XK72 | Java and Adobe Flash Software

In December last year Rich Riendeau reported excessive packet fragmentation from the throttling implementation in Charles v2.6.1. While the simulation of the bandwidth was accurate, it didn’t necessarily reflect the structure of the packets as they would actually exist on the network in a bandwidth limited situation.

Packets are usually be sent out onto the network in fragments equal to the MTU, however Charles tried to share the throttled bandwidth between multiple connections by allowing data through in chunks of approximately 1/10 of the available bandwidth. Therefore depending upon your throttling configuration, and the number of concurrent connections, Charles could produce more packet fragments than would be expected. So as of Charles v2.6.2 the throttling system has been tweaked to use MTU-size packets.

As a consequence of this there is an MTU setting as part of the throttling configuration. On a broadband connection (and on any Ethernet network) the MTU is often 1500 bytes, however on a modem or PPP connection the MTU is 576 bytes and packets must be fragmented to fit. Charles now has the capability to simulate the MTU differences on different types of connections and thus effect the appropriate realistic fragmentation.

Looking to the future: a feature that has been requested a few times is random throttling to simulate variable Internet conditions. I’ve spoken to several people about it and I think that its time may be near! If you’ve got any thoughts about it I’d be pleased to hear them.

This entry was posted on Monday, February 12th, 2007 at 8:29 pm and is filed under Charles. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.