1. Carnegie Mellon Introduction to Computer Systems 15-213/18-243, spring 2009 22 nd Lecture, Apr. 9 th Instructors: Gregory Kesden and Markus Püschel

2. Carnegie Mellon Exam 2 (This Section)

3. Carnegie Mellon Last Time: Open Files in Unix Two descriptors referencing two distinct open disk files. Descriptor 1 (stdout) points to terminal, and descriptor 4 points to open disk file fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table [one table per process] Open file table [shared by all processes] v-node table [shared by all processes] File pos refcnt=1... File pos refcnt=1... stderr stdout stdin File access... File size File type File access... File size File type File A (terminal) File B (disk) Info in stat struct

4. Carnegie Mellon Last Time: Unix I/O file fd memory pospos (after read or write) n bytes buf readwrite ssize_t read(int fd, void *buf, size_t n) ssize_t write(int fd, void *buf, size_t n)

5. Carnegie Mellon Last Time: Standard I/O (Buffering) memory freadfwrite file fd buffer data transferred in chunks abstracted as stream: FILE

6. Carnegie Mellon Last Time: Robust I/O rio_readn /* * rio_readn - robustly read n bytes (unbuffered) */ ssize_t rio_readn(int fd, void *usrbuf, size_t n) { size_t nleft = n; ssize_t nread; char *bufp = usrbuf; while (nleft > 0) { if ((nread = read(fd, bufp, nleft)) < 0) { if (errno == EINTR) /* interrupted by sig handler return */ nread = 0; /* and call read() again */ else return -1; /* errno set by read() */ } else if (nread == 0) break; /* EOF */ nleft -= nread; bufp += nread; } return (n - nleft); /* return >= 0 */ }

7. Carnegie Mellon Today System level I/O  Unix I/O  Standard I/O  RIO (robust I/O) package  Conclusions and examples Internetworking  Networks  Global IP Internet

8. Carnegie Mellon Buffered I/O: Motivation I/O Applications Read/Write One Character at a Time  getc, putc, ungetc  gets  Read line of text, stopping at newline Implementing as Calls to Unix I/O Expensive  Read & Write involve require Unix kernel calls  > 10,000 clock cycles Buffered Read  Use Unix read() to grab block of bytes  User input functions take one byte at a time from buffer  Refill buffer when empty unreadalready read Buffer

9. Carnegie Mellon unread Buffered I/O: Implementation For reading from file File has associated buffer to hold bytes that have been read from file but not yet read by user code Layered on Unix File already read Buffer rio_buf rio_bufptr rio_cnt unreadalready readnot in bufferunseen Current File Position Buffered Portion

10. Carnegie Mellon Buffered I/O: Declaration All information contained in struct typedef struct { int rio_fd; /* descriptor for this internal buf */ int rio_cnt; /* unread bytes in internal buf */ char *rio_bufptr; /* next unread byte in internal buf */ char rio_buf[RIO_BUFSIZE]; /* internal buffer */ } rio_t; unreadalready read Buffer rio_buf rio_bufptr rio_cnt

11. Carnegie Mellon RIO: Robustness and Buffering Robust Buffered read rio_read rio_readn rio_readnb

12. Carnegie Mellon Buffered RIO Input Functions Efficiently read text lines and binary data from a file partially cached in an internal memory buffer  rio_readlineb reads a text line of up to maxlen bytes from file fd and stores the line in usrbuf  Especially useful for reading text lines from network sockets  Stopping conditions  maxlen bytes read  EOF encountered  Newline (‘ \n ’) encountered #include "csapp.h" void rio_readinitb(rio_t *rp, int fd); ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen); Return: num. bytes read if OK, 0 on EOF, -1 on error

13. Carnegie Mellon Buffered RIO Input Functions (cont)  rio_readnb reads up to n bytes from file fd  Stopping conditions  maxlen bytes read  EOF encountered  Calls to rio_readlineb and rio_readnb can be interleaved arbitrarily on the same descriptor  Warning: Don’t interleave with calls to rio_readn #include "csapp.h" void rio_readinitb(rio_t *rp, int fd); ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen); ssize_t rio_readnb(rio_t *rp, void *usrbuf, size_t n); Return: num. bytes read if OK, 0 on EOF, -1 on error

