IPQoS

[Chris Wilson]

Hi Saku,

On Thu, 13 Feb 2014, Saku Ytti wrote:

>> OpenSSH is conforming to this "informal standard", and with its huge 
>> installed user base, helping to define it as well. It already does set 
>> a high-priority TOS flag on interactive sessions, and low-priority on 
>> non-interactive ones:
>
> I've never ever seen DSCP 0x4 used for anything.

That's because it's not a DSCP at all. It would be 000001 in the 
"Experimental or Local Use" block, and it has no standardised meaning 
in DSCP at the moment.

What it is, is TOS 0x4 or "High Reliability" (which I take to mean as "we 
want this packet delivered, no matter how late).

But OpenSSH apparently doesn't use it. According to djm, it uses 
IPTOS_THROUGHPUT which is 0x8 not 0x4. 
<http://www.gossamer-threads.com/lists/openssh/dev/48410>. (It would be 
0x4 if you disregard the "must be 0" field and shift the TOS value one bit 
to the right).

And according to the Linux manual page: "IP_TOS (since Linux 1.0) Set or 
receive the Type-Of-Service (TOS) field that is sent with every IP packet 
originating from this socket. It is used to prioritize packets on the 
network. TOS is a byte. There are some standard TOS flags defined: 
IPTOS_LOWDELAY to minimize delays for interactive traffic, 
IPTOS_THROUGHPUT to optimize throughput..." Nothing about DSCP there. 
IETF's attempt to kill it notwithstanding, this is what people actually 
use on the Internet.

> And PREC 0 is BE.

If you pay attention to PREC, which I don't think anyone does. I've never 
seen a packet with PREC bits set either. But I get plenty of packets with 
TOS bits set.

> I also can't right now remember buying QoS from company which didn't by 
> default map PREC 5/CS5 to high-priorioty/low-delay (it's used often by 
> default in phones and conferencing systems). So if you're not going for 
> DSCP 0x0, you should go with 0x28, i.e. CS5, and it would work in vast 
> majority of networks which implement QoS, out of the box.

And what networks are those? I don't think you can buy a connection that 
actually uses QoS in the UK unless you buy both ends and install your own 
routers on them.

Cheers, Chris.
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