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Course Review

Course Review. Outline. Networks: A top down view (for a change). Other topics. Security QoS Multicast Questions?. Protocol Stacks. The Open Systems Interconnection (OSI) Model. Application. Application. 7. Presentation. Presentation. 6. Session. Session. 5. Transport.

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Course Review

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  1. Course Review

  2. Outline • Networks: A top down view (for a change). • Other topics. • Security • QoS • Multicast • Questions? Final Review: 12/10/2001

  3. Protocol Stacks The Open Systems Interconnection (OSI) Model. Application Application 7 Presentation Presentation 6 Session Session 5 Transport Transport 4 Network Network Network 3 Data link Data link Data link Data link 2 Physical Physical Physical Physical 1 Final Review: 12/10/2001

  4. Browsing the Web Web Server Web Browser 7 Presentation Presentation 6 ? Session Session 5 Transport Transport 4 Network Network Network 3 Data link Data link Data link Data link 2 Physical Physical Physical Physical 1 Client Server Final Review: 12/10/2001

  5. HTTP Request Example GET / HTTP/1.1 Accept: */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Host: www.seshan.org Connection: Keep-Alive Final Review: 12/10/2001

  6. HTTP Response Example HTTP/1.1 200 OK Date: Tue, 27 Mar 2001 03:49:38 GMT Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1 OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24 Last-Modified: Mon, 29 Jan 2001 17:54:18 GMT ETag: "7a11f-10ed-3a75ae4a" Accept-Ranges: bytes Content-Length: 4333 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html ….. Final Review: 12/10/2001

  7. Single Transfer Example Client Server 0 RTT SYN Client opens TCP connection SYN 1 RTT ACK DAT Client sends HTTP request for HTML ACK Server reads from disk DAT FIN 2 RTT ACK Client parses HTML Client opens TCP connection FIN ACK SYN SYN 3 RTT ACK Client sends HTTP request for image DAT Server reads from disk ACK 4 RTT DAT Image begins to arrive Final Review: 12/10/2001

  8. Persistent Connection Example Client Server 0 RTT DAT Client sends HTTP request for HTML ACK Server reads from disk DAT 1 RTT ACK Client parses HTML Client sends HTTP request for image DAT Server reads from disk ACK DAT 2 RTT Image begins to arrive Final Review: 12/10/2001

  9. DNS Server • A DNS server is responsible for maintaining the name-address mapping in a specific domain. • E.g. cs.cmu.edu • The network manager can add, remove, or change mappings. • Computers can send requests to the server to translate a name into an address. • But how do you find the server? • Recursively contact the parent in the hierarchical name space • Caching is used to speed up the lookup of frequently used names. Other DNS servers cs.cmu.edu hawaii.cs.cmu.edu 128.17.4.174 Final Review: 12/10/2001

  10. Sender creates and initializes a socket. Sender issues an open connection command. Specifies destination IP and application port addresses Sender blocks while connection is established If the connection succeeds, data exchange can start. Lots of things can go wrong: wrong addresses, receiver or network down. Receiver creates and initializes a socket. Receiver listens on the socket for a connection request. Can sometimes restrict the type of connection If receiver accepts the connection and the connection succeeds, data exchange can start. Communication typically uses a different socket Typical Exchange Final Review: 12/10/2001

  11. Browsing the Web Web Server Web Browser 7 Presentation Presentation 6 Session Session 5 Transport Transport 4 ? Network Network Network 3 Data link Data link Data link Data link 2 Physical Physical Physical Physical 1 Client Server Final Review: 12/10/2001

  12. Connection management Sender Receiver Establish Initial Sequence Numbers syn Open syn/ack ack Data fin ack Close fin ack Time Final Review: 12/10/2001

  13. Reliability • Checksum guarantees end-end data integrity. • Sequence numbers detect packet sequencing problems: • duplicate: ignore • reordered: reorder or drop • lost: retransmit • Lost packets detected by sender. • uses time out to detect lack of acknowledgment • requires reliable roundtrip time estimate • Retransmission requires that sender keeps copy of the data until ACK is received. • performance issue Final Review: 12/10/2001

