# The Z39.50 ASN.1 Module

## Introduction

The &asn; module provides you with a set of C struct definitions for the various PDUs of the Z39.50 protocol, as well as for the complex types appearing within the PDUs. For the primitive data types, the C representation often takes the form of an ordinary C language type, such as `int`. For ASN.1 constructs that have no direct representation in C, such as general octet strings and bit strings, the &odr; module (see section The ODR Module) provides auxiliary definitions.

The &asn; module is located in sub directory `z39.50`. There you'll find C files that implements encoders and decoders for the Z39.50 types. You'll also find the protocol definitions: `z3950v3.asn`, `esupdate.asn`, and others.

## Preparing PDUs

A structure representing a complex ASN.1 type doesn't in itself contain the members of that type. Instead, the structure contains pointers to the members of the type. This is necessary, in part, to allow a mechanism for specifying which of the optional structure (SEQUENCE) members are present, and which are not. It follows that you will need to somehow provide space for the individual members of the structure, and set the pointers to refer to the members.

The conversion routines don't care how you allocate and maintain your C structures - they just follow the pointers that you provide. Depending on the complexity of your application, and your personal taste, there are at least three different approaches that you may take when you allocate the structures.

You can use static or automatic local variables in the function that prepares the PDU. This is a simple approach, and it provides the most efficient form of memory management. While it works well for flat PDUs like the InitReqest, it will generally not be sufficient for say, the generation of an arbitrarily complex RPN query structure.

You can individually create the structure and its members using the `malloc(2)` function. If you want to ensure that the data is freed when it is no longer needed, you will have to define a function that individually releases each member of a structure before freeing the structure itself.

You can use the `odr_malloc()` function (see section Using ODR for details). When you use `odr_malloc()`, you can release all of the allocated data in a single operation, independent of any pointers and relations between the data. `odr_malloc()` is based on a "nibble-memory" scheme, in which large portions of memory are allocated, and then gradually handed out with each call to `odr_malloc()`. The next time you call `odr_reset()`, all of the memory allocated since the last call is recycled for future use (actually, it is placed on a free-list).

You can combine all of the methods described here. This will often be the most practical approach. For instance, you might use `odr_malloc()` to allocate an entire structure and some of its elements, while you leave other elements pointing to global or per-session default variables.

The &asn; module provides an important aid in creating new PDUs. For each of the PDU types (say, `Z_InitRequest`), a function is provided that allocates and initializes an instance of that PDU type for you. In the case of the InitRequest, the function is simply named `zget_InitRequest()`, and it sets up reasonable default value for all of the mandatory members. The optional members are generally initialized to null pointers. This last aspect is very important: it ensures that if the PDU definitions are extended after you finish your implementation (to accommodate new versions of the protocol, say), you won't get into trouble with uninitialized pointers in your structures. The functions use `odr_malloc()` to allocate the PDUs and its members, so you can free everything again with a single call to `odr_reset()`. We strongly recommend that you use the `zget_*` functions whenever you are preparing a PDU (in a C++ API, the `zget_` functions would probably be promoted to constructors for the individual types).

The prototype for the individual PDU types generally look like this:

        Z_<type> *zget_<type>(ODR o);

eg.:

        Z_InitRequest *zget_InitRequest(ODR o);

The &odr; handle should generally be your encoding stream, but it needn't be.

As well as the individual PDU functions, a function `zget_APDU()` is provided, which allocates a top-level Z-APDU of the type requested:

        Z_APDU *zget_APDU(ODR o, int which);

The `which` parameter is (of course) the discriminator belonging to the `Z_APDU` `CHOICE` type. All of the interface described here is provided by the &asn; module, and you access it through the `proto.h` header file.

## Object Identifiers

When you refer to object identifiers in your application, you need to be aware that SR and Z39.50 use two different set of OIDs to refer to the same objects. To handle this easily, &yaz; provides a utility module to &asn; which provides an internal representation of the OIDs used in both protocols. Each oid is described by a structure:

    typedef struct oident
    {
        enum oid_proto proto;
        enum oid_class class;
        enum oid_value value;
        int oidsuffix[OID_SIZE];
        char *desc;
    } oident;

The `proto` field can be set to either `PROTO_SR` or `PROTO_Z3950`. The `class` might be, say, `CLASS_RECSYN`, and the `value` might be `VAL_USMARC` for the USMARC record format. Functions

    int *oid_ent_to_oid(struct oident *ent, int *dst);
    struct oident *oid_getentbyoid(int *o);

are provided to map between object identifiers and database entries. If you store a member of the `oid_proto` type in your association state information, it's a simple matter, at runtime, to generate the correct OID when you need it. For decoding, you can simply ignore the proto field, or if you're strict, you can verify that your peer is using the OID family from the correct protocol. The `desc` field is a short, human-readable name for the PDU, useful mainly for diagnostic output.

The old function `oid_getoidbyent` still exists but is not thread safe. Use `oid_ent_to_oid` instead and pass an array of size `OID_SIZE`.

Plans are underway to merge the two protocols into a single definition, with one set of object identifiers. When this happens, the oid module will no longer be required to support protocol independence, but it should still be useful as a simple OID database.

