Re: iSCSI: Next-Qualifiers and PDU diagrams

[julian_satran@il.ibm.com]

David,

I will try to lessen confusion somehow - I am not sure that adding the NQ
everywhere is the answer.

The trouble with TLV formats is that they need at least two passes. With
the NQ I use the fact that
the basic header is always there and the qualifier will be needed do
describe the next segment (its type and length).

Julo

Black_David@emc.com on 02/03/2001 21:23:10

Please respond to Black_David@emc.com

To:   ips@ece.cmu.edu
cc:
Subject:  iSCSI: Next-Qualifiers and PDU diagrams




In working with some other folks, I stumbled across
a format issue that's more confusing than it needs
to be in the current iSCSI draft.  The issue is that
Next-Qualifiers (WN, etc.) and the PDUs they go with
are not documented/diagrammed together, making it
hard to figure out how to parse iSCSI PDUs.

For an example of how this causes confusion, consider
the Text, Login and NOP PDU diagrams in Sections 2.8
through 2.13.  Starting from one of these diagrams,
how does one figure out the size of the Text or
Ping data that's appended?

The answer's not in the diagrams -- there's a 4-byte
Next Qualifier preceding the PDU shown in each diagram.
The Text or Ping data is considered to be a data
segment, and hence the format of the Next Qualifier
is that given in Section 2.2.2.3, including a 3-byte
length field that answers the question.

That's not an obvious answer, and there are several
factors that contribute to it being potentially confusing:
- None of the PDU diagrams show the Next-Qualifiers
- Byte 0 means different things - at the top of
     Section 2.2, it's the start of the Next-Qualifier,
     but by Section 2.2.3, it's shifted down 4 bytes
     to be the start of the Basic Header Segment.
- Nothing obvious seems to say that the Text or the
     Ping Data is a "data segment".  The latter
     might be obvious, but the former isn't.
- The Next-Qualifier structure is unexpected to
     those familiar with other protocols.  A
     common protocol structure is T/L/V (Type/
     Length/Value).  Because the BHS type and
     length are implicit, the iSCSI structure
     is actually Type N+1/Length N+1/Value N.
So, much as I realize the busy work involved in
revising ASCII graphics, I think all the PDU diagrams
in Section 2 need to be revised to show the Next-
Qualifiers.  This would also take care of the second
item above, because then byte 0 in all the PDU
diagrams will be the first byte of the Next-Qualifier
that precedes the Basic Header Segment.  Addressing
the third item involves a few sentences in to
explain what the "data segment" indicated by the
Next Qualifier before the BHS actually covers/
contains in those cases.

The fourth item is a matter of taste.  It would
certainly be cleaner if iSCSI used a TLV structure,
rather than a T+1/L+1/V structure.  Much of the
network community is familiar with TLV and will
find the T+1/L+1/V structure confusing.  Are the
4 bytes saved by not having a descriptor for the
BHS really worth it?

On a related topic, it would be considerably
cleaner if the type (WN) field were a single byte
that was completely enumerated, rather than the
current bit field structure, and if the
peculiar convention for the AddCDB length in
2.2.2.1 were dropped in favor of the 3-byte
length field in 2.2.2.3 to remove a case
from the header parse logic.

Mindful of my WG co-chair role, I really think
the diagram revisions and explanations of what
a "data segment" is need to be done.  OTOH,
the above two paragraphs on whether to have a BHS
descriptor and how to format the Next-Qualifiers
are suggestions for further discussion.

Thanks,
--David

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