14. Carnegie Mellon RIO Example Copying the lines of a text file from standard input to standard output #include "csapp.h" int main(int argc, char **argv) { int n; rio_t rio; char buf[MAXLINE]; Rio_readinitb(&rio, STDIN_FILENO); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) Rio_writen(STDOUT_FILENO, buf, n); exit(0); }

15. Carnegie Mellon Today System level I/O  Unix I/O  Standard I/O  RIO (robust I/O) package  Conclusions and examples Internetworking  Networks  Global IP Internet

16. Carnegie Mellon Choosing I/O Functions General rule: use the highest-level I/O functions you can  Many C programmers are able to do all of their work using the standard I/O functions When to use standard I/O  When working with disk or terminal files When to use raw Unix I/O  When you need to fetch file metadata  In rare cases when you need absolute highest performance When to use RIO  When you are reading and writing network sockets or pipes  Never use standard I/O or raw Unix I/O on sockets or pipes

17. Carnegie Mellon For Further Information The Unix bible:  W. Richard Stevens & Stephen A. Rago, Advanced Programming in the Unix Environment, 2 nd Edition, Addison Wesley, 2005  Updated from Stevens’ 1993 book Stevens is arguably the best technical writer ever.  Produced authoritative works in:  Unix programming  TCP/IP (the protocol that makes the Internet work)  Unix network programming  Unix IPC programming Tragically, Stevens died Sept. 1, 1999  But others have taken up his legacy

18. Carnegie Mellon Fun with File Descriptors (1) What would this program print for file containing “abcde”? #include "csapp.h" int main(int argc, char *argv[]) { int fd1, fd2, fd3; char c1, c2, c3; char *fname = argv[1]; fd1 = Open(fname, O_RDONLY, 0); fd2 = Open(fname, O_RDONLY, 0); fd3 = Open(fname, O_RDONLY, 0); Dup2(fd2, fd3); Read(fd1, &c1, 1); Read(fd2, &c2, 1); Read(fd3, &c3, 1); printf("c1 = %c, c2 = %c, c3 = %c\n", c1, c2, c3); return 0; }

19. Carnegie Mellon Fun with File Descriptors (2) What would this program print for file containing “abcde”? #include "csapp.h" int main(int argc, char *argv[]) { int fd1; int s = getpid() & 0x1; char c1, c2; char *fname = argv[1]; fd1 = Open(fname, O_RDONLY, 0); Read(fd1, &c1, 1); if (fork()) { /* Parent */ sleep(s); Read(fd1, &c2, 1); printf("Parent: c1 = %c, c2 = %c\n", c1, c2); } else { /* Child */ sleep(1-s); Read(fd1, &c2, 1); printf("Child: c1 = %c, c2 = %c\n", c1, c2); } return 0; }

20. Carnegie Mellon Fun with File Descriptors (3) What would be the contents of the resulting file? #include "csapp.h" int main(int argc, char *argv[]) { int fd1, fd2, fd3; char *fname = argv[1]; fd1 = Open(fname, O_CREAT|O_TRUNC|O_RDWR, S_IRUSR|S_IWUSR); Write(fd1, "pqrs", 4); fd3 = Open(fname, O_APPEND|O_WRONLY, 0); Write(fd3, "jklmn", 5); fd2 = dup(fd1); /* Allocates descriptor */ Write(fd2, "wxyz", 4); Write(fd3, "ef", 2); return 0; }

21. Carnegie Mellon Accessing Directories Only recommended operation on a directory: read its entries  dirent structure contains information about a directory entry  DIR structure contains information about directory while stepping through its entries #include { DIR *directory; struct dirent *de;... if (!(directory = opendir(dir_name))) error("Failed to open directory");... while (0 != (de = readdir(directory))) { printf("Found file: %s\n", de->d_name); }... closedir(directory); }

22. Carnegie Mellon Unix I/O Key Characteristics Classic Unix/Linux I/O: I/O operates on linear streams of bytes  Can reposition insertion point and extend file at end I/O tends to be synchronous  Read or write operation block until data has been transferred Fine grained I/O  One key-stroke at a time  Each I/O event is handled by the kernel and an appropriate process Mainframe I/O: I/O operates on structured records  Functions to locate, insert, remove, update records I/O tends to be asynchronous  Overlap I/O and computation within a process Coarse grained I/O  Process writes “channel programs” to be executed by the I/O hardware  Many I/O operations are performed autonomously with one interrupt at completion

