draft-ietf-sip-manyfolks-resource-05.txt   draft-ietf-sip-manyfolks-resource-06.txt 
Internet Engineering Task Force SIP WG Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo (Editor) Internet Draft G. Camarillo (Editor)
Ericsson Ericsson
W. Marshall (Editor) W. Marshall (Editor)
AT&T AT&T
Jonathan Rosenberg Jonathan Rosenberg
dynamicsoft dynamicsoft
draft-ietf-sip-manyfolks-resource-05.txt draft-ietf-sip-manyfolks-resource-06.txt
March 1, 2002 March 25, 2002
Expires: September, 2002 Expires: September, 2002
Integration of Resource Management and SIP Integration of Resource Management and SIP
STATUS OF THIS MEMO STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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1 Introduction ........................................ 3 1 Introduction ........................................ 3
2 Terminology ......................................... 3 2 Terminology ......................................... 3
3 Overview ............................................ 3 3 Overview ............................................ 3
4 SDP parameters ...................................... 4 4 SDP parameters ...................................... 4
5 Usage of preconditions with offer/answer ............ 7 5 Usage of preconditions with offer/answer ............ 7
5.1 Generating an offer ................................. 8 5.1 Generating an offer ................................. 8
5.1.1 SDP encoding ........................................ 9 5.1.1 SDP encoding ........................................ 9
5.2 Generating an Answer ................................ 10 5.2 Generating an Answer ................................ 10
6 Suspending and Resuming Session Establishment ....... 12 6 Suspending and Resuming Session Establishment ....... 12
7 Status Confirmation ................................. 13 7 Status Confirmation ................................. 13
8 Refusing an offer ................................... 13 8 Refusing an offer ................................... 14
8.1 Rejecting a Media stream ............................ 14 8.1 Rejecting a Media Stream ............................ 15
9 Multiple Preconditions per Media Stream ............. 15 9 Unknown Precondition Type ........................... 15
10 Option Tag for Preconditions ........................ 15 10 Option Tag for Preconditions ........................ 15
11 Examples ............................................ 15 11 Indicating Capabilities ............................. 16
11.1 End-to-end Status Type .............................. 16 12 Examples ............................................ 16
11.2 Segmented Status Type ............................... 20 12.1 End-to-end Status Type .............................. 16
11.3 Offer in a SIP response ............................. 21 12.2 Segmented Status Type ............................... 21
12 Security Considerations ............................. 24 12.3 Offer in a SIP response ............................. 22
13 IANA considerations ................................. 24 13 Security Considerations ............................. 26
14 Contributors ........................................ 24 14 IANA considerations ................................. 26
15 Acknowledgments ..................................... 26 15 Contributors ........................................ 26
16 Authors' Addresses .................................. 26 16 Acknowledgments ..................................... 27
17 Bibliography ........................................ 27 17 Authors' Addresses .................................. 27
18 Bibliography ........................................ 28
1 Introduction 1 Introduction
Some architectures require that at session establishment time, once Some architectures require that at session establishment time, once
the callee has been alerted, the chances of a session establishment the callee has been alerted, the chances of a session establishment
failure are minimum. One source of failure is the inability to failure are minimum. One source of failure is the inability to
reserve network resources for a session. In order to minimize "ghost reserve network resources for a session. In order to minimize "ghost
rings", it is necessary to reserve network resources for the session rings", it is necessary to reserve network resources for the session
before the callee is alerted. However, the reservation of network before the callee is alerted. However, the reservation of network
resources frequently requires learning the IP address, port, and resources frequently requires learning the IP address, port, and
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2 Terminology 2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3]. document are to be interpreted as described in RFC 2119 [3].
3 Overview 3 Overview
In order to ensure that session establishment does not take place In order to ensure that session establishment does not take place
until certain preconditions are met, we distinguish between two until certain preconditions are met we distinguish between two
different state variables that affect a particular media stream: different state variables that affect a particular media stream:
current status and desired status. This document defines quality of current status and desired status. This document defines quality of
service status. service status.
The desired status consists of a threshold for the current status. The desired status consists of a threshold for the current status.
Session establishment stops until the current status reaches or Session establishment stops until the current status reaches or
surpasses this threshold. Once this threshold is reached or surpasses this threshold. Once this threshold is reached or
surpassed, session establishment resumes. surpassed, session establishment resumes.
For example, the following values for current and desired status For example, the following values for current and desired status
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current status = resources reserved in the send direction current status = resources reserved in the send direction
desired status = resources reserved in both (sendrecv) directions desired status = resources reserved in both (sendrecv) directions
On the other hand, the values of the example below would make session On the other hand, the values of the example below would make session
establishment resume: establishment resume:
current status = resources reserved in both (sendrecv) directions current status = resources reserved in both (sendrecv) directions
desired status = resources reserved in the send direction desired status = resources reserved in the send direction
These two state variables define a piece of state of a media stream. These two state variables define a certain piece of state of a media
This is similar to the way the direction attributes, or the codecs in stream the same way as the direction attribute or the codecs in use,
use, define other pieces of state. Consequently, we treat these two define other pieces of state. Consequently, we treat these two new
new variables in the same way as other SDP media attributes are variables in the same way as other SDP media attributes are treated
treated in the offer/answer model used by SIP [4]: they are exchanged in the offer/answer model used by SIP [4]: they are exchanged between
between two user agents using an offer and an answer in order to have two user agents using an offer and an answer in order to have a
a shared view of the status of the session. shared view of the status of the session.
