On Mon, Oct 25, 2021 at 9:47 AM Andrew Alston <Andrew.Alston= [email protected]> wrote:
> > Of course there is. You cannot distinguish routing from host without > looking at external control channels, such as a routing or configuration > protocol; and you certainly cannot determine the subnet mask of a network > without that external information, since it's not in the ?> packet. And > it's not even in the control plane if the route has been aggregated. Does > that make the information "ambiguous"? The point is that the subnet mask > of a network is part of a context that you discussed, and you might not > have it. > > This analogy doesn't really work for me. In the case of an SRH - the > router acts on the packet based on local configuration - how it gets that > config doesn't really matter. The SID size in use by each router may or > may not be exposed by an IGP for reading - and could differ at each node. I think Eliot's main point is that there are aspects of a flow's routing through a network which are observable in a packet and there are aspects which are not. Think about a network which implements a flow-based equal-cost multipath; the determination of which flows take which path uses information in the packet but the algorithm that hashes it together and chooses a next hop is not. The practices about what to choose can vary significantly and to debug (or plan), you have to know a lot more about the network than what's in the packet itself. regards, Ted Hardie > In the case of standard forwarding you have a FIB entry - inserted from > the RIB - that goes a certain place. In the case of this - a router could > be using either 16bit SID or 32bit SID - entirely configured locally - and > dependent on the configuration - will determine if the node in question > shifts the information by 16 or 32 bits - before the FIB lookup. > > Therefore - in a standard scenario - you know that packet X is going to be > forwarded using a FIB entry lookup of Y - in the case of this - you have no > idea what the router is actually going to be looking up - because you don't > know if its going to be doing a 16 or 32 bit sid without knowing the > specific local configuration on the router. That’s a very different level > of complexity in terms of operations and debugging on a large network > > Andrew > > > -------------------------------------------------------------------- > IETF IPv6 working group mailing list > [email protected] > Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 > -------------------------------------------------------------------- >
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