| SIPCORE Working Group | C. Holmberg |
| Internet-Draft | Ericsson |
| Updates: 4028 (if approved) | March 5, 2018 |
| Intended status: Standards Track | |
| Expires: September 6, 2018 |
Session Initiation Protocol (SIP) Session Timer Glare Handling
draft-ietf-sipcore-sessiontimer-race-01
This document updates RFC 4028, by clarifying the procedures for negotiating usage of the Session Initiation Protocol (SIP) session timer mechansim, in order to avoid a race condition where both endpoints trigger simultaneous negotiations.
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[RFC4028] defines a mechanism, where periodic SIP requests are sent in order to allow SIP user agents and proxies to determine whether a SIP session is still active.
The usage of the mechanism is negotiated using two new SIP header fields (Session-Expires and Min-SE), a new option tag ('timer') and a new SIP response code (422).
1. Section 7.4 of [RFC4028] says that, in a session refresh request sent within a dialog with an active session timer, the Session-Expires header field should be included in the request. However, the text is unclear regarding including the Session-Expires header field in a session refresh request when a negotiation of session timer usage is still ongoing, e.g., if one user agent is sending a request that contains a Session-Expires header field and, before it receives the associated 2xx response, receives a request from the peer user agent that also contains a Session-Expires header field. Such scenario might cause a glare situation, with conflicting negotiation parameters.
2. The absence of the Session-Expires header field in a 2xx response to an (re-)INVITE request [RFC3261] or UPDATE request [RFC3311] means that the mechanism isn't used. This can be used to reject the usage of the mechanism when sending a response. However, the text is unclear that this only applies to cases where the associated SIP request contained a Session-Expires header field.
This document updates [RFC4028], by clarifying the procedures for negotiating usage of the Session Initiation Protocol (SIP) [RFC3261] session timer mechanism [RFC4028]. The following clarifications are provided:
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].
This section updates the third paragraph in section 7.2 of [RFC4028], by clarifying that the absence of a Session-Expires header field in a response will disable usage of the session timer mechanism only if the associated request contained a Session-Expires header field.
OLD TEXT:
If the 2xx response did not contain a Session-Expires header field,
there is no session expiration. In this case, no refreshes need to
be sent. A 2xx without a Session-Expires can come for both initial
and subsequent session refresh requests. This means that the session
timer can be 'turned-off' in mid dialog by receiving a response
without a Session-Expires header field.
NEW TEXT:
If the request contained a Session-Expires header field, and the
associated 2xx response did not contain a Session-Expires header
field, there is no session expiration. In this case, no refreshes
need to be sent. A 2xx without a Session-Expires can come for both
initial and subsequent session refresh requests. This means that
the session timer can be 'turned-off' in mid dialog by receiving a
response without a Session-Expires header field.
This section updates the fifth paragraph in section 7.4 of [RFC4028], by clarifying that the Session-Expires header field can only be included in a refresh request if there is no ongoing negotiation of usage of the session timer mechanism.
OLD TEXT:
In a session refresh request sent within a dialog with an active
session timer, the Session-Expires header field SHOULD be present.
When present, it SHOULD be equal to the maximum of the Min-SE header
field (recall that its default value when not present is 90 seconds)
and the current session interval. Inclusion of the Session-Expires
header field with this value avoids certain denial-of-service
attacks, as documented in Section 11. As such, a UA should only
ignore the SHOULD in unusual and singular cases where it is
desirable to change the session interval mid-dialog.
NEW TEXT:
In a session refresh request sent within a dialog with an active
session timer, if there is no ongoing negotiation of usage of the
session timer mechanism and if there is no ongoing INVITE
transaction (with or without session timer negotiation), the
Session-Expires header field SHOULD be present. If there is an
ongoing negotiation, or if there is an ongoing INVIET transaction,
the Session-Expires header field MUST NOT be present. When present,
it SHOULD be equal to the maximum of the Min-SE header field (recall
that its default value when not present is 90 seconds) and the
current session interval. Inclusion of the Session-Expires header
field with this value avoids certain denial-of-service attacks, as
documented in Section 11. As such, a UA should only ignore the
SHOULD in unusual and singular cases where it is desirable to
change the session interval mid-dialog.
This section updates the second paragraph section 8.1 of [RFC4028], by clarifying that a proxy can insert a Session-Expires header field in a request only if there is no ongoing negotiation of usage of the session timer mechanism and if there is no ongoing INVITE transaction.
OLD TEXT:
To request a session timer for a session, a proxy makes sure that a
Session-Expires header field is present in a session refresh request
for that session. A proxy MAY insert a Session-Expires header field
in the request before forwarding it if none was present in the
request. This Session-Expires header field may contain any desired
expiration time the proxy would like, but not with a duration lower
than the value in the Min-SE header field in the request, if it is
present. The proxy MUST NOT include a refresher parameter in the
header field value.
NEW TEXT:
To request a session timer for a session, a proxy makes sure that a
Session-Expires header field is present in a session refresh request
for that session. A proxy MAY insert a Session-Expires header field
in the request before forwarding it if none was present in the
request, if there is no ongoing negotiation of usage of the
session timer mechanism and if there is no ongoing INVITE
transaction (with or without session timer negotiation). This
Session-Expires header field may contain any desired expiration time
the proxy would like, but not with a duration lower than the value
in the Min-SE header field in the request, if it is present. The
proxy MUST NOT include a refresher parameter in the header field
value.
This section updates section 8.1 of [RFC4028], by clarifying that a session refresh request that is received while there is an ongoing negotiation of usage of the session timer mechanism shall be rejected by a 491 (Request Pending) response. The clarification is done by adding a new paragraph between the fouth and fifth paragraphs of the section.
NEW TEXT:
If an incoming request contains a Session-Expires header field,
and there is on ongoing negotiation of usage of the session timer
mechanism, or if there is an ongoing INVITE transaction (with or
without session timer negotiation), the UAS MUST reject the request
with a 491 (Request Pending) response.
The security considerations associated with the SIP Session-Timer mechanism are described in [RFC4028]. This specification adds clarification text for avoiding session-timer negotiation race conditions, and does not introduce new security considerations.
This specification makes no requests from IANA.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 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, DOI 10.17487/RFC3261, June 2002. |
| [RFC3311] | Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October 2002. |
| [RFC4028] | Donovan, S. and J. Rosenberg, "Session Timers in the Session Initiation Protocol (SIP)", RFC 4028, DOI 10.17487/RFC4028, April 2005. |