A Network of Interconnected Objects The TCP/IP stack

Transcription

1 Lecture 1 A Network of Interconnected Objects The TCP/IP stack Part of the material used for this slides has been obtained from: Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith Ross. Addison-Wesley, July 2007.

2 Starting Point Wireless Networks as an Access Technology to Internet Internet Protocol (IP) everywhere

3 Wireless Access Networks Access Points = {WIFI, WIMAX, GSM/GPRS, UMTS, HSDPA, LTE...}

4 Wireless Access Networks

5 Mobile Communication Networking Mobile Internet Applications Johan Zuidweg (4th part) IMS Miquel Oliver (3rd part) SIP Anna Sfairopoulou (2nd part) TCP/UDP IP LINK WLANs PHY Boris Bellalta (1st part)

6 Some Internet Objects

7 What's the Internet Millions of connected computing devices: Hosts = end systems running network applications Routers / Switches = network elements

8 Internet Structure

9 What's the Internet: a service view The Network provides communication services to applications. The different services are characterized by their capability to provide: Data loss Timing (delay) Throughput Security What services provide the current Internet? IP is a best effort (there are no loss & delay guarantees) A reliable service (based on TCP) A non reliable service (based on UDP)

10 Types of Traffic vs User perception Elastic flows (TCP based): use all the available bandwidth, fair. Streaming/Rigid flows (UDP based): use only the required bandwidth. User Perception Rigid Traffic (UDP) Video/audio streaming Voice over IP Good Bad B Minimum Bandwidth Multimedia User Perception Elastic Traffic (TCP) Better Web (HTTP) FTP, e mail,... Worst B 10

11 Transport service requirements of common apps

12 Circuit switching At each link between the source and destination a portion of bandwidth is reserved for that communication (data transfer). Before to start the communication, the circuit must be established (e.g. mobile phone call). Benefits: Once the circuit is created, the communication does not suffer any interference from other communications. Disadvantages: Very low efficiency (if there is no data to be transmitted, nobody can take profit of the reserved bandwidth). Low network capacity.

13 Packet switching There is no bandwidth reservation at any link between the source and destination. All decision (routing to next hop) are taken based on each individual packet. Benefits: Simple, does not require any extra signaling. Better use of the transmission resources Higher capacity. Disadvantages: Unpredictable network performance (for each communication) as it depends on the behavior of the others.

14 Packet switching voice packets bits Codec L bits Packetizer streaming flow Time beetwen packets bits File 10 MBytes t packets L bits Packetizer elastic flow A packetizer function encapsulates (adds the necessary headers) a sequence of bits in a packet. The overall packet has length of L (data + header) bits. t

15 Packet switching Optical Fiber links PC Router A Web Server ADSL/Switch Router B Switch Router C VoIP/WLAN phone Access Point VoIP phone Network elements forward packets to its destination through other network elements (routing). End points (final systems) generate the data. VoIP/WLAN phone

16 Internet: a network of queues PC Router A Web Server Queue HTTP request ADSL/Switch Router B Switch Router C VoIP/WLAN phone HTTP response Access Point VoIP/WLAN phone Request delay? Response delay? HTTP request HTTP response Router A Router C Router B Router B Router C Web Server PC clients If the link is full, packets have to wait its turn in a queue!!!!

17 End to end vs Hop by Hop retransmissions Hop by hop Data 1 Data 2 ACK/NAK Data 3 Data 4 ACK/NAK 5 ACK/NAK ACK/NAK End to end ACK/NAK 1 2 Data 3 Data 5 4 Data Data 17

18 What is a protocol? network protocols: between objects (machines) all communication activity in Internet is governed by protocols protocols define format, order of msgs sent and received among network entities, actions taken on msg transmission, receipt

19 TCP/IP protocol stack application: supporting network applications FTP, SMTP, HTTP transport: process process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements Ethernet, WLANs physical: wired / wireless

20 Layer information: Headers Each layer can only modify its own headers bit

21 Internet Protocol Stack User space (software) Operating System (software) Hardware

22 Internet Protocol Stack application: supporting network applications FTP, SMTP, HTTP transport: process process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements Ethernet, WLAN physical: bits on the wire/wireless

23 Link Layer Frame transfer between two directly connected (1 hop) node. The services provided at the link layer depend on the specific link layer protocol that is employed over the link.

