1.  Inter process communication (IPC) between
processes on different hosts over the network.
 IPC has Two Forms :
◦ Local IPC
◦ Network IPC

3. ◦ Local IPC
 Communication between local processes (on same host)
 PIPE
 FIFO
 System V IPC
 Message queues
 Semaphores
 Shared Memory
◦ Network IPC
 Communication between processes on different host
 socket

5.  Pipe
 FIFO
 Message Queues
 Shared Memory
 Semaphores
 Sockets

6.  Client / Server
Communication link
Client Server
Figure 1.1 Network application : client and
server
Client
...
Client Server
...
Client
Figure 1.2 Server handling multiple clients at the same
time.

7.  Example : Client and Server on the same Ethernet
communication using TCP
User Web Application protocol Web Application
process Client server layer
TCP protocol
TCP TCP transport layer
Protocol
stack
IP IP protocol IP network layer
within
kernel
Ethernet Ethernet protocol Ethernet datalink layer
driver driver
Actual flow between client and server
Ethernet
Figure 1.3 Client and server on the same Ethernet
communicating using TCP

8.  Example : Client and Server on different LANs
connected through WAN.
client server
application application
Host Host
with with
TCP/IP TCP/IP
LAN LAN
router router
WAN
router router router router
Figure 1.4 Client and server on different LANs connected through a
WAN

9. application
7 Application details
user
6 Presentation process
Application
5 Session Sockets
XTI
4 Transport TCP | | UDP
3 Network IPv4, IPv6 kernel
communication
2 Datalink Device driver details
1 Physical and Hardware
OSI Model Internet protocol
suite
Figure 1.14 Layers on OSI model and Internet protocol suite
 First, the upper three layers handle all the details of the application and The lower four layers
handle all the communication details.
 Second, the upper three layers is called a user process while the lower four layers are
provided as part of the operating system kernel.

10.  POSIX
◦ POSIX is an acronym for Portable Operating System Interface.
◦ POSIX is not a single standard, but a family of standards being developed by the Institute for
Electrical and Electronics Engineers, Inc., normally called the IEEE.
◦ The POSIX standards have also been adopted as international standards by ISO and the International
Electro technical Commission (IEC), called ISO/IEC.
 Open group
◦ The Open Group was formed in 1996 by the consolidation of the X/Open Company and the Open Software
Foundation.
◦ It is an international association of vendors and end-user customers from industry, government, and
academia.
 IETF
◦ The Internet Engineering Task Force (IETF) is a large, open, international community of network designers,
operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth
operation of the Internet.
◦ It is open to any interested individual

11.  TCP provides connections between clients and servers (connection oriented
protocol).
 TCP also provides reliability.
 TCP contains algorithms to estimate the round-trip time (RTT) between a client
and server dynamically so that it knows how long to wait for an
acknowledgment.
 TCP also sequences the data by associating a sequence number with every
byte that it sends.
 TCP provides flow control. TCP always tells its peer exactly how many bytes of
data it is willing to accept from the peer at any one time.
 TCP connection is full-duplex.

12.  UDP provides a connectionless service, as there need not be any long-term
relationship between a UDP client and server.
 UDP provides no flow control.
 UDP supports multicasting.
 UDP is a simple transport-layer protocol.
 The application writes a message to a UDP socket, which is then encapsulated in a
UDP datagram, which is then further encapsulated as an IP datagram, which is then
sent to its destination.
 There is no guarantee that a UDP datagram will ever reach its final destination, that
order will be preserved across the network, or that datagram arrive only once.

13. TCP UDP
Binding between Yes (connection- No (connection-
client and server oriented) less)
Data Byte-stream Record
Reliability Yes (ack, time- No
out, retx)
Sequencing Yes No
Flow control Yes (window- No
based)
Full-duplex Yes Yes
13

14.  The server must be prepared to accept an incoming connection. (By calling socket,
bind, and listen )and is called a passive open.
 The client issues an active open by calling connect. This causes the client TCP to
send a "synchronize" (SYN) segment, which tells the server the client's initial
sequence number for the data that the client will send on the connection.
 The server must acknowledge (ACK) the client's SYN and the server must also
send its own SYN containing the initial sequence number for the data that the
server will send on the connection. The server sends its SYN and the ACK of the
client's SYN in a single segment.
 The client must acknowledge the server's SYN.

