Review of networking concepts

1. Review of networking concepts Prof. Malathi Veeraraghavan University of Virginia

2. Outline • Review of basic concepts in networking • Prerequisite: A first course on networking • Communication links and switches • Types of networks • Shared links: media access control (MAC)

3. End-user equipment End-user equipment What is a communication network? • Simplest “network” • Single link between two pieces of end-user equipment (e.g., PC, telephone) • Types of communication links • Twisted pair • Coaxial cable • Optical fiber • Wireless links • Radio frequencies • Infra-red frequencies

4. What is needed to send data on communication links? • Error control • Error detection: • Parity checks, Checksum, Cyclic Redundancy Code (CRC) • Error correction: • ARQ (Automatic Repeat reQuest) • FEC (Forward Error Correction) • Flow control: handles rate mismatch between sender and receiver • x-ON/x-OFF • Window based flow control • Rate based flow control

5. End-user equipment End-user equipment Switch End-user equipment End-user equipment Switches • Connect multiple links and route traffic from one link to another

6. Why use a switch? • If there are N endpoints (end-user equipment), then how many links are needed for full mesh connectivity? • How many physical links are needed if these endpoints are connected through a switch?

7. Answers • Number of direct links needed to connect N nodes is • N links – since we only need one link from an endpoint to a switch

8. Cost of using a switch? • Switch cost • Can all endpoints have full connectivity at all times to all other endpoints? • Yes, with multiplexing on the links

9. Concept of multiplexing • Time division multiplexing • Allows data from different sessions to be combined at different times on to the same line • How many DS0s in a T1? • Wavelength division multiplexing • Difference between FDM (Frequency Division Multiplexing) and WDM? • Relation between frequency and wavelength

10. Answers • 24 DS0s in a T1 • Term WDM is the same as FDM at optical frequencies – see EM spectrum chart • Speed of light c = f • : wavelength; f: frequency

11. Transceiver rate • Rate of transmission and reception at endpoints and the switch • Needs to be sufficient for “full mesh” connectivity “all the time” • e.g., if DS0s used between endpoints in full mesh network, then T1s can be used in 25 endpoint network with a switch for full mesh connectivity

12. Types of switches • Circuit switches: Position-based switching • Switch consults a table to determine output port on which to send data bits based on their arriving position • “Position”: Interface (space), time slot and/or wavelength • Space division switch: switch based on input interface • Time division switching: interface + time slot • Wavelength division switching: interface + wavelength • No buffers • Packet switches: Label-based switching • Switch consults a table to determine output port on which to send the packet based on value of label (in packet header) • Label could be changed on outgoing port or could stay the same • Have buffers to hold packets

13. Switch designs • See lectures on circuit switching and packet switching in Course on Data Networks • Compare unfolded view of a CS with that of a PS • See relevance of queueing theory to delays of calls or packets through switches

14. Network of switches • Expand 1-switch network to a multi-switch network • Why not build one gigantic switch? • Scalability limitations Switch End-user equipment Switch End-user equipment Switch End-user equipment

15. Networking modes Switching modes Connection-oriented Connectionless Packet-switching Circuit-switching Different types of networks • A network is defined by its “switching mode” and its “networking mode” • Circuit switching vs. packet switching • Circuit-switching: switching based on position (space, time, ) of arriving bits • Packet-switching: switching based on information in packet headers • Connectionless vs. connection-oriented networking: • CL: Packets routed based on address information in headers • CO: Connection set up (resources reserved) prior to data transfer MPLS IP + RSVP ATM, X.25 IP, SS7 Telephone network, SONET/SDH, WDM

16. Consuming end Stored Live Sending end Live Stored Types of data transfers • An application could consist of different types of data transfers • An http session has an interactive component, but could also have a non-real-time transfer Interactive/ Live streaming Recording Stored streaming File transfers

17. Consuming end Stored Live Sending end Live Stored Types of data transfers • An application could consist of different types of data transfers • An http session has an interactive component, but could also have a non-real-time transfer Interactive/ Live streaming Recording Stored streaming File transfers

18. Non-real-time (stored at sender and receiver ends) Real-time (consumed or sent live) Streaming (one-way) (consumed live; sent from live or stored source) e.g. radio/TV broadcasts Interactive (two-way) (consumed and sent live) e.g. telephony, telnet, ftp, http Short transfers (e.g. short email) Long transfers (e.g. large image, audio, video or data) Recording (one-way) (stored at receiver end; sent from live source); e.g. Replay Connectionless networks Circuit-switched networks Packet-switched CO networks Matching applications & networks Data transfers Ideal networks

19. Congestion control • What is it? • The purpose of a network is to allow sharing of resources • This means if demand is high, there could be competition for resources from multiple users • What are network resources: • Link capacity (bandwidth) • Switch buffer space (only in packet switches)

20. Congestion control • In CO networks • Congestion control: mostly preventive • Connection Admission Control (CAC) • Check availability of bandwidth and buffer resources before admitting a connection • CS CO networks: congestion will not occur once circuits are admitted • PS CO networks: congestion can occur after connection is admitted if connection admission is based on statistical multiplexing • Have some supplemental reactive congestion control scheme

