Internet Engineering Task Force (IETF)
Request for Comments: 6223
Category: Standards Track
ISSN: 2070-1721

C. Holmberg
Ericsson
April 2011

Indication of Support for Keep-Alive

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 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 5741.

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/rfc6223.

Copyright Notice

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.

   1. Introduction ....................................................2
      1.1. Use-Case: Dialog 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 .....................................................3
   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. Connection Reuse ................................................9
   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 ...11
      7.4. Keep-Alive Negotiation Associated with Dialog: UA-UA ......13
   8. Grammar ........................................................15
      8.1. General ...................................................15
      8.2. ABNF ......................................................15
   9. IANA Considerations ............................................15
      9.1. "keep" Via Header Field Parameter .........................15
   10. Security Considerations .......................................15
   11. Acknowledgements ..............................................16
   12. References ....................................................17
      12.1. Normative References .....................................17
      12.2. Informative References ...................................17

1. Introduction

Section 3.5 of SIP Outbound [RFC5626] defines two keep-alive mechanisms. Even though 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. In some cases, usage of keep-alives can be implicitly negotiated as part of the SIP Outbound negotiation.

However, there are SIP Outbound use-cases where usage of keep-alives is not implicitly negotiated as part of the SIP Outbound negotiation. In addition, there are cases where SIP Outbound is not supported, or where it cannot be applied, but where there is still a need to be able to negotiate usage of keep-alives. Last, SIP Outbound only allows keep-alives to be negotiated between a User Agent (UA) and an edge proxy, and not between other SIP entities.

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: Dialog from Non-Registered UAs

In some cases, a User Agent Client (UAC) does not register itself before it establishes a dialog, but in order to maintain NAT bindings open during the lifetime of the dialog, it still needs to be able to negotiate the sending of keep-alives towards its adjacent downstream 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, some SIP entities that need to be able to negotiate the use 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 RFC 5626 [RFC5626], usage of keep-alives is not implicitly negotiated for such flows.

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 RFC 5626, 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 be physically located in the same SIP entity as the Authoritative Proxy.

Keep-alives:

The keep-alive messages defined in RFC 5626.

"keep" parameter: A SIP Via header field parameter that a SIP entity can insert in the topmost Via header field that it adds to the request, to explicitly indicate willingness to send keep-alives towards its adjacent downstream SIP entity. A SIP entity can add a parameter value to the "keep" parameter in a response to explicitly indicate willingness to receive keep-alives from its adjacent upstream SIP entity.

SIP entity: SIP User Agent (UA), or proxy, as defined in RFC 3261.

Adjacent downstream SIP entity: The adjacent SIP entity in the direction towards which a SIP request is sent.

Adjacent upstream SIP entity: The adjacent SIP entity in the direction from which a SIP request is received.

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 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 downstream SIP entity using SIP requests. The associated responses are used by SIP entities to indicate willingness to receive 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.

In general, it can be useful for SIP entities to indicate willingness to send keep-alives, even if they are not aware of any necessity for them to send keep-alives, since the adjacent downstream SIP entity might have knowledge about the necessity. Similarly, if the adjacent upstream SIP entity has indicated willingness to send keep-alives, it can be useful for SIP entities to indicate willingness to receive keep-alives, even if they are not aware of any necessity for the adjacent upstream SIP entity to send them.

NOTE: Usage of keep-alives is negotiated per direction. If a SIP entity has indicated willingness to receive keep-alives from an adjacent SIP entity, the sending of keep-alives towards that adjacent 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 double-CRLF keep- alive messages towards an adjacent SIP entity even if usage of keep-alives with that SIP entity has not been negotiated. However, the "keep" parameter is still important in order for a SIP entity to indicate that it supports the sending of double-CRLF keep-alive messages, so that the adjacent downstream 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. This 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 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 the sending of keep-alives associated with the registration.

NOTE: The sending of keep-alives associated with a registration can only be negotiated in the direction from the registering SIP entity towards the registrar.