23. Carnegie Mellon Unix I/O vs. Standard I/O vs. RIO Standard I/O and RIO are implemented using low-level Unix I/O Which ones should you use in your programs? Unix I/O functions (accessed via system calls) Standard I/O functions C application program fopen fdopen fread fwrite fscanf fprintf sscanf sprintf fgets fputs fflush fseek fclose open read write lseek stat close rio_readn rio_writen rio_readinitb rio_readlineb rio_readnb RIO functions

24. Carnegie Mellon Pros and Cons of Unix I/O Pros  Unix I/O is the most general and lowest overhead form of I/O  All other I/O packages are implemented using Unix I/O functions  Unix I/O provides functions for accessing file metadata Cons  Dealing with short counts is tricky and error prone  Efficient reading of text lines requires some form of buffering, also tricky and error prone  Both of these issues are addressed by the standard I/O and RIO packages

25. Carnegie Mellon Pros and Cons of Standard I/O Pros  Buffering increases efficiency by decreasing the number of read and write system calls  Short counts are handled automatically Cons  Provides no function for accessing file metadata  Standard I/O is not appropriate for input and output on network sockets  There are poorly documented restrictions on streams that interact badly with restrictions on sockets

26. Carnegie Mellon Working with Binary Files Binary File Examples  Object code  Images (JPEG, GIF)  Arbitrary byte values Functions you shouldn’t use  Line-oriented I/O  fgets, scanf, printf, rio_readlineb –use rio_readn or rio_readnb instead  Interprets byte value 0x0A (‘ \n ’) as special  String functions  strlen, strcpy  Interprets byte value 0 as special

27. Carnegie Mellon Java I/O Standard Java Streams are Unbuffered  Every read/write call invokes OS  Preferable to “wrap” stream with buffered stream Java Distinguishes Characters from Bytes  Characters: Various encodings to allow more than ASCII characters  Bytes: Always 8 bits. Used for binary data BufferedReader in = new BufferedReader(new FileReader("char-in.txt")); BufferedWriter out = new BufferedWriter(new FileWriter("char-out.txt")); BufferedInputStream in = new BufferedInputStream(new FileInputStream("binary-in.txt")); BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream("binary-out.txt"));

28. Carnegie Mellon Today System level I/O  Unix I/O  Standard I/O  RIO (robust I/O) package  Conclusions and examples Internetworking  Networks  Global IP Internet

29. Carnegie Mellon A Client-Server Transaction Client process Server process 1. Client sends request 2. Server handles request 3. Server sends response 4. Client handles response Resource Most network applications are based on the client-server model:  A server process and one or more client processes  Server manages some resource  Server provides service by manipulating resource for clients  Server activated by request from client (vending machine analogy) Note: clients and servers are processes running on hosts (can be the same or different hosts)

30. Carnegie Mellon Hardware Organization of a Network Host main memory I/O bridge MI ALU register file CPU chip system busmemory bus disk controller graphics adapter USB controller mousekeyboardmonitor disk I/O bus Expansion slots network adapter network

31. Carnegie Mellon Computer Networks A network is a hierarchical system of boxes and wires organized by geographical proximity  SAN (System Area Network) spans cluster or machine room  Switched Ethernet, Quadrics QSW, …  LAN (Local Area Network) spans a building or campus  Ethernet is most prominent example  WAN (Wide Area Network) spans country or world  Typically high-speed point-to-point phone lines An internetwork (internet) is an interconnected set of networks  The Global IP Internet (uppercase “I”) is the most famous example of an internet (lowercase “i”) Let’s see how an internet is built from the ground up

32. Carnegie Mellon Lowest Level: Ethernet Segment Ethernet segment consists of a collection of hosts connected by wires (twisted pairs) to a hub Spans room or floor in a building Operation  Each Ethernet adapter has a unique 48-bit address (MAC address)  Hosts send bits to any other host in chunks called frames  Hub slavishly copies each bit from each port to every other port  Every host sees every bit  Note: Hubs are on their way out. Bridges (switches, routers) became cheap enough to replace them (means no more broadcasting) host hub 100 Mb/s port

33. Carnegie Mellon Next Level: Bridged Ethernet Segment Spans building or campus Bridges cleverly learn which hosts are reachable from which ports and then selectively copy frames from port to port host hub bridge 100 Mb/s host hub 100 Mb/s 1 Gb/s host bridge host hub AB C X Y

34. Carnegie Mellon Conceptual View of LANs For simplicity, hubs, bridges, and wires are often shown as a collection of hosts attached to a single wire: host...

