What if I created a packet with a terminal destination IP address that belongs to a router? That is, what would happen if I sent a packet to a router as my terminal destination? How would the router process it?
A router consists of 2 major components.
1) The "forwarding plane".
2) The "control plane".
The forwarding plane is software or hardware that does nothing but:
a) receive incoming packets
b) look at the destination ip-address
c) look-up the destination ip-address in the forwarding table (aka the "FIB")
d) find the outgoing interface and next-hop ip-address in that "FIB-entry"
e) forward the packet out that interface to the next-hop as specified in the FIB-entry
The "forwarding-plane" can be a process running in user-space on a Linux-box. It can be code inside the kernel on a Linux box. It can be an interrupt-routine in the kernel in a Linux-box. Or it can be c-code running on a cpu on a line-card in a bigger router. Or it can be firmware running on an ASIC or a FPGA. Or even firmware running on a NPU (special chip, like a CPU, but for packet-forwarding). The whole idea of a "forwarding-plane" is that it is simple. All state it has is inside the FIB (Forwarding Information Base). All it does is forward packets.
The control-plane does not forward packets. The control-plane is very much like a regular computer. It has a CPU and RAM. Maybe long-term memory too (flash, or an SSD, or whatever). It runs a full-blow OS. Based on Unix, Linux, or some real-time version of Unix. It might be an (old) type of OS, like vXworks. Or a home-grown OS (like Classic IOS). It has a scheduler, runs processes (or Tasks). Maybe does multi-threading. It runs processes that do routing-protocols. The routing-protocols build a routing-table (the RIB). The RIB is simplified and copied to the forwarding-plane, where it is called the FIB. The control-plane also runs CLI-processes (like Unix-shells), it runs an SNMP-demon, etc, etc.
When you send a packet to a router, destined for one of its own ip-addresses, this happens:
1) the router receives the packet
2) does a lookup for the destination ip-address in the FIB
3) find an entry in the FIB that indicates that the ip-address belongs to the router itself
4) the forwarding-plane *punts* the packet up, to the control-plane
(The word "punt" is used for that: a packet is not forwarded, but given to the control-plane)
5) the control-plane looks at the destination-ip-address, the tcp-portnumber, etc
6) the control-plane gives the packet to the TCP component
7) the TCP component gives the packet to one of the tasks running on the control-plane
e.g. the sshd demon, the BGP task, the HTTP demon, etc
I hope this de-mystifies what a router does.
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