Protocols and Interaction Models for Web Services

1. Protocols and Interaction Models for Web Services CSCI 8710 Fall 2006

2. Outline • Networks • Client/Server • Peer-to-Peer • Web Service Protocols

3. Networks • Originated with ARPANET • Packet-switched • Started in late 1960’s • ARPA - Advanced Research Projects Agency (now known as DARPA) • Then: about 10 nodes (~100 million today) • Goal: resource sharing • Result: reached goal + demonstrated importance of networks as tool for communication and interaction via email

4. Networks • Kleinrock at UCLA • Did much of the early work in queueing models of networks, measurement and management of networks and network protocols

5. Networks • 1983: ARPANET split into two networks: • MILNET (military purposes) • ARPANET (reduced version) • Term “Internet” first introduced • -- came into being in 1983

6. Today’s Internet • Worldwide collection of interconnected WANs • Characterized by: • IP - networking protocol • TCP - process-to-process protocol

7. Types of Networks • WANs • LANs • LAN-to-WAN connection • Home-to-WAN connection

8. WANs • Wide-area network • City-wide, muti-city, country-wide, continent … • Uses packet switching: • Messages transmitted between hosts are broken down into chunks called packets with some max size • Packet header has routing and sequencing info • Consists of: • Packet switches or routers • High speed links connecting the routers

9. Routers • Communication computers that: • Store incoming packets • Examine headers • Look up routing tables • Decide next router to send packet to • Place packet on output queue for selected link • Known as “store and forward”

10. Technologies used to build WANs • X25 - a standard of the ITU (International Telecommunications Union) • ISDN - Integrated Services Digital Network • Service offered by telephone companies; integrates voice and data over ordinary telephone lines • Frame Relay • A high-speed WAN service offered by long-distance carriers

11. Technologies used to build WANs (cont’d) • SMDS - Switched Multi-megabit Data Service • Another high-speed WAN service offered by long-distance carriers • ATM - Asynchronous Transfer Mode • Packet-switched technology that uses small fixed-size packets (53 bytes), called cells, to provide fast switching to voice, video, and data over WANs

12. LANs • Typically confined to a building or set of closely located buildings • Both wired and wireless • Most popular: • 10-Mbps Ethernet • 100-Mbps Ethernet • 4- or 16-Mbps Token Ring • 100-Mbps FDDI

13. Ethernet • Invented by Metcalfe in the early 1970s • Have bus topology: Shared bus computer Network interface card (NIC)

14. Ethernet • Computers connect to a shared coaxial cable through NICs • Packets transmitted by one NIC can be received by all others: broadcast communication • But, since packets have destination addresses, only destination NIC will (typically) copy the packet to the computer’s main memory

15. Ethernet • All NICs can try to talk at once • which reminds EK of a faculty meeting ;^) • No central coordinator • CSMA/CD is used: • Carrier Sense with Multiple Access / Collision Detection • NIC that wants to transmit “listens” to see if a transmission is in progress (carrier sense) • If so, it waits • But two may start “talking” at about the same time (collision) • Collisions are detected and NICs stop “talking”, wait for a randomly selected time period(so they don’t just collide again), and then try to retransmit

16. Ethernet • What happens as number of nodes and/or traffic increases? • probability of collision increases • Network throughput decreases • Because more bandwidth spent on collisions and retransmissions

17. Token Ring • Invented at IBM Research Labs • Based on a ring topology: = computer = Network interface card (NIC) ring

18. Token Ring • Access to ring is controlled by a token (special bit pattern that circulates in the ring) • NIC with token can transmit • If NIC has nothing to transmit, just passes the token

19. Token Ring • Sender (NIC with token) inserts the bits representing its packet into the ring • Packet goes around the ring, is copied by the NIC of the destination address • Packet flows back around to sender; sender removes packet and performs error checking (compares received packet with sent packet)

