CS4550 Computer Networks II IP : internet protocol, part 2 : packet formats, routing, routing tables, ICMP
IP topics IP topics brief history (1) basic function (1) names and addresses (1) packet format (2) packet routing, routing tables (2) lnternet Control Message Protocol (2) routing (calculation) : RIP, OSPF (2) IPv6 : the IP of the future (2)
TCP/IP IP TCP/IP internet TCP/IP internet IP WAN WAN IP TCP/IP IP... TCP/IP
TCP/IP protocol suite TCP/IP protocol suite telnet, FTP, SMTP,etc. other apps TFTP, BOOTP, DNS, etc. TCP UDP ICMP IP IGMP ARP LAN/WAN RARP media
IP packet format IP packet format 0 34 78 10 15 16 31 vers. HLEN pre. TOS total length identification flags fragment offset TTL protocol header checksum source IP address destination IP address options, (if any) DATA
IP packet - explanation IP packet - explanation version -- currently 4; next - 6. HLEN - header length; 20 to 60 bytes. total length - packet length in bytes. precedence (3 bits) - designed for priority, but no standard procedure for this; little used. TOS - type of service TTL - time to live (die). Standard specified seconds, but in practice - router hops.
IP packet - explanation IP packet - explanation ID - numbers each datagram sent by a host. ( fragmentation/reassembly) flags - 3 bits. DF, don t fragment; MF, more fragments. (1st bit unused=0). frag offset - ( fragmentation/reassembly) protocol - indicates TCP, UDP, etc. header checksum - done on header only; recomputed at each hop.
IP routing IP routing routing mechanism - the mechanics of routing; simply, IP routs packets according to a routing table, in memory. routing policy - how the paths in the networking are calculated- i.e., how the entries in the table are determined. Two separate procedures. mechanism - differs slightly, depending on whether in a host or a router; simpler for hosts.
IP routing IP routing basic IP routing mechanism: given an IP DA (destination IP address), 1. search table for complete IP DA; if found, send to next hop indicated. 2. search table for network ID; if found, send to next hop indicated. 3. search for default entry; if found, send to next hop indicated. 4. discard the packet.
IP routing : in a host IP routing : in a host IP (in host) receives packets to send from TCP, UDP, ICMP, IGMP. upon receipt of a packet to send, IP will 1. check mask (determine net/host parts). 2. if destination directly connected (point-to-point link/ same subnet), then send packet to it. 3. otherwise, send packet to the default router (routing table).
IP routing : in a host IP routing : in a host fundamental difference :a host never forwards a packet; IP packets received not for this host are discarded. note : if sending to a host on same subnet (e.g. ethernet), the MAC address corresponds to the IP DA; if sending to default router, the MAC/hardware DA is the router s, while the IP DA is that of the final destination.
routing table IP (host) IP routing : in a host IP routing : in a host TCP, UDP, etc. yes get next hop this IP DA or broadcast packet? input queue NW interface no bit bucket
IP routing : in routers IP routing : in routers Same basic algorithm as stated, but : routing tables bigger, generally ; more overhead in maintaining routing tables, exchanging information with other routers; more network interfaces, generally ; usually at least 2 (hosts may have only 1) forward packets received onto other routers. (fundamental difference)
routing table IP (router) IP routing : in routers IP routing : in routers TCP, UDP, etc. yes get next hop no this IP DA or broadcast packet? input queue NW interfaces
IP routing tables IP routing tables series of entries which contain destination - IP address of distant location (either network or host) gateway(router) - IP address of router to send the packet to flags - 5 of these which give additional info refcnt - number of active uses use - number of packets sent this route interface - the outgoing interface for this route; (e.g., ethernet, a direct link, etc. )
IP routing tables IP routing tables IP routing table flags (5) U route is up G route is to a gateway H route is to a host D created by a redirect (see ICMP) M modified by a redirect
ICMP : internet control message ICMP : internet control message protocol required & essential companion protocol to IP sends error messages to report problems on internets, such as destination unreachable time exceeded parameter problem source quench redirect