[GRO] check whether ip_id continuity needs to be checked when two TCP packets are merged.

[Stephen Hemminger]

On Thu, 20 Apr 2023 02:30:41 +0000
"Hu, Jiayu" <jiayu.hu at intel.com> wrote:

> Hi Cheng,
> 
> > -----Original Message-----
> > From: jiangheng (G) <jiangheng14 at huawei.com>
> > Sent: Saturday, April 15, 2023 10:46 PM
> > To: users at dpdk.org; Hu, Jiayu <jiayu.hu at intel.com>; dev at dpdk.org
> > Subject: [GRO] check whether ip_id continuity needs to be checked when
> > two TCP packets are merged.
> > 
> > Hi jiayu.hu
> > 
> > It cannot be guaranteed that 16bit identification field of ip packets in the
> > same tcp stream will be continuous.
> > Please help check whether ip_id continuity needs to be checked when two
> > TCP packets are merged?
> > Seems to modify the following code, gro will aggregate better, and work
> > better:
> > 
> > diff --git a/lib/gro/gro_tcp4.h b/lib/gro/gro_tcp4.h index
> > 212f97a042..06faead7b5 100644
> > --- a/lib/gro/gro_tcp4.h
> > +++ b/lib/gro/gro_tcp4.h
> > @@ -291,12 +291,10 @@ check_seq_option(struct gro_tcp4_item *item,
> >         /* check if the two packets are neighbors */
> >         len = pkt_orig->pkt_len - l2_offset - pkt_orig->l2_len -
> >                 pkt_orig->l3_len - tcp_hl_orig;
> > -       if ((sent_seq == item->sent_seq + len) && (is_atomic ||
> > -                               (ip_id == item->ip_id + 1)))
> > +       if (sent_seq == item->sent_seq + len)  
> 
> For atomic packets, the IP ID field is ignored, as it can be set in various ways.
> For non-atomic packets, it follows Linux kernel tcp_gro_receive().
> 
> Is this change specific to your case? Can you give more details on why it helps?

Many OS's don't change IP ID if DF bit is set.
See RFC 6864 for details
   >> The IPv4 ID field MUST NOT be used for purposes other than
      fragmentation and reassembly.