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  • MTU doubts

    Hi,

    I've following queries:

    1. Consider in a TCP/IP network, two PCs are connected via router. PC1 has MTU=2000 and PC2 has MTU=500. I try to send a packet of size 2000 from PC1 to PC2. What happens when:
    • (a)Dont fragment bit is not set (DF=0)?
      (b)Dont fragment bit is set (DF=1)?



    2. What happens if a try to ping, with packet size 2000, from PC1 to PC2??

    Waiting for explanations..

    -Muks

  • #2
    Re: MTU doubts

    If I'm not mistaken, the MTU of the sending host has nothing to do with the MSS of the receiving host (although the MSS is usually based on the MTU). So the question here is what will the router do based on it's MTU and MSS since it is both receiving the packet from PC1 and sending it to PC2. If the DF flag is 0 and the packet is too large for the router it will tell PC1 to fragment the packet. If the DF flag is 1 and the packet is too large for the router it will tell PC1 that it can't fragment the packet and then the router will drop the packet.

    Can anyone confirm my understanding of this?

    Comment


    • #3
      Re: MTU doubts

      Originally posted by joeqwerty View Post
      If I'm not mistaken, the MTU of the sending host has nothing to do with the MSS of the receiving host (although the MSS is usually based on the MTU). So the question here is what will the router do based on it's MTU and MSS since it is both receiving the packet from PC1 and sending it to PC2.
      MTU and MSS are related in IP. So MTU does make a difference.

      Originally posted by joeqwerty View Post
      If the DF flag is 0 and the packet is too large for the router it will tell PC1 to fragment the packet.
      Can anyone confirm my understanding of this?
      Are you sure that router will ask PC1 to fragment?? I think Router itself will fragment the packet.

      -Muks

      Comment


      • #4
        Re: MTU doubts

        Of course the MSS (receive window) of the receiving host will have some bearing on the MTU (send window) of the sending host but I was looking at it from the perspective of the router (although the receiving host would do the same thing if a router was not involved). So my assumption is the same: If the DF flag is 0 and the packet is too large then the router or receiving host will tell PC1 to fragment the packet. If the DF flag is 1 and the packet is too large then the router or receiving host will tell PC1 that it can't fragment the packet and then drop the packet.

        You could set up a packet sniffer to confirm what actually happens.
        Last edited by joeqwerty; 7th September 2008, 19:04.

        Comment


        • #5
          Re: MTU doubts

          Originally posted by joeqwerty View Post
          So my assumption is the same: If the DF flag is 0 and the packet is too large then the router or receiving host will tell PC1 to fragment the packet.
          How router will tell PC1 to fragment the packet?

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          • #6
            Re: MTU doubts

            Well I'm trying to put this together with my understanding of TCP/IP, so here goes:

            1. If the packet sent from PC1 to PC2 is too large for the router and DF = 0 then the router will fragment the packet.

            2. If the packet sent from PC1 to PC2 is too large for the router and DF = 1 then the router will send an ICMP type 3 code 4 packet to PC1 telling PC1 that the packet needed to be fragmented but that DF = 1.

            3. If the packet is not too large for the router but is too large for PC2 and DF = 0 then PC2 will advertise it's receive window to PC1 and PC1 will adjust accordingly.

            4. If the packet is not too large for the router but is too large for PC2 and DF = 1 then PC2 will send an ICMP type 3 code 4 packet to PC1 telling PC1 that the packet needed to be fragmented but that the DF = 1.

            Have you tested any of this with a packet sniffer yet?

            Comment


            • #7
              Re: MTU doubts

              Originally posted by joeqwerty View Post
              Well I'm trying to put this together with my understanding of TCP/IP, so here goes:

              1. If the packet sent from PC1 to PC2 is too large for the router and DF = 0 then the router will fragment the packet.

              2. If the packet sent from PC1 to PC2 is too large for the router and DF = 1 then the router will send an ICMP type 3 code 4 packet to PC1 telling PC1 that the packet needed to be fragmented but that DF = 1.

              3. If the packet is not too large for the router but is too large for PC2 and DF = 0 then PC2 will advertise it's receive window to PC1 and PC1 will adjust accordingly.

              4. If the packet is not too large for the router but is too large for PC2 and DF = 1 then PC2 will send an ICMP type 3 code 4 packet to PC1 telling PC1 that the packet needed to be fragmented but that the DF = 1.

              Have you tested any of this with a packet sniffer yet?
              Hey joe,
              Thanks a ton!
              Now, you've explained perfectly. Thanks for keeping the patience and answer my queries.
              Currently, I dont have access to lab so I cannot test this with a packet sniffer Will surely give it a try once i get back to my stuff.

              Regards
              Muks

              Comment


              • #8
                Re: MTU doubts

                Glad to help. I'm certainly not a packet analysis expert but I hope you find what I've said holds pretty true in your lab.

                Comment

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