Carnegie MellonIntroduction to Computer Systems15‐213, fall 200921stLecture, Nov. 9thInstructors:Majd Sakr and Khaled HarrasCarnegie MellonToday Programmer’s view of the internet Sockets interfaceCarnegie MellonA 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 connectionCarnegie MellonIP 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 orderhtons: convert short int from host to network byte orderntohl: convert long int from network to host byte orderntohs: convert short int from network to host byte orderCarnegie MellonDotted Decimal Notation By convention, each byte in a 32‐bit IP address is represented by a string: decimal values for bytes, separated by a period IP address: 0x8002C2F2 = 128.2.194.242Blackboard?Carnegie MellonDotted Decimal Notation By convention, each byte in a 32‐bit IP address is represented by a string: decimal values for bytes, 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 representationCarnegie MellonIP Address Structure IP (V4) Address space divided into classes: Network ID written in form w.x.y.z/n n = number of bits in net id (yellow part above) E.g., CMU written as 128.2.0.0/16 Which class is that? Unrouted (private) IP addresses: 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16 Nowadays: CIDR (Classless interdomain routing)Class AClass BClass CClass DClass E0 1 2 3 8 16 24 310Net ID Host IDHost IDHost IDNet IDNet IDMulticast addressReserved for experiments1 01 011 1 011 1 11Carnegie MellonInternet Domain Names.net .edu .gov .comcmu berkeleymitcs ecekittyhawk128.2.194.242cmclunnamed rootpdlimperial128.2.189.40 amazonwww208.216.181.15First‐level domain namesSecond‐level domain namesThird‐level domain namesCarnegie MellonDomain Naming System (DNS) The Internet maintains a mapping between IP addresses and domain names in a huge worldwide distributed DNS database 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*/ };Carnegie MellonProperties 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 localhostwhich 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.eduCarnegie MellonA Program That Queries DNSint 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);elsehostp = 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));}}Carnegie MellonQuerying DNS from the Command Line Domain Information Groper (dig) provides a scriptable command line interface to DNSlinux> dig +short kittyhawk.cmcl.cs.cmu.edu128.2.194.242 linux> dig +short -x 128.2.194.242 KITTYHAWK.CMCL.CS.CMU.EDU. linux> dig +short aol.com205.188.145.215 205.188.160.121 64.12.149.24 64.12.187.25 linux> dig +short -x 64.12.187.25 aol-v5.websys.aol.com.Carnegie MellonInternet 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)Carnegie MellonPutting it all Together: Anatomy of an Internet ConnectionConnection socket pair(128.2.194.242:51213, 208.216.181.15:80)Server(port 80)ClientClient socket address128.2.194.242:51213Server socket addr ess208.216.181.15:80Client host address128.2.194.242 Server host address208.216.181.1551213 is an ephemeral port allocated by the kernel 80 is a well‐known portassociated with Web serversCarnegie MellonClients Examples of client programs Web browsers, ftp, telnet, ssh How
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