In order to achieve extensibility and adaptability to different
    application domains, the new version of the protocol defines many
    structures outside of the main ASN.1 specification, referencing them
    through ASN.1 EXTERNAL constructs. To simplify the construction and
    access to the externally referenced data, the Z39.50 ASN.1 module defines a
    specialized version of the EXTERNAL construct, called
    Z_External.It is defined thus:
   
typedef struct Z_External
{
    Odr_oid *direct_reference;
    int *indirect_reference;
    char *descriptor;
    enum
    {
        /* Generic types */
        Z_External_single = 0,
        Z_External_octet,
        Z_External_arbitrary,
        /* Specific types */
        Z_External_SUTRS,
        Z_External_explainRecord,
        Z_External_resourceReport1,
        Z_External_resourceReport2
    ...
    } which;
    union
    {
        /* Generic types */
        Odr_any *single_ASN1_type;
        Odr_oct *octet_aligned;
        Odr_bitmask *arbitrary;
        /* Specific types */
        Z_SUTRS *sutrs;
        Z_ExplainRecord *explainRecord;
        Z_ResourceReport1 *resourceReport1;
        Z_ResourceReport2 *resourceReport2;
        ...
    } u;
} Z_External;
   
    When decoding, the Z39.50 ASN.1 module will attempt to determine which
    syntax describes the data by looking at the reference fields
    (currently only the direct-reference). For ASN.1 structured data, you
    need only consult the which field to determine the
    type of data. You can the access  the data directly through the union.
    When constructing data for encoding, you set the union pointer to point
    to the data, and set the which field accordingly.
    Remember also to set the direct (or indirect) reference to the correct
    OID for the data type.
    For non-ASN.1 data such as MARC records, use the
    octet_aligned arm of the union.
   
    Some servers return ASN.1 structured data values (e.g. database
    records) as BER-encoded records placed in the
    octet-aligned branch of the EXTERNAL CHOICE.
    The ASN-module will not automatically decode
    these records. To help you decode the records in the application, the
    function
   
Z_ext_typeent *z_ext_gettypebyref(const oid *oid);
    can be used to retrieve information about the known, external data
    types. The function returns a pointer to a static area, or NULL, if no
    match for the given direct reference is found. The
    Z_ext_typeent
    is defined as:
   
typedef struct Z_ext_typeent
{
    int oid[OID_SIZE]; /* the direct-reference OID. */
    int what;          /* discriminator value for the external CHOICE */
    Odr_fun fun;       /* decoder function */
} Z_ext_typeent;
   
    The what member contains the
    Z_External union discriminator value for the
    given type: For the SUTRS record syntax, the value would be
    Z_External_sutrs.
    The fun member contains a pointer to the
    function which encodes/decodes the given type. Again, for the SUTRS
    record syntax, the value of fun would be
    z_SUTRS (a function pointer).
   
    If you receive an EXTERNAL which contains an octet-string value that
    you suspect of being an ASN.1-structured data value, you can use
    z_ext_gettypebyref to look for the provided
    direct-reference.
    If the return value is different from NULL, you can use the provided
    function to decode the BER string (see Section 2, “Using ODR”
    ).
   
If you want to send EXTERNALs containing ASN.1-structured values in the octet-aligned branch of the CHOICE, this is possible too. However, on the encoding phase, it requires a somewhat involved juggling around of the various buffers involved.
    If you need to add new, externally defined data types, you must update
    the struct above, in the source file prt-ext.h, as
    well as the encoder/decoder in the file prt-ext.c.
    When changing the latter, remember to update both the
    arm array and the list
    type_table, which drives the CHOICE biasing that
    is necessary to tell the different, structured types apart
    on decoding.
   
Eventually, the EXTERNAL processing will most likely automatically insert the correct OIDs or indirect-refs. First, however, we need to determine how application-context management (specifically the presentation-context-list) should fit into the various modules.