Search requests can have multiple responses (zero or more search result entries, zero or more search result references, and the final search result done), but the add, bind, compare, delete, extended, modify, and modify DN operation types all have one response message.

Except that’s not entirely true. The server can also return any number of intermediate response messages in response to any of the above types of operations, although RFC 4511 section 4.13 states that intermediate responses should only be used in circumstances where the client explicitly indicates that they’re prepared to accept them. This is generally done by requesting a type of extended operation that may return them, or by including a request control that may cause intermediate response messages to be returned.

RFC 4511 section 4.13 defines the intermediate response protocol operation as follows:

IntermediateResponse ::= [APPLICATION 25] SEQUENCE {
        responseName     [0] LDAPOID OPTIONAL,
        responseValue    [1] OCTET STRING OPTIONAL }

And the LDAPOID type is defined in RFC 4511 section 4.1.2 as:

LDAPOID ::= OCTET STRING -- Constrained to <numericoid>
           -- [RFC4512]

That is, it’s a sequence with BER type 0x78 (application class, constructed, tag number twenty-five), that may include an OID that identifies the type of intermediate response, and it may contain a value that provides additional data for the response.

There aren’t many intermediate response types defined in official LDAP specifications (although some servers define their own proprietary intermediate responses for various purposes). But RFC 4533, which describes the LDAP content synchronization operation, does define a Sync Info intermediate response with an OID of and a value with the following encoding:

syncInfoValue ::= CHOICE {
    newcookie      [0] syncCookie,
    refreshDelete  [1] SEQUENCE {
        cookie         syncCookie OPTIONAL,
        refreshDone    BOOLEAN DEFAULT TRUE
    refreshPresent [2] SEQUENCE {
        cookie         syncCookie OPTIONAL,
        refreshDone    BOOLEAN DEFAULT TRUE
    syncIdSet      [3] SEQUENCE {
        cookie         syncCookie OPTIONAL,
        refreshDeletes BOOLEAN DEFAULT FALSE,
        syncUUIDs      SET OF syncUUID

A Sync Info intermediate response with message ID two that only provides a syncCookie with cookie value NomNomNom would be encoded as:

30 2c -- Begin the LDAPMessage sequence
   02 01 02 -- The message ID (integer value 2)
   79 27 -- Begin the intermediate response protocol op
      80 18 31 2e 33 2e 36 2e 31 2e -- The responseName (octet string
            34 2e 31 2e 34 32 30 33 -- "")
            2e 31 2e 39 2e 31 2e 34
      81 0b -- Begin the responseValue element
         80 09 4e 6f 6d 4e 6f 6d 4e 6f -- The syncCookie value
 6d                      -- (octet string "NomNomNom")

Intermediate response messages can only be sent after a request that indicates that the client is willing to accept intermediate responses, and before the final response to that request. The inclusion of intermediate response messages in response to a request does not affect the encoding of the final response to that request.

Previous: The LDAP Unbind Operation