Introduction to This Class • • • • • Instructor: Stephan Bohacek bohacek@udel.edu, 302-831-4274 http://www.eecis.udel.edu/~bohacek Textbook: Kurose and Ross: Computer Networks (6th edition) Web page has – – – – – – syllabus class notes videos lectures of some topics (most, but not all topics are covered) homework assignments project assignments announcements • Issues – Programming languages: C++ or java? – Computers: • >=2GB ram? • Windows or Linux? Introduction to Data Networking Kurose and Ross: Computer Networking: A Top Down Approach • Chapter 1: Overview and general principles – Protocol stack – Sharing • Statistic multiplexing • Packet switching • Circuit switching – Performance of packet switching networks • Chapter 2: Application layer – TCP and UDP, multiplexing and ports – Applications • http, ftp, email, DNS, P2P, DHT • Chapter 3: Transport layer – Tools for reliable transport – TCP • Chapter 4: Network layer – IP and IPv6 – NAT – Routing • Intra-network routing • Inter-network routing • Chapter 5: Datalink and MAC layer – – – – – Multiple access ARP Ethernet Switches and hubs Link layer routing The Internet • What is the longest time that you have gone without using “the Internet?” • What is the Internet? • How do you use the Internet? • Do you only watch TV over the Internet, or do you have cable? • Skype? • Facebook? • IM? • Twitter? • Smart phone? • Do you only have a data plan on your phone? What is the Internet Networking Basics • • • • Core components of the Internet – the protocol stack Multiplexing, circuit switching, and packet switching Loss and delays The structure of the Internet Networking Basics • • • • Core components of the Internet – the protocol stack Multiplexing, circuit switching, and packet switching Loss and delays The structure of the Internet The Internet • Objective: split an application across multiple machines – Web client and web server • Option 1: users get a DVD of the NY Times every day • Option 2: Split the NY Times application into 2 parts. The client part of the application runs on the users machine and the server part runs in NY Times headquarters. NY Times reporters can load articles on to the server and the client can get articles from the server and read them on their machine – Email • Option 1: Write a letter on paper and mail it to its destination. • Option 2: Write an email on one machine and allow this message to be sent to the destination machine – Buy something • Option 1: go to the store, see item, pick up item and pay for it • Option 2: The client machine can explore all items via a GUI. When an item is selected for purchase, the vendor is notified, credit card is charged, and the item is sent • Key derived requirement: send message between machines – These are distributed applications • The task of sending messages between machine is very complicated – To solve this task, we subdivide it into subtasks. Wht? The Internet • Objective: split an application across multiple machines • Key derived requirement: send message between machines – These are distributed applications • The task of sending messages between machine is very complicated – To solve this task, we subdivide it into subtasks. Why – Each task is easier to solve – If we have multiple solutions for a subtask, perhaps we can use different solutions at different times • E.g., a solution for wired networks and a solution for wireless. It is easier to solved these specific tasks then a very general one (e.g., one that must for for wired and wireless networks) • Subtasks are called the protocol stack – – – – – Application layer Transport layer Network layer Link layer Physical Layer (Not covered in this course. Communication theory covers this topic) Application Layer – where the applications live Protocol Stack 7. Application 4. Transport 3. Network 2. Data link 1. Physical • Email: – • Web: – • Rules/protocols for how the end-hosts gets a web page from the web servers Question: – – • Rules/protocols for how an end-host gets mail from the mail server How is a networking application different from a non-networking application (e.g., MS Word). That is, why, when talking about networking application, do we focus on protocols, but do not focus on protocols when discussing nonnetworked applications such as MS-Word? Answer: The networking applications must communicate, and rules are required to define the communication. Roles that end-hosts play: – – Client, server, and peer The client asks the server for a service. • • • – • The protocol stack E.g., The client asks the server to send a mail for it. The client asks the server for a web page The client asks the server to translate a web address to an IP address. Peer: A host can act as both a client and a server. But usually in one transaction, the host takes only one role Application layer defined messages Layers 2-4 Protocol Stack 7. Application 4. Transport 3. Network 2. Data link 1. Physical Definition: End-host Which are the end-host? client server Routers The protocol stack Layers 2-4 • • End-hosts Applications – – – – • Packets Routers and gateways and groups of routers (ISPs) Links Protocols – – • • – – – – • Web Email File transfer File sharing Goal: move messages from between end-hosts Approach: break the problem into little pieces. Each piece is a layer in the “protocol stack” TCP UDP Fiber Coaxial Twisted pair Wireless The protocol stack client server Layers 2-4 • • End-hosts Applications – – – – • • • • Web Email File transfer File sharing Packets client server Routers and gateways and groups of routers (ISPs) Links Protocols – – TCP UDP – – – – Fiber Coaxial Twisted pair Wireless The protocol stack Top down approach of breaking problems into small pieces 4. Transport layer • Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) (http://www.youtube.com/watch?v=RjrEQaG5jPM • Congestion Control: The server should send data as fast as possible, but not too fast • TCP provides these features (services), while UDP does not 3. Network layer (could be called the routing layer, but it isn’t) • The packets must find their way through the network. • Each packet has the IP address of the destination • By examining the IP address, routers decide where to send the packet next 2. Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include • congestion control, media access, error detection/correction Layers 2-4 • • End-hosts Applications – – – – • • • • Web Email File transfer File sharing Top down approach of breaking problems into small pieces ….. 2. Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include congestion control, media access, error detection/correction Packets Routers and gateways and groups of routers (ISPs) Links Protocols – – TCP UDP – – – – Fiber Coaxial Twisted pair Wireless The protocol stack • • Media access. The “air” is a shared medium. If two nodes transmit at the same time, there will be a collision. Thus, a scheme must be developed to determine which node transmits when. Error detection/correction. If interference does occur, then errors might occur. If an error is detected, then 1. the error could be corrected with forward error correction, or 2. the receiving link could request a retransmission Layers 2-4 • • End-hosts Applications – – – – • • • • Web Email File transfer File sharing Packets Routers and gateways and groups of routers (ISPs) Links Protocols – – TCP UDP – – – – Fiber Coaxial Twisted pair Wireless The protocol stack client server Top down approach of breaking problems into small pieces 4. Transport layer 1. Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) 2. Congestion Control: The server should send data as fast as possible, but not too fast 3. TCP provides these features (services), while UDP does not 3. Network layer (could be called the routing layer, but it isn’t) 1. The packets must find their way through the network. 2. Each packet has the IP address of the destination 3. By examining the IP address, routers decide where to send the packet next 2. Link Layer or MAC layer 1. Links connect the routers/gateways and end-hosts 2. This layer provides logical and control for communicating across links. 3. Services that this layer might provide include 1. congestion control, media access, error detection/correction 1. Physical layer 1. Logical bits are encoded as physical quantities, e.g., as voltage levels, as shifts in phase, … 2. This course does not cover the physical layer Protocols • • End-hosts Applications – – – – • Packets – – • • TCP UDP Routers and gateways and groups of routers (ISPs) Links – – – – • Web Email File transfer File sharing Fiber Coaxial Twisted pair Wireless Protocols The protocol stack protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Hi TCP connection request Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 <file> time Core components • • End-hosts Applications – • Packets – – • • TCP UDP Routers and gateways and groups of routers (ISPs) Links – • ? ? Protocols Core components • • End-hosts Applications – – – – • Packets – – • • TCP UDP Routers and gateways and groups of routers (ISPs) Links – – – – • Web Email File transfer File sharing Fiber Coaxial Twisted pair Wireless Protocols Application Layer – where the applications live • • End-hosts Applications – – – – • • – • • TCP UDP Fiber Coaxial Twisted pair Wireless Protocols The protocol stack Rules/protocols for how an end-host gets mail from the mail server Web: – Routers and gateways and groups of routers • (ISPs) Links – – – – • Email: Rules/protocols for how the end-hosts gets a web page from the web servers Question: – Packets – – • Web Email File transfer File sharing • – How is a networking application different from a non-networking application (e.g., MS Word). That is, why, when talking about networking application, do we focus on protocols, but do not focus on protocols when discussing nonnetworked applications such as MS-Word? Answer: The networking applications must communicate, and rules are required to define the communication. Roles that end-hosts play: – – Client, server, and peer The client asks the server for a service. • • • – E.g., The client asks the server to send a mail for it. The client asks the server for a web page The client asks the server to translate a web address to an IP address. Peer: A host can act as both a client and a server. But usually in one transaction, the host takes only one role Layers 2-4 • • End-hosts Applications – – – – • • Routers and gateways and groups of routers (ISPs) Links Protocols – – – – • Web Email File transfer File sharing Packets – – • Which are the end-host? TCP UDP Fiber Coaxial Twisted pair Wireless The protocol stack client server Routers Layers 2-4 • • End-hosts Applications – – – – • • Routers and gateways and groups of routers (ISPs) Links Protocols – – – – • Why is this a good approach? 1.Small problems are easier to understand/solve. 