TCP/IP Programming Basics Recall the Basics of TCP/IP

TCP/IP Programming Basics recall the basics of TCP/IP

IP uses datagrams

TCP uses segments

TCP is stream-oriented (programs read and write byte streams to communicate)

“client” initiates connection with “server”

multiplexing (demultiplexing) uses ports

programs use sockets what is the difference between socket and port?

1 the socket approach client and server (these are just names for the two applications at the endpoints of a connection) read from sockets, write to

sockets, in the same way programs read from files and write to ¡ ¢¥¤ £ ¥¢ files using file descriptors like ¡£¢¥¤§¦©¨ , , etc

client server sockets

ports

connection

Host A Host B

2 useful programming background network programming is complicated because two programs are running at the same time (client and server)

useful to have two windows, one for client, one for server

can debug on one host, using loopback address

name — ¥ ¥¡£¢

address — port — any number greater than 1024; usually choose some number greater than 5000

system call to get time of day

system call to “timeout” if server unavailable

networking calls to convert name into address, etc (see Pocket Socket Guide)

know how to cast byte string in various ways

be aware of different binary representations on different hardware

multi-tasking basics needed if multiple concurrent clients per server

3 Linux and non-LAN Links modems, ISDN lines, etc — point to point links

not covered here, but textbook covers some of these

SLIP (Serial Line IP)

PPP (Point-to-Point Protocol)

SLiRP (TCP/IP emulator turning an ordinary shell into (C)SLIP/PPP) problem suppose host B is on the Internet, and we wish to have host A dial up (using SLIP/PPP) host B and then use Internet applications (www, etc)

host A does not have an IP address (and even if DHCP used, host B may not have an extra IP address to give to host A) — what can be done?

4 A B C SLIP Internet host A unknown in global Internet (only dialed in to B) — how can C and A communicate?

primitive solution: user on A uses ¢ ¨!"¢ to B (user must have account on host B), then all subsequent com- mands appear to originate at B

note: even X-windows is possible

¤§¦&¡('§") *,+- (use command such as ¡#"¢ $¨ % )

A B C packets for X-windows ftp, www, etc

drawback: can’t copy ﬁle from C to A easily

5 proxy ARP

idea: change ARP so that more than one IP address can be associated with an interface

motivation: to “extend” LAN to hosts that aren’t really on the LAN

x y dialup LAN

host x has two IP addresses I1 and I2, one for itself (I1) and I2 for y, so that the any datagram with destination I2 will arrive on x’s interface

if y isn’t dialed in when datagram with destination I2

arrives at x, then datagram will be discarded / note: for any host t in LAN, t . x, the ARP table for t can have two IP addresses (I1 and I2) for x’s hardware Ethernet address host x is similar to a gateway — but no need for a new subnet

6 more complicated use of proxy ARP

x LAN not on Internet y1 y3

LAN on Internet y2 y4

proxy ARP associates all IP addresses of y1–y4 with x’s interface on primary (Internet) LAN — so other hosts send anything for y1–y4 to x’s interface host x can use netmask in routing table to route y1, y2, etc via secondary interface with one row in x’s routing table

not a true subnet for more details, see Linux HOWTO for Proxy ARP

7 HTTP proxy

www server http proxy web users (yahoo)

using HTTP proxy server, many web users all appear (to yahoo) to have the same IP address — namely the proxy’s IP address

acts as “concentrator” of web requests

can be a shared cache of web pages

security possibilities (hide real IP address of user)

control possibilities (block some kinds of web pages)

transparent to users (proxy is almost invisible) what are shared themes of Proxy ARP and HTTP Proxy?

1. one host acts on behalf of others ( / proxy) 2. to outside hosts, proxy is not visible implies no need to change Internet protocols and servers! this is important design principle!