issues with SO_PRIORITY and IP_TOS

From: Chris Friesen (cfriesen_at_nortelnetworks.com)
Date: 10/30/03

  • Next message: Bruce Perens: "Crash-on-boot in init_l440gx SMP" 
    Date:	Thu, 30 Oct 2003 14:59:34 -0500
To: netdev@oss.sgi.com, linux-kernel@vger.kernel.org

    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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/
  • Next message: Bruce Perens: "Crash-on-boot in init_l440gx SMP"

    Relevant Pages