After a bit of a break due to current pandemic we decided to carry on and continue our meetings as virtual calls. Ignoring the usual initial hiccups and the missing whiteboard the medium worked well for us.
We started off with a quick recap of the previous session covering why we need congestion control, how one can view a multi-hop connection as a single hop connection with a single bottleneck and most importantly the fact that the Internet is the largest distributed system that most of the time “just works” due to congestion control.
The key takeaways are best understood through figure 1 of the ACM Queue article . A TCP connection can be in 3 states:
A connection starts off in the app limited stage where increasing the amount of inflight packets does not increase the round-trip-time.
Once one surpases the bandwidth delay product one enters the bandwidth limited stage. With each additional packet inflight, the round-trip-time increases, given that the buffer length at the bottleneck grows.
With reaching the limit of the buffer one enters the buffer limited stage. At this point packets are dropped and eventually packet loss is detected on the sender side.
Loss-based congestion control algorithms, as their name implies, operate based on loss detection, thus they only act once going from bandwidth limited to buffer limited. The key take-away and the novelty of BBR is that it does not operate based on loss-detection, but rather on perceived round-trip-time and bottleneck-bandwidth. With this mechanism BBR promises to operate closer to the optimal - the bandwidth delay product - preventing packet-loss in the first place instead of operating based on it.
For people wanting to dive deeper here is a list of resources mentioned during the session:
The IETF draft.
BBR uses ACKs to estimate round-trip-time and bandwidth. On Wifi ACKs can be aggregated, thus distorting these measurements. The below link shows the corresponding kernel patch tackling this issue.
Following up on the concerns above there is a PacketPushers podcast covering the topic.
While BBR might work well for the connection itself, it is quite aggressive and thus can have an impact on loss-based congestion control algorithms like Rino. The following paper provides more details in section 4.C.2.
For the next session we will stay within the realm of networking and talk about packet scheduling and queuing in Linux and how one can implement QoS (Quality of Service).
 Cardwell, Neal, et al. “BBR: Congestion-based congestion control.” Queue 14.5 (2016): 20-53.
Author Max Inden