Re: Questions about FCIP

[Matt Wakeley <matt_wakeley@agilent.com>]

"Kavi, Prabhu" wrote:
> 
> I have some questions about the Fibre Channel over TCP/IP I-D.
> I understand IP networks well, but am a Fibre Channel newbie and
> am asking these questions to learn more about Fibre Channel's
> requirements rather than to challenge the document.  Here are
> my questions:
> 
> 1.  Why Fibre Channel over TCP, rather than over UDP?  Is it
>     because of an expectation of lossy IP networks?  Or is it
>     equally valid to put Fibre Channel over UDP (and therefore
>     can be the subject of another I-D).

Because UDP does not implement congestion management.

> 2.  What are the typical values of the ED_TOV and R_A_TOV timers?
>     Do I understand this right to say that these timers effectively
>     limit the latency across the IP network?

In Fibre Channel, the ED_TOV and R_A_TOV timer values are advertised from the
switches to the N_Ports during fabric login.  If a switch knows it has a FCIP
port, it could increase theses values as needed (but they are already
unreasonably huge to begin with)

> 3.  What is an acceptable loss ratio of Fibre Channel datagrams
>     that still allows the SAN devices to deliver appropriate
>     end user performance (order of magnitude estimate is
>     sufficient).
> 
> The reasons I ask are:
> 
> * I believe it is invalid to assume that IP networks are
>   necessarily more lossy than Fibre Channel networks.  A
>   well-engineered IP network, using a combination of both
>   Diff-Serv and MPLS, can achieve (close to) zero loss.

> * Stacking one reliable delivery protocol on top of another can
>   possibly lead to interesting performance problems after a loss
>   occurs, depending upon the timer values for TCP and Fibre Channel.

You talk about stacking one reliable protocol on another.  TCP is one, what is
the other?  Don't kid yourself into thinking fibre channel is reliable!

-Matt Wakeley
Agilent Technologies


> 
> Thanks in advance for your answers.
> 
> Prabhu Kavi
> ----------------------------------------------------------------------
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