SIPCORE Working Group C. Holmberg
Internet-Draft Ericsson
Intended status: Informational May 31, 2010
Expires: December 2, 2010
Indication of support for keep-alive
draft-ietf-sipcore-keep-04.txt
Abstract
This specification defines a new Session Initiation Protocol (SIP)
Via header field parameter, "keep", which allows adjacent SIP
entities to explicitly negotiate usage of the Network Address
Translation (NAT) keep-alive mechanisms defined in SIP Outbound, in
cases where SIP Outbound is not supported, cannot be applied, or
where usage of keep-alives is not implicitly negotiated as part of
the SIP Outbound negotiation.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 2, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Use-case: Session from non-registered UAs . . . . . . . . 3
1.2. Use-case: SIP Outbound not supported . . . . . . . . . . . 3
1.3. Use-case: SIP dialog initiated Outbound flows . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. User Agent and Proxy behavior . . . . . . . . . . . . . . . . 4
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Lifetime of keep-alives . . . . . . . . . . . . . . . . . 5
4.2.1. General . . . . . . . . . . . . . . . . . . . . . . . 5
4.2.2. Keep-alives associated with registration . . . . . . . 5
4.2.3. Keep-alives associated with dialog . . . . . . . . . . 6
4.3. Behavior of a SIP entity willing to send keep-alives . . . 6
4.4. Behavior of a SIP entity willing to receive keep-alives . 7
5. Keep-alive frequency . . . . . . . . . . . . . . . . . . . . . 8
6. Overlap with connection reuse . . . . . . . . . . . . . . . . 8
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. Keep-alive negotiation associated with registration:
UA-proxy . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.3. Keep-alive negotiation associated with dialog: UA-proxy . 10
7.4. Keep-alive negotiation associated with dialog: UA-UA . . . 12
8. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9.1. keep . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10. Security Considerations . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
Section 3.5 of SIP Outbound [RFC5626] defines two keep-alive
mechanisms. Eventhough the keep-alive mechanisms are separated from
the rest of the SIP Outbound mechanism, SIP Outbound does not define
a mechanism to explicitly negotiate usage of the keep-alive
mechanisms, since usage of keep-alives in most cases are implicitly
negotiated as part of the SIP Outbound negotiation.
However, there are SIP Outbound use-cases where usage of keep-alives
are not implicitly negotiated as part of the SIP Outbound
negotiation. In addition, there are cases where SIP Outbound is not
supported, where it cannot be applied, but where there is still a
need to be able to negotiate usage of keep-alives.
This specification defines a new Session Initiation Protocol (SIP)
[RFC3261] Via header field parameter, "keep", which allows adjacent
SIP entities to explicitly negotiate usage of the NAT keep-alive
mechanisms defined in SIP Outbound. The "keep" parameter allows SIP
entities to indicate willingness to send keep-alives, to indicate
willingness to receive keep-alives, and for SIP entities willing to
receive keep-alives to provide a recommended keep-alive frequency.
The following sections describe use-cases where a mechanism to
explicitly negotiate usage of keep-alives is needed.
1.1. Use-case: Session from non-registered UAs
In some cases a User Agent Client (UAC) does not register itself
before it establishes a session, but in order to maintain NAT
bindings open during the session it still needs to be able to
negotiate sending of keep-alives towards its adjacent upstream SIP
entity. A typical example is an emergency call, where a registration
is not always required in order to make the call.
1.2. Use-case: SIP Outbound not supported
In some cases all SIP entities that need to be able to negotiate the
usage of keep-alives might not support SIP Outbound. However, they
might still support the keep-alive mechanisms defined in SIP
Outbound, and need to be able to negotiate usage of them.
1.3. Use-case: SIP dialog initiated Outbound flows
SIP Outbound allows the establishment of flows using the initial
request for a dialog. As specified in [RFC5626], usage keep-alives
is not implicitly negotiated for such flows, why usage needs to be
explicitly negotiated.
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2. Conventions
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 BCP 14, RFC 2119
[RFC2119].
3. Definitions
Edge proxy: As defined in [RFC5626], a SIP proxy that is located
topologically between the registering User Agent (UA) and the
Authoritative Proxy.
NOTE: In some deployments the edge proxy might physically be located
in the same entity as the Authoritative Proxy.
Keep-alives: Refers to keep-alive messages as defined in SIP Outbound
[RFC5626].