## EXTERNAL Data

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 &asn; 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 &asn; 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 (eg. 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(oid_value ref);

Can be used to retrieve information about the known, external data types. The function return 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
    {
        oid_value dref;    /* the direct-reference OID value. */
        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 Using ODR).

If you want to send EXTERNALs containing ASN.1-structured values in the occtet-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` arrary 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.

## PDU Contents Table

We include, for reference, a listing of the fields of each top-level PDU, as well as their default settings.

### Default settings for PDU Initialize Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
protocolVersion | Odr_bitmask | Empty bitmask
options | Odr_bitmask | Empty bitmask
preferredMessageSize | int | 30*1024
maximumRecordSize | int | 30*1024
idAuthentication | Z_IdAuthentication | NULL
implementationId | char* | "81"
implementationName | char* | "YAZ"
implementationVersion | char* | YAZ_VERSION
userInformationField | Z_UserInformation | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for PDU Initialize Response

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
protocolVersion | Odr_bitmask | Empty bitmask
options | Odr_bitmask | Empty bitmask
preferredMessageSize | int | 30*1024
maximumRecordSize | int | 30*1024
result | bool_t | TRUE
implementationId | char* | "id)"
implementationName | char* | "YAZ"
implementationVersion | char* | YAZ_VERSION
userInformationField | Z_UserInformation | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for PDU Search Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
smallSetUpperBound | int | 0
largeSetLowerBound | int | 1
mediumSetPresentNumber | int | 0
replaceIndicator | bool_t | TRUE
resultSetName | char * | "default"
num_databaseNames | int | 0
databaseNames | char ** | NULL
smallSetElementSetNames | Z_ElementSetNames | NULL
mediumSetElementSetNames | Z_ElementSetNames | NULL
preferredRecordSyntax | Odr_oid | NULL
query | Z_Query | NULL
additionalSearchInfo | Z_OtherInformation | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for PDU Search Response

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
resultCount | int | 0
numberOfRecordsReturned | int | 0
nextResultSetPosition | int | 0
searchStatus | bool_t | TRUE
resultSetStatus | int | NULL
presentStatus | int | NULL
records | Z_Records | NULL
additionalSearchInfo | Z_OtherInformation | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for PDU Present Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
resultSetId | char* | "default"
resultSetStartPoint | int | 1
numberOfRecordsRequested | int | 10
num_ranges | int | 0
additionalRanges | Z_Range | NULL
recordComposition | Z_RecordComposition | NULL
preferredRecordSyntax | Odr_oid | NULL
maxSegmentCount | int | NULL
maxRecordSize | int | NULL
maxSegmentSize | int | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for PDU Present Response

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
numberOfRecordsReturned | int | 0
nextResultSetPosition | int | 0
presentStatus | int | Z_PRES_SUCCESS
records | Z_Records | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Delete Result Set Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
deleteFunction | int | Z_DeleteRequest_list
num_ids | int | 0
resultSetList | char** | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Delete Result Set Response

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
deleteOperationStatus | int | Z_DeleteStatus_success
num_statuses | int | 0
deleteListStatuses | Z_ListStatus** | NULL
numberNotDeleted | int | NULL
num_bulkStatuses | int | 0
bulkStatuses | Z_ListStatus | NUL L
deleteMessage | char* | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Scan Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
num_databaseNames | int | 0
databaseNames | char** | NULL
attributeSet | Odr_oid | NULL
termListAndStartPoint | Z_AttributesPlus... | NULL
stepSize | int | NULL
numberOfTermsRequested | int | 20
preferredPositionInResponse | int | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Scan Response

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
stepSize | int | NULL
scanStatus | int | Z_Scan_success
numberOfEntriesReturned | int | 0
positionOfTerm | int | NULL
entries | Z_ListEntris | NULL
attributeSet | Odr_oid | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Trigger Resource Control Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
requestedAction | int | Z_TriggerResourceCtrl_resou..
prefResourceReportFormat | Odr_oid | NULL
resultSetWanted | bool_t | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Resource Control Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
suspendedFlag | bool_t | NULL
resourceReport | Z_External | NULL
partialResultsAvailable | int | NULL
responseRequired | bool_t | FALSE
triggeredRequestFlag | bool_t | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Resource Control Response

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
continueFlag | bool_t | TRUE
resultSetWanted | bool_t | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Access Control Request

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
which | enum | Z_AccessRequest_simpleForm;
u | union | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Access Control Response

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
which | enum | Z_AccessResponse_simpleForm
u | union | NULL
diagnostic | Z_DiagRec | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Segment

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
numberOfRecordsReturned | int | value=0
num_segmentRecords | int | 0
segmentRecords | Z_NamePlusRecord | NULL
otherInfo | Z_OtherInformation | NULL

### Default settings for Close

Field | Type | Default Value
--- | --- | ---
referenceId | Z_ReferenceId | NULL
closeReason | int | Z_Close_finished
diagnosticInformation | char* | NULL
resourceReportFormat | Odr_oid | NULL
resourceFormat | Z_External | NULL
otherInfo | Z_OtherInformation | NULL