Re: CSMA/CD

From: K sPecial (xzziroz_at_linuxmail.org)
Date: 08/22/03

  • Next message: K sPecial: "Re: traceroute-like tool for UDP or TCP packet" 
    To: "Ansgar Wiechers" <bugtraq@planetcobalt.net>, "Security Focus" <security-basics@securityfocus.com>
Date: Sat, 23 Aug 2003 01:03:18 +0800

    Also take into recognition that there is a difference in length between what's called a "short" jam and a "long" jam. The difference is how far along the line from the sending host that the jam happens, usualy past are worst since if the line is too long and Machine A sends a message to machine B, the interval to wait is 0.9, if the line is long enough where as they had both waited that long And machine A had sent the message to B, B could send a message (legaly) that had followed the rule of waiting the .9, and still have a collision because of the ammount of time it had taken for machine A's message to get to machine B. This is a long collision and the problem here is that if its very long in some cases Machine A can go back into listen mode and not know there was ever a collision if it has transmitted all of it's data before the JAM signal propogates to himself. NICS only recognize a JAM when they are in send mode.

    --K-sPecial

    ----- Original Message -----
    From: Ansgar Wiechers <bugtraq@planetcobalt.net>
    Date: Fri, 22 Aug 2003 10:30:55 +0200
    To: Security-basics@securityfocus.com
    Subject: Re: CSMA/CD

    > On 2003-08-21 David Gillett wrote:
    > > Before a CSMA/CD device transmits, it listens to hear that the wire is
    > > clear. (If it isn't, it waits for a while and listens again.)
    > > There's a potential race condition where two (or more!) devices
    > > listen, find that the wire is clear, and both decide to transmit. Part
    > > of resolving this race condition is for each frame transmission to
    > > start with a "pad" of known filler.
    >
    > I may be wrong here, but AFAIK this is not the way padding works.
    >
    > Say you have two stations located at the two furthermost ends of the
    > ether. If one station starts sending, the signal takes some time to
    > propagate (since it travels with approximately 0,6 light speed only).
    > Now assume the other station starts sending right before the signal
    > reaches its port. In the next moment it will receive the first signal,
    > detect a collision (this is implemented in hardware AFAIK - something
    > like an XOR gate between input and output - since software would not be
    > responsive enough to accomplish this task) and a jam signal is sent.
    > This station is aware of the collision now, but the first station still
    > is not! Only when it receives the signal the second box sent before
    > finishing its own transmission, it will be able to detect that a
    > collision occured. This is accomplished by defining the minimum frame
    > size (64 byte) large enough to make sure a station keeps sending at
    > least the time a signal needs to cover twice the maximum segment size
    > ("there and back again").
    >
    > So where does pad fit in here? Unfortunately, en empty eternet frame
    > (headers only) is smaller than 64 byte (just 18 byte: destination
    > address, source address, type, FCS), so every frame smaller than 64 byte
    > is filled up with zeroes (or maybe even garbage, I'm not sure) to have
    > the required minimum size. This fillup is called pad.
    >
    > Regards
    > Ansgar Wiechers
    >
    > ---------------------------------------------------------------------------
    > ----------------------------------------------------------------------------
    > 

    -- 
______________________________________________
http://www.linuxmail.org/
Now with e-mail forwarding for only US$5.95/yr
Powered by Outblaze
---------------------------------------------------------------------------
----------------------------------------------------------------------------
  • Next message: K sPecial: "Re: traceroute-like tool for UDP or TCP packet"

    Relevant Pages

    • Re: CSMA/CD
      ... > Before a CSMA/CD device transmits, it listens to hear that the wire is ... > of resolving this race condition is for each frame transmission to ... Now assume the other station starts sending right before the signal ... This is accomplished by defining the minimum frame ...
      (Security-Basics)
    • Re: CSMA/CD
      ... > Before a CSMA/CD device transmits, it listens to hear that the wire is ... > of resolving this race condition is for each frame transmission to ... Now assume the other station starts sending right before the signal ... This is accomplished by defining the minimum frame ...
      (Security-Basics)
    • Re: Digital is at full strength, but reception may improve on Feb. 17.
      ... I-forget-the-brand DVDR with HardDrive is capable of receiving the ... same digitial station at different times on different frequencies?? ... Analog TV signals transmit a full frame of video 30 times a second ... saves a lot of transmission bandwidth, which is part of how it's ...
      (sci.electronics.repair)
    • Re: CSMA/CD
      ... > Before a CSMA/CD device transmits, it listens to hear that the wire is ... > of resolving this race condition is for each frame transmission to ... Now assume the other station starts sending right before the signal ... This is accomplished by defining the minimum frame ...
      (Security-Basics)
    • Re: A little challenge for relativists.
      ... Clocks and rulers in motion wrt the ether frame behave as ... Of course any observer can consider himself to be at rest. ... The light is emitted from the station WHEN the train's caboose ... So the same light leaves the same source at two different speeds ...
      (sci.physics.relativity)