Re: FCIP: A question about framing

["Randall R. Stewart" <randall@stewart.chicago.il.us>]

Robert:

I am a bit confused on how the URGENT pointer can be used
to do record marking... which is what is being attempted with
the PUSH bit. 

In RFC793 Section 3.7 it discusses the use of Urgent data. Here it
states:

"
  This mechanism permits a point in the data stream to be designated as
  the end of urgent information.  Whenever this point is in advance of
  the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
  must tell the user to go into "urgent mode"; when the receive sequence
  number catches up to the urgent pointer, the TCP must tell user to go


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Transmission Control Protocol
Functional Specification



  into "normal mode".  If the urgent pointer is updated while the user
  is in "urgent mode", the update will be invisible to the user.
"


Now if I am sending 3 FC frames and I mark each with the Urgent pointer.
the the last sentence there..

"If the urgent pointer is updated while the user is in "urgent mode",
 the update will be invisible to the user.
"

Pretty much means that if the receiver gets its urgent mode signal,
and then goes in to urgent mode reading... but the other two frames
arrive behind .. or possibly together with the first.. I fail to
see how one can tell the 3 frames apart...


I do NOT see how either PUSH or URGENT markers can be used for
something they were never intended for.. i.e. changing TCP
into a datagram oriented protocol..


R

"Robert B. Harmon" wrote:
> 
> I thought URGENT would be a more appropriate mechanism, why was URG
> expressly
> forbidden?
> 
> Also I've heard that other protocols have wanted to keep frames
> together in one IP packet.  Anyone remember which ones?
> 
> I can think of several other tunnelling protocol which use IP only
> IP-IP, Ether-o-IP, PPP-o-Eth...  I can't think of any which use TCP.
> 
> "KRUEGER,MARJORIE (HP-Roseville,ex1)" wrote:
> >
> > Do TCP RFCs specify the behavior of the receiver regarding when to deliver
> > data to the application?  From what I've read, different implementations act
> > differently - some buffer data received data and deliver it to the app when
> > the buffer's full, others deliver data immediately when each packet is
> > received.  In either implementation, when the PUSH flag is set, all data is
> > delivered immediately to the application.  In a FCoIP device, could't the
> > receipt of the packet with the PUSH flag set indicate that this is the "end
> > of a frame"?  Assuming that the FC aware code can also view the TCP header.
> >
> > I understand the truth of what you are saying when TCP is delivering data to
> > an "application", but I'm assuming that FCoTCP implementations will have
> > some level of mapping interaction between the TCP and the FC layer?
> >
> > > -----Original Message-----
> > > From: Randall R. Stewart [mailto:randall@stewart.chicago.il.us]
> > > Sent: Thursday, November 02, 2000 11:34 AM
> > > To: Murali Rajagopal
> > > Cc: Douglas Otis; Merhar, Milan; ips@ece.cmu.edu
> > > Subject: Re: FCIP: A question about framing
> > > Murali:
> > >
> > > Using Push does not guarantee that you will get "datagram-like"
> > > behavior. You
> > > must have some mechanism to find your "message" amongst a stream of
> > > bytes... this is what TCP presents you with. This usually means a
> > > message/finder that can parse the incoming stream and find the
> > > beginning/ending of the message boundaries... Some apps have
> > > been known
> > > to write every message in two pieces... write the number of bytes
> > > in this message in a 4 byte int, followed by the message with the PUSH
> > > flag set.  Even at that the reader side must read the 4 byte value
> > > and then continue to read until it has assembled a complete message.
> > >
> > > The PUSH flag is just a hint to TCP it does not assure you
> > > that you will
> > > get a datagram behavior depending on PUSH will not help you... been
> > > there ..
> > > . done that.. it don't work :/
> > >
> > > R
> > >
> >
> > Marjorie Krueger
> > Networked Storage Architecture
> > Hewlett-Packard Storage Organization
> > tel: +1 916 785 2656
> > fax: +1 916 785 0391
> > email: marjorie_krueger@hp.com

-- 
Randall R. Stewart
randall@stewart.chicago.il.us or rrs@cisco.com
815-342-5222 (cell) 815-477-2127 (work)