Rensselaer Polytechnic Institute
CSCI-4220 – Network Programming
David Goldschmidt, Ph.D.

In reality, not
all layers are
always used....

The Internet
uses only
four layers:
 Application
 Transport
 Network
 Physical

Each layer prepends or appends its
information in a header or trailer
HTTP Request
TCP Hdr | HTTP Request
IP Hdr | TCP Hdr | HTTP Request
Ethernet Hdr | IP Hdr | TCP Hdr | HTTP Request | Cksum
P

Ethernet encompasses both
the data link layer and the
physical layer
 Ethernet (really IEEE 802.3)
 Ethernet provides a shared
medium (multi-access)
Ethernet

Every Ethernet interface is assigned a
unique 48-bit (hardware) address
 Represented as a sequence of 6 hexadecimal
bytes delimited by ':' characters:
▪ 00:50:56:9F:27:3E
▪ (use /sbin/ifconfig to see network interfaces)
 Also known as MAC address or physical address
 Ethernet addresses are assigned to
vendors by a central authority

Messages are sent over Ethernet via frames:
Preamble
Destination
Address
Source
Address
Len
Data
CRC
8 bytes
6 bytes
6 bytes
2
≤ 1500 bytes
4 bytes
 The Preamble is a sequence of
alternating 1s and 0s for synchronization
▪ In hex: AAAAAAAAAAAAAAAB
 Transmitters are required to send
an Interframe Gap of at least 12 octets

How does an Ethernet interface know when
it can transmit a frame?
 CSMA/CD
 Carrier Sense (CS): a transmitter can tell when
another host is transmitting data
 Multiple Access (MA): many transmitters
 Collision Detection (CD): a transmitter can tell
when another host is transmitting data that
collides with its own transmission

How does an Ethernet interface know when
it should receive a frame?
 Receivers look at all frames, discarding frames
not addressed to them
 Receivers only receive frames addressed to them
(or broadcast messages)

Each layer on the client side logically
communicates with the same layer on
the server side
Q
server
client
intermediate router
P

IP is the network layer
 IP provides host-to-host delivery service
of packets (called datagrams)
 IP is connectionless
 IP is unreliable
 IP also provides translation between
different data link protocols

IP addresses are logical addresses at
the network layer
 IP addresses are not Ethernet addresses

Why is this important?
 IP must be able to provide communication
between hosts on different types of networks
(i.e. different data link implementations)

An IP address contains information about
what network the destination host is on
 This enables routing at the network layer
 An IP address (IPv4) is a series of 4 octets
▪ 128.113.2.68 (www.rpi.edu)
▪ 128.113.126.13 (www.cs.rpi.edu)
▪ 128.213.56.14 (mary-kate.cs.rpi.edu)

Each IP address contains information that
identifies the network ID and the host ID
 Routing tables rely on IP address classes:
from, you guessed it, Wikipedia!
Decoding IP Addresses:
A
0
B
10
C
110
NetID
D
1110
Multicast Address
NetID
HostID
NetID
HostID
CLASS
8 bits
8 bits
HostID
8 bits
8 bits

An organization can further subdivide its
host (HostID) space into subnets:
B
10
NetID
10
NetID
8 bits
8 bits
HostID
SubnetID
HostID
8 bits
8 bits
router
Subnet Q
128.213.1.x
Subnet R
128.213.2.x
Subnet S
128.213.3.x

Other subnetting notes:
 Subnets can simplify routing
 IP subnet broadcasts have a HostID of all 1s
 It is possible to have
a single wire network
with multiple subnets:
▪ How would this work?

IP addresses are logical addresses; therefore
we need to map them to physical addresses
 If we know the IP address of a host,
how do we find out the hardware address?
 Address resolution is the process of
finding the hardware address of a
host from a given IP address

ARP is used by a host when it needs to
determine a physical (Ethernet) address
from a given IP address
 ARP is what’s known as a broadcast protocol
 Every host on the network
That’s
me!
receives the ARP request
 Only the “right” one responds

Address resolution does not need to be
performed every time an IP datagram is sent
 Hosts cache IP-to-hardware
I know!
I remember you.
address mappings
 This happens on both ends
▪ i.e. the receiver also caches
the IP-to-hardware address
mapping of the transmitter
Me too!
Hey everyone!
Will IP 128.213.56.14 send
me your Ethernet address?
Not me.
Not me.
Not me.
Sure, that’s me!
My Ethernet address is
3F:55:83:02:AB:79.

IP provides the following:
 Connectionless delivery
▪ i.e. each datagram is individually routed
 Unreliable (non-guaranteed) delivery
 Fragmentation and reassembly
▪ Only at the source and destination hosts
▪ If fragments are lost, the entire datagram is discarded
 Routing
 Error detection (via ICMP)
http://tools.ietf.org/html/rfc1122
8 bits
VERS
HL
8 bits
8 bits
Service
Fragment Length
Datagram ID
TTL
8 bits
FLAG
Protocol
Fragment Offset
Header Checksum
Source Address
Destination Address
Options (if any)
Data
http://tools.ietf.org/html/rfc792

Internet Control Message Protocol (ICMP):
 ICMP is used to transmit control
(or out-of-band) messages
 ICMP uses IP for delivery
 ICMP messages are usually
generated and processed
by the IP software (not user processes)

ICMP message types include:
 Echo Request (ping)
 Echo Response (ping)
 Destination Unreachable
 Redirect
 Time Exceeded
 etc.

Two protocols at the Transport Layer:
 Transmission Control Protocol (TCP)
▪ Connection-oriented
▪ Reliable byte-streaming
▪ Full-duplex
▪ 3-way handshake (SYN/ACK/ACK)
to establish a connection
 User Datagram Protocol (UDP)
▪ Connectionless and unreliable
click me
http://www.ietf.org/rfc/rfc768.txt
Source Port
Destination Port
Length
Checksum
Data
8 bits
8 bits
8 bits
8 bits
http://www.ietf.org/rfc/rfc793.txt
8 bits
8 bits
8 bits
Source Port
8 bits
Destination Port
Sequence Number
Acknowledgment Number
Data
Offset
Reserved
Window
Control
Urgent Pointer
Checksum
Options (if any)
Data

Which is better? TCP or UDP?
 Serving up Web pages
 Financial transaction processing
 Video-streaming server
 File transfer
 Email
 Chat
 Robotic surgery
controlled remotely
Get out of my
way, humans!