Compaq OSI/FTAM D43 Limits on Small String-Length Values With Large PDUs, Use of Format Effectors

Conformance and Interoperatility Data-Transfer Considerations

Limits on Small String-Length Values With Large PDUs

When the Compaq FTAM responder receives data from a remote initiator, it decodes the data and stores it, as a sequence of strings, in an internal buffer with a maximum size of 25 KB. According to the NIST FTAM Phase 2 agreements, P-DATA carrying encoded FTAM PDUs or data elements cannot exceed 16 KB; however, string-header information in the buffer can cause the data in the buffer to be much larger than the maximum size of the encoded data. Because each string in the buffer includes a fixed number of bytes of header information, packing small strings into a large PDU can cause the 25 KB buffer size to be exceeded.

During data decoding, the responder checks the length of the data. If the decoded data cannot be accommodated in the 25 KB buffer, the responder generates a provider abort.

To avoid exceeding the buffer-size limit for writes to the Compaq responder, you can either send a smaller number of strings per PDU or send larger strings, as described in Section 4.

Handling of Escape Sequences

When writing data to the Guardian file system from a remote initiator, the responder first removes any escape sequences contained in each string it receives before enforcing the maximum-string-length limitation. When sending data to the remote initiator, it does not check for escape sequences, but simply counts all bytes and packages them into strings.

Use of Format Effectors

Format effectors are characters such as carriage returns and line feeds, which control the formatting of information on character-imaging devices. To interoperate successfully, application programmers must understand what an implementation expects as an end-of-line symbol and how it interprets format effectors. For example, some implementations use the FTAM-1 document type to transfer binary data as opposed to text. Some implementations recognize carriage returns and line feeds as format effectors and discard them if binary data, not text, is being transferred. Others see the format effectors as data and transfer them as such.

In the Compaq FTAM responder’s virtual filestore (VFS), FTAM-1 files are treated as documents and are implemented as Guardian EDIT files, which have a maximum record length of 239 characters. The responder interprets carriage return-line feed combinations (CR/LFs) as end-of-line indicators. If a file being written to the responder’s VFS does not contain CR/LFs, the file is written in 239-character records.

Character Sets

The Compaq FTAM responder does no character-set verification. For FTAM-1 and FTAM-2 files, to ensure that the file being transferred contains the correct character- string type as specified in the universal-class parameter, your remote application should verify characters as it sends or receives the data.

OSI/FTAM Responder Manual—425199-001

2-5