Re: [net-next v5 1/2] seg6: inherit DSCP of inner IPv4 packets

[Ahmed Abdelsalam]

On 26/08/2020 21:41, David Ahern wrote:
> On 8/26/20 6:12 AM, Ahmed Abdelsalam wrote:
> > On 26/08/2020 02:45, David Ahern wrote:
> > > 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.
> > > SRv6 does not have an issue of having this per route.
> > Actually, as SRv6 leverage IPv6 encapsulation, I would say it should
> > consistent with ip6_tunnel not MPLS.
> >
> > In ip6_tunnel, both ttl and flowinfo (tclass and flowlabel) are provided.
> >
> > Ideally, SRv6 code should have done the same with:
> > TTL       := VLAUE | DEFAULT | inherit.
> > TCLASS    := 0x00 .. 0xFF | inherit
> > FLOWLABEL := { 0x00000 .. 0xfffff | inherit | compute.
>
> New attributes get added all the time. Why does something like this now
> work for these features:
>
> diff --git a/include/uapi/linux/seg6_iptunnel.h
> b/include/uapi/linux/seg6_iptunnel.h
> index eb815e0d0ac3..b628333ba100 100644
> --- a/include/uapi/linux/seg6_iptunnel.h
> +++ b/include/uapi/linux/seg6_iptunnel.h
> @@ -20,6 +20,8 @@
>   enum {
>          SEG6_IPTUNNEL_UNSPEC,
>          SEG6_IPTUNNEL_SRH,
> +       SEG6_IPTUNNEL_TTL,      /* u8 */
> +       SEG6_IPTUNNEL_TCLASS,   /* u8 */
>          __SEG6_IPTUNNEL_MAX,
>   };
>   #define SEG6_IPTUNNEL_MAX (__SEG6_IPTUNNEL_MAX - 1)
> diff --git a/net/ipv6/seg6_iptunnel.c b/net/ipv6/seg6_iptunnel.c
> index 897fa59c47de..7cb512b65bc3 100644
> --- a/net/ipv6/seg6_iptunnel.c
> +++ b/net/ipv6/seg6_iptunnel.c
> @@ -46,6 +46,11 @@ static size_t seg6_lwt_headroom(struct
> seg6_iptunnel_encap *tuninfo)
>
>   struct seg6_lwt {
>          struct dst_cache cache;
> +       u8      ttl_propagate;  /* propagate ttl from inner header */
> +       u8      default_ttl;    /* ttl value to use */
> +       u8      tclass_inherit; /* inherit tclass from inner header */
> +       u8      tclass;         /* tclass value to use */
> +
>          struct seg6_iptunnel_encap tuninfo[];
>   };
>
> @@ -61,7 +66,10 @@ seg6_encap_lwtunnel(struct lwtunnel_state *lwt)
>   }
>
>   static const struct nla_policy seg6_iptunnel_policy[SEG6_IPTUNNEL_MAX +
> 1] = {
> -       [SEG6_IPTUNNEL_SRH]     = { .type = NLA_BINARY },
> +       [SEG6_IPTUNNEL_UNSPEC]          = { .strict_start_type =
> SEG6_IPTUNNEL_SRH + 1 },
> +       [SEG6_IPTUNNEL_SRH]             = { .type = NLA_BINARY },
> +       [SEG6_IPTUNNEL_TTL]             = { .type = NLA_U8 },
> +       [SEG6_IPTUNNEL_TCLASS]          = { .type = NLA_U8 },
>   };
>
>   static int nla_put_srh(struct sk_buff *skb, int attrtype,
> @@ -460,6 +468,22 @@ static int seg6_build_state(struct net *net, struct
> nlattr *nla,
>
>          memcpy(&slwt->tuninfo, tuninfo, tuninfo_len);
>
> +       if (tb[SEG6_IPTUNNEL_TTL]) {
> +               slwt->default_ttl = nla_get_u8(tb[SEG6_IPTUNNEL_TTL]);
> +               slwt->ttl_propagate = slwt->default_ttl ? 0 : 1;
> +       }
> +       if (tb[SEG6_IPTUNNEL_TCLASS]) {
> +               u32 tmp = nla_get_u32(tb[SEG6_IPTUNNEL_TCLASS]);
> +
> +               if (tmp == (u32)-1) {
> +                       slwt->tclass_inherit = true;
> +               } else if (tmp & <some valid range mask>) {
> +                       error
> +               } else {
> +                       slwt->tclass = ...
> +               }
> +       }
> +
>          newts->type = LWTUNNEL_ENCAP_SEG6;
>          newts->flags |= LWTUNNEL_STATE_INPUT_REDIRECT;
>
>
> And the use the values in slwt as needed.

Thanks for the suggestions and the code example
I will write the patches and submit to net-next.