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CHAPTER 5: LINK LAYER & LANS

CHAPTER 5: LINK LAYER & LANS. Flow Control Error Control Data Link Protocols Medium Access Control IEEE 802 Standards Multiprotocol Label Switching Example: Tracing A Web Page Request. FLOW CONTROL. Network Node. Network Node.

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CHAPTER 5: LINK LAYER & LANS

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  1. CHAPTER 5: LINK LAYER & LANS • Flow Control • Error Control • Data Link Protocols • Medium Access Control • IEEE 802 Standards • Multiprotocol Label Switching • Example: Tracing A Web Page Request

  2. FLOW CONTROL Network Node Network Node When a network node transmits frames faster than the next network node can “digest” them, the receiver will usually just discard the excess frames. frame frame frame frame frame frame frame frame frame frame To combat this problem, the Data Link Layer protocol usually contains some kind of “flow control” mechanism. CS 447 Chapter 5 Page 117

  3. STOP-AND-WAIT Network Node Network Node One approach to Data Link Layer flow control is for the receiving node to respond with an acknowledgement whenever it has finished dealing with the previous frame and is ready to receive the next frame. frame #1 frame #2 frame #3 frame #6 frame #5 frame #4 ACK #6 ACK #5 ACK #4 ACK #2 ACK #1 ACK #3 Variations on this approach include: • “Piggybacking” ACKs on frames going the opposite direction on the link, in order to reduce line utilization. • Sending negative acknowledgements whenever a received frame is corrupted or whenever a frame is not received in a timely fashion. • Retransmitting a frame automatically whenever an ACK is not received in a timely fashion. CS 447 Chapter 5 Page 118

  4. SLIDING WINDOWS Sending Node (window size 8) Receiving Node Sending Node (window size 0) Receiving Node STOP F4 F2 F7 F3 F1 F6 F0 F5 Sending Node (window size 2) Receiving Node Sending Node (window size 4) Receiving Node F5 F2 F2 F7 F4 F1 ACK (awaiting F7) F1 F3 F0 F0 Sending Node (window size 5) Receiving Node Sending Node (window size 0) Receiving Node STOP F5 F0 F6 F3 F4 ACK (awaiting F3) F7 F5 F2 F3 F4 F1 By increasing the memory on each network node and using a more sophisticated algorithm for keeping track of which frames have and haven’t been accepted, traffic can flow more smoothly between the nodes. CS 447 Chapter 5 Page 119

  5. ERROR CONTROL Network Node Network Node Network Node Network Node Error control at the Data Link Layer is concerned with the detection (and, if possible, the correction) of lost or corrupted frames between consecutive network nodes. frame frame faerm??? CS 447 Chapter 5 Page 120

  6. ERROR DETECTION: PARITY CHECK One way to detect transmission errors is by using parity bits to ensure that each segment of data has an even number of 1’s (even parity) or an odd number of 1’s (odd parity), depending on which type of parity the protocol is using. Wants to send message “YO!” in ASCII, using even parity. ASCII ‘Y’ is 1011001, so tack on a ___ ASCII ‘O’ is 1001111, so tack on a ___ ASCII ‘!’ is 0010001, so tack on a ___ 0 Received message is: 10110010 10011111 00110010 First byte is 10110010, with even parity, so it’s ASCII ‘Y’ Second byte is 10011111, with even parity, so it’s ASCII ‘O’ Third byte is 00110010, with odd parity, so it’s an error!!! 1 0 So, the transmitted message is: 101100101001111100100010 Network Node Network Node 101100101001111100100011 One major problem with parity checking: if a segment has an even number of corrupted bits, no error is detected! CS 447 Chapter 5 Page 121

  7. ERROR DETECTION: CYCLIC REDUNDANCY CHECK For more effective error detection, the cyclic redundancy check was developed. 111001000010011111 11100100001001111 11100100001001 1 1110010000100111 11 111 111001 111001000010011 11100100001 110101 10001101100100011100000 110101 1. Both stations agree upon a binary “generator”, for example: 110101 101100 110101 110011 2.The sending station tacks len(generator)-1 0’s onto its binary message and does a modulo-2 division by the generator. For example, if the original message is 100011011001000111 with generator 110101, then the division at right is performed: 110101 00110100 110101 0000110001 110101 00100110 110101 100110 110101 100110 110101 100110 110101 3.The sending station transmits its message, with the remainder of the above quotient added as a suffix. 100110 110101 10011 Actual transmission: 10001101100100011110011 CS 447 Chapter 5 Page 122

