Why we need add :: at the last when encap seg6 with inline mode?

Why we need add :: at the last when encap seg6 with inline mode?

[Hangbin Liu]

Hi David,

When I tried encap seg6 inline mode via iproute, I saw it added a :: address
at the last.

# ip -6 route add 1234::1234 encap seg6 mode inline segs 1::1 dev dum1
# ip -6 route show dev dum1
1234::1234  encap seg6 mode inline segs 2 [ 1::1 :: ] metric 1024 pref medium

In the iproute2 code

```
static struct ipv6_sr_hdr *parse_srh(char *segbuf, int hmac, bool encap)
{
        ...
        if (!encap)
                nsegs++;
        ...
```

So is there any reason we add this?

Thanks
Hangbin

Re: R: Why we need add :: at the last when encap seg6 with inline mode?

[Hangbin Liu]

Hi Paolo,
On Thu, Jun 29, 2023 at 10:57:32PM +0000, paolo lungaroni wrote:
> Just for the sake of clarity let's consider Example 1 where the SID List is
> built as follows: <S3,S2,S1>.
> 
> Before applying H.Insert
>    IPv6 DA=B2
> 
> After applying H.Insert:
>    IPv6 DA=S1 | SRH <B2,S3,S2,S1>
> 
> In this representation, we stick with the draft conventions where the SID List
> is represented in the reversed order. That means that S1 will be the first SID,
> S2 the second one, etc. B2 is the original IPv6 DA that is pushed into the
> SID List and will be restored once the SRH is removed.
> 
> Now let's consider the H.Insert implementation in Linux.
> 
> User space
> =========
> 
> Currently, when a H.Insert tunnel is instantiated the userspace (iproute2, in
> this case) acquires all the info (e.g. inline, segs, etc) from the CLI and
> builds directly the required SRH header [2].
> Considering the H.Insert tunnel (i.e. inline mode), parse_srh() (line 894)
> accommodates for an extra space required to store the original IPv6 DA (i.e.
> B2 considering the Example 1) which is not known at the moment of the tunnel
> creation.
> For this reason, iproute2 fills the reserved space with 16 bytes set to zero
> (and that's why we see ::).

Thanks for the explanation. Compared with dealing each segment route and
adding the original DA in the middle of struct ipv6_sr_hdr, this way looks
more easier. Although the users may feel a little confused :)

Regards
Hangbin