"keep" parameter: A SIP Via header field parameter that a SIP entity
can insert in its Via header field of a request to explicitly
indicate willingness to send keep-alives towards it adjacent upstream
SIP entity. If a SIP entity is willing to receive keep-alives from
its adjacent downstream SIP entity, and the Via header field
associated with the adjacent downstream SIP entity contains a "keep"
parameter, the SIP entity willing to receive keep-alives can add an
integer parameter value to that "keep" parameter. The integer
parameter value can be used to indicate a recommended keep-alive
frequency. A zero value indicates that the SIP entity is willing to
receive keep-alives, but that a recommended keep-alive frequency is
not provided.
SIP entity: SIP User Agent (UA), or proxy, as defined in [RFC3261].
4. User Agent and Proxy behavior
4.1. General
This section describes how SIP UAs and proxies negotiate usage of
keep-alives associated with a registration, or a dialog, which types
of SIP requests types can be used in order to negotiate the usage,
and the lifetime of the negotiated keep-alives.
SIP entities indicate willingness to send keep-alives towards the
adjacent upstream SIP entity using SIP requests. The associated
responses are used by SIP entities to indicate willingness to receive
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keep-alives. SIP entities that indicate willingness to receive keep-
alives can provide a recommended keep-alive frequency.
The procedures to negotiate usage of keep-alives are identical for
SIP UAs and proxies.
NOTE: Usage of keep-alives is negotiated per direction. If a SIP
entity has indicated willingness to receive keep-alives from its
adjacent downstream SIP entity, sending of keep-alives towards the
same SIP entity needs to be separately negotiated.
NOTE: Since there are SIP entities that already use a combination of
Carriage Return and Line Feed (CRLF) as keep-alive messages, and SIP
entities are expected to be able to receive those, this specification
does not forbid the sending of CRLF keep-alive messages towards an
adjacent upstream SIP entity even if usage of keep-alives have not
been negotiated. However, the "keep" parameter is still important in
order for a SIP entity to indicate that it supports sending of CRLF
keep-alive messages, so that the adjacent upstream SIP entity does
not use other mechanisms (e.g. short registration refresh intervals)
in order to keep NAT bindings open.
4.2. Lifetime of keep-alives
4.2.1. General
The lifetime of negotiated keep-alives depends on whether the keep-
alives are associated with a registration or a dialog. The section
describes the lifetime of negotiated keep-alives.
4.2.2. Keep-alives associated with registration
SIP entities use a registration request in order to negotiate usage
of keep-alives associated with a registration. Usage of keep-alives
can be negotiated when the registration is established, or later
during the lifetime of the registration. Once negotiated, keep-
alives are sent until the registration is terminated, or until a
subsequent registration refresh request is sent or forwarded. When a
subsequent registration refresh request is sent or forwarded, if a
SIP entity is willing to continue sending keep-alives associated with
the registration, usage of keep-alives MUST be re-negotiated. If
usage is not successfully re-negotiated, the SIP entity MUST cease
sending of keep-alives associated with the registration.
In case a SIP entity establishes multiple registration flows
[RFC5626], usage of keep-alives needs to be negotiated separately for
each individual registration flow. A SIP entity MUST NOT send keep-
alives associated with a registration flow for which usage of keep-
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alives has not been negotiated.
4.2.3. Keep-alives associated with dialog
SIP entities use an initial request for a dialog, or a mid-dialog
target refresh request [RFC3261], in order to negotiate sending and
receiving of keep-alives associated with a dialog. Usage of keep-
alives can be negotiated when the dialog is established, or later
during the lifetime of the dialog. Once negotiated, keep-alives MUST
be sent for the lifetime of the dialog, until the dialog is
terminated. Once usage of keep-alives associated with a dialog has
been negotiated, it is not possible to re-negotiate the usage
associated with the dialog.
4.3. Behavior of a SIP entity willing to send keep-alives
As defined in [RFC5626], a SIP entity that supports sending of keep-
alives must act as a Session Traversal Utilities for NAT (STUN)
client [RFC5389]. The SIP entity must support the amount of STUN
which is required to apply the STUN keep-alive mechanism defined in
[RFC5626], and it must support the CRLF keep-alive mechanism defined
in [RFC5626].
