ISP Column - March 2021
IETF 110 was held virtually in March 2020. These are some notes I took on the topic of current research activities in the area of transport protocol flow control at the meeting of the Internet Congestion Control Research Group at that meeting.
In the early days of congestion control signaling, we originally came up with the rather crude "Source Quench" control message, an ICMP control message sent from an interior gateway to a packet source that directed the source to reduce its packet sending rate. It was unrelated to the transport protocol that the end host might be using, and intentionally operated at the level of the IP layer within the context of the protocol stack. It was a decent first effort at a rather complex problem, but it was always going to be replaced by successive refinements as we gained experience in this area. The IETF got around to formally deprecating the Source Quench mechanism in 2012 in RFC 6633, though it had lapsed into disuse around two decades earlier. Then there was Explicit Congestion Notification (ECN) for TCP (RFC 3168) where a gateway marked one or more packets in a TCP data flow when it was experiencing congestion and the ECN signal was echoed in the reverse TCP ACK packets so that the signal was passed back to the sender. It’s an improvement without doubt, as it allows the sender to respond to the onset of congestion prior to the more disruptive signal of packet drop, but at the same time it’s a single bit signal, so the signal can signal the presence of a congestion condition or not, but not much more.
It is always fascinating how we continue to refine silicon in ways previously considered impossible or completely impractical. New commodity ASICs have in-band telemetry ability. This can be used to formulate precise feedback for congestion control. The idea is to replace the coarse single-bit feedback in ECN with more information, including queue length, link capacity, timestamp and similar. Packets in the data forwarding path have switch telemetry attached to the packet by every switch, and this telemetry is copied to the corresponding ACK packet being passed in the reverse direction (Figure 1). The objective here is to eliminate the extended process of searching for a stable flow profile with a process of rapid adjustment to a desired profile, while at the same time making minimal queue demands as a latency reduction mechanism.
