Title: ADS - Non-canonical encoding
     
PDR Reference:       			M0030001
Originator Reference:          	-
SARPs Document Reference: 		Doc 9705 edition 2, 2.2.1.4.2
Status:                   		ADOPTED (extral line with a comma in ASN.1)
Impact:                   		C (Clarification)
PDR Revision Date:     			30/01/02 (RESOLVED -> ADOPTED)
						25/08/00 (PROPOSED -> RESOLVED)
						23/05/00 (SUBMITTED -> ACCEPTED -> PROPOSED)
PDR Submission Date: 			02/03/00
Submitting State/Organization:    	Eurocontrol
Submitting Author Name:          	Kerr, A
Submitting Author E-mail Address: 	tony.kerr@ecsoft.co.uk
Submitting Author Supplemental:   	ECsoft UK Ltd. 
Contact Information:           	Tel: +44 1344 867199
                               	Fax: +44 1344 868442
SARPs Date:          			Doc 9705 Edition 2
						Doc 9705 Draft Edition 3
SARPs Language:      			English
     
Summary of Defect:
     
The ASN.1 definitions for ADS allow some options to be applied when encoding according to BASIC-PER.  This in general results in non-canonical encodings, which could cause future problems for applications, such as security, that depend upon a unique encoding to generate some derived value from the encoded bitstream.  The problem would occur if the encoded datastream is decoded, then re-encoded before the check on the derived value takes place.
     
Assigned SME:             		Sub-Volume II SME
     
Proposed SARPs amendment:
     
1) Replace SET OF in the ASN.1 definition of NoncomplianceNotification:
     
NoncomplianceNotification ::= CHOICE 
{
   demand-ncn        [0]  SET OF ReportType, 
   event-ncn         [1]  SET OF EventTypeContracted, 
   periodic-ncn      [2]  SET OF ReportTypeAndPeriod,
   ...
}
     
with SEQUENCE OF, as follows:

NoncomplianceNotification ::= CHOICE 
{
   demand-ncn        [0]  SEQUENCE OF ReportType, 
   event-ncn         [1]  SEQUENCE OF EventTypeContracted, 
   periodic-ncn      [2]  SEQUENCE OF ReportTypeAndPeriod,
   ...
}
     
2) Replace the DEFAULT construction of the reporting-interval element in the 
ASN.1 PeriodicContract definition:
     
PeriodicContract ::= SEQUENCE
{
   reporting-interval     [0]  ReportingInterval
                               DEFAULT minutes-scale: 5,
   (Etc..)
     
with the following:
     
PeriodicContract ::= SEQUENCE
{
   reporting-interval     [0]  ReportingInterval OPTIONAL
   -- The absence of the above element indicates a 5 minute reporting interval. 
   (Etc..)
     
Note: the file M0030001.asn attached to this PDR contains the ASCII DOS text of the ASN.1 resulting from the resolution of this PDR.
     
Impact on Interoperability:
     
The proposed changes have no impact on bits-on-the wire interoperability, and result in the BASIC-PER encoded ADS datastream being fully canonical. CENA tested the change with a COTS ASN.1 compiler, with the result that the resulting encoding from both ASN.1 descriptions is identical.
     
SME Recommendation to CCB:		-
     
CCB Decision:				atnp_ccb_chair: SUBMITTED (02/03/00)
						atnp_ccb_chair: PROPOSED (23/05/00)
						CCB-12 (Berlin): RESOLVED (25/08/00)