Re: a vote for asymmetric connections in a session

["Matt Wakeley" <matt_wakeley@agilent.com>]

Mike Kazar wrote:

> Folks,
>
> Here are my reasons for preferring the asymmetric connection model to the
> symmetric connection model, in decreasing order of importance.
>
> 1.  Implementing a sliding window protocol for seqRn processing is likely
> to be as hard, or harder, as implementing TCP.  Getting something that
> works will be easy, but getting something that works well, even when
> various iSCSI TCP connections are running at different rates is likely to
> be an "interesting" research problem.  Yet connections running at different
> rates is likely to occur in real life quite frequently, sometimes due to
> different connections seeing different packet drop events in a somewhat
> congested network, sometimes due to different connections taking different
> paths (the whole *goal* of multiple connections in a session, after all).

Well, I disagree.  Implementing sliding windows is not that hard.  If sliding
windows were the toughest thing to implement in iSCSI, this would be a cake
walk.

>
>
> Furthermore, the sliding window state machine for a given session's seqRn
> processing is running at N times the event rate of an individual TCP
> connection's state machine (N is the # of connections in a session), making
> it even more challenging to implement.

I disagree again.  The TCP sliding window SM is running at once per TCP
segment.  If just iSCSI command messages where crammed down the connection,
that means the session's seqRn processing will be done once every 48 bytes.
Every time an iSCSI message is received, there is a lot of processing invoked.
Adding to the list a simple sliding window algorithm is not a challenge.

>
>
> 2.  It is easier to implement target-specific load sharing mechanisms if
> the target gets to choose, perhaps with input from the host, the connection
> on which a particular transfer should be performed.  In the symmetric
> proposal, the host chooses the connection to use completely on its own.

And it's good that the host chooses the connection.  It is the one that has set
up DMA structures on a particular connection to get the data in/out with
minimal CPU intervention.

>
>
>         Mike
>                 (kazar@spinnakernet.com)

In my opinion, these are not "good" reasons to vote for one or the other.

Matt Wakeley
Agilent Technologies