CS 498 Lecture 14The Internet Protocol V4.ppt

CS 498 Lecture 14 The Internet Protocol V4 Jennifer Hou Department of Computer Science University of Illinois at Urbana-Champaign Reading:Chapter 14, The Linux Networking Architecture: Design and Implementation of Network Protocols in the Linux Kernel First Possible Path of an IP Packet Packets arrive on an interface and are stored in the input queue of the respective CPU. Once the layer-3 protocol in the LLC has been determined (e.g., ETH_PROTO_IP), the packet is passed to the ip_rcv() function. Recall the Packet Path in LLC Internet Protocol Implementation in Linux 2nd/3rd Possible Path of an IP Packet TCP/UDP packets are packed into an IP packet and passed down to IP via ip_queue_xmit(). The IP layer generates IP packets itself, e.g., multicast packets or fragmentaion of a large packet, or ICMP/IGMP packets. Path of Incoming IP Packets ip_rcv(skb,dev,pkt_type) Packets that are not addressed to the host (packets received in the promiscuous mode) are rejected. A sanity check is performed Does the packet have at least the size of an IP header? Is this IP Version 4? Is the checksum correct? Does the packet have a wrong length? If the actual packet size skb?len, then invoke skb_trim(skb,iph?total_len) The netfilter hook NF_IP_PRE_ROUTING is invoked Packet Filtering Architecture in Linux ip_rcv_finish(skb) ip_route_input() is invoked to determine the route of a packet. skb?dst is set to an entry in the routing cache which stores both the destination IP and the pointer to an entry in the hard header cache (cache for the layer 2 frame packet header) If the IP packet header includes options, an ip_option structure is created. skb?dst?input() points to the function that should be used to handle the packet (delivered locally or forwarded further) ip_local_deliver(), ip_forward(), ip_mr_input() IP Forwarding To activate IP packet forwarding, do echo ‘1’ /proc/sys/net/ipv4/ip_forward ip_forward(skb) Step 1: Packet not marked with pkt_type == PACKET_HOST are deleted.