Network Model. Why a Layered Model? All People Seem To Need Data Processing

Transcription

1 Network Model Why a Layered Model? All People Seem To Need Data Processing

2 Layers with Functions Packet Propagation Each router provides its services to support upper-layer functions.

3 Headers (Encapsulation De-Encapsulation) Process DATA Process Transport H DATA Transport Network H H DATA Network Data Link H H H DATA Data Link OSI Model and TCP/IP Model

4 TCP/IP Protocol Suite Application Layer DHCP DNS SNMP HTTP SMTP POP Transport Layer UDP TCP ICMP IGMP Network Layer IP Link Layer ARP ARP Ethernet/FastEthernet/802.11/PPP Network Infrastructure

5 LAN Devices Repeater Copies bits from one network to another Does not look at any bits Allows the extension of a network beyond physical length limitations REPEATER

6 Bridge/Switch Copies frames from one network to another Can operate selectively - does not copy all frames (must look at data-link headers). Extends the network beyond physical length limitations. BRIDGE Router Copies packets from one network to another. Makes decisions about what route a packet should take (looks at network headers). ROUTER

7 Ethernet LAN Segmentation Switches break collision domains. Routers break collision domains as well as broadcast domains. Ethernet Multi-access (shared medium). Every Ethernet interface has a unique 48 bit address (a.k.a. hardware address). Example: C0:B3:44:17:21:17 The broadcast address is all 1 s. Addresses are assigned to vendors by a central authority.

8 CSMA/CD Carrier Sense Multiple Access with Collision Detection Carrier Sense: can tell when another host is transmitting Multiple Access: many hosts share one wire Collision Detection: can tell when another host transmits at the same time. An Ethernet Frame Preamble Destination Address Source Address Type DATA CRC 8 bytes The preamble is a sequence of alternating 1s and 0s used for synchronization. CRC is Cyclic Redundency Check

9 WAN Devices WAN Technologies Include Analog modems Integrated Services Digital Network (ISDN) Digital Subscriber Line (DSL) Frame Relay Asynchronous Transfer Mode (ATM) T (US) and E (Europe) carrier series: T1, E1, T3, E3 Synchronous Optical Network (SONET) Cellular Network Satellites WiMAX

10 Introduction to IP, Addressing, DHCP, ARP

11 Anatomy of an IP Packet Version IP header length (HLEN) Type-of-service Total length Identification Flags Fragment offset Time-to-live Protocol Header checksum Source address Destination address Options Padding Data IP Address as a 32-Bit Binary Number

12 Private Addresses Dynamic Host Configuration Protocol Allows a host to obtain an IP address using a defined range of IP addresses on a DHCP server. As hosts come online, contact the DHCP server, and request an address.

13 DHCP at a Glance Client Actions Request lease for an address for a period of time (LEASE TIME) Renew address lease prior to LEASE TIME expiration, or allow the lease to EXPIRE Release lease once no longer needed Reject offered lease if it is already in use Remark: clients can bypass the DHCP server to access the network Determines Configuration Commits Configuration DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK Configures Self Address Resolution Protocol (ARP) t ARP enables a computer to find the MAC address of the computer that is associated with an IP address.

14 ARP Operation Within a Subnet All devices on the network receive the packet and pass to network layer; only one device responds with an ARP reply. ARP Process

15 Default Gateway A default gateway is the IP address of the interface on the router that connects to the network segment on which the source host is located. Network Address Translation

16 Virtual Local Area Networks (VLAN)

17 VLAN Communication Typical Deployment Networks

18 Typical Network Design

19 UDP, TCP, DNS Simple Demultiplexor (UDP) Unreliable and unordered datagram service Adds multiplexing No flow control Endpoints identified by ports servers have well-known ports see /etc/services on Unix Header format SrcPort Length DstPort Checksum Data Optional checksum psuedo header + UDP header + data

20 TCP Overview Connectionoriented Byte-stream app writes bytes TCP sends segments app reads bytes Full duplex Flow control: keep sender from overrunning receiver Congestion control: keep sender from overrunning network Application process Application process Write bytes Read bytes TCP Send buffer TCP Receive buffer Segment Segment Segment Transmit segments TCP Header SrcPort DstPort SequenceNum Acknowledgment HdrLen 0 Flags Checksum AdvertisedWindow UrgPtr Options (variable) Data Flags: SYN, FIN, RESET, PUSH, URG, ACK Checksum: IP pseudo header + TCP header + data

21 IP Fragmentation IP Header Payload IP Header Payload More Fragements is Set IP Header More Fragements is NOT Set Payload IP Header Payload DNS: Root name servers contacted by local name server that can not resolve name root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server ~ 13 root name servers worldwide Further information about the root name servers can be found at:

22 DNS Queries root name server recursive query: puts burden of name resolution on contacted name server heavy load? iterated query: contacted server replies with name of server to contact I don t know this name, but ask this server local name server dns.lopsys.net requesting host Sun.lopsys.net iterated query intermediate name server dns.wmich.edu 5 6 authoritative name server dns.cs.wmich.edu Csy01.cs.wmich.edu