Synopsis:

What value do we give the integer in the Extension.identifier field?

Response:

The integer is the identifier of the extension. It is (or should be) defined in the protocol, and an amendment (ISO 10161-1:1997/pDAM1) is being processed to do just that. The value is 1, for "Externally defined extension". This is so that an extension with identifier 1 requires the ANY to be an EXTERNAL data type.

Clause 9.1.2
Change the definition of the Extension data type (lines 462-466) to:
Extension ::= SEQUENCE {
	identifier	[0]	IMPLICIT INTEGER,
	-- value 1 = implementor extensions			--
	critical	[1]	IMPLICIT BOOLEAN DEFAULT FALSE,
	item		[2]	ANY DEFINED BY identifier
	-- if identifier = 1 then item ::= EXTERNAL		--
}

iLL-Request-Extension {
    identifier   1,
    critical      FALSE,
    item         EXTERNAL :  {    -- universal tag 8
        direct-reference {1 0 10161 13 3},
        encoding  single-asn1-type : APDU-DeliveryInfo : {
          sender-info  {
            encoding   2,
            transport  {
              telecom-service-identifier  "TCP/IP",
              telecom-service-address  "library.college.edu:1410" },
            aliases  {
               person-or-institution-symbol
		institution-symbol : "NUC:ABCD"},
                     {
               person-or-institution-symbol
		institution-symbol : "OCLC:XYZ"} },
          recipient-info  {
            encoding  ...,
            transport ...,
            aliases  ... },
          transponder-info  {
            encoding  ...,
            transport ...,
            aliases  ...  } } } }