Comment by toast0

Most intermediate routers don't care much. Lookup the destination IP in the routing table, forward to the next hop, no time for anything else.

Classic congestion control is done on the sender alone. The router's job is simply to drop packets when the queue is too large.

Maybe the router supports ECN, so if there's a queue going to the next hop, it will look for protocol specific ECN headers to manipulate.

Some network elements do more than the usual routing work. A traffic shaper might have per-user queues with outbound bandwidth limits. A network accelerator may effectively reterminate TCP in hopes of increasing acheivable bandwidth.

Often, the router has an aggregated connection to the next hop, so it'll use a hash on the addresses in the packet to choose which of the underlying connections to use. That hash could be based on many things, but it's not uncommon to use tcp or udp port numbers if available. This can also be used to chose between equally scored next hops and that's why you often see several different paths during a traceroute. Using port numbers is helpful to balance connections from IP A to IP B over multiple links. If you us an unknown protocol, even if it is multiplexed into ports or similar (like tcp and udp), the different streams will likely always hash onto the same link and you won't be able to exceed the bandwidth of a single link and a damaged or congested link will affect all or none of your connections.