  8. CRC AT THE RECEIVER 4. The receiving station performs a modulo-2 division by the generator on the received message (including the appended CRC suffix). 111001000010011111 11100100001001111 1110010000100111 11100100001 111001000010011 11100100001001 11 111 111001 1 10001101100100011110011 110101 110101 101100 110101 110011 110101 00110100 110101 0000110001 110101 00100111 110101 100100 110101 100010 5. If the remainder of this quotient is non-zero, then a transmission error has occurred. Otherwise, we’re reasonably certain that there’s been no error! 110101 101111 110101 110101 110101 00000 CS 447 Chapter 5 Page 123

  9. FRAME FORMATS Network Layer Packet Network Layer Packet Special Data Link Header Special Data Link Header Network Layer Packet Special Data Link Header When formatting the Data Link Layer’s frames for transmission on the Physical Layer, it’s necessary to mark the frame with a header so the receiving network node will recognize the beginning of the frame. Network Layer Packet Special Data Link Header To enable the receiver to recognize the end of the frame, several options exist. Include a size field in the header Use a standard size for all frames Include a special trailer coded with a bit sequence that’s guaranteed not to occur in the rest of the frame Special Trailer CS 447 Chapter 5 Page 124

  10. SPECIFIC DATA LINK PROTOCOLS A variety of Data Link Layer framing formats have been developed. Developed by the OSI folks, the High-Level Data Link Control protocol is commonly used in traditional packet-switching networks, like X.25. HDLC Developed by the TCP/IP folks, the Serial Line Internet Protocol and the Point-To-Point Protocol are used to send IP datagrams across slow serial lines. SLIP PPP ATM Asynchronous Transfer Mode was developed to address the transition of communication data from voice and text to multimedia. CS 447 Chapter 5 Page 125

  11. HIGH-LEVEL DATA LINK CONTROL 01111110 01111110 Address Control Data CRC Checksum 01111110 01111110 Address Control Data CRC Checksum Delimiting fields to mark the beginning and ending of the frame. Require bit stuffing! Address field to identify the specific node with which communication is occurring in a multipoint line. (Not used in a meaningful way in a point-to-point line.) • Control field to identify the type of frame being transmitted: • Information frames start with a zero, followed by a 3- or 7-bit sequence number (for sliding window purposes), a bit to indicate whether this is a polling frame (from the multipoint primary station) or a final frame in a sequence (from a multipoint secondary station), and a 3- or 7-bit ACK sequence number. • Supervisory frames start with a 10, followed by a 2-bit type (ACK, NAK/Go-Back-N, ACK/Halt, or NAK/S-R), a Poll/Final bit, and an ACK sequence number. • Unnumbered frames start with a 11, followed by a 1-bit Poll/Final bit, and a 5-bit command (Frame Reject, Disconnect, Set Up Link w/Asynchronous Response Mode, Set Up Link w/Normal Response Mode, etc.) Data field containing the Network Layer bits that were handed down to it. Checksum field to perform the good ol’ Cyclic Redundancy Check! CS 447 Chapter 5 Page 126

  12. SERIAL LINE INTERNET PROTOCOL 11000000 “Stuffed” IP Datagram 11000000 11000000 11000000 “Stuffed” IP Datagram Delimiting fields at the beginning and ending of the frame. Require byte stuffing! The Network Layer’s data (i.e., IP’s datagram), stuffed to ensure that the delimiter never occurs: whenever 11000000 occurs in the data, it’s replaced with 11011011 11011100, and whenever 11011011 occurs in the data, it’s replaced with 11011011 11011101. • Note that SLIP has several problems: • It only works with IP; no other Network Layer protocol is supported. • It does no error checking, leaving such problems to the higher layers. • Addresses must be known in advance by both communicating nodes, since no address fields are available. • It isn’t an approved IP standard, so numerous, incompatible versions exist. • Why is SLIP popular in spite of these problems? • There are free versions readily available, all working with the ubiquitous EIA-232D modem interface! CS 447 Chapter 5 Page 127