Figure 1 shows a typical message exchange between two SIP user agents Figure 1 shows a typical message exchange between two SIP user agents
using preconditions. A includes quality of service preconditions in using preconditions. A includes quality of service preconditions in
the SDP of the initial INVITE. A does not want B to be alerted until the SDP of the initial INVITE. A does not want B to be alerted until
there is network resources reserved in both directions (sendrecv) there is network resources reserved in both directions (sendrecv)
end-to-end. B agrees to reserve network resources for this session end-to-end. B agrees to reserve network resources for this session
before alerting the callee. B will handle resource reservation in the before alerting the callee. B will handle resource reservation in the
B->A direction, but needs A to handle the A->B direction. To indicate B->A direction, but needs A to handle the A->B direction. To indicate
so, B returns a 183 response to A asking A to start resource so, B returns a 183 response to A asking A to start resource
reservation and to confirm to B as soon as the A->B direction is reservation and to confirm to B as soon as the A->B direction is
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current-status = "a=curr:" precondition-type current-status = "a=curr:" precondition-type
SP status-type SP direction-tag SP status-type SP direction-tag
desired-status = "a=des:" precondition-type desired-status = "a=des:" precondition-type
SP strength-tag SP status-type SP strength-tag SP status-type
SP direction-tag SP direction-tag
confirm-status = "a=conf:" precondition-type confirm-status = "a=conf:" precondition-type
SP status-type SP direction-tag SP status-type SP direction-tag
precondition-type = "qos" | token precondition-type = "qos" | token
strength-tag = ("mandatory" | "optional" | "none" strength-tag = ("mandatory" | "optional" | "none"
= | "failure") = | "failure" | "unknown")
status-type = ("e2e" | "local" | "remote") status-type = ("e2e" | "local" | "remote")
direction-tag = ("none" | "send" | "recv" | "sendrecv") direction-tag = ("none" | "send" | "recv" | "sendrecv")
Current status: The current status attribute carries the current Current status: The current status attribute carries the current
status of network resources for a particular media stream. status of network resources for a particular media stream.
Desired status: The desired status attribute carries the Desired status: The desired status attribute carries the
preconditions for a particular media stream. When the preconditions for a particular media stream. When the
current status value has the same or a better value than direction-tag of the current status attribute with a given
the desired status value, the preconditions are considered precondition-type/status-type for a particular stream is
to be met for each stream. equal to (or better than) the direction-tag of the desired
status attribute with the same precondition-type/status-
type for that stream, then the preconditions are considered
to be met for that stream.
Confirmation status: The confirmation status attribute carries Confirmation status: The confirmation status attribute carries
threshold conditions for a media stream. When the status of threshold conditions for a media stream. When the status of
network resources reach these conditions, the peer user network resources reach these conditions, the peer user
agent will send an update of the session description agent will send an update of the session description
containing an updated current status attribute for this containing an updated current status attribute for this
particular media stream. particular media stream.
Precondition type: This document defines quality of service Precondition type: This document defines quality of service
preconditions. Extensions may define other types of preconditions. Extensions may define other types of
preconditions. preconditions.
Strength tag: The strength tag indicates whether or not the Strength tag: The strength-tag indicates whether or not the
callee can be alerted in case the network fails to meet the callee can be alerted in case the network fails to meet the
preconditions. preconditions.
Status type: We define two types of status: end-to-end and Status type: We define two types of status: end-to-end and
segmented. The end-to-end status reflects the status of the segmented. The end-to-end status reflects the status of the
end-to-end reservation of resources. The segmented status end-to-end reservation of resources. The segmented status
reflects the status of the access network reservations of reflects the status of the access network reservations of
both user agents. The end-to-end status corresponds to the both user agents. The end-to-end status corresponds to the
tag "e2e" defined above and the segmented status to the tag "e2e" defined above and the segmented status to the
tags "local" and "remote". End-to-end status is useful when tags "local" and "remote". End-to-end status is useful when
end-to-end resource reservation mechanisms are available.
The segmented status is useful when one or both UAs perform
resource reservations on their respective access networks.
A B A B
| | | |
|-------------(1) INVITE SDP1--------------->| |-------------(1) INVITE SDP1--------------->|
| | | |
|<------(2) 183 Session Progress SDP 2-------| |<------(2) 183 Session Progress SDP 2-------|
| *** *** | | *** *** |
|--*R*-----------(3) PRACK-------------*R*-->| |--*R*-----------(3) PRACK-------------*R*-->|
| *E* *E* | | *E* *E* |
|<-*S*-------(4) 200 OK (PRACK)--------*S*---| |<-*S*-------(4) 200 OK (PRACK)--------*S*---|
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| | | |
| | | |
| | | |
|<-----------(10) 200 OK (INVITE)------------| |<-----------(10) 200 OK (INVITE)------------|
| | | |
|------------------(11) ACK----------------->| |------------------(11) ACK----------------->|
| | | |
| | | |
Figure 1: Basic session establishment using preconditions Figure 1: Basic session establishment using preconditions
Note that the use of the segmented status-type does not end-to-end resource reservation mechanisms are available.
The segmented status is useful when one or both UAs perform
resource reservations on their respective access networks.
Note that the use of the segmented status type does not
prevent bottlenecks in the backbone, only in the access prevent bottlenecks in the backbone, only in the access
networks. networks.
Direction tag: The direction tag indicates the direction a Direction tag: The direction-tag indicates the direction a
particular attribute (current, desired or confirmation particular attribute (current, desired or confirmation
status) is applicable to. status) is applicable to.
The values of the tags "send", "recv", "local" and "remote" represent The values of the tags "send", "recv", "local" and "remote" represent
the point of view of the entity generating the SDP description. In an the point of view of the entity generating the SDP description. In an
offer, "send" is the direction offerer->answerer and "local" is the offer, "send" is the direction offerer->answerer and "local" is the
offerer's access network. In an answer, "send" is the direction offerer's access network. In an answer, "send" is the direction
answerer->offerer and "local" is the answerer's access network. answerer->offerer and "local" is the answerer's access network.
The following example shows these new SDP attributes in two media The following example shows these new SDP attributes in two media
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reserved in the corresponding direction. "No" indicates that reserved in the corresponding direction. "No" indicates that
resources have not been reserved yet. The "Desired Strength" field resources have not been reserved yet. The "Desired Strength" field
indicates the strength of the preconditions in the corresponding indicates the strength of the preconditions in the corresponding
direction. The table for the segmented status type contains four direction. The table for the segmented status type contains four
rows: both directions in the local access network and in the peer's 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- access network. The meaning of the fields is the same as in the end-
to-end case. to-end case.
Before generating an offer, the offerer MUST build a transaction Before generating an offer, the offerer MUST build a transaction
status table with the current and the desired status for each media status table with the current and the desired status for each media
stream. The different values of the strength tag for the desired stream. The different values of the strength-tag for the desired
status attribute have the following semantics: status attribute have the following semantics:
o None: no resource reservation is needed. o None: no resource reservation is needed.
o Optional: the user agents SHOULD try to provide resource o Optional: the user agents SHOULD try to provide resource
reservation, but the session can continue regardless of reservation, but the session can continue regardless of
whether this provision is possible or not. whether this provision is possible or not.
o Mandatory: the user agents MUST provide resource reservation. o Mandatory: the user agents MUST provide resource reservation.