2.Different solutions can be mixed and matched Packets – – • Web Email File transfer File sharing Goal: move messages from server to the client Approach: break the problem into little pieces. Each piece is a layer in the “protocol stack” TCP UDP Fiber Coaxial Twisted pair Wireless The protocol stack client server Layers 2-4 • • End-hosts Applications – – – – • • • • Web Email File transfer File sharing Packets client server Routers and gateways and groups of routers (ISPs) Links Protocols – – TCP UDP – – – – Fiber Coaxial Twisted pair Wireless The protocol stack Top down approach of breaking problems into small pieces 4. Transport layer • Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) (http://www.youtube.com/watch?v=RjrEQaG5jPM • Congestion Control: The server should send data as fast as possible, but not too fast • TCP provides these features (services), while UDP does not 3. Network layer (could be called the routing layer, but it isn’t) • The packets must find their way through the network. • Each packet has the IP address of the destination • By examining the IP address, routers decide where to send the packet next 2. Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include • congestion control, media access, error detection/correction Layers 2-4 • • End-hosts Applications – – – – • • • • Web Email File transfer File sharing Top down approach of breaking problems into small pieces ….. 2. Link Layer or MAC layer (link layer and MAC layer) • Links connect the routers/gateways and end-hosts • This layer provides logical and control for communicating across links. • Services that this layer might provide include congestion control, media access, error detection/correction Packets Routers and gateways and groups of routers (ISPs) Links Protocols – – TCP UDP – – – – Fiber Coaxial Twisted pair Wireless The protocol stack • • Media access. The “air” is a shared medium. If two nodes transmit at the same time, there will be a collision. Thus, a scheme must be developed to determine which node transmits when. Error detection/correction. If interference does occur, then errors might occur. If an error is detected, then 1. the error could be corrected with forward error correction, or 2. the receiving link could request a retransmission Layers 2-4 • • End-hosts Applications – – – – • • • • Web Email File transfer File sharing Packets Routers and gateways and groups of routers (ISPs) Links Protocols – – TCP UDP – – – – Fiber Coaxial Twisted pair Wireless The protocol stack client server Top down approach of breaking problems into small pieces 4. Transport layer 1. Reliability: The server must make sure that the client gets the data Congestion control (or lack there of) 2. Congestion Control: The server should send data as fast as possible, but not too fast 3. TCP provides these features (services), while UDP does not 3. Network layer (could be called the routing layer, but it isn’t) 1. The packets must find their way through the network. 2. Each packet has the IP address of the destination 3. By examining the IP address, routers decide where to send the packet next 2. Link Layer or MAC layer 1. Links connect the routers/gateways and end-hosts 2. This layer provides logical and control for communicating across links. 3. Services that this layer might provide include 1. congestion control, media access, error detection/correction 1. Physical layer 1. Logical bits are encoded as physical quantities, e.g., as voltage levels, as shifts in phase, … 2. This course does not cover the physical layer Protocols • • End-hosts Applications – – – – • Packets – – • • TCP UDP Routers and gateways and groups of routers (ISPs) Links – – – – • Web Email File transfer File sharing Fiber Coaxial Twisted pair Wireless Protocols The protocol stack protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Hi TCP connection request Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 <file> time Internet protocol stack • application: supporting network applications – FTP, SMTP, HTTP • transport: process-process data transfer application – 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” transport network link physical The protocol stack ISO/OSI reference model • • • presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions session: synchronization, checkpointing, recovery of data exchange Internet stack “missing” these layers! – these services, if needed, must be implemented in application – needed? application presentation session transport network link physical The protocol stack Layers 1-5 (7) • Why is L3 Communications called L3? • What does the L7 filter web page discuss? Why is it called L7 switch router Link PHY pkt pkt Application Transport Network Link PHY Link PHY Link PHY It was a dark and… pkt pkt pkt pkt pkt Link PHY It was a dark and… pkt pkt pkt Link PHY pkt pkt Network Link Link PHY PHY pkt pkt pkt pkt Link PHY Link PHY pkt pkt pkt pkt pkt Link PHY Network Link Link PHY PHY pkt pkt Application Transport Network Link PHY Internet components pkt pkt It was a dark and… It was a dark and… pkt pkt pkt Today – networking basics • • • • • Core components of the Internet – the protocol stack Multiplexing, circuit switching, and packet switching Loss and delays The structure of the Internet This lecture covers much of chapter 1 in the textbook. Circuit switching versus Packet switching • Packet switching brought the networking revolution • Circuit switching • Virtual circuit networking – A half-way point between packet switched and circuit switched networking Circuit switching • Circuit switching – Old style phone system – Each connection gets its own wire or bandwidth – Note: calls must be set-up. – E.g., • Me: operator, get my the president. • Operator: one moment please. • Then she plugs a cable into a socket so now I have a physical wired between me and the president. – Instead of each connection getting a whole wire, connections can share a wire via