ep0y+@ANDREW.CMU.EDU ("Erik D. Perkins") (08/02/89)
I am implementing the MTU discovery IP options for our gateway software. My question is: should the IP MTU Request option be copied into fragments upon fragmentation of the original datagram? RFC 1063 doesn't specify. It is "obvious" that the reply option shouldn't be copied, because it isn't changed. Can anyone suggest a compelling reason to copy the request option? The individual fragments could conceivably travel over different routes, so MINMTU values may differ. Would this information be useful when the original datagram is reassembled? The general feeling around here is that the answer is no. There doesn't seem to be any real benefit to justify the extra gateway/host overhead. Any other thoughts??? (BTW, what is IP option 10? Strict Source Routing is option 9 and MTU Request is option 11. Just wondering...) thanks in advance, erik perkins network development carnegie mellon
ww0n+@ANDREW.CMU.EDU (Walter Lloyd Wimer III) (08/02/89)
Upon closer examination of RFC 791 (Internet Protocol), it is clear that the MTU discovery options should definitely not be copied on fragmentation since an IP option-type encodes the most-significant-bit as the "copy" bit. As specified in RFC 1063, the MTU option numbers do not have this bit set. (As is often the case, this answer was discovered mere moments after the question was posted.) Walt Wimer Network Development Carnegie Mellon University
cpw%sneezy@LANL.GOV (C. Philip Wood) (08/02/89)
The absense of the 'copy on fragmentation' flag indicates that there is
no need to copy on fragmentation. See my IP option wall chart below:
(Please send flaming corrections to cpw@lanl.gov)
Two types of options fixed and variable:
TYPE MNE LEN First Octet Second Octet Function
C CL NUM OLEN
F EOL 1 0 00 00000 N/A End of Option List.
F NOP 1 0 00 00001 N/A No OPeration
V/F SECURITY 11 1 00 00010 00001011 SECURITY.
V LSRR var 1 00 00011 Loose Source and RR
V TS var 0 10 00100 Time Stamp.
V XSECURITY var 1 00 00101 eXtended SECURITY.
xxx x xx 00110 not defined
V RR var 0 00 00111 Record Route.
V/F SATID 4 1 00 01000 00000100** SATnet stream ID.
V SSRR var 1 00 01001 Strict Source and RR
xxx x xx 01010 not defined
V/F PMTU 4 1 00 01011 00000100 Probe MTU
V/F RMTU 4 1 00 01100 00000100 Reply MTU
TYPE (F = Fixed), (V = Variable), (V/F = Fixed but with length octet).
MNE mneumonic
LEN length of option
C copied flag (copy on fragmentation, if set to 1)
CL option class
0 = control
1 = reserved for future use
2 = debugging and measurement
3 = reserved for future use
NUM option number
OLEN value of option length field for variable length options
** RFC 791 indicates 00000010 which must be a typo.