35. Carnegie Mellon Next Level: internets Multiple incompatible LANs can be physically connected by specialized computers called routers The connected networks are called an internet host... host... WAN LAN 1 and LAN 2 might be completely different, totally incompatible (e.g., Ethernet and Wifi, 802.11*, T1-links, DSL, …) router LAN

36. Carnegie Mellon Logical Structure of an internet Ad hoc interconnection of networks  No particular topology  Vastly different router & link capacities Send packets from source to destination by hopping through networks  Router forms bridge from one network to another  Different packets may take different routes router host

37. Carnegie Mellon The Notion of an internet Protocol How is it possible to send bits across incompatible LANs and WANs? Solution:  protocol software running on each host and router  smooths out the differences between the different networks Implements an internet protocol (i.e., set of rules)  governs how hosts and routers should cooperate when they transfer data from network to network  TCP/IP is the protocol for the global IP Internet

38. Carnegie Mellon What Does an internet Protocol Do? Provides a naming scheme  An internet protocol defines a uniform format for host addresses  Each host (and router) is assigned at least one of these internet addresses that uniquely identifies it Provides a delivery mechanism  An internet protocol defines a standard transfer unit (packet)  Packet consists of header and payload  Header: contains info such as packet size, source and destination addresses  Payload: contains data bits sent from source host

39. Carnegie Mellon LAN2 Transferring Data Over an internet protocol software client LAN1 adapter Host A LAN1 data (1) dataPHFH1 (4) dataPHFH2 (6) data (8) dataPHFH2 (5) LAN2 frame protocol software LAN1 adapter LAN2 adapter Router dataPH (3) FH1 dataPHFH1 (2) internet packet LAN1 frame (7) dataPHFH2 protocol software server LAN2 adapter Host B PH: Internet packet header FH: LAN frame header

40. Carnegie Mellon Other Issues We are glossing over a number of important questions:  What if different networks have different maximum frame sizes? (segmentation)  How do routers know where to forward frames?  How are routers informed when the network topology changes?  What if packets get lost? These (and other) questions are addressed by the area of systems known as computer networking

41. Carnegie Mellon Today System level I/O  Unix I/O  Standard I/O  RIO (robust I/O) package  Conclusions and examples Internetworking  Networks  Global IP Internet

42. Carnegie Mellon Global IP Internet Most famous example of an internet Based on the TCP/IP protocol family  IP (Internet protocol) :  Provides basic naming scheme and unreliable delivery capability of packets (datagrams) from host-to-host  UDP (Unreliable Datagram Protocol)  Uses IP to provide unreliable datagram delivery from process-to-process  TCP (Transmission Control Protocol)  Uses IP to provide reliable byte streams from process-to-process over connections Accessed via a mix of Unix file I/O and functions from the sockets interface

43. Carnegie Mellon Hardware and Software Organization of an Internet Application TCP/IP Client Network adapter Global IP Internet TCP/IP Server Network adapter Internet client hostInternet server host Sockets interface (system calls) Hardware interface (interrupts) User code Kernel code Hardware and firmware

44. Carnegie Mellon Basic Internet Components Internet backbone:  collection of routers (nationwide or worldwide) connected by high-speed point-to-point networks Network Access Point (NAP):  router that connects multiple backbones (often referred to as peers) Regional networks:  smaller backbones that cover smaller geographical areas (e.g., cities or states) Point of presence (POP):  machine that is connected to the Internet Internet Service Providers (ISPs):  provide dial-up or direct access to POPs

45. Carnegie Mellon NAP-Based Internet Architecture NAPs link together commercial backbones provided by companies such as AT&T and Worldcom Currently in the US there are about 50 commercial backbones connected by ~12 NAPs (peering points) Similar architecture worldwide connects national networks to the Internet

