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Departamento de
Tecnología Electrónica
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Chapter 1
Introduction to
network architecture
Computer Networking:
A Top Down Approach
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, April
2009.
Introduction to network architecture
1
Chapter 1: Introduction to network
architecture
 Chapter goals:
 Remembering networking basic concepts that are
key for the subject.
o
o
o
o

OSI Model and TCP/IP architecture
Transport layer concepts
Network layer concepts
Data link layer concepts
Introducing Virtual Local Area Networks (VLAN)
Introduction to network architecture
2
Chapter 1: Introduction to network
architecture
 1.1 OSI Model. TCP/IP
Architecture
 1.2. Transport layer
 1.3. Network layer
 1.4. Data link layer
 1.5 Virtual Local Area
Network, VLANs
Introduction to network architecture
3
Chapter 1: Introduction to
network architecture
 1.1 OSI Model. TCP/IP
Architecture
 1.2. Transport layer
 1.3. Network layer
 1.4. Data link layer
 1.5 Virtual Local Area
Network, VLANs
Introduction to network architecture
4
TCP/IP Architecture
 application: supporting network
applications

FTP, SMTP, HTTP
 transport: process-process data
transfer

TCP, UDP
 network: addressing and routing of
datagrams from source to
destination

IP, routing protocols
 data link: data transfer between
application
A_PDU
message
transport
T_PDU
segment
network
R_PDU
Datagram/
packet
data link
E_PDU
frame
physical
neighboring network elements

PPP, Ethernet
 physical: bits “on the wire”
Introduction to network architecture
5
OSI Model
 presentation: allow applications to
interpret meaning of data, e.g.,
encryption, compression, machinespecific conventions
 session: synchronization,
checkpointing, recovery of data
exchange
 Internet stack “missing” these
layers!
 these services, if needed, must
be implemented in application
Introduction to network architecture
application
presentation
session
transport
network
link
physical
6
Encapsulation
Transmitter
(N+1)-PDU
Level N+1
Encapsulation
Level N
(N)-SDU
(N)-PCI
(N)-DU
(N)-PDU
Simplified encapsulation model
Introduction to network architecture
7
Decapsulation
Receiver
(N+1)-PDU
Level N+1
Decapsulation
Level N
(N)-SDU
(N)-PCI
(N)-DU
(N)-PDU
Introduction to network architecture
8
Example of layer architecture:
Internet
Source
application
transport
network
data link
physical
Example: Two hosts connected by a router.
Destination
message
segment
M
Ht
M
Ht
M
datagram
M
M
Hr Ht
He Hr Ht
M
frame
Hrn Ht
He H r H t
Hr Ht
Nota
M
Hx = X_PCI
M = A_PCI(Ha) + User data (UD)
Example UD:
e_mail subject/body
Text of a WhatsApp message
M
application
transport
network
data link
physical
Mnetwork
data link
He
physical
Hr Ht
M
router
Introduction to network architecture
Phy medium
9
Multiplexion
 more than one transport protocol
available to apps

FTP
Internet: TCP and UDP
HTTP
SMTP
DNS
TFTP
Application
How do we identify the client
protocol?
 TCP & UDP: Port field.
 IP: Protocol field
 Ethernet: Ethertype field
TCP
UDP
IP
Link layer (LLC & MAC)
Transport
Network
(Type/length)
 IEEE 802.3 (MAC) uses LLC (IEEE
802.2)
 IEEE 802.2: DSAP y SSAP
SNAP may be used together with
IEEE 802.2 to identify with Ethertype
Physical
Layer
Introduction to network architecture
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Multiplexion. Example (I)
Introduction to network architecture
11
Multiplexion. Example (II)
Data link
Message
H
Introduction to network architecture
C
Data
12
Multiplexion. Example (III)
Data link
Network
Message
H
Data
H
Introduction to network architecture
E
Data
13
Multiplexion. Example (IV)
Data link
Network
Transport
H
Data
H
Introduction to network architecture
Data
14
Chapter 1: Introduction to
network architecture
 1.1 OSI Model. TCP/IP
Architecture
 1.2. Transport layer
 1.3. Network layer
 1.4. Data link layer
 1.5 Virtual Local Area
Network, VLANs
Introduction to network architecture
15
Transport services and protocols
 provide logical communication between app
processes running on different hosts
 transport protocols run in end systems

breaks app messages into segments, passes to
network layer
Host A
Network
Host B
Introduction to network architecture
16
Internet transport-layer
protocols
 more than one
transport
protocol available
to apps

