Whenever someone asks me about LISP, I answer, “it’s a nice idea, but cache-based forwarding never worked well.” Oldtimers familiar with the spectacular failures of fast switching and various incarnations of flow switching usually need no further explanation. Unfortunately, that lore is quickly dying out, so let’s start with the fundamentals: how does packet forwarding work?
Packet forwarding used by bridges and routers (or Layer-2/3 switches if you believe in marketing terminology) is just a particular case of statistical multiplexing – a mechanism where many communication streams share the network resources by slicing the data into packets that are sent across the network. The packets are usually forwarded independently; every one of them must contain enough information to be propagated by each intermediate device it encounters on its way across the network.
In the previous blog post in this series I described how convoluted routing table lookups could become when you have to deal with numerous layers of indirection (BGP prefix ⇨ BGP next hop ⇨ IGP next hop ⇨ link bundle ⇨ outgoing interface). Modern high-end hardware can deal with the resulting complexity; decades ago we had to use router CPU to do multiple (potentially recursive) lookups in the IP routing table (there was no FIB at that time).
Network devices were always pushed to the bleeding edge of performance, and smart programmers always tried to optimize the CPU-intensive processes. One of the obvious packet forwarding optimizations relied on the fact that within a short timeframe most packets have to be forwarded to a small set of destinations. Welcome to the wonderful world of cache-based forwarding.
In the Cache-Based Packet Forwarding blog post I described what happens when someone tries to bypass the complexities of IP routing table lookup with a forwarding cache.
Now imagine you want to implement full-featured fast packet forwarding including ingress- and egress ACL, NAT, QoS… but find the required hardware (TCAM) too expensive. Wouldn’t it be nice if we could send the first packet of every flow to a CPU to figure out what to do with it, and download the results into a high-speed flow cache where they could be used to switch the subsequent packets of the same flow. Welcome to flow-based packet forwarding.