1、CS61C Anatomy of I/O Devices: Networks Lecture 14,March 5, 1999 Dave Patterson (http.cs.berkeley.edu/patterson) www-inst.eecs.berkeley.edu/cs61c/schedule.html,Review 1/2,Operating System started as shared I/O library Support for OS abstraction: Kernel/User bit, stacked KU bits, syscall MIPS follows
2、coprocessor abstraction to add resources, instructions for OS Interrupt control: Interrupt Enable bit, stacked IE bits, Interrupt Priority Levels, Interrupt Mask Re-entrant via restricting int. to higher priority DMA to accelerate data movement,Outline,Buses Why Networks? A Simple Example: Derive Ne
3、twork Basics Protocol, Ethernet Administrivia, “Computers in the News” Internetworking, Protocol Suites, TCP/IP Performance Pitfalls Conclusion,Recall : 5 components of any Computer,Processor(active),Computer,Control (“brain”),Datapath (“brawn”),Memory (passive)(where programs, data live when runnin
4、g),Devices,Input,Output,Keyboard, Mouse,Display, Printer,Disk, Network,Connecting to Networks (and Other I/O),Bus - shared medium of communication that can connect to many devices Hierarchy of Buses in a PC,Buses in a PC,Data rates Memory: 100 MHz, 8 bytes 800 MB/s (peak) PCI: 33 MHz, 4 bytes wide 1
5、32 MB/s (peak) SCSI: “Ultra2” (40 MHz), “Wide” (2 bytes) 80 MB/s (peak),Why Networks?,Originally sharing I/O devices between computers (e.g., printers) Then Communicating between computers (e.g, file transfer protocol) Then Communicating between people (e.g., email) Then Communicating between networ
6、ks of computers Internet, WWW,Types of Networks,Local Area Network (Ethernet) Inside a building: Up to 1 km (peak) Data Rate: 10 Mbits/sec, 100 Mbits/sec,1000 Mbits/sec Run, installed by network administrators Wide Area Network Across a continent (10km to 10000 km) (peak) Data Rate: 1.5 Mbits/sec to
7、 2500 Mbits/sec Run, installed by telephone companies,ABCs of Networks: 2 Computers,Starting Point: Send bits between 2 computersQueue (First In First Out) on each end Can send both ways (“Full Duplex”) Information sent called a “message” Note: Messages also called packets,A Simple Example: 2 Comput
8、ers,What is Message Format? (Similar in idea to Instruction Format) Fixed size? Number bits?,0: Please send data from address in your memory 1: Packet contains data corresponding to request,Header(Trailer): information to deliver message Payload: data in message (1 word above),Questions About Simple
9、 Example,What if more than 2 computers want to communicate? Need computer “address field” in packet to know which computer should receive it (destination), and to which computer it came from for reply (source),Questions About Simple Example,What if message is garbled in transit? Add redundant inform
10、ation that is checked when message arrives to be sure it is OK 8-bit sum of other bytes: called “Check sum”; upon arrival compare check sum to sum of rest of information in message,Questions About Simple Example,What if message never arrives? If tell sender it has arrived (and tell receiver reply ha
11、s arrived), can resend upon failure Dont discard message until get “ACK”; (Also, if check sum fails, dont send ACK),Observations About Simple Example,Simple questions such as those above lead to more complex procedures to send/receive message and more complex message formats Protocol: algorithm for
12、properly sending and receiving messages (packets),Ethernet Packet Format,Preamble,Dest Addr,Src Addr,Length of Data 2 Bytes,Data,Check,Preamble to recognize beginning of packet Unique Address per Ethernet Network Interface Card so can just plug in & use Pad ensures minimum packet is 64 bytes Easier
13、to find packet on the wire Header+ Trailer: 24B + Pad,Pad,8 Bytes,6 Bytes,6 Bytes,0-1500B,0-46B,4B,Software Protocol to Send and Receive,SW Send steps 1: Application copies data to OS buffer 2: OS calculates checksum, starts timer 3: OS sends data to network interface HW and says start SW Receive st
14、eps 3: OS copies data from network interface HW to OS buffer 2: OS calculates checksum, if OK, send ACK; if not, delete message (sender resends when timer expires) 1: If OK, OS copies data to user address space, & signals application to continue,Administrivia 1/2,6th homework: Due 3/10 7PM Exercises
15、 8.3, 8.29 (skip challenge), Ap A.3 4th Project: Friday 3/12 7PM (absolute latest: 3/13 8AM) Readings: Cache Memory 7.1, 7.2 Upcoming events Midterm Review Sunday 3/14 2PM, 1 Pimentel Midterm on Wed. 3/17 5pm-8PM, 1 Pimentel No discussion sections 3/18, 3/19 Friday before Break 3/19: video tape by G
