Re: [storm] iSER - what to do

Alexander Nezhinsky <nezhinsky@gmail.com> Sun, 15 July 2012 07:26 UTC

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Date: Sun, 15 Jul 2012 10:27:24 +0300
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From: Alexander Nezhinsky <nezhinsky@gmail.com>
To: storm@ietf.org, David Black <david.black@emc.com>, Mike Ko <mkosjc@gmail.com>, Paul Koning <Paul_Koning@dell.com>, Mallikarjun Chadalapaka <cbm@chadalapaka.com>, Or Gerlitz <ogerlitz@mellanox.com>, Mike Christie <michaelc@cs.wisc.edu>
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Subject: Re: [storm] iSER - what to do
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Hi, all

Sorry for a late answer (again).

I have been thinking over this issue hesitantly for a long time being
close to just agree with the latest set of suggestions.
But then I realized there is a simple counter-argument which
complicates things even more.

When the initiator sends its final Login Request it is not guaranteed
that the next Login Response it receives is the "final" one, too.
If the target has more text data to send than the hardcoded 8KB, it
will split it into two (or more) PDUs by raising Continue bit in all
its responses except the last.

This is a rare event but it means that to be fully compliant and
full-proof the initiator can't just post another N buffers to anticipate
all "unexpected" PDUs from target.

It posts one 8KB buffer for the next Login Response, but it should be
ready for the case where the response contains C=1. In such case
it would post another 8KB buffer and answer ok to continue.

Regular initiator rx-buffers are much smaller than 8KB.
Implementation-wise they are usually allocated from a separate pool or
some other kind of discrimination is made between the login and
full-featured-phase buffers.

As there is no acceptable way to reclaim the buffers after they have
been posted, the only way out is to post a few 8KB buffers, but it will
make the implementation even more complicated and cumbersome.

All in all, I suggest that we bite the bullet, complete the spec and head
towards fully spec-compliant implementations of both initiator and target
as soon as possible.
On pratical grounds we can address the distro maintainers to employ all
possible means to distribute compliant updates sooner than later,
as those will represent a special, critical change.

To minimize the damages i suggest taking the following path:

1. iSERHelloRequired remains defined as is, with default=No.

2. It becomes *mandatory* for a fully-spec compliant initiator
   implementation to communicate iSERHelloRequired=Yes.
   * If this key is not sent then the "new" target knows that it has
     encountered an "old" initiator.
   * If the initiator sends  iSERHelloRequired=No, it means it choses
     (for some bizarre reasons) to behave as an "old" one - while
     such behavior is strongly discouraged.
     I guess the requirement that:
     "the initiator SHOULD send iSERHelloRequired=Yes"
     reflects the situation, correct me if i'm wrong.

3. "New" initiator will recognize an "old" target by receiving
   "NotUnderstood" in response to iSERHelloRequired=Yes.
   Then it can either refuse to deal with it, or to employ a range of
   tricky means used until now.
   We can describe those means as the guidelines, e.g. :
   * posting one or better MaxOutstandingUnexpectedPDUs buffers
   * to be really on the safe side, having those buffers at least 8KB long.

   As we are trying to neutralize the shortcomings of the existing
   targets, the initiator can bet that the target won't send split
   login responses, as it regularly does not do so today.

4. "New" target will recognize an "old" initiator by having received
   iSERHelloRequired=No either implicitly or explicitly. Then it must
   ignore the iSERHello absense and may also take some precautions,
   like:
   * delaying sending any "unexpected" PDUs until the first PDU is
     received from the initiator after the final login response
     has been sent
   * taking a reasonable timeout, say a second (the exact value
     does not matter as the initiator can't count on it anyway and
     no value will solve the problem in full, theoretically).
   * doing both, that is waiting for the first incoming PDU and
     taking a timer to start sending NOP-INs in case no PDUs arrived
     during the timeout period, to be able to detect silent connection
     failures.

5. "New" target and "new" initiator will count on ISERHello as the
   guarantee of proper buffer posting

6. "Old" target and "old" initiator will work as they do now, in their
   double bliss of ignorance.

By the way, the initiator patch alleviating the problem by posting one
additional login buffer was submitted relatively recently and all previous
deployed implementations of the initiator are exposed.
Eventually, the new better code is making its way to the users of all distros.
This is a common situation encountered by the linux kernel community
quite often. Let's take this as a working example, make the spec fool-proof
and advise the implementors how to minimize the damages with the old
software, while keeping everything as simple as possible under these
already over-complicated circumstances.

Alexander