Otherwise, session establishment MUST NOT continue. Otherwise, session establishment MUST NOT continue.
The offerer then decides whether it is going to use the end-to-end 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 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 media line will be end-to-end, the user agent generates records with
the desired status and the current status for each direction (send the desired status and the current status for each direction (send
and recv) independently, as shown in table 1: and recv) independently, as shown in table 1:
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
Direction Current Desired Strength Direction Current Desired Strength
____________________________________ ____________________________________
send no mandatory send no mandatory
recv no mandatory recv no mandatory
Table 1: Table for the end-to-end status type Table 1: Table for the end-to-end status type
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: remote) independently, as shown in table 2:
Direction Current Desired Strength Direction Current Desired Strength
______________________________________ ______________________________________
local send no none local send no none
local recv no none local recv no none
remote send no optional remote send no optional
remote recv no none remote recv no none
Table 2: Table for the segmented status type Table 2: Table for the segmented status type
At the time of sending the offer, the offerer's local status table At the time of sending the offer, the offerer's local status table
and the transaction status table contain the same values. and the transaction status table contain the same values.
With the transaction status table, the user agent generates the With the transaction status table, the user agent MUST generate the
current-status and the desired status lines following the syntax of current-status and the desired status lines following the syntax of
Section 4 and the rules described below in Section 5.1.1. Section 4 and the rules described below in Section 5.1.1.
5.1.1 SDP encoding 5.1.1 SDP encoding
For the end-to-end status type, the user agent MUST generate one 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 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 strength-tags for both directions are equal (e.g., both "mandatory")
the transaction status table, the user agent MUST add one desired in the transaction status table, the user agent MUST add one desired
status line with the tag "sendrecv". If both tags are different, the status line with the tag "sendrecv". If both tags are different, the
user agent MUST include two desired status lines, one with the tag user agent MUST include two desired status lines, one with the tag
"send" and the other with the tag "recv". "send" and the other with the tag "recv".
The semantics of two lines with the same strength tag, one The semantics of two lines with the same strength-tag, one
with a "send" tag and the other with a "recv" tag, is the with a "send" tag and the other with a "recv" tag, is the
same as one "sendrecv" line. However, in order to achieve a same as one "sendrecv" line. However, in order to achieve a
more compact encoding, we have chosen to make mandatory the more compact encoding, we have chosen to make mandatory the
latter format. latter format.
For the segmented status type, the user agent MUST generate two For the segmented status type, the user agent MUST generate two
current status lines: one with the tag "local" and the other with the 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 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 per segment (i.e., local and remote). If for a particular segment
(local or remote) the tags for both directions in the transaction (local or remote) the tags for both directions in the transaction
status table are equal (e.g., both mandatory), the user agent MUST 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 add one desired status line with the tag "sendrecv". If both tags are
different, the user agent MUST include two desired status lines, one different, the user agent MUST include two desired status lines, one
with the tag "send" and the other with the tag "recv". with the tag "send" and the other with the tag "recv".
Note that the rules above apply to the desired strength tag "none" as 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 well. This way, a user agent that supports quality of service but
does not intend to use them, adds desired status lines with the 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 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 described in Section 5.2, the answerer can request quality of service
reservation without a need of another offer/answer exchange. reservation without a need of another offer/answer exchange.
The example below shows the SDP corresponding to tables 1 and 2. The example below shows the SDP corresponding to tables 1 and 2.
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
a=curr:qos e2e none a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
m=audio 20002 RTP/AVP 0 m=audio 20002 RTP/AVP 0
a=curr:qos local none a=curr:qos local none
a=curr:qos remote none a=curr:qos remote none
a=des:qos optional remote send a=des:qos optional remote send
a=des:qos none remote recv a=des:qos optional local none
a=des:qos none local sendrecv
5.2 Generating an Answer 5.2 Generating an Answer
When the answerer receives the offer, it recreates the transaction When the answerer receives the offer, it recreates the transaction
status table using the SDP attributes contained in the offer. The status table using the SDP attributes contained in the offer. The
answerer updates both its local status and the transaction status answerer updates both its local status and the transaction status
table following the rules below: table following the rules below:
Desired Strength: We define an absolute ordering for the Desired Strength: We define an absolute ordering for the
strength tags: none, optional and mandatory. Mandatory is strength-tags: "none", "optional" and "mandatory".
the tag with highest grade and none the tag with lowest "Mandatory" is the tag with highest grade and "none" the
grade. An answerer MAY upgrade the desired strength in any tag with lowest grade. An answerer MAY upgrade the desired
entry of the transaction status table, but it MUST NOT strength in any entry of the transaction status table, but
downgrade it. Therefore, it is OK to upgrade a row from it MUST NOT downgrade it. Therefore, it is OK to upgrade a
none to optional, from none to mandatory or from optional row from "none" to "optional", from "none" to "mandatory"
to mandatory, but not the other way around. or from "optional" to "mandatory", but not the other way
around.
Current Status: For every row, the value of the "Current" field Current Status: For every row, the value of the "Current" field
in the transaction status table and in the local status in the transaction status table and in the local status
table of the answerer have to be compared. Table 3 shows table of the answerer have to be compared. Table 3 shows
the four possible combinations. If both fields have the the four possible combinations. If both fields have the
same value (two first rows of table 3), nothing needs to be same value (two first rows of table 3, nothing needs to be
updated. If the "Current" field of the transaction status updated. If the "Current" field of the transaction status
table is "Yes" and the field of the local status table is table is "Yes" and the field of the local status table is
"No" (third row of table 3), the latter MUST be set to "No" (third row of table 3), the latter MUST be set to
"Yes". If the "Current" field of the transaction status "Yes". If the "Current" field of the transaction status
table is "No" and the field of the local status table is table is "No" and the field of the local status table is
"Yes" (forth row of table 3), the answerer needs to check "Yes" (forth row of table 3), the answerer needs to check
if it has local information (e.g., a confirmation of a if it has local information (e.g., a confirmation of a
resource reservation has been received) about that resource reservation has been received) about that
particular current status. If it does, the "Current" field particular current status. If it does, the "Current" field
of the transaction status table is set to "Yes". If the of the transaction status table is set to "Yes". If the
answerer does not have local information about that current answerer does not have local information about that current
status, the "Current" field of the local status table MUST status, the "Current" field of the local status table MUST
be set to "No". be set to "No".
