From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Ananyev, Konstantin" Subject: Re: A question about GRO neighbor packet matching Date: Thu, 7 Dec 2017 00:19:46 +0000 Message-ID: <2601191342CEEE43887BDE71AB9772585FAC57C1@irsmsx105.ger.corp.intel.com> References: <4F9781B2-338C-4154-BDA1-BC24D1B2B689@gmail.com> <20171206101200.031afa39@shemminger-XPS-13-9360> <2111ED2C-DB90-4AE3-893E-2406EFE129AD@gmail.com> <20171206151532.3abaf2fb@xeon-e3> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Cc: "dev@dpdk.org" , "Hu, Jiayu" To: Stephen Hemminger , Ilya Matveychikov Return-path: Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) by dpdk.org (Postfix) with ESMTP id 03471E5D for ; Thu, 7 Dec 2017 01:19:49 +0100 (CET) In-Reply-To: <20171206151532.3abaf2fb@xeon-e3> Content-Language: en-US List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" > -----Original Message----- > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Stephen Hemminger > Sent: Wednesday, December 6, 2017 11:16 PM > To: Ilya Matveychikov > Cc: dev@dpdk.org; Hu, Jiayu > Subject: Re: [dpdk-dev] A question about GRO neighbor packet matching >=20 > On Wed, 6 Dec 2017 22:38:12 +0400 > Ilya Matveychikov wrote: >=20 > > > On Dec 6, 2017, at 10:12 PM, Stephen Hemminger wrote: > > > > > > On Wed, 6 Dec 2017 18:02:21 +0400 > > > Ilya Matveychikov 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 enumerat= ion > > >> (i.e. iph-next.id =3D iph-prev.id + 1)? > > > > > > > > > No. > > > > > >> ~~~ > > >> lib/librte_gro/gro_tcp4.c:check_seq_option() > > >> ... > > >> /* check if the two packets are neighbors */ > > >> tcp_dl0 =3D pkt0->pkt_len - pkt0->l2_len - pkt0->l3_len - tcp_hl0; > > >> if ((sent_seq =3D=3D (item->sent_seq + tcp_dl0)) && > > >> (ip_id =3D=3D (item->ip_id + 1))) > > >> /* append the new packet */ > > >> return 1; > > >> else if (((sent_seq + tcp_dl) =3D=3D item->sent_seq) && > > >> ((ip_id + item->nb_merged) =3D=3D 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 Frag= ment bit. > > > With DF, the IP id is unused (since no fragmentation). > > > Many implementations just send 0 since generating unique IP id requir= es an > > > atomic operation which is potential bottleneck. > > > > So, is my question correct and the code is wrong? > > >=20 > 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. >=20 >=20 > 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 =3D ntohl(*(__be32 *)&iph->id); flush =3D (u16)((ntohl(*(__be32 *)iph) ^ skb_gro_len(skb)) | (id & ~IP_DF)= ); id >>=3D 16; ... NAPI_GRO_CB(p)->flush_id =3D ((u16)(ntohs(iph2->id) + NAPI_GRO_CB(p)->count) ^ id); NAPI_GRO_CB(p)->flush |=3D 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 =3D 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 -=20 DPDK GRO library doesn't strip off IPv4 header, instead it has to merge the= m 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=20 =20