When a SIP entity sends or forwards a request, if it wants to
negotiate the sending of keep-alives for the lifetime of a
registration (in case of a REGISTER request), or a dialog (in case of
an initial request for a dialog, or a mid-dialog target refresh
request), it MUST insert a "keep" parameter in its Via header field
of the request.
When the SIP entity receives the associated response, if the "keep"
parameter in its Via header field of the response contains a
parameter value, it MUST start to send keep-alives towards the same
destination where it would send a subsequent request (e.g. REGISTER
requests and initial requests for dialog) associated with the
registration (if the keep-alive negotiation is for a registration),
or where it would send subsequent mid-dialog requests (if the keep-
alive negotiation is for a dialog). Subsequent mid-dialog requests
are addressed based on the dialog route set.
Once a SIP entity has negotiated sending of keep-alives associated
with a dialog towards its adjacent upstream SIP entity, it MUST NOT
insert a "keep" parameter in any subsequent SIP requests associated
with the dialog. Such "keep" parameter MUST be ignored, if received.
Since an ACK request does not have an associated response, it can not
be used to negotiate usage of keep-alives. Therefore, a SIP entity
MUST NOT insert a "keep" parameter in its Via header field of an ACK
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request. Such "keep" parameter MUST be ignored, if received.
When a SIP entity is about to send a keep-alive, if the SIP entity at
the same time is also about to send or forward a SIP request
associated with the same registration or dialog, for which the keep-
alive is to be sent, the SIP entity MAY choose not to send the keep-
alive, as the SIP request will perform the same keep-alive action.
NOTE: When a SIP entity sends an initial request for a dialog, if the
adjacent upstream SIP entity does not insert itself in the dialog
route set using a Record-Route header field [RFC3261], the adjacent
upstream SIP entity will change once the dialog route set has been
established. If a SIP entity inserts a "keep" parameter in its Via
header field of an initial request for a dialog, and the "keep"
parameter in the associated response does not contain a parameter
value, the SIP entity might choose to insert a "keep" parameter in
its Via header field of a subsequent SIP request associated with the
dialog, in case the new adjacent SIP entity (based on the dialog
route set) is willing to receive keep-alives (in which case it will
add a parameter value to the "keep" parameter).
NOTE: If a SIP entity inserts a "keep" parameter in its Via header
field of an INVITE request, and it receives multiple responses
(provisional or final) associated with the request, as long as at
least one of the responses, per dialog, contains a "keep" parameter
with a parameter value it is seen as an indication that the adjacent
upstream SIP entity is willing to receive keep-alives associated with
the dialog.
4.4. Behavior of a SIP entity willing to receive keep-alives
As defined in [RFC5626], a SIP entity that supports receiving of
keep-alives must act as a STUN server [RFC5389]. The SIP entity must
support the amount of STUN which is required to apply the STUN keep-
alive mechanism defined in [RFC5626], and it must support the CRLF
keep-alive mechanism defined in [RFC5626].
When a SIP entity creates or receives a response to a request that
can be used in order to indicate willingness to send keep-alives
associated with a registration or dialog, and the Via header field
associated with the adjacent downstream SIP entity (that is, the top-
most Via header field once a SIP entity that received the response
has removed its own Via header field from the response) contains a
"keep" parameter, if the SIP entity is willing to receive keep-alives
associated with the registration (in case of a REGISTER response), or
the dialog (in case of a response to an initial request for a dialog,
or a response to a mid-dialog target refresh request), from the
adjacent downstream SIP entity it MUST add a parameter value to the
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"keep" parameter, before sending or forwarding the response. The
parameter can contain a recommended keep-alive frequency, or a zero
value.
5. Keep-alive frequency
If a SIP entity receives a SIP response, where its Via header field
contains a "keep" parameter with a non-zero value that indicates a
recommended keep-alive frequency, it MUST use the procedures defined
for the Flow-Timer header field [RFC5626]. According to the
procedures, the SIP entity must send keep-alives at least as often as
the indicated recommended keep-alive frequency, and if the SIP entity
uses the recommended keep-alive frequency then it should send its
keep-alives so that the interval between each keep-alive is randomly
distributed between 80% and 100% of the recommended keep-alive
frequency.
If the received "keep" parameter value is zero, the SIP entity can
send keep-alives at its discretion. [RFC5626] provides additional
guidance on selecting the keep-alive frequency in case a recommended
keep-alive frequency is not provided.
