# Performance Impact of Nested Congestion Control on Transport-Layer Multipath Tunneling
## 3 Related Work
The above examples of multipath proxies and anchors are either at a layer _other than_ the transport layer or are based on _converting_ a single-path protocol to its multipath analogue at a proxy, rather than tunneling at an anchor. If a multipath transport-layer solution is implemented _as a tunnel rather than a protocol converter_, the tunnel may serve _all_ traffic types, but as demonstrated in this paper, issues of adverse interactions between nested CC instances arise. When transport-layer tunnels have been considered, they have typically been _single-path_ tunnels. Issues explored include things like how to use TCP tunnels to force CC upon UDP [13], the usage of TCP-tunnels with network coding over high loss links [33], the performance difference between TCP and UDP tunnels [12], or when TCP-over-TCP is helpful and when it is harmful [11]. Obviously, that research lacks the multipath component which is a key aspect of this paper. A very recent development is _Multiplexed Application Substrate over QUIC Encryption_ (MASQUE) [34; 35], whereby multiple end-to-end connections may use a single QUIC tunnel. One possible use case is for end-to-end QUIC; as end-to-end QUIC traffic is encrypted, a tunnel is required to support network functionality that has historically been implemented as a TCP proxy [36]. With a future multipath QUIC, MASQUE could serve a function very much similar to MP-DCCP in this paper. There are several CC algorithms (CCA) in common use today. These typically regulate the sending rate by limiting the quantity of in-flight data to be no more than the congestion window (CWND). NewReno increases its CWND linearly and is thereby often bad at utilizing _high BDP_ paths, _i.e., paths having a high throughput or a long RTT_. Cubic [37], on the other hand, increases its CWND using a cubic function. The rationale behind this design choice is that a cubic function helps the CCA to plateau at a CWND that ensures the full utilization of the path, without consistently filling the bottleneck buffer. Using a cubic function also more rapidly utilizes high BDP paths. BBR is designed based on a run-time model of the path that BBR maintains [38]. This requires BBR to monitor the achievable throughput and the minimum round-trip time. By multiplying these two into a BDP, BBR may achieve the full utilization of the link by having a number of bytes in-flight which is equal to or greater than that of the BDP. BBR version 2 (BBRv2) is currently being developed as an improvement on the original BBR and will, among other things, react to packet loss, _unlike BBR_. To understand the issues that arise in the paper, and how to solve them, it becomes necessary to understand and analyze the CC algorithms used. The particulars of a given tunnel CCA may well drive certain problems, as well as constitute a solution in other circumstances; as our results demonstrate. Although we do not take this step in the paper, it is fairly clear that the problems we have illustrated could be solved by using a CCA designed for the transport-layer multipath tunneling context. Packet scheduling algorithms are actively being researched. It is known that a bad traffic scheduling strategy cannot fully utilize multipath resources, and that too many data sent over a long-RTT path may cause blocking on that path [39]. The min RTT scheduler [40] is the default scheduler for MPTCP and prioritizes the path with the shortest estimated RTT. However, min RTT does not estimate how much traffic should be sent over each path and can be improved, for example, by using a Blocking Estimation-based MPTCP scheduler algorithm [41], which adds such an estimation. More recent work consider how to avoid blocking for streams in QUIC [42]. Other scheduling algorithms include Earliest Completion First [43], Peekaboo [44], DEMS [45], and QSAT [46]. Our work here focus on the CPF scheduler, which, much like nested CC, is made particularly relevant given the 5G ATSSS architecture, where a cheap path, e.g., WiFi, is often available.