Transac. status table Local status table New values transac./local Transac. status table Local status table New values transac./local
____________________________________________________________________ ____________________________________________________________________
no no no/no no no no/no
yes yes yes/yes yes yes yes/yes
yes no yes/yes yes no yes/yes
no yes no/no OR yes/yes no yes depends on local info
Table 3: Possible values for the "Current" fields Table 3: Possible values for the "Current" fields
Once both tables have been updated, an answer is generated following Once both tables have been updated, an answer MUST be generated
the rules described in Section 5.1.1 and taking into account that following the rules described in Section 5.1.1 and taking into
"send", "recv", "local" and "remote" tags have to be inverted in the account that "send", "recv", "local" and "remote" tags have to be
answer, as shown in table 4. inverted in the answer, as shown in table 4.
Offer Answer Offer Answer
______________ ______________
send recv send recv
recv send recv send
local remote local remote
remote local remote local
Table 4: Values of tags in offer and answers Table 4: Values of tags in offer and answers
At the time the answer is sent, the transaction status table and the At the time the answer is sent, the transaction status table and the
answerer's local status table contain the same values. Therefore, answerer's local status table contain the same values. Therefore,
this answer contains the shared view of the status of the media line this answer contains the shared view of the status of the media line
in the current-status attribute and the negotiated strength and in the current-status attribute and the negotiated strength and
direction tags in the desired-status attribute. direction-tags in the desired-status attribute.
If the resource reservation mechanism used requires participation of If the resource reservation mechanism used requires participation of
both user agents, the answerer SHOULD start resource reservation both user agents, the answerer SHOULD start resource reservation
after having sent the answer and the offerer SHOULD start resource after having sent the answer and the offerer SHOULD start resource
reservation as soon as the answer is received. If participation of reservation as soon as the answer is received. If participation of
the peer user agent is not needed (e.g., segmented status type), the the peer user agent is not needed (e.g., segmented status type), the
offerer MAY start resource reservation before sending the offer and offerer MAY start resource reservation before sending the offer and
the answerer MAY start it before sending the answer. the answerer MAY start it before sending the answer.
The status of the resource reservation of a media line can change The status of the resource reservation of a media line can change
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response. The user agent client will send the answer in another SIP response. The user agent client will send the answer in another SIP
request (i.e., the PRACK for the 1xx). If the offer and the answer request (i.e., the PRACK for the 1xx). If the offer and the answer
contain preconditions, the user agent server SHOULD NOT alert the contain preconditions, the user agent server SHOULD NOT alert the
user until all the mandatory preconditions in the answer are met. user until all the mandatory preconditions in the answer are met.
Note that in this case, a user agent server providing a Note that in this case, a user agent server providing a
initial offer with preconditions, a 180 (Ringing) response initial offer with preconditions, a 180 (Ringing) response
will never be sent, since the user agent server cannot will never be sent, since the user agent server cannot
alert the user until all the preconditions are met. alert the user until all the preconditions are met.
A UAS that is not capable of unilaterally meeting all of the
mandatory preconditions MUST include a confirm-status attribute in
the SDP (offer or answer) that it sends (see Section 7). Further, the
SDP (offer or answer) that contains this confirm-status attribute
MUST be sent as soon as allowed by the SIP offer/answer rules.
While session establishment is suspended, user agents SHOULD not send While session establishment is suspended, user agents SHOULD not send
any data over any media stream. In the case of RTP [6], neither RTP any data over any media stream. In the case of RTP [6], neither RTP
nor RTCP packets are sent. nor RTCP packets are sent.
A user agent server knows that all the preconditions are met for a 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 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 the rows whose strength-tag is "mandatory". When the preconditions of
all the media lines of the session are met, session establishment all the media lines of the session are met, session establishment
SHOULD resume. SHOULD resume.
For an initial INVITE suspending and resuming session establishment For an initial INVITE suspending and resuming session establishment
is very intuitive. The callee will not be alerted until all the is very intuitive. The callee will not be alerted until all the
mandatory preconditions are met. However, offers containing mandatory preconditions are met. However, offers containing
preconditions sent in the middle of an ongoing session need further preconditions sent in the middle of an ongoing session need further
explanation. Both user agents SHOULD continue using the old session explanation. Both user agents SHOULD continue using the old session
parameters until all the mandatory preconditions are met. At that parameters until all the mandatory preconditions are met. At that
moment, the user agents SHOULD begin using the new session moment, the user agents can begin using the new session parameters.
parameters. Section 11 contains an example of this situation. Section 12 contains an example of this situation.
7 Status Confirmation 7 Status Confirmation
The confirm-status attribute MAY be used in both offers and answers. The confirm-status attribute MAY be used in both offers and answers.
This attribute represents a threshold for the resource reservation. This attribute represents a threshold for the resource reservation.
When this threshold is reached or surpassed, the user agent MUST send 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 an offer to the peer user agent reflecting the new current status of
the media line as soon as allowed by the SIP offer/answer rules. If the media line as soon as allowed by the SIP offer/answer rules. If
this threshold is crossed again (e.g., the network stops providing this threshold is crossed again (e.g., the network stops providing
resources for the media stream), the user agent MUST send a new offer resources for the media stream), the user agent MUST send a new offer
skipping to change at page 13, line 40 skipping to change at page 14, line 4
For example, if a user agent receives an SDP description with the For example, if a user agent receives an SDP description with the
following attributes: following attributes:
m=audio 20002 RTP/AVP 0 m=audio 20002 RTP/AVP 0
a=curr:qos local none a=curr:qos local none
a=curr:qos remote none a=curr:qos remote none
a=des:qos mandatory local sendrecv a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv a=des:qos mandatory remote sendrecv
a=conf:qos remote sendrecv a=conf:qos remote sendrecv
It will send an offer as soon as it reserves resources in its access It will send an offer as soon as it reserves resources in its access
network ("remote" tag in the received message) for both directions network ("remote" tag in the received message) for both directions
(sendrecv). (sendrecv).
8 Refusing an offer 8 Refusing an offer
We define a new SIP status code: We define a new SIP status code:
Server-Error = "580" ;Precondition Failure Server-Error = "580" ;Precondition Failure
When a UAS acting as an answerer cannot or is not willing to meet the When a UAS acting as an answerer cannot or is not willing to meet the
preconditions in the offer it SHOULD reject the offer by returning a preconditions in the offer it SHOULD reject the offer by returning a
580 (Precondition-Failure) response. This response SHOULD contain an 580 (Precondition-Failure) response. This response SHOULD contain an
SDP description indicating which desired status triggered the SDP description indicating which desired status triggered the
failure. The corresponding desired status line MUST use the "failure" failure. The corresponding desired status line MUST use the "failure"
value of the strength tag, as shown in the example below: strength-tag, as shown in the example below:
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
a=des:qos failure e2e send a=des:qos failure e2e send
SDP description indicating this type of failure MUST follow the SDP description indicating this type of failure MUST follow the
format for describing media capabilities defined in the SIP format for describing media capabilities defined in the SIP
offer/answer model [4]. offer/answer model [4].
