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Chapter 5 The Network Layer The IP Protocol The IPv4 (Internet Protocol) header. IP datagram format IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to 32 bits head. type of length ver len service fragment 16-bit identifier flgs offset upper time to Internet layer live checksum total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) E.g. timestamp, record route taken, specify list of routers to visit. IP Addresses IP address formats. IP Addresses (2) Special IP addresses. Subnets A campus network consisting of LANs for various departments. Subnets (2) A class B network subnetted into 64 subnets. CDR – Classless InterDomain Routing 5-59 A set of IP address assignments. NAT – Network Address Translation Placement and operation of a NAT box. The TCP Segment Header TCP Header. NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.4 10.0.0.1 10.0.0.2 138.76.29.7 10.0.0.3 All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) NAT: Network Address Translation Motivation: local network uses just one IP address as far as outside word is concerned: no need to be allocated range of addresses from ISP: - just one IP address is used for all devices can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network devices inside local net not explicitly addressable, visible by outside world (a security plus). NAT: Network Address Translation Implementation: NAT router must: outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #) . . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr. remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table NAT: Network Address Translation 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table 2 NAT translation table WAN side addr LAN side addr 1: host 10.0.0.1 sends datagram to 128.119.40, 80 138.76.29.7, 5001 10.0.0.1, 3345 …… …… S: 10.0.0.1, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3: Reply arrives dest. address: 138.76.29.7, 5001 3 1 10.0.0.4 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 10.0.0.1 10.0.0.2 4 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 NAT – Network Address Translation Placement and operation of a NAT box. Internet Control Message Protocol 5-61 The principal ICMP message types. ICMP: Internet Control Message Protocol used by hosts & routers to communicate network-level information Type 0 3 error reporting: unreachable host, 3 network, port, protocol 3 echo request/reply (used by ping) 3 network-layer “above” IP: 3 ICMP msgs carried in IP datagrams 3 4 ICMP message: type, code plus first 8 bytes of IP datagram causing error 8 9 10 11 12 Code 0 0 1 2 3 6 7 0 0 0 0 0 0 description echo reply (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable dest network unknown dest host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header Traceroute and ICMP Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number When nth datagram arrives to nth router: Router discards datagram And sends to source an ICMP message (type 11, code 0) Message includes name of router& IP address When ICMP message arrives, source calculates RTT Traceroute does this 3 times Stopping criterion UDP segment eventually arrives at destination host Destination returns ICMP “host unreachable” packet (type 3, code 3) When source gets this ICMP, stops. ARP– The Address Resolution Protocol Three interconnected /24 networks: two Ethernets and an FDDI ring. Dynamic Host Configuration Protocol Operation of DHCP. The Main IPv6 Header The IPv6 fixed header (required). Extension Headers 5-69 IPv6 extension headers. Extension Headers (2) The hop-by-hop extension header for large datagrams (jumbograms). Extension Headers (3) The extension header for routing.