  14. When to Send a Packet? • End-to-end flow control. • avoid buffer overflow on receiver • receiver advertizes a window size • Congestion control. • estimates amount of data that can be in network • implemented using the congestion window, slow start, and fast retransmit/recovery mechanisms • Efficiency considerations. • try to send large packets (if possible) • more efficient in the network and on end points • piggybacking of acks Final Review: 12/10/2001

  15. Window Size versus Throughput Sender Receiver Time Window Size Throughput = Roundtrip Time Final Review: 12/10/2001

  16. TCP Congestion Avoidance • Congestion avoidance limits how fast TCP can send data. • Implemented using a congestion window that limits how much data can be in the network • independent from flow control window • transmission is limited by minimum of the two windows • window grows in response to acknowledgement • Packet loss is seen as sign of congestion. • multiplicative decrease of the congestion window • have to cut back fast since cost of congestion is high • How do you detect when more bandwidth becomes available? • gradually increment congestion window (probing) • results in oscillation around congestion window size! Final Review: 12/10/2001

  17. TCP Saw Tooth Behavior Congestion Window Timeouts may still occur Time Slowstart to pace packets Fast Retransmit and Recovery Initial Slowstart Final Review: 12/10/2001

  18. Browsing the Web Web Server Web Browser 7 Presentation Presentation 6 Session Session 5 Transport Transport 4 Network Network Network ? 3 Data link Data link Data link Data link 2 Physical Physical Physical Physical 1 Client Server Final Review: 12/10/2001

  19. Hop-by-Hop PacketForwarding in the Internet Ethernet Packets over SONET Mixed Ethernet Host Host 7 .. 3 2 1 Final Review: 12/10/2001

  20. Addressing in IP v4 (Basic) • Each host has an Internet address. • Addresses are hierarchical. • address contains hint about location • Address space is divided in three classes of point-to-point addresses, multicast addresses, and some special addresses. type network host A 1 7 24 B 2 14 16 C 3 21 8 D 4 (multicast) 28 Example: 128.2.209.19 Final Review: 12/10/2001

  21. Net ID Next Net ID Next Net ID Next Net ID Next Forwarding Table Routing based on Network Identifier AN 3 Host Host AN 4 AN 2 ISP 1 Host Host ISP 3 ISP 2 AN 5 AN 1 Host Host Net.Host Final Review: 12/10/2001

  22. Problems with Simple Address Structure • Running out of addresses. • Especially true for mid-sized networks • Routing tables are becoming too big. • 100 of thousands of entries • Temporary solution: classless inter-domain routing. • Use address space more efficiently by relaxing the strict address structure, • length of network address is variable • generalization of subnetting idea • have internet service providers hand out blocks of addresses to their customers Final Review: 12/10/2001

  23. Route Lookup with CIDR • Problem: with CIDR there can be multiple matches when looking up an address. • Can for example happen when a customer switches ISPs but keeps addresses • Solution: lookup is based on longest prefix match. • If there are multiple matches in the lookup, the longest match (longest netmask) wins 10110110 hosts 10110110 010 hosts 10110110 010 0100011 Final Review: 12/10/2001

  24. What Does Routing Do? • Routing protocol specifies how routers jointly collect information about the network. • Routing protocols must be standardized • Routing algorithm uses network information to select appropriate routes and to set up the routing table. • The data forwarding engine performs route lookup in the routing table. • through which interface should a packet be forwarded? Other routers Routing protocol Routing Protocol Routing Algorithms Route Lookup Final Review: 12/10/2001

  25. 5, E 5, E 3, F 3, F 5, E 3, F B 5 D 3 1 3 A F 2 3 6, B 4 2 6, B C E 2 4 6, E 2, F 2, F 6, E 6, E 6, E Dijkstra’s Algorithm(Link State) 6, B Final Review: 12/10/2001

