Internet Engineering Task Force (IETF) P. Jones
Request for Comments: 7989 G. Salgueiro
Obsoletes: 7329 C. Pearce
Category: Standards Track P. Giralt
ISSN: 2070-1721 Cisco Systems, Inc.
October 2016
End-to-End Session Identification in
IP-Based Multimedia Communication Networks
Abstract
This document describes an end-to-end session identifier for use in
IP-based multimedia communication systems that enables endpoints,
intermediary devices, and management systems to identify a session
end-to-end, associate multiple endpoints with a given multipoint
conference, track communication sessions when they are redirected,
and associate one or more media flows with a given communication
session. While the identifier is intended to work across multiple
protocols, this document describes its usage in the Session
Initiation Protocol (SIP).
This document also describes a backwards-compatibility mechanism for
an existing session identifier implementation (RFC 7329) that is
sufficiently different from the procedures defined in this document.
This document obsoletes RFC 7329.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7989.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
2. Conventions Used in This Document ...............................5
3. Session Identifier Definitions, Requirements, and Use Cases .....5
4. Constructing and Conveying the Session Identifier ...............5
4.1. Constructing the Session Identifier ........................5
4.2. Conveying the Session Identifier ...........................6
5. The Session-ID Header Field .....................................8
6. Endpoint Behavior ...............................................9
7. Processing by Intermediaries ...................................11
8. Handling of Remote UUID Changes ................................14
9. Associating Endpoints in a Multipoint Conference ...............16
10. Examples of Various Call Flow Operations ......................17
10.1. Basic Call with Two UUIDs ................................18
10.2. Basic Call Transfer Using REFER ..........................22
10.3. Basic Call Transfer Using Re-INVITE ......................24
10.4. Single Focus Conferencing ................................26
10.5. Single Focus Conferencing Using a Web-Based
Conference Service .......................................28
10.6. Cascading Conference Bridges .............................30
10.6.1. Establishing a Cascaded Conference ................30
10.6.2. Calling Into Cascaded Conference Bridges ..........31
10.7. Basic 3PCC for Two UAs ...................................33
10.8. Handling in 100 Trying SIP Response and CANCEL Request ...33
10.8.1. Handling in a 100 Trying SIP Response .............34
10.8.2. Handling a CANCEL SIP Request .....................35
10.9. Out-of-Dialog REFER Transaction ..........................36
11. Compatibility with a Previous Implementation ..................37
12. Security and Privacy Considerations ...........................39
13. IANA Considerations ...........................................40
13.1. Registration of the "Session-ID" Header Field ............40
13.2. Registration of the "remote" Parameter ...................40
14. References ....................................................41
14.1. Normative References .....................................41
14.2. Informative References ...................................42
Acknowledgements ..................................................44
Dedication ........................................................44
Authors' Addresses ................................................45
1. Introduction
IP-based multimedia communication systems, such as Session Initiation
Protocol (SIP) [RFC3261] and [H.323], have the concept of a "call
identifier" that is globally unique. The identifier is intended to
represent an end-to-end communication session from the originating
device to the terminating device. Such an identifier is useful for
troubleshooting, session tracking, and so forth.
For several reasons, however, the current call identifiers defined in
SIP and H.323 are not suitable for end-to-end session identification.
A fundamental issue in protocol interworking is the fact that the
syntax for the call identifier in SIP and H.323 is different. Thus,
if both protocols are used in a call, it is impossible to exchange
the call identifier end-to-end.
Another reason why the current call identifiers are not suitable to
identify a session end-to-end is that, in real-world deployments,
devices such as session border controllers [RFC7092] often change the
session signaling, including the value of the call identifier, as it
passes through the device. While this is deliberate and useful, it
makes it very difficult to track a session end-to-end.
This document defines a new identifier, referred to as the "session
identifier", that is intended to overcome the issues that exist with
the currently defined call identifiers used in SIP and other IP-based
communication systems. The identifier defined here has been adopted
by the ITU ([H.460.27]) for use in H.323-based systems, allowing for
the ability to trace a session end-to-end for sessions traversing
both SIP and H.323-based systems. This document defines its use in
SIP.
The procedures specified in this document attempt to comply with the
requirements specified in [RFC7206]. The procedures also specify
capabilities not mentioned in [RFC7206], shown in the call flows in
Section 10. Additionally, this specification attempts to account for
a previous, pre-standard version of a SIP session identifier header
[RFC7329], specifying a backwards-compatibility approach in
Section 11.
2. Conventions Used in This Document
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 [RFC2119] when they
appear in ALL CAPS. These words may also appear in this document in
lowercase, absent their normative meanings.
The term "session identifier" refers to the value of the identifier,
whereas "Session-ID" refers to the header field used to convey the
identifier. The session identifier is a set of two Universally
Unique Identifiers (UUIDs) and each element of that set is simply
referred to herein as a "UUID".
Throughout this document, the term "endpoint" refers to a SIP User
Agent (UA) that either initiates or terminates a SIP session, such as
a user's mobile phone or a conference server, but excludes entities
such as Back-to-Back User Agents (B2BUAs) that are generally located
along the call-signaling path between endpoints. The term
"intermediary" refers to any entity along the call-signaling path
between the aforementioned endpoints, including B2BUAs and SIP
proxies. In certain scenarios, intermediaries are allowed to
originate and terminate SIP messages without an endpoint being part
of the session or transaction. An intermediary may be performing
interworking between different protocols (e.g., SIP and H.323) that
support the session identifier defined in this document.
3. Session Identifier Definitions, Requirements, and Use Cases
Requirements and use cases for the end-to-end session identifier,
along with the definition of "session identifier", "communication
session", and "end-to-end" can be found in [RFC7206]. Throughout
this document, the term "session" refers to a "communication session"
as defined in [RFC7206].
As mentioned in Section 6.1 of [RFC7206], the ITU-T undertook a
parallel effort to define compatible procedures for an H.323 session
identifier. They are documented in [H.460.27].
4. Constructing and Conveying the Session Identifier
4.1. Constructing the Session Identifier
The session identifier comprises two UUIDs [RFC4122], with each UUID
representing one of the endpoints participating in the session.
The version number in the UUID indicates the manner in which the UUID
is generated, such as using random values or using the Media Access
Control (MAC) address of the endpoint. To satisfy the requirement
that no user or device information be conveyed, endpoints MUST
generate version 4 (random) or version 5 (SHA-1) UUIDs to address
privacy concerns related to the use of MAC addresses in UUIDs.
When generating a version 5 UUID, endpoints or intermediaries MUST
utilize the procedures defined in Section 4.3 of [RFC4122] and employ
the following "namespace ID":
uuid_t NameSpace_SessionID = {
/* a58587da-c93d-11e2-ae90-f4ea67801e29 */
0xa58587da,
0xc93d,
0x11e2,
0xae, 0x90, 0xf4, 0xea, 0x67, 0x80, 0x1e, 0x29
};
Further, the "name" to utilize for version 5 UUIDs is the
concatenation of the Call-ID header-value and the "tag" parameter
that appears on the "From" or "To" line associated with the device
for which the UUID is created. Once an endpoint generates a UUID for
a session, the UUID never changes, even if values originally used as
input into its construction change over time.
Stateless intermediaries that insert a Session-ID header field into a
SIP message on behalf of an endpoint MUST utilize version 5 UUIDs to
ensure that UUIDs for the communication session are consistently
generated. If a stateless intermediary does not know the tag value
for the endpoint (e.g., a new INVITE request without a To: tag value
or an older SIP implementation [RFC2543] that did not include a "tag"
parameter), the intermediary MUST NOT attempt to generate a UUID for
that endpoint. Note that, if an intermediary is stateless and the
endpoint on one end of the call is replaced with another endpoint due
to some service interaction, the values used to create the UUID
should change and, if so, the intermediary will compute a different
UUID.
4.2. Conveying the Session Identifier
The SIP User Agent (UA) initiating a new session by transmitting a
SIP request ("Alice"), i.e., a User Agent Client (UAC), MUST create a
new, previously unused UUID and transmit that to the ultimate
destination UA ("Bob"). Likewise, the destination UA ("Bob"), i.e.,
a User Agent Server (UAS), MUST create a new, previously unused UUID
and transmit that to the first UA ("Alice"). These two distinct
UUIDs form what is referred to as the "session identifier" and is
represented in this document in set notation of the form {A,B}, where
"A" is UUID value created by UA "Alice" and "B" is the UUID value
created by UA "Bob". The session identifier {A,B} is equal to the
session identifier {B,A}. Section 6 describes how the UUIDs selected
by the source and destination UAs persist for the duration of the
session.
