1. Introduction to Networks
CSEN 404
Introduction to Networks
Amr El Mougy
Ali Saudi
** Slides are attributed to J. F. Kurose

2. Link Layer
Application
Transport
Network
Data Link
PHY

3. Link Layer: Introduction
Some terminology:
hosts and routers are nodes
communication channels that connect adjacent nodes along communication path are links
wired links
wireless links
LANs
layer-2 packet is a frame, encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link

4. Link Layer Functions Framing Medium Access Control (MAC)
Reliable delivery
Flow control
Error detection and correction

5. IP Address vs. MAC Address
32-bit IP address:
network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address:
function: get frame from one interface to another physically- connected interface (same network)
48 bit MAC address (for most LANs)
burned in NIC ROM, also sometimes software settable
Each network card (adapter) has a permanent unique MAC address
(a.k.a hardware address, physical address, LAN address)
represented in hexadecimal notation
5: DataLink Layer

6. IP Address vs. MAC Address
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx
223
191
056
008
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx
xxxxxxxx 1C 22 F3 D1 42 A9
5: DataLink Layer

7. LAN Addresses and ARP Broadcast address = FF-FF-FF-FF-FF-FF LAN
Each adapter on LAN has unique LAN address
1A-2F-BB AD
Broadcast address =
FF-FF-FF-FF-FF-FF
LAN
(wired or
wireless)
= adapter
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
5: DataLink Layer

8. LAN Address (more) MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy:
(a) MAC address: like Social Security Number, SIN
(b) IP address: like postal address
MAC flat address ➜ portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable
address depends on IP subnet to which node is attached
5: DataLink Layer

9. MACs in Frame Starting at A, given IP datagram addressed to B:
look up net. address of B, find B on same net. as A
link layer send datagram to B inside link-layer frame

10. ARP: Address Resolution Protocol
Each IP node (host, router) on LAN has ARP table
ARP table: IP/MAC address mappings for some LAN nodes
< IP address; MAC address; TTL>
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
1A-2F-BB AD
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98

11. ARP protocol: Same LAN (network)
A wants to send datagram to B, and B’s MAC address not in A’s ARP table.
A broadcasts ARP query packet, containing B's IP address
dest MAC address =
FF-FF-FF-FF-FF-FF
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state: information that times out (goes away) unless refreshed
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator

12. Addressing: routing to another LAN
walkthrough: send datagram from A to B via R
assume A knows B’s IP address
two ARP tables in router R, one for each IP network (LAN)
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D A
C-E8-FF-55
88-B2-2F-54-1A-0F B
49-BD-D2-C7-56-2A
LAN
5: DataLink Layer

13. R A B A creates IP datagram with source A, destination B
A uses ARP to get R’s MAC address for
A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
A’s NIC sends frame
R’s NIC receives frame
R removes IP datagram from Ethernet frame, sees its destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B R
1A-23-F9-CD-06-9B
E6-E BB-4B
CC-49-DE-D0-AB-7D A
C-E8-FF-55
88-B2-2F-54-1A-0F B
49-BD-D2-C7-56-2A
5: DataLink Layer

14. Switches and Routers
Routers
Switches
Traffic Isolation
Yes
Plug and Play No
Optimal Routing
Larger networks require routers as they have better traffic isolation and can find better routes

15. A Day in the Life of a Webpage Request
DNS Server
GUC
/24
Bob
00:16:D3:23:63:8A
ISP
/13
00:22:6B:45:1F:1B
Bob wishes to retrieve a page from
Bob has just booted up the computer
Google
/19

16. A Day in the Life of a Webpage Request68 67
00:16:D3:23:63:8A
FF:FF:FF:FF:FF:FF
DNS Server
GUC
/24
Bob
00:16:D3:23:63:8A
ISP
/13
00:22:6B:45:1F:1B
Yiaddr: 67 68
00:22:6B:45:1F:1B
FF:FF:FF:FF:FF:FF
DHCP ACK also includes IP of default gateway (router in this case) and DNS server
Google
/19

17. A Day in the Life of a Webpage Request
ARP Query
00:16:D3:23:63:8A
FF:FF:FF:FF:FF:FF
DNS Server
GUC
/24
Bob
00:16:D3:23:63:8A
ISP
/13
00:22:6B:45:1F:1B
ARP Reply
00:22:6B:45:1F:1B
00:16:D3:23:63:8A
Before sending a DNS query, Bob must first discover the MAC address of the router through ARP
Google
/19

18. A Day in the Life of a Webpage Request
DNS query:
61334 53
00:16:D3:23:63:8A
00:22:6B:45:1F:1B
DNS reply: 53
61334
MAC of DNS
MAC of default R
DNS Server
GUC
/24
Bob
00:16:D3:23:63:8A
ISP
/13
00:22:6B:45:1F:1B
To send HTTP request, Bob’s browser has to create a TCP socket. Thus, it needs the IP address of
Google
/19

19. A Day in the Life of a Webpage Request
52122 80
00:16:D3:23:63:8A
00:22:6B:45:1F:1B
DNS Server
GUC
/24
Bob
00:16:D3:23:63:8A
ISP
/13
00:22:6B:45:1F:1B
To establish TCP connection, TCP SYN packet is sent
Google
/19 80
52122
MAC of web server
MAC of default R

20. A Day in the Life of a Webpage Request
HTTP Request: google search
52122 80
00:16:D3:23:63:8A
00:22:6B:45:1F:1B
DNS Server
GUC
/24
Bob
00:16:D3:23:63:8A
ISP
/13
00:22:6B:45:1F:1B
Finally HTTP request can be sent
HTTP Reply: search results 80
52122
MAC of web server
MAC of default R
Google
/19