Hi,
I need to prioritize data sent on a socket over a ppp link
so it is transmitted before some other data sharing that link.
I googled around for a few days and I thought I understood
how I might go about doing this, but my attempts have
failed.
I thought the default qdisc (pfifo_fast) would prioritize data
flagged as "lowdelay" by putting it in a different band that has
preferential queuing priority over data in other bands. I
attempted to configure the socket like this:
int lowdelay = IPTOS_LOWDELAY;
return setsockopt(servFd_, IPPROTO_IP, IP_TOS,
(void *)&lowdelay, sizeof(lowdelay));
And I verified that the data had TOS=0x10 using tcpdump:
tcpdump -v -v -i ppp0
13:47:00.259910 IP (tos 0x10, ttl 64, id 26932, offset 0, flags [DF],
proto TCP (6), length 67)
10.10.12.2.irisa > 10.10.12.1.41152: Flags [P.], cksum 0x178a
(correct), seq 480:495, ack 1, win 91, options [nop,nop,TS val
10108331 ecr 683540551], length 15
...
But when I browse a web page through that same ppp link,
the 5Hz data that I'm expecting through the priority socket
has delays of up to a second. This is well over the maximum
delay I expected, which was around MTU/baud rate assuming
one big message was queued just before I send the next 5
Hz priority message. I tried UDP sockets too in case it
was the low priority ack that was causing the problem, but that
didn't seem to make a difference.
Any ideas what I'm doing wrong?
Thanks,
David
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