On Wed, 6 Dec 2017 22:38:12 +0400 Ilya Matveychikov <matvejchi...@gmail.com> wrote:
> > On Dec 6, 2017, at 10:12 PM, Stephen Hemminger <step...@networkplumber.org> > > wrote: > > > > On Wed, 6 Dec 2017 18:02:21 +0400 > > Ilya Matveychikov <matvejchi...@gmail.com> wrote: > > > >> Hello all, > >> > >> > >> My question is about neighbor packet matching algorithm for TCP. Is it > >> correct to expect that IP packets should have continuous ID enumeration > >> (i.e. iph-next.id = iph-prev.id + 1)? > > > > > > No. > > > >> ~~~ > >> lib/librte_gro/gro_tcp4.c:check_seq_option() > >> ... > >> /* check if the two packets are neighbors */ > >> tcp_dl0 = pkt0->pkt_len - pkt0->l2_len - pkt0->l3_len - tcp_hl0; > >> if ((sent_seq == (item->sent_seq + tcp_dl0)) && > >> (ip_id == (item->ip_id + 1))) > >> /* append the new packet */ > >> return 1; > >> else if (((sent_seq + tcp_dl) == item->sent_seq) && > >> ((ip_id + item->nb_merged) == item->ip_id)) > >> /* pre-pend the new packet */ > >> return -1; > >> else > >> return 0; > >> ~~~ > >> > >> As per RFC791: > >> > >> Identification: 16 bits > >> > >> An identifying value assigned by the sender to aid in assembling the > >> fragments of a datagram. > > > > The IP header id is meaningless in most TCP sessions. > > Good TCP implementations use PMTU discovery which sets the Don't Fragment > > bit. > > With DF, the IP id is unused (since no fragmentation). > > Many implementations just send 0 since generating unique IP id requires an > > atomic operation which is potential bottleneck. > > So, is my question correct and the code is wrong? > Yes. This code is wrong on several areas. * The ip_id on TCP flows is irrelevant. * packet should only be merged if TCP flags are the same. The author should look at Linux net/ipv4/tcp_offload.c
