Forcing a higher MSS to improve TCP performance on links with asymmetric MTUs

An interesting networking problem was recently posed to me. As context, suppose two computers, call them “A” and “B”, are networked together such that the physical link from A to B is separate from the physical link from B to A. The A-B link has a link-layer Maximum Transmission Unit (MTU) of, say, 1500 bytes (normal for Ethernet). However, the B-A link has a much smaller MTU, say 500 bytes:

-----              -----

|   |-- 1500 MTU ->|   |

| A |              | B |

|   |<-- 500 MTU --|   |

-----              -----

Here’s the problem: when transferring large volumes of data from A to B over a TCP connection, the transfer rate is much slower than what is expected given a 100Mbps link speed from A to B. The question is why, and what can be done about it? Read on for the discussion, and a proposed and tested solution.

Weekend Diversion - Conway's Game of Life using ncurses

Seeking a bit of for-fun programming for the weekend and an excuse to refresh myself on the basics of ncurses, I decided to write my own small implementation of Conway's Game of Life. If you've never had a chance to become acquainted with this "game," I highly recommend taking the time to read about it or to play with one of the many implementations (including mine below if you'd like). It is a great study in how a small number of simple rules can lead to some amazing emergent phenomena when applied at a larger scale.

In a nutshell, the "game" (I use quotes because it is not a game in the traditional, competitive sense, although some people have created variants that allow two players to "compete") is staged in a 2-D world that one can think of as a grid of "cells." The life of each cell depends upon its neighbors; cells like some company, but not too much! For each "step" in the game, each cell's life is evaluated based on the number of living neighbors it has (above and below, left and right, and the four diagonals). The life of the cell following the step is determined by three rules:

1. If the number of living neighbors is exactly 2, the cell remains alive or dead (whichever it was previously).
2. If the number of living neighbors is exactly 3, the cell is "born" (or remains alive).
3. Otherwise, the cell "dies" (or remains dead).

Pretty simple, right? So simple that an entire implementation with an ncurses interface takes less than 200 lines of code (as sloccount reports; minus whitespace and comments). So, let's get to it!