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OSI Model
}
User
Level
Software
Interoper-
ability
Kernel
Level
Network
Support
OSI = Open Systems Interconnection (Introduced in late 1970s by ISO)
ISO = International Standardization Organization
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Summary of layers
Session: synchronize and maintain data stream state.
Presentation: data marshaling to maintain to architecture
independence.
OSI layers 2-3
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Example Scenario: Secure File Transfer
User initiates scp from the command line: scp file host: O/S User Interface
scp calls network APIs (i.e. Berkeley sockets Or Unix System V STREAMS) Application Layer 7
Network APIs call compression and encryption system functions Presentation Layer 6
Originating host opens connection to remote host Session Layer 5
Datastream is broken into segments or datagrams and encapsulated into a
Transport Layer 4
transport layer protocol (TCP or UDP, respectively)
Segments or datagrams are further encapsulated into routable IP packets each
Network Layer 3
having a source and destination IP address.
Network stack hands IP datagrams to the Ethernet interface which
Data Link Layer 2
encapsulates each datagram in a frame with an Ethernet header and footer
Ethernet transceiver reads each frame and generates appropriate
Physical Layer 1
voltages that correspond to binary 1s or 0s
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Questions:
Which layer is responsible for managing the following processes?
1) Route determination
2) Flow control
3) Ethernet interface to twisted pair cable
4) Framing of IP packets
5) Process to process message delivery
6) Error correction and retransmission of lost segments
7) Marshalling and unmarshalling of binary data
8) Establishment and management of multiple, concurrent, and independent
connections from a client process to a server process
9) A Mail User Agent (MUA) establishing a connection to a Mail Transfer
Agent (MTA)
An exchange using the OSI model
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Reliable end-to-end delivery of a message
Process to Process
Communication
TCP/IP vs. OSI model 2-8
“Upper
Layers”
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SMTP Simple Mail Transport Protocol
FTP File Transfer Protocol
TFTP Trivial File Transfer Protocol
DNS Domain Name System
SNMP Simple Network Management Protocol
NFS Network File System
RPC Remote Procedure Call
TCP Transmission Control Protocol
UDP User Datagram Protocol
IP Internet Protocol
ICMP Internet Control Message Protocol
IGMP Internet Group Management Protocol
ARP Address Resolution Protocol
RARP Reverse ARP
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Addresses in TCP/IP
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Layers and Addresses in TCP/IP
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Physical addresses
Most local area networks use a 48-bit (6 bytes) physical or MAC
addresses. Example:
07:01:02:01:2C:4B
Media Access Control (MAC) address (along with other relevant network
configuration data, like IP address, default gateway etc.) of a Network Interface Card
(NIC) can be displayed using the command:
ipconfig /all (Windows cmd.exe), ifconfig -a (Unix/Linux)
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Example
# ifconfig -a
lo0: flags=849 mtu 8232
inet 127.0.0.1 netmask ff000000
hme0:
flags=863
mtu 1500
inet 130.191.166.200 netmask ffffff00 broadcast 130.191.166.255
ether 8:0:20:ad:a5:c7
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Internetworking Protocol (IP)
Addresses
An Internet address (in IPv4)
is 4 bytes = 32 bits in length.
Example:
Router
132.24.75.9
LANs
in IPv6 the IP address is 4
times longer => 16 bytes or
128 bits.
Port Addresses
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j - port address of the sending process A port address is a 16-bit address.
k - port address of the receiving process Example: 0x0014 (FTP Data) =
2010