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The Future

networking is still evolving. Major changes at the kernel layer will bring a very flexible configuration scheme that will allow you to configure the network devices at run time. For instance, the ifconfig command will take arguments that set the IRQ line and DMA channel.

Another change to come soon is the additional mtu flag to the route command which will set the Maximum Transmission Unit for a particular route. This route-specific MTU overrides the MTU specified for the interface. You will typically use this option for routes through a gateway, where the link between the gateway and the destination host requires a very low MTU. For instance, assume host wanderer is connected to vlager through a SLIP link. When sending data from vstout to wanderer, the networking layer on wanderer would would use packets of up to 1500 bytes, because packets are sent across the Ethernet. The SLIP link, on the other hand, is operated with an MTU of 296, so the network layer on vlager would have to break up the IP packets into smaller fragments that fit into 296 bytes. If instead, you would have configured the route on vstout to use a MTU of 296 right from the start, this relatively expensive fragmentation could be avoided:

           # route add wanderer gw vlager mtu 296
Note that the mtu option also allows you to selectively undo the effects of the `Subnets Are Local' Policy (SNARL). This policy is a kernel configuration option and is described in chapter-gif.

Andrew Anderson
Thu Mar 7 23:22:06 EST 1996