In the case where only one UUID is known, such as when a UA first
initiates a potentially dialog-initiating SIP request, the session
identifier would be {A,N}, where "A" represents the UUID value
transmitted by the UA "Alice", and "N" is what is referred to as the
"nil UUID" [RFC4122] (see Section 5 of this document).
Since SIP sessions are subject to any number of service interactions,
SIP INVITE requests might be forked as sessions are established, and
since conferences might be established or expanded with endpoints
calling in or the conference focus calling out, the construction of
the session identifier as a set of UUIDs is important.
To understand this better, consider that an endpoint participating in
a communication session might be replaced with another, such as the
case where two "legs" of a call are joined together by a Private
Branch Exchange (PBX). Suppose "Alice" and "Bob" both call UA "C"
("Carol"). There would be two distinctly identifiable session
identifiers, namely {A,C} and {B,C}. Then, suppose that "Carol" uses
a local PBX function to join the call between herself and "Alice"
with the call between herself and "Bob", resulting in a single
remaining call between "Alice" and "Bob". This merged call can be
identified using two UUID values assigned by each entity in the
communication session, namely {A,B} in this example.
In the case of forking, "Alice" might send an INVITE request that
gets forked to several different endpoints. A means of identifying
each of these separate communication sessions is needed; since each
of the destination UAs will create its own UUID, each communication
session would be uniquely identified by the values {A, B1}, {A, B2},
{A, B3}, and so on, where each of the Bn values refers to the UUID
created by the different UAs to which the SIP session is forked.
For conferencing scenarios, it is also useful to have a two-part
session identifier where the conference focus specifies the same UUID
for each conference participant. This allows for correlation among
the participants in a single conference. For example, in a
conference with three participants, the session identifiers might be
{A,M}, {B,M}, and {C,M}, where "M" is assigned by the conference
focus. Only a conference focus will purposely utilize the same UUID
for more than one SIP session and, even then, such reuse MUST be
restricted to the participants in the same conference.
How a device acting on session identifiers processes or utilizes the
session identifier is outside the scope of this document. However,
devices storing a session identifier in a log file SHOULD follow the
security considerations outlined in [RFC6872]. Note that the primary
intent of a session identifier is for troubleshooting; therefore, it
should be included in logs at rest that will be used for
troubleshooting purposes.
5. The Session-ID Header Field
This document replaces the definition of the "Session-ID" token that
was added to the definition of the element "message-header" in the
SIP message grammar by [RFC7329]. The Session-ID header is a single-
instance header.
Each endpoint participating in a communication session has a
distinct, preferably locally generated UUID associated with it. The
endpoint's UUID value remains unchanged throughout the duration of
the communication session. Multipoint conferences can bridge
sessions from multiple endpoints and impose unique requirements
defined in Section 9. An intermediary MAY generate a UUID on behalf
of an endpoint that did not include a UUID of its own.
The UUID values for each endpoint are inserted into the Session-ID
header field of all transmitted SIP messages. The Session-ID header
field has the following ABNF [RFC5234] syntax:
session-id = "Session-ID" HCOLON session-id-value
session-id-value = local-uuid *(SEMI sess-id-param)
local-uuid = sess-uuid / nil
remote-uuid = sess-uuid / nil
sess-uuid = 32(DIGIT / %x61-66) ;32 chars of [0-9a-f]
sess-id-param = remote-param / generic-param
remote-param = "remote" EQUAL remote-uuid
nil = 32("0")
The productions "SEMI", "EQUAL", and "generic-param" are defined in
[RFC3261]. The production DIGIT is defined in [RFC5234].
The Session-ID header field MUST NOT have more than one "remote"
parameter. In the case where an entity compliant with this
specification is interworking with an entity that implemented a
session identifier as defined in [RFC7329], the "remote" parameter
may be absent; otherwise, the "remote" parameter MUST be present.
The details under which those conditions apply are described in
Section 11. Except for backwards compatibility with [RFC7329], the
"remote" parameter MUST be present.
A special nil UUID value composed of 32 zeros is required in certain
situations. A nil UUID is expected as the "remote-uuid" of every
initial standard SIP request since the initiating endpoint would not
initially know the UUID value of the remote endpoint. This nil value
will get replaced by the ultimate destination UAS when that UAS
generates a response message. One caveat is explained in Section 11
for a possible backwards-compatibility case. A nil UUID value is
also returned by some intermediary devices that send provisional or
other responses as the "local-uuid" component of the Session-ID
header field value, as described in Section 7.
The "local-uuid" in the Session-ID header field represents the UUID
value of the endpoint transmitting a message and the "remote-uuid" in
the Session-ID header field represents the UUID of the endpoint's
peer. For example, a Session-ID header field might appear like this:
Session-ID: ab30317f1a784dc48ff824d0d3715d86;
remote=47755a9de7794ba387653f2099600ef2
While this is the general form of the Session-ID header field,
exceptions to syntax and procedures are detailed in subsequent
sections.
The UUID values are presented as strings of lowercase hexadecimal
characters, with the most significant octet of the UUID appearing
first.
6. Endpoint Behavior
To comply with this specification, endpoints (non-intermediaries)
MUST include a Session-ID header field value in all SIP messages
transmitted as a part of a communication session. The locally
generated UUID of the transmitter of the message MUST appear in the
"local-uuid" portion of the Session-ID header field value. The UUID
of the peer device, if known, MUST appear as the "remote" parameter
following the transmitter's UUID. The nil UUID value MUST be used if
the peer device's UUID is not known.
Once an endpoint allocates a UUID value for a communication session,
the endpoint originating the request MUST NOT change that UUID value
for the duration of the session, including when:
o communication attempts are retried due to receipt of 4xx messages
or request timeouts;
o the session is redirected in response to a 3xx message;
o a session is transferred via a REFER message [RFC3515]; or
o a SIP dialog is replaced via an INVITE request with Replaces
[RFC3891].
An endpoint that receives a Session-ID header field MUST take note of
any non-nil "local-uuid" value that it receives and assume that is
the UUID of the peer endpoint within that communication session.
Endpoints MUST include this received UUID value as the "remote"
parameter when transmitting subsequent messages, making sure not to
change this UUID value in the process of moving the value internally
from the "local-uuid" field to the "remote-uuid" field.
If an endpoint receives a 3xx message, a REFER that directs the
endpoint to a different peer, or an INVITE request with Replaces that
also potentially results in communicating with a new peer, the
endpoint MUST complete any message exchanges with its current peer
using the existing session identifier, but it MUST NOT use the
current peer's UUID value when sending the first message to what it
believes may be a new peer endpoint (even if the exchange results in
communicating with the same physical or logical entity). The
endpoint MUST retain its own UUID value, however, as described above.
It should be noted that messages received by an endpoint might
contain a "local-uuid" value that does not match what the endpoint
expected its peer's UUID to be. It is also possible for an endpoint
to receive a "remote-uuid" value that does not match its generated
UUID for the session. Either might happen as a result of service
interactions by intermediaries and MUST NOT affect how the endpoint
processes the session; however, the endpoint may log this event for
troubleshooting purposes.
An endpoint MUST assume that the UUID value of the peer endpoint may
change at any time due to service interactions. Section 8 discusses
how endpoints must handle remote UUID changes.
It is also important to note that if an intermediary in the network
forks a session, the endpoint initiating a session may receive
multiple responses back from different endpoints, each of which
contains a different UUID ("local-uuid") value. Endpoints MUST
ensure that the correct UUID value is returned in the "remote"
parameter when interacting with each endpoint. The one exception is
when the endpoint sends a CANCEL request, in which case the Session-
ID header field value MUST be identical to the Session-ID header
field value sent in the original request.
If an endpoint receives a message that does not contain a Session-ID
header field, that message must have no effect on what the endpoint
believes is the UUID value of the remote endpoint. That is, the
endpoint MUST NOT change the internally maintained "remote-uuid"
value for the peer.
If an endpoint receives a SIP response with a non-nil "local-uuid"
that is not 32 octets long, this response comes from a misbehaving
implementation, and its Session-ID header field MUST be discarded.
That said, the response might still be valid according to the rules
within SIP [RFC3261], and it SHOULD be checked further.
A Multipoint Control Unit (MCU) is a special type of conferencing
endpoint and is discussed in Section 9.
7. Processing by Intermediaries
The following applies only to an intermediary that wishes to comply
with this specification and does not impose a conformance requirement
on intermediaries that elect not to provide any special treatment for
the Session-ID header field. Intermediaries that do not comply with
this specification might pass the header unchanged or drop it
entirely.
The Call-ID often reveals personal, device, domain, or other
sensitive information associated with a user, which is one reason why
intermediaries, such as session border controllers, sometimes alter
the Call-ID. In order to ensure the integrity of the end-to-end
session identifier, it is constructed in a way that does not reveal
such information, removing the need for intermediaries to alter it.
