iSCSI: Negotiation clarifications still needed

[Martins Krikis <mkrikis@yahoo.com>]

The previous thread went on too long, but since it
has now quieted down, I'll conjecture that the
following are the aspects that still need to be
addressed.

1. Not everybody seemed to have noticed that it is
   NOT legal to send the same key again, if it
   has once been negotiated (including negotiations
   that end with a reserved value (Reject, 
   Irrelevant or NotUnderstood).

I think it would benefit the draft to add the
sentence that Pat proposed to paragraph 5 or page
72 in 12-92: "Sending the key again would be a
re-negotiation". That I think would make it crystal
clear.

2. When the key=value pairs that Originator is
   sending are broken across multiple PDUs, it
   is not clear whether the Responder may start
   responding to the keys as soon as it receives
   them or whether it should send blank PDUs back
   (as in the example on page 164 of 12-92) until
   it gets a PDU, the data part of which ends in
   a NUL byte (thus signaling that there are no
   broken key=value pairs at the end of it).

I am proposing that the draft should make it explicit
that only blank PDUs are to be sent. This allows
decoupling of key=value generation from
their encapsulation in PDUs (i.e., the generating
logic need not worry about whether a key=value
pair will fit and go out in this PDU or has to
be retained to go out in the next). I can explain
in detail why this is important (it has to do
with teh possibility of receiving the "just-about
outgoing" keys) but I'm keeping this "brief".

Furthermore, it is my feeling that instead of
checking the last bytes of a PDU for NUL, it
would be better if the end-of-data was marked by
a flag in the header. This way encapsulation will
be simpler---just put as much data in the PDU as
fits there and raise the flag if it isn't all,
instead of checking whether it ends in a NUL and
possibly shortening data to make it not end like
that.

3. There is an opinion that on page 73 of 12-92,
   the phrase that says "or the responder may
   select an admissible value" is in contradiction
   to the very next sentence. There is also an
   opinion that this phrase is entirely unnecessary
   and detrimental to achieving broad 
   interoperability (I call it "cutting slack to
   misbehaving or incompatible originators").

I don't have a suggestion since I consider the
"feature" that this phrase allows of little 
importance to a properly built iSCSI node.

4. This is new. When doing Text Request/Response
negotiations (i.e., in FFP), it seems 
that the Initiator commits to the new values
when it receives a response from the Target with
the F bit set. It is unclear when the Target should
commit. Should it switch to using the new values
as soon as it sends its response with the F-bit
set, or should it do so only when it knows that
the Initiator received its response? 

Commiting right away is simpler and since responses
with F-bits set have TTT=0xffffffff and thus
may not be reset, sounds plausible. If the
values have importance on the next reception,
it may also be important to commit timely. However,
what if the Initiator doesn't get this response? 
Target now has committed, Initiator hasn't. Committing
later puts the burden on Initiator to send something
effectively telling "I've received your final
response". Otherwise the Target will time out and
not commit. This response can get lost too.

Basically, it is beginning to look a bit like
(what was it called?) "distributed consensus problem"?
I think it goes like this:

Two generals that are on oposite sides of the
enemy want to synchronize their attack, and
start sending messengers through with messages
like 
  "attack at dawn->", 
  "<-ok, attack at dawn",
  "I know you know we attack at dawn->",
  "<-, I know you know I know we attack at dawn",
  etc., etc., ...
But at no point can they commit yet...

Is anybody else worried about this?
Anyway, so when should a target commit? Page 83
of 12-92 is the relevant reference.

Thanks,

  Martins Krikis

Disclaimer: these opinions are my own and may
            not be those of my employer.


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