  13. POINT-TO-POINT PROTOCOL 01111110 01111110 Address Control Protocol Payload CRC Checksum 01111110 01111110 Address Control Protocol Payload CRC Checksum Delimiting fields at the beginning and ending of the frame. Just require bit stuffing! Address field always uses the value 11111111, signifying that every transmission is a broadcast! Control field always uses the value 00000011, signifying that every transmission is unnumbered (i.e., sliding windows are not supported!). • Protocol field to identify the type of data in the Payload field: • Protocol 00000000 00100001 means the payload is an IP datagram. • Protocol 11000000 00100001 means the payload is link control data, used to establish, configure, and test the particular link being used. • Protocol 10000000 00100001 means the payload is network control data, used to identify the Network Layer protocol being used (e.g., IP, AppleTalk, OSI, DECnet). Payload field containing the Network Layer bits that were handed down to it, or the link control or network control message being relayed. Checksum field to again perform our old friend, the Cyclic Redundancy Check! CS 447 Chapter 5 Page 128

  14. ASYNCHRONOUS TRANSFER MODE VPI VCI PTI CLP HEC Payload VPI VCI PTI CLP HEC Payload The Virtual Path Identifier (VPI) specifies the number of a particular path that several virtual circuits take through the network node; by hierarchically identifying such routes, individual nodes may just use this prefix to forward a cell, rather than having to examine the entire path/circuit sequence. The Virtual Circuit Identifier (VCI) completes the identification of the particular virtual circuit being used. • The Payload Type Identifier (PTI) signifies the sort of data being transmitted: • 000 and 001 signify uncongested user data cells (with a Final bit on the end). • 010 and 011 signify congested user data cells (with a Final bit on the end). • 100 and 101 signify maintenance info (locally or end-to-end). • 110 signifies the cell is relaying end-to-end congestion info. The Cell Loss Priority (CLP) bit is used to distinguish high- and low-priority traffic. The Header Error Check (HEC) byte is a CRC remainder for just the header. The 48-byte Payload includes the data and any AAL (ATM Adaptation Layer) headers that might have been added at the protocol layer above ATM. CS 447 Chapter 5 Page 129

  15. MEDIUM ACCESS CONTROL 00:00:00 00:00:01 00:00:02 00:00:03 00:00:04 00:00:05 00:00:02 00:00:03 00:00:04 00:00:05 00:00:03 00:00:04 00:00:05 frame w/reserv. Various algorithms have been formulated for providing access to a shared transmission channel to multiple independent stations. frame frame frame frame Contention Systems frame frame frame Carrier Sense Systems reserv. frame Reservation Systems CS 447 Chapter 5 Page 130

  16. CSMA/CD 110100010101001010101010 I haven’t heard anything for a while, so I’m sending! 0011100100001010111100101001011011110100010101001010101010 111000111110100111100 Everything I’ve heard so far is exactly what I’ve been sending! I haven’t heard anything for a while, so I’m sending! 100010101110100101010101101001011010101000010110101010101010101001010111010101000110111001010 That’s not what I sent! COLLISION!!! That’s not what I sent! COLLISION!!! CS 447 Chapter 5 Page 131

  17. IEEE 802 STANDARDS The IEEE 802 Local and Metropolitan Area Network Standards Committee has the basic charter to create, maintain, and encourage the use of IEEE/ANSI and equivalent IEC/ISO JTC 1 standards primarily within layers 1 and 2 of the OSI (Open System Interconnection) Reference Model. The committee was formed in February 1980 and met at least three times per year as a Plenary body ever since that time. An explicit objective since inception has been the goal of establishing international standards in JTC 1. The IEEE series of standards are known as IEEE 802.xxx and the JTC 1 series of equivalent standards are known as ISO 8802-nnn. In the IEEE 802 context, "local" means campus and "metropolitan" means intra-city. CS 447 Chapter 5 Page 132

  18. IEEE 802.1: HIGHER-LAYER LAN PROTOCOLS Ethernet Internetworking standards for bridging different LAN and MAN protocols. FDDI DQDB Token Ring CS 447 Chapter 5 Page 133

  19. IEEE 802.2: LOGICAL LINK CONTROL Application Layer Presentation Layer • The LLC sits on top of the Medium Access Control sublayer of the Data Link Layer, and is responsible for: • Framing Network Layer packets • Link synchronization • Message acknowledgement • Error detection and possible recovery • Flow control Session Layer Transport Layer Network Layer Data Link Layer Logical Link Control Sublayer Medium Access Control Sublayer Current Status: INACTIVE Physical Layer CS 447 Chapter 5 Page 134