Using the 580 (Precondition Failure) status code to refuse an offer 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. is useful when the offer came in an INVITE or in an UPDATE request.
skipping to change at page 14, line 41 skipping to change at page 15, line 5
applies here: applies here:
"The UAS MUST ensure that the session description overlaps "The UAS MUST ensure that the session description overlaps
with its previous session description in media formats, with its previous session description in media formats,
transports, other parameters that require support from the transports, other parameters that require support from the
peer. This is to avoid the need for the peer to reject the peer. This is to avoid the need for the peer to reject the
session description. If, however, it is unacceptable to A, session description. If, however, it is unacceptable to A,
A SHOULD generate an answer with a valid session A SHOULD generate an answer with a valid session
description, and then send a BYE to terminate the session." description, and then send a BYE to terminate the session."
8.1 Rejecting a Media stream 8.1 Rejecting a Media Stream
In the offer/answer model when an answerer wishes to reject a media In the offer/answer model when an answerer wishes to reject a media
stream it sets its port to zero. The presence of preconditions does stream it sets its port to zero. The presence of preconditions does
not change this behaviour; streams are still rejected by setting not change this behaviour; streams are still rejected by setting
their port to zero. their port to zero.
Both the offerer and the answerer MUST ignore all the preconditions Both the offerer and the answerer MUST ignore all the preconditions
that affect a stream with its port set to zero. They are not taken that affect a stream with its port set to zero. They are not taken
into consideration to decide whether or not session establishment can into consideration to decide whether or not session establishment can
resume. resume.
9 Multiple Preconditions per Media Stream 9 Unknown Precondition Type
A media stream MAY contain multiple preconditions. Different This document defines the "qos" tag for quality of service
preconditions MAY have the same precondition-type and different preconditions. New precondition-types defined in the future will have
status-types (e.g., end to end and segmented quality of service new associated tags. A UA that receives an unknown precondition-type
preconditions) or different precondition-types (this document only with a "mandatory" strength-tag in an offer MUST refuse the offer
defines the "qos" precondition type, but extensions may define more unless the only unknown mandatory preconditions have the "local" tag.
precondition-types in the future). In this case, the UA does not need to be involved in order to meet
the preconditions. The UA will ask for confirmation of the
preconditions and, when the confirmation arrives, it will resume
session establishment.
All the preconditions for a media stream MUST be met in order to A UA refusing an offer follows the rules described in section 8, but
resume session establishment. The following example shows a session instead of the tag "failure", it uses the tag "unknown", as shown in
description that uses both end-to-end and segmented status-types for the example below:
a media stream.
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
a=curr:qos local none a=des:foo unknown e2e send
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=curr:qos e2e none
a=des:qos optional e2e sendrecv
10 Option Tag for Preconditions 10 Option Tag for Preconditions
We define the option tag "precondition" for use in the Require and We define the option tag "precondition" for use in the Require and
Supported header fields. An offerer MUST include this tag in the Supported header fields. An offerer MUST include this tag in the
Require header field if the offer contains one or more strength tags Require header field if the offer contains one or more "mandatory"
with the value "mandatory". If all the strength tags in the strength-tags. If all the strength-tags in the description are
description are "optional" or "none" the offerer MUST include this "optional" or "none" the offerer MUST include this tag either in a
tag either in a Supported header field or in a Require header field. Supported header field or in a Require header field. It is, however,
It is, however, RECOMMENDED, that the Supported header field is used RECOMMENDED, that the Supported header field is used in this case.
in this case. The lack of preconditions in the answer would indicate The lack of preconditions in the answer would indicate that the
that the answerer did not support this extension. answerer did not support this extension.
The mapping of offers and answers to SIP requests and responses is The mapping of offers and answers to SIP requests and responses is
performed following the rules given in [5]. Therefore, a user agent performed following the rules given in [5]. Therefore, a user agent
including preconditions in the SDP MUST include both "100rel" [7] and including preconditions in the SDP MUST support the PRACK method, and
"update" [5] tags in the Require header field. consequently, MUST include the "100rel" [7] tag in the Require header
field.
11 Examples A user agent including preconditions in the SDP SHOULD support the
UPDATE method. If it is supported, an "update" [5] tag MUST be
included in the Require header field.
User agents that use preconditions but do not support
UPDATE can only be used in a limited set of scenarios, such
as the one described in figure 4.
11 Indicating Capabilities
The offer/answer model [4] describes the format of a session
description to indicate capabilities. This format is used in
responses to OPTIONS requests. A UA that supports preconditions
SHOULD add desired status lines indicating the precondition-types
supported for each media stream. These lines MUST have the "none"
strength-tag, as shown in the example below:
m=audio 0 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=des:foo none e2e sendrecv
a=des:qos none local sendrecv
Note that when this document was published, the precondition-type
"foo" has not been registered. It is used here in the session
description above to provide an example with multiple precondition-
types.
A UA that supports this framework SHOULD add a "precondition" tag to
the Supported header field of its responses to OPTIONS requests.
12 Examples
The following examples cover both status types; end-to-end and The following examples cover both status types; end-to-end and
segmented. segmented.
11.1 End-to-end Status Type 12.1 End-to-end Status Type
The call flow of figure 2 shows a basic session establishment using The call flow of figure 2 shows a basic session establishment using
the end-to-end status type. The SDP descriptions of this example are the end-to-end status type. The SDP descriptions of this example are
shown below: shown below:
SDP1: A includes end-to-end quality of service preconditions in the SDP1: A includes end-to-end quality of service preconditions in the
initial offer. initial offer.
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
skipping to change at page 16, line 32 skipping to change at page 17, line 26
in the other direction. B requests confirmation for resource in the other direction. B requests confirmation for resource
reservations in its "recv" direction to the peer user agent A in its reservations in its "recv" direction to the peer user agent A in its
answer. answer.