multiplexing – The first automatic circuit switching was developed by Almon Strowger – an undertaker. There were two undertakers in a small town and the switch board operator was the wife of the other undertaker. So Strowger invented an automatic circuit switch to rid both husband and wife of employment. Division of resources: FDM, TDM Frequency division multiplexing On each hop, the connection gets its own bandwidth phone end office 300 3400 toll office 100300 103400 end office 200300 203400 phone 300 3400 Frequency division multiplexing is used in •TV & radio •Cell phones (not so much today) Division of resources: FDM, TDM Time division multiplexing phone 1 byte for each channel every 1/8000 seconds Or 24×7×8000+overhead = 1.544Mbps (DS1 or T1) bytes 4321 4321 3 2 1 byte for each channel every 1/8000 seconds Or 28×24×7×8000+overhead = 44.736Mbps (DS-3) 1 4321 1 byte every 1/8000 seconds Or 7×8000=56Kbps (1Kbps=1000bps) (7 bits of data & 1 bit of control) Division of resources: FDM, TDM There are standard bit-rates that support multiplexing different numbers of calls Multiplex 28 DS1 = 28*24*64kbps + overhead = 44.736Mbps DS-3 Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1 STS is electrical and oc is optical OC3 = 155.52Mbps (150.336 payload) OC12 = 633.08 Mbps (601.344 payload) OC48 = 2.488Gbps (2.405Gbps) OC192 = 9.953Gbps (9.6Gbps payload) Time division multiplexing phone 1 byte for each channel every 1/8000 seconds Or 24×7×8000+overhead = 1.544Mbps (DS1 or T1) bytes 4321 4321 3 2 1 byte for each channel every 1/8000 seconds Or 28×24×7×8000+overhead = 44.736Mbps (DS-3) 1 4321 1 byte every 1/8000 seconds Or 7×8000=56Kbps (1kbps=1000bps) (7 bits of data 1 bit of control) • • • • Note all the control overhead: if the bit is 1, then payload is control. Lots of control is needed to setup a circuit. How is it possible to get channels at each hop? Also, if there is not data, then nothing is sent. This wastes data. But the circuit is yours, guaranteed! Division of resources: FDM, TDM There are standard bit-rates that support multiplexing different numbers of calls Multiplex 28 DS1 = 28*24*64kbps + overhead = 44.736Mbps DS-3 Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1 STS is electrical and oc is optical OC3 = 155.52Mbps (150.336 payload) OC12 = 633.08 Mbps (601.344 payload) OC48 = 2.488Gbps (2.405Gbps) OC192 = 9.953Gbps (9.6Gbps payload) Packet switching - Statistical multiplexing • • • • Data is in packets, not streams. Must be digital Each packet has an address A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward Packet format specification specifies where the address is data 1 client Server: address = 1 Division of resources: Packet Switching Packet switching - Statistical multiplexing • • • • Data is in packets, not streams. Must be digital Each packet has an address A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward If destination If destination is 1, then next hop is B A data 1 is 1, then next hop is C B If destination is 1, then next data 1 hop is data 1 C data 1 D client Server: address = 1 F Division of resources: Packet Switching E Packet switching - Statistical multiplexing • • • • • • Data is in packets, not streams. Must be digital Each packet has an address A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward No reservations are needed. First come first serve. Major benefit: – • If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else can use it. Major drawback: – What happens if two packets arrive at a switch and both need to go to the same output interface. Picture. One packet is either dropped, or is placed in a buffer. Either way, something bad has happened, the packet is gone or is delayed. This would never happen on a circuit switched network. queuing delay and packet loss Division of resources: Packet Switching Packet switching - Statistical multiplexing • • • • Data is in packets, not streams. Must be digital Each packet has an address A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward data 1 client other 1 other client Division of resources: Packet Switching data 1 Server: address = 1 Packet switching - Statistical multiplexing • • • • Data is in packets, not streams. Must be digital Each packet has an address A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward data 1 other data 1 client other 1 other client Division of resources: Packet Switching other 1 data 1 data 1 other Server: address = 1 Packet switching - Statistical multiplexing • • • • • • Data is in packets, not streams. Must be digital Each packet has an address A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward No reservations are needed. First come first serve. Major benefit: – • If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else can use it. Major drawback: – What happens if two packets arrive at a switch and both need to go to the same output interface. One packet is either dropped, or is placed in a buffer. Either way, something bad has happened, the packet is gone or is delayed. This would never happen on a circuit switched network. queuing delay and packet loss Division of resources: Packet Switching Packet vs. Circuit Switching If usage is random (e.g., web surfing) statistical multiplexing is better. Suppose that 1. We have a 5Mbps link 2. Each user needs 50kbps 3. And each user is active 20% of the time. (Note that this condition does not matter for circuit switching. Why?) How many users can be accommodated under circuit switching and how many can be accommodated under packet switching? Circuit switching case The total number of users that can be accommodated with circuit switching is 5×106/50×103 = 100 users Division of resources: Packet Switching Packet Switching Case Now suppose there are 200 users, what is the probability that there are 150 or more active users? In this case, there would be a problem, since the network cannot support more than 100 active users. Simpler questions: What is the probability of 150 particular users being active and 50 other being inactive? 