Re: TCP_MAXSEG vs TCP/generic segmentation offload (tso/gso)

Eric Dumazet
2010-11-25T16:44:45Z

Le jeudi 25 novembre 2010 à 17:25 +0100, Niels Möller a écrit :

>
> I was under the impression that TSO (and maybe GSO) implied more
> cleverness in the network card; that the network card more or less gets
> to decide by itself how to divide a tcp stream into segments. And for
> example in the atl1c driver which I looked a bit into, this was what the
> REG_MTU register was for. Seems I have gotten this totally wrong.
>

You were not totally wrong, but device does not use its own MTU to
perform the split : We give it the MSS of the flow.

You can have multiple flows in parallel, each with its own MSS, while
device has a single MTU.

> Maybe Documentation/networking/netdevices.txt could clarify how it
> works. Currently, it says
>
> : Segmentation Offload (GSO, TSO) is an exception to this rule. The
> : upper layer protocol may pass a large socket buffer to the device
> : transmit routine, and the device will break that up into separate
> : packets based on the current MTU.

MTU means : maximum transmission unit. But each layer has its own :)

In this context, TCP protocol, so MSS should be taken into account.

By default, MSS derives from device MTU (ipv4 without options case :
MSS = MTU - 40), but user can change it with TCP_MAXSEG.