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
a=curr:qos e2e none a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
a=conf:qos e2e recv a=conf:qos e2e recv
After having sent the answer, B starts reserving network resources After having sent the answer B starts reserving network resources for
for the media stream. When A receives this answer (2) it starts the media stream. When A receives this answer (2) it starts
performing resource reservation as well. Both UAs use RSVP, so A performing resource reservation as well. Both UAs use RSVP, so A
sends PATH messages towards B and B sends PATH messages towards A. sends PATH messages towards B and B sends PATH messages towards A.
As time passes by, B receives RESV messages confirming the As time passes by, B receives RESV messages confirming the
reservation. However, B waits until resources in the other direction reservation. However, B waits until resources in the other direction
as reserved as well since it did not receive any confirmation and the are reserved as well since it did not receive any confirmation and
preconditions still have not been met. the preconditions still have not been met.
SDP3: When A receives RESV messages it sends an updated offer (5) to SDP3: When A receives RESV messages it sends an updated offer (5) to
B: B:
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=curr:qos e2e send a=curr:qos e2e send
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
SDP4: B responds with an answer (6) which contains the current status SDP4: B responds with an answer (6) which contains the current status
of the resource reservation (i.e., sendrecv): of the resource reservation (i.e., sendrecv):
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
a=curr:qos e2e sendrecv a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
At this point of time, session establishment resumes and B returns a At this point of time, session establishment resumes and B returns a
180 (Ringing) response (7). 180 (Ringing) response (7).
skipping to change at page 17, line 22 skipping to change at page 18, line 17
At this point of time, session establishment resumes and B returns a At this point of time, session establishment resumes and B returns a
180 (Ringing) response (7). 180 (Ringing) response (7).
Note that now the media stream has been already established, and A Note that now the media stream has been already established, and A
has received a 180 (Ringing) response. Since the direction of the has received a 180 (Ringing) response. Since the direction of the
stream is "sendrecv", A will not generate local ringback, since it stream is "sendrecv", A will not generate local ringback, since it
assumes that it will receive early media over this stream. assumes that it will receive early media over this stream.
However, if B wants A to generate local ringback, it can put the 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 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). stream off hold by sending an offer in an UPDATE request which would
The contents of the messages for this alternative flow is shown be sent at the same time as the 200 OK for the INVITE (10). The
below: contents of the messages for this alternative flow are shown below:
SDP4 (on hold): SDP4 (on hold):
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
a=recvonly a=recvonly
a=curr:qos e2e sendrecv a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
SDP5 in the 200 OK (10): SDP5 in an UPDATE:
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
a=sendrecv a=sendrecv
a=curr:qos e2e sendrecv a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
SDP6 in the ACK (11): SDP6 in the 200 OK for the UPDATE:
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=sendrecv a=sendrecv
a=curr:qos e2e sendrecv a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
Let's assume that in the middle of the session A wishes to change the
IP address where it is receiving media. Figure 3 shows this scenario.
A B A B
| | | |
|-------------(1) INVITE SDP1--------------->| |-------------(1) INVITE SDP1--------------->|
| | | |
|<------(2) 183 Session Progress SDP 2-------| |<------(2) 183 Session Progress SDP 2-------|
| *** *** | | *** *** |
|--*R*-----------(3) PRACK-------------*R*-->| |--*R*-----------(3) PRACK-------------*R*-->|
| *E* *E* | | *E* *E* |
|<-*S*-------(4) 200 OK (PRACK)--------*S*---| |<-*S*-------(4) 200 OK (PRACK)--------*S*---|
skipping to change at page 19, line 4 skipping to change at page 20, line 4
| | | |
| | | |
| | | |
|<-----------(10) 200 OK (INVITE)------------| |<-----------(10) 200 OK (INVITE)------------|
| | | |
|------------------(11) ACK----------------->| |------------------(11) ACK----------------->|
| | | |
| | | |
Figure 2: Example using the end-to-end status type Figure 2: Example using the end-to-end status type
Let's assume that in the middle of the session A wishes to change the
IP address where it is receiving media. Figure 3 shows this scenario.
A B A B
| | | |
|-------------(1) INVITE SDP1--------------->| |-------------(1) INVITE SDP1--------------->|
| | | |
|<------(2) 183 Session Progress SDP 2-------| |<------(2) 183 Session Progress SDP 2-------|
| *** *** | | *** *** |
|--*R*-----------(3) PRACK-------------*R*-->| |--*R*-----------(3) PRACK-------------*R*-->|
| *E* *E* | | *E* *E* |
|<-*S*-------(4) 200 OK (PRACK)--------*S*---| |<-*S*-------(4) 200 OK (PRACK)--------*S*---|
skipping to change at page 20, line 33 skipping to change at page 21, line 30
SDP4: B responds with an answer (6) indicating that the preconditions SDP4: B responds with an answer (6) indicating that the preconditions
have been met (current status "sendrecv). It is now when B begins have been met (current status "sendrecv). It is now when B begins
sending media to the new remote IP address (192.0.2.2). sending media to the new remote IP address (192.0.2.2).
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
a=curr:qos e2e sendrecv a=curr:qos e2e sendrecv
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
11.2 Segmented Status Type 12.2 Segmented Status Type
The call flow of figure 4 shows a basic session establishment using The call flow of figure 4 shows a basic session establishment using
the segmented status type. The SDP descriptions of this example are the segmented status type. The SDP descriptions of this example are
shown below: shown below:
SDP1: B includes local and remote QoS preconditions in the initial SDP1: A includes local and remote QoS preconditions in the initial
offer. Before sending the initial offer, A reserves resources in its offer. Before sending the initial offer, A reserves resources in its
access network. This is indicated in the local current status of the access network. This is indicated in the local current status of the
SDP below: SDP below:
m=audio 20000 RTP/AVP 0 8 m=audio 20000 RTP/AVP 0 8
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=curr:qos local sendrecv a=curr:qos local sendrecv
a=curr:qos remote none a=curr:qos remote none
a=des:qos mandatory local sendrecv a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv a=des:qos mandatory remote sendrecv
skipping to change at page 21, line 42 skipping to change at page 22, line 37
B would generate an answer for this offer and place it in the 200 OK B would generate an answer for this offer and place it in the 200 OK
for the UPDATE. for the UPDATE.
Note that this last offer/answer to reduce the number of supported 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 codecs may arrive to the user agent server after the 200 OK response
has been generated. This would mean that the session is established has been generated. This would mean that the session is established
before A has reduced the number of supported codecs. To avoid this before A has reduced the number of supported codecs. To avoid this
situation, the user agent client could wait for the first answer from situation, the user agent client could wait for the first answer from
the user agent before setting its local current status to "sendrecv". the user agent before setting its local current status to "sendrecv".