Internet: TCP
and UDP
TCP
UDP
Connection- oriented Non-connectionoriented
Reliable
Unreliable
Segment grouping
Unfragmented
messages
Rcv orders segments User datagram
ACKs and timers
No ACKs
Flow control
No flow control
Congestion control
No congestion control
Introduction to network architecture
17
Internet transport-layer
protocols
 Port: identifies application
 Port numbers: http://www.iana.org/assignments/port-
numbers
Application protocol
Port numbers
Transport protocol
FTP
20, 21
TCP
Telnet
23
TCP
SMTP
25
TCP
DNS
53
UDP (TCP (*))
TFTP
69
UDP
HTTP
80
TCP
POP3
110
TCP
RIP
520
UDP
Introduction to network architecture
18
UDP_PDU
32 bits
T_PCI
T_UD
Header (T_PCI)
has only 4 fields.
Lenght is in bytes
and deals with the
whole T_PDU,
including the
header.
Source port
Dest port
length
checksum
App level data
(message)
UDP T_PDU format
Introduction to network architecture
19
TCP: Overview
 point-to-point:
 one sender, one receiver
 reliable, in-order byte
stream:

no “message boundaries”
 pipelined:
 TCP congestion and flow
control set window size
 send & receive buffers
socket
door
application
writes data
application
reads data
TCP
send buffer
TCP
receive buffer
RFCs: 793, 1122, 1323, 2018, 2581
 full duplex data:
 bi-directional data flow
in same connection
 MSS: maximum segment
size
 connection-oriented:
 handshaking (exchange
of control msgs) init’s
sender, receiver state
before data exchange
 flow controlled:
 sender will not
socket
door
overwhelm receiver
segment
Introduction to network architecture
20
TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
Receive window
Urg data pnter
Options (variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
application
data
(variable length)
Introduction to network architecture
21
TCP seq. #’s and ACKs
Host A
Host B
Client starts
active open
Server is in passive open,
starts connection and
confirms client open
Client confirms
server open
Connection
established
Introduction to network architecture
time
22
Chapter 1: Introduction to
network architecture
 1.1 OSI Model. TCP/IP
Architecture
 1.2. Transport layer
 1.3. Network layer
 1.4. Data link layer
 1.5 Virtual Local Area
Network, VLANs
Introduction to network architecture
23
Network layer
Network layer
functions:
•Packet routing
•Logical addressing
•Multiplexion
•Packet segmentation
(fragmentation)
Adressing: Network & Host
Network
Host
-Network address: Used by the router
-Host address: used by a determined host
Which route?
Introduction to network architecture
24
IPv4 protocol
IP addressing
 Most used network layer
protocol
 32-bit address


Network
Host
Network part
Host part
 Network and host parts are
determined by the subnet
mask.


First x bits are set to 1 and
define the number of bits
of the network part.
Last 32-x bits are set to 0
and define the number of
bits of the host part
Introduction to network architecture
25
IPv4 special addresses
Addresses
Meaning
Use
0.0.0.0/32
The own host inside the own
network
As source address if a host does not know its
IP address
10.0.0.0/8
172.16.0.0/12
192.168.0.0/16
Private addresses
IP connectivity, but no access to the
Internet
127.0.0.0/8
Loopback inteface
Ip use without access to the physical medium.
169.254.0.0/16
Autoconfiguration
A host does not have an IP address, neither
manually nor by means of a DHCP server
224.0.0.0/4
Multicast
IP Multicast (D class)
240.0.0.0/4
Reserved
For the use of IANA (E class)
255.255.255.255
Limited broadcast
Broadcast destination for the network
Introduction to network architecture
26
IP_PDU format (datagram/packet)
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
how much overhead
with TCP?
 20 bytes of TCP
 20 bytes of IP
 = 40 bytes + app
layer overhead
32 bits
type of
ver head.
len service
length
fragment
16-bit identifier flgs
offset
upper
time to
header
layer
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
Introduction to network architecture
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
27
Routing table (host)