0.2150 1 0.2 200150 How many different ways can I select these 150 active users? n n! k k! n k ! 200 150 Is the number of ways that you can select k out of n Is the number of ways that you can select 150 people out of 200 The probability of any 150 users being active and the rest in active is 200 150 0. 2 150 1 0. 2200150 This is the Binomial distribution Division of resources: Packet Switching Packet Switching Case What is the probability of more than 100 users being active? The probability of 101 users being active plus, 102 users being active, plus …., plus 200 users being active, which is 200 k k 101 k 0.2 1 0.2200k 1014 This is the binomial complementary cumulative distribution 200 We conclude that if there are 200 users, then in “pretty much always” things will work fine 300 k k 101 k 0.2 1 0.2300k 108 300 Suppose that there are 300 users: 400 k 101 k 0.2k 1 0.2400k 0.004 400 Suppose that there are 400 users: Still pretty good Might be acceptable performance (if there is some other mechanism to recover!) Therefore: circuit switching could support 100 users, while packet switching can support 400 users. A factor of 4 more!!! Division of resources: Packet Switching Packet Switching vs. Circuit Switching A couple of things: What does this probability really mean? 400 k 400k 0.004 0.2 1 0.2 k 101 k 400 This means that • when you walk into the switching center, the probability of finding overload is 10 -8. • Or, if you random access the link, the probability of finding it in overload. • Once you find it in overload, or not, the probability that is will be in overload in the next second is more complicated and requires queuing theory. This analysis might reveal worst performance. In this example, we assumed 20% user utilization (they were active 20% of the time) If it the user utilization is smaller, then the difference between packet switching and circuit switching is even greater. But if it is larger, then there is less of a difference. What is your user utilization? •For web surfing •For cell phone usage •VoIP call •For music streaming •P2P Division of resources: Packet Switching Packet Switching vs. Circuit Switching • If loss and delay are permissible and usage is random, then packet switching is better than circuit switching. • If usage is very regular (e.g. TV!), circuit switching is best. • If losses and delay are not permissible, then circuit switching is best (e.g., remote controlled surgery). • With packet switching, congestion control is required. Also, there is more overhead for each packet. • For circuit switching, once the circuit is setup, it can be very efficient. But circuits must be set-up. • So, for short file transfer, packet switching is good but for long file transfers, circuit switching might be better. Division of resources: Packet Switching Packet Switching vs Statistical Multiplexing • • • • • • There is a subtle difference between packet switching and statistical multiplexing. Statistical multiplexing means to use the resource as needed. • This leads to the performance improvements mentioned but also the complications (delay and loss). Statistical multiplexing requires packet switching to put data into chunks Circuit switching can work with data packets/chunks, but there is no need for an address The phone network uses circuit switching, but the circuits are statistically multiplexed between users. In packet switching, links are statistically multiplexed. Division of resources: Packet Switching Packet Switching: Statistical Multiplexing 100 Mb/s Ethernet A B statistical multiplexing C 1.5 Mb/s queue of packets waiting for output link D E Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. Division of resources: Packet Switching Time division multiplexing phone 1 byte for each channel every 1/8000 seconds Or 24×7×8000+overhead = 1.544Mbps (DS1 or T1) bytes 4321 4321 3 2 1 byte for each channel every 1/8000 seconds Or 28×24×7×8000+overhead = 44.736Mbps (DS-3) 1 4321 1 byte every 1/8000 seconds Or 7×8000=56Kbps (1kbps=1000bps) (7 bits of data 1 bit of control) • • • • Note all the control overhead: if the bit is 1, then payload is control. Lots of control is needed to setup a circuit. How is it possible to get channels at each hop? Also, if there is not data, then nothing is sent. This wastes data. But the circuit is yours, guaranteed! Division of resources: Packet Switching There are standard bit-rates that support multiplexing different numbers of calls Multiplex 28 DS1 = 28*24*64kbps + overhead = 44.736Mbps DS-3 Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1 STS is electrical and oc is optical OC3 = 155.52Mbps (150.336 payload) OC12 = 633.08 Mbps (601.344 payload) OC48 = 2.488Gbps (2.405Gbps) OC192 = 9.953Gbps (9.6Gbps payload) Packet-switching: store-and-forward L R R R Example: • L = 7.5 Mbits • R = 1.5 Mbps • transmission delay = 15 sec • takes L/R seconds to transmit (push out) packet of L bits on to link at R bps • store and forward: entire packet must arrive at router before it can be transmitted on next link • delay = 3L/R (assuming more on delay shortly … zero propagation delay) Division of resources: Packet Switching Today – networking basics • • • • • Core