15. TCP Connection: Establishment
Three-way handshake
client server
socket,bind,listen
socket LISTEN(passive open)
connect (blocks) SYN j accept (blocks)
(active open) SYN_SENT
SYN k, ack j+1 SYN_RCVD
ESTABLISHED
connect returns
ack k+1 ESTABLISHED
accept returns
read (blocks)
TCP options (in SYN): MSS (maximum segment size) option,
window scale option (advertized window up to 65535x2^14, 1GB),
timestamp option (the latter two: long fat pipe options)

16.  One application calls close first, and we say that this end performs the
active close. This end's TCP sends a FIN segment, which means it is
finished sending data.
 The other end that receives the FIN performs the passive close. The
received FIN is acknowledged by TCP. The receipt of the FIN is also
passed to the application as an end-of-file, since the receipt of the FIN
means the application will not receive any additional data on the connection.
 Sometime later, the application that received the end-of-file will close its
socket. This causes its TCP to send a FIN.
 The TCP on the system that receives this final FIN acknowledges the FIN.

17. TCP Connection: Termination
Four-way handshake
client server
FIN m
close CLOSE_WAIT (passive close)
(active close) FIN_WAIT_1 read returns 0
ack m+1
FIN_WAIT_2 close
FIN n LAST_ACK
TIME_WAIT
1~4 mins ack n+1 CLOSED
CLOSED
TIME_WAIT to allow old duplicate segment to expire for reliable termination
(the end performing active close might have to retx the final ACK)

18. s ta rtin g p o in t
C LO SED
a p p l: p a s s iv e o p e n
ap
s e n d : < n o t h in g >
pl
:a
se
ct
nd
K
ive N
AC L IS T E N
: S
N,
op
SY
Y
:
nd p a s s iv e o p e n
en
se T
;
YN RS
: S v:
v rec
re c
re c v : S Y N a p p l: c lo s e
SYN _R C VD SYN _SEN T
send: SYN , AC K K o r tim e o u t
se re s im u lta n e o u s o p e n AC a c tiv e o p e n
n d c v: N,
:< AC
: SY AC K
no K
cv :
th
in re nd
g> se
re c v : F IN
E S T A B L IS H E D C L O S E _ W A IT
send: AC K
d a ta tra n s f e r s ta te
e
lo s re c v : c lo s e
pl :c IN send: F IN
ap : F
nd
- State se
re c v : F IN
s im u lta n e o u s c lo s e
re c v : A C K
transition F IN _ W A IT _ 1
send: AC K
C L O S IN G LA S T_A C K
s e n d : < n o th in g >
diagram re
v: c p a s s iv e c lo s e
se F
re c v : AC K n d IN , re c v : A C K
send: < n o th in g > : A AC s e n d : < n o th in g >
CK K
re c v : F IN
F IN _ W A IT _ 2 T IM E _ W A IT
send: AC K
2 M S L tim e o u t
a c t iv e c lo s e
F ig u re 2 . 4 T C P s t a t e t ra n s it io n d ia g ra m

19. c lie n t s e rv e r
socket,bind,listen
socket LISTEN(passive open)
c o n n e c t ( b lo c k s ) a c c p e t ( b lo c k s )
( a c t io n o p e n ) SYN J , m
s s = 14
S YN _S EN T 60
 Watching SYN K, ac k J +
1 , ms
s = 10
24
the Packets
ESTABLISHED
c o n n e c t re tu rn s
a c k K+ 1
<client forms request> ESTABLISHED
a c c e pt re tu rn s
write r e a d ( b lo c k s )
r e a d ( b lo c k s ) data(re
q uest)
r e a d re tu rn s
<server processes request>
write
p ly)
d ata (re r e a d ( b lo c k s )
uest
ac k o f req
r e a d re tu rn s
ac k o f
rep ly
close
( a c t iv e c lo s e )
FIN M
F IN _ W A IT _ 1
C L O S E _ W A IT
( p a s s iv e c lo s e )
1 r e a d re tu rn s 0
ac k M +
close
F IN _ W A IT _ 2
FIN N LA S T _A C K
T IM E _ W A IT
ac k N +
1
C LO S E D
F ig u re 2 . 5 P a c k e t e x c h a n g e f o r T C P c o n n e c tio n
UNIX Network Programming 19