When an intermediary receives messages from one endpoint in a
communication session that causes the transmission of one or more
messages toward the second endpoint in a communication session, the
intermediary MUST include the Session-ID header field in the
transmitted messages with the same UUID values found in the received
message, except as outlined in this section and in Section 8.
If the intermediary aggregates several responses from different
endpoints, as described in Section 16.7 of [RFC3261], the
intermediary MUST set the local-uuid field to the nil UUID value when
forwarding the aggregated response to the endpoint since the true
UUID value of the peer is undetermined at that point. Note that an
intermediary that does not implement this specification might forward
a non-nil value, resulting in the originating endpoint receiving
different UUID values in the responses. It is possible for this to
result in the endpoint temporarily using the wrong remote UUID.
Subsequent messages in the dialog should resolve the temporary
mismatch as long as the endpoint follows the rules outlined in
Section 8 dealing with the handling of remote UUID changes.
Intermediary devices that transfer a call, such as by joining
together two different "call legs", MUST properly construct a
Session-ID header field that contains the UUID values associated with
the endpoints involved in the joined session and correct placement of
those values. As described in Section 6, the endpoint receiving a
message transmitted by the intermediary will assume that the first
UUID value belongs to its peer endpoint.
If an intermediary receives a SIP message without a Session-ID header
field or valid header field value from an endpoint for which the
intermediary is not storing a "remote-uuid" value, the intermediary
MAY assign a "local-uuid" value to represent that endpoint and,
having done so, MUST insert that assigned value into all signaling
messages on behalf of the endpoint for that dialog. In effect, the
intermediary becomes dialog-stateful, and it MUST follow the endpoint
procedures in Section 6 with respect to Session-ID header field value
treatment with itself acting as the endpoint (for the purposes of the
Session-ID header field) for which it inserted a component into the
Session-ID header field value. If the intermediary is aware of the
UUID value that identifies the endpoint to which a message is
directed, it MUST insert that UUID value into the Session-ID header
field value as the "remote-uuid" value. If the intermediary is
unaware of the UUID value that identifies the receiving endpoint, it
MUST use the nil UUID value as the "remote-uuid" value.
If an intermediary receives a SIP message without a Session-ID header
field or a valid Session-ID header field value from an endpoint for
which the intermediary has previously received a Session-ID and is
storing a "remote-uuid" value for that endpoint, the lack of a
Session-ID must have no effect on what the intermediary believes is
the UUID value of the endpoint. That is, the intermediary MUST NOT
change the internally maintained "remote-uuid" value for the peer.
When an intermediary originates a response, such as a provisional
response or a response to a CANCEL request, the "remote-uuid" field
will contain the UUID value of the receiving endpoint. When the UUID
of the peer endpoint is known, the intermediary MUST insert the UUID
of the peer endpoint in the "local-uuid" field of the header value.
Otherwise, the intermediary MAY set the "local-uuid" field of the
header value to the "nil" UUID value.
When an intermediary originates a request message without first
having received a SIP message that triggered the transmission of the
message (e.g., sending a BYE message to terminate a call for policy
reasons), the intermediary MUST, if it has knowledge of the UUID
values for the two communicating endpoints, insert a Session-ID
header field with the "remote-uuid" field of the header value set to
the UUID value of the receiving endpoint and the "local-uuid" field
of the header value set to the UUID value of the other endpoint.
When the intermediary does not have knowledge of the UUID value of an
endpoint in the communication session, the intermediary SHOULD set
the unknown UUID value(s) to the "nil" UUID value. (If both are
unknown, the Session-ID header value SHOULD NOT be included at all,
since it would have no practical value.)
With respect to the previous two paragraphs, note that if an
intermediary transmits a "nil" UUID value, the receiving endpoint
might use that value in subsequent messages it sends. This
effectively violates the requirement of maintaining an end-to-end
session identifier value for the communication session if a UUID for
the peer endpoint had been previously conveyed. Therefore, an
intermediary MUST only send the "nil" UUID when the intermediary has
not communicated with the peer endpoint to learn its UUID. This
means that intermediaries SHOULD maintain state related to the UUID
values for both ends of a communication session if it intends to
originate messages (versus merely conveying messages). An
intermediary that does not maintain this state and that originates a
message as described in the previous two paragraphs MUST NOT insert a
Session-ID header field in order to avoid unintended, incorrect
reassignment of a UUID value.
The Session-ID header field value included in a CANCEL request MUST
be identical to the Session-ID header field value included in the
corresponding request being cancelled.
If a SIP intermediary initiates a dialog between two endpoints in a
third-party call control (3PCC [RFC3725]) scenario, the initial
INVITE request will have a non-nil, locally fabricated "local-uuid"
value; call this temporary UUID "X". The request will still have a
nil "remote-uuid" value; call this value "N". The SIP server MUST be
transaction-stateful. The UUID pair in the INVITE request will be
{X,N}. A 1xx or 2xx response will have a UUID pair {A,X}. This
transaction-stateful, dialog-initiating SIP server MUST replace its
own UUID, i.e.,"X", with a nil UUID (i.e., {A,N}) in the INVITE
request sent towards the other UAS as expected (see Section 10.7 for
an example).
Intermediaries that manipulate messages containing a Session-ID
header field SHOULD be aware of what UUID values it last sent towards
an endpoint and, following any kind of service interaction initiated
or affected by the intermediary, what UUID values the receiving
endpoint should have knowledge of to ensure that both endpoints in
the session have the correct and same UUID values. If an
intermediary can determine that an endpoint might not have received a
current, correct Session-ID field, the intermediary SHOULD attempt to
provide the correct Session-ID header field to the endpoint such as
by sending a re-INVITE request. Failure to take such measures may
make troubleshooting more difficult because of the mismatched
identifiers; therefore, it is strongly advised that intermediaries
attempt to provide the correct session identifier if able to do so.
If an intermediary receives a SIP response with a non-nil "local-
uuid" that is not 32 octets long, this response comes from a
misbehaving implementation, and its Session-ID header field MUST be
discarded. That said, the response might still be valid according to
the rules within SIP [RFC3261], and it SHOULD be checked further.
An intermediary MUST assume that the UUID value of session peers may
change at any time due to service interactions and MAY itself change
UUID values for sessions under its control to ensure that end-to-end
session identifiers are consistent for all participants in a session.
Section 8 discusses how intermediaries must handle remote UUID
changes if they maintain state of the session identifier.
An intermediary may perform protocol interworking between different
IP-based communications systems, e.g., interworking between H.323 and
SIP. If the intermediary supports the session identifier for both
protocols for which it is interworking, it SHOULD pass the identifier
between the two call legs to maintain an end-to-end identifier,
regardless of protocol.
8. Handling of Remote UUID Changes
It is desirable to have all endpoints and intermediaries involved in
a session agree upon the current session identifier when these
changes occur. Due to race conditions or certain interworking
scenarios, it is not always possible to guarantee session identifier
consistency; however, in an attempt to ensure the highest likelihood
of consistency, all endpoints and intermediaries involved in a
session MUST accept a peer's new UUID under the following conditions:
o When an endpoint or intermediary receives a mid-dialog request
containing a new UUID from a peer, all responses to that request
MUST contain the new UUID value as the "remote" parameter unless a
subsequent successful transaction (for example, an UPDATE)
contains a different UUID, in which case, the newest UUID MUST be
used.
o If an endpoint or intermediary sends a successful (2xx) or
redirection (3xx) response to the request containing the new UUID
value, the endpoint or intermediary MUST accept the peer's UUID
and include this new UUID as the "remote" parameter for any
subsequent messages unless the UUID from a subsequent transaction
has already been accepted. The one exception is a CANCEL request,
as outlined below.
o If the endpoint or intermediary sends a failure (4xx, 5xx, or 6xx)
response, it MUST NOT accept the new UUID value and any subsequent
messages MUST contain the previously stored UUID value in the
"remote" parameter for any subsequent message. Note that the
failure response itself will contain the new UUID value from the
request in the "remote" parameter.
o When an endpoint or intermediary receives an ACK for a successful
(2xx) or redirection (3xx) response with a new UUID value, it MUST
accept the peer's new UUID value and include this new UUID as the
"remote" parameter for any subsequent messages. If the ACK is for
a failure (4xx, 5xx, or 6xx) response, the new value MUST NOT be
used.
o As stated in Sections 6 and 7, the Session-ID header field value
included in a CANCEL request MUST be identical to the Session-ID
header field value included in the corresponding INVITE request.
