TCP - LRO/TSO techniques

Why i开发者_C百科s it must that all interfaces (routers and bridges) involved support LRO/TSO technique ?

Routers don't. Bridges do.

External routers, hubs, switches or anything else that is externally connected to the network will not see the effects of TSO, only interfaces inside the device with TSO will experience any effects - it's a software thing.

A router is an external device which is connected to the network by ethernet cables, fibre optic cables, wireless comms etc. These communication mediums adhere to internation standards such as 803.2 for ethernet or 803.11 for wireless - they're hardware devices, and hardware devices have very strict rules on how they communicate.

A bridge is an internal software construct and is specific to your OS.

Let's use 803.2 (ethernet) and a linux host for an example.

An application calls for a socket to be created and then pushes a large data chunk into the socket. The linux kernel determines which interface this data should be transitted on. The kernel will next interrogate the driver for this interface to determine its capabilities, if the interface is TSO capable the kernel will pass an sk_buff with a single "template" header and a huge chunk of data (more than 1 packets worth) to the interface driver.

Let's consider a standard interface straight to a hardware NIC first:

Some interfaces have fake TSO (they segment the packet in the driver) and some have true TSO (the template header and data are passed to the hardware with minimal alterations). At this point ether the driver or the NIC hardware will convert this large segment of data into multiple, standard compliant, 803.2 ethernet frames, it is these compliant frames that an external device, such as a router, hub, switch, modem or other host will see on the wire.

Now let's consider several NICs behind a software bridge:

Although the kernel is aware of each NIC at a low level, the network stack is only aware of the bride, thus only capabilities that ALL of the underlying NICs have should be passed up to the bridge. If an sk_buff is passed to a bridge, then ALL the interfaces in the bridge will receive the same sk_buff. We'll assume that the kernal has once again passed our large TSO sk_buff to a bridge, if any of the underlying interfaces does not support TSO then the packet will most likely be dropped by the hardware NIC in question.

In summary:

Worst case scenario is the bridge will repeatedly retry to send the same data chunk on the broken interface and the whole bridge will lock up until the application decides to give up. Best case scenario, the non TSO NIC will simply appear to be dead.

That said, if the NIC has unsafe code in its driver then this could cause a segmentation fault that could bring the whole system down.