e9272af9   
   From: barmar@alum.mit.edu   
      
   In article   
   ,   
    acadian  wrote:   
      
   > Greetings-   
   >      I am looking at rfc 791, and just do not follow the algorithm   
   > they use on page 25 to calculate the TL when a packet is fragmented.   
   > I specially do not follow step 9.  Would someone please explain?   
   > IF TL =< MTU THEN Submit this datagram to the next step   
   >              in datagram processing ELSE IF DF = 1 THEN discard the   
   >         datagram ELSE   
   >         To produce the first fragment:   
   >         (1)  Copy the original internet header;   
   >         (2)  OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF;   
   >         (3)  NFB <- (MTU-IHL*4)/8;   
   >         (4)  Attach the first NFB*8 data octets;   
   >         (5)  Correct the header:   
   >              MF <- 1;  TL <- (IHL*4)+(NFB*8);   
   >              Recompute Checksum;   
   >         (6)  Submit this fragment to the next step in   
   >              datagram processing;   
   >         To produce the second fragment:   
   >         (7)  Selectively copy the internet header (some options   
   >              are not copied, see option definitions);   
   >         (8)  Append the remaining data;   
   >         (9)  Correct the header:   
   >              IHL <- (((OIHL*4)-(length of options not copied))+3)/4;   
      
   Some IP options are copied into all fragments, some are only put in the   
   first fragment.  If you don't copy all the options, the header may be   
   shorter.  However, IHL counts in 4-octet words, while (length of options   
   not copied) is in octets.  Step 9 figures out the new number of words   
   based on the decrease in octets.   
      
   --   
   Barry Margolin, barmar@alum.mit.edu   
   Arlington, MA   
   *** PLEASE don't copy me on replies, I'll read them in the group ***   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)