Modularized TCP congestion implementation
Linux TCP implementation has revolutionized by modularizing various TCP congestion control flavors. Linux kernel 2.6.13 onwards we see this kind of organization. There are different modules introduced for various TCP flavors like vegas, westwood, higspeed TCP, reno etc., One file for each TCP flavor exist under the directory -
net/ipv4/
Flexibility for applications to select TCP flavor -
The biggest flexibility that Linux offers by modularizing various TCP congestion flavors is that each TCP connection can choose congestion algorithm according to it's own requirements. A given application can have multiple TCP connections each one using different link to connect to it's peer. These connections may be on LAN, slow WAN, high speed optical network and on wireless networks. All these connection require different congestion algorithms to be active because of varying network characteristics. So, application has flexibility of selecting congestion algorithm. If an application does not select any specific TCP congestion flavor, reno is a default selection. New sysctl is introduced to change the default congestion flavor!!!!
Following socket option can be exploited to change congestion flavor for your connection -
name of the TCP flavor needs to be passed to the socket option and that flavor need to be present as loadable module on the system.
Inside the kernel -
Each TCP flavor module has to register struct tcp_congestion_ops of it's own and register with the system. The key to find specific TCP congestion module is name of the flavor like vegas, highspeed etc., This is stored in name field of struct tcp_congestion_ops. Different congestion parameters like slowstart threshold, congestion window etc., are evaluated by routines registered with each module and at the same time we have congestion avoidance algorithms and recovery algorithms are also implemented as part of these modules.
More on the way .....
