On Tue, 19 Mar 2002, Ed Day wrote: > Hi, > > There seems to be some question among some groups that we have dealt > with whether PER unaligned is always unaligned. It would seem to me > that clause 7.7 is quite clear on this point that padding bits are > never inserted,
It is true that padding bits are never inserted in the case of PER unaligned, except in one case. See X.691 clause 10.1 where it makes it clear that padding bits are inserted when the encoding is that of the outermost type. In other words, when the entire message has been encoded, pad bits are added if needed to have the completed message end on an octet boundary. This also applies to the contents of an open type, which is always encoded in PER as if it were a complete message. > but others we have dealt with claim the note in clause > 16 on OCTET STRING's indicates padding bits should be inserted because > it does not discern between aligned and unaligned forms. It simply > states that "Octet strings of fixed length less than or equal to two > octets are not octet aligned. All other octet strings are aligned. > etc..". In the above you are pointing to the tutorial note at the top of clause 16, "Encoding the octetstring type", which is simply meant to give a tutorial summary of what the clause covers in depth. Please read the normative text - the body of clause 16. Note that it consistently speaks of addition of the octet string value to either the field-list as a bit-field or an octet-aligned bit-field. Once the field-list has been fully created, the fields within it are concatenated with or without padding to form the complete encoding, as specified by clause 10.1. Clause 10.1.2 makes it clear that for the UNALIGNED variant of PER no padding ever occurs, except for the encoding of the outermost type itself. It is also useful to read the definition of "field-list" in clause 3.7.12, as well as the clarifying note attached to it. This clause makes a clear distinction between adding an encoding into the field-list as either aligned or unaligned, and creation of the complete encoding where pad bits are or are not inserted, as specified by clause 10.1 ------------------------------------------------------------------------- Bancroft Scott Toll Free :1-888-OSS-ASN1 OSS Nokalva International:1-732-302-0750 [EMAIL PROTECTED] Tech Support :1-732-302-9669 x-1 1-732-302-9669 x-200 Fax :1-732-302-0023 http://www.oss.com
