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    SubjectRe: [net-next v5 1/2] seg6: inherit DSCP of inner IPv4 packets
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    On 8/25/20 5:45 PM, Ahmed Abdelsalam wrote:
    >
    > Hi David
    >
    > The seg6 encap is implemented through the seg6_lwt rather than
    > seg6_local_lwt.

    ok. I don't know the seg6 code; just taking a guess from a quick look.

    > We can add a flag(SEG6_IPTUNNEL_DSCP) in seg6_iptunnel.h if we do not
    > want to go the sysctl direction.

    sysctl is just a big hammer with side effects.

    It struck me that the DSCP propagation is very similar to the TTL
    propagation with MPLS which is per route entry (MPLS_IPTUNNEL_TTL and
    stored as ttl_propagate in mpls_iptunnel_encap). Hence the question of
    whether SR could make this a per route attribute. Consistency across
    implementations is best.

    > Perhaps this would require various changes to seg6 infrastructure
    > including seg6_iptunnel_policy, seg6_build_state, fill_encap,
    > get_encap_size, etc.
    >
    > We have proposed a patch before to support optional parameters for SRv6
    > behaviors [1].
    > Unfortunately, this patch was rejected.
    >

    not sure I follow why the patch was rejected. Does it change behavior of
    existing code?

    I would expect that new attributes can be added without affecting
    handling of current ones. Looking at seg6_iptunnel.c the new attribute
    would be ignored on older kernels but should be fine on new ones and
    forward.

    ###

    Since seg6 does not have strict attribute checking the only way to find
    out if it is supported is to send down the config and then read it back.
    If the attribute is missing, the kernel does not support. Ugly, but one
    way to determine support. The next time an attribute is added to seg6
    code, strict checking should be enabled so that going forward as new
    attributes are added older kernels with strict checking would reject it.

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