An Analysis of TCP Maximum Segment Sizes

The Maximum Segment Size (MSS) is a basic yet critical property of any TCP connection. Yet surprisingly, there have been no recent studies examining the MSS values that are announced and utilised by TCP senders in any detail. This paper analyses packet traces from several different locations and investigates the advertised and observed MSS values for each captured TCP connection. In particular, this study focuses on the range of MSS values that are announced by TCP endpoints and the relationship between the advertised MSS value and the MSS used by TCP senders. We find that although 1460 bytes remains the most commonly announced MSS, it accounts for less than half of all MSS advertisements. Instead, slightly smaller MSS values are increasing in popularity due to the rise in PPPoE and VPN technologies. We also find that many TCP sessions do not utilise either of the announced MSS values, possibly due to application behaviour, and that a small but not insignificant number of TCP senders transmit segments larger than permitted by the MSS announced to them.