11.3 Offer in a SIP response 12.3 Offer in a SIP response
The call flow of figure 5 shows a basic session establishment where The call flow of figure 5 shows a basic session establishment where
the initial offer appears in a reliable 1xx response. This example the initial offer appears in a reliable 1xx response. This example
uses the end-to-end status type. The SDP descriptions of this example uses the end-to-end status type. The SDP descriptions of this example
are shown below: are shown below:
The first INVITE) (1) does not contain a session description.
Therefore, the initial offer is sent by B in a reliable 183 response.
SDP1: B includes end-to-end quality of service preconditions in the
initial offer. Since B uses RSVP, it can know when resources in its
A B A B
| *** | | *** |
| *R* | | *R* |
| *E* | | *E* |
| *S* | | *S* |
| *E* | | *E* |
| *R* | | *R* |
| *V* | | *V* |
| *A* | | *A* |
skipping to change at page 23, line 4 skipping to change at page 24, line 4
|<-----------(4) 200 OK (PRACK)--------------| |<-----------(4) 200 OK (PRACK)--------------|
| | | |
| | | |
|<-----------(5) 200 OK (INVITE)-------------| |<-----------(5) 200 OK (INVITE)-------------|
| | | |
|------------------(6) ACK------------------>| |------------------(6) ACK------------------>|
| | | |
| | | |
Figure 4: Example using the segmented status type Figure 4: Example using the segmented status type
The first INVITE) (1) does not contain a session description. A B
Therefore, the initial offer is sent by B in a reliable 183 response.
SDP1: B includes end-to-end quality of service preconditions in the | |
initial offer. Since B uses RSVP, it can know when resources in its |----------------(1) INVITE----------------->|
| |
|<------(2) 183 Session Progress SDP 1-------|
| |
|---------------(3) PRACK SDP 2------------->|
| *** *** |
|<-*R*--------(4) 200 OK (PRACK)-------*R*---|
| *E* *E* |
| *S* *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 5: Example of an initial offer in a 1xx response
"send" direction are available, because it will receive RESV messages "send" direction are available, because it will receive RESV messages
from the network. However, it does not know the status of the from the network. However, it does not know the status of the
reservations in the other direction. B requests confirmation for reservations in the other direction. B requests confirmation for
resource reservations in its "recv" direction to the peer user agent resource reservations in its "recv" direction to the peer user agent
A in its offer. A in its answer.
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
a=curr:qos e2e none a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
a=conf:qos e2e recv a=conf:qos e2e recv
SDP2: A includes its answer in the PRACK for the 183 response. SDP2: A includes its answer if the PRACK for the 183 response.
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=curr:qos e2e none a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
After having sent the answer, A starts reserving network resources After having sent the answer A starts reserving network resources for
for the media stream. When B receives this answer (3) it starts the media stream. When B receives this answer (3) it starts
performing resource reservation as well. Both UAs use RSVP, so A performing resource reservation as well. Both UAs use RSVP, so A
sends PATH messages towards B and B sends PATH messages towards A. sends PATH messages towards B and B sends PATH messages towards A.
SDP3: When A receives RESV messages it sends an updated offer (5) to SDP3: When A receives RESV messages it sends an updated offer (5) to
B: B:
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=curr:qos e2e send a=curr:qos e2e send
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
skipping to change at page 24, line 10 skipping to change at page 26, line 7
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4 c=IN IP4 192.0.2.4
a=curr:qos e2e recv a=curr:qos e2e recv
a=des:qos mandatory e2e sendrecv a=des:qos mandatory e2e sendrecv
As time passes by, B receives RESV messages confirming the As time passes by, B receives RESV messages confirming the
reservation. At this point of time, session establishment resumes and reservation. At this point of time, session establishment resumes and
B returns a 180 (Ringing) response (7). B returns a 180 (Ringing) response (7).
12 Security Considerations 13 Security Considerations
An entity in the middle of two user agents establishing a session may An entity in the middle of two user agents establishing a session may
add desired-status attributes making session establishment add desired-status attributes making session establishment
impossible. It could also modify the content of the current-status impossible. It could also modify the content of the current-status
parameters so that the session is established without meeting the parameters so that the session is established without meeting the
preconditions. Integrity protection can be used to avoid these preconditions. Integrity protection can be used to avoid these
attacks. attacks.
An entity performing resource reservations upon reception of An entity performing resource reservations upon reception of
unathenticated requests carrying preconditions can be an easy target unathenticated requests carrying preconditions can be an easy target
for a denial of service attack. Requests with preconditions SHOULD be for a denial of service attack. Requests with preconditions SHOULD be
authenticated. authenticated.
13 IANA considerations 14 IANA considerations
This document defines three media level SDP attributes: desired- This document defines three media level SDP attributes: desired-
status, current-status and conf-status. Their format is defined in status, current-status and conf-status. Their format is defined in
Section 4. Section 4.
Section 4 also defines one standard precondition-type related to the Section 4 also defines one standard precondition-type related to the
attributes above: "qos". If in the future it was needed to attributes above: "qos". If in the future it was needed to
standardize further precondition-types, they would need to be defined standardize further precondition-types, they would need to be defined
in a standards track document. Future precondition-types MUST define in a standards track document. Future precondition-types MUST define
the semantics with respect to the offer/answer model, as this the semantics with respect to the offer/answer model, as this
document defined these semantics for quality of service preconditions document defined these semantics for quality of service preconditions
in Section 5. in Section 5.
This document also defines a new SIP status code (580). Its default This document also defines a new SIP status code (580). Its default
reason phrase (Precondition Failure) is defined in section 8. reason phrase (Precondition Failure) is defined in section 8.
This document defines a SIP option tag (precondition) in section 10. This document defines a SIP option tag (precondition) in section 10.
