IP fragmentation disagreement between current and stable

[Maxim Konovalov]

On 10:12-0700, Apr 22, 2003, Don Lewis wrote:

> On 22 Apr, To: freebsd-net at freebsd.org wrote:
>
> > # ping -c 1 -s 3176 192.168.101.3
>
> > If I increase the packet size by any multiple of 1480 bytes (which
> > results in the same final fragment size), I see the same symptoms.
> > Interestingly, I don't see any problems if I decrease the packet size by
> > 1480 bytes to 1696, everything works just fine.
>
> I explored the vicinity of 3176 and found that 3174 works, 3175 through
> 3177 don't work, and 3178 works.  I'm smelling a packet length
> calculation error.  In the case of a 3176 byte ping, the final fragment
> will contain 222 bytes of data.  With a 14 byte Ethernet header and a 20
> byte IP header, that just fits into one 256 byte mbuf.  I wonder if
> there is a divide by 4 to calculate the number of words, and the
> remainder is being discarded instead of being rounded up?  And why does
> this problem only occur if there are three or more fragments?
>
> Could this be a driver problem instead of a problem in the stack itself?
> Both ends are fxp cards.

can't reproduce with ref5.freebsd.org:

stable# ping -c 1 -s 3176 ref5.freebsd.org
PING ref5.freebsd.org (216.136.204.102): 3176 data bytes
3184 bytes from 216.136.204.102: icmp_seq=0 ttl=46 time=194.385 ms

ref5$ netstat -s -p ip | grep frag
        7496159 fragments received
        0 fragments dropped (dup or out of space)
        0 fragments dropped after timeout
        38994 output datagrams fragmented
        231719 fragments created
        0 datagrams that can't be fragmented
ref5$ netstat -s -p ip | grep frag
        7496162 fragments received
        0 fragments dropped (dup or out of space)
        0 fragments dropped after timeout
        38995 output datagrams fragmented
        231722 fragments created
        0 datagrams that can't be fragmented

It got three fragments, defragmented them, sent icmp echo reply in
three fragments.

-- 
Maxim Konovalov, maxim at macomnet.ru, maxim at FreeBSD.org