Host IP addr?
Subnet mask? Network Id.?
MAC address?
Default gateway? Gateway for 10.10.63.20?
Gateway for 150.214.141.120?
MAC for 10.10.63.255?
Introduction to network architecture
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Routing table (router)
RT Router 1
RT Router 2
Network
Subnet mask
Next hop
Interface
Network
Subnet mask
Next hop
Interface
192.1.1.0
255.255.255.224
-
E0
192.1.1.0
255.255.255.224
192.1.4.2
E0
192.1.2.0
255.255.255.192
192.1.4.1
E1
192.1.2.0
255.255.255.192
-
E1
192.1.3.0
255.255.255.128
-
E2
192.1.3.0
255.255.255.128
192.1.4.2
E0
192.1.4.0
255.255.255.0
-
E1
192.1.4.0
255.255.255.0
-
E0
192.1.1.2
E0
NETWORK 1
E1
192.1.3.1
Router 2
192.1.4.2
192.1.1.1
E2
192.1.4.1
NETWORK
2
192.1.2.1
…
192.1.2.3

192.1.1.31
…
192.1.3.2
E1
E0
NETWORK
3
…
192.1.1.3
192.1.2.2
Router 1
192.1.3.127
192.1.2.63
When receiving a packet, the router
makes the AND operation between
IP dest addr and the different
subnet masks in the routing table.
Finally, it sends the packet for the
interface pointed out by the routing
table.
Introduction to network architecture
29
Difference between logical addresses
(IP) and physical addresses (MAC)
Host Y
150.214.141.19
00:1C:27:18:00:01
Host A
142.128.1.11
00:1C:27:56:34:AA
Router
142.128.1.1
150.214.141.1
12:34:56:78:90:AB
Source IP addr
142.128.1.11
Dest IP addr
Source MAC addr
150.214.141.19
00:1C:27:56:34:AA
IP packet
Dest MAC addr
12:34:56:78:90:AB
Source IP addr
142.128.1.11
Dest IP addr
Source MAC addr
Dest MAC addr
150.214.141.19
12:34:56:78:90:AB
00:1C:27:18:00:01
IP packet
Introduction to network architecture
30
Socket
 A process sends/recieves messages
by/from its socket
 A socket is identified by:
IP address.
 Port number.

Port number examples:

HTTP: port 80

DNS: port 53
Introduction to network architecture
31
Chapter 1: Introduction to
network architecture
 1.1 OSI Model. TCP/IP
Architecture
 1.2. Transport layer
 1.3. Network layer
 1.4. Data link layer
 1.5 Virtual Local Area
Network, VLANs
Introduction to network architecture
32
LAN (Local Area Network)
 Local Area Networks are the most used network technology.
 They allow the connection between hosts and routers inside
a broadcast domain.
 Most used LAN standards are:
 802.3, Ethernet.
 802.11, WI-FI (WLAN, Wireless LAN).
Data link layer is divided into two sublevels:
o LLC (Link Layer Control). Its functions
are flow control and error correction.
o MAC (Medium Access Control). Foer
frame synchronism, error detection,
medium access control, and physical
addressing.
Implmented in NIC (Network Interface Card)
up to MAC sublevel.
OSI
LAN
Data link
LLC
MAC
Physical
Introduction to network architecture
33
MAC addresses

Size -> 48 bits.