21. Congestion control • In CL networks • Have packet switches detect congestion and send reactive messages asking sender to slow down • e.g., datagram routers in SS7 networks send such messages; SRP (Spatial Reuse Protocol) switches in 802.17 MANs send such messages • IP routers implement Explicit Congestion Notification (ECN) procedures

22. End-to-end path • Transport protocols • Ensure reliable transfer across a communication path consisting of many links (“zero” loss) • OR ensure delay-controlled path across a communication path consisting of many links • Error control and flow control • Delay control (e.g., RTP) • Congestion control and connection control – special in TCP

23. End-user equipment Email-sending clients (outlook, messenger) Network Network Outgoing email servers (pop, imap) Incoming email servers (smtp) Applications • Most Internet applications are client-server based Network End-user equipment Web server (Usually runs on fixed hosts) Web clients Network End-user equipment Email-receiving clients (outlook, messenger)

24. AL TL NL NL NL DLL DLL DLL DLL DLL PHY PHY PHY PHY PHY Switch Switch Endpoint Protocol Stacks AL • OSI model: two more layers between AL and TL • Session layer and presentation layer • PHY: Physical; DLL: Data Link Layer; NL: Network Layer; TL: Transport Layer; AL: Application Layer TCP/UDP IP DLL PHY Endpoint

25. Example protocols • AL protocols: http, smtp, ftp, PCM voice • TL protocols: TCP, UDP, RTP, AAL • NL protocols: IP, ATM • DLL protocols: PPP, HDLC • PHY protocols: DS0, DS1 • Ethernet: PHY+DLL+NL

26. Functions of protocol layers • PHY: sends bits across a link • DLL: error control and flow control on a link • NL: switching (routing), multiplexing, congestion control • TL: error control and flow control on an end-to-end basis • AL: Functions specific to the application

27. Congestion control and connection control in TCP • IP routers did not implement ECN until recently • TCP performs congestion control • Senses whether network switches (routers) are congested or not • Adjusts rate accordingly • Slow start and congestion avoidance • Concept of a “connection” at the TL • End hosts maintain state information regarding a TCP connection to track sequence numbers and ACKs • Connection open (SYN) and close (FIN) procedures • Contrast with a “connection” at the NL, where each switch maintains state about the connection

28. User plane, control plane, and management plane • Management plane: consists of all the protocols needed to “configure” data tables for the operation of the network • For example, protocols for routing data dissemination (distributed or centralized) • Other functions: performance, fault mgmt., accounting, security • Control plane: • Connection control protocols • in CO networks, this includes connection setup at each switch (connections at the network layer) • in CL networks, this includes connection setup only at the endpoints (connections at the transport layer, if the TL protocol is reliable) • Call control protocols • User plane: protocols for the actual flow of data

29. Routing protocol Routing protocol Routing protocol Dest. Dest. Dest. Next hop Next hop Next hop III-* III-* B III IV B Routing tables Routing protocol in all three types of networks - Phase 1 II III Host B I Host A V IV • Routing protocols exchange topology/loading/reachability information • Routes to destinations are precomputed and stored in routing tables

30. Connection setup (B) b a a Connection setup Connection setup b c b d c c a Connection setup d IN Port /Label IN Port /Label IN Port /Label OUT Port/Label OUT Port/Label OUT Port/Label d/L1 a/L1 a/L2 b/L3 c/L2 c/L1 Virtual circuit Signaling protocol for NL connection setup in a PS CO network - Phase 2 • Connection setup consists of each switch on the path • Route lookup for next hop node to reach destination • CAC (Connection Admission Control) for buffer and BW • Writing the input/output label mapping tables and programming the scheduler II III I Host A Host B V IV

31. Connection setup (B) b a a Connection setup Connection setup b c b d c c a Connection setup d IN Port /Timeslot OUT Port/Timeslot IN Port /Timeslot IN Port /Timeslot OUT Port/Timeslot OUT Port/Timeslot a/1 c/2 d/2 a/2 b/1 c/2 Circuit Signaling protocol for NL connection setup in a CS CO network - Phase 2 • Connection setup consists of each switch on the path • Route lookup for next hop node to reach destination • CAC (Connection Admission Control) for BW (note: no buffers) • Writing the port/timeslot/l mapping table II III I Host A Host B V IV

32. Dest. Dest. Dest. Next hop Next hop Next hop SYN ACK B B B II III B Routing tables SYN TL connection setup in a CL PS network - Phase 2 • Notion of transport layer connections • Exchange initial sequence numbers end-to-end to allow for ARQ (Automatic Repeat reQuest) based error correction, i.e., retransmissions in case of errors II III Host B I Host A V IV

33. IN Port /Label OUT Port/Label L3 L1 L1 L2 a/L1 c/L2 User-plane packet forwarding in a PS CO network - Phase 3 • Labels are VPI/VCIs in ATM • Labels are translated from link-to-link II b a a III I Host A b c Host B b d c c V IV a d

