netdev - RE: 2.6.20.7 mss negotiation and path mtu discovery mostly broken?

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Date:	Thu, 26 Apr 2007 11:53:04 -0400
From:	"Ristuccia, Brian" <bristuccia@...rentnetworks.com>
To:	"Herbert Xu" <herbert@...dor.apana.org.au>
Cc:	<netdev@...r.kernel.org>
Subject: RE: 2.6.20.7 mss negotiation and path mtu discovery mostly broken?

> > 
> > 08:39:55.649689 IP 10.2.10.254.22 > 12.33.234.69.35026: .
> > 5906:10250(4344) ack 1
> > 794 win 674 <nop,nop,timestamp 413790 3873837887>
> > 08:39:55.650532 IP 10.2.10.1 > 10.2.10.254: icmp 92: 12.33.234.69 
> > unreachable - need to frag (mtu 1500)
> 
> Where was this dump taken, on 10.2.10.254?
> 

Yes. It appears that if tcpdump is run on the sending system it sees the
packets before segmentation offload does its thing. 

> If so could youd either take the dump further down the route 
> or show the full contents (with tcpdump -x) of the ICMP error 
> here so that we can see what the actual packet size was?
> 

Here's an example IP packet followed by the resulting ICMP as seen with
tcpdump on  10.2.10.254. ICMP is complaining about a 2896 byte packet,
which leads me to believe that the TSO is using the wrong mss or simply
isn't doing anything at all. 

11:36:12.006330 IP (tos 0x8, ttl  64, id 20681, offset 0, flags [DF],
length: 29
48) 10.2.10.254.22 > 12.33.234.69.47703: . 91224:94120(2896) ack 49 win
674 <nop
,nop,timestamp 24659048 3970819017>
        0x0000:  4508 0b84 50c9 4000 4006 d33c 0a02 0afe
E...P.@.@..<....
        0x0010:  0c21 ea45 0016 ba57 09f8 1838 7577 90cb
.!.E...W...8uw..
        0x0020:  8010 02a2 16dd 0000 0101 080a 0178 4468
.............xDh
        0x0030:  ecad e3c9 96f8 ad0d e909 4e89 202a 9d85
..........N..*..
        0x0040:  4b58 a65f 53d9 07f4 0435 2557 970b 943d
KX._S....5%W...=
        0x0050:  1eba                                     ..
11:36:12.007019 IP (tos 0x0, ttl  64, id 43865, offset 0, flags [DF],
length: 11
2) 10.2.10.1 > 10.2.10.254: icmp 92: 12.33.234.69 unreachable - need to
frag (mt
u 1500) for IP (tos 0x8, ttl  64, id 20681, offset 0, flags [DF],
length: 2948)
10.2.10.254.22 > 12.33.234.69.47703: . 91224:94120(2896) ack 49 win 674
<nop,nop
,timestamp 24659048 3970819017>
        0x0000:  4500 0070 ab59 4000 4001 6631 0a02 0a01
E..p.Y@.@.......
        0x0010:  0a02 0afe 0304 7cc8 0000 05dc 4508 0b84
......|.....E...
        0x0020:  50c9 4000 4006 d33c 0a02 0afe 0c21 ea45
P.@.@..<.....!.E
        0x0030:  0016 ba57 09f8 1838 7577 90cb 8010 02a2
...W...8uw......
        0x0040:  706d 0000 0101 080a 0178 4468 ecad e3c9
pm.......xDh....
        0x0050:  96f8             

-Brian

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