1. TCP and Sockets Lecture 3
cs193i – Internet Technologies
Summer 2004
Stanford University

2. Sending a Message Ron Leslie Leland.Stanford.edu Arachne . Berkeley
Network Layer
Link Layer
Ron
Leslie
Leland.Stanford.edu
Arachne .
Berkeley
.edu
Application Layer
Transport Layer
O.S.
Header
Data H D

3. Protocol Stack Application End-to-End Network Link Level
Ethernet, token ring
Network IP
End-to-End
TCP, UDP
Application
HTTP, SMTP, FTP,
TELNET, DNS

4. Protocol Stack Application End-to-End Network Link Level
Ethernet, token ring
Network IP
End-to-End
TCP, UDP
Application
HTTP, SMTP, FTP,
TELNET, DNS

5. Protocol Stack Application End-to-End Network Link Level
Ethernet, token ring
Network IP
End-to-End
TCP, UDP
Application
HTTP, SMTP, FTP,
TELNET, DNS

6. Protocol Stack Application End-to-End Network Link Level
Ethernet, token ring
Network IP
End-to-End
TCP, UDP
Application
HTTP, SMTP, FTP,
TELNET, DNS

7. TCP Connection-Oriented Reliable Byte-Stream Flow Control
Port on “client” connects to port on “server”
Reliable
3-way handshake
Byte-Stream
To application, looks like stream of bytes flowing between two hosts
Flow Control
Prevents overrunning receiver / network capacity
First, it is connection-oriented, unlike IP.
IP, where each packet is conceptually separate... TCP packets are part of one connection
In Addition, the connection is two-way. Software can write bytes to the connection, and it shows up other side.
Second, it’s End-to-End Reliable
The destination will reassemble and error check packets
Lost, Corrupted packets will be resent by the sender
Third, TCP provides illusion of continuous, byte stream from sender to recipient.
Bytes written 1234 by the sender... will show up 1234 at the receiver
Finally, TCP has Flow Control which means that
Sender wills slow down to a packet rate that the receiver and network can cope with. The way it does this is with ACK packets. Helps to share network capacity between users...

8. Establishing the Connection
Client
Sequence # (x)
Server
ACK (x+1)
Own sequence # (y)
ACK (y+1)
Sequence # (x+1)

9. Maintaining the “Connection”IP
Hdr
IP Data
TCP
TCP Data
Src
port
Dst port
Sequence #
Ack
HLEN 4
RSVD 6
URG
ACK
PSH
RST
SYN
FIN
Flags
Window Size
Checksum
Urg
Pointer
(TCP Options) 15 31
/dst port numbers
and IP addresses
uniquely identify socket

10. Terminating the Connection
Connection Close/Teardown
2 x 2-way handshake
(Active)
Client
(Passive)
Server
Fin
(Data +)
Ack

11. Another Transport Layer Protocol: UDP
Basically allows access to IP functionality without TCP overhead...

12. User Datagram Protocol (UDP)
Characteristics
Connectionless, Datagram, Unreliable
Adds only application multiplexing/demultiplexing and checksumming to IP
Good for Streaming Media, Real-time Multiplayer Networked Games, VoIP
No connection establishment.... packets can show up at any time.... the App has to handle whether you are prepared for the incoming data or not...
Datagram.... means UDP packets are self contained no sequencing like in TCP
Unreliable like IP, no ACKs... no flow control, no automatic retransmission...
Adds checksumming the headers.... to prevent errors... Adds application multiplexing, to send the UDP packet to the correct service. Both TCP and UDP perform this demultiplexing, and they use port numbers to do it....

13. Summary IP is the basis of Internetworking
TCP builds on top of IP adds reliable, congestion-controlled, connection-oriented byte-stream.
UDP builds on top of IP allows access to IP functionality

14. Brief Return to Addressing
Broadcast
Send message to everyone on LAN
Address Resolution Protocol (ARP)
Broadcast “Who has IP address ?”
Owner of IP address answers with LAN address

15. Addressing in Action IP Address Subnet Mask Gateway DNS server
Last 8 bits used for host
Gateway
Local router to forward traffic to
DNS server
Translates names to IP addresses

16. Domain Name Service (DNS)
Distributed database maps
host names --> numerical IP Address
Hierarchical Namespace
Top-level domain (root domain)
.com, .net, .org
Second-level domain
hotmail.com, stanford.edu
Sub domains
movies.yahoo.com

17. DNS and Routing Both are Hierarchical
IP Routing Hierarchy is left to right
( )
DNS Hierarchy is right to left (
Root name server & delegate name server
Backbone router & delegate router

18. Network Address Translation (NAT)
Hosts share single IP address
Hosts IP addresses in *.*
Router between LAN and Internet
IP address ( )
Translates to host address before forwarding

19. Five Minute Break

20. Sockets Basic Concepts & Important Issues
Client Socket Code & Server Socket Code
Client-Server Interaction

21. What is a Socket? Interface to the TCP Byte-Stream (2-way)
Both sides think they are writing to Files!
Example Code
write(SOCK, “Hello\n”);
print SOCK “Hello\n”;

22. TCP Byte Stream (Virtual Circuit)

23. Protocol Stack
SOCKETS

24. What is a Port?
Transport address to which processes can listen for connection requests
Local to host: ports
Well-known ports
Below 1024
Standard services
Only supervisor privileged
enough to access
Examples
FTP: 21
HTTP: 80
TELNET: 23

25. Server and Client host:port host:port host:port Server listens
on a port #
host:port
Clients make “calls”
to that port #

26. Server and client can be the same machine!
SSH
SSHD
Client Process
Server Process

27. Client Side

28. Establishing a Connection
Establish Socket Connection
Send & Receive Data
Close Socket Connection
Remember how TCP is connection-oriented,
and has three phases of a connection?

