I experiment on the kernel of linux environment.

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Scott Andrews
5 years ago
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1 I experiment on the kernel of linux environment. RX checksum offload ==================================== The linux kernel can t calculate TCP and UDP checksum if skb->ip_summed is CHECKSUM_UNNECESSARY. But the kernel always calculates the IP layer checksum whenever skb->ip_summed is any value. If you want that kernel doesn t calculate IP layer checksum, you have to modify the linux-<version>/net/ipv4/ip_input.c file. In linux/net/ipv4/ip_input.c int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt) #line 415 if (ip_fast_csum((u8 *)iph, iph->ihl)!= 0) goto inhdr_error; if (ip_fast_csum((u8 *)iph, iph->ihl)!= 0) goto inhdr_error; TX checksum offload ======================================== In the linux/include/linux/netdevice.h, the flag, (struct net_device *)dev->features, supports checksum offload capability. /* Net device features */ int features; #define NETIF_F_IP_CSUM 2 /* Can checksum only TCP/UDP over IPv4. */ #define NETIF_F_NO_CSUM 4 /* Does not require checksum. F.e. loopack. */ #define NETIF_F_HW_CSUM 8 /* Can checksum all the packets. */ When (struct net_device *)dev->features = NETIF_F_HW_CSUM, the TCP/UDP/IP checksum of packets were calculated by kernel. To eliminate the TCP/UDP/IP checksum overhand, we have to modify the kernel source. To have kernel stop the IP layer checksum calculation for transmit packet, we have to modify the source code here.

2 In linux/net/ipv4/ip_output.c inline void ip_send_check(struct iphdr *iph) iph->check = 0; was: int ip_build_xmit() # line 758: was: To have kernel stop the TCP checksum calculation for transmit packet, we have to modify the source code here In linux/net/ipv4/tcp_ipv4.c /* This routine computes an IPv4 TCP checksum. */ void tcp_v4_send_check(struct sock *sk, struct tcphdr *th, int len, struct sk_buff *skb) if (skb->ip_summed == CHECKSUM_HW) th->check = ~tcp_v4_check(th, len, sk->saddr, sk->daddr, 0); skb->csum = offsetof(struct tcphdr, check); else th->check = tcp_v4_check(th, len, sk->saddr, sk->daddr, csum_partial((char *)th, th->doff<<2, skb->csum));

3 Is : if (skb->ip_summed == CHECKSUM_HW) static void tcp_v4_send_reset(struct sk_buff *skb) #line 1269 arg.csum = csum_tcpudp_nofold(skb->nh.iph->daddr, skb->nh.iph->saddr, /*XXX*/ sizeof(struct tcphdr), IPPROTO_TCP, 0); rg.csumoffset = offsetof(struct tcphdr, check) / 2; Is : arg.csum = 0; static void tcp_v4_send_ack(struct sk_buff *skb, u32 seq, u32 ack, u32 win, u32 ts) #line 1326 arg.csum = csum_tcpudp_nofold(skb->nh.iph->daddr, skb->nh.iph->saddr, arg.iov[0].iov_len, IPPROTO_TCP, 0); arg.csumoffset = offsetof(struct tcphdr, check) / 2; Is: arg.csum = 0;

4 static int tcp_v4_send_synack(struct sock *sk, struct open_request *req, struct dst_entry *dst) #line 1430 th->check = tcp_v4_check(th, skb->len, req->af.v4_req.loc_addr, req->af.v4_req.rmt_addr, csum_partial((char *)th, skb->len, skb->csum)); th->check=0; To have kernel stop the UDP checksum calculation for transmit packet, we have to modify the source code here. In linux/net/ipv4/udp.c static int udp_push_pending_frames(struct sock *sk, struct udp_sock *up) if (sk->sk_no_check == UDP_CSUM_NOXMIT) skb->ip_summed = CHECKSUM_NONE; goto send; Is : sk->sk_no_check = UDP_CSUM_NOXMIT; if (sk->sk_no_check == UDP_CSUM_NOXMIT) skb->ip_summed = CHECKSUM_NONE; goto send;

5 How to build linux kernel : =============================================== You can reference README in kernel tarball. -- If you install the full sources, put the kernel tarball in a directory where you have permissions (eg. your home directory) and unpack it: tar zxvf linux-<version>.tar.gz -- Make sure you have no stale.o files and dependencies lying around: cd linux-<version> make mrproper -- Do a "make menuconfig" to configure the basic kernel. Alternate configuration commands are: "make menuconfig" Text based color menus, radiolists & dialogs. "make xconfig" X windows based configuration tool. "make oldconfig" Default all questions based on the contents of your existing./.config file. -- Do a "make dep" to set up all the dependencies correctly. -- Do a "make bzimage" to create a compressed kernel image. -- If you configured any of the parts of the kernel as `modules', you will have to do "make modules" followed by "make modules_install". -- Copy the kernel image bzimage and System.map to /boot area. -- Modify boot management program (ex. LILO, GRUB, etc.) -- Reboot

I experiment on the kernel of linux environment.

I experiment on the kernel of linux 2.4.29 environment. RX checksum offload ==================================== The linux kernel can t calculate TCP and UDP checksum if skb->ip_summed is CHECKSUM_UNNECESSARY.