Transcript Evolution of Data Networks - Electrical & Computer

ELEN602 Lecture 2
• Review of Last Lecture
• Layering
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Communication Networks
• Many forms of communication networks around
– Postal mail, telephone, Internet etc.
• Generic steps involved in communication
– Addressing, name-to-address translation, transport,
routing, delivery
• Forms of delivery
– Reliable, in-order
– Unreliable, no acknowledgement
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Steps in Web Surfing
1.
2.
3.
4.
5.
6.
The user clicks on a link to indicate which document is to be
retrieved.
The browser must determine the address that contains the
document. It does this by sending a query to its local name
server.
Once the address is known the browser establishes a connection to the
specified machine, usually a TCP connection. In order for the
connection to be successful, the specified machine must be ready to
accept TCP connections.
The browser runs a client version of HTTP, which issues a request
specifying both the name of the document and the possible document
formats it can handle.
The machine that contains the requested document runs a server
version of HTTP. It reacts to the HTTP request by sending an
HTTP response which contains the desired document in the
appropriate format.
The TCP connection is then closed and the user may view
the document.
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Application level View
Request
HTTP
client
HTTP
server
Response
4
Steps in HTTP Transfer
• User selects document
• Networking software locates the remote machine address
& establishes a 2-way connection
• HTTP client sends request for document (GET )
• HTTP daemon on remote machine interprets the GET
message
• Remote HTTP daemon sends the result of GET operation
– Result code, Description of the information, requested
file
• Remote HTTP daemon disconnects the connection
• HTTP client displays the HTML content in the browser.
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Domain Name Service
• DNS servers help resolve name-to-address translation
– Given a name such as ee.tamu.edu, find its IP address
• IP addresses = telephone numbers in phone network
• Machines (computers/network elements) know each other
by IP addresses
• IP addresses are 32bits in IPv4.
• DNS protocol allows name-to-IP address resolution
– Similar to phone directory assistance (411 or 555-1212)
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DNS Example (cont’d)
• QNAME = ee.tamu.edu, QCLASS = IN QTYPE = A
• QNAME = ee.tamu.edu QCLASS = IN QTYPE = A
– IN A 165.91.241.199
• The application will now use 165.91.241.199 whenever it
needs to contact ee.tamu.edu
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A more detailed view - Transport protocol
HTTP
server
HTTP
client
Ephemeral
Port 80
Port #
GET
80, #
TCP
TCP
#, 80
STATUS
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Email and SMTP
• Web transfers use HTTP, email uses SMTP (Simple Mail
Transfer Protocol) - Application level protocols
• Sender - Local SMTP Server -- Remote SMTP Server Receiver
• SMTP uses Port 25, HTTP uses port 80
• POP (Post Office Protocol) may be used if receiver not on
all the time.
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Sender establishes TCP Connection to SMTP server
Sends HELO message -identifying itself to SMTP server
Client sends sender’s email address
SMTP server acknowledges with 250 if successful
Client sends receiver’s email address
SMTRP server acknowledges with 250
Client sends a DATA message - requesting permission to
send email message
SMTP server sends OK-to-send message
Client transfers the email message
SMTP server returns ack with a message ID
Client indicates that the transfer is complete.
SMTP confirms the receipt of End-of-transfer message
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Abstract View
n-PDUs
n
n
entity
entity
11
Layering in General
n+1
entity
n+1
entity
n-SDU
n-SDU
n-SAP
n-SAP
n-SDU
H
n entity
n entity
H
n-SDU
n-PDU
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Data Units at different layers
(a)
n-PDU
(b)
Segmentation
Reassembly
n-SDU
n-SDU
n-PDU
n-PDU
n-PDU
Blocking
n-SDU
n-SDU
n-PDU
n-PDU
n-PDU
Unblocking
n-SDU
n-SDU
n-SDU
n-SDU
n-PDU
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OSI Seven-layer Model
Application A
Application B
Application
Layer
Application
Layer
Presentation
Layer
Presentation
Layer
Session
Layer
Session
Layer
Transport
Layer
Communication Network
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Electrical and/or Optical Signals
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Packet-Switched Network
PS = packet switch
C
PS
C
C = computer
PS
PS
PS
C
C
C
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An Internetwork
net 3
G
net 1
G
G
G
net 2
net 5
G
net 4
G
G = gateway/router
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Headers and Trailers
Application A
data
Application
Layer
data
Transport
Layer
data
Network
Layer
Physical
Layer
data
dt
Presentation
Layer
ph
data
Session
Layer
Application
Layer
ah
data
Presentation
Layer
Data Link
Layer
Application B
data
bits
sh
Session
Layer
th
Transport
Layer
Network
Layer
nh
dh
Data Link
Layer
Physical
Layer
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TCP/IP Network Architecture
Application
Layer
Transport
Layer
Internet
Layer
Network
Interface
(a)
Application
Layer
Transport
Layer
Internet
Layer
Network
Interface
(b)
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IP Layer and Network Interface Layer
Machine B
Machine A
Application
Application
Transport
Router/Gateway
Transport
Internet
Internet
Internet
Network Interface
Network Interface
Network 1
Network Interface
Network 2
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IP Hour-Glass Protocol Graph
HTTP
SMTP
RTP
DNS
TCP
UDP
IP
Network
Network
Network
Interface 1
Interface 2
Interface 3
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(a)
(1,1)
(2,1)
(2,2)
router
s
Ethernet
PPP
(1,3) r
w
(1,2)
(b)
Server
HTTP
PC
HTTP
TCP
Router
IP
IP
IP
Net Interface
Net Interface
Net Interface
Ethernet
TCP
PPP
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IP Datagram on an Ethernet
IP
Header
Header contains
source and destination
physical addresses;
network protocol type
Ethernet
Header
Frame
Check
Sequence
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HTTP Request
Header contains source and
destination port numbers
Header contains source and
destination IP addresses;
transport protocol type
Header contains source
and destination physical
addresses; network
protocol type
Ethernet
Header
TCP
Header
IP
Header
Frame
Check
Sequence
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socket
interface
Application 1
Application 2
user
user
kernel
kernel
Socket
socket
interface
Socket
Underlying
communication
Protocols
Underlying
communication
Protocols
Communications
network
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Server
socket()
bind()
listen()
Client
accept()
blocks until server receives
a connect request from client
read()
write()
close()
socket()
connect negotiation
connect()
data
write()
data
read()
close()
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Server
socket()
Client
socket()
bind()
bind()
recvfrom()
blocks until server
receives data from client
data
sendto()
data
sendto()
recvfrom()
close()
close()
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User
Interface
Control
Server PI
Server
DTP
Server FTP
Connection
Data
User PI
User DTP
Connection
User FTP
PI = Protocol interpreter
DTP = Data transfer process
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C:\WINDOWS>ping nal.toronto.edu
Pinging nal.toronto.edu [128.100.244.3] with 32 bytes of data:
Reply
Reply
Reply
Reply
from
from
from
from
128.100.244.3:
128.100.244.3:
128.100.244.3:
128.100.244.3:
bytes=32
bytes=32
bytes=32
bytes=32
time=118ms
time=118ms
time=118ms
time=118ms
TTL=243
TTL=243
TTL=243
TTL=243
C:\WINDOWS>
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