TCP differences in 7.2 vs 7.1

[Xin LI]

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Hi, David,

David Samms wrote:
> Xin LI wrote:
>> Hi David,
>>
>> David Samms wrote:
>>> After upgrading to 7.2 (amd64) some customers complained of very poor
>>> bandwidth.  Upon investigation all the effected customers were ATT DSL
>>> clients located all over the USA, not in a single city, nor were other
>>> ISPs effected.  The server is a Supermicro with  dual (quad core)
>>> processors with a single Intel fxp network card on a 100mbit connection.
>>
>> Could you please try if this would help:
>>
>>     sysctl net.inet.tcp.tso=0
>>
>> Cheers,
>> - --
>> Xin LI <delphij at delphij.net>    http://www.delphij.net/
>> FreeBSD - The Power to Serve!
> 
> Xin LI,
> 
> Thank you for your help.
> 
> Setting sysctl net.inet.tcp.tso=0 resolved the issue completely.   What
> does sysctl net.inet.tcp.tso=0 do?  Where can I read more about the
> option?  I captured tcpdumps of a single file transfer to 7.1, 7.2 and
> 7.2 with sysctl net.inet.tcp.tso=0, but they are to large to attach to
> this list.  Let me know if you are interested in viewing the dump files.
> 
> Thanks again for your assistance!

Thanks for the offer but I think this is a known problem so perhaps the
dump files are no longer necessary.  The problem was caused by the
reciever side (usually PPPoE clients, e.g. DSL users) which proposes a
smaller MSS than the interface MTU, the previous implementation sets the
packet length to interface MTU instead of the negotiated one, which
would cause problem.

Setting net.inet.tcp.tso=0 would turn off TCP Segment Offloading
completely.  The previous release of FreeBSD does not include this feature.

I think yongari@ has committed a fix as revision 191867 (RELENG_7) and
190982 (HEAD):

Index: if_fxp.c
===================================================================
- --- if_fxp.c	(revision 190981)
+++ if_fxp.c	(revision 190982)
@@ -1485,7 +1485,8 @@
 		 * checksum in the first frame driver should compute it.
 		 */
 		ip->ip_sum = 0;
- -		ip->ip_len = htons(ifp->if_mtu);
+		ip->ip_len = htons(m->m_pkthdr.tso_segsz + (ip->ip_hl << 2) +
+		    (tcp->th_off << 2));
 		tcp->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
 		    htons(IPPROTO_TCP + (tcp->th_off << 2) +
 		    m->m_pkthdr.tso_segsz));

To re@:

Perhaps we should issue an errata for this, at least document it in
errata (I can do this)?

Cheers,
- --
Xin LI <delphij at delphij.net>	http://www.delphij.net/
FreeBSD - The Power to Serve!
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