Upon receiving a CANCEL request, an endpoint or intermediary would
normally send a 487 Request Terminated response (see
Section 15.1.2 of [RFC3261]) which, by the rules outlined above,
would result in the endpoint or intermediary not storing any UUID
value contained in the CANCEL request. Section 3.8 of [RFC6141]
specifies conditions where a CANCEL request can result in a 2xx
response. Because a CANCEL request is not passed end-to-end and
will always contain the UUID from the original INVITE request,
retaining a new UUID value received in a CANCEL request may result
in inconsistency with the Session-ID value stored on the endpoints
and intermediaries involved in the session. To avoid this
situation, an endpoint or intermediary MUST NOT accept the new
UUID value received in a CANCEL request and any subsequent
messages MUST contain the previously stored UUID value in the
"remote" parameter". Note that the response to the CANCEL request
will contain the UUID value from the CANCEL request in the
"remote" parameter.
o When an endpoint or intermediary receives a response containing a
new UUID from a peer, the endpoint or intermediary MUST accept the
new UUID as the peer's UUID and include this new UUID as the
"remote" parameter for any subsequent messages.
When an intermediary accepts a new UUID from a peer, the intermediary
SHOULD attempt to provide the correct Session-ID header field to
other endpoints involved in the session, for example, by sending a
re-INVITE request. If an intermediary receives a message with a
"remote" parameter in the session identifier that does not match the
updated UUID, the intermediary MUST update the "remote" parameter
with the latest stored UUID.
If an intermediary is performing interworking between two different
protocols that both support the session identifier defined in this
document (e.g., SIP to H.323), UUID changes SHOULD be communicated
between protocols to maintain the end-to-end session identifier.
9. Associating Endpoints in a Multipoint Conference
Multipoint Control Units (MCUs) group two or more sessions into a
single multipoint conference and have a conference focus responsible
for maintaining the dialogs connected to it [RFC4353]. MCUs,
including cascaded MCUs, MUST utilize the same UUID value ("local-
uuid" portion of the Session-ID header field value) with all
participants in the conference. In so doing, each individual session
in the conference will have a unique session identifier (since each
endpoint will create a unique UUID of its own), but will also have
one UUID in common with all other participants in the conference.
When creating a cascaded conference, an MCU MUST convey the UUID
value to be utilized for a conference via the "local-uuid" portion of
the Session-ID header field value in an INVITE request to a second
MCU when using SIP to establish the cascaded conference. A
conference bridge, or MCU, needs a way to identify itself when
contacting another MCU. [RFC4579] defines the "isfocus" Contact
header field value parameter just for this purpose. The initial MCU
MUST include the UUID of that particular conference in the "local-
uuid" of an INVITE request to the other MCU(s) participating in that
conference. Also included in this INVITE request is an "isfocus"
Contact header field value parameter identifying that this INVITE
request is coming from an MCU, and that this UUID is to be given out
in all responses from endpoints into those MCUs participating in this
same conference. This ensures that a single UUID is common across
all participating MCUs of the same conference, but that it is unique
between different conferences.
In the case where two existing conferences are joined, there should
be a session between the two MCUs where the session identifier is
comprised of the UUID values of the two conferences. This session
identifier can be used to correlate the sessions between participants
in the joined conference. This specification does not impose any
additional requirements when two existing conferences are joined.
Intermediary devices or network-diagnostic equipment might assume
that when they see two or more sessions with different session
identifiers but with one UUID in common, the sessions are part of the
same conference. However, the assumption that two sessions having
one common UUID being part of the same conference is not always
correct. In a SIP-forking scenario, for example, there might also
exist what appears to be multiple sessions with a shared UUID value;
this is intended. The desire is to allow for the association of
related sessions, regardless of whether a session is forked or part
of a conference.
10. Examples of Various Call Flow Operations
Seeing something frequently makes understanding easier. With that in
mind, this section includes several call flow examples with the
initial UUID and the complete session identifier indicated per
message, as well as examples of when the session identifier changes
according to the rules within this document during certain
operations/functions.
This section is for illustrative purposes only and is non-normative.
In the following flows, "RTP" refers to the Real-time Transport
Protocol [RFC3550].
In the examples in this section, "N" represents a nil UUID and other
letters represent the unique UUID values corresponding to endpoints
or MCUs.
10.1. Basic Call with Two UUIDs
Session-ID
--- Alice B2BUA Bob Carol
{A,N} |---INVITE F1--->| |
{A,N} | |---INVITE F2--->|
{B,A} | |<---200 OK F3---|
{B,A} |<---200 OK F4---| |
{A,B} |-----ACK F5---->| |
{A,B} | |-----ACK F6---->|
|<==============RTP==============>|
Figure 1: Session-ID Creation When Alice Calls Bob
General operation of this example:
o UA-Alice populates the "local-uuid" portion of the Session-ID
header field value.
o UA-Alice sends its UUID in the SIP INVITE request and populates
the "remote" parameter with a nil value (32 zeros).
o The B2BUA receives an INVITE request with both a "local-uuid"
portion of the Session-ID header field value from UA-Alice as well
as the nil "remote-uuid" value and transmits the INVITE request
towards UA-Bob with an unchanged Session-ID header field value.
o UA-Bob receives the Session-ID and generates its "local-uuid"
portion of the Session-ID header field value UUID to construct the
whole/complete Session-ID header field value, at the same time
transferring UA-Alice's UUID unchanged to the "remote-uuid"
portion of the Session-ID header field value in the 200 OK SIP
response.
o The B2BUA receives the 200 OK response with a complete Session-ID
header field value from UA-Bob and transmits the 200 OK response
towards UA-Alice with an unchanged Session-ID header field value.
o UA-Alice, upon reception of the 200 OK from the B2BUA, transmits
the ACK towards the B2BUA. The construction of the Session-ID
header field in this ACK is that of UA-Alice's UUID is the "local-
uuid", and UA-Bob's UUID populates the "remote-uuid" portion of
the header-value.
o The B2BUA receives the ACK with a complete Session-ID header field
from UA-Alice and transmits the ACK towards UA-Bob with an
unchanged Session-ID header field value.
Below is a SIP message exchange illustrating proper use of the
Session-ID header field. For the sake of brevity, non-essential
headers and message bodies are omitted.
F1 INVITE Alice -> B2BUA
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.example.com
;branch=z9hG4bK776asdhds
Max-Forwards: 70
To: Bob <sip:bob@biloxi.example.com>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.example.com
Session-ID: ab30317f1a784dc48ff824d0d3715d86
;remote=00000000000000000000000000000000
CSeq: 314159 INVITE
Contact: <sip:alice@pc33.atlanta.example.com>
Content-Type: application/sdp
Content-Length: 142
(Alice's SDP not shown)
F2 INVITE B2BUA -> Bob
INVITE sip:bob@192.168.10.20 SIP/2.0
Via: SIP/2.0/UDP server10.biloxi.example.com
;branch=z9hG4bK4b43c2ff8.1
Via: SIP/2.0/UDP pc33.atlanta.example.com
;branch=z9hG4bK776asdhds;received=10.1.3.33
Max-Forwards: 69
To: Bob <sip:bob@biloxi.example.com>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.example.com
Session-ID: ab30317f1a784dc48ff824d0d3715d86
;remote=00000000000000000000000000000000
CSeq: 314159 INVITE
Contact: <sip:alice@pc33.atlanta.example.com>
Record-Route: <sip:server10.biloxi.example.com;lr>
Content-Type: application/sdp
Content-Length: 142
(Alice's SDP not shown)
F3 200 OK Bob -> B2BUA
SIP/2.0 200 OK
Via: SIP/2.0/UDP server10.biloxi.example.com
;branch=z9hG4bK4b43c2ff8.1;received=192.168.10.1
Via: SIP/2.0/UDP pc33.atlanta.example.com
;branch=z9hG4bK776asdhds;received=10.1.3.33
To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.example.com
Session-ID: 47755a9de7794ba387653f2099600ef2
;remote=ab30317f1a784dc48ff824d0d3715d86
CSeq: 314159 INVITE
Contact: <sip:bob@192.168.10.20>
Record-Route: <sip:server10.biloxi.example.com;lr>
Content-Type: application/sdp
Content-Length: 131
(Bob's SDP not shown)
F4 200 OK B2BUA -> Alice
SIP/2.0 200 OK
Via: SIP/2.0/UDP pc33.atlanta.example.com
;branch=z9hG4bK776asdhds;received=10.1.3.33
To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.example.com
Session-ID: 47755a9de7794ba387653f2099600ef2
;remote=ab30317f1a784dc48ff824d0d3715d86
CSeq: 314159 INVITE
Contact: <sip:bob@192.168.10.20>
Record-Route: <sip:server10.biloxi.example.com;lr>
Content-Type: application/sdp
Content-Length: 131
(Bob's SDP not shown)
F5 ACK Alice -> B2BUA
ACK sip:bob@192.168.10.20 SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.example.com
;branch=z9hG4bKnashds8
Route: <sip:server10.biloxi.example.com;lr>
Max-Forwards: 70
To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.example.com
Session-ID: ab30317f1a784dc48ff824d0d3715d86
;remote=47755a9de7794ba387653f2099600ef2
CSeq: 314159 ACK
Content-Length: 0
F6 ACK B2BUA -> Bob
ACK sip:bob@192.168.10.20 SIP/2.0
Via: SIP/2.0/UDP server10.biloxi.example.com
;branch=z9hG4bK4b43c2ff8.2
Via: SIP/2.0/UDP pc33.atlanta.example.com
;branch=z9hG4bKnashds8;received=10.1.3.33
Max-Forwards: 70
To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.example.com
Session-ID: ab30317f1a784dc48ff824d0d3715d86
;remote=47755a9de7794ba387653f2099600ef2
CSeq: 314159 ACK
Content-Length: 0
The remaining examples in this section do not display the complete
SIP message exchange. Instead, they simply use the set notation
described in Section 4.2 to show the session identifier exchange
throughout the particular call flow being illustrated.
