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Governing Communication


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  • 1. CMPE 151: Network Administration Lecture 2
  • 2. Review?
    • Network protocols.
    • TCP/IP.
  • 3. Outline
    • Network protocols.
    • IP
    • TCP
  • 4. What are protocols?
    • Set of rules governing communication between network elements (applications, hosts, routers).
    • Protocols define:
      • Format and order of messages.
      • Actions taken on receipt of a message.
    • Protocols are hard to design
      • We need design guidelines!
  • 5. Protocol stack Host Host Application Transport Network Link User A User B Teleconferencing Layering: technique to simplify complex systems Peers
  • 6. Layering Characteristics
    • Each layer relies on services from layer below and exports services to layer above.
    • Interface defines interaction,
    • Hides implementation - layers can change without disturbing other layers (black box).
  • 7. Encapsulation
  • 8. OSI Model: 7 Protocol Layers
    • Physical: how to transmit bits
    • Data link: how to transmit frames
    • Network: how to route packets hop2hop
    • Transport: how to send packets end2end
    • Session: how to tie flows together
    • Presentation: byte ordering, security
    • Application: everything else!
  • 9. Layering Functionality
    • Reliability
    • Flow control
    • Fragmentation
    • Multiplexing
    • Connection setup (handshaking)
    • Addressing/naming (locating peers)
  • 10. Example: Transport layer
    • First end-to-end layer.
    • End-to-end state.
    • May provide reliability, flow and congestion control.
  • 11. Example: Network Layer
    • Point-to-point communication.
    • Network and host addressing.
    • Routing.
  • 12. Internetworking
  • 13. Internetworking
    • Interconnection of 2 or more networks forming an internetwork, or internet.
      • LANs, MANs, and WANs.
    • Different networks mean different protocols.
      • TCP/IP, IBM’s SNA, DEC’s DECnet, ATM, Novell and AppleTalk.
  • 14. Internetworks (cont’d)
  • 15. TCP/IP
    • TCP/IP is the most widely used internetworking protocol suite
      • Initially funded through ARPA.
      • Picked up by NSF.
      • Used in the Internet.
    • Other internetworking protocols exist but are less used
      • Example: AppleTalk, X.25, etc.
  • 16. IP
  • 17. The Internet Protocol: IP
    • Glues Internet together.
    • Common network-layer protocol spoken by all Internet participating networks.
    • Best effort datagram service:
      • No reliability guarantees.
      • No ordering guarantees.
  • 18. IP (cont’d)
    • IP is responsible for datagram routing.
    • Important : each datagram is routed independently!
      • Two different datagrams from same source to same destination can take different routes!
      • Why?
      • Implications?
  • 19. IP (cont’d)
    • IP provides a best effort delivery mechanism
      • Does not guarantee to prevent duplicate datagrams, delayed and out-of-order delivery, corruption of data or datagram loss
    • Reliable delivery is provided by the transport layer , not the network layer (IP)
    • Network layer (IP) can detect and report errors without actually fixing them
  • 20. The Internet Protocol Router Router Host Host Application Transport Network IP IP IP IP Network
  • 21. Datagrams
    • Transport layer breaks data streams into datagrams which are transmitted over Internet, possibly being fragmented.
    • When all datagram fragments arrive at destination, reassembled by network layer and delivered to transport layer at destination host.
  • 22. IP Datagram Format
    • IP datagram consists of header and data (or payload).
    • Header:
      • 20-byte fixed (mandatory) part.
      • Variable length optional part.
  • 23. IP Versions
    • IPv4: IP version 4.
      • Current, predominant version.
      • 32-bit long addresses.
    • IPv6: IP version 6.
      • Evolution of IPv4.
      • Longer addresses (16-byte long).
  • 24. IP(v4) Header Format Header Payload
  • 25. Encapsulation
    • Each datagram is encapsulated within a data link layer frame
      • The whole datagram is placed in the data area of the frame.