20.  The end that performs the active close is the end that remains in the
TIME_WAIT state=>because that end is the one that might have to
retransmit the final ACK.
 The MSL is the maximum amount of time that any given IP
datagram can live in a network.
 There are two reason for TIME_WAIT state
◦ to implement TCP’s full-duplex connection termination reliably
◦ to allow old duplicate segments to expire in the network

21.  TCP,UDP define a group of well known port to identify well known
services.
 Clients normally use ephemeral ports, that is short lived ports.
 These port no are normally assigned automatically by the transport
protocol to the client.
 IANA maintains list of port numbers assignments.
1. Well-known ports: 0 to 1023controlled and assigned by IANA.
2. Registered ports: 1024 to 49151. These are not controlled by
IANA.
3. Dynamic or private port:49152 to 65,535

23.  The socket pair for a TCP connection is the four-
tuple that defines the two endpoints of the
connection:
◦ local IP address, local port, foreign IP, Foreign port.
 A socket pair uniquely identifies every TCP
connection on a network.
 Two values that identify each endpoint, an IP
address and a port number are often called a
socket.

24. 206.62.226.35
206.62.226.66 198.69.10.2
c o n n e c t io n re q u e s t to
s e rv e r c lie n t
2 0 6 . 6 2 . 2 2 6 . 3 5 , p o rt 2 1
lis te n in g s o c k e t (* .21, * .* ) { 198.69.10.2.1500,
206.62.226.35.21}
F ig u re 2 . 8 C o n n e c t io n re q u e s t f ro m c lie n t t o s e rv e r
206.62.226.35
206.62.226.66 198.69.10.2
s e rv e r c lie n t
lis te n in g s o c k e t (* .21, * .* ) { 198.69.10.2.1500,
on 206.62.226.35.21}
e cti
fork nn
co
s e rv e r
( c h ild )
c o n n e c te d
socket { 206.62.22 6.35 .21,
1 98.6 9.10.2 .150 0}
F ig u re 2 . 9 C o n c u rre n t s e rv e r h a s c h ild h a n d le c lie n t

25. 206.62.226.35
206.62.226.66 198.69.10.2
s e rv e r c lie n t 1
lis te n in g s o c k e t (* .21, * .* ) { 198.69.10.2.1500,
io n
ec t 206.62.226.35.21}
nn
fork co
s e rv e r
c lie n t 2
( c h ild 1 )
c o n n e c te d
socket { 206.62.226.35.21, { 198.69.10.2.1500,
io n
198.69.10 .2.1 500} ec t 206.62.226.35.21}
nn
co
s e rv e r
( c h ild 2 )
c o n n e c te d
socket { 206.62.226.35.21,
198.69.10 .2.1 501}
F ig u re 2 . 1 0 S e c o n d c lie n t c o n n e c t io n w it h s a m e s e rv e r

26.  Maximum size of IPv4 => 65535 byte
 Maximum size of IPv6 => 65575 byte
 MTU(maximum transmit unit) => fragmentation
 The smallest MTU in the path between two hosts is called
the path MTU
 Today, the Ethernet MTU of 1,500 bytes is often the path
MTU. The path MTU need not be the same in both
directions between any two hosts
 When an IP datagram is to be sent out an interface, if the
size of the datagram exceeds the link MTU, fragmentation
is performed by both IPv4 and IPv6.

27.  The fragments are not normally reassembled until they reach the final
destination.
 IPv4 hosts perform fragmentation on datagrams that they generate and
IPv4 routers perform fragmentation on datagrams that they forward.
 But with IPv6, only hosts perform fragmentation on datagrams that they
generate; IPv6 routers do not fragment datagrams that they are forwarding.

28.  DF (don’t fragment)
◦ A router that receives an IPv4 datagram with the DF bit set whose size exceeds
the outgoing link's MTU generates an ICMPv4 "destination unreachable,
fragmentation needed but DF bit set" error message
◦ TCP decreases the amount of data it sends per datagram and retransmits.
◦ TCP has a maximum segment size (MSS) that announces to the peer TCP the
maximum amount of TCP data that the peer can send per segment. The goal of
the MSS is to tell the peer the actual value of the reassembly buffer size and to try
to avoid fragmentation. The MSS is often set to the interface MTU minus the fixed
sizes of the IP and TCP headers.

31. Notice: TCP and UDP port number is same.