CS4550 Computer Networks II IP : internet protocol, part 2 : packet formats, routing, routing tables, ICMP read feit chapter 6

IP packet format source IP address destination IP address options, (if any) DATA total length identification protocolheader checksum TTL fragment offset vers. HLEN flags pre. TOS

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  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  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  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 (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  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.

IP routing : in a host routing table IP (host) TCP, UDP, etc. get next hop NW interface input queue this IP DA or broadcast packet? yes no bit bucket

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)

IP routing : in routers routing table IP (router) TCP, UDP, etc. get next hop NW interfaces input queue this IP DA or broadcast packet? yes no

IP routing tables  series of entries(non standard) typically(F 8.1) destination - IP address of distant location (either network or host) route mask - AND with packet destination to get matching route destination gateway(router) - IP address of router to send the packet to route metric - metric to use for shortest route calculation, often hop count interface - the outgoing interface for this route; (e.g., ethernet, a direct link, etc. ) Route Age - how many seconds since last update

ICMP : internet control message protocol  required & essential companion protocol to IP  purpose : to provide a tool for notifying routers / hosts of problems (e.g., router down, packet discarded, etc.)  ICMP messages wrapped in IP packets with protocol = 1  ICMP protocol specifies that messages “should” or “may” be sent; doesn’t require every error be reported

ICMP  sends error messages to report problems on internets, such as u destination unreachable u time exceeded...{ dead packet :-) } u parameter problem (in IP header) u source quench (router/host is congested) u redirect (host sent a packet to wrong local router)

ICMP  ICMP message NOT sent when --- u routing/delivering ICMP messages u for broadcast/multicast packets u datagram fragments except 1st u source address not unique (e.g., )

ICMP message contents “destination unreachable” example  type field (3)  code (some error information)  checksum  IP header and 1st 8 data bytes  other information according to the code field