Lectures 24 & 25 Higher Layer Protocols: TCP/IP and ATM Eytan Modiano Massachusetts Institute of Technology Laboratory for Information and Decision Systems Eytan Modiano Slide 1Outline • Network Layer and Internetworking • The TCP/IP protocol suit • ATM • MPLS Eytan Modiano Slide 2Higher Layers Virtual link for reliable packets Application Presentation Session Transport Network Data link Control Application Presentation Session Transport Network Data link Control Network Network DLC DLC DLC DLC Virtual bit pipe Virtual link for end to end packets Virtual link for end to end messages Virtual session Virtual network service physical interfacephys. int. phys. int. phys. int. phys. int.physical interface TCP, UDP IP, ATM Physical link External subnet subnet External Site node node site Eytan Modiano Slide 3Packet Switching • Datagram packet switching – Route chosen on packet-by-packet basis – Different packets may follow different routes – Packets may arrive out of order at the destination – E.g., IP (The Internet Protocol) • Virtual Circuit packet switching – All packets associated with a session follow the same path – Route is chosen at start of session – Packets are labeled with a VC# designating the route – The VC number must be unique on a given link but can change fromlink to link Imagine having to set up connections between 1000 nodes in a mesh Unique VC numbers imply 1 Million VC numbers that must be representedand stored at each node – E.g., ATM (Asynchronous transfer mode) Eytan Modiano Slide 4Virtual Circuits Packet Switching • For datagrams, addressing information must uniquely distinguisheach network node and session – Need unique source and destination addresses • For virtual circuits, only the virtual circuits on a link need be distinguished by addressing – Global address needed to set-up virtual circuit – Once established, local virtual circuit numbers can then be used to represent the virtual circuits on a given link: VC number changes fromlink to link • Merits of virtual circuits – Save on route computation Need only be done once at start of session – Save on header size – More complex – Less flexible 3 6 5 8 2 9 VC3 VC13 VC7 VC4 VC3 VC7 Node 5 table (3,5) VC13 -> (5,8) VC3 (3,5) VC7 -> (5,8) VC4(6,5) VC3 -> (5,8) VC7 Eytan Modiano Slide 5The TCP/IP Protocol Suite • Transmission Control Protocol / Internet Protocol • Developed by DARPA to connect Universities and Research Labs Four Layer model Telnet, FTP, email, etc. TCP, UDP IP, ICMP, IGMP �Device drivers, interface cards TCP - Transmission Control Protocol UDP - User Datagram Protocol IP - Internet Protocol Applications Transport Network Link Eytan Modiano Slide 6Internetworking with TCP/IP FTP FTP Protocol FTP client server TCP Protocol TCPTCP IP IP Protocol IP Protocol Ethernet Ethernet Protocol token driver token ring Protocol Ethernet driver IP ROUTER IP Ethernet driver token driver token ring ring ring Eytan Modiano Slide 7Encapsulation 1420 20 4 Ethernet frame 46 to 1500 bytes Ethernet Ethernet Application user data Appluser data header TCPheader application header IP TCP header application IP datagram TCP header application header IP Ethernet header Ethernet trailer driver IP TCP TCP segment data datadata Eytan Modiano Slide 8Bridges, Routers and Gateways • A Bridge is used to connect multiple LAN segments – Layer 2 routing (Ethernet) – Does not know IP address – Varying levels of sophistication Simple bridges just forward packets smart bridges start looking like routers • A Router is used to route connect between different networks using network layer address – Within or between Autonomous Systems – Using same protocol (e.g., IP, ATM) • A Gateway connects between networks using different protocols – Protocol conversion – Address resolution • These definitions are often mixed and seem to evolve! Eytan Modiano Slide 9Bridges, routers and gateways Ethernet A Ethernet B Bridge IP Router Small company Gateway Service provider’s ATM backbone ATM switches (routers) Gateway Another provider’s Frame Relay Backbone Eytan Modiano Slide 10IP addresses • 32 bit address written as four decimal numbers – One per byte of address (e.g., 155.34.60.112) • Hierarchical address structure – Network ID/ Host ID/ Port ID – Complete address called a socket – Network and host ID carried in IP Header – Port ID (sending process) carried in TCP header • IP Address classes: 8 32 Net ID Host ID Net ID Net ID Host ID Host ID 0 10 110 16 32 24 32 Class A Nets Class B Nets Class C Nets Class D is for multicast traffic Eytan Modiano Slide 11Host Names • Each machine also has a unique name • Domain name System: A distributed database that provides amapping between IP addresses and Host names • E.g., 155.34.50.112 => plymouth.ll.mit.edu Eytan Modiano Slide 12Internet Standards • Internet Engineering Task Force (IETF) – Development on near term internet standards – Open body – Meets 3 times a year • Request for Comments (RFCs) – Official internet standards – Available from IETF web page: http://www.ietf.org Eytan Modiano Slide 13The Internet Protocol (IP) • Routing of packet across the network • Unreliable service – Best effort delivery – Recovery from lost packets must be done at higher layers • Connectionless – Packets are delivered (routed) independently – Can be delivered out of order – Re-sequencing must be done at higher layers • Current version V4 • Future V6 – Add more addresses (40 byte header!) – Ability to provide QoS Eytan Modiano Slide 14Header Fields in IP 1 4 8 16 32 Protocol Note that the minimum size header is 20 bytes; TCP also has 20 byte header Ver Header length type of service Total length (bytes) 16 - bit identification Flags 13 - bit fragment offset TTL Header Checksum Source IP Address Destination IP Address Options (if any) Data Eytan Modiano Slide 15IP HEADER FIELDS • Vers: Version # of IP (current version is 4) • HL: Header Length in 32-bit words • Service: Mostly Ignored • Total length Length of IP datagram • ID Unique datagram ID • Flags: NoFrag, More • FragOffset: Fragment offset in units of 8 Octets • TTL: Time to Live in "seconds” or Hops • Protocol: Higher Layer Protocol ID # • HDR Cksum: 16 bit 1's complement checksum (on header only!)