46. Carnegie Mellon Internet Connection Hierarchy NAP Backbone NAP POP Regional net POP Small Business Big BusinessISP POP Pgh employee Cable modem DC employee POP T3 T1 ISP (for individuals) POP DSL T1 Colocation sites Private “peering” agreements between two backbone companies often bypass NAP

47. Carnegie Mellon Network Access Points (NAPs) Source: Boardwatch.com Note: Peers in this context are commercial backbones (droh)

48. Carnegie Mellon Source: http://personalpages.manchester.ac.uk/staff/m.dodge/cybergeography/atlas/ MCI/WorldCom/UUNET Global Backbone

49. Carnegie Mellon Naming and Communicating on the Internet Original Idea  Every node on Internet would have unique IP address  Everyone would be able to talk directly to everyone  No secrecy or authentication  Messages visible to routers and hosts on same LAN  Possible to forge source field in packet header Shortcomings  There aren't enough IP addresses available  Don't want everyone to have access or knowledge of all other hosts  Security issues mandate secrecy & authentication

50. Carnegie Mellon Evolution of Internet: Naming Dynamic address assignment  Most hosts don't need to have known address  Only those functioning as servers  DHCP (Dynamic Host Configuration Protocol)  Local ISP assigns address for temporary use Example:  My laptop at CMU  IP address 128.2.220.249 ( bryant-tp3.cs.cmu.edu )  Assigned statically  My laptop at home  IP address 205.201.7.7 ( dhcp-7-7.dsl.telerama.com)  Assigned dynamically by my ISP for my DSL service

51. Carnegie Mellon Evolution of Internet: Firewalls Firewalls  Hides organizations nodes from rest of Internet  Use local IP addresses within organization  For external service, provides proxy service 1. Client request: src=10.2.2.2, dest=216.99.99.99 2. Firewall forwards: src=176.3.3.3, dest=216.99.99.99 3. Server responds: src=216.99.99.99, dest=176.3.3.3 4. Firewall forwards response: src=216.99.99.99, dest=10.2.2.2 Corporation X Firewall Internet 10.2.2.2 1 4 2 3 176.3.3.3 216.99.99.99

52. Carnegie Mellon Virtual Private Networks Supporting road warrior  Employee working remotely with assigned IP address 198.3.3.3  Wants to appear to rest of corporation as if working internally  From address 10.6.6.6  Gives access to internal services (e.g., ability to send mail) Virtual Private Network (VPN)  Overlays private network on top of regular Internet Corporation X Internet 10.x.x.x 198.3.3.3 Firewall 10.6.6.6

53. Carnegie Mellon A Programmer’s View of the Internet Hosts are mapped to a set of 32-bit IP addresses  128.2.203.179 The set of IP addresses is mapped to a set of identifiers called Internet domain names  128.2.203.179 is mapped to www.cs.cmu.edu A process on one Internet host can communicate with a process on another Internet host over a connection

54. Carnegie Mellon IP Addresses 32-bit IP addresses are stored in an IP address struct  IP addresses are always stored in memory in network byte order (big-endian byte order)  True in general for any integer transferred in a packet header from one machine to another.  E.g., the port number used to identify an Internet connection. /* Internet address structure */ struct in_addr { unsigned int s_addr; /* network byte order (big-endian) */ }; Useful network byte-order conversion functions: htonl: convert long int from host to network byte order htons: convert short int from host to network byte order ntohl: convert long int from network to host byte order ntohs: convert short int from network to host byte order

55. Carnegie Mellon Dotted Decimal Notation By convention, each byte in a 32-bit IP address is represented by its decimal value and separated by a period  IP address: 0x8002C2F2 = 128.2.194.242 Functions for converting between binary IP addresses and dotted decimal strings:  inet_aton : dotted decimal string → IP address in network byte order  inet_ntoa : IP address in network byte order → dotted decimal string  “n” denotes network representation  “a” denotes application representation Blackboard?