10.2. Basic Call Transfer Using REFER
From the example built within Section 10.1, we proceed to this 'Basic
Call Transfer using REFER' example. Note that this is a mid-dialog
REFER in contrast with the out-of-dialog REFER in Section 10.9.
Session-ID
--- Alice B2BUA Bob Carol
| | | |
|<==============RTP==============>| |
{B,A} | |<---re-INVITE---| |
{B,A} |<---re-INVITE---| (puts Alice on Hold) |
{A,B} |-----200 OK---->| | |
{A,B} | |-----200 OK---->| |
{B,A} | |<-----ACK-------| |
{B,A} |<-----ACK-------| | |
| | | |
{B,A} | |<----REFER------| |
{B,A} |<----REFER------| | |
{A,B} |-----200 OK---->| | |
{A,B} | |-----200 OK---->| |
{A,B} |-----NOTIFY---->| | |
{A,B} | |-----NOTIFY---->| |
{B,A} | |<----200 OK-----| |
{B,A} |<----200 OK-----| | |
| | | |
{A,N} |-----INVITE---->| |
{A,N} | |-----INVITE-------------------->|
{C,A} | |<----200 OK---------------------|
{C,A} |<----200 OK-----| |
{A,C} |------ACK------>| |
{A,C} | |------ACK---------------------->|
| | | |
|<======================RTP======================>|
| | | |
{A,B} |-----NOTIFY---->| | |
{A,B} | |-----NOTIFY---->| |
{B,A} | |<----200 OK-----| |
{B,A} |<----200 OK-----| | |
{B,A} | |<-----BYE-------| |
{B,A} |<-----BYE-------| | |
{A,B} |-----200 OK---->| | |
{A,B} | |-----200 OK---->| |
| | | |
Figure 2: Call Transfer Using REFER
General operation of this example:
Starting from the existing Alice/Bob call described in Figure 1 of
this document, which established an existing Session-ID header field
value:
o UA-Bob requests Alice to call Carol, using a REFER transaction, as
described in [RFC3515]. UA-Alice is initially put on hold, then
told in the REFER who to contact with a new INVITE, in this case
UA-Carol. This Alice-to-Carol dialog will have a new Call-ID;
therefore, it requires a new Session-ID header field value. The
wrinkle here is we can, and will, use Alice's UUID from her
existing dialog with Bob in the new INVITE request to Carol.
o UA-Alice retains her UUID from the Alice-to-Bob call {A} when
requesting a call with UA-Carol. This is placed in the "local-
uuid" portion of the Session-ID header field value, at the same
time inserting a nil "remote-uuid" value (because Carol's UA has
not yet received the UUID value). This same UUID traverses the
B2BUA unchanged.
o UA-Carol receives the INVITE request with a session identifier
UUID {A,N}, replaces the "A" UUID value into the "remote-uuid"
portion of the Session-ID header field value and creates its own
UUID {C}, and places this value in the "local-uuid" portion of the
Session-ID header field value, thereby removing the "N" (nil)
value altogether. This combination forms a full session
identifier {C,A} in the 200 OK to the INVITE. This Session-ID
header field traverses the B2BUA unchanged towards UA-Alice.
o UA-Alice receives the 200 OK with the session identifier {C,A} and
responds to UA-Carol with an ACK (just as in Figure 1, this
switches the places of the two UUID fields), and generates a
NOTIFY request to Bob with a session identifier {A,B} indicating
that the call transfer was successful.
o It does not matter which UA terminates the Alice-to-Bob call;
Figure 2 shows UA-Bob terminating the call.
10.3. Basic Call Transfer Using Re-INVITE
From the example built within Section 10.1, we proceed to this 'Basic
Call Transfer using re-INVITE' example.
Alice is talking to Bob. Bob pushes a button on his phone to
transfer Alice to Carol via the B2BUA (using re-INVITE).
Session-ID
--- Alice B2BUA Bob Carol
| | | |
|<==============RTP==============>| |
| | | |
| | <--- (non-standard signaling) |
{A,B} | |---re-INVITE--->| |
{B,A} | |<-----200 OK----| |
{A,B} | |-----ACK------->| |
| | | |
{A,N} | |-----INVITE-------------------->|
{C,A} | |<----200 OK---------------------|
{A,C} | |------ACK---------------------->|
| | | |
|<======================RTP======================>|
| | | |
{A,B} | |------BYE------>| |
{B,A} | |<----200 OK-----| |
| | | |
{C,A} |<--re-INVITE----| | |
{A,C} |----200 OK----->| | |
{C,A} |<-----ACK-------| | |
| (Suppose Alice modifies the session) |
{A,C} |---re-INVITE--->| | |
{A,C} | |---re-INVITE------------------->|
{C,A} | |<---200 OK----------------------|
{C,A} |<---200 OK------| | |
{A,C} |------ACK------>| | |
{A,C} | |------ACK---------------------->|
| | | |
Figure 3: Call Transfer Using Re-INVITE
General operation of this example:
o We assume the call between Alice and Bob from Section 10.1 is
operational with session identifier {A,B}.
o Bob uses non-standard signaling to the B2BUA to initiate a call
transfer from Alice to Carol. This could also be initiated via a
REFER message from Bob, but the signaling that follows might still
be similar to the above flow. In either case, Alice is completely
unaware of the call transfer until a future point in time when
Alice receives a message from Carol.
o The B2BUA sends a re-INVITE request with the session identifier
{"local-uuid" = "A", "remote-uuid" = "B"} to renegotiate the
session with Bob.
o The B2BUA sends a new INVITE request with Alice's UUID {"local-
uuid" = "A"} to Carol.
o Carol receives the INVITE request and accepts the request and adds
her UUID {C} to the session identifier for this session {"local-
uuid" = "C", "remote-uuid" = "A"}.
o The B2BUA then terminates the call to Bob with a BYE using the
session identifier {"local-uuid" = "A", "remote-uuid" = "B"}.
o The B2BUA sends a re-INVITE request to Alice to update Alice's
view of the session identifier.
o When Alice later attempts to modify the session with a re-INVITE,
Alice will send "remote-uuid" = "C" toward Carol because it had
previously received the updated UUID in the re-INVITE request from
the B2BUA. The B2BUA maintains the session identifier {"local-
uuid" = "A", "remote-uuid" = "C"}. Carol replies with the "local-
uuid" = "C", "remote-uuid" = "A" to reflect what was received in
the INVITE request (which Carol already knew from previous
exchanges with the B2BUA). Alice then includes "remote-uuid" =
"C" in the subsequent ACK message.
10.4. Single Focus Conferencing
Multiple users call into a conference server (for example, an MCU) to
attend one of many conferences hosted on or managed by that server.
Each user has to identify which conference they want to join, but
this information is not necessarily in the SIP messaging. It might
be done by having a dedicated address for the conference or via an
Interactive Voice Response (IVR), as assumed in this example and
depicted with the use of M1, M2, and M3. Each user in this example
goes through a two-step process of signaling to gain entry onto their
conference call, which the conference focus identifies as "M".
