Hello,
a small update: Please disregard my oservation #3.
Explanation:
IP framents have, by protocol standard, a size of n * 8 (8 is a fixed
defined blocksize).
IP header is not part of the fragmentation blocks (of course) and counts 20.
=> division (256-20)/8.0 = 29.5 has a rest => length of packet:
29*8+20 = 252. That's the observed value.
Btw, on netrom, the rest is 0 (netrom overhead 20 => netrom iface mtu 236):
(236-20)/8.0 = 27.0
=> Netrom payload with IP fragmented packets match exact the mtu length.
vy 73,
- Thomas dl9sau
On Mon, Feb 15, 2016 at 03:00:36PM +0100, Thomas Osterried wrote:
[..]
> 3. IP fragmentation in Mode DG and VC
> -------------------------------------
>
>
> MTU of interface is 256 (== paclen).
>
[..]
> # ifconfig bpq2 mtu 256
>
> bpq2: fm DL9SAU to DL9SAU-15 ctl I00^ pid=CC(IP) len 252
> IP: len 252 44.128.128.1->44.128.128.10 ihl 20 ttl 64 id 52882 offs 0 MF prot
> ICMP
> ICMP: type Echo Request id 5113 seq 1
> 0000 ...V............................ !"#$%&'()*+,-./0123456789:;<=>?
> 0040 @ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~.
> 0080 ................................................................
> 00C0 ................................
> bpq2: fm DL9SAU to DL9SAU-15 ctl I01+ pid=CC(IP) len 25
> IP: len 25 44.128.128.1->44.128.128.10 ihl 20 ttl 64 id 52882 offs 232
> 0000 .....
>
> => On both, Mode VC and Mode DG, the IP fragmentation makes packets 252 bytes
> long.
> IP packets with length of 256 pass the interface with len 256, which is
> correct.
[..]
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