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Networks Overview

Networks Overview. OSI Architecture. The OSI 7-layer Model OSI – Open Systems Interconnection. Description of Layers. Physical Layer Handles the transmission of raw bits over a communication link Data Link Layer Collects a stream of bits into a larger aggregate called a frame

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Networks Overview

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  1. Networks Overview

  2. OSI Architecture The OSI 7-layer Model OSI – Open Systems Interconnection

  3. Description of Layers • Physical Layer • Handles the transmission of raw bits over a communication link • Data Link Layer • Collects a stream of bits into a larger aggregate called a frame • Network adaptor along with device driver in OS implement the protocol in this layer • Frames are actually delivered to hosts • Network Layer • Handles routing among nodes within a packet-switched network • Unit of data exchanged between nodes in this layer is called a packet The lower three layers are implemented on all network nodes

  4. Description of Layers • Transport Layer • Implements a process-to-process channel • Unit of data exchanges in this layer is called a message • There is a disagreement on the top three • Session, Presentation, and Application • Mainly because the are not always present

  5. Description of Layers • Session Layer • Provides a name space that is used to tie together the potentially different transport streams that are part of a single application • Ex., tie the audio and video together in videoconference • Presentation Layer • Concerned about the format of data exchanged between peers • Ex., integer formats, audio/video format, most/least significant • Application Layer • Include things like the Hypertext Transfer Protocol • Basis the world wide web • Used by web browsers The transport layer and the higher layers typically run only on end-hosts and not on the intermediate switches and routers

  6. Internet Architecture • Sometimes called TCP/IP • Evolved from an earlier packet-switched network called ARPANET • Internet and ARPANET were funded by ARPA (Advanced Research Projects Agency) • Both existed before the OSI architecture • Both affected the OSI model

  7. Internet Architecture Alternative view of the Internet architecture. The “Network” layer shown here is sometimes referred to as the “sub-network” or “link” layer. Internet Protocol Graph

  8. Internetworking • What is internetwork • An arbitrary collection of networks interconnected to provide some sort of host-to-host packet delivery service A simple internetwork where H represents hosts and R represents routers

  9. Internetworking • What is IP • IP stands for Internet Protocol • Key tool used today to build scalable, heterogeneous internetworks • It runs on all the nodes in a collection of networks and defines the infrastructure that allows these nodes and networks to function as a single logical internetwork A simple internetwork showing the protocol layers

  10. IP Service Model • Two parts • Global Addressing Scheme • Provides a way to identify all hosts in the network • Datagram (Connectionless) model for data delivery • Best-effort delivery (unreliable service) • packets are lost • packets are delivered out of order • duplicate copies of a packet are delivered • packets can be delayed for a long time

  11. Packet Format

  12. Global Addresses • IP addresses Properties • globally unique • hierarchical: network + host • Network part: identifies the network the host is attached to • Host: identifies a unique host on that network • Ethernet addresses, even globally unique, are flat (no structure and thus no meaning) and can not be use for routing • Note that a router is attached to at least twonetworks, so it must have an IP address on each port/interface • Thus it is more precise to think of IP addresses as belonging to interfaces rather than to hosts

  13. Global Addresses • Approximately, 4 Billion IP address, half are A type, ¼ is B type, and 1/8 is C type (a) Class A (b) Class B (c) Class C

  14. Global Addresses • Class A was intended for Wide Area Networks • Thus there should a very few of them • Class B was intended for a modest size networks (like a campus) • Class C is for the large number of LANs • However, these classifications are not flexible and today’s IP addresses are normally “classless” as we will see • Format • 4 bytes, each byte is represented by a decimal number • Dot notation • 10.3.2.4 • 128.96.33.81 • 192.12.69.77 (

  15. IP Datagram Forwarding • Strategy • every datagram contains destination's address • if directly connected to destination network, then forward to host • if not directly connected to destination network, then forward to some router • forwarding table maps network number into next hop • each host has a default router • each router maintains a forwarding table • Example (router R2)

  16. Subnetting • The network number part was designed to uniquely identify exactly one physical network • However, this approach has some problems • A network with only 2 hosts has to have at least a class C network!! • A network with only 256 hosts has to have at least a class B network!! • Thus, we will waste our valuable IP address space • Solution • Subnetting

  17. Subnetting • Key Idea • Allocate a single network number and use it for several physical networks • called subnets • Several things need to be done • Subnets need to be physicallyclose to each other • From the Internet point of view, they all look ONEnetwork • A perfect situation to use subnetting is for large campus or corporation • Configure all nodes on each subnet with a subnet mask • It masks the network part • Introduces the subnet number • All nodes on the same subnet have the same subnet number and the same mask • The IP address of a nodes ANDedwith the subnet mask give the subnet number • IP AND subnet mask  subnet number

  18. Subnetting Increases the number of networks and reduces the number of hosts

  19. Classless Addressing • Subnetting has a counterpart, sometimes called supernetting, but often called Classless Interdomain Routing, CIDR(pronounced cider)

  20. Classless Addressing • Requires to hand out blocks of class C addresses that share a common prefix • The convention is to place a /X after the prefix where X is the prefix length in bits • For example, the 20-bit prefix for all the networks 192.4.16 through 192.4.31 is represented as 192.4.16/20 • By contrast, if we wanted to represent a single class C network number, which is 24 bits long, we would write it 192.4.16/24

  21. Classless Addressing Route aggregation with CIDR

  22. Different Protocols • ARP (Address Resolution Protocol) • DHCP (Dynamic Host Configuration Protocol) • ICMP (Internet Control Message Protocol)

  23. Simple Demultiplexer (UDP) Format for UDP header (Note: length and checksum fields should be switched)

  24. Simple Demultiplexer (UDP) UDP Message Queue

  25. TCP Header TCP Header Format

  26. Connection Establishment/Termination in TCP Timeline for three-way handshake algorithm

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