      • The data link layer addresses for source and destination included in the frame header.
  • 26. Encapsulation - Example
  • 27. Encapsulation Across Multiple Hops
    • Each router in the path from source to destination:
      • Decapsulates datagram from incoming frame.
      • Forwards datagram - determines next hop.
      • Encapsulate datagram in outgoing frame.
  • 28. Encapsulation Across Multiple Hops - Example
  • 29. Maximum Transfer Unit
    • Each data link layer technology specifies the maximum size of a frame.
      • Called the Maximum Transfer Unit (MTU).
        • Ethernet: 1,500 bytes.
        • Token Ring: 2048 or 4096 bytes.
    • What happens when large packet wants to travel through network with smaller MTU?
      • Maximum payloads (data portion of datagram) range from 48 bytes (ATM cells) to 64Kbytes (IP packets).
  • 30. Fragmentation
    • Another solution (used by IP): fragmentation.
    • Gateways break packets into fragments to fit the network’s MTU ; each sent as separate datagram.
    • Gateway on the other side have to reassemble fragments into original datagram.
  • 31. Keeping Track of Fragments
    • Fragments must be numbered so that original data stream can be reconstructed.
    • Define elementary fragment size that can pass through every network.
    • When packet fragmented, all pieces equal to elementary fragment size, except last one (may be smaller).
    • Datagram may contain several fragments.
  • 32. Fragmentation - Example
  • 33. Addressing
  • 34. Universal Addressing
    • One key aspect of internetworks is unique addresses.
    • Sending host puts destination internetworking address in the packet.
    • Destination addresses can be interpreted by any intermediate router/gateway.
    • Router/gateway examines address and forwards packet on to the destination.
  • 35. IP Addresses
    • Each machine on the Internet has a unique IP address.
    • The IP address is different from the “physical” /“MAC” address.
      • The “physical address” is the address of a computer (actually, of a NIC) in the LAN.
        • It is only know within the LAN.
      • The IP address is a universal address.
      • When a packet arrives in a LAN, there needs to be a conversion from IP to MAC address (local “ address resolution ”).
  • 36. IP Addresses (cont’d)
    • An IP address is represented by a binary number with 32 bits (in IPv4).
      • Meaning that there are around 4 billion addresses.
      • Often IP addresses are represented in “dotted decimal”, such as
        • Each group of numbers can go from 0 to 255.
  • 37. IP Address Organization
    • Each IP address is divided into a prefix and a suffix
      • Prefix identifies network to which computers are attached.
      • Suffix identifies computers within that network.
  • 38. Network and Host Numbers
    • Every network in a TCP/IP internet is assigned a unique network number.
    • Each host on a specific network is assigned a host address that is unique within that network.
    • Host’s IP address is the combination of the network number (prefix) and host address (suffix).
    • Assignment of network numbers must be coordinated globally; assignment of host addresses can be managed locally.
  • 39. IP Address Format
    • IP address are 32 bits long.
    • There are different classes of addresses , corresponding to different subdivisions of the 32 bits into prefix and suffix.
      • Some address classes have large prefix , small suffix.
        • Many such networks, few hosts per network.
      • Other address classes have small prefix, large suffix.
        • Few such networks, many hosts per network.
  • 40. IP Address Format (cont’d)
    • How can we recognize to which class an IP address belongs to?
      • Look at the first 4 bits!
  • 41. IP Address Format (cont’d)
    • Class A, B and C are primary classes.
      • Used for ordinary addressing.
    • Class D is used for multicast , which is a limited form of broadcast.
      • Internet hosts join a multicast group.
      • Packets are delivered to all members of the group.
      • Routers manage delivery of single packets from source to all members of multicast group.
    • Class E is reserved.
