* *O* |
| *N* *N* |
| *** *** |
| *** |
| *** |
|-------------(5) UPDATE SDP3--------------->|
| |
|<--------(6) 200 OK (UPDATE) SDP4-----------|
| |
|<-----------(7) 200 OK (INVITE)-------------|
| |
|------------------(8) ACK------------------>|
| |
| |
Figure 3: Session modification with preconditions
SDP1: A includes an offer in a re-INVITE (1). A continues to receive
media on the old IP address (192.0.2.1), but it is ready to receive
media on the new one as well (192.0.2.2):
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.2
a=curr:qos e2e none
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a=des:qos mandatory e2e sendrecv
SDP2: B includes a "confirm" tag in its answer. B continues sending
media to the old remote IP address (192.0.2.1)
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv confirm
SDP3: When A receives RESV messages it sends an updated offer (5) to
B:
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.2
a=curr:qos e2e send
a=des:qos mandatory e2e sendrecv
SDP4: B responds with an answer (6) indicating that the preconditions
have been met (current status "sendrecv). It is now when B begins
sending media to the new remote IP address (192.0.2.2).
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv
10.2 Segmented Status Type
The call flow of figure 4 shows a basic session establishment using
the segmented status type. The SDP descriptions of this example are
shown below:
SDP1: B includes local and remote QoS preconditions in the initial
offer. Before sending the initial offer, A reserves resources in its
access network. This is indicated in the local current status of the
SDP below:
m=audio 20000 RTP/AVP 0 8
c=IN IP4 192.0.2.1
a=curr:qos local sendrecv
a=curr:qos remote none
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a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
SDP2: B reserves resources in its access network and, since all the
preconditions are met, returns an answer in a 180 (Ringing) response
(3).
m=audio 30000 RTP/AVP 0 8
c=IN IP4 192.0.2.4
a=curr:qos local sendrecv
a=curr:qos remote sendrecv
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
Let's assume that after receiving this response A decides that it
wants to use only PCM u-law (payload 0), as opposed to both PCM u-law
and A-law (payload 8). It would send an UPDATE to B possibly before
receiving the 200 OK for the INVITE (5). The SDP would look like:
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos local sendrecv
a=curr:qos remote sendrecv
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
B would generate an answer for this offer and place it in the 200 OK
for the UPDATE.
Note that this last offer/answer to reduce the number of supported
codecs may arrive to the user agent server after the 200 OK response
has been generated. This would mean that the session is established
before A has reduced the number of supported codecs. To avoid this
situation, the user agent client could wait for the first answer from
the user agent before setting its local current status to "sendrecv".
11 Security Considerations
An entity in the middle of two user agents establishing a session may
add desired-status attributes making session establishment
impossible. It could also modify the content of the current-status
parameters so that the session is established without meeting the
preconditions. Integrity protection can be used to avoid this
attacks.
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Internet Draft SIP February 25, 2002
A B
| *** |
| *R* |
| *E* |
| *S* |
| *E* |
| *R* |
| *V* |
| *A* |
| *T* |
| *I* |
| *O* |
| *N* |
| *** |
|-------------(1) INVITE SDP1--------------->|
| *** |
| *R* |
| *E* |
| *S* |
| *E* |
| *R* |
| *V* |
| *A* |
| *T* |
| *I* |
| *O* |
| *N* |
| *** |
|<----------(2) 180 Ringing SDP2-------------|
| |
|----------------(3) PRACK------------------>|
| |
|<-----------(4) 200 OK (PRACK)--------------|
| |
| |
|<-----------(5) 200 OK (INVITE)-------------|
| |
|------------------(6) ACK------------------>|
| |
| |
Figure 4: Example using the segmented status type
An entity performing resource reservations upon reception of
unathenticated requests carrying preconditions can be an easy target
for a denial of service attack. Requests with preconditions SHOULD be
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12 IANA considerations
This document defines three media level SDP attributes: desired-
status, current-status and conf-status. Their format is defined in
Section 4.
Section 4 also one standard precondition-type related to the
attributes above: "qos". If in the future it was needed to
standardize further precondition-types, they would need to be defined
in a standards track document.
This document also defines a new SIP status code (580). Its default
reason phrase (Precondition Failure) is defined in section 8.
13 Contributors
The following persons contributed to early versions of this spec:
K. K. Ramakrishnan (TeraOptic Networks), Ed Miller (Terayon), Glenn
Russell (CableLabs), Burcak Beser (Pacific Broadband Communications),
Mike Mannette (3Com), Kurt Steinbrenner (3Com), Dave Oran (Cisco),
Flemming Andreasen (Cisco), Michael Ramalho (Cisco), John Pickens
(Com21), Poornima Lalwaney (Nokia), Jon Fellows (Copper Mountain
Networks), Doc Evans (D. R. Evans Consulting), Keith Kelly
(NetSpeak), Adam Roach (dynamicsoft), Dean Willis (dynamicsoft),
Steve Donovan (dynamicsoft), Henning Schulzrinne (Columbia
University).
14 Acknowledgments
The Distributed Call Signaling work in the PacketCable project is the
work of a large number of people, representing many different
companies. The authors would like to recognize and thank the
following for their assistance: John Wheeler, Motorola; David
Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jay Strater,
Jeff Ollis, Clive Holborow, General Instruments; Doug Newlin, Guido
Schuster, Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay Networks; Farzi
Khazai, Nortel; John Chapman, Bill Guckel, Cisco; Chuck Kalmanek,
Doug Nortz, John Lawser, James Cheng, Tung-Hai Hsiao, Partho Mishra,
AT&T; Telcordia Technologies; and Lucent Cable Communications.
15 Authors' Addresses
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
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Internet Draft SIP February 25, 2002
electronic mail: Gonzalo.Camarillo@ericsson.com
Bill Marshall
AT&T
Florham Park, NJ 07932
USA
electronic mail: wtm@research.att.com
Jonathan Rosenberg
dynamicsoft
West Orange, NJ 07052
USA
electronic mail: jdrosen@dynamicsoft.com
16 Bibliography
[1] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation
protocol," Internet Draft, Internet Engineering Task Force, Feb.
2002. Work in progress.
[2] M. Handley and V. Jacobson, "SDP: session description protocol,"
Request for Comments 2327, Internet Engineering Task Force, Apr.
1998.
[3] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments 2119, Internet Engineering Task Force,
Mar. 1997.
[4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
SDP," Internet Draft, Internet Engineering Task Force, Jan. 2002.
Work in progress.
[5] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
transport protocol for real-time applications," Request for Comments
1889, Internet Engineering Task Force, Jan. 1996.
[6] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
responses in SIP," Internet Draft, Internet Engineering Task Force,
Sept. 2001. Work in progress.
Full Copyright Statement
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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
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Internet Draft SIP February 25, 2002
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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G. Camarillo (Editor) et. al. [Page 23]
