1/21/10 1 MS in Telecommunications TCOM 500: Modern Telecommunications Dr. Bernd-Peter Paris George Mason University Spring 2009 MS in Telecommunications INTRODUCTION Paris 2 TCOM 500: Modern Telecommunications1/21/10 2 MS in Telecommunications Protocol “Layers” Networks are complex! many “pieces”: • hosts • routers • links of various media • applications • protocols • hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? Paris 3 TCOM 500: Modern Telecommunications MS in Telecommunications Organization of air travel a series of steps ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing airplane routing Paris 4 TCOM 500: Modern Telecommunications1/21/10 3 MS in Telecommunications ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing departure airport arrival airport intermediate air-traffic control centers airplane routing airplane routing ticket (complain) baggage (claim gates (unload) runway (land) airplane routing ticket baggage gate takeoff/landing airplane routing Layering of airline functionality Layers: each layer implements a service • via its own internal-layer actions • relying on services provided by layer below Paris 5 TCOM 500: Modern Telecommunications MS in Telecommunications Layers Provide Services Example: • The network layer provides end-to-end delivery of packets • Using the ability to physically transmit packets between adjacent nodes. • Note: network layer does not need to know how packets are sent. Paris 6 TCOM 500: Modern Telecommunications Layer Provides service to upper layer Using service provided by lower layer1/21/10 4 MS in Telecommunications Why layering? Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieces • layered reference model for discussion modularization eases maintenance, updating of system • change of implementation of layer’s service transparent to rest of system • e.g., change in gate procedure doesn’t affect rest of system layering considered harmful? Paris 7 TCOM 500: Modern Telecommunications MS in Telecommunications Internet protocol stack application: supporting network applications • FTP, SMTP, HTTP transport: host-host data transfer • TCP, UDP network: routing of datagrams from source to destination • IP, routing protocols link: data transfer between neighboring network elements • PPP, Ethernet physical: bits “on the wire” application transport network link physical Paris 8 TCOM 500: Modern Telecommunications1/21/10 5 MS in Telecommunications For Reference: OSI model Paris 9 TCOM 500: Modern Telecommunications MS in Telecommunications message segment datagram frame source application transport network link physical Ht Hn Hl M Ht Hn M Ht M M destination application transport network link physical Ht Hn Hl M Ht Hn M Ht M M network link physical link physical Ht Hn Hl M Ht Hn M Ht Hn Hl M Ht Hn M Ht Hn Hl M Ht Hn Hl M router switch Encapsulation Paris 10 TCOM 500: Modern Telecommunications1/21/10 6 MS in Telecommunications Where do Layers Live? Paris 11 TCOM 500: Modern Telecommunications Host Hardware (Network Interface Cards) Operating System Application (Program) Application (Program) Socket Socket Driver Application Transport Network Link Physical MS in Telecommunications Illustration: Wireshark Packet Capture Paris 12 TCOM 500: Modern Telecommunications Downloading a single webpage creates a lot of traffic.1/21/10 7 MS in Telecommunications The HTTP Request • The 4th packet contains the application layer request message. • Application layer protocol is HTTP. • Note: HTTP is in human-readable format. MS in Telecommunications The HTTP Response Paris 14 TCOM 500: Modern Telecommunications • The response from the server begins in the 6th packet. • Actual data is preceded by HTTP header. • Again, HTTP protocol is human readable.1/21/10 8 MS in Telecommunications TCP header in Request Packet Paris 15 TCOM 500: Modern Telecommunications TCP header contains: • Port numbers • Sequence and acknowledgement numbers • Header length • Flags • Window size • Checksum MS in Telecommunications Services Provided by TCP • TCP stands for Transport Control Protocol. • It is a standard protocol in the TCP/IP suite of Internet protocols. • TCP provides reliable, connection-oriented transport service to application layer programs. • TCP transports streams of data, not just isolated packets. • The fields present in the TCP header provide a lot of information about what this service entails. • TCP is easily the most complex protocol in the TCP/IP suite. Paris 16 TCOM 500: Modern Telecommunications1/21/10 9 MS in Telecommunications TCP Port Numbers • Port numbers (in conjunction with IP addresses) provide addressing capabilities within a host. • Usually a host runs multiple networked applications at the same time. • The operating system decides based on the two port numbers and IP addresses which application should receive an incoming packet. • This is also called “multiplexing”, it enables multiple simultaneous connections. Paris 17 TCOM 500: Modern Telecommunications MS in Telecommunications TCP Reliable Transport Service • TCP promises the application layer that it will deliver packets to the destination correctly and in order. • To do so, it employs: • Sequence numbers – for correct ordering, • Acknowledgment numbers – for informing other side which packets have been received correctly, • Checksum – allows receiver to determine if packet is correct. Paris 18 TCOM 500: Modern Telecommunications1/21/10 10 MS in Telecommunications TCP Flow Control • It is possible that one side of a TCP connection may send packets faster than they can be processed by the receiver. • E.g., a heavily loaded server. • The “window size” in TCP specifies how many unacknowledged bytes may be sent. • If acknowledgement does not arrive, sender is essentially stalled. • Timeouts protect against deadlock. • Equally important, Window Flow Control provides distributed load balancing to protect the network. • Makes it difficult to