  • 42. IP Addresses (cont’d)
    • Another way to determine the address class is by looking at the first group of numbers in the dotted decimal notation
  • 43. Networks and Hosts in Each Class
  • 44. Understanding IP Addresses
    • Examples:
      • 10 . 0.0.37 (class A)
      • 128.10 . 0.1 (class B)
      • 192.5.48 . 3 (class C)
  • 45. IP addresses: how to get one?
    • ICANN (Internet Corporation for Assigned Names and Numbers) coordinate IP address assignment.
    • How does host get its IP address in the network? 2 possibilities:
      • 1: Hard-coded by system administrator in a file inside the host.
      • 2: DHCP : “Dynamic Host Configuration Protocol”
        • Dynamically get address: “plug-and-play”.
  • 46. DHCP
    • DHCP allows a computer to join a new network and automatically obtain an IP address The network administrator establishes a pool of addresses for DHCP to assign.
    • When a computer boots, it broadcasts a DHCP request to which a server sends a DHCP reply.
  • 47. DHCP (Cont’d)
    • DHCP allows non-mobile computers that run server software to be assigned a permanent address (won’t change when the computer reboots).
      • The permanent address actually needs to be re-negotiated after a certain period of time.
  • 48. The Internet Transport Protocols: TCP and UDP
    • UDP: user datagram protocol (RFC 768).
      • Connection-less protocol.
    • TCP: transmission control protocol (RFCs 793, 1122, 1323).
      • Connection-oriented protocol.
  • 49. UDP
    • Provides connection-less, unreliable service.
      • No delivery guarantees.
      • No ordering guarantees.
      • No duplicate detection.
    • Low overhead.
      • No connection establishment/teardown.
    • Suitable for short-lived connections.
      • Example: client-server applications.
  • 50. TCP
    • Reliable end-to-end communication.
    • TCP transport entity:
      • Runs on machine that supports TCP.
      • Interfaces to the IP layer.
      • Manages TCP streams.
        • Accepts user data, breaks it down and sends it as separate IP datagrams.
        • At receiver, reconstructs original byte stream from IP datagrams.
  • 51. TCP Reliability
    • Reliable delivery.
      • ACKs.
      • Timeouts and retransmissions.
    • Ordered delivery.
  • 52. TCP Service Model 1
    • Obtained by creating TCP end points.
      • Example: UNIX sockets.
      • Socket number or address: IP address + 16-bit port number (TSAP).
      • Multiple connections can terminate at same socket.
      • Connections identified by socket ids at both ends.
      • Port numbers below 1024: well-known ports reserved for standard services.
        • List of well-known ports in RFC 1700.
  • 53. TCP Service Model 2
    • TCP connections are full-duplex and point-to-point.
    • Byte stream (not message stream).
      • Message boundaries are not preserved e2e.
    A B C D 4 512-byte segments sent as separate IP datagrams A B C D 2048 bytes of data delivered to application in single READ
  • 54. TCP Byte Stream
    • When application passes data to TCP, it may send it immediately or buffer it.
    • Sometimes application wants to send data immediately.
      • Example: interactive applications.
      • Use PUSH flag to force transmission.
      • TCP could still bundle PUSH data together (e.g., if it cannot transmit it right away).
    • URGENT flag.
      • Also forces TCP to transmit at once.
  • 55. TCP Protocol Overview 1
    • TCP’s TPDU: segment.
      • 20-byte header + options.
      • Data.
    • TCP entity decides the size of segment.
      • 2 limits: 64KByte IP payload and MTU.
      • Segments that are too large are fragmented.
        • More overhead by addition of IP header.
  • 56. TCP Protocol Overview 2
    • Sequence numbers.
      • Reliability, ordering, and flow control.
      • Assigned to every byte .
      • 32-bit sequence numbers.
  • 57. TCP Connection Setup
    • 3-way handshake.
    Host 1 Host 2 SYN (SEQ=x) SYN(SEQ=y,ACK=x+1) (SEQ=x+1, ACK=y+1)
  • 58. TCP Connection Release 1
    • Abrupt release:
      • Send RESET.
