Re: [Int-area] Continuing the addressing discussion: what is an address anyway?

[Geoff Huston <[e-mail]>]

> On 26 Jan 2022, at 5:47 am, Stewart Bryant <[e-mail]> wrote:
> 
> There is both a topological view of an address and a protocol view.
> 
> The topological view is some place in the network, be that a node or an interface.
> 
> The protocol view is that it is an instruction, for example to deliver the packet to the place identified by the address field lookup. In IP this was originally an interface somewhere in the network. In MPLS we generalised and abstracted this to make the label an instruction what sometimes said deliver the packet closer to some network object, but sometimes was an instruction to do something else.
> 
> In SRv6 they designed a hybrid of this with the prefix having the original IP meaning, and the suffix providing some other instruction together with some parameter such as send the packet to this VPN.
> 
> Thus in my mind the address field in a packet is an opaque instruction that is looked up in some large table and causes the forwarder to take some set of actions that are referenced by the table.
> 

Stewart,

There is an alternative view here that inverts this perspective. From the perspective of the network, a packet's IP address is a token that helps the network determine which egress interface is to be used for the packet to be kicked out of the network! MPLS, next hops addresses, encapsulation, virtual circuits are all somewhat isomorphic from such a perspective. The differences lies in the amount of work the network performs on acceptance of the packet. In some models it performs an initial lookup to select the egress point and then uses this initially computed egress tag to guide the packet’s journey through this network (MPLS, virtual circuits, encap). Other models, including hop-by-hop destination based stateless forwarding, perform this same egress point computation at every internal switching point.
 
My point is that the semantic construct of an “address” does not limit or predetermine which of these models your network must use, nor does a network’s choice of switching architecture necessarily redefine the semantics of an IP address.


Geoff