  20. IEEE 802.3: ETHERNET 10Base-T twisted pair Ethernet connection with RJ-45 jack 10Base-2 coax Ethernet connection with T-junction tap 10Base-5 coax Ethernet cable, capable of 10Mbps 10Base-F fiber optics Ethernet hub CS 447 Chapter 5 Page 135

  21. ETHERNET CONFIGURATIONS CS 447 Chapter 5 Page 136

  22. ETHERNET FRAME FORMAT Preamble Start Destination Address Source Address Length Data Padding Checksum Preamble Start Destination Address Source Address Length Data Padding Checksum Preamble: Seven Manchester-encoded 10101010-bytes to enable synchronization. Start: One Manchester-encoded 10101011-byte to delimit the start of the frame. Destination Address: 2- or 6-byte Ethernet card address, burned into the card. Source Address: 2- or 6-byte Ethernet card address, burned into the card. Length: 2-byte length of the data field (range: 0-1500 bytes). Data: The actual data handed down from the Network Layer. Padding: 0-46 bytes of dummy info, to ensure a 64-byte minimum frame length. Checksum: 4-byte Cyclic Redundancy Check. CS 447 Chapter 5 Page 137

  23. BINARY EXPONENTIAL BACKOFF ALGORITHM COLLISION! Pick a wait time between 0 and 7: COLLISION! Pick a wait time between 0 and 3: COLLISION! Pick a wait time between 0 and 1: COLLISION! Pick a wait time between 0 and 3: COLLISION! Pick a wait time between 0 and 1: COLLISION! Pick a wait time between 0 and 7: 0 1 5 2 1 3 00:00:02 00:00:00 00:00:01 00:00:02 00:00:00 00:00:00 00:00:01 00:00:01 00:00:03 00:00:00 00:00:03 00:00:04 00:00:02 00:00:01 00:00:00 00:00:01 00:00:00 00:00:05 frame frame frame frame frame frame frame frame When a collision does occur on Ethernet, each station must retransmit, but they’d like to avoid another collision, so they independently generate random wait times before their attempted retransmissions. CS 447 Chapter 5 Page 138

  24. IEEE 802.4: TOKEN BUS P=4; S=7 P=7; S=6 P=2; S=9 P=4; S=7 P=1; S=3 P=5; S=4 P=8; S=5 P=9; S=1 P=3; S=8 P=6; S=2 Current Status: DISBANDED To preserve the simplicity of the bus structure, while eliminating the unpleasantness of collisions, 802.4 passes a special “token” from station to station, using a prearranged predecessor/successor numbering system. When a station receives the token, it has “permission” to transmit normally across the bus. It is allowed to transmit for a certain length of time, then it must pass the token to its successor. CS 447 Chapter 5 Page 139

  25. IEEE 802.5: TOKEN RING Current Status: DISBANDED CS 447 Chapter 5 Page 140

  26. IEEE 802.6: DISTRIBUTED QUEUE/DUAL BUS empty slot empty slot empty slot fullslot fullslot fullslot Access Node Access Node Access Node Access Node Access Node Access Node Slot Generator Bus Terminator Bus Terminator Slot Generator empty slot empty slot empty slot fullslot fullslot fullslot Two buses are maintained, with data flowing in opposite directions, and every networked machine connected to both buses. Each bus has a head-end that generates 53-byte cells, which can be filled by the machines that are passed according to an access protocol. Current Status: DISBANDED CS 447 Chapter 5 Page 141

  27. MORE DISBANDED GROUPS • IEEE 802.7: Broadband • Implementing broadband on LANs using coaxial cable • IEEE 802.8: Fiber Optics • Physical Layer interfaces and MAC sublayer protocols • IEEE 802.9 Integrated Data And Voice • Bundling ISDN and Ethernet onto a single cable • IEEE 802.10: Interoperable LAN Security • Security functions that could be used in LANs and MANs • IEEE 802.12: Demand Priority Access Method • Effort to combine benefits of Ethernet and Token Ring • IEEE 802.13: The Group That Shall Not Be Named • Triskaidekaphobia • IEEE 802.14: Cable Modems • Cable operators ultimately set up their own standard Current Status: DISBANDED CS 447 Chapter 5 Page 142

