iscsi: header digest issue

[pat_thaler@agilent.com]

We have a concern about the header digest location in the PDU. Currently,
the header digest is located after the AHS. This is convenient for the
transmitter since it calculates the header digest over the BHS and AHS and
then appends the digest. 

The problem is that the receiver must read a field that is covered by the
header digest in order to know the position of the digest. This has two
problems: it reduces the hamming distance protection of the digest to one
and it can cause an error recovery problem when an error changes the length
field to a value that is beyond the end of the transmitted data stream.

The CRC generally has a hamming distance of 4 and we went to some effort to
pick a CRC that was robust in the presence of small numbers of bad bits.
With the current PDU definition, an error in the TotalAHSLength field will
cause the wrong value of the stream being read as the AHS. There is a
possibility that this location contains a value that is the same as the CRC
of the bytes up to that point. A single bit error has a 2^-32 chance of
producing an undetectable error.

The second concern is on error recovery when an error occurs in a PDU that
is not followed by other PDUs. The error may change the length field so that
it indicates a position beyond the end of the actual PDU. The receiver waits
for more data to arrive so that it can check the digest and then process the
packet. Since there is no more data arriving it will hang waiting until a
timeout occurs. Therefore, when this kind of error occurs the error recovery
doesn't function.

Both problems could be solved by placing the header digest after the BHS and
before the AHS headers. 

Regards,
Pat Thaler