      • May cause data loss.
  • 59. TCP Connection Release 2
    • Graceful release:
      • Each side of the connection released independently.
        • Either side send TCP segment with FIN=1.
        • When FIN acknowledged, that direction is shut down for data.
        • Connection released when both sides shut down.
      • 4 segments: 1 FIN and 1 ACK for each direction; 1st. ACK+2nd. FIN combined.
  • 60. TCP Connection Release 3
    • Timers to avoid 2-army problem.
      • If response to FIN not received within 2*MSL (maximum segment lifetime), FIN sender releases connection.
    • After connection released, TCP waits for 2*MSL (e.g., 120 sec) to ensure all old segments have aged.
  • 61. TCP Transmission
    • Sender process initiates connection.
    • Once connection established, TCP can start sending data.
    • Sender writes bytes to TCP stream.
    • TCP sender breaks byte stream into segments.
      • Each byte assigned sequence number.
      • Segment sent and timer started.
  • 62. TCP Transmission (cont’d)
    • If timer expires, retransmit segment.
      • After retransmitting segment for maximum number of times, assumes connection is dead and closes it.
    • If user aborts connection, sending TCP flushes its buffers and sends RESET segment.
    • Receiving TCP decides when to pass received data to upper layer.
  • 63. TCP Flow Control
    • Sliding window.
      • Receiver’s advertised window .
        • Size of advertised window related to receiver’s buffer space.
        • Sender can send data up to receiver’s advertised window.
  • 64. TCP Flow Control: Example 2K;SEQ=0 ACK=2048; WIN=2048 2K; SEQ=2048 ACK=4096; WIN=0 ACK=4096; WIN=2048 1K; SEQ=4096 App. writes 2K of data 4K 2K 0 App. reads 2K of data 2K 1K App. does 3K write Sender blocked Sender may send up to 2K
  • 65. TCP Flow Control: Observations
    • TCP sender not required to transmit data as soon as it comes in from application.
      • Example: when first 2KB of data comes in, could wait for more data since window is 4KB.
    • Receiver not required to send ACKs as soon as possible.
      • Wait for data so ACK is piggybacked.
  • 66. Congestion Control
    • Why do it at the transport layer?
      • Real fix to congestion is to slow down sender.
    • Use law of “conservation of packets”.
      • Keep number of packets in the network constant.
      • Don’t inject new packet until old one leaves.
    • Congestion indicator: packet loss.
  • 67. TCP Congestion Control
    • Like, flow control, also window based.
      • Sender keeps congestion window (cwin) .
      • Each sender keeps 2 windows: receiver’s advertised window and congestion window.
      • Number of bytes that may be sent is min(advertised window, cwin).
  • 68. TCP Congestion Control (cont’d)
    • Slow start [Jacobson 1988]:
      • Connection’s congestion window starts at 1 segment.
      • If segment ACKed before time out, cwin=cwin+1.
      • As ACKs come in, current cwin is increased by 1.
      • Exponential increase.
  • 69. TCP Congestion Control (cont’d)
    • Congestion Avoidance:
      • Third parameter: threshold .
      • Initially set to 64KB.
      • If timeout, threshold=cwin/2 and cwin=1.
      • Re-enters slow-start until cwin=threshold.
      • Then, cwin grows linearly until it reaches receiver’s advertised window.
  • 70. TCP Congestion Control: Example threshold timeout threshold cwin time
  • 71. TCP Retransmission Timer
    • When segment sent, retransmission timer starts.
      • If segment ACKed, timer stops.
      • If time out, segment retransmitted and timer starts again.
  • 72. How to set timer?
    • Based on round-trip time: time between a segment is sent and ACK comes back.
    • If timer is too short, unnecessary retransmissions.
    • If timer is too long, long retransmission delay.
  • 73. Jacobson’s Algorithm 1
    • Determining the round-trip time:
      • TCP keeps RTT variable.