A SIP entity that uses the "keep" parameter to indicate willingness
to receive keep-alives MUST NOT use the Flow-Timer header field in
order to provide a recommended keep-alive frequency.
SIP Outbound uses the Flow-Timer header field to indicate the server-
recommended keep-alive frequency. However, it will only be sent
between a UA and an edge proxy. Using the "keep" parameter, however,
the sending and receiving of keep-alives might be negotiated between
multiple entities on the signalling path. Since the server-
recommended keep-alive frequency might vary between different SIP
entities, those would have to re-write the Flow-Timer header field
value. In addition, if a SIP entity does not indicate willingness to
receive keep-alives from its adjacent downstream SIP entity, and
receives a Flow-Timer header field in a response, it would have to
remove the Flow-Timer header field from the response. This issue
does not exist for the "keep" parameter, as each SIP entity has its
own individual Via header field.
6. Overlap with connection reuse
The connect-reuse specification [I-D.ietf-sip-connect-reuse]
specifies how to use connection-oriented transports to send requests
in the reverse direction. SIP entity A opens a connection to entity
B in order to send a request. Under certain conditions entity B can
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reuse that connection for sending requests in the backwards direction
to A as well. However, the connect-reuse specification does not
define a keep-alive mechanism for this connection.
The mechanism specified in this draft is thus orthogonal to the
purpose of connection reuse. An entity that wants to use connection-
reuse as well as indicate keep-alive mechanism on that connection
will insert both the "alias" parameter defined in [connect-reuse] as
well as the "keep" parameter defined in this memo. Inserting only
one of these parameters is not a substitute for the other. Thus,
while the presence of a "keep" parameter will indicate that the
entity supports keep-alives in order to keep the connection open, no
inference can be drawn on whether that connection can be used for
requests in the backwards direction.
7. Examples
7.1. General
This section shows example flows where usage of keep-alives,
associated with a registration and a dialog, is negotiated between
different SIP entities.
7.2. Keep-alive negotiation associated with registration: UA-proxy
The figure shows an example where Alice sends an REGISTER request.
She indicates willingness of sending keep-alive by inserting a "keep"
parameter in her Via header field of the request. The edge proxy
(P1) forwards the request towards the registrar.
P1 is willing to receive keep-alives from Alice for the duration of
the registration, so When P1 receives the associated response it adds
a keep parameter value, which indicates a recommended keep-alive
frequency of 30 seconds, to Alice's Via header field, before it
forwards the response towards Alice.
When Alice receives the response, she determines from her Via header
field that P1 is willing to receive keep-alives associated with the
registration. For the lifetime of the registration, Alice then sends
periodic keep-alives (in this example using the STUN keep-alive
technique) towards P1, using the recommended keep-alive frequency
indicated by the keep parameter value.
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Alice P1 REGISTRAR
| | |
|--- REGISTER------------->| |
| Via: Alice;keep | |
| |--- REGISTER-------------->|
| | Via: P1 |
| | Via: Alice;keep |
| | |
| |<-- 200 OK ----------------|
| | Via: P1 |
| | Via: Alice;keep |
|<-- 200 OK ---------------| |
| Via: Alice;keep=30 | |
| | |
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
Figure 1: Example call flow
7.3. Keep-alive negotiation associated with dialog: UA-proxy
The figure shows an example where Alice sends an initial INVITE
request for a dialog. She indicates willingness to send keep-alive
by inserting a "keep" parameter in her Via header field of the
request. The edge proxy (P1) adds itself to the dialog route set by
adding itself to a Record-Route header field, before it forwards the
request towards Bob.
P1 is willing to receive keep-alives from Alice for the duration of
the dialog, so When P1 receives the associated response it adds a
keep parameter value, which indicates a recommended keep-alive
frequency of 30 seconds, to Alice's Via header field, before it
forwards the response towards Alice.
When Alice receives the response, she determines from her Via header
field that P1 is willing to receive keep-alives associated with the
dialog. For the lifetime of the dialog, Alice then sends periodic
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keep-alives (in this example using the STUN keep-alive technique)
towards P1, using the recommended keep-alive frequency indicated by
the keep parameter value.