Session-ID Conference
--- Alice Focus Bob Carol
| | | |
| | | |
{A,N} |----INVITE----->| | |
{M1,A} |<---200 OK------| | |
{A,M1} |-----ACK------->| | |
|<====RTP=======>| | |
{M',A} |<---re-INVITE---| | |
{A,M'} |-----200 OK---->| | |
{M',A} |<-----ACK-------| | |
| | | |
| | | |
{B,N} | |<----INVITE-----| |
{M2,B} | |-----200 OK---->| |
{B,M2} | |<-----ACK-------| |
| |<=====RTP======>| |
{M',B} | |---re-INVITE--->| |
{B,M'} | |<----200 OK-----| |
{M',B} | |------ACK------>| |
| | | |
| | | |
{C,N} | |<--------------------INVITE-----|
{M3,C} | |---------------------200 OK---->|
{C,M3} | |<---------------------ACK-------|
| |<=====================RTP======>|
{M',C} | |-------------------re-INVITE--->|
{C,M'} | |<--------------------200 OK-----|
{M',C} | |----------------------ACK------>|
Figure 4: Single Focus Conference Bridge
General operation of this example:
Alice calls into a conference server to attend a certain conference.
This is a two-step operation since Alice cannot include the
conference ID at this time and/or any passcode in the INVITE request.
The first step is Alice's UA calling another UA to participate in a
session. This will appear to be similar as the call flow in Figure 1
(in Section 10.1). What is unique about this call is the second
step: the conference server sends a re-INVITE request with its second
UUID, but maintaining the UUID Alice sent in the first INVITE. This
subsequent UUID from the conference server will be the same for each
UA that calls into this conference server participating in this same
conference bridge/call, which is generated once Alice typically
authenticates and identifies which bridge she wants to participate
on.
o Alice sends an INVITE request to the conference server with her
UUID {A} and a "remote-uuid" = "N".
o The conference server responds with a 200 OK response, which
replaces the "N" UUID with a temporary UUID ("M1") as the "local-
uuid" and a "remote-uuid" = "A".
NOTE: this 'temporary' UUID is a real UUID; it is only temporary to
the conference server because it knows that it is going to generate
another UUID to replace the one just sent in the 200 OK response.
o Once Alice, the user, gains access to the IVR for this conference
server, she enters a specific conference ID and whatever passcode
(if needed) to enter a specific conference call.
o Once the conference server is satisfied Alice has identified which
conference she wants to attend (including any passcode
verification), the conference server re-INVITEs Alice to the
specific conference and includes the Session-ID header field value
component "local-uuid" = "M'" (and "remote-uuid" = "A") for that
conference. All valid participants in the same conference will
receive this same UUID for identification purposes and to better
enable monitoring and tracking functions.
o Bob goes through this two-step process of an INVITE transaction,
followed by a re-INVITE transaction to get this same UUID ("M'")
for the conference.
o In this example, Carol (and each additional user) goes through the
same procedures as Alice and Bob to get on this same conference.
10.5. Single Focus Conferencing Using a Web-Based Conference Service
Alice, Bob, and Carol call into the same web-based conference. Note
that this is one of many ways of implementing this functionality, and
it should not be construed as the preferred way of establishing a
web-based conference.
Session-ID Conference
--- Alice Focus Bob Carol
| | | |
|<** HTTPS *****>| | |
| Transaction | | |
| | | |
{M,N} |<----INVITE-----| | |
{A,M} |-----200 OK---->| | |
{M,A} |<-----ACK-------| | |
|<=====RTP======>| | |
| | | |
| |<** HTTPS *****>| |
| | Transaction | |
| | | |
{M,N} | |-----INVITE---->| |
{B,M} | |<----200 OK-----| |
{M,B} | |------ACK------>| |
| |<=====RTP======>| |
| | | |
| |<****************** HTTPS *****>|
| | Transaction |
| | | |
{M,N} | |--------------------INVITE----->|
{C,M} | |<-------------------200 OK------|
{M,C} | |---------------------ACK------->|
| |<====================RTP=======>|
Figure 5: Single Focus Web-Based Conference
General operation of this example:
o Alice communicates with the web server that she wants to join a
certain meeting by using a meeting number and including UA-Alice's
contact information (phone number, URI, and/or IP address, etc.)
for each device she wants for this conference call. For example,
the audio and video (A/V) play-out devices could be separate
units.
o The Conference Focus server sends the INVITE request (Session-ID
header field value components "local-uuid" = "M" and a remote UUID
of "N", where "M" equals the "local-uuid" for each participant on
this conference bridge) to UA-Alice to start a session with that
server for this A/V conference call.
o Upon receiving the INVITE request from the conference focus
server, Alice responds with a 200 OK. Her UA moves the "local-
uuid" unchanged into the "remote-uuid" field, generates her own
UUID, and places that into the "local-uuid" field to complete the
Session-ID construction.
o Bob and Carol perform same function to join this same A/V
conference call as Alice.
10.6. Cascading Conference Bridges
10.6.1. Establishing a Cascaded Conference
Expanding conferencing capabilities requires cascading conference
bridges. A conference bridge, or MCU, needs a way to identify itself
when contacting another MCU. [RFC4579] defines the "isfocus" Contact
header field value parameter just for this purpose.
Session-ID
--- MCU-1 MCU-2 MCU-3 MCU-4
| | | |
{M',N} |----INVITE----->| | |
{J,M'} |<---200 OK------| | |
{M',J} |-----ACK------->| | |
Figure 6: MCUs Communicating Session Identifier UUID for Bridge
Regardless of which MCU (1 or 2) a UA contacts for this conference,
once the above exchange has been received and acknowledged, the UA
will get the same {M',N} UUID pair from the MCU for the complete
session identifier.
A more complex form would be a series of MCUs all being informed of
the same UUID to use for a specific conference. This series of MCUs
can be informed in one of two ways:
o All by one MCU (that initially generates the UUID for the
conference).
o The MCU that generates the UUID informs one or several MCUs of
this common UUID, and then they inform downstream MCUs of this
common UUID that each will be using for this one conference.
Session-ID
--- MCU-1 MCU-2 MCU-3 MCU-4
| | | |
{M',N} |----INVITE----->| | |
{J,M'} |<---200 OK------| | |
{M',J} |-----ACK------->| | |
| | | |
{M',N} |---------------------INVITE----->| |
{K,M'} |<--------------------200 OK------| |
{M',K} |----------------------ACK------->| |
| | | |
{M',N} |-------------------------------------INVITE----->|
{L,M'} |<------------------------------------200 OK------|
{M',L} |--------------------------------------ACK------->|
Figure 7: MCU Communicating
Session Identifier UUID to More Than One MCU
General operation of this example:
o The MCU generating the session identifier UUID communicates this
in a separate INVITE, having a Contact header with the "isfocus"
Contact header field value parameter. This will identify the MCU
as what [RFC4579] calls a "conference-aware" SIP entity.
o An MCU that receives this {M',N} UUID pair in an inter-MCU
transaction can communicate the M' UUID in a manner in which it
was received to construct a hierarchical cascade (though this time
this second MCU would be the UAC MCU).
o Once the conference is terminated, the cascaded MCUs will receive
a BYE message to terminate the cascade.
10.6.2. Calling Into Cascaded Conference Bridges
Here is an example of how a UA, Robert for example, calls into a
cascaded conference focus. Because MCU-1 has already contacted MCU-3
(the MCU where Robert is going to join the conference), MCU-3 already
has the Session-ID (M') for this particular conference call.
Session-ID
--- MCU-1 MCU-2 MCU-3 Robert
| | | |
{M',N} |----INVITE----->| | |
{J,M'} |<---200 OK------| | |
{M',J} |-----ACK------->| | |
| | | |
{M',N} |---------------------INVITE----->| |
{K,M'} |<--------------------200 OK------| |
{M',K} |----------------------ACK------->| |
| | | |
{R,N} | | |<---INVITE-----|
(M',R} | | |----200 OK---->|
{R,M'} | | |<----ACK-------|
Figure 8: A UA Calling Into a Cascaded MCU UUID
General operation of this example:
o The UA, Robert in this case, INVITEs the MCU to join a particular
conference call. Robert's UA does not know anything about whether
this is the main MCU of the conference call or a cascaded MCU.
Robert likely does not know MCUs can be cascaded, he just wants to
join a particular call. As is the case with any standard
implementation, he includes a nil "remote-uuid".
o The cascaded MCU, upon receiving this INVITE request from Robert,
replaces the nil UUID with the UUID value communicated from MCU-1
for this conference call as the "local-uuid" in the SIP response,
thus moving Robert's UUID "R" to the "remote-uuid" value.
o The ACK has the Session-ID {R,M'}, completing the three-way
handshake for this call establishment. Robert has now joined the
conference call originated from MCU-1.
o Once the conference is terminated, the cascaded MCUs will receive
a BYE message to terminate the cascade.
10.7. Basic 3PCC for Two UAs
An external entity sets up calls to both Alice and Bob for them to
talk to each other.
