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CS4550 Computer Networks II IP : internet protocol, part 2 : packet formats, routing, routing tables, ICMP read feit chapter 6 IP packet format 0 34 78 15 16 10 vers. HLEN pre. TTL total length TOS identification flags protocol 31 fragment offset header checksum source IP address destination IP address options, (if any) DATA 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 header - indicates TCP, UDP, etc. 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 TCP, UDP, etc. routing table yes get next hop this IP DA or broadcast packet? input queue IP (host) NW interface 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 TCP, UDP, etc. routing table yes get next hop no this IP DA or broadcast packet? input queue IP (router) NW interfaces 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 destination unreachable time exceeded ...{ dead packet :-) } parameter problem (in IP header) source quench (router/host is congested) redirect (host sent a packet to wrong local router) ICMP ICMP message NOT sent when -- routing/delivering ICMP messages for broadcast/multicast packets datagram fragments except 1st source address not unique (e.g., 0.0.0.0) 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