Alice P1 Bob
| | |
|--- INVITE -------------->| |
| Via: Alice;keep | |
| |--- INVITE --------------->|
| | Via: P1 |
| | Via: Alice;keep |
| | Record-Route: P1 |
| | |
| |<-- 200 OK ----------------|
| | Via: P1 |
| | Via: Alice;keep |
| | Record-Route: P1 |
|<-- 200 OK ---------------| |
| Alice: UAC;keep=30 | |
| Record-Route: P1 | |
| | |
|--- ACK ----------------->| |
| | |
| |--- ACK ------------------>|
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
| | |
|--- BYE ----------------->| |
| | |
| |--- BYE ------------------>|
| | |
| |<-- 200 OK ----------------|
| | |
Figure 2: Example call flow
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7.4. Keep-alive negotiation associated with dialog: UA-UA
The figure shows an example where Alice sends an initial INVITE
request for a dialog. She indicates willingness to send keep-alive
by inserting a "keep" parameter in her Via header field of the
request. The edge proxy (P1) does not add itself to the dialog route
set, by adding itself to a Record-Route header field, before it
forwards the request towards Bob. .
When Alice receives the response, she determines from her Via header
field that P1 is not willing to receive keep-alives associated with
the dialog from her. When the dialog route set has been established,
Alice sends a mid-dialog UPDATE request towards Bob (since P1 did not
insert itself in the dialog route set), and she once again indicates
willingness to send keep-alives by inserting a "keep" parameter in
her Via header field of the request. Bob supports the keep-alive
mechanism, and is willing to receive keep-alives associated with the
dialog from Alice, so he creates a response and adds a keep parameter
value, which indicates a recommended keep-alive frequency of 30
seconds, to Alice's Via header field, before he forwards the response
towards Alice.
When Alice receives the response, she determines from her Via header
field that P1 is willing to receive keep-alives associated with the
dialog. For the lifetime of the dialog, Alice then sends periodic
keep-alives (in this example using the STUN keep-alive technique)
towards Bob, using the recommended keep-alive frequency indicated by
the keep parameter value.
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Alice P1 Bob
| | |
|--- INVITE -------------->| |
| Via: Alice;keep | |
| |--- INVITE --------------->|
| | Via: P1 |
| | Via: Alice:keep |
| | |
| |<-- 200 OK ----------------|
| | Via: P1 |
| | Via: Alice;keep |
|<-- 200 OK ---------------| |
| Via: Alice;keep | |
| | |
| |
|--- ACK --------------------------------------------->|
| |
|--- UPDATE ------------------------------------------>|
| Via: Alice;keep |
| |
|<-- 200 OK ------------------------------------------>|
| Via: UAC;keep=30 |
| |
| |
| *** Timeout *** |
| |
|=== STUN request ====================================>|
|<== STUN response ====================================|
| |
| *** Timeout *** |
| |
|=== STUN request ====================================>|
|<== STUN response ====================================|
| |
| |
|--- BYE --------------------------------------------->|
| |
|<-- 200 OK -------------------------------------------|
| |
Figure 3: Example call flow
8. Grammar
This specification defines a new Via header field parameter, "keep".
The grammar includes the definitions from [RFC5626].
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The ABNF [RFC5234] is:
via-params =/ keep
keep = "keep" [ EQUAL 1*(DIGIT) ]
9. IANA Considerations
9.1. keep
This specification defines a new Via header field parameter called
keep in the "Header Field Parameters and Parameter Values" sub-
registry as per the registry created by [RFC5626]. The syntax is
defined in Section 8. The required information is:
Predefined
Header Field Parameter Name Values Reference
---------------------- --------------------- ---------- ---------
Via keep No [RFCXXXX]
10. Security Considerations
This specification does not introduce security considerations in
addition to those specified in [RFC5626].
11. Acknowledgements
Thanks to Staffan Blau, Francois Audet, Hadriel Kaplan, Sean Schneyer
and Milo Orsic for their comments on the initial draft. Thanks to
Juha Heinaenen, Jiri Kuthan, Dean Willis and John Elwell for their
comments on the list. Thanks to Vijay Gurbani for providing text
about the relationship with the connect-reuse specification.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
12.2. Informative References
[I-D.ietf-sip-connect-reuse]
Gurbani, V., Mahy, R., and B. Tate, "Connection Reuse in
the Session Initiation Protocol (SIP)",
draft-ietf-sip-connect-reuse-14 (work in progress),
August 2009.
Author's Address
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Holmberg Expires December 2, 2010 [Page 15]