  28. IEEE 802.11: WIRELESS LANS This “Wi-Fi” protocol divide the world into three regions for the purpose of frequency allocation. • Region 1 (includes Europe and Africa) • Digital European Cordless Telecommunications • High Performance European Radio LAN • GroupeSpeciale Mobile • Region 2 (includes United States) • FCC-governed Spread Spectrum • Personal Communication Services Region 3 (includes Japan and Australia) • The standard includes specs on: • MAC protocol (no collision detection) • Encryption algorithm • Minimal battery capabilities • Vendor licensing CS 447 Chapter 5 Page 143

  29. IEEE 802.15: WIRELESS PANS Personal area networks include Bluetooth’s “piconets”, small localized networks of devices that communicate with each other by perpetually hopping between frequencies in a manner that prevents mutual interference as well as external eavesdropping. By sharing “slave” devices across piconets, “master” devices can form larger “scatternets”. CS 447 Chapter 5 Page 144

  30. IEEE 802.16: BROADBAND WIRELESS ACCESS With new high-speed wireless techniques being developed, it’s possible to advance from the small wireless LAN system to a more sophisticated wireless MAN system, potentially supplying powerful, upgradeable communications systems to residential and industry customers. Local Multipoint Distribution System WirelessHUMAN (Wireless High-Speed Unlicensed Metropolitan Network) Systems CS 447 Chapter 5 Page 145

  31. IEEE 802.17: RESILIENT PACKET RING A ring-based protocol with prioritization and fairness built in, it eliminates the token used in FDDI and the need for master nodes of DQDB. • TDM channels are established dynamically. • Channel bandwidths are allowed to change dynamically. • Multicasting is built in to the protocol. • Simplex channels are used, supporting asymmetric communication and high bandwidth utilization simultaneously. CS 447 Chapter 5 Page 146

  32. AND THE REST... IEEE 802.18: Radio Regulatory Technical Advisory Group IEEE 802.21: Media Independent Handoff Monitor and actively participate in ongoing radio regulatory activities, at the national and international levels. Develop and supports algorithms enabling seamless handover between networks of the same type as well as handover between different network types (e.g., cellular, mobile, packet radio, wireless LAN, and wireless PAN). IEEE 802.19: Coexistence Technical Advisory Group Develop and maintain policies defining the responsibilities of 802 standards developers to address issues of coexistence with existing standards and other standards under development. IEEE 802.22: Wireless Regional Area Network Develop a standard for a cognitive radio-based PHY/MAC/air interface for use by license-exempt devices on a non-interfering basis in spectrum that is allocated to the TV Broadcast Service. IEEE 802.20: Mobile Broadband Wireless Access Develop the specification for an efficient packet-based air interface that is optimized for the transport of IP-based services. The goal is to enable worldwide deployment of affordable, ubiquitous, always-on and interoperable multi-vendor mobile broadband wireless access networks that meet the needs of business and residential end user markets. IEEE 802.23: Emergency Service Working Group Define a media-independent framework to provide consistent access and data that facilitate compliance to applicable civil authority requirements for transferring data required by an emergency services request. CS 447 Chapter 5 Page 147

  33. VIRTUAL LANS VLAN 1 VLAN 2 VLAN 4 VLAN 1 VLAN 2 Router VLAN 1 VLAN 2 VLAN 3 VLAN 1 VLAN Switch VLAN 2 VLAN 3 VLAN Switch VLAN 4 • There are occasions when it is beneficial to give a collection of endstations the characteristics of a LAN, in spite of their lack of physical proximity. • The Benefits of the VLAN Approach: • VLANs improve security by isolating groups. High-security users can be grouped into a VLAN, possibly on the same physical segment, and no users outside that VLAN can communicate with them. • VLANs facilitate broadcast control by allowing stations to be grouped by functionality instead of physical location. • VLANs also facilitate network management by allowing configuration changes to take place without recabling. CS 447 Chapter 5 Page 148

  34. MULTIPROTOCOL LABEL SWITCHING MPLS was originally designed as a mechanism for speeding up the routing of IP packets over ATM networks. Advances in switching hardware have transformed the main advantage of MPLS into its ability to support multiple service models and to perform traffic management. CS 447 Chapter 5 Page 149

