Packet Trimming to Reduce Buffer Sizes and Improve Round-Trip Times - Extended Abstract

The size of the buffers in a wide area network is intrinsically linked to the behavior of the end-to-end transport protocol. Indeed, the buffer’s role is dual: reducing the number of packet drops in the network when multiple flows are contending for the same constrained resource, and ensuring that the outgoing link is fully utilized. Large buffers help with both these objectives, but introduce congestion delays in the network. The larger the buffer, the longer the potential wait of a packet before being sent on the wire. In order to reduce this congestion latency, shallow buffers are preferred, but they introduce packet drops and retransmission. In particular, the feedback loop of the transport protocol are typically not quick enough to reduce the transmission rate when congestion occurs. In [9], a mechanism was suggested to selectively drop chunks of a packet in order to reduce congestion in the network by reducing the payload size. In one instance, the payload may be constituted of several chunks, some with higher priority/importance than others, and the lowest priority chunks can be dropped preferrentially upon congestion. In another instance, the payload may be constituted of chunks drawn from a fountain code so that any number of chunks may be dropped along the way. As long as sufficiently many reach the receiver, the payload can be decoded. The idea of dropping part of the packet has been explored in the context of the datacenter. Trimming methods have been proposed for data centers before, such as "cut payload" (CP)[4] and NDP [7]. The goal in these prior schemes is to achieve fast retransmissions for data center workloads with very shallow buffers. They work by trimming the whole payload and only keep the header. This work in the Data Center, since the dropped payload may be retransmitted fast enough to be re-sent to the node that dropped the payload before this buffer has emptied. This does not generalize to the case of the WAN. We contend that selectively trimming the packet rather than the whole payload offers a less drastic mechanism that would work in the wide area network. It is an immediate reaction to congestion that still sends a signal to the end-to-end congestion feedback loop. For this reason, it is better suited to shallow buffers in the WAN. This is work in progress, and designing an overall end-to-end transport protocol that provides the rate adaptation to minimize the number of packets being trimmed is an open issue. Related Works: Considerable work has been reported on transportlayer protocols (see, for instance, [10] for a recent survey), and reducing Internet delays has been a major goal in this space [2]. Buffer sizing [1, 5, 11] and reducing the buffer size [6] have been studied previously as well. This prior work established a rule that the buffer depth should be C × RTT / √ N , where C is the link capacity, and N is the number of concurrent flows. Large buffers, and bufferbloat in particular, have introduced unwanted latency in the network [3]. We contend that packet trimming, associated with a properly designed transport protocol, can reduce buffer depth to reduce latency due to congestion. Our work is inspired by the work by Li et al. [9], which uses the concept of selectively dropping chunks in a packet based upon some congestion criteria.