29. Once the connection is established...
Protocol (RFC) dictates turn taking between Server and Client
Write by saying in Perl:
print SOCK “Hello\n”;
Read by saying in Perl:
$incoming_data = <SOCK>;

30. Example Sockets in Perl
$ipaddr = inet_aton(”
$sockaddr = sockaddr_in(80, $ipaddr);
socket($sock, .... );
connect($sock, $a);
print $sock “Hi There!”

31. Socket Example in C struct sockaddr_in peer;
peer.sin_addr.s_addr = inet_addr(” ”);
peer.sin_port = htons(7500);
peer.sin_family = AF_INET;
s = socket(AF_INET, SOCK_STREAM,0);
connect(s, (struct sockaddr *)&peer, sizeof(peer));
send(s, “Hi There!”, 9, 0);

32. Demo Code in Java myClient = new Socket(”www.yahoo.com”, 80);
outputStream = new PrintStream(myClient.getOutputStream());
outputStream.println(”Hello There!”);

33. Blocking vs. Non-Blocking
Blocking Function Call
Waits if necessary
<SOCK> blocks if no data to read
OS will wake up the process to read
Also write blocks
(send faster than recipient can handle)
Non-Blocking Function Call
Returns immediately
May need a while loop to check for data

34. Buffering Why is it good normally?
Batch the work for efficiency (Harddrives...)
Prevent Failures (CD Players, etc...)
Concept of flushing the buffer (write to disk/network)
Why is it a problem for networking?

35. Autoflush Automatically flush after every write
use FileHandle; # FileHandle module...
....
autoflush SOCK, 1; # set autoflush

36. Other Issues Irregular Timing (CPU fast, mostly blocked)
Irregular Sizing
Line Endings
\r\n -- most common on Internet, oldest
\n -- Unix way (single char, nice)
\n may get remapped! Messes up portability.
In your code... really is \015\012

37. Setting Up a Socket Program
1. Hostname to IPAddr conversion
$ip = inet_aton($hostname);
2. Create Socket Address
$sockaddr = sockaddr_in($port, $ip);
3. Allocate Socket
socket(SOCK, PF_INET, SOCK_STREAM, 0);
4. Connect
connect(SOCK, $sockaddr);

38. Reading from a Socket connect(SOCK, $sockaddr); $line = <SOCK>;
$line =~ s/\015\012//g;
Turn-taking via EOF
while ($line = <SOCK>) {
..... }
OR sysread(SOCK, str, length); ## efficient

39. sysread vs <> Read bytes sysread(SOCK, ...)
Read line by line <SOCK>
Do things in larger chunks!
Just like in Buffering...

40. Writing to a Socket print SOCK “Hello!\012”;
OR syswrite(SOCK, str, length);

41. Close the Socket close(SOCK); ## sends EOF to other end
Other side sees all data, then EOF... then quits
Consider it an end “marker” that is sent across

42. Server Side

43. Server Listens on Port Create the Socket as before Bind socket
bind(...) associates port & socket
Listen at a port
Does not block.
Accept an incoming connection
(so server must sit on the Internet all day)
This call blocks! Returns when client connects...

44. Server Listens on Port Look at Client Address (caller id) Read & Write
Close
Loop and listen again...

45. Perl Server Setup Example
sub CreateServerSocket {
my($sock, $port)
socket($sock, PF_INET, SOCK_STREAM, 0);
setsockopt($sock, SOL_SOCKET, SO_REUSEADDR, pack("l",1));
bind($sock, sockaddr_in($port, INADDR_ANY));
autoflush $sock, 1;
return(""); }

46. Listen and Accept listen (SERVER, ...); accept (CLIENT, SERVER);
autoflush CLIENT, 1;

47. Server Pseudo-code $serverport = 3456;
CreateServerSocket(SERVER, $serverport);
listen(SERVER,...);
while($clientaddr = accept(CLIENT, SERVER)) {
$incomingline = <CLIENT>;
print CLIENT “$outgoingline\n”;
close(CLIENT); }

48. Telnet Trick
Many Services just use ASCII text dialog between server and client
> telnet host port
Bottom Line: Text based protocols are easier to debug because their state is visible and accessible

49. Design of a Chat Room Server/Client
Server listens
Clients call

50. Chat Server Design Server State Server Abilities Who is connected
Max clients connected
Server Abilities
Accept a New Connection
Close a Connection
Send Message from A --> B
Send Message from A --> All

51. Chat Server Design Main Thread Handler Threads Handles connections
Calls handler threads
Handler Threads
<CLIENT>
print CLIENT “Hi!\n”;
close CLIENT;

52. Chat Client Design Client State Client Abilities
Server’s IP address : port
Who is connected to server
Client Abilities
Connect to server
Send message to A (thru server handler)
Send message to all (thru server handler)
Leave server
(e.g. print SOCK “BYE”, close SOCK)

53. Peer-to-Peer No fixed Server & Client
Anybody can be Server or Client at any time
Example P2P software
File Sharing (Bittorrent, FastTrack, Gnutella)

54. Socket Summary Client Socket Server Socket
Setup/Create, Connect, Read/Write, Close
Server Socket
Setup/Create, Bind, Listen, Accept, Read/Write, Close
It’s just a Programmer’s Application Programming Interface (API) to TCP or UDP on top of IP