56. Carnegie Mellon Dotted Decimal Notation By convention, each byte in a 32-bit IP address is represented by its decimal value and separated by a period  IP address: 0x8002C2F2 = 128.2.194.242 Functions for converting between binary IP addresses and dotted decimal strings:  inet_aton : dotted decimal string → IP address in network byte order  inet_ntoa : IP address in network byte order → dotted decimal string  “n” denotes network representation  “a” denotes application representation

57. Carnegie Mellon IP Address Structure IP (V4) Address space divided into classes: Network ID Written in form w.x.y.z/n  n = number of bits in host address  E.g., CMU written as 128.2.0.0/16  Class B address Unrouted (private) IP addresses: 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 Class A Class B Class C Class D Class E 0 1 2 3 8 16 24 31 0 Net IDHost ID Net ID Multicast address Reserved for experiments 10 101 1101 1111

58. Carnegie Mellon Internet Domain Names.net.edu.gov.com cmuberkeleymit csece kittyhawk 128.2.194.242 cmcl unnamed root pdl imperial 128.2.189.40 amazon www 208.216.181.15 First-level domain names Second-level domain names Third-level domain names

59. Carnegie Mellon Domain Naming System (DNS) The Internet maintains a mapping between IP addresses and domain names in a huge worldwide distributed database called DNS  Conceptually, programmers can view the DNS database as a collection of millions of host entry structures: Functions for retrieving host entries from DNS:  gethostbyname: query key is a DNS domain name.  gethostbyaddr: query key is an IP address. /* DNS host entry structure */ struct hostent { char *h_name; /* official domain name of host */ char **h_aliases; /* null-terminated array of domain names */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of an address, in bytes */ char **h_addr_list; /* null-terminated array of in_addr structs */ };

60. Carnegie Mellon Properties of DNS Host Entries Each host entry is an equivalence class of domain names and IP addresses Each host has a locally defined domain name localhost which always maps to the loopback address 127.0.0.1 Different kinds of mappings are possible:  Simple case: one-to-one mapping between domain name and IP address:  kittyhawk.cmcl.cs.cmu.edu maps to 128.2.194.242  Multiple domain names mapped to the same IP address:  eecs.mit.edu and cs.mit.edu both map to 18.62.1.6  Multiple domain names mapped to multiple IP addresses:  aol.com and www.aol.com map to multiple IP addresses  Some valid domain names don’t map to any IP address:  for example: cmcl.cs.cmu.edu

61. Carnegie Mellon A Program That Queries DNS int main(int argc, char **argv) { /* argv[1] is a domain name */ char **pp; /* or dotted decimal IP addr */ struct in_addr addr; struct hostent *hostp; if (inet_aton(argv[1], &addr) != 0) hostp = Gethostbyaddr((const char *)&addr, sizeof(addr), AF_INET); else hostp = Gethostbyname(argv[1]); printf("official hostname: %s\n", hostp->h_name); for (pp = hostp->h_aliases; *pp != NULL; pp++) printf("alias: %s\n", *pp); for (pp = hostp->h_addr_list; *pp != NULL; pp++) { addr.s_addr = ((struct in_addr *)*pp)->s_addr; printf("address: %s\n", inet_ntoa(addr)); }

62. Carnegie Mellon Querying DNS from the Command Line Domain Information Groper ( dig ) provides a scriptable command line interface to DNS linux> dig +short kittyhawk.cmcl.cs.cmu.edu 128.2.194.242 linux> dig +short -x 128.2.194.242 KITTYHAWK.CMCL.CS.CMU.EDU. linux> dig +short aol.com 205.188.145.215 205.188.160.121 64.12.149.24 64.12.187.25 linux> dig +short -x 64.12.187.25 aol-v5.websys.aol.com.

63. Carnegie Mellon Internet Connections Clients and servers communicate by sending streams of bytes over connections:  Point-to-point, full-duplex (2-way communication), and reliable. A socket is an endpoint of a connection  Socket address is an IPaddress:port pair A port is a 16-bit integer that identifies a process:  Ephemeral port: Assigned automatically on client when client makes a connection request  Well-known port: Associated with some service provided by a server (e.g., port 80 is associated with Web servers) A connection is uniquely identified by the socket addresses of its endpoints (socket pair)  (cliaddr:cliport, servaddr:servport)

64. Carnegie Mellon Putting it all Together: Anatomy of an Internet Connection Connection socket pair (128.2.194.242:51213, 208.216.181.15:80) Server (port 80) Client Client socket address 128.2.194.242:51213 Server socket address 208.216.181.15:80 Client host address 128.2.194.242 Server host address 208.216.181.15

65. Carnegie Mellon Next Time How to use the sockets interface to establish Internet connections between clients and servers How to use Unix I/O to copy data from one host to another over an Internet connection