Tuesday, July 21, 2009

More about Chimney and TCPOffload in Hyper-V

Here are some definitions that help to clarify the TCPOffload and Chimney thing really well.

I have Don from the Hyper-V networking team to thank for the detail.  Being a hard-core networking guy he knows his packets.

Chimney, also known as TCP Chimney Offload, is the offloading of all IP and TCP processing to the NIC.  This means the NIC receives the packet, processes the headers, generates the ACKs, and keeps all the state.  On the outbound side it receives a block of data from the app, packetizes it, generates the headers, generates the IP layer, and ships it.  Chimney is available on some Broadcom NICs.

Checksum offload is the offloading to the NIC the responsibility for generating the header checksums (outbound side) and verifying the header checksums (inbound side).  No header processing is done other than the checksum processing.  No state is maintained in the NIC.  Nearly all server class NICs support checksum offload.

Large Send Offload (LSO and LSOv2) is the offloading, on the send side, of the packetization and header generation.  The hardware takes a large data block and, using state information from the stack, generates appropriate size data packets (including the headers).  The state is kept in the stack.  LSO and LSO v2 are different versions of this feature.  LSOv2 is supported in R2.

In summary: if you are using Chimney you receive the benefits of the other two.  Disabling Chimney does not disable either LSO/LSOv2 or checksum offload.


BrianEh said...


Anonymous said...

offloading in windows totally sucks! and most have to disable this crap

BrianEh said...

Offloading is not in the control of MSFT, it has been left in the control of the NIC vendors.

The combination of the NIC, NIC firmware, and drivers determines whether you have a good experience or bad.

I am a person that has disabled these settings by default for years and years. And lately with new Intel and HP NICs I find I can leave it alone. Broadcom is still flakey (watch for rebranded Broadcom).