Example: 1B:03:F2:45:78:25
There are three types of MAC addresses:

Unicast: To send DL_PDUs to an only destination.



All the network interfaces have a default factory MAC address.
Broadcast: As a destination, it is used to send DL_PDUs to ALL the
hosts in a broadcast domain (FF:FF:FF:FF:FF:FF).
Multicast: As a destination, it is used to send DL_PDUs to SOME of
the hosts in a broadcast domain .

Configurable. The least significant bit in the first byte of the MAC address
is set to 1.
 IEEE manages unicast factory MAC addresses.


Every manufacturer has a range of MAC addresses (to assure MAC
addresses are unique)
It is possible to change them.
Introduction to network architecture
34
Standards
LAN/MAN standards
Introduction to network architecture
35
MAC_PDU (frame)
1 byte
Preamble
(8 bytes)
Dest MAC address
(6 bytes)
Source MAC address
(6 bytes)
Length/Type
(2 bytes)
MAC_UD
(46-1500 bytes)
Trailer
CRC
(4 bytes)
7 bytes with 10101010.
1 byte (the last one) con 10101011.
•<=1500
meaning length:
- number of bytes for MAC_UD
- Sublevel LLC is present.
•>=1536
meaning type:
Multiplexion and demultiplexion
Data corresponding to the upper
layer, generally IP, ARP or LLC.
If MAC_UD size is below 46
bytes, there is a trailer of bytes
set to 0.
Note
MTU for Ethernet is 1500 bytes
Introduction to network architecture
36
Chapter 1: Introduction to
network architecture
 1.1 OSI Model. TCP/IP
Architecture
 1.2. Transport layer
 1.3. Network layer
 1.4. Data link layer
 1.5 Virtual Local Area
Network, VLANs
Introduction to network architecture
37
VLAN
Introduction:
 Hierarchic topology for institutional modern LANs
 Every working group has its own switched LAN
 Switched LANs may be interconnected by a
hierarchy of switches.
S4
S1
S2
A
B
S3
C
F
D
E
I
G
H
Introduction to network architecture
38
VLAN
Disadvantages:
 Traffic is not isolated


Broadcast traffic
Traffic must be limited for security and confidentiality
reasons
 Inefficient use of switches
 User management
Introduction to network architecture
39
VLAN
VLAN:
 Port-based VLAN




A
Switch ports are divided into groups
Every group constitutes a VLAN
Every VLAN is a broadcast domain
User management -> Change in switch configuration
B
C
D
G
E
F
H
I
Introduction to network architecture
40
VLAN
VLAN:
 How is information between groups sent?




A
Connect switch port to an external router
Configure that port as a member of both groups
Logical configuration -> Separate switches connected by a router
Habitually, manufacturers include VLAN and router in an only
device
B
C
D
G
E
F
H
I
Introduction to network architecture
41
VLAN
VLAN:
 Different location



A
Members of the same group in different buildings
Several switches needed
Connecting group ports between switches -> Non scalable
B
D
E
C
GIntroduction to network architecture
I
H
F
42
VLAN
VLAN:
 Different location



VLAN Trunking
Trunk port belongs to all VLANs
Destination VLAN? 802.1Q frame format
Trunk
link
A
B
D
E
G
C
I
Introduction to network architecture
H
F
43
VLAN
IEEE 802.1Q:

IEEE 802.3 (Ethernet)
Preamble

Dest
addr
Source
addr
Type
Data
CRC
IEEE 802.1Q
Preamble
Dest
addr
Source
TPID TCI Type
Data
addr
New
CRC
Tag Control Information
Tag Protocol Identifier
Introduction to network architecture
44
VLAN
VLAN:
 MAC-based VLAN


The network administrator creates VLAN groups based on
MAC address ranges.
Switch port is connected to the VLAN that corresponds to
the associated host’s MAC address.
 IP-based VLAN
 Based on IPv4 or IPv6 addresses
 Based on network protocols (Appletalk, IPX, TCP/IP)
Introduction to network architecture
45