34. 1 2 IN Port /Timeslot OUT Port/Timeslot a/1 c/2 1 1 2 2 1 2 User-plane actions in a circuit-switched network - Phase 3 II • Bits arriving at switch I on time slot 1 on port a are switched to time slot 2 of port c b a a III I Host A b c Host B b d c c V IV a d

35. B B B B User-plane packet forwarding in a CL PS network - Phase 3 II • Packet headers carry destination host address (unchanged as it passes hop by hop) • Each CL packet switch does a route lookup to determine the outgoing port/next hop node b a a III I Host A b c Host B b d c c V IV a d

36. Addressing • Where are endpoint addresses used: • In CL PS networks, endpoint addresses are carried in packet headers • In CO networks, be it PS or CS, endpoint addresses are carried in connection setup messages

37. Summarized addresses • What are summarized addresses? • Why summarize addresses?

38. Summarized addresses • What are summarized addresses? • An address that represents a group of endpoint addresses • e.g., all 212 numbers, 128.238 IP addresses • Why summarize addresses? • Reduces routing table sizes – hold one entry for a summarized address instead of a large number of individual addresses • Reduces routing message lengths that convey reachability information

39. Examples of signaling protocols • SS7 (Signaling System No. 7) network (with its SS7 protocol stack) carries signaling messages to set up and release circuits in a telephone network

40. Examples of routing protocols • In an Ethernet network • Spanning tree algorithm and address learning • In the Internet: • Link-state routing protocols, such as Open Path Shortest First (OSPF) • Distance-vector based routing protocols, such as Routing Information Protocol (RIP) • In telephone networks: • Real-Time Network Routing (RTNR)

41. Examples of addressing schemes • Internet • 4-byte IP addresses • Telephone networks • 8-byte E.164 address (telephone number) • ATM networks • 20-byte ATM End System Address (AESA)

42. Dest: A Ethernet switch (packet switch) Broadcast links • Wireless • Copper: ethernet hubs • Optical fiber: Passive star couplers A Ethernet hub or WDM Passive Star Coupler Blind broadcast

43. B’s MAC layer checks destination address to determine whether the packet should be “switched” to the application or dropped C’s MAC layer checks destination address to determine whether the packet should be “switched” to the application or dropped End-user equipment B End-user equipment A End-user equipment C To B To B MAC protocols • Medium Access Control (MAC) protocols are used in broadcast links to allow a node to access medium and send information • As if “switch” is in endpoints • Wasteful of resources because all endpoints receive all packets

44. Endpoint Endpoint Endpoint Consider wireless links • Naturally broadcast medium • One transmitter sends data; multiple receivers can receive the signal and obtain the data • Need a MAC (Medium Access Control) protocol to share the “naturally broadcast” wireless medium

45. outbound Hub or optical passive star coupler inbound Host Host Host Multipoint drops: potential interference on inbound line – polling; e.g. multidrop telephone lines Hubs/Optical passive star couplers: any data received on one line is broadcast to all other lines Shared links in wired domain • Distance limitation between farthest hosts – Shannon’s capacity; SNR; attenuation

46. Classification of MAC protocols MAC protocols Fixed-assignment schemes Random-access schemes Demand assignment schemes Circuit-switched (e.g., FDMA, TDMA) Connectionless packet-switched (e.g., Ethernet, 802.11) Connection-oriented packet-switched (e.g., CDMA, polling) Channelization

47. Shared link as a LAN:relation between MAC protocols and LANs • A shared link allows multiple end stations to hear a transmission from any station • No node is serving as a “forwarding engine” for packets in a controlled fashion • hubs, passive star couplers, ring adapters, taps blindly send data UNLIKE switches, routers, bridges • This shared link concept works well as a local area network • if too large a network – with many hosts – each host will get a small percentage of bandwidth

48. Shared links as “access” links • Two reasons for using shared links on the access segment • individual endpoints (hosts/phones) generate small quantities of data traffic • Costs should be kept low for end users • Consequence: access links are often shared • MAC protocols in the upstream direction

49. Shared link in the presence of basestations/APs? • Is it still one shared link if basestations/APs forward data between two endpoints that cannot “hear” each other • No, basestations/APs become forwarding engines, i.e., switches • If a cell phone under one basestation calls another cell phone under the same basestation and the basestation allocates frequencies for both ends and forwards data bits • Not different from a circuit switch forwarding bits received on one DS0 to another DS0 • Same thing when an AP uses destination addresses to rebroadcast data – it acts as a packet switch

50. Compare TDMA on an access link with TDM on an inter-switch link • Similar in concept: sharing resources on one link among many users • Difference: • Multiple senders on access link • One sender in each direction on inter-switch link Basestation Circuit switch Circuit switch T1 line carrying 24 different DS0s (phone calls) Endpoint Endpoint Endpoint Timeslot 1 Timeslot 2 Timeslot 3