16、ordon Moore, “Nanometers and Gigabucks”,Administrivia 2/2: Survey Results,Survey Results,“Computers in the News”,“A Growing Compatibility Issue in the Digital Age: Computers and Their Users Privacy”, NYT, 3/2/99“. an imminent privacy threat or simply part of the foundation of advanced computer syste
17、ms” “Privacy groups argue fiercely that the merger of computers and the Internet has brought the specter of a new surveillance society (anything)” “.network designers argue that identity information is a vital aspect of modern security design because it is necessary to authenticate an individual in
18、a network, thereby preventing fraud or intrusion” “all devices connected to networks require identification simply to function correctly. for 20 years a requirement for any computer connected to an Ethernet”,Protocol for Networks of Networks?,Internetworking: allows computers on independent and inco
19、mpatible networks to communicate reliably and efficiently; Enabling technologies: SW standards that allow reliable communications without reliable networks Hierarchy of SW layers, giving each layer responsibility for portion of overall communications task, called protocol families or protocol suites
20、 Abstraction to cope with complexity of communication vs. Abstraction for complexity of computation,Protocol for Network of Networks,Transmission Control Protocol/Internet Protocol (TCP/IP) This protocol family is the basis of the Internet, a WAN protocol IP makes best effort to deliver TCP guarante
21、es delivery TCP/IP so popular it is used even when communicating locally: even across homogeneous LAN,FTP From Stanford to Berkeley,BARRNet is WAN for Bay Area T3 is 45 Mbit/s leased line (WAN); FDDI is 100 Mbit/s LAN IP sets up connection, TCP sends file,T3,FDDI,FDDI,Ethernet,Ethernet,Ethernet,Henn
22、essy,Patterson,FDDI,Protocol Family Concept,Message,Message,Message,Protocol Family Concept,Key to protocol families is that communication occurs logically at the same level of the protocol, called peer-to-peer, but is implemented via services at the lower level Danger is each level lower performanc
23、e if family is implemented as hierarchy (e.g., multiple check sums),TCP/IP packet, Ethernet packet, protocols,Application sends message,TCP breaks into 64KB segments, adds 20B header,IP adds 20B header, sends to network,If Ethernet, broken into 1500B packets with headers, trailers (24B),All Headers,
24、 trailers have length field, destination, .,Shared vs. Switched Based Networks,Shared Media vs. Switched: pairs communicate at same time: “point-to-point” connections Aggregate BW in switched network is many times shared point-to-point faster since no arbitration, simpler interface,Example of Networ
25、k Performance,Compare Ethernet, ATM sending a message with a 5 MB payload Time to send message: Overhead + Size/Data Rate Ethernet Overhead: 500 microseconds BW is 1.25 Mbyte/sec ATM Overhead: 600 microseconds BW is 10 Mbyte/sec Transmission time = Overhead + Size/(Data Rate),Example of Network Perf
26、ormance,Compare Ethernet (10 Mbit/sec), ATM (800 Mbit/sec) sending a message with a 250 Byte payload Time to send message: Overhead + Size/Data Rate Ethernet Overhead: 460 microseconds BW is 1.25 Mbyte/sec Time = 460 + 250/1.25 = 660 usecs ATM Overhead: 630 microseconds BW is 10 Mbyte/sec Time = 630
27、 + 250/10 = 655 microseconds,Limits to Performance of Networks,Layers of protocol Processor speed for protocol processing Memory speed for transferring messages Bus speed for connecting to computer Sharing of Network by multiple computers,I/O Pitfall: Relying on Peak Data Rates,Using the peak transf
28、er rate of a portion of the I/O system to make performance projections or performance comparisons Peak bandwidth measurements often based on unrealistic assumptions about system or unattainable because of other system limitations In example, Peak Bandwidth FDDI vs. 10 Mbit Ethernet = 10:1, but deliv
29、ered BW ratio (due to software overhead) is 1.01:1 Peak PCI BW is 132 MByte/sec, but combined with memory often 80 MB/s,Network Media (if time),“And in Conclusion” 1/1,Protocol suites allow heterogeneous networking Another use of principle of abstraction Protocols operation in presence of failures Standardization key for LAN, WAN Integrated circuit revolutionizing network switches as well as processors Switch just a specialized computer High bandwidth networks with slow SW overheads dont deliver their promise Next: Anatomy of disks, RAID,
