Preferred Message Size

Issue raised by: Curt Ellmann, 21 Aug 1995

Does "message size" (as in Preferred-message-size) apply to the size of the Present/Search Response PDU or only to the record data within the PDU? For example if the negotiated Preferred-message- size is 32767, must the target ensure that the PDU does not exceed that value, or just that the sum of the sizes of the records do not?

Formally: the sum of the sizes (in bytes) of the response records (retrieval and diagnostic records) should not exceed the value. Informally: if it does exceed it, it should not exceed it substantially.
Formally, message size is the sum of the sizes of the records "not including protocol control information (PCI)". However, what is and what is not PCI is not clearly defined. PCI was better understood (or thought to be better understood) when the only records of concern were MARC, but with the emergence of syntaxes described in ASN.1 PCI becomes less clear. The sentiment among implementors seems to be, for example, if the target thinks it sent 32767 bytes, but the origin thinks it sent 32770 (because the target considered something to be data that the origin though was protocol control information) the target complies in spirit, and the origin should be forgiving.
It is recommended that a client propose a value of Preferred-message- size that is less than the actual limit that the client can support.

Status: Approved (2/96)
Library of Congress
Library of Congress Help Desk (04/10/96)