Re: [v6ops] WG Doc? draft-gont-v6ops-ipv6-ehs-packet-drops

[Nick Hilliard <[e-mail]>]

[e-mail] wrote:
> in every router / middlebox on the planet. (and somewhat tangential
> for any RFC describing shared IPv4 addresses)
> 
> e.g (from VPP):

i.e. when the packet processor can see the tcp/udp ports, the flow hash
generally ends up being an n-tuple similar to:

fh(flowlabel, srcip, dstip, srcport, dstport)

In this case, protocol is also added in, but this value will be tied
tightly to similar srcport and dstport values, so its overall
contribution to entropy is low.

If the packet processor can't see the tcp/udp src/dst ports due to
excessive amounts of EHs, then this n-tuple drops to:

fh(flowlabel, srcip, dstip, 0, 0)

In practice, the flow label is often set to zero, giving:

fh(0, srcip, dstip, 0, 0)

The point of the draft is to note that excessive EHs will contribute to
a deterioration of entropy in flow hash inputs and that this has
operational consequences.

Nick

> 
> /* Compute flow hash.  We'll use it to select which Sponge to use for this
>    flow.  And other things. */
> always_inline u32
> ip6_compute_flow_hash (ip6_header_t * ip, u32 flow_hash_config)
> {
>     tcp_header_t * tcp = (void *) (ip + 1);
>     u64 a, b, c;
>     u64 t1, t2;
>     uword is_tcp_udp = (ip->protocol == IP_PROTOCOL_TCP
> 			|| ip->protocol == IP_PROTOCOL_UDP);
> 
>     t1 = (ip->src_address.as_u64[0] ^ ip->src_address.as_u64[1]);
>     t1 = (flow_hash_config & IP_FLOW_HASH_SRC_ADDR) ? t1 : 0;
> 
>     t2 = (ip->dst_address.as_u64[0] ^ ip->dst_address.as_u64[1]);
>     t2 = (flow_hash_config & IP_FLOW_HASH_DST_ADDR) ? t2 : 0;
> 
>     a = (flow_hash_config & IP_FLOW_HASH_REVERSE_SRC_DST) ? t2 : t1;
>     b = (flow_hash_config & IP_FLOW_HASH_REVERSE_SRC_DST) ? t1 : t2;
>     b ^= (flow_hash_config & IP_FLOW_HASH_PROTO) ? ip->protocol : 0;
> 
>     t1 = is_tcp_udp ? tcp->ports.src : 0;
>     t2 = is_tcp_udp ? tcp->ports.dst : 0;
> 
>     t1 = (flow_hash_config & IP_FLOW_HASH_SRC_PORT) ? t1 : 0;
>     t2 = (flow_hash_config & IP_FLOW_HASH_DST_PORT) ? t2 : 0;
> 
>     c = (flow_hash_config & IP_FLOW_HASH_REVERSE_SRC_DST) ?
>         ((t1<<16) | t2) : ((t2<<16) | t1);
> 
>     hash_mix64 (a, b, c);
>     return (u32) c;
> }
> 
> Best,
> Ole
> 
> _______________________________________________
> v6ops mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/v6ops