15.4. Prioritizing interactive traffic
If lots of data is coming down your link, or going up for that matter, and
you are trying to do some maintenance via telnet or ssh, this may not go too
well. Other packets are blocking your keystrokes. Wouldn't it be great if
there were a way for your interactive packets to sneak past the bulk
traffic? Linux can do this for you!
As before, we need to handle traffic going both ways. Evidently, this works
best if there are Linux boxes on both ends of your link, although other
UNIX's are able to do this. Consult your local Solaris/BSD guru for this.
The standard pfifo_fast scheduler has 3 different 'bands'. Traffic in band 0
is transmitted first, after which traffic in band 1 and 2 gets considered.
It is vital that our interactive traffic be in band 0!
We blatantly adapt from the (soon to be obsolete) ipchains HOWTO:
There are four seldom-used bits in the IP header, called the Type of Service
(TOS) bits. They effect the way packets are treated; the four bits are
"Minimum Delay", "Maximum Throughput", "Maximum Reliability" and "Minimum
Cost". Only one of these bits is allowed to be set. Rob van Nieuwkerk, the
author of the ipchains TOS-mangling code, puts it as follows:
Especially the "Minimum Delay" is important for me. I switch it on for
"interactive" packets in my upstream (Linux) router. I'm
behind a 33k6 modem link. Linux prioritizes packets in 3 queues. This
way I get acceptable interactive performance while doing bulk
downloads at the same time. |
The most common use is to set telnet & ftp control connections to "Minimum
Delay" and FTP data to "Maximum Throughput". This would be
done as follows, on your upstream router:
# iptables -A PREROUTING -t mangle -p tcp --sport telnet \
-j TOS --set-tos Minimize-Delay
# iptables -A PREROUTING -t mangle -p tcp --sport ftp \
-j TOS --set-tos Minimize-Delay
# iptables -A PREROUTING -t mangle -p tcp --sport ftp-data \
-j TOS --set-tos Maximize-Throughput |
Now, this only works for data going from your telnet foreign host to your
local computer. The other way around appears to be done for you, ie, telnet,
ssh & friends all set the TOS field on outgoing packets automatically.
Should you have an application that does not do this, you can always do it
with netfilter. On your local box:
# iptables -A OUTPUT -t mangle -p tcp --dport telnet \
-j TOS --set-tos Minimize-Delay
# iptables -A OUTPUT -t mangle -p tcp --dport ftp \
-j TOS --set-tos Minimize-Delay
# iptables -A OUTPUT -t mangle -p tcp --dport ftp-data \
-j TOS --set-tos Maximize-Throughput |
