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Hi all, I am trying to understand the current state of TCP segmentation offload to Network cards. in Linux I have read posts that tells that Intel e1000 driver supports TSO (TCP segmentation offload). This link tells that TSO is supported in Linux: http://lwn.net/Articles/9129/ I see the define NETIF_F_TSO in the linux kernel, and I see that the e1000 driver is setting this flag in its capabilities list. But for TSO to work, TCP code should send the big segment (say 64K) directly to the NIC, bypassing the IP layer. Else, the IP layer would fragment the packets, and the whole purpose of TSO will be lost. I cannot see any code in tcp layer which bypasses the IP layer and goes directly to the NIC. It seems to me that the TCP/IP stack in Linux always does the segmentation, since there is no code under the NETIF_F_TSO define which bypasses the IP layer. Or am I missing something? Can anyone please tell me whether the Linux TCP/IP stack support TSO so that TCP segments bigger than MSS is segmented by the NIC, not the software? If not, is there plans to support this, since this is stateless offload? Any pointers would be greatly appreciated. Regards, Prodyuthttp://lwn.net/Articles/9129/ I see the define NETIF_F_TSO in the linux kernel, and I see that the e1000 driver is setting this flag in its capabilities list. But for TSO to work, TCP code should send the big segment (say 64K) directly to the NIC, bypassing the IP layer. Else, the IP layer would fragment the packets, and the whole purpose of TSO will be lost. I cannot see any code in tcp layer which bypasses the IP layer and goes directly to the NIC. It seems to me that the TCP/IP stack in Linux always does the segmentation, since there is no code under the NETIF_F_TSO define which bypasses the IP layer. Or am I missing something? Can anyone please tell me whether the Linux TCP/IP stack support TSO so that TCP segments bigger than MSS is segmented by the NIC, not the software? If not, is there plans to support this, ...
The IP layer does not need to be bypassed. The TCP layer will set the proper headers for TSO to work properly and continue sending the skb down the stack. IP will not fragment a packet on transmit. The only way an skb will be fragmented into smaller chunks is if gso_segment() is called, which will call the proper upper-layer segmenter routine. This case only happens when a packet was coalesced by the TCP layer and sent to the core stack, which then ran GSO (generic segmentation offload) in net/core/dev.c, dev_hard_start_xmit(), which would be the case if the underlying NIC didn't have TSO enabled, but still wanted to have the TCP Yes, TSO is fully supported. The default is 64k frames, and in the upstream tree, netif_set_gso_max_size(). I know most 1 GbE and beyond parts and Linux drivers fully support TSO. I hope this helps. Cheers, -PJ Waskiewicz --
Thanks for your reply, Peter. I would appreciate if you could clarify Which flag/header does the TCP layer set to indicate to IP that it should not fragment a 64K segment, but rather offload the whole segment to NIC? I tried to look at the TCP code in linux stack, but could not figure the flag or the logic that sets the flag/header you mentioned. The NIC driver must indicate that it is capable of doing TSO by setting NETIF_F_TSO flag, but NETIF_F_TSO capability is not checked in IP/TCP layer. I would really appreciate if you could point me to the function in TCP code in linux. Thanks for your time, Prodyut --
It's checked using sk_can_gso() (defined in <net/sock.h>) which Ben. -- Ben Hutchings, Senior Software Engineer, Solarflare Communications Not speaking for my employer; that's the marketing department's job. --
To add: the flag that sk_can_gso() checks is set on the socket creation, based on the underlying netdevice's TSO capabilities. There are a bunch of flags in the sk struct that get set on TCP socket creation that are then used elsewhere in the TCP stack. Cheers, -PJ --