14 Contributors 15 Contributors
The following persons contributed and were co-authors on earlier The following persons contributed and were co-authors on earlier
versions of this spec: versions of this spec:
K. K. Ramakrishnan (TeraOptic Networks), Ed Miller K. K. Ramakrishnan (TeraOptic Networks), Ed Miller
(Terayon), Glenn Russell (CableLabs), Burcak Beser (Pacific (Terayon), Glenn Russell (CableLabs), Burcak Beser (Pacific
A B
| |
|----------------(1) INVITE----------------->|
| |
|<------(2) 183 Session Progress SDP 1-------|
| |
|---------------(3) PRACK SDP 2------------->|
| *** *** |
|<-*R*--------(4) 200 OK (PRACK)-------*R*---|
| *E* *E* |
| *S* *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 5: Example of an initial offer in a 1xx response
Broadband Communications), Mike Mannette (3Com), Kurt Broadband Communications), Mike Mannette (3Com), Kurt
Steinbrenner (3Com), Dave Oran (Cisco), Flemming Andreasen Steinbrenner (3Com), Dave Oran (Cisco), Flemming Andreasen
(Cisco), Michael Ramalho (Cisco), John Pickens (Com21), (Cisco), Michael Ramalho (Cisco), John Pickens (Com21),
Poornima Lalwaney (Nokia), Jon Fellows (Copper Mountain Poornima Lalwaney (Nokia), Jon Fellows (Copper Mountain
Networks), Doc Evans (D. R. Evans Consulting), Keith Kelly Networks), Doc Evans (D. R. Evans Consulting), Keith Kelly
(NetSpeak), Adam Roach (dynamicsoft), Dean Willis (NetSpeak), Adam Roach (dynamicsoft), Dean Willis
(dynamicsoft), Steve Donovan (dynamicsoft), Henning (dynamicsoft), Steve Donovan (dynamicsoft), Henning
Schulzrinne (Columbia University). Schulzrinne (Columbia University).
This "manyfolks" draft is the culmination of over two years of work This "manyfolks" draft is the culmination of over two years of work
by many individuals, most are listed here and in the following by many individuals, most are listed here and in the following
acknowledgements section. A special note is due to Flemming acknowledgements section. A special note is due to Flemming
Andreasen, Burcak Beser, Dave Boardman, Bill Guckel, Chuck Kalmanek, Andreasen, Burcak Beser, Dave Boardman, Bill Guckel, Chuck Kalmanek,
Keith Kelly, Poornima Lalwaney, John Lawser, Bill Marshall, Mike Keith Kelly, Poornima Lalwaney, John Lawser, Bill Marshall, Mike
Mannette, Dave Oran, K.K. Ramakrishnan, Michael Ramalho, Adam Roach, Mannette, Dave Oran, K.K. Ramakrishnan, Michael Ramalho, Adam Roach,
Jonathan Rosenberg, and Henning Schulzrinne for spearheading the Jonathan Rosenberg, and Henning Schulzrinne for spearheading the
initial "single Invite" quality of service preconditions work from initial "single INVITE" quality of service preconditions work from
previous, non-SIP compatible, "two-stage Invite" proposals. These previous, non-SIP compatible, "two-stage Invite" proposals. These
"two-stage Invite" proposals had their origins from Distributed Call "two-stage INVITE" proposals had their origins from Distributed Call
Signaling work in PacketCable, which, in turn, had architectural Signaling work in PacketCable, which, in turn, had architectural
elements from AT&T's Distributed Open Systems Architecture (DOSA) elements from AT&T's Distributed Open Systems Architecture (DOSA)
work [8]. work [8].
15 Acknowledgments 16 Acknowledgments
The Distributed Call Signaling work in the PacketCable project is the The Distributed Call Signaling work in the PacketCable project is the
work of a large number of people, representing many different work of a large number of people, representing many different
companies. The authors would like to recognize and thank the companies. The authors would like to recognize and thank the
following for their assistance: John Wheeler, Motorola; David following for their assistance: John Wheeler, Motorola; David
Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jay Strater, Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jay Strater,
Jeff Ollis, Clive Holborow, General Instruments; Doug Newlin, Guido Jeff Ollis, Clive Holborow, General Instruments; Doug Newlin, Guido
Schuster, Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay Networks; Farzi Schuster, Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay Networks; Farzi
Khazai, Nortel; John Chapman, Bill Guckel, Cisco; Chuck Kalmanek, Khazai, Nortel; John Chapman, Bill Guckel, Cisco; Chuck Kalmanek,
Doug Nortz, John Lawser, James Cheng, Tung-Hai Hsiao, Partho Mishra, Doug Nortz, John Lawser, James Cheng, Tung-Hai Hsiao, Partho Mishra,
AT&T; Telcordia Technologies; and Lucent Cable Communications. AT&T; Telcordia Technologies; and Lucent Cable Communications.
16 Authors' Addresses Miguel Angel Garcia, Rohan May and Mark Watson provided helpful
comments and suggestions.
17 Authors' Addresses
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Advanced Signalling Research Lab. Advanced Signalling Research Lab.
FIN-02420 Jorvas FIN-02420 Jorvas
Finland Finland
electronic mail: Gonzalo.Camarillo@ericsson.com electronic mail: Gonzalo.Camarillo@ericsson.com
Bill Marshall Bill Marshall
AT&T AT&T
skipping to change at page 27, line 6 skipping to change at page 28, line 4
Advanced Signalling Research Lab. Advanced Signalling Research Lab.
FIN-02420 Jorvas FIN-02420 Jorvas
Finland Finland
electronic mail: Gonzalo.Camarillo@ericsson.com electronic mail: Gonzalo.Camarillo@ericsson.com
Bill Marshall Bill Marshall
AT&T AT&T
Florham Park, NJ 07932 Florham Park, NJ 07932
USA USA
electronic mail: wtm@research.att.com electronic mail: wtm@research.att.com
Jonathan Rosenberg Jonathan Rosenberg
dynamicsoft dynamicsoft
West Orange, NJ 07052 West Orange, NJ 07052
USA USA
electronic mail: jdrosen@dynamicsoft.com electronic mail: jdrosen@dynamicsoft.com
17 Bibliography 18 Bibliography
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[3] S. Bradner, "Key words for use in RFCs to indicate requirement [3] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments 2119, Internet Engineering Task Force, levels," Request for Comments 2119, Internet Engineering Task Force,
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[4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with [4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
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[8] C. Kalmanek, W. Marshall, P. Mishra, D. Nortz, and K. K. [8] C. Kalmanek, W. Marshall, P. Mishra, D. Nortz, and K. K.
Ramakrishnan, "DOSA: an architecture for providing robust IP Ramakrishnan, "DOSA: an architecture for providing robust IP
telephony service," in telephony service," in Proceedings of the Conference on Computer
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Full Copyright Statement Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved. Copyright (c) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
 End of changes. 

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