Transcript ICOM 6115
ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 21 ICOM 6115 ©Manuel Rodriguez-Martinez Lecture Objectives • Introduction to Global Internetworking – Layer 3 – The Network Layer • Store-and-Forward Service – IP Protocol • IP addressing • ARP, ICMP – Routing ICOM 6115 ©Manuel Rodriguez-Martinez Internetworking • What is this? – Process of connecting independent and possibly heterogeneous LANs into a larger network • Again: Independent LANs – Each LAN might have a different Layer 2 • Ethernet • Token Ring • 802.11b • What Layer takes care of this service? – Network Layer (Layer 3) ICOM 6115 ©Manuel Rodriguez-Martinez Example internetwork H1 H2 Network 1 (Ethernet) H7 R4 Network 5 (Point-to-Point) Network 4 (Ethernet) R1 Network 3 (FDDI) R3 H3 H4 H6 H5 ICOM 6115 R2 Network 2 (802.11b) ©Manuel Rodriguez-Martinez H8 Layer 3 and LAN Integration Multiple, heterogeneous LANs can become Interoperable via IP (Layer 3 example) ICOM 6115 ©Manuel Rodriguez-Martinez Layer 3: Packet Switching • Layer 3 deals with getting packets forwarded from source to destination – Across LANs • Layer 2 was switching within a LAN • Routers are computers that provide service of packet forwarding – Must understand the topology of internet – Forwarding must be fast – Routing should produce accurate forwarding tables ICOM 6115 ©Manuel Rodriguez-Martinez Store-and-Forward Service • Packets are send by end-host to routers • Routers – Store each packet until fully arrives – Make checksum calculations – Lookup forwarding tables to determine next hop • A router or final destination host – Forward the packet along the line (port) of next hop ICOM 6115 ©Manuel Rodriguez-Martinez Store-and-Forward Packet Switching Routers form a subnet of forwarding elements ICOM 6115 ©Manuel Rodriguez-Martinez Layer 3 Service Principles • Layer 3 will provide Layer 4 (Transport Layer) packet forwarding services – Connectionless – Connection-oriented • Driving Principles – Should independent of router technology – Transparency from routers for Layer 4 • Type, number and topology – Uniform addressing scheme across network • LANs and WANs ICOM 6115 ©Manuel Rodriguez-Martinez Connectionless Service • Advocated by Internet community – IP Protocol • Data is broken into packets, that are moved independently over the network – Datagram – Each one can take a different path • Most often they all take the same path • Each router moves the datagram along path from src to dst ICOM 6115 ©Manuel Rodriguez-Martinez Route Information Routers typically store routes to networks as the router next router to get there ICOM 6115 ©Manuel Rodriguez-Martinez Example Forwarding Tables A’s Forwarding Table C’s Forwarding Table E’s Forwarding Table A - A A A C B B B A B D C C C - C C D B D D D D E B E E E - F C F E F F ICOM 6115 ©Manuel Rodriguez-Martinez Connection-Oriented Service • Advocated by Telephone companies – ATM Protocol • Data is broken into packets, that are moved along a virtual circuit (VC) over the network – Circuit must be first set up – Each packet takes the same path • Driven by performance guarantees – Bounds on delays for packet arrival ICOM 6115 ©Manuel Rodriguez-Martinez Virtual Circuit based network ICOM 6115 ©Manuel Rodriguez-Martinez IP: Network Layer on Internet ICOM 6115 ©Manuel Rodriguez-Martinez IP features • Connectionless • Best effort delivery – Every effort is made to deliver datagram • No guarantee is made – Can be lost or delivered out of order – Simplicity – Unreliable • Need transport to add reliability (TCP) • Runs over “anything” – Support many Layer 2 protocols • Many invented after IP (e.g. 802.11b) ICOM 6115 ©Manuel Rodriguez-Martinez ICOM 6115 65KB Max Length Variable length IP v4 Packet Format ©Manuel Rodriguez-Martinez Some Issues • Header length is measured in terms of 32bit word • Datagram length is measured in terms of number of bytes – Max size is 64KB • Fragments indicates offset in terms of 8byte chunks – Only have 13 bits to expresses offset ICOM 6115 ©Manuel Rodriguez-Martinez Fragmentation and Reassembly • Packets might get fragmented as they travel over the net • Why? – Layer 2 might have different max frame size • Example: – 10Mbps Ethernet – max frame size is 1500 bytes – 100Mbps FDDI – max frame size is 4500 bytes – 56Kbps PPP – max frame size is 532 bytes – Packet might get cut to make it fit into Layer 2 frame ICOM 6115 ©Manuel Rodriguez-Martinez Example of Fragmentation Packet gets broken along the way to make it fit Layer 2 frame. It must be reassembled by end-receiver ICOM 6115 ©Manuel Rodriguez-Martinez Network MTU • Each network has a maximum transmission unit (MTU) – Biggest IP datagram that be fit into a frame • IP datagram can have variable size • Good idea is to keep it under MTU – Avoid fragmentation whenever possible • Let it happen on a case by case basis (be flexible) – Send as much data as possible in each frame • Good use of bandwidth ICOM 6115 ©Manuel Rodriguez-Martinez Reassembly • Routers can fragment packet – They won’t reassemble it – Each fragment gets forwarded independently • End-receiver deals with the problem of reassembling the packet – Putting pieces back in order – Lost fragment causes retransmission of entire packet! ICOM 6115 ©Manuel Rodriguez-Martinez Control Information for reassembly • Each packet carries an identification number – Sequence number • Fragments are identified as parts of packet with given id number – Fragment identifies offset bytes for a packet – Example: offset for individual bytes • Offset 0 of packet 10287 • Offset 512 of packet 10287 • Offset 1024 of packet 10287 – In reality, offset counts groups of 8-bytes ICOM 6115 ©Manuel Rodriguez-Martinez IP Addresses • Unique, Global identifier for a network card – Machines with multiple card get multiple IPs • e.g. routers • IP address is a 32 bit number – For simplicity, expressed in dotted-decimal notation • e.g. 128.34.5.92 Bits 32 128 ICOM 6115 34 0 5 ©Manuel Rodriguez-Martinez 92 What are benefits of IP Addresses? • Hierarchical structure – Provide way to identify network and host • Ethernet are flat – Example: 135.134.9.20 • Host 20 • Network 135.134.9.0 • Routers need only to keep information about network – subnets ICOM 6115 ©Manuel Rodriguez-Martinez IP Address Formats ICOM 6115 ©Manuel Rodriguez-Martinez Fragmentation in packets Start of header Start of header Id = X 0 Offset=0 Fragmentation 1 Offset=0 Id = X Rest of Header Rest of Header 1400 data bytes 512 data bytes Fragment 1 Start of header Id = X 1 512 Offset Start of header Id = X 0 1024 Rest of Header Rest of Header 512 data bytes 512 data bytes Fragment 2 ICOM 6115 Fragment 3 ©Manuel Rodriguez-Martinez Offset