      • When segment sent, TCP measures how long it takes to get ACK back ( M ).
      • RTT = alpha*RTT + (1-alpha)M.
      • alpha: smoothing factor; determines weight given to previous estimate.
      • Typically, alpha=7/8.
  • 74. Jacobson’s Algorithm 2
    • Determining timeout value:
      • Measure RTT variation, or |RTT-M|.
      • Keeps smoothed value of cumulative variation D=alpha*D+(1-alpha)|RTT-M|.
      • Alpha may or may not be the same as value used to smooth RTT.
      • Timeout = RTT+4*D.
  • 75. Client-Server Model Client Kernel File Server Kernel Printer Server Kernel
  • 76. File Transfer
    • Sharing remote files: “on-line” access versus “file transfer”.
    • “ On-line” access transparent access to shared files, e.g., distributed file system.
    • Sharing through file transfer: user copies file then operates on it.
  • 77. The Web and HTTP
  • 78. The Web
    • WWW, or the world-wide web is a resource discovery service.
      • Resource space is organized hierarchically, and resources are linked to one another according to some relation.
      • Hypertext organization: link “granularity”; allows links within documents.
      • Graphical user interface.
  • 79. The Client Side
    • Users perceive the Web as a vast collection of information.
      • Page is the Web’s information transfer unit.
      • Each page may contain links to other pages.
      • Users follow links by clicking on them which takes them to the corresponding page.
      • This process can go on indefinetly, traversing several pages located in different places.
  • 80. The Browser
    • Program running on client that retrieves and displays pages.
      • Interacts with server of page.
      • Interprets commands and displays page.
    • Examples: Mosaic, Netscape’s Navigator and Communicator, Microsoft Internet Explorer.
    • Other features: back, forward, bookmark, caching, handle multimedia objects.
  • 81. Domain Name System (DNS)
    • Basic function: translation of names (ASCII strings) to network (IP) addresses and vice-versa.
    • Example:
      • <->
  • 82. DNS
    • Hierarchical name space.
    • Distributed database.
    • RFCs 1034 and 1035.
  • 83. How is it used?
    • Client-server model.
      • Client DNS (running on client hosts), or resolver.
      • Application calls resolver with name.
      • Resolver contacts local DNS server (using UDP) passing the name.
      • Server returns corresponding IP address.
  • 84. Name Resolution 1
    • Application wants to resolve name.
    • Resolver sends query to local name server.
      • Resolver configured with list of local name servers.
      • Select servers in round-robin fashion.
    • If name is local, local name server returns matching authoritative RRs.
      • Authoritative RR comes from authority managing the RR and is always correct.
      • Cached RRs may be out of date.
  • 85. Name Resolution 2
    • If information not available locally (not even cached), local NS will have to ask someone else.
      • It asks the server of the top-level domain of the name requested.
  • 86. Electronic Mail
    • Non-interactive.
      • Deferred mail (e.g., destination temporarily unavailable).
    • Spooling:
      • Message delivery as background activity.
      • Mail spool: temporary storage area for outgoing mail.
  • 87. Mail system User interface User sends mail User reads mail Outgoing mail spool Mailboxes incoming mail Client (send) Server (receive) TCP connection (outgoing) TCP connection (incoming)
  • 88. Observations
    • When user sends mail, message stored is system spool area.
    • Client transfer runs on background.
    • Initiates transfer to remote machine.
    • If transfer succeeds, local copy of message removed; otherwise, tries again later (30 min) for a maximum interval (3 days).
  • 89. Remote access
  • 90. Telnet User’s machine Telnet client OS TCP connection over Internet Telnet server OS
  • 91. Telnet basic operation
    • When user invokes telnet, telnet client on user machine establishes TCP connection to specified server.
    • TCP connection established; user’s keystrokes sent to remote machine.
    • Telnet server sends back response, echoed on user’s terminal.
    • Telnet server can accept multiple concurrent connections.