Messages in this thread | | | Subject | Re: TCP_MAXSEG vs TCP/generic segmentation offload (tso/gso) | From | Eric Dumazet <> | Date | Thu, 25 Nov 2010 17:44:45 +0100 |
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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.
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