Assymetric connections [WAS: Re: iSCSI Autosense Consensus, Connection next steps]

[csapuntz@cisco.com]

Hi Randall,

> > * iSCSI data/ready to transmit (rtt)
> >         - potentially high bandwidth
> >         - with appropriate headers, segments can be
> >           processed entirely out-of-order
> >         - simple data transfer state machine
> >         - no flow control on data needed, just congestion control
> >                    - rtt's need to be flow-controlled, but
> >                      that can be done with a simple credit mechanism
> >         - data needs to end up in special buffers (e.g. buffer cache)
> >
> I understand how you will obtain all of the above with 
> two TCP connecions. I can even support this, considering
> the 1 for control is "low bandwidth" (note: I can't support
> N data connections.. only 1). 

Thanks for pointing this out. The wording was a bit loose.

How does this sound?

Multiple parallel data connections should only be used in the
presence of an ECM-like congestion layer that does congestion
control on macro-flows.

> But my question is to the Data connection. How do you get 
> "no flow control on data needed, just congestion control" on
> a TCP connection?

You don't, but you can approximate it by opening the window to 1-2
gigabytes.

A longer answer for why no flow control is needed. Excuse me if
this is redundant:

I was assuming that all data transfers on the control connection are
requested by one of the parties. Since they are requested, there is a
buffer waiting to receive the result of the data transfer.

For example, let's say an initiator issues a SCSI READ of 64k
bytes. It is fairly safe to assume that the initiator has a 64k
byte buffer to receive the data.

In the case of writes, the initiator does not send the data along with
the SCSI WRITE (as is the case in NFS, CIFS, and HTTP PUT). Instead,
the initiator waits for the target to request the data. This is done
using the request to transmit (RTT) command. By the time a target
issues a request to transmit, it presumably has sufficient buffer to
receive the response.

There is, of course, flow control on data transfers in this system,
but it is done at the higher layers and need not be done at the
transport. Congestion control, of course, still needs to be done
at the transport layer.

_Costa