Bug 5610

Summary:	IP MTU Path Discovery not working properly
Product:	Networking	Reporter:	Rui Ferrao (mail)
Component:	IPV4	Assignee:	Stephen Hemminger (stephen)
Status:	REJECTED INSUFFICIENT_DATA
Severity:	normal	CC:	bunk
Priority:	P2
Hardware:	i386
OS:	Linux
Kernel Version:	2.6.12	Subsystem:
Regression:	---	Bisected commit-id:
Attachments:	Traffic capture file showing the problem

Description Rui Ferrao 2005-11-15 02:56:23 UTC

Most recent kernel where this bug did not occur:
Distribution: Ubuntu
Hardware Environment: Laptop Acer Aspire 3500
Software Environment: 
Problem Description: Performing ftp upload on a network that has an MTU of 1400
along the way. Local interface MTU is set to 1500. Laptop receives from the 
network ICMP Destination Unreachable (Fragmentation Needed), MTU of next hop
1400, but i see no reduction on the IP packet size sent by the Laptop. 

Because the IP MTU Path Discovery mechanism is enabled, all the outoing packets
have the DF flag set, causing this TCP session to stall.

Steps to reproduce:

Comment 1 Rui Ferrao 2005-11-15 03:02:14 UTC

Created attachment 6590 [details]
Traffic capture file showing the problem

Comment 2 Stephen Hemminger 2005-11-15 16:09:55 UTC

What network driver is used?

Is TCP Segmentation Offload enabled for the network interface?
    ethtool -k eth0

Comment 3 Rui Ferrao 2005-11-15 16:28:04 UTC

Driver is sis900, compiled into the kernel.

ferrao@meter:~$ sudo ethtool -k eth0
Offload parameters for eth0:
Cannot get device rx csum settings: Operation not supported
Cannot get device tx csum settings: Operation not supported
Cannot get device scatter-gather settings: Operation not supported
Cannot get device tcp segmentation offload settings: Operation not supported
no offload info available

Comment 4 Rui Ferrao 2005-11-15 16:40:51 UTC

According to my ethtool (compiled latest version) docs, the only thing i seem to
be able to do on this driver is :

ferrao@meter:/usr/share/doc/ethtool$ sudo ethtool -i eth0
driver: sis900
version: v1.08.08 Jan. 22 2005
firmware-version: 
bus-info: 0000:00:04.0

Comment 5 Rui Ferrao 2005-11-16 01:44:49 UTC

My email is getting rejected at due to administrative prohibition, so i'm
answering here.

I can try to capture it again, but won't tcpdump -nr trace_file.cap -x give you
the same result?

Thanx

Herbert Xu wrote:

>>David S. Miller wrote:
>>  
>>
>
>>>>Alternatively, we're ignoring the PMTU message for one reason or
>>>>another.  Perhaps the quoted TCP packet in the ICMP pmtu message
>>>>has an incorrect sequence number or is truncated for some reason.
>>>>    
>>>>
>
>>
>>That's what I think too.
>>
>>Can we please get the same packet dump with -x?
>>
>>Thanks,
>>

Comment 6 Stephen Hemminger 2006-05-12 11:02:16 UTC

This bug is getting little attention, does it still exist with 2.6.16 or 2.6.17-rc?

Comment 7 Adrian Bunk 2006-07-10 13:17:19 UTC

Please reopen this bug if it's still present in kernel 2.6.17.