  26. B/3 B A/3 - B/4 B/7 A B/3 A C/5 B/3 - - A/3 A/9 B/4 - A A/3 - C/2 C B/1 C/1 - C - B - B/4 B/4 C/1 B/1 C/1 B/4 C/2 B/1 - C/9 B D - - C/1 D - D - C - D/4 C/1 B/7 C/1 D/1 D/1 C/2 B/5 C/2 D/1 Distance Vector RoutingExample B 3 4 1 A D 9 1 C Final Review: 12/10/2001

  27. Hierarchical Routing • Two level routing based on intra-domain and inter-domain routing to improve scalability. • Matches the structure of the address space. • Driven in part by business/management concerns. • Local network information is kept internal • Agreements with specific service providers at boundaries Host Host Host Host Host Host Final Review: 12/10/2001

  28. Browsing the Web Web Server Web Browser 7 Presentation Presentation 6 Session Session 5 Transport Transport 4 Network Network Network 3 ? Data link Data link Data link Data link 2 Physical Physical Physical Physical 1 Client Server Final Review: 12/10/2001

  29. Datalink in the Backbone • Routers are connected by point-point links or by (datalink layer) switched clouds. • Point-point links typically based on SONET. • E.g. Packets over SONET • Switched clouds often uses virtual connection datalink technologies. • E.g., ATM, frame relay Point-Point link Switched Cloud Router PCs at Work PC at Home Final Review: 12/10/2001

  30. 802.3 Ethernet • Carrier-sense multiple access with collision detection (CSMA/CD). • 10Mbps cable rate. • Maximum diameter 2.5km. • Minimum frame = 64 bytes. • Thick or thin coax; 10Base-T unshielded twisted pair in star configuration using hub. Broadcast technology host host host host host host host host Hub Final Review: 12/10/2001

  31. Ethernet Switches • Bridges make it possible to increase LAN capacity. • Packets are no longer broadcasted - they are only forwarded on selected links • Adds a switching flavor to the broadcast LAN • Ethernet switch is a special case of a bridge: each bridge port is connected to a single host. • Simplifies the protocol and hardware used (only two stations on the link) • Can make the link full duplex (really simple protocol!) • Can have different port speeds Final Review: 12/10/2001

  32. Framing • A link layer function, defining which bits have which function. • Minimal functionality: mark off units of transmission. • Some techniques: • frame delimiter characters with character stuffing • frame delimiter codes with bit stuffing • out of band delimiters (e.g. FDDI control symbols) • synchronous transmission (e.g. SONET) Final Review: 12/10/2001

  33. Browsing the Web Web Server Web Browser 7 Presentation Presentation 6 Session Session 5 Transport Transport 4 Network Network Network 3 Data link Data link Data link Data link 2 ? Physical Physical Physical Physical 1 Client Server Final Review: 12/10/2001

  34. The Frequency Domain • A (periodic) signal can be viewed as a sum of sine waves of different strengths. • Every signal has an equivalent representation in the frequency domain. • What frequencies are present and what is their strength • Similar to radio and TV signals Amplitude Time Frequency Final Review: 12/10/2001

  35. Wireless: Good News Bad News • Great technology: no wires to install, convenient mobility, .. • High attenuation limits distances. • Wave propagates out as a sphere • Signal strength reduces quickly (1/distance)2 • High noise due to interference from other transmitters. • Use MAC and other rules to limit interference • Aggressive encoding techniques to make signal less sensitive to noise • Other effects: multipath fading, security, .. • Ether has limited bandwidth. • Try to maximize its use Final Review: 12/10/2001

  36. TCP Problems Over Noisy Links • Wireless links are inherently error-prone • Fades, interference, attenuation • Errors often happen in bursts • TCP cannot distinguish between corruption and congestion • TCP unnecessarily reduces window, resulting in low throughput and high latency • Burst losses often result in timeouts • Sender retransmission is the only option • Inefficient use of bandwidth Final Review: 12/10/2001

  37. Proposed Solutions • End-to-end protocols • Selective ACKs, Explicit loss notification • Split-connection protocols • Separate connections for wired path and wireless hop • Reliable link-layer protocols • Error-correcting codes • Local retransmission Final Review: 12/10/2001

