Merge pull request #118 from Metaswitch/clarify_rfc4028_support

[Reviewer: Andy] Update our documented support for RFC4028

docs/SIP_Interface_Specifications.md

@@ -107,8 +107,8 @@ The following RFCs are already supported by Clearwater.  Note that a number of t
 ### Session-expiry ([RFC 4028](http://www.ietf.org/rfc/rfc4028.txt))
 
 *   Covers periodic reINVITE or UPDATE messages used to allow UEs or call stateful proxies to detect when sessions have ended.  Includes Min-SE and Session-Expires headers, which are used to negotiate the frequency of keepalives.
-*   Currently supported transparently by Clearwater.  Neither is call stateful, so neither monitors the messages or participates actively in the negotiation.
-*   Optional according to [TS 24.229](http://www.3gpp.org/ftp/Specs/html-info/24229.htm), so would only need to implement active support if we have a functional need (for example, if in future Clearwater needed some form of dialog tracking in Bono for privacy or quiescing support).
+*   Clearwater supports session-expiry as a proxy per section 8 of the RFC. It does not have a minimum acceptable session interval, so does not make use of the Min-SE header, though does honor it.
+*   Clearwater requests a session interval of 10 minutes (or as close to 10 minutes as possible given pre-existing headers) in order to properly perform session-based billing.
 
 ### Early session disposition type ([RFC 3959](http://www.ietf.org/rfc/rfc3959.txt))
 
@@ -256,7 +256,7 @@ The following RFCs are already supported by Clearwater.  Note that a number of t
 
 ### Fixes to issues with SIP non-INVITE transactions ([RFC 4320](http://www.ietf.org/rfc/rfc4320.txt))
 
-*   Defines changes to [RFC 3261](http://www.ietf.org/rfc/rfc3261.txt) procedures for handling non-INVITE transactions to avoid some issues - in particular the potential for an O(N^2) storm of 408 responses if a transaction times out.  
+*   Defines changes to [RFC 3261](http://www.ietf.org/rfc/rfc3261.txt) procedures for handling non-INVITE transactions to avoid some issues - in particular the potential for an O(N^2) storm of 408 responses if a transaction times out.
 *   Mandatory for all SIP nodes according to [TS 24.229](http://www.3gpp.org/ftp/Specs/html-info/24229.htm).
 *   Supported in Clearwater.
 
@@ -288,7 +288,7 @@ The following RFCs are already supported by Clearwater.  Note that a number of t
     *   as a public user identity associated with a subscription (although a subscription must have at least on public user identity which is a SIP URI)
     *   as the target URI for a call.
 *   Other URIs can be specified as Request URI for a SIP message.
-*   Clearwater supports SIP and Tel URIs but not mailto, pres or im URIs. 
+*   Clearwater supports SIP and Tel URIs but not mailto, pres or im URIs.
 
 ### Number portability parameters in Tel URI ([RFC 4694](http://www.ietf.org/rfc/rfc4694.txt))
 
@@ -320,7 +320,7 @@ These are the RFCs which are relevant to Clearwater and not yet supported.
 ### P-Media-Authorization header ([RFC 3313](http://www.ietf.org/rfc/rfc3313.txt))
 
 *   According to [TS 24.229](http://www.3gpp.org/ftp/Specs/html-info/24229.htm), only required if P-CSCF supporting IMS AKA authentication with IPsec ESP encryption, or SIP digest authentication with TLS encryption.
-*   Not supported, as this is P-CSCF only (and Bono doesn't support AKA). 
+*   Not supported, as this is P-CSCF only (and Bono doesn't support AKA).
 
 ### Signalling Compression aka SigComp ([RFC 3320](http://www.ietf.org/rfc/rfc3320.txt), [RFC 3485](http://www.ietf.org/rfc/rfc3485.txt), [RFC 3486](http://www.ietf.org/rfc/rfc3486.txt), [RFC 4077](http://www.ietf.org/rfc/rfc4077.txt), [RFC 4896](http://www.ietf.org/rfc/rfc4896.txt), [RFC 5049](http://www.ietf.org/rfc/rfc5049.txt), [RFC 5112](http://www.ietf.org/rfc/rfc5112.txt))
 
@@ -337,7 +337,7 @@ These are the RFCs which are relevant to Clearwater and not yet supported.
 
 *   Defines headers that can be used to negotiate authentication mechanisms.
 *   Only required if P-CSCF supporting IMS AKA authentication with IPsec ESP encryption, or SIP digest authentication with TLS encryption.
-*   Not supported, as these headers are always caught by the P-CSCF (and Bono doesn't support AKA).  
+*   Not supported, as these headers are always caught by the P-CSCF (and Bono doesn't support AKA).
 
 ### SMS over IP ([RFC 3862](http://www.ietf.org/rfc/rfc3862.txt) and [RFC 5438](http://www.ietf.org/rfc/rfc5438.txt))