Session-ID
--- Alice B2BUA Bob Carol
| | |
{X,N} |<----INVITE-----| |
{A,X} |-----200 OK---->| |
{A,N} | |----INVITE----->|
{B,A} | |<---200 OK------|
{B,A} |<-----ACK-------| |
{A,B} | |------ACK------>|
|<==============RTP==============>|
Figure 9: 3PCC-Initiated Call between Alice and Bob
General operation of this example:
o Some out-of-band procedure directs a B2BUA (or other SIP server)
to have Alice and Bob talk to each other. In this case, the SIP
server has to be transaction stateful, if not dialog stateful.
o The SIP server INVITEs Alice to a session and uses a temporary
UUID {X} and a nil UUID pairing.
o Alice receives and accepts this call setup and replaces the nil
UUID with her UUID {A} in the session identifier, now {A,X}.
o The transaction-stateful SIP server receives Alice's UUID {A} in
the local UUID portion and keeps it there; and it discards its own
UUID {X}, replacing this with a nil UUID value in the INVITE
request to Bob as if this came from Alice originally.
o Bob receives and accepts this INVITE request and adds his own UUID
{B} to the session identifier, now {B,A}, for the response.
o The session is established.
10.8. Handling in 100 Trying SIP Response and CANCEL Request
The following two subsections show examples of the session identifier
for a 100 Trying response and a CANCEL request in a single call flow.
10.8.1. Handling in a 100 Trying SIP Response
The following 100 Trying response is taken from [RFC5359],
Section 2.9 ("Call Forwarding - No Answer").
Session-ID Alice SIP Server Bob-1 Bob-2
| | | |
{A,N} |----INVITE----->| | |
{A,N} | |---INVITE---->| |
{N,A} |<--100 Trying---| | |
{B1,A} | |<-180 Ringing-| |
{B1,A} |<--180 Ringing--| | |
| | | |
| *Request Timeout* |
| | | |
{A,N} | |---CANCEL---->| |
{B1,A} | |<--200 OK-----| |
{B1,A} | |<---487-------| |
{A,B1} | |---- ACK ---->| |
| | | |
{N,A} |<-181 Call Fwd--| | |
| | | |
{A,N} | |------------------INVITE------>|
{B2,A} | |<----------------180 Ringing---|
{B2,A} |<-180 Ringing---| | |
{B2,A} | |<-----------------200 OK ------|
{B2,A} |<--200 OK-------| | |
{A,B2} |----ACK-------->| | |
{A,B2} | |------------------ACK--------->|
| | | |
|<=========== Both way RTP Established =========>|
| | | |
{A,B2} |----BYE-------->| | |
{A,B2} | |--------------------BYE------->|
{B2,A} | |<------------------200 OK------|
{B2,A} |<--200 OK-------| | |
| | | |
Figure 10: Session Identifier in the 100 Trying and CANCEL Messaging
Below is the explanatory text from RFC 5359, Section 2.9, detailing
what the desired behavior is in the above call flow (i.e., what the
call flow is attempting to achieve).
Bob wants calls to B1 forwarded to B2 if B1 is not answered
(information is known to the SIP server). Alice calls B1, and no
one answers. The SIP server then places the call to B2.
General operation of this example:
o Alice generates an INVITE request because she wants to invite Bob
to join her session. She creates a UUID as described in
Section 10.1, and she places that value in the "local-uuid" field
of the Session-ID header field value. Alice also generates a
"remote-uuid" of nil and sends this along with the "local-uuid".
o The SIP server (imagine this is a B2BUA), upon receiving Alice's
INVITE request, generates the optional provisional response 100
Trying. Since the SIP server has no knowledge of Bob's UUID for
his part of the session identifier value, it cannot include his
"local-uuid". Rather, any 100 Trying response includes Alice's
UUID in the "remote-uuid" portion of the Session-ID header-value
with a nil "local-uuid" value in the response. This is consistent
with what Alice's UA expects to receive in any SIP response
containing this UUID.
10.8.2. Handling a CANCEL SIP Request
In the same call flow example as the 100 Trying response is a CANCEL
request. Please refer to Figure 10 for the CANCEL request example.
General operation of this example:
o In Figure 10 above, Alice generates an INVITE request with her
UUID value in the Session-ID header field.
o Bob-1 responds to this INVITE request with a 180 Ringing. In that
response, he includes his UUID in the Session-ID header field
value (i.e., {B1,A}); thus completing the Session-ID header field
for this session, even though no final response has been generated
by any of Bob's UAs.
o While this means that if the SIP server were to generate a SIP
request within this session it could include the complete
SessionID, the server sends a CANCEL request and a CANCEL request
always uses the same Session-ID header field as the original
INVITE request. Thus, the CANCEL request would have a session
identifier with the "local-uuid" = "A", and the "remote-uuid" =
"N".
o As it happens with this CANCEL, the SIP server intends to invite
another UA of Bob's (i.e., B2) for Alice to communicate with.
o In this example call flow, taken from RFC 5359, Section 2.9, a 181
Call is Being Forwarded response is sent to Alice. Since the SIP
server generated this SIP request, and has no knowledge of Bob-2's
UUID value, it cannot include that value in this 181. Thus, and
for the exact reasons the 100 Trying including the session
identifier value, only Alice's UUID is included in the remote-uuid
component of the Session-ID header field value, with a nil UUID
present in the "local-uuid" component.
10.9. Out-of-Dialog REFER Transaction
The following call flow was extracted from Section 6.1 of [RFC5589]
("Successful Transfer"), with the only changes being the names of the
UAs to maintain consistency within this document.
Alice is the transferee
Bob is the transferer
and Carol is the transfer-target
Session-ID Bob Alice Carol
| | |
{A,N} |<-----INVITE--------| |
{B,A} |------200 OK------->| |
{A,B} |<------ACK----------| |
| | |
{B,A} |--INVITE {hold}---->| |
{A,B} |<-200 OK------------| |
{B,A} |--- ACK ----------->| |
| | |
{B,A} |--REFER------------>|(Refer-To:Carol) |
{A,B} |<-202 Accepted------| |
| | |
{A,B} |<NOTIFY {100 Trying}| |
{B,A} |-200 OK------------>| |
| | |
{A,N} | |--INVITE------------>|
{C,A} | |<-200 OK-------------|
{A,C} | |---ACK-------------->|
| | |
{A,B} |<--NOTIFY {200 OK}--| |
{B,A} |---200 OK---------->| |
| | |
{B,A} |--BYE-------------->| |
{A,B} |<-200 OK------------| |
{C,A} | |<------------BYE-----|
{A,C} | |-------------200 OK->|
Figure 11: Out-Of-Dialog Call Transfer
General operation of this example:
o Just as in Section 10.2, Figure 2, Alice invites Bob to a session,
and Bob eventually transfers Alice to communicate with Carol.
o What is different about the call flow in Figure 11 is that Bob's
REFER is not in-dialog. Even so, this is treated as part of the
same communication session and, thus, the session identifier in
those messages is {A,B}.
o Alice will use her existing UUID and the nil UUID ({A,N}) in the
INVITE request towards Carol (who generates UUID "C" for this
session), thus maintaining the common UUID within the session
identifier for this new Alice-to-Carol session.
11. Compatibility with a Previous Implementation
There is a much earlier document that specifies the use of a Session-
ID header field (namely, [RFC7329]) that we will herewith attempt to
achieve backwards compatibility. Neither Session-ID header field has
any versioning information, so merely adding that this document
describes "version 2" is insufficient. This section contains the set
of rules for compatibility between the two specifications. Although
the previous version was never standardized, it has been heavily
implemented and adopted by other standards development organizations.
For the purposes of this discussion, we will label the pre-standard
specification of the Session-ID as the "old" version and this
specification as the "new" version of the Session-ID.
The previous (i.e., "old") version only has a single UUID value as a
Session-ID header field value, but has a generic-parameter value that
can be of use.
In order to have an "old" version talk to an "old" version
implementation, nothing needs to be done as far as the IETF is
concerned.
In order to have a "new" version talk to a "new" version
implementation, both implementations need to follow this document (to
the letter) and everything should be just fine.
