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Three Packets Walk into a Tunnel

MPLS is a tunneling technology that was designed in such a way that traceroute can operate correctly, but it is configurable

So let’s wind our time machine back to 1996 and I will reconstruct the process that led to the MPLS header being what it is today, complete with its configurable support of traceroute. We later realized that the central idea of Tag switching, which was to associate fixed-length labels with variable-length IP prefixes from the routing table, had been invented and published by Girish Chandranmenon and George Varghese in SIGCOMM 1995. But the pressure to minimize the header along with the lack of widespread usage of ToS led to us compromising on 3 bits, initially called “Class of Service” and later renamed to “Experimental” in RFC 3032.

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