20. Token Ring • As more stations are added to the token ring … • Delay in obtaining token increases • Token must circulate through more NICs • Probability that token is used by other NICs increases

21. FDDI • Uses optical fibers and token-passing • Differs in that it uses 2 rings = computer = Network interface card (NIC) ring

22. FDDI • Benefits of 2nd ring? • Data flows in opposite directions in the 2 rings • If a station fails, the hardware can reconfigure the ring and turn it into a single functioning ring by bypassing the malfunctioning station

23. LANs: limits on size • Two types of limits: • Physical limits • Performance limits • Examples: • Ethernet physical limits: • Cable no more than 500 m in length • Mininum separation of 3 m between stations

24. Managing performance limits on LANs • Number of stations may be limited because of adverse impact of additional stations on overall performance • Approach: • Can divide larger LAN into LAN segments with fewer stations each • Segments joined by connecting devices such as routers and bridges • Stations that communicate frequently should be in same segment

25. Wireless LANs • Stations communicate via RF (radio frequency) • Modulation of transmitted wave is interpreted as sequence of 0s and 1s • IEEE standard for LANs is the IEEE 802.11 protocol • Can transmit data at 1 or 2 Mbps depending on underlying modulation technology

26. IEEE 802.11 protocol • Provides a carrier sense signal that indicates if a transmission is in progress • Data sent by one station can be received by all stations in the coverage area • Subject to the “hidden terminal” problem • Happens when walls or other structures obstruct the RF signals • Station C may hear A and B, but they may not be able to communicate with one another

27. IEEE 802.11 protocol • within their cells or basic service set (BSS), stations ( ) can communicate with one another and with an access point (AP). • Through the AP stations may communicate with stations in another BSS • Stations may also form an ad-hoc network (without an AP) BSS AP BSS AP

28. IEEE 802.11 protocol: handling interference • Uses CSMA/CA • Carrier Sense Multiple Access/Collision Avoidance • If channel is sensed idle for time equal to DIFS (distributed inter frame space), station may transmit • Receiver of a correct frame then sends ack frame to sender after short time (SIFS=short interframe spacing) • If channel is busy: • sender defers access, listens again • If quiet for DIFS, xmits after random backoff time expires • Use of random prevents all waiting from sending at same time • Doesn’t detect collisions, tries to avoid • Transmitted frame contains transmission duration; others know how long to wait

29. IEEE 802.11:handling the hidden station problem • Two stations (A and B) hidden from one another may transmit to same station (C) • Can use RTS (request to send) and CTS (clear to send) exchange of frames before actual transmission • B will “hear” C send the CTS to A and will not send.

30. LAN-to-WAN connection • LANS usually connect to WANS through dedicated leased lines at T1 (1.544 Mbps) or T3 (45 Mbps) speeds • LANS may be of any type • Example: • 3 LANS: 1 FDDI, 1 ethernet, 1 Token ring • Each connects to Frame Relay WAN through router and T1 line

31. Home to WAN connection • Many alternatives: • Dialup modem, 14.4 - 15.6 Kbps • simple, cheap • ISDN Basic Rate Interface (BRI) • Dialup digital modem • Speed up to 128 Kbps • ISDN Primary Rate Interface (PRI) • 1.544 Mbps • Leased T1 line • 1.544 Mbps

32. Home-WAN connection, continued • High Bit Rate Digital Subscriber Line (HDSL) • 1.544 Mbps • Asymmetric Digital Subscribe Line (ADSL) • 640 Kbps outbound • 6 Mbps inbound • Good for web access (http requests are small, returned images, videos, etc. may be large) • Cable modems • Cable is shared; actual bandwidth seen by customer depends on load on network • Most cable modems are asymmetric • Typical speeds 1- 10 Mbps downstream, 128 Kbps upstream