components of the Internet – the protocol stack Multiplexing, circuit switching, and packet switching Loss and delays The structure of the Internet This lecture covers much of chapter 1 in the textbook. Losses and delay in packet switched networks • Losses – Transmission losses • In fiber links, bit-error is 10-12 or better (i.e., less). – What is the probability of packet error when there are 1500 bytes in a packet? » 1 - (1 - 10-12)1500×8 = 1.2×10-8 • In wireless links, the bit-error rate can be very high – Congestion losses. • If too many packets arrive at the same time, then the buffers will fill up and packets are lost. • Increasing the link speeds or reducing the number of users can reduce the probability of loss. • Increasing the size of the buffer reduces losses, but also increases delay. • Delay – – – – Queuing delay Transmission delay Propagation delay Processing delay packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Loss and Delay Queuing delay packet being transmitted (delay) A B • • • packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Queuing delay occurs for the same reason as congestion losses. The more the network is utilized, the high the queueing delay (and losses) Utilization = := actual use / maximum possible use Suppose that • the link bit-rate is Z, • there are X users • Each users uses data rate Y with probability p, and use no bandwidth with probability 1-p. = X×(Y×p)/Z Loss and Delay: Queuing delay Queuing delay Is it possible to have a network run at full utilization? No! The average delay would be infinite! From queuing theory Delay = /(1- ) Loss and Delay: Queuing delay Delay in packet switched networks • Delay – – – – Queuing delay Transmission delay Propagation delay Processing delay How long does it take to transmit a packet? How long does it take to get all the bits from node on to the wire/air/fiber? Suppose •Link bit rate is 10 Mbps •Packet size is 1400 bytes How long to transmit the packet? 1400 *8 bits / packet 10*10^6 bits / sec Loss and Delay: Transmission and Propagation Delay = .0011 sec = 1.1 ms Delay in packet switched networks • Delay – – – – Queuing delay Transmission delay Propagation delay Processing delay – Suppose • • How long does it take for a bit to travel along a wire/fiber/through the air? Speed of light in a vacuum 3108 m/s while in a fiber it is 2108 m/s How long does it take to transmit a bit from NY to LA = 3962km – 20ms propagation delay • How about from NY to Jakarta, Indonesia = 16,179km • How about to a Geostationary satellite? – – – – – – • 35,786,000 m above the equator 35,786,000 m /3e8 = 120ms (each way), 240ms up and back On the edge of coverage, the delay can be 280ms Note: for voice, the maximum delay is 250ms one-way For the Iraq war, two satellite hops were often used, resulting in a one-way delay over500ms Medium orbit satellites (e.g., GPS) – • 80ms 60ms (each way) Low-earth orbit satellites (low-earth? What about middle-earth?) – – Iridium at 10ms (each way_ » Note, Iridium paid 5 billion for the network and sold for 25million (1/2%->on sale 99.5% off, everything must go) – Teledesic. 10ms Solar powered aircraft – 0.125ms (each way) Loss and Delay: Transmission and Propagation Delay transmitter Loss and Delay: Transmission and Propagation Delay receiver Start of bit 0 transmitter0 Loss and Delay: Transmission and Propagation Delay receiver Transmitter is transmitting bit 0 transmitter 0 Loss and Delay: Transmission and Propagation Delay receiver Transmitter has finished transmitting bit 0, and is starting to transmit bit 1 Start of bit 1 transmitter1 0 Loss and Delay: Transmission and Propagation Delay receiver Transmitter is transmitting bit 2 transmitter 2 1 0 Loss and Delay: Transmission and Propagation Delay receiver … and so on … Loss and Delay: Transmission and Propagation Delay The transmitter is transmitting bit 8 The receiver is starting to receive bit 0 transmitter 8 7 6 5 4 Loss and Delay: Transmission and Propagation Delay 3 2 1 0 receiver The transmitter is transmitting bit 9. The receiver has completed receiving bit 0 and is now starting to receive bit 1 transmitter 9 8 7 6 5 Loss and Delay: Transmission and Propagation Delay 4 3 2 1 0receiver … and so on … Loss and Delay: Transmission and Propagation Delay Question: what is the transmission delay? Answer: The time to transmit a bit = 1/bit-rate If bit rate = 10Mbps, then the time to transmit a bit is 1/10×106=10-7sec=100nsec Question: What is this? transmitter 9 8 7 6 5 4 3 2 1 0receiver Answer: This is the length of a bit on the wire (in in the air) Question: How is the length of a bit related to the bit rate and transmission delay? Question: How far does the beginning of the bit travel while rest of the bit is being transmitted? Question: How long does it take to transmit the bit? Answer: If the link is wireless, then the signal is moving at 3108m/sec. In this case, the start of the bit has traveled 3108m/sec 1/bit-rate e.g., bit-rate is 10Mbps, the beginning of the bit has traveled: 3108m/sec / (10010-9sec) = 30m e.g., bit-rate is 10Gbps, the beginning of the bit has traveled: 3108m/sec / (10010-12sec) = 0.03m Loss and Delay: Transmission and Propagation Delay If the propagation delay is fixed, but the bit-rate is increased. Or, equivalently If the propagation delay is fixed, but the transmission delay is decreased. transmitter16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 receiver Higher bit rate => low transmission delay (less time to transmit a bit) => More bits fit on the wire (or the air) Loss and Delay: Transmission and Propagation Delay If the