querying TOS/DSCP at accept() time?

querying TOS/DSCP at accept() time?

[Jeremy Jackson]

I would like to allow clients connecting to webserver to choose TOS/DSCP
value they want in reply packets, based on TOS setting received by
webserver in initial SYN-SYN/ACK-ACK exchange.  Linux samples the TOS
byte of the ACK that moves the state to CONNECTED, presumably to use for
later ECN processing, but I cannot find a way to get this from
userspace.

net/ipv4/tcp_ipv4.c:1645
    TCP_SKB_CB(skb)->flags   = iph->tos;


Am I missing something in getsockopt(TCP_INFO perhaps?) or accept(), or
is this currently not possible?



-- 
Jeremy Jackson
Coplanar Networks
(519)489-4903
http://www.coplanar.net
jerj@coplanar.net

Re: querying TOS/DSCP at accept() time?

[Jeremy Jackson]

It seems this question was asked before in 2001:

http://www.squid-cache.org/mail-archive/squid-dev/200103/0035.html

http://lkml.indiana.edu/hypermail/linux/kernel/0103.0/1211.html

And apparently Squid developers made a kernel patch

http://zph.bratcheda.org/tos_marking_with_tos_preserving.htm
http://zph.bratcheda.org/

It seems like this is generally useful to a number of applications,
perhaps it should be included in mainstream kernel?

I see there's a structure member added, which will likely be frowned
upon, but perhaps it could be reworked to use the existing TOS byte
captured 

net/ipv4/tcp_ipv4.c:1645
     TCP_SKB_CB(skb)->flags   = iph->tos;

Cheers,

Jeremy

The kernel patch adds 
On Sun, 2009-12-27 at 09:57 -0500, Jeremy Jackson wrote:
> I would like to allow clients connecting to webserver to choose TOS/DSCP
> value they want in reply packets, based on TOS setting received by
> webserver in initial SYN-SYN/ACK-ACK exchange.  Linux samples the TOS
> byte of the ACK that moves the state to CONNECTED, presumably to use for
> later ECN processing, but I cannot find a way to get this from
> userspace.
> 
> net/ipv4/tcp_ipv4.c:1645
>     TCP_SKB_CB(skb)->flags   = iph->tos;
> 
> 
> Am I missing something in getsockopt(TCP_INFO perhaps?) or accept(), or
> is this currently not possible?
> 
> 
> 


-- 
Jeremy Jackson
Coplanar Networks
(519)489-4903
http://www.coplanar.net
jerj@coplanar.net