readme file to 25.331 v10.9.0 (2012-09): 2012-10-25

- EXPLANATION:
  10.9.0 is missing 1 keyword "OPTIONAL" in the ASN.1 text clause 11.2 in message RRCConnectionRequest
  which can cause severe backward/forward compatibility problems.

  It is therefore NOT recommended to use 25.331 v10.9.0 for implementation/testing purposes.

  In case where the use of 25.331 v10.9.0 can not be avoided this 1 "OPTIONAL" SHALL be added according to the PATCH below.


- PATCH:
  a. search for message "RRCConnectionRequest ::= SEQUENCE {" under clause 11.2 in 25.331
  b. replace the current spec text:
=========================================================================================================
		va40NonCriticalExtensions
			RRCConnectionRequest-NonCriticalExts-va40-IEs
=========================================================================================================
     by the following text adding one OPTIONAL:
==========================================================================================================
		va40NonCriticalExtensions
			RRCConnectionRequest-NonCriticalExts-va40-IEs	OPTIONAL
==========================================================================================================
     Note: Some TAB stops were removed in the citation above. 


- Background information:
  The reason for this compatibility problem is explained in LS R2-125168 of RAN WG2 meeting #79bis (2012-10):
  - 25.331 REL-10 CR4669r1 approved at RAN #52 (June 2011) in RP-110838
    missed to keep the OPTIONAL in the message RRCConnectionRequest
    which created a problem in 25.331 v10.4.0 (2011-06) and later REL-10 versions (up to v10.9.0).
  - With introduction of 25.331 v11.0.0 after RAN #54 in Dec. 2011 based on v10.5.0 the
    problem propagated into v11.0.0 and later REL-11 versions (up to v11.3.0).
  - 25.331 REL-9 CR5026r1 approved at RAN #56 (June 2012) in RP-120808
    and the solution of one ASN.1 error of this CR during the CR implementation review
    transferred the problem into 25.331 REL-9 affecting v9.11.0 and v9.12.0.
  - In the RRCConnectionRequest message an IE is optional in earlier versions but mandatory in later versions of this release.
     - If the UE signals the IE as Mandatory but the NW expects it as Optional,
       this may lead to unreliable decoding in the NW for any later non critical extension (NCE)
       as the UE will omit the optionality bit.
     - If the UE signals the IE as Optional but NW expects it as Mandatory,
       this may lead to a decoding error in the NW (in case of not enough padding)
       or unreliable decoding in the NW for any later non critical extension (NCE). 
    As a consequence the UE may not obtain a connection to the network

  The problem is solved as follows:
  - v9.11.0, v9.12.0, v10.4.0 to v10.9.0 will have a readme.txt file included that explains how the 25.331
    version has to be patched to avoid this problem.
  - A CR set to v9.13.0, 10.10.0 and v11.4.0 will add the missing OPTIONAL at RAN #58.
 