Comment by Twisol

They probably can do deep/shallow packet inspection for that purpose (being one of the non-nefarious applications I alluded to), but that's not to say their correct functioning relies on it. Those routers also need to support at least UDP, and UDP provides almost no extra information at that level -- just the source and destination ports (so, perhaps QoS prioritization) and the inner payload's length and checksum (so, perhaps dropping bad packets quickly).

If middleware decides to do packet inspection, it better make sure that any behavioral differences (relative to not doing any inspection) is strictly an optimization and does not impact the correctness of the link.

Also, although I'm not a network operator by any stretch, my understanding is that TCP congestion control is primarily a function of the endpoints of the TCP link, not the IP routers along the way. As Wikipedia explains [0]:

> Per the end-to-end principle, congestion control is largely a function of internet hosts, not the network itself.

[0]: https://en.wikipedia.org/wiki/TCP_congestion_control