CS 152 Computer Architecture and Engineering Lecture 24 Networks 2005 4 19 John Lazzaro www cs berkeley edu lazzaro TAs Ted Hong and David Marquardt www inst eecs berkeley edu cs152 CS 152 L24 Networks UC Regents Spring 2005 UCB Last Time Making Mac Mini Size fixed by the form factor physical size of desktop DIMMS Laptop DRAM is smaller but too expensive for 499 price CS 152 L24 Networks UC Regents Spring 2005 UCB Why are networks different from buses Serial Data is sent bit by bit over one logical wire Network Primary purpose is to connect computers to computers CS 152 L24 Networks USB FireWire Primary purpose is to connect devices to a computer UC Regents Spring 2005 UCB Today Networks Link layers Using physics to send bits from place to place Internet A network of networks Routing Inside the cloud CS 152 L24 Networks UC Regents Spring 2005 UCB Networking bottom up Link two endpoints Q1 How far away are the endpoints Japan US undersea cable network Physical media optical fiber photonics Distance WiFi wireless mobility from hotel bandwidth bed to influences access point choice of medium Physical media unlicensed radio spectrum CS 152 L24 Networks UC Regents Spring 2005 UCB Networking bottom up Link two endpoints Q2 Initial investment cost for the link 1B USD A ship lays cable on ocean floor For expensive media much of the price goes to pay off loans The price of the WiFi laptop card the base station Unlicensed radio no fee to the FCC CS 152 L24 Networks UC Regents Spring 2005 UCB Networking bottom up Link two endpoints Q3 How is the link imperfect A steady bitstream circuit No packets to lose Only one bit flips per 10 000 000 000 000 sent Undersea failure is catastrophic Someone walks by and the network stops working fading CS 152 L24 Networks Solution Short packets spaced in time to escape the fade If lost do retransmits UC Regents Spring 2005 UCB Networking bottom up Link two endpoints Q4 How does link perform BW 640 Gb s ping irt1 ge1 1 tdc noc sony co jp CA JP cable Latency PING irt1 ge1 1 tdc noc sony co jp 211 125 132 198 56 data bytes 64 bytes from 211 125 132 198 icmp seq 0 ttl 242 time 114 571 ms round trip Compare Light speed in vacuum SFOTokyo 63ms RT In general risky to halve the round trip time for oneway latency paths are often different each direction BW In theory 801 11b offers 11 Mb s Users are lucky to see 3 5 Mb s in practice Latency If there is no fading quite good I ve measured 2 ms RTT on a short hop CS 152 L24 Networks UC Regents Spring 2005 UCB There are dozens of link networks Protocol Complexity email WWW phone SMTP HTTP RTP TCP UDP IP Ethernet Wi Fi Link networks CSMA async sonet copper fiber radio Diagram Credit Steve Deering CS 152 L24 Networks The undersea cable the hotel WiFi and many others DSL Ethernet UC Regents Spring 2005 UCB Web browsers do not know about link nets Protocol Complexity Applications email WWW phone SMTP HTTP RTP TCP UDP IP Ethernet Wi Fi CSMA async sonet Link networks copper fiber radio Diagram Credit Steve Deering CS 152 L24 Networks App authors do not want to add support for N different network types The undersea cable the hotel WiFi and many others DSL Ethernet UC Regents Spring 2005 UCB The Internet A Network of Networks Protocol Complexity Internet Protocol IP An abstraction for applications to target and for link networks to support Very simple very successful email WWW phone SMTP HTTP RTP TCP UDP IP Ethernet Wi Fi CSMA async sonet copper fiber radio IP presents link network errors losses in an abstract way not a link specific way Link layer is not expected to be perfect Diagram Credit Steve Deering CS 152 L24 Networks UC Regents Spring 2005 UCB The Internet interconnects hosts IP4 number for this computer 198 211 61 22 Every directly connected host has a unique IP number Upper limit of 2 32 IP4 numbers some are reserved for other purposes Next generation IP IP6 limit 2 128 198 211 61 22 A user friendly form of the 32 bit unsigned value 3335732502 which is 198 2 24 211 2 16 61 2 8 22 CS 152 L24 Networks UC Regents Spring 2005 UCB Internet Sends Packets Between Hosts IP4 IP6 etc How the destination should interpret the payload data 2 0 1 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Version IHL Type of Service Total Length Identification Flags Fragment Offset Time to Live Protocol Header Checksum From IP number Source Address Note Could be a lie To IP number Destination Address Payload data size implied by Total Length header field IHL field of words in header The typical header IHL 5 words is shown Longer headers code add extra fields after the destination address CS 152 L24 Networks Bitfield numbers Header Data UC Regents Spring 2005 UCB Link networks transport IP packets ISO Layer Names IP packet Layer 3 WiFi and Cable Modem packets Layer 2 Radio cable waveforms Layer 1 801 11b WiFi packet IP Packet For this hop IP packet sent inside of a wireless 801 11b packet CS 152 L24 Networks Cable modem packet IP Packet For this hop IP packet sent inside of a cable modem DOCSIS packet UC Regents Spring 2005 UCB Link layers maximum packet size vary Maximum IP packet size 64K bytes Maximum Transmission Unit MTU generalized packet size of link networks may be much less often 2K bytes or less Efficient uses of IP3sense MTU 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Version IHL Type of Service Total Length Identification Flags Fragment Offset Time to Live Protocol Header Checksum Source Address Destination Address Payload data size implied by Total Length header field Header Data Fragment fields Link layer splits up big IP packets into many link layer packets reassembles IP packet on arrival CS 152 L24 Networks UC Regents Spring 2005 UCB IP abstraction of non ideal link networks A sent packet may never arrive lost If packets sent P1 P2 P3 they may arrive P2 P1 P3 out of order Best Effort The link networks and other parts of the cloud do their best to meet the ideal But no promises Relative timing of packet stream not necessarily preserved late packets IP payload bits received may not match payload bits sent IP header protected by checksum almost always correct CS 152 L24 Networks UC Regents Spring 2005 UCB How do apps deal with this abstraction Computing apps use the TCP Transmission Control Protocol Protocol Complexity email WWW phone SMTP HTTP RTP TCP UDP TCP lets host A send a reliable IP byte stream to host …
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