  38. Browsing the Web Web Server Web Browser 7 Presentation Presentation 6 Session Session 5 Transport Transport 4 Network Network Network 3 Data link Data link Data link Data link 2 Physical Physical Physical Physical 1 Client Server Everything Cleared Up! Final Review: 12/10/2001

  39. Security Threats • Impersonation. • Pretend to be another user with the intent of getting access to information or services • Secrecy. • Get access to the contents of packets • Message integrity. • Change a message unbeknownst to the sender or receiver • Repudiation • Denying to have sent a message • Denial of service. • Flooding the system so users with legitimate needs cannot get service • Range of other threats: password guessing, exploiting programming bugs, … Final Review: 12/10/2001

  40. Encryption ciphertext = E(plaintext, k) plaintext = D(ciphertext, k’) • Private key (symmetric, e.g. DES) • the two parties share a common private key k • Public key (asymmetric, e.g. RSA) • derive two keys, kprivate and kpublic • kprivate is kept private by its owner • kpublic is published • Tradeoffs between private and public key cryptography. • Key management, speed • Challenge: key management. Final Review: 12/10/2001

  41. Example Applications • Kerberos. • Support security in corporate environment • Based on key distribution center that knows all the entities • Know = share secret • Secure socket layer (SSH). • Support secure channels in open internet environment • Based on certificates and certification authorities • Provides privacy, but trust is limited • Pretty good privacy (PGP). • Provides privacy, authentication, repudiation in internet environment • Key management based on a “web of trust” Final Review: 12/10/2001

  42. How to Provide QoS? • Admission control limits number of users. • You cannot provide guarantees if there are too many users sharing the same set of resources (bandwidth) • For example, telephone networks - busy tone • This implies that your request for service can be rejected • Traffic enforcement limits how much traffic users can inject based on predefined limits. • Make sure user respects the traffic contract • Data outside of contract can be dropped (before entering the network!) or can be sent at a lower priority • Scheduling support in the routers guarantee that users get their share of the bandwidth. • Again based on pre-negotiated bounds • Signaling protocol gives routers the information they need to provide QoS. • E.g. RSVP Final Review: 12/10/2001

  43. Qos Summary Final Review: 12/10/2001

  44. IETF QoS Models • Integrated services: diverse QoS at the micro-flow level. • Range of QoS: best effort, controlled load, guaranteed • Specific end-to-end service defined for each class • Requires end-to-end support, e.g. edge and core routers • Concern about complexity, cost, marketing/charging • Differentiated services: QoS at the aggregate flow level. • Defines range of “forwarding behaviors”, but services are defined by the providers • Pushes most complexity to the edge of the network – fast core routers work only with small number of traffic classes • Based on the same building blocks. Final Review: 12/10/2001

  45. Multimedia Challenges • TCP/UDP/IP suite provides best-effort, no guarantees on expectation or variance of packet delay • Streaming applications delay of 5 to 10 seconds is typical and has been acceptable, but performance deteriorate if links are congested (transoceanic) • Real-Time Interactive requirements on delay and its jitter have been satisfied by over-provisioning (providing plenty of bandwidth), what will happen when the load increases?... Final Review: 12/10/2001

  46. Multicast – Efficient Data Distribution Src Src Final Review: 12/10/2001

  47. IP Multicast Architecture Service model Hosts Host-to-router protocol(IGMP) Routers Multicast routing protocols(various) Final Review: 12/10/2001

  48. Multicast Routing • Basic objective – build distribution tree for multicast packets • Core based protocols • Examples: CBT, PIM-SM • Flood and prune • Examples: DVMRP, PIM-DM • Link-state multicast protocols • Example: MOSPF Final Review: 12/10/2001

  49. Shared vs. Source-based Trees • Source-based trees • Separate shortest path tree for each sender • DVMRP, MOSPF, PIM-DM, PIM-SM • Shared trees • Single tree shared by all members • Data flows on same tree regardless of sender • CBT, PIM-SM Final Review: 12/10/2001

  50. Questions? Final Review: 12/10/2001

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