6.263 Data Communication Networks

Transcription

1 6.263 Data s Lecture 1: Computer s (some slides are taken from I. Stoica and N. Mckewon) Dina Katabi dk@mit.edu

2 Administrative Information Instructors Dina Katabi Office Hours in 32-G936 on Tue 2:30pm-3:30pm Muriel Médard ) Office Hours in 32-D626 on Wed 1:45pm-2:245pm Michael Lewy Phone (617) Course Webpage 2

3 Grades HW (weekly) 10% Project 30% Midterm 25% Final 35% 3

4 Syllabus Routing & Switching Queuing Wireless Congestion Control Compression & Coding Security 4

5 A Taxonomy of s networks can be classified based on the way in which the nodes exchange information: Switched Broadcast Circuit-Switched Packet-Switched Datagram Virtual Circuit 5

6 Broadcast vs. Switched s Broadcast networks Information transmitted by any node is received by every node in the network Ex: Broadcast Ethernet, wireless LANs Need to coordinate the access to the shared medium MAC Does it scale? Switched networks Links are point-to-point Ex: WANs (Telephony, Internet) Routing becomes harder 6

7 A Taxonomy of s networks can be classified based on the way in which the nodes exchange information: Switched Broadcast Circuit-Switched Packet-Switched Datagram Virtual Circuit 7

8 Circuit Switching Three phases: 1. Circuit Establishment: allocates certain bandwidth and establish a path 2. Data Transfer 3. Circuit Termination If circuit not available: Busy signal Ex: Telephone networks (1) (2) Host 1 Host 2 Node 1 Node 2 processing delay at Node 1 DATA propagation delay between Host 1 and Node 1 (3) 8

9 Circuit Switching: Multiplexing/Demultiplexing Switch Frames Slots = One way for circuit switching is TDM: Time divided into frames and frames divided in slots Relative slot position inside a frame determines which conversation the data belongs to E.g., slot 0 belongs to the red conversation Needs synchronization between sender and receiver If a conversation does not use its circuit the capacity is wasted! 9

10 A Taxonomy of s networks can be classified based on the way in which the nodes exchange information: Switched Broadcast Circuit-Switched Packet-Switched Datagram Virtual Circuit 10

11 Datagram Packet Switching Data is sent in Packets (header and trailer contain control info, e.g., source and destination addresses) Host 1 Host 2 Node 1 Node 2 Header Data Trailer Per-packet routing At each node the entire packet is received, stored, and then forwarded (Store-and- Forward s) No capacity is allocated E.g. The Internet transmission time of Packet 1 at Host 1 Packet 1 Packet 2 Packet 3 propagation delay between Host 1 & Node 2 Packet 1 Packet 2 Packet 3 processing delay of Packet 1 at Node 2 Packet 1 Packet 2 Packet 3 11

12 Datagram Packet Switching: Multiplexing/Demultiplexing Router Queue Multiplex using a queue Routers need memory Queuing introduces jitters Demultiplex using packet headers 12

13 Datagram Packet Switching Packets in a flow may not follow the same path (depends on routing as we will see later) Host C Host A Host D Node 1 Node 2 Node 3 Node 5 Host B Node 4 Node 6 Node 7 Host E 13

14 Some Differences Between Datagram & Circuit Switching Circuit-switching Datagram-Switching Guaranteed capacity No guarantees (best effort) Capacity is wasted if data is bursty Before sending data establishes a path All data in a single flow follows one path No Reordering of data More efficient Send data immediately Different packets might follow different paths Packets may be reordered 14

15 A Taxonomy of s networks can be classified based on the way in which the nodes exchange information: Switched Broadcast Circuit-Switched Packet-Switched Datagram Virtual Circuit 15

16 Virtual-Circuit Packet Switching Hybrid of circuit switching and packet switching Data is transmitted as packets All packets from one packet stream are sent along a pre-established path (i.e., a virtual circuit) No capacity guarantees, but guarantees no reordering of packets E. g., ATM networks 16

17 Timing of Virtual-Circuit Packet Switching Host 1 Host 2 Node 1 Node 2 VC establishment propagation delay between Host 1 and Node 1 Packet 1 Data transfer Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 VC termination Packet 3 17

18 Virtual Circuits guarantees all packets in a flow follow one path Host C Host A Host D Node 1 Node 2 Node 3 Node 5 Host B Node 4 Node 6 Node 7 Host E 18

19 In Reality The Internet is a datagram network But part of the Internet use circuit-switching (Phone links) or virtual circuit (ATM) The Internet works by abstracting an ATM region or a circuit-switched region as a single link 19

20 Routing: How to deliver packets from source to destination? 20

21 Addressing A network is a graph Each node should have an address E.g., IP addresses, Phone numbers 21

22 How can we deliver packets? We need to send packets form source to destination Hierarchical Addressing Source Routing Virtual Circuit Routing & forwarding are both done by the routers 22

23 Internet Routing (see BG for Algs.) 23

24 Forwarding A What s the difference between routing and forwarding? Routing is finding the path Router R1 R2 C F B Forwarding is the action of sending the packet to the next-hop toward its destination R3 E G Each router has a forwarding table Forwarding tables are created by a routing protocol Forwarding: For every packet, we need to look up the next hop toward its destination in the table Dest. Addr A B C E F Next-hop R1 R2 R1 R3 R1 Forwarding needs to be fast G R3 24

25 Picture of the Internet Interior router AS-1 Border router AS-3 AS-2 Internet: A collection of ASes Routing: Intra-Domain Routing Inter-Domain Routing 25

26 Intra-Domain Routing Algorithms Routing algorithms view the network as a graph A node is a router or a whole network region Examples of link cost: Distance, load, price, congestion/delay, Problem: find minimum cost path between two nodes (Shortest path)» Factors Semi-dynamic topology (deal with link failures) Dynamic load Policy C 4 B 3 9 A 1 D 1 1 E 6 2 F 26

27 Problem: Shortest Path Routing Objective: Determine the route from (R 1,, R 7 ) to R 8 that minimizes the cost. R R 2 R R R R R 6 R

28 Solution is simple by inspection... (in this case) R R 3 R R 5 R 4 3 R R 6 R 8 2 The shortest paths from all sources to a destination (e.g., R 8 ) is the spanning tree routed at that destination. 28