24 Data Link Layer functions Link Layer Hop to hop addressing Framing Error control (detection and correction) Retransmissions and flow control Reduce errors. Flow control (avoid to saturate the receiver). Medium Access Control PHY Layer Transmission / Reception 24

25 Why addresses? Address = identifier Layer 2 address: Broadcast medium: to identify the sender and destination nodes. Point to point: not required 0A:1C:2E:AA:02:55 B1:CB:11:A0:25:11 0A:1C:2E:AA:04:25 All nodes will inspect the destination address before to discard or to process the received packet. 25

26 Segmentation & Reassembly To accommodate arbitrary message size, a layer may have to deal with messages that are too long or too short for its protocol Segmentation & Reassembly: a layer breaks long messages into smaller blocks and reassembles these at the destination 1 long message 2 or more blocks 2 or more short messages 1 block 26

27 Error Control and Detection Let assume: Pb Probability of bit error F Frame size in bits Pr{no errors}= (1 Pb)F Pr{one or more bits in error} = 1 (1 Pb)F Example: Pb = 10 6, F = 8*1500 bits Pr {Frame error} = 1 (1 10 6) 8*1500 = 1.2*

28 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction otherwise 28

29 Automatic Repeat Request (ARQ) Purpose: to ensure a sequence of information packets is delivered in order and without errors or duplications despite transmission errors & losses Three basic schemes: Stop and Wait ARQ Go Back N ARQ Selective Repeat ARQ Basic elements of ARQ: Error detecting code with high error coverage ACKs (positive acknowledgments NAKs (negative acknowlegments) Timeout mechanism 29

30 Automatic Repeat Request (ARQ) Efficiency and other metrics: computed in full duplex channels. Different channels from Tx to Rx and from Rx to Tx. Selective Repeat is the most efficient. Real broadcast systems: half duplex channel. There is no possibility to transmit simultaneously in both cases. Stop & Wait option becomes the unique solution. Improvements: Block transmissions with Stop & Wait ARQs, Piggy backing, Hybrid ARQs... Data and ACKs in the same bandwidth Data and ACKs in separated channels f C [bps] f C [bps] 30

31 ARQ: Stop & Wait Sender Receiver Packet In this case: Same propagation delay. Why? Different transmission delay. Why? ACK time time 31

32 Multiuser Environments A group of stations share the communication medium Broadcast networks Each transmission is received by all the stations Circuit switching (TDM) vs Packet switching (e.g. random access). The packets are distributed in different nodes... which complicates the MAC...

33 Multiple Access protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time multiple access protocol (MAC) Coordinates the access to the shared medium Defines when and how the stations should transmit Performance: Depending on the technique used the performance of the network differs. Metrics: Total number of packets/s, delay, reliability, cost, max. number of stations, etc. 33

34 MAC Techniques

35 Static Channelization Bandwidth allocation: Give a portion of the shared medium to each user Highly efficient for constant bit rate traffic Examples:

36 FDMA Divide channel into M frequency bands Each station transmits and listens on assigned bands Each station transmits at most C/M bits/s

37 TDMA Dedicate 1 slot per station in transmission cycles Each station transmits at C bps 1/M of the time

38 FDMA/TDMA

39 Random Access When a node transmits uses all the available bandwidth If more than one node transmit at the same time a collision occurs Random access protocols Determine the way nodes access the channel and share it Minimize and specify how to recover from collisions

40 Aloha

41 Aloha Performance S G Only 18.4% maximum performance!! S: Efficiency,

42 S Aloha

43 S Aloha Performance S Aloha S Aloha G High performance increase compared with Aloha. At which cost?? Synchronization is needed

44 CSMA NP

45 CSMA Performance S G The performance depends on the propagation time Better than Aloha for low propagation times

46 Reservation MAC protocols channel partitioning MAC protocols: share channel efficiently and fairly at high load inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols efficient at low load: single node can fully utilize channel high load: collision overhead taking turns protocols look for best of both worlds! 46

47 Reservation

48 Internet Protocol Stack application: supporting network applications FTP, SMTP, HTTP transport: process process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements Ethernet, WLAN physical: bits on the wire/wireless

49 Network Layer Transport layer: TCP, UDP (reliability, flow control) Network layer IP protocol addressing conventions datagram format packet handling conventions Routing protocols path selection RIP, OSPF, BGP forwarding table ICMP protocol error reporting router signaling Link layer (node-to-node communication) Physical layer (bits coding and modulation)

50 Network layer The network layer receives a data segment from transport Network layer encapsulates the data segments into datagrams All routers have a network layer, they check the datagram and route it according to its destination segment datagram application transport network data link physical network data link physical network data link physical network data link physical data gram datagram dat agr am network data link physical application transport network data link physical Sending host network data link physical Internet Protocol (IP) is the worldwide used as network protocol Receiving host