33. Protocols • Purpose • IP • TCP

34. Protocols • Protocol:(in this context) • A set of rules governing communication between two computers or two processes over a computer network • Consists of functions/rules for: • Addressing • Routing • Together ensure that message from A to B arrives at B • Error detection • Error recovery • Sequence control • Together handle situation in which messages from A are lost or corrupted due to noise or network failures • Flow control • To handle situation in which A sends at faster rate than B can consume

35. Protocols • Connectionless • Messages from A to B are independent of one another; may arrive at destination in order different from transmission order • Think of mailing off a batch of postcards that together contain the content of a novel • Good when data to be exchanged fit into maximum data unit (all fits on one postcard)

36. Protocols • Connection-oriented • Used when messages that are much larger than the maximum data unit are transmitted • Sequencing and data recovery important • Think of making a phone call: a connection is set up, and the channel remains open for transmission until you disconnect

37. Protocol specification • Syntax • Specifies the types of messages that can be sent, the format of those messages, and the meaning of each field in the message • Semantics • Specifies the actions taken by each entity when specific events occur • Example: when a message arrives, when a message times out, etc.

38. Protocol specification • ISO (International Standards Organization) defined a seven-layer model, the Reference Model for Open Systems Interconnection • Physical • Data link • Network • Transport • Session • Presentation • Application

39. (ISO) OSI • Each entity at layer n communicates only with remote nth-layer entities • Layer n uses local services provided by layer n-1 N-th layer protocol N-th layer N-th layer (N-1)th layer (N-1)th layer network

40. Protocol Layers • Data exchanged between nth-layer entities have to be: • physically processed by layers n to 1 at the sending computer • Transported through the network • Moved from layer 1 to n at the receiving end

41. Protocol Layers • Each entity at layer n exchanges a Protocol Data Unit (PDU) with a remote layer n entity • PDU has: • Layer n data • Layer n header • The layer n PDU becomes layer (n-1) data: Layer (n-1) header Layer n header Layer n data <---------- layer (n-1) data ---------------------->

42. TCP/IP • IP = Internet Protocol • a network layer protocol • TCP = Transmission Control Protocol • A transport layer protocol • Connection-oriented • UDP = User Datagram Protocol • A transport layer protocol • A connectionless protocol • Together: TCP/IP protocol suite; forms the core of the internet

43. On top of TCP: • HTTP- hypertext transfer protocol (web) • FTP - file transfer protocol • SMTP - simple mail transfer protocol • Telnet - an interactive login protocol

44. On top of UDP: • RPC - remote procedure call • NFS - network file sytem - runs on top of RPC • DNS - Domain Name Server • SNMP - Simple Network Management Protocol

45. Internet Protocol (IP) • Specifies: • the formats of packets sent across the Internet, • the mechanisms used to forward these packets through a collection of networks • Routers from source to destination

46. Internet Protocol (IP) • Every host connected to the internet has a unique address: an IP address • A 32-bit number • Represented by a dotted notation: 129.192.4.5, for example • Each of the four numbers represents the value of 8 bits in the address • Divided into prefix and suffix • Prefix: indicates the network • Suffix: host within the network

47. Internet Protocol (IP) • Number of bits allocated to prefix determines number of unique network numbers • Number of bits allocated to suffix determines number of hosts per network • Currently, IPv4 uses 32-bit address field • But … may be approaching limits of number of servers to be on … • IP v 6 uses 128 bits

48. Internet Protocol (IP) • IP datagram: the data unit transported by IP • IP is connectionless and • can “lose” datagrams • can deliver datagrams out-of-order (may travel to destination by different routes) • is known as “best effort” service

49. Internet Protocol (IP) • Header is 20 bytes long • 4 bytes for IP address of source • 4 bytes for IP address of destination • Performs routing of datagrams from source to destination • IP implementation at router maintains an in-memory routing table; used to search for next router or host to which to forward the datagram.

50. Tuesday’s class • Kelly, • Stop here … • On Thursday, pick up here with TCP… • Thanks! • Ek