> -----Original Message----- > From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Stephen Hemminger > Sent: Wednesday, December 6, 2017 11:16 PM > To: Ilya Matveychikov <matvejchi...@gmail.com> > Cc: dev@dpdk.org; Hu, Jiayu <jiayu...@intel.com> > Subject: Re: [dpdk-dev] A question about GRO neighbor packet matching > > 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
As I remember, linux GRO implementation *does* require that IP IDs of the merging packets to be continuous. net/ipv4/af_inet.c: static struct sk_buff **inet_gro_receive(struct sk_buff **head, struct sk_buff *skb) { ... id = ntohl(*(__be32 *)&iph->id); flush = (u16)((ntohl(*(__be32 *)iph) ^ skb_gro_len(skb)) | (id & ~IP_DF)); id >>= 16; ... NAPI_GRO_CB(p)->flush_id = ((u16)(ntohs(iph2->id) + NAPI_GRO_CB(p)->count) ^ id); NAPI_GRO_CB(p)->flush |= flush; .... And then at net/ipv4/tcp_offload.c: struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb) { ... /* Include the IP ID check below from the inner most IP hdr */ flush = NAPI_GRO_CB(p)->flush | NAPI_GRO_CB(p)->flush_id; ... if (flush || skb_gro_receive(head, skb)) { ... The reason why we do need to check that IP ID is continuous - DPDK GRO library doesn't strip off IPv4 header, instead it has to merge them into one. If IP ID would be non-contiguous it is unclear which one should be to used. By same reason packets with different IP/TCP options are not allowed. So in that case GRO lib makes a decision that it isn't safe to merge these packets. As I understand linux does pretty much the same. Konstantin