propagation delay is fixed, but the bit-rate is deceased. Or, equivalently If the propagation delay is fixed, but the transmission delay is increased. transmitter 1 2 1 receiver Lower bit rate => higher transmission delay (more time to transmit a bit) => Fewer bits fit on the wire (or the air) Loss and Delay: Transmission and Propagation Delay If the propagation delay is fixed, but the bit-rate is deceased even more. Or, equivalently If the propagation delay is fixed, but the transmission delay is increased even more. transmitter 1 receiver The receiver receives the bits as the transmitter is still transmitting them. The wire (or air) doesn’t hold any complete bits Loss and Delay: Transmission and Propagation Delay Question: what is the propagation delay? Answer: The time for the start (or end) of a bit to move from the transmitter to receiver The beginning of bit 0 6857946835794683579468357946835729468357294transmitter 68357294683572946183572946183572946183572946183507246183507246183507246183507246135072461350724613507246135024613502461350246135024135024135024135024130241302413024130213021302130210210210210101010 0 0 0 receiver 00:00:00 00:00:21 00:00:20 00:00:19 00:00:18 00:00:17 00:00:16 00:00:15 00:00:14 00:00:13 00:00:12 00:00:11 00:00:10 00:00:09 00:00:08 00:00:07 00:00:06 00:00:05 00:00:04 00:00:03 00:00:02 00:00:01 00:00:30 00:00:29 00:00:28 00:00:27 00:00:26 00:00:25 00:00:24 00:00:23 00:00:22 Of course, the time for the beginning of a bit to travel from the transmitter to receiver is the same as the time for the end of a bit to travel from the transmitter to receiver Loss and Delay: Transmission and Propagation Delay Question: How long to get a bit from transmitter to receiver? Or: Once a host starts transmitting, how long until the bit is received? transmitter 0 Three things 1. Transmit the bit 2. Bit must propagate to receiver 3. Receive bit Duration = 2TtransmissionDelay + TpropagationDelay ??? No Loss and Delay: Transmission and Propagation Delay receiver Question: How long to get a bit from transmitter to receiver? Or: Once a host starts transmitting, how long until the bit is received? transmitter 0 0 receiver Three things 1. Transmit the bit 2. The end of the bit must propagate to receiver – at which point, the bit has been received 3. Receive bit (already done) Duration = TtransmissionDelay + TpropagationDelay Loss and Delay: Transmission and Propagation Delay Question: How long to get a packet from transmitter to receiver? Or: Once a host starts transmitting, how long until the packet is received? transmitter 0 0 receiver Three things 1. Transmit the packet 2. The end of the packet must propagate to receiver – at which point, the packet has been received 3. Receive packet (already done) Duration = packetSize TtransmissionDelayPerBit + TpropagationDelay Important: This delay is incurred at each hop. a N hop path will have a transmission delay at each hop Loss and Delay: Transmission and Propagation Delay Mild correction: Maybe a bit that has value 1 looks like this voltage transmitter receiver x length of bit Loss and Delay: Transmission and Propagation Delay Delay in packet switched networks • Delay – – – – Queuing delay Transmission delay Propagation delay Processing delay Loss and Delay: Processing Delay Routers take a bit of time to process packets. • moving packets inside the router • Finding which is the next hop • Applying security or QoS How to measure delay? • • • • Ping: > ping 216.109.124.73 Ping gives help (linux) Ping –I 10 216.109.124.73 > file.txt Then read it in excel and plot delay • • Traceroute (linux), tracert (windows) Traceroute 216.109.124.73 gives the routers and an estimate of the delay to each router. • • • • (Question? Does it take larger packets longer to transmit than shorter packets? Of course, it does. Can we test it with ping? Not really. But try it with ping and wireshark) 1.Open wireshark 2.Select correct interface 3.Start recording 4.There are too many packets 5.Filter out everything that is not icmp 6.Run ping for a bit 7.In wireshark export only what is displayed Loss and Delay: Lab-Ping 8. 9. 10. 11. 12. 13. Open file in excel Delete things we don’t want Save file Open in matlab Plot Repeat for large packets Estimating the distribution of queuing delay with Wireshark and other tools you google RTT = 2Tpropagation + Ttransmisison_1 + Ttransmisison_2 +…+ Q1 + Q2 + … 1. ping -n 250 google.com – type “ping” for help 2. Get/open wireshark 3. Find the correct interface 4. Start collecting data – Notice packet are being captured 5. Start ping 6. When ping finishes, stop capture 7. Export data – Select displayed packets option (lower left) – Deselect Packet Details (lower right) Loss and Delay: Lab-Ping 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. Open data in something like excel Remove everything but the times Resave as times.txt Load times.txt in matlab u = diff(times); plot(u) u = u(u<.2); plot(u) What is min(u)? q = u – min(u); hist(q,20) Little’s Theorem • Let N = number of items in the system – • Let be the rate that items arrive into the system – • E.g., the rate that packets at the output of the interface Let T be the duration an item spends in the system – • Any system. E.g., the number of packets in the output interface of a router E.g., the time a packet spends in the output of the interface Little’s theorem – N = T • is a rate, so T is the number of items that arrive during period T • Suppose that N is the average number of packets in the router and suppose that it takes seconds to transmit a packet – – – • • • • What is the duration that a packet