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To: cs551@merlot.usc.edu
Subject: Re: Snoop operation 
Date: Thu, 03 May 2007 22:45:55 -0700
From: william@bourbon.usc.edu

Someone wrote:

  > What I mean is that whenever a SNOOP device receives
  > a new packet out of order, how can it immediately determine
  > that it's caused by congestion and not by the wireless
  > environment?

Slide 34 of lecture 24 is about "FH-to-MH Snoop Data
Processing".  It's assume that there is no wireless
connection from FH all the way to the basestation (running
Snoop).  The only wireless connection is from the basestation 
to the MH.  So, if a data packet from FH to MH skip a
sequence number, it's assume that the packet before was lost
in the Internet (and all packet loss in the Internet are
assumed to due to congestion).
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>





  On 5/3/07, william@bourbon.usc.edu <william@bourbon.usc.edu> wrote:
  > Someone wrote:
  >
  >   > On p. 34 of Lec. 24, what does "greater than last ACKed,
  >   > pass on" correspond to in the flowchart on the left?
  >
  > This corresponds to the "No" arrow out of "In-sequence".
  > This is the case of a congestion loss.
  >
  >   > Also, in the flowchart, if a new packet is out of sequence,
  >   > how could we immediately say that it is "congestion loss"
  >   > (the red font under the box)? I think the idea of snoop
  >   > is to abate TCP's overreaction to packet loss (some
  >   > caused by wireless environment not congestion).
  >
  > "Congestion loss" means that a packet is dropped by a
  > congested router in the Internet.  This is what TCP assumes
  > when any packet is lost.
  > --
  > Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>