Exercise 3: drop the header

Question: change one table-1 route on h1 to "mode encap.red" and capture again. What disappears, and why does
it still work?

Answer: the Segment Routing Header disappears. With "mode encap" the outer IPv6 header says next-header
Routing (43) and the payload is 108 bytes (24-byte SRH + the 84-byte IPv4 packet). With "mode encap.red" it's
next-header IPIP (4) and the payload is 84 bytes: just IPv6 + the original IPv4 packet inside.
It still works because with uSID the whole path already lives in the destination address
(fcbb:bb00:100:3:fe00::); the SRH is only needed when the path is longer than one 128-bit container.
That's why MRC uses uSID: zero extra header bytes per packet.

Reproduce:  sudo ./solutions/exercise3/run.sh