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 the 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 the 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 RFC 5626, a SIP entity that supports the sending of keep-alives must act as a Session Traversal Utilities for NAT (STUN) client [RFC5389]. The SIP entity must support those aspects of STUN that are required in order to apply the STUN keep-alive mechanism defined in RFC 5626, and it must support the CRLF keep-alive mechanism defined in RFC 5626. RFC 5626 defines when to use STUN and when to use double-CRLF for keep-alives.

When a SIP entity sends or forwards a request, if it wants to negotiate the sending of keep-alives associated with a registration or a dialog, it MUST insert a "keep" parameter in the topmost Via header field that it adds to the request, to indicate willingness to send keep-alives.

When the SIP entity receives the associated response, if the "keep" parameter in the topmost Via header field of the response contains a "keep" parameter value, it MUST start sending 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 the sending of keep-alives associated with a dialog towards an adjacent SIP entity, it MUST NOT insert a "keep" parameter in any subsequent SIP requests associated with that dialog towards that adjacent SIP entity. Such "keep" parameters MUST be ignored, if received.

Since an ACK request does not have an associated response, it cannot be used to negotiate usage of keep-alives. Therefore, a SIP entity MUST NOT insert a "keep" parameter in the topmost Via header field of an ACK request. Such "keep" parameters MUST be ignored, if received.

A SIP entity MUST NOT indicate willingness to send keep-alives associated with a dialog, unless it has also inserted itself in the dialog route set [RFC3261].

NOTE: When a SIP entity sends an initial request for a dialog, if the adjacent downstream SIP entity does not insert itself in the dialog route set using a Record-Route header field [RFC3261], the adjacent downstream SIP entity will change once the dialog route set has been established. If a SIP entity inserts a "keep" parameter in the topmost 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 the topmost Via header field of a subsequent SIP request associated with the dialog, in case the new adjacent downstream 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).

If an INVITE request is used to indicate willingness to send keep- alives, as long as at least one response (provisional or final) to the INVITE request contains a "keep" parameter with a parameter value, it is seen as an indication that the adjacent downstream SIP entity is willing to receive keep-alives associated with the dialog on which the response is received.

4.4. Behavior of a SIP Entity Willing to Receive Keep-Alives

As defined in RFC 5626, a SIP entity that supports the receiving of keep-alives must act as a STUN server [RFC5389]. The SIP entity must support those aspects of STUN that are required in order to apply the STUN keep-alive mechanism defined in RFC 5626, and it must support the CRLF keep-alive mechanism defined in RFC 5626.

When a SIP entity sends or forwards a response, and the adjacent upstream SIP entity has indicated willingness to send keep-alives, if the SIP entity is willing to receive keep-alives associated with the registration or with the dialog from that adjacent upstream SIP entity, then it MUST add a parameter value to the "keep" parameter before sending or forwarding the response. The parameter value, if present and with a value other than zero, represents a recommended keep-alive frequency, given in seconds.

There might be multiple responses to an INVITE request. When a SIP entity indicates willingness to receive keep-alives in a response to an INVITE request, it MUST add a parameter value to the "keep" parameter in at least one reliable response to the request. The SIP entity MAY add identical parameter values to the "keep" parameters in other responses to the same request. The SIP entity MUST NOT add different parameter values to the "keep" parameters in responses to the same request. The SIP entity SHOULD indicate the willingness to receive keep-alives as soon as possible.

A SIP entity MUST NOT indicate willingness to receive keep-alives associated with a dialog, unless it has also inserted itself in the dialog route set [RFC3261].

5. Keep-Alive Frequency

If a SIP entity receives a SIP response, where the topmost Via header field contains a "keep" parameter with a non-zero value that indicates a recommended keep-alive frequency, given in seconds, 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. RFC 5626 provides additional guidance on selecting the keep-alive frequency in case a recommended keep-alive frequency is not provided.