  35. MPLS LABEL STACK Label TC BS TTL Internal MPLS routers examine only the top label in a packet’s label stack, disregarding all lower level information (ATM, PPP, Frame Relay, etc.). Label TC BS TTL Label: Label lookup ID number, which is swapped as the packet progresses from router to router within the MPLS network. Traffic Class: Signifies quality of service priority and explicit congestion notification. Bottom-of-Stack Flag: When set, indicates the last label in the packet’s label stack. Time-to-Live: 8-bit hop countdown before packet is discarded as undeliverable. The label stack essentially establishes a hierarchical virtual private network (VPN) that provides traffic isolation and differentiation without substantial overhead. CS 447 Chapter 5 Page 150

  36. TRACING A WEB PAGE REQUEST browser Having examined the protocol stack from the Application Layer, through the Transport and Network Layers, and all the way down to the Data Link Layer, let’s pull it all together and trace what happens as a student attaches a laptop to the campus network and accesses Google... DNS server Charter network 142.45.0.0/15 SIUE network 148.63.13.0/11 web page web server Google’s network 64.233.160.0/19 64.233.169.105 CS 447 Chapter 5 Page 151

  37. STUDENT CONNECTS TO INTERNET DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP 1. The connecting laptop needs to get its own IP address, the address of a first-hop router, and the address of a DNS server: use DHCP (Dynamic Host Configuration Protocol) • The DHCP request is encapsulated in UDP, then in IP, and ultimately in Ethernet • An Ethernet frame is broadcast (with destination FFFFFFFFFFFF) on the LAN, and is received at the router running the DHCPserver router (runs DHCP) • The Ethernet payload is demultiplexedto IP, then to UDP, and finally to DHCP CS 447 Chapter 5 Page 152

  38. STUDENT CONNECTS TO INTERNET (continued) DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP 2. The DHCP server formulates a DHCP ACK , containing the client’s IP address, the IP address of the first-hop router for client, and the name and IP address of the DNS server • The DHCP server encapsulates the ACK, that frame is forwarded through the LAN, back to the student’s machine, where it is demultiplexed router (runs DHCP) • The DHCP client (i.e., the student’s laptop) has now received the DHCP ACK reply, with its own IP address, the IP address of its first-hop router, and the name and address of the DNS server CS 447 Chapter 5 Page 153

  39. RETRIEVING GOOGLE’S ADDRESS ARP ARP Eth Phy ARP reply ARP query DNS UDP IP Eth Phy DNS DNS DNS 3. Before sending its HTTP request, the student’s laptop needs the IP address of www.google.com: Use DNS • A DNS query is created and encapsulated in UDP, IP, and Ethernet. In order to send the frame to the router, the MAC address of the router interface is needed: use ARP • An ARP queryis broadcast and received by the router, which replies with an ARP replygiving the MAC address of the router interface • The client now knows the MAC address of its first-hop router, so it can now send a frame containing DNS query CS 447 Chapter 5 Page 154

  40. RETRIEVING GOOGLE’S ADDRESS (continued) DNS UDP IP Eth Phy DNS UDP IP Eth Phy DNS UDP IP Eth Phy DNS DNS DNS DNS DNS DNS DNS DNS DNS DNS server Charter network 142.45.0.0/15 • An IP datagram containing the DNS query is forwarded via the LAN switch from the client to its first-hop router • The IP datagram is forwarded from the campus network into the Charter network, and then routed (via tables created by RIP, OSPF, IS-IS and/or BGProuting protocols) to the DNS server • The datagram is demultiplexedto the DNS server, which replies to the client with the IP address of www.google.com CS 447 Chapter 5 Page 155

  41. ESTABLISHING TCP CONNECTION SYN SYN SYN SYN SYN SYN SYN HTTP TCP IP Eth Phy TCP IP Eth Phy HTTP SYNACK SYNACK SYNACK SYNACK SYNACK SYNACK SYNACK • To send its HTTP request, the client first opens a TCP socketto the web server • A TCP SYN segment(step 1 in the 3-way handshake) is inter-domain routed to the web server • The web server responds with a TCP SYNACK (step 2 in the 3-way handshake) • The TCP connection is now established web server 64.233.169.105 CS 447 Chapter 5 Page 156

  42. INVOKING THE APPLICATION HTTP TCP IP Eth Phy HTTP TCP IP Eth Phy HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP HTTP • TheHTTP requestis sent into the TCP socket • An IP datagram containing the HTTP request routed to Goggle’s web server • That web server responds with the HTTP reply (containing the Google web page) • An IP datagram containing the HTTP reply is routed back to the client web server • The Google web page isdisplayed 64.233.169.105 CS 447 Chapter 5 Page 157

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