[LARTC] IPv6 priority in tc filter

[Devanshu Mehta <mehta@ll.mit.edu>]

Hi,
I have recently started to use 'tc' on ipv6 traffic and have come across 
an issue with tc filter. As per tc_filter.c in iproute2-050816, to match 
'priority' of an ipv6 packet, the u32 filter matches against the first 8 
bits of the packet.

 From f_u32.c:

        if (strcmp(*argv, "priority") = 0) {
                NEXT_ARG();
                res = parse_u8(&argc, &argv, sel, 0, 0);
                goto done;
        }

According to RFC2460 for IPv6 (http://www.ietf.org/rfc/rfc2460.txt), the 
first 4 bits of the packet are the version number and the subsequent 8 
bits are the traffic class. Shouldn't the u32 filter be matching against 
the 8 bits of the traffic class, not the 4bit version+first 4bits of 
traffic class?

The reason this is relevant is because I am trying to match EF traffic 
with a tc filter command:
tc filter add dev eth2.1070 parent 10: protocol ipv6 prio 1 u32 match 
ip6 priority 0xb8 0xff flowid 10:1

i.e. 0xb8 for EF. This, however, does not match my EF packets. If I send 
the following command:
tc filter add dev eth2.1070 parent 10: protocol ipv6 prio 1 u32 match 
ip6 priority 0x6b 0xff flowid 10:1

then it actually does match EF packets. That is, it matches the 4 bits 
of the version (IP v6) and the first 4 bits of the traffic class. I 
believe, the code above should actually read:

        if (strcmp(*argv, "priority") = 0) {
                NEXT_ARG();
                res = parse_u8(&argc, &argv, sel, 4, 0);
                goto done;
        }

though I may be mistaken as to the actual syntax of parse_u8().

-- 
Devanshu Mehta
Advanced Networks and Applications Group
MIT Lincoln Laboratory
(781) 981-1233

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