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Network Layer 3 OSI Model Application Presentation Session Transport Network Data Link Physical Overview Layer 3 Network Layer • Layer 3 responsibilities • IP as an example Network Layer Overview Responsible to route data (datagrams) through an internetwork to destination host. • Data packets User data • Route update packets Information to map and navigate layer 3 networks Not responsible for error-free communications Communications are connectionless Communications are not ordered Defines broadcast domains Requires an addressing scheme Primary device is router Examples: IP and IPX TCP/IP Suite Layer 3 protocols IP (Internet Protocol) ARP (Address Resolution Protocol) ICMP (Internet Control Messaging Protocol) IGMP (Internet Group Messaging Protocol) The Internet Protocol IP The Internet Protocol (IP) is the Network Layer protocol used in the Internet suite of protocols. IP provides: • • • • • • Datagram Delivery without reliability checking Addressing through a fixed length IP address No method of flow control No ordering of packets Connectionless service Fragmentation and reassembly of long packets The full specification is described in RFC 7911 1All internet protocols are described by specifications called Request for comments (RFC) IP Packet Structure 8 bits Version Header Length Type of Service Total Length Identifier Flags Fragment Offset Time to Live Protocol Header Checksum Source Address Destination Address Options + Padding 0 10 0 00 0 = 1 bit 16 bits 24 bits 32 bits Address Resolution Protocol (ARP) ARP is used to discover the MAC address for the destination device or the “Next Hop” router on the path to the destination. It does this by broadcasting a message to all asking for the MAC address that matches the known IP address. The device who owns the IP address answers back with its own MAC address Some argue that ARP should be at layer 2 ARP Packet Structure Hardware Type 08 00 Hardware Address Length 06 Protocol Address Length 04 Protocol Type Operation (Opcode) Sender's Hardware Address Sender's IP Address Target's Hardware Address Target's IP Address Internet Control Messaging Protocol (ICMP) Sends control and error messages throughout the IP network Examples: • Echo and Echo reply Ping is a utility that sends an ICMP echo to the destination host. The host replies with an ICMP echo reply • Source Quench If a device is sending data faster than it can be received, a source squelch message is sent to the sender telling it to SLOW DOWN. • Destination Unreachable There is no way to get to the network you have requested Internet Group Messaging Protocol (IGMP) Used between IP hosts and their immediate neighbor multicast agents to support the allocation of temporary group addresses and the addition and deletion of members of a group. A multicast group is a set of hosts that wish to receive the same data as others within their group. • A multicast transmission is addressed to a group, not to an individual machine. Streaming media is a good example of a use for multicasting Unicast Datagrams A unicast message is sent from one host to exactly one host. Host A wants to send a 5 meg message to hosts B,C and D. The message is sent 3 times requiring 15 meg of bandwidth Multicast Datagrams Multicast datagrams are sent once to the multicast group address and received by all hosts in the multicast group. Host A wants to send a 5 meg message to hosts B,C and D. IGMP is used to create a multicast group Host A sends the message to the Multicast group address Hosts B,C and D receive the datagram(s) Total bandwidth used is 5 meg.