51 Network functions 1. routing: determine route taken by packets from source to dest. (routing algorithms) 2. forwarding: move packets from router s input to appropriate router output

52 Addressing Address: interface unique identifier. Network address: Network interface identifier. A node can have multiple interfaces. Each interface has a link layer and a network layer address: link layer (MAC or physical) address: EA 2E C5 84 network layer (logical or IP) address: Binary: 32 bits Decimal with dots 4 numbers dot separated Name (easy for humans)

53 Routing: Motivation How to route the packet along the network: Several ways to reach the destination, but which one is better? According to the cost (that means the delay, throughput, reliability, money, a combination of them ) The nodes should decide how to route each packet routing algorithms! The network is alive, decisions on routing may change at any moment

54 Routing: Graph abstraction 5 Graph: G = (N,E) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Link cost c(x,x ) = cost of link (x,x ) e.g., c(w,z) = 5 cost could always be 1, or inversely related to bandwidth, or inversely related to congestion Cost of path (x1, x2, x3,, xp) = = c(x1,x2) + c(x2,x3) + + c(xp 1,xp) 3 v w 2 u x 1 5 y z 2 Remark: Graph abstraction is also useful in other network contexts Example: P2P, where N is set of peers and E is set of TCP connections Question: What s the least-cost path between u and z? Routing algorithm: algorithm that finds least-cost path

55 Internet Protocol Stack application: supporting network applications FTP, SMTP, HTTP transport: process process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements Ethernet, WLANs physical: bits on the wire/wireless

56 Transport protocols provide logical communication between application processes running on different hosts. transport protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer Two transport protocol available to apps: TCP and UDP

57 Logical end to end transport logical : it is independent of the physical path or the Low layer protocols followed by packets.

58 Internet Transport services 1) reliable, in order delivery (TCP) congestion control flow control connection setup 2) unreliable, unordered delivery: UDP no frills extension of best effort IP Transport services not available: delay guarantees bandwidth guarantees

59 Transport vs. Network layer network layer: data communication between hosts (end to end) transport layer: data communication between processes (end to end) Multiplexing Proc. 1 port/socket Proc. M? Proc. 1 port/socket Proc. M? Host 1 Host 2 IP address IP address

60 Application / Process Application: group or processes (at least 1 process) Process: part of an application which handles with some specific functions. Example: Application: Remote Weather Monitoring Two process: measure the weather (e.g. temperature) send the information to the its destination through a network. Some times: 1 application = 1 process

61 Mux / Demux Mux: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) Demux: delivering received segments to correct socket

62 Connection / Connectionless Connection oriented: there are state information about transfer status in both emitter and receiver. The TCP protocol. Required for congestion control, flow control, retransmissions, etc. Connectionless oriented: the opposite case The UDP protocol. 62

63 Principles of Congestion Control Congestion: informally: too many sources sending too much data too fast for network to handle manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) 63

64 Causes/costs of congestion: scenario 1 two senders, two receivers one router, infinite buffers no retransmission Host A Host B λ in : original data λ out unlimited shared output link buffers large delays when congested maximum achievable throughput For connection 64

65 Causes/costs of congestion: scenario 2 two senders, two receivers one router, finite buffers sender retransmits lost packet > more traffic to the network Host A λ in : original data λ out λ 'in : original data, plus retransmitted data Host B finite shared output link buffers 65

66 Fairness (only TCP based service) Fairness goal: if K TCP sessions share same bottleneck link of bandwidth C, each should have average rate of C/K TCP connection 1 TCP connection 2 bottleneck router capacity R 66

67 Transport layer end to end functionalities End to end error detection End to end retransmissions Flow control Congestion control Fairness 67

68 Internet transport protocols services TCP service: connection oriented: setup required between client and server processes reliable transport between sending and receiving process flow control: sender won t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum throughput guarantees, security UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security

69 Internet apps: application, transport protocols

70 Internet Protocol Stack application: supporting network applications FTP, SMTP, HTTP transport: process process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements Ethernet, WLANs physical: bits on the wire/wireless

71 Aplicacions Internet write programs that run on (different) end systems communicate over network e.g., web server software communicates with browser software application transport network data link physical No need to write software for network core devices Network core devices do not run user applications applications on end systems allows for rapid app development, propagation application transport network data link physical application transport network data link physical 71

72 Application Architectures Client server Peer to peer (P2P) Hybrid of client server and P2P 72

73 Client server Architecture server: always on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other

74 P2P Architecture no always on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses Highly scalable but difficult to manage

75 Example: requesting a web page Warriors of the Net: Dzs