This specification does not specify actions to take if negotiated keep-alives are not received. As defined in RFC 5626, the receiving SIP entity may consider a connection to be dead in such situations.

If a SIP entity that adds a parameter value to the "keep" parameter in order to indicate willingness to receive keep-alives also inserts a Flow-Timer header field (that can happen if the SIP entity is using both the Outbound mechanism and the keep-alive mechanism) in the same SIP message, the header field value and the "keep" parameter value MUST be identical.

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. On the other hand, by using the "keep" parameter, the sending and receiving of keep-alives can be negotiated between multiple entities on the signalling path. In addition, since the server-recommended keep-alive frequency might vary between different SIP entities, a single Flow-Timer header field cannot be used to indicate all the different frequency values.

6. Connection Reuse

Keep-alives are often sent in order to keep NAT bindings open, so that SIP requests sent in the reverse direction will pass by the NAT and reuse the same connection. In the case of non-connection- oriented transport protocols, keep-alives would permit the same path to be reused. This specification does not define such a connection reuse mechanism. The keep-alive mechanism defined in this specification is only used to negotiate the sending and receiving of keep-alives. Entities that want to reuse connections need to use another mechanism to ensure that security aspects associated with connection reuse are taken into consideration.

RFC 5923 [RFC5923] specifies a mechanism for using connection- oriented transports to send requests in the reverse direction, and an entity that wants to use connection reuse as well as indicate support of keep-alives on that connection will insert both the "alias" parameter defined in RFC 5923 and the "keep" parameter defined in this specification.

SIP Outbound specifies how registration flows are used to send requests in the reverse 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.

NOTE: The examples do not show the actual syntactical encoding of the request lines, response lines, and the Via header fields, but rather a pseudocode in order to identify the message type and also identify to which SIP entity a Via header field is associated.

7.2. Keep-Alive Negotiation Associated with Registration: UA-Proxy

Figure 1 shows an example where Alice sends a REGISTER request. She indicates willingness to send keep-alives by inserting a "keep" parameter in the Via header field of her 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. Until either the registration expires or Alice sends a registration refresh request, 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.

     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

Figure 2 shows an example where Alice sends an initial INVITE request for a dialog. She indicates willingness to send keep-alives by inserting a "keep" parameter in the Via header field of her 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 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 ---------------|                           |
       |    Via: Alice;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

7.4. Keep-Alive Negotiation Associated with Dialog: UA-UA

Figure 3 shows an example where Alice sends an initial INVITE request for a dialog. She indicates willingness to send keep-alives by inserting a "keep" parameter in the Via header field of her request. In this scenario, the edge proxy (P1) does not add itself to a Record-Route header field (and so will not be added to the dialog route set) before forwarding the request towards Bob.

When Alice receives the response, she determines from the 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 the Via header field of her 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 Bob 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.

     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: Alice;keep=30                                |
       |                                                      |
       |                                                      |
       |                   *** Timeout ***                    |
       |                                                      |
       |=== STUN request ====================================>|
       |<== STUN response ====================================|
       |                                                      |
       |                   *** Timeout ***                    |
       |                                                      |
       |=== STUN request ====================================>|
       |<== STUN response ====================================|
       |                                                      |
       |                                                      |
       |--- BYE --------------------------------------------->|
       |                                                      |
       |<-- 200 OK -------------------------------------------|
       |                                                      |

Figure 3: Example Call Flow

8. Grammar

8.1. General

This section extends the ABNF definition of via-params from [RFC3261] by adding a new Via header field parameter, "keep". The ABNF defined in this specification is conformant to RFC 5234 [RFC5234]. "EQUAL" is defined in RFC 3261. "DIGIT" is defined in RFC 5234.

