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Subjectissues with SO_PRIORITY and IP_TOS

I've been doing some experimenting with both of the options mentioned in
the subject line, and it seems that there is some strangeness in the
current handling.

First, setting IP_TOS sets the whole 8 bits of the tos field in the
packet header. However, the code then uses the 4 bits defined as the
tos field to generate the packet priority value. This is bad for two
reasons. Firstly, if we're using the old bit fields it should be the
precedence bits that are used for the skb priority rather than the tos
field. Secondly, the whole precedence/tos thing has been obsoleted by
the 6-bit DSCP field, of which the first 3 bits are supposed to be
backwards compatible with the old precedence field. Shouldn't we
properly handle that?

Secondly, for vlan priority tagging there are only 3 bits available.
This means that practically speaking anyone using vlan priorities needs
to limit themselves to priorities 0-7.

Currently, for me to send a packet with IP precedence bits set to a
nonzero value *and* vlan priority set to the same value, I have to do
the following:

int opt = PRIORITY << 5;
setsockopt(mysocks[i], SOL_IP, IP_TOS, &opt, sizeof(opt));
opt = PRIORITY;
setsockopt(mysocks[i], SOL_SOCKET, SO_PRIORITY, &opt, sizeof(opt));

The first call sets the IP precedence bits, and also incorrectly sets
the socket priority. The second call sets the proper socket priority so
that the vlan egress mapping works properly.

This is kind of ugly. I propose adding a new IP socket option, IP_DSCP,
which would let you set the 6-bit DSCP value (which is then shifted by
two bits in the kernel to generate the 8-bit value for the header
field). The high-order 3 bits would then be automatically used to set
the socket priority to make a vlan egress mapping simple.

Does this make any sense?

Chris


--
Chris Friesen | MailStop: 043/33/F10
Nortel Networks | work: (613) 765-0557
3500 Carling Avenue | fax: (613) 765-2986
Nepean, ON K2H 8E9 Canada | email: cfriesen@nortelnetworks.com

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