Re: [dpdk-dev] two tso related questions

[Alex Markuze <alex@weka.io>]

On Tue, Dec 16, 2014 at 2:24 PM, Helmut Sim <simhelmut@gmail.com> wrote:
>
> Thanks Alex,
>
> So i probably miss something...
> what you are saying is correct for IP segmentation where the segmentation
> is at the IP level, and all segments are identified according to the
> Identification field in the IP header.
>
> However in TCP segmentation the segments are at the TCP level (isn't it?),
> where each frame is at a size of
> MSS+sizeof(tcp_hdr)+sizeof(ip_hdr)+sizeof(eth_hdr).
> Hence, for each of the sent packets, the IP Identification is 0 and the IP
> total length is MSS+sizeof(tcp_hdr)+sizeof(ip_hdr).
>
> Please correct me if i am wrong.
>
TSO - takes a one packet max size 64KB(not counting mac/vlan size). and
brakes it into valid mtu sized packets each with its one IP and TCP header.
I'm not sure what how the identificayion/Frag off fields are filled. you
can easily check it by running a short tcp stream(perf/netperf) between two
machines and capturing the packets with tcpdump (wireshark to open) use
ethanol -K to disable LRO/GRO (the receive side kernel driver will
rearrange the headers otherwise).

I hope this helps.

>
>
thanks.
>
> On Tue, Dec 16, 2014 at 11:10 AM, Alex Markuze <alex@weka.io> wrote:
>>
>>
>>
>> On Mon, Dec 15, 2014 at 10:20 PM, Helmut Sim <simhelmut@gmail.com> wrote:
>>>
>>> Hi,
>>>
>>> While working on TSO based solution I faced the following two questions:
>>>
>>> 1.
>>> is there a maximum pkt_len to be used with TSO?, e.g. let's say if seg_sz
>>> is 1400 can the entire segmented pkt be 256K (higer than 64K) ?, then the
>>> driver gets a list of chanined mbufs while the first mbuf is set to TSO
>>> offload.
>>>
>>
>> TSO segments a TCP packet into mtu sied bits. The TCP/IP protocols are
>> limited to 64K due to the length fields being 16bit wide. You can't build a
>> valid packet longer then 64K regardless of the NIC.
>>
>>
>>> 2.
>>> I wonder, Is there a specific reason why TSO is supported only for IXGBE
>>> and not for IGB ? the 82576 NIC supports TSO though.
>>> Is it due to a kind of tecnical barrier or is it because of priorities?
>>>
>>> It will be great if someone from the forum could address this.
>>>
>>> Thanks,
>>> Sim
>>>
>>