8.2. ABNF

   via-params =/ keep
   
   keep       = "keep" [ EQUAL 1*(DIGIT) ]

9. IANA Considerations

9.1. "keep" Via Header Field Parameter

   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 [RFC3968].  The syntax is
   defined in Section 8 of this document.  IANA has registered the
   following:
                                                  Predefined
   Header Field            Parameter Name         Values      Reference
   ----------------------  ---------------------  ----------  ---------
   Via                     keep                   No          [RFC6223]

10. Security Considerations

SIP entities that send or receive keep-alives are often required to use a connection reuse mechanism, in order to ensure that requests sent in the reverse direction, towards the sender of the keep-alives, traverse NATs, etc. This specification does not define a connection reuse mechanism, and it does not address security issues related to connection reuse. SIP entities that wish to reuse connections need to use a dedicated connection reuse mechanism, in conjunction with the keep-alive negotiation mechanism.

Unless SIP messages are integrity protected hop-by-hop, e.g., using Transport Layer Security (TLS) [RFC5246] or Datagram Transport Layer Security (DTLS) [RFC4347], a man-in-the-middle can modify Via header fields used by two entities to negotiate the sending of keep-alives, e.g., by removing the designations used to indicate willingness to send and receive keep-alives, or by decreasing the timer value to a very low value, which might trigger additional resource consumption due to the frequently sent keep-alives.

The behaviors defined in Sections 4.3 and 4.4 require a SIP entity using the mechanism defined in this specification to place a value in the "keep" parameter in the topmost Via header field value of a response the SIP entity sends. They do not instruct the entity to place a value in a "keep" parameter of any request it forwards. In particular, a SIP proxy MUST NOT place a value into the "keep" parameter of the topmost Via header field value of a request it receives before forwarding it. A SIP proxy implementing this specification SHOULD remove any "keep" parameter values in any Via header field values below the topmost one in responses it receives before forwarding them.

When requests are forwarded across multiple hops, it is possible for a malicious downstream SIP entity to tamper with the accrued values in the Via header field. The malicious SIP entity could place a value, or change an existing value in a "keep" parameter in any of the Via header field values -- not just the topmost value. A proxy implementation that simply forwards responses by stripping the topmost Via header field value and not inspecting the resulting new topmost Via header field value risks being adversely affected by such a malicious downstream SIP entity. In particular, such a proxy may start receiving STUN requests if it blindly forwards a response with a "keep" parameter with a value it did not create in the topmost Via header field.

To lower the chances of the malicious SIP entity's actions having adverse effects on such proxies, when a SIP entity sends STUN keep- alives to an adjacent downstream SIP entity and does not receive a response to those STUN messages (as described in Section 7.2.1 of RFC 5389 [RFC5389], it MUST stop sending keep-alives for the remaining duration of the dialog (if the sending of keep-alives were negotiated for a dialog) or until the sending of keep-alives is re-negotiated for the registration (if the sending keep-alives were negotiated for a registration).

Apart from the issues described above, this specification does not introduce security considerations in addition to those specified for keep-alives in [RFC5626].

11. Acknowledgements

Thanks to Staffan Blau, Francois Audet, Hadriel Kaplan, Sean Schneyer, and Milo Orsic for their comments on the initial draft version of this document. Thanks to Juha Heinanen, Jiri Kuthan, Dean Willis, John Elwell, Paul Kyzivat, Peter Musgrave, Dale Worley, Adam Roach, and Robert Sparks for their comments on the sipcore mailing 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.
   
   [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., Ed., 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., Ed., Mahy, R., Ed., and F. Audet, Ed.,
              "Managing Client-Initiated Connections in the Session
              Initiation Protocol (SIP)", RFC 5626, October 2009.

12.2. Informative References

   [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Header Field Parameter Registry for the Session
              Initiation Protocol (SIP)", BCP 98, RFC 3968,
              December 2004.
   
   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.
   
   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.
   
   [RFC5923]  Gurbani, V., Ed., Mahy, R., and B. Tate, "Connection Reuse
              in the Session Initiation Protocol (SIP)", RFC 5923,
              June 2010.

Author's Address

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland
   
   EMail: christer.holmberg@ericsson.com