TCP Splitting for Performance Optimization

As a case study, consider the delay in receiving a response for a search query. Typically, the server requires three TCP windows during slow start to deliver the response [Pathak 2010]. Thus the time from when an end system initiates a TCP connection until the time when it receives the last packet of the response is roughly 4 # RTT (one RTT to set up the TCP connection plus three RTTs for the three windows of data) plus the processing time in the data center. These RTT delays can lead to a noticeable delay in returning search results for a significant fraction of queries. Moreover, there can be significant packet loss in access networks, leading to TCP retransmissions and even larger delays. One way to mitigate this problem and improve user-perceived performance is to (1) deploy front-end servers closer to the users, and (2) utilize TCP splitting by break- ing the TCP connection at the front-end server. With TCP splitting, the client establishes a TCP connection to the nearby front-end, and the front-end maintains a persistent TCP connection to the data center with a very large TCP congestion window [Tariq 2008, Pathak 2010, Chen 2011]. With this approach, the response time roughly becomes 4 # RTTFE + RTTBE + processing time, where RTTFE is the round-trip time between client and front-end server, and RTTBE is the round-trip time between the front-end server and the data center (back-end server). If the front-end server is close to client, then this response time approximately becomes RTT plus processing time, since RTTFE is negligibly small and RTTBE is approximately RTT. In summary, TCP splitting can reduce the networking delay roughly from 4 # RTT to RTT, significantly improving user-perceived performance, particularly for users who are far from the nearest data center. TCP splitting also helps reduce TCP retransmission delays caused by losses in access networks. Google and Akamai have made extensive use of their CDN servers in access networks (recall our discussion in Section 2.6) to perform TCP splitting for the cloud services they support [Chen 2011].