Encapsulation

1
An example of how a "typical" file might be sent
between PC's across the Internet
http//homepage.ntlworld.com/shaun_d_smith/tcpip/t
cpip.html
2
This process is called
3
http//www.tcpipguide.com/free/t_IPDatagramEncapsu
lation.htm
4
Encapsulation des données
clement.reinier.free.fr/
Lors d'une transmission, les données traversent
chacune des couches au niveau de la machine
émettrice. A chaque couche, une information est
ajoutée au paquet de données, il s'agit d'un
en-tête, (ensemble d'informations qui garantit la
transmission). Au niveau de la machine
réceptrice, lors du passage dans chaque couche,
l'en-tête est lu, puis supprimé. Ainsi, à la
réception, le message est dans son état
originel...
5
Ping cs-server.humboldt.edu from Boston University

• At BU, greens software must look up the IP
  address of the domain name cs-server.humboldt.edu
  
• It uses a Domain Name Service (DNS) server to
  make this mapping

R1
R2
R6
R8
R13
6
Ping cs-server.humboldt.edu from Boston University
2. Greens network layer gives packet to Greens
layer 2 to send to R1 Green knows the IP address
of R1. Greens NIC sends frame to Router R1s MAC
address (lets assume Green already knows the MAC
address of R1)
R1
R2
R6
R8
R13
7
Ping cs-server.humboldt.edu from Boston University
3. R1 has an internal routing table that says,
Of all the places I can send this, it needs to
go to R2. How the frame gets to R2 is probably a
WAN issue.
R1
R2
R6
R8
R13
8
Ping cs-server.humboldt.edu from Boston University
4. R2 has an internal routing table that says,
Of all the places I can send this, it needs to
go to R6. How the frame gets to R6 is probably a
WAN issue.
R1
R2
R6
R8
R13
9
Ping cs-server.humboldt.edu from Boston University
5. R6 has an internal routing table that says,
Of all the places I can send this, it needs to
go to R8. How the frame gets to R8 is probably a
WAN issue.
R1
R2
R6
R8
R13
10
Ping cs-server.humboldt.edu from Boston University
6. R8 has an internal routing table that says,
Of all the places I can send this, it needs to
go to R13. How the frame gets to R13 is probably
a WAN issue.
R1
R2
R6
R8
R13
11
Ping cs-server.humboldt.edu from Boston University
7. R13has an internal routing table that says,
Oh, this packet is addressed to a computer on my
network! I need to deliver it locally. But
what is the MAC address of 137.150.30.197? Do I
already know it? Ill look in my ARP table for
the MAC address of 137.150.30.197.
R1
R2
R6
R8
R13
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Ping cs-server.humboldt.edu from Boston University
8. Nope, cant find the MAC address for
137.150.30.197. Ill have to ask everyone on my
network. Ill use an ARP request
R1
R2
R6
R8
R13
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Ping cs-server.humboldt.edu from Boston University

• cs-server (137.150.30.197) hears the ARP request
  and says, Thats me! Ill send an ARP Reply

R1
R2
R6
R8
R13
14
Ping cs-server.humboldt.edu from Boston University
10. R13 gets the ARP Reply and puts the result
into its ARP table. Now it can put the Layer 2
headers onto the Ping packet. R13 gives the frame
with the encapsulated Ping packet to the LAN
R1
R2
R6
R8
R13
15
Ping cs-server.humboldt.edu from Boston University
11. Yellow (csserver.humboldt.edu) gets the Ping
packet and constructs a Ping response and the
process now goes in the opposite direction.
R1
R2
R6
R8
R13
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Ping cs-server.humboldt.edu from Boston University
Blue BUs LAN, BUs ISP, BUs ISPs backbone
provider (BBN)
R1
R2
R6
Peering point
R8
Purple HSUs LAN, HSUs ISP, HSUs ISPs
backbone provider (Sprint)
R13
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(No Transcript)
18
ARP only works locally (on a single network).
ARP gets a level 2 address so Ethernet can be
used to send packets. Level 2 is only point to
point it is NOT NOT NOT an internetworking
protocol like TCP/IP.
http//www.erg.abdn.ac.uk/users/gorry/course/inet-
pages/arp.html