spends in the router? When the packet arrives, there are N packets already in the router. So the delay is N + , where the N is the time this packets spends in the router, and is the time sitting in the transmitter Thus, T = N + Hence, T=N => (N + )=N ( N + )=N N (1- ) = N = /(1- ) – Which is the same formula that shows that the buffer occupancy grows to infinity as the utilization, , goes to 1 Loss and Delay: Lab-Ping Today – networking basics • • • • • Core components of the Internet – the protocol stack Multiplexing, circuit switching, and packet switching Loss and delays The structure of the Internet This lecture covers much of chapter 1 in the textbook. Internet structure: network of networks • • roughly hierarchical at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage – treat each other as equals Tier-1 providers interconnect (peer) privately Tier 1 ISP Tier 1 ISP The components of the Internet Tier 1 ISP Tier-1 ISP: e.g., Sprint POP: point-of-presence to/from backbone peering … … . … … … to/from customers The components of the Internet Internet structure: network of networks • “Tier-2” ISPs: smaller (often regional) ISPs – Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider Tier-2 ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP The components of the Internet Tier 1 ISP Tier-2 ISP Tier-2 ISPs also peer privately with each other. Tier-2 ISP Internet structure: network of networks • “Tier-3” ISPs and local ISPs – last hop (“access”) network (closest to end systems) local ISP Local and tier3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier 3 ISP Tier-2 ISP local ISP local ISP local ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local local ISP ISP The components of the Internet Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP Internet structure: network of networks • a packet passes through many networks! local ISP Tier 3 ISP Tier-2 ISP local ISP local ISP local ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local local ISP ISP The components of the Internet Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP ISPs and the structure of the Internet • http://som.csudh.edu/fac/lpress/netapps/hout/oneWilshire/index.htm The components of the Internet MEET ME ROOM Said to be the most interconnected space in the world and the most expensive real estate in North America, the “Meet Me Room” (a telco industry term) is the heart of One Wilshire. Here the primary fiber optic cables are routed, split, and shared. Because of the presence of so many telcos in this room and the ability to freely interconnect between them, rackspace here becomes extremely valuable. For comparison, the average price for office space in downtown Los Angeles is $1.75 per square foot per month. At the Meet Me Room, $250 per square foot would be a bargain. CABLE RISERS Some 1,800 known conduits contain the fiber optic cables that flow through the building’s stairwells and vertical utility corridors, called “risers.” Cable connects the commercial telco tenants on floors 5 through 29 to the 4th floor Meet Me Room, and to a new, “wireless” Meet Me Room constructed on the 30th floor. SURFACE CABLE MAP Whenever a permit is pulled by a city contractor for any underground repairs outside One Wilshire, the various telco companies with cable in the area come out and paint the cable routes on the asphalt, creating a visible graphic of the complexity of what lies just under the surface. HVAC Computers generate a lot of heat, and maintaining a stable, cool temperature and a low humidity is essential in telco hotels, so tenants sometimes demand to install their own cooling systems to safeguard their equipment. At One Wilshire, these units are installed primarily on the third floor roof. A new closed loop cooling system has been installed on the 30th floor roof. CABLE MINING As tenants’ needs change, cables can go unused. Cable mining is performed to thin out the obsolete cables and future congestion is alleviated through the installation of dedicated new ducts. ELECTRICITY Power is supplied by DWP, but in the event of a blackout, the building’s five generators will kick in. It takes the generators three seconds to start up and stabilize. During this brief period, the entire building runs on batteries. There are 11,000 gallons of diesel stored on site, enough to run the generators for 24 hours before being refueled. MICROWAVE On the roof, microwave antennas link up One Wilshire to transmission towers located around the city. Though fiber’s higher capacity has given it dominance over microwave at One Wilshire, microwave’s relatively low cost over long distances continues to make it economical for some applications. The roof’s clear line of sight to the south, west, and to other high-rises, along with the ability to interface with the fiber inside, continues to make One Wilshire an attractive location for microwave-based transmission. READING A ROOF Much can be learned about a building’s function by examining its roof. The existence of telco hotels in the region around One Wilshire is indicated by the presence of new and extensive cooling units on the roofs of adjacent buildings, many of which were nearly vacant until the telco companies moved in. POINT OF ENTRY The main fiber optic cables connecting One Wilshire to the world enter the building from under the street through closets in the walls of the building’s parking garage. Given the importance of the building to the global communications network, access to the parking garage is controlled, and the building is said to be monitored continuously by federal security officials.
