question about trimning data "len" conditions in TSO in tcp_output.c

[Cui, Cheng]

On Nov 6, 2015, at 4:16 PM, Hans Petter Selasky <hps at selasky.org> wrote:

> On 11/06/15 21:46, Cui, Cheng wrote:
>> Hello Hans,
>> 
>> Sorry if my previous email does not reach you because of a bad subject.
>> 
>> This is Cheng Cui. I am reading the CURRENT FreeBSD code in tcp_output.c, and find this question regarding your change in revision 271946.
>> https://svnweb.freebsd.org/base/head/sys/netinet/tcp_output.c?r1=271946&r2=271945&pathrev=271946
>> 
>> trim data "len" under TSO:
>> 
>> 885	                        /*
>> 886	                         * Prevent the last segment from being
>> 887	                         * fractional unless the send sockbuf can be
>> 888	                         * emptied:
>> 889	                         */
>> 890	                        max_len = (tp->t_maxopd - optlen);
>> 891	                        if ((off + len) < sbavail(&so->so_snd)) {    <==
>> 892	                                moff = len % max_len;
>> 893	                                if (moff != 0) {
>> 894	                                        len -= moff;
>> 895	                                        sendalot = 1;
>> 896	                                }
>> 897	                        }
>> 
>> Is there a specific reason that it should skip trimming the data "len" under the condition of "(off + len) == sbavail(&so->so_snd)" in TSO?
> > Because I am wondering if we can trim the data "len" directly without checking the "(off + len)" condition.
> 
> Hi Cheng,
> 
> I believe the reason is to avoid looping one more time outputting a single packet containing the remainder of the available data, with regard to max_len. How did you envision the removal of this check would influence the generated packet sequence?
> 
> --HPS
> 
Hi Hans,

I may be wrong but my assumption is that the remainder of the available data may be larger than one single packet.

Suppose max_len==1500, sb_acc==3001, off==2, and (off+len)==3001. In this case, the current code will not trim the "len" and let it go directly to the NIC. I think it skips the Nagle's algorithm. As len==2999, the last packet is 1499, it is supposed to be held until all outstanding data are ACKed, but it has been sent out.

Cheng