For this "new" implementation to work with the "old" implementation
and an "old" implementation to work with "new" implementations, there
needs to be a set of rules that all "new" implementations MUST follow
if the "new" implementation will be communicating with devices that
have implemented the "old" implementation.
o Since no option tags or feature tags are to be used for
distinguishing versions, the presence and order of any "remote-
uuid" value within the Session-ID header field value is to be used
to distinguish implementation versions.
o If a SIP request has a "remote-uuid" value, this comes from a
standard implementation, and not a pre-standard one.
o If a SIP request has no "remote-uuid" value, this comes from a
pre-standard implementation, and not a standard one. In this
case, one UUID is used to identify this dialog, even if the
responder is a standard implementor of this specification.
o If a SIP response has a non-nil "local-uuid" that is 32 octets
long and differs from the endpoint's own UUID value, this response
comes from a standard implementation.
o If a SIP response arrives that has the same value of Session-ID
UUIDs in the same order as was sent, this comes from a pre-
standard implementation and MUST NOT be discarded even though the
"remote-uuid" may be nil. In this case, any new transaction
within this dialog MUST preserve the order of the two UUIDs within
all Session-ID header fields, including the ACK, until this dialog
is terminated.
o If a SIP response only contains the "local-uuid" that was sent
originally, this comes from a pre-standard implementation and MUST
NOT be discarded for removing the nil "remote-uuid". In this
case, all future transactions within this dialog MUST contain only
the UUID received in the first SIP response. Any new transaction
starting a new dialog from the standard Session-ID implementation
MUST include a "local-uuid" and a nil "remote-uuid", even if that
new dialog is between the same two UAs.
o Standard implementations should not expect pre-standard
implementations to be consistent in their implementation, even
within the same dialog. For example, perhaps the first, third,
and tenth responses contain a "remote-uuid", but all the others do
not. This behavior MUST be allowed by implementations of this
specification.
o The foregoing does not apply to other, presently unknown
parameters that might be defined in the future. They are ignored
for the purposes of interoperability with previous
implementations.
12. Security and Privacy Considerations
The session identifier MUST be constructed in such a way that does
not convey any user or device information as outlined in Section 4.1.
This ensures that the data contained in the session identifier itself
does not convey user or device information; however, the session
identifier may reveal relationships between endpoints that might not
be revealed by messages without a session identifier.
Section 4.2 requires that a UA always generate a new, previously
unused UUID when transmitting a request to initiate a new session.
This ensures that two unrelated sessions originating from the same UA
will never have the same UUID value, thereby removing the ability for
an attacker to use the session identifier to identify the two
unrelated sessions as being associated with the same user.
Because of the inherent property that session identifiers are
conveyed end-to-end and remain unchanged by a UA for the duration of
a session, the session identifier could be misused to discover
relationships between two or more parties when multiple parties are
involved in the same session such as the case of a redirect,
transfer, or conference. For example, suppose that Alice calls Bob
and Bob, via his PBX (acting as a B2BUA), forwards or transfers the
call to Carol. Without use of the session identifier, an
unauthorized third party that is observing the communications between
Alice and Bob might not know that Alice is actually communicating
with Carol. If Alice, Bob, and Carol include the session identifier
as a part of the signaling messages, it is possible for the third
party to observe that the UA associated with Bob changed to some
other UA. If the third party also has access to signaling messages
between Bob and Carol, the third party can then discover that Alice
is communicating with Carol. This would be true even if all other
information relating to the session is changed by the PBX, including
both signaling information and media address information. That said,
the session identifier would not reveal the identity of Alice, Bob,
or Carol. It would only reveal the fact that those endpoints were
associated with the same session.
This document allows for additional parameters (generic-param) to be
included in the Session-ID header. This is done to allow for future
extensions while preserving backward compatibility with this
document. To protect privacy, the data for any generic-param
included in the Session-ID header value MUST NOT include any user or
device information. Additionally, any information conveyed through
an additional parameter MUST NOT persist beyond the current session,
and therefore MUST NOT be reused between unrelated sessions.
Additional parameters MAY be used by future extensions of this
document to correlate related communication sessions that cannot
already be correlated by the procedures described in this document as
long as the requirements regarding privacy and persistence defined
above are followed.
An intermediary implementing a privacy service that provides user
privacy as per Section 5.3 of [RFC3323] MAY choose to consider the
Session-ID header as being a nonessential informational header with
the understanding that doing so will impair the ability to use the
session identifier for troubleshooting purposes.
13. IANA Considerations
13.1. Registration of the "Session-ID" Header Field
The following is the registration for the Session-ID header field to
the "Header Name" registry at
<http://www.iana.org/assignments/sip-parameters>:
RFC number: RFC 7989
Header name: 'Session-ID'
Compact form: none
Note: This document replaces the Session-ID header originally
registered via [RFC7329].
13.2. Registration of the "remote" Parameter
The following parameter has been added to the "Header Field
Parameters and Parameter Values" section of the "Session Initiation
Protocol (SIP) Parameters" registry:
+--------------+----------------+-------------------+-----------+
| Header Field | Parameter Name | Predefined Values | Reference |
+--------------+----------------+-------------------+-----------+
| Session-ID | remote | No | [RFC7989] |
+--------------+----------------+-------------------+-----------+
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, DOI 10.17487/RFC3515, April 2003,
<http://www.rfc-editor.org/info/rfc3515>.
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891,
DOI 10.17487/RFC3891, September 2004,
<http://www.rfc-editor.org/info/rfc3891>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<http://www.rfc-editor.org/info/rfc4122>.
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents",
BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006,
<http://www.rfc-editor.org/info/rfc4579>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC7206] Jones, P., Salgueiro, G., Polk, J., Liess, L., and H.
Kaplan, "Requirements for an End-to-End Session
Identification in IP-Based Multimedia Communication
Networks", RFC 7206, DOI 10.17487/RFC7206, May 2014,
<http://www.rfc-editor.org/info/rfc7206>.
14.2. Informative References
[H.323] International Telecommunications Union, "Packet-based
multimedia communications systems", ITU-T
Recommendation H.323, December 2009.
[H.460.27] International Telecommunications Union, "End-to-End
Session Identifier for H.323 Systems", ITU-T
Recommendation H.460.27, November 2015.
[RFC2543] Handley, M., Schulzrinne, H., Schooler, E., and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
DOI 10.17487/RFC2543, March 1999,
<http://www.rfc-editor.org/info/rfc2543>.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323,
DOI 10.17487/RFC3323, November 2002,
<http://www.rfc-editor.org/info/rfc3323>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004,
<http://www.rfc-editor.org/info/rfc3725>.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
DOI 10.17487/RFC4353, February 2006,
<http://www.rfc-editor.org/info/rfc4353>.
[RFC5359] Johnston, A., Ed., Sparks, R., Cunningham, C., Donovan,
S., and K. Summers, "Session Initiation Protocol Service
Examples", BCP 144, RFC 5359, DOI 10.17487/RFC5359,
October 2008, <http://www.rfc-editor.org/info/rfc5359>.
[RFC5589] Sparks, R., Johnston, A., Ed., and D. Petrie, "Session
Initiation Protocol (SIP) Call Control - Transfer",
BCP 149, RFC 5589, DOI 10.17487/RFC5589, June 2009,
<http://www.rfc-editor.org/info/rfc5589>.
[RFC6141] Camarillo, G., Ed., Holmberg, C., and Y. Gao, "Re-INVITE
and Target-Refresh Request Handling in the Session
Initiation Protocol (SIP)", RFC 6141,
DOI 10.17487/RFC6141, March 2011,
<http://www.rfc-editor.org/info/rfc6141>.
[RFC6872] Gurbani, V., Ed., Burger, E., Ed., Anjali, T., Abdelnur,
H., and O. Festor, "The Common Log Format (CLF) for the
Session Initiation Protocol (SIP): Framework and
Information Model", RFC 6872, DOI 10.17487/RFC6872,
February 2013, <http://www.rfc-editor.org/info/rfc6872>.
[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
<http://www.rfc-editor.org/info/rfc7092>.
[RFC7329] Kaplan, H., "A Session Identifier for the Session
Initiation Protocol (SIP)", RFC 7329,
DOI 10.17487/RFC7329, August 2014,
<http://www.rfc-editor.org/info/rfc7329>.
Acknowledgements
The authors would like to thank Robert Sparks, Hadriel Kaplan,
Christer Holmberg, Paul Kyzivat, Brett Tate, Keith Drage, Mary
Barnes, Charles Eckel, Peter Dawes, Andrew Hutton, Arun Arunachalam,
Adam Gensler, Roland Jesske, and Faisal Siyavudeen for their
invaluable comments during the development of this document.
Dedication
This document is dedicated to the memory of James Polk, a long-time
friend and colleague. James made important contributions to this
specification, including being one of its primary editors. The IETF
global community mourns his loss, and he will be missed dearly.
Authors' Addresses
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
United States of America
Phone: +1 919 476 2048
Email: paulej@packetizer.com
Gonzalo Salgueiro
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
United States of America
Phone: +1 919 392 3266
Email: gsalguei@cisco.com
Chris Pearce
Cisco Systems, Inc.
2300 East President George Bush Highway
Richardson, TX 75082
United States of America
Phone: +1 972 813 5123
Email: chrep@cisco.com
Paul Giralt
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
United States of America
Phone: +1 919 991 5644
Email: pgiralt@cisco.com
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