Sharing small optical buffers between real-time and TCP traffic

Buffers in emerging optical packet routers are expensive resources, and it is expected that they would be able to store at most a few tens of KiloBytes of data in the optical domain. When TCP and real-time (UDP) traffic multiplex at an optical router with such small buffers (less than 50 KiloBytes), we recently showed that UDP packet loss can increase with increasing buffer size. This anomalous loss behaviour can negatively impact the investment made in larger buffers and degrade quality of service. In this paper we explore if this anomalous behaviour can be alleviated by dedicating (i.e., pre-allocating) buffers to UDP traffic. Our contributions within this context are two fold. First, we show using extensive simulations that there would seem to be a critical buffer size above which UDP benefits with dedicated buffers that protect it from the aggressive nature of TCP. However, for smaller buffers that are below this critical value, UDP can benefit by time-sharing the buffers with TCP. Second, we develop a simple linear model that quantitatively captures the combined utility of TCP and real-time traffic for shared and dedicated buffers, and propose a method to optimise the buffer split ratio with the objective of maximising the overall network utility. Our study equips designers of optical packet switched networks with quantitative tools to tune their buffer allocation strategies subject to various system parameters such as the ratio of traffic mix and relative weights associated with TCP and UDP traffic.