2 Replies Latest reply: Jan 16, 2012 1:58 PM by tariqccie RSS

    Need clarification on Ip Fragmentation offset value


      Scenario A:


      Suppose consider the Datagram size of 1700. MTU =1500



      1. first fragment packet will be


      identifier = 1,  R=0, DF=0, MF=1, Flag offset = 0


      Header size[20] + data gram size[1480] = 1500



      2. Second fragment packet will be


      identifier = 2, R=0 DF=0 MF=0 Flag offset = 1480



      Header size[20] + Data gram size[220] = 240



      a) In cisco routers, flag offset is calculated as sum of datagram size that has been sent. i.e., 1480


      b) But it RFC 791, Flag offset is calculated as sum of datagram size divisible by 8. i.e., 1480/8 = 185



      Which one of the above is correct??????? either a or b. Please clarify it.




      Scenario B:


      I have one more question W.r.to RFC791.



      In RFC791, they given fragmentation example, Datagram size = 452 and MTU = 280.


      First fragmented packet size is 276[header 20+ datagram 256]. Even though the MTU is 280. RFC provided the data gram size as 276.

      Because it is the nearest value[256] that divisible by 8. so it can be used as offset value for next packet.




      When i try the same on cisco router, first fragmentated packet size is 280. 280 is offset value for second packet on cisco routers.


      This is checked by enabling "debug ip packet detail" on the router.


      Which one is correct??????