On Thu, 1 Feb 2001, Augustin.Chevalier wrote:
> Hello,
>
> I don't really understand why using TYPE-IDENTIFIER.&Type(IFPPacket) instead
> of IFPPacket itself in the syntax (extracted from T38) below! If I'm not
> mistaken the result should be the same : only the values of the IFPPacket
> type are expected here.
> Have someone an explanantion.
>
> IFPPacket::=SEQUENCE
> {
> type-of-msg Type-of-msg,
> data-field Data-field OPTIONAL
>
> }
> UDPTLPacket::=SEQUENCE
> {
> seq-number INTEGER(0..65535),
> primary-ifp-packet TYPE-IDENTIFIER.&Type (IFPPacket),
> error-recovery CHOICE
> {
> secondary-ifp-packets SEQUENCE OF TYPE-IDENTIFIER.&Type(IFPPacket),
> fec-info SEQUENCE
> {
> fec-npackets INTEGER,
> fec-data SEQUENCE OF OCTET STRING
> }
> }
> }
This is a mechanism that is sometimes employed when PER ALIGNED is in use
and there is a need to ensure that the encoded data be aligned on a byte
boundary and ends on a byte boundary. This sort of syntax is also used in
security-related protocols (typically with DER) when cryptogrhic
enhancements are to be made to the data, such as creation of a digital
signature, for it avoids the need to encode IFPPacket twice - once to
generate the signature and a second time for transmission.
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