The document discusses how TCP guarantees reliable transmission of data over the internet. It explains that TCP breaks messages into packets and tracks which packets are received successfully, resending any lost packets and assembling the data in the right order. This allows communication to work reliably even though the underlying internet protocol (IP) is sometimes unreliable and packets can get lost. The document also mentions how TCP includes additional header information with each packet to help the receiver reconstruct the original message properly.

1. L.O: STUDENTS WILL BE ABLE TO EXPLAIN
HOW TRANSMISSION CONTROL
PROTOCOL (TCP) GUARANTEES RELIABLE
TRANSMISSION.
20-90 minutes (<1 class period, unless Take It Further is used)
DO NOW: READ
Unit 4 Lab 3: Communication
Protocols, Page 2

2. When any information is sent over the
internet, it is NOT sent WHOLE and
DIRECTLY to you like in this picture..
When you send a message over the Internet,
your computer divides it into small chunks
called packets that it sends individually, each
on its own path.
A packet can include any kind of
data: text, numbers, lists, etc.
Computers, servers, and routers are fairly
reliable, but every once in a while a packet will
be lost, and devices on the Internet need to
tolerate these faults.

3. The Transmission Control Protocol (TCP) guarantees
reliable transmission by breaking messages into packets,
keeping track of which packets have been received
successfully, resending any that have been lost, and
specifying the order for reassembling the data on the
other end. This process is what makes the Internet a
packet switching network.
• The computers (including servers) at the two endpoints of a
communication run the Transmission Control Protocol (TCP) that
divides up the packets and guarantees reliable transmission.
• The routers at every connection-point on the Internet run the
Internet Protocol (IP) that transmits packets from one IP address to
another (not caring that sometimes a packet will be lost and not
knowing anything about the purpose or meaning of a packet).

5. Load this project. It provides a simulation
of unreliable data transmission by
Internet Protocol.
• Click the green flag to initialize the incoming
transmission variables before each experiment.
• Click either character to enter a message for it to
send to the other one.
In this simulation, the complete message
is a string of text that is divided into
packets of one letter each. In reality, the
packet length is not so strictly limited
and messages are usually much longer.

6. TCP works by including additional information along
with each packet so that the receiving computer can
keep track of how many packets it has received, re-
request any missing packets, and reorder the packets
to reconstruct the original message.

7. Now let’s Read Blown to Bits pages 306-
309.

8. Take it further (extension
activities)
A. Build a simple TCP. Resolve the unreliability so
that messages are received reliably despite the
limitations of IP packets. You'll need to change
the definitions of:

9. Do not change the definition of
That block simulates the unreliable network. You could
"solve" the problem by rewriting this block to simulate a
perfect network instead of an imperfect one, but that misses
the point.
To solve this problem, you'll need a way to keep track of the
order of the data and a way to re-request missing packets:
First, solve the problem of packets arriving out of
order. You can include extra header information in
addition to the packet data in order to help the
receiver reconstruct the message. This will require
cooperation by both sender and receiver (that is,
changes to both grey blocks).
Then, solve the problem of packets not arriving
at all. That is, make the transmission reliable
even though IP is unreliable. This, too, will
require changing both sender and receiver.

10. Enduring Understandings:
• EU 6.1 The Internet is a network
of autonomous systems.
• EU 6.2 Characteristics of the
Internet influence the systems
built on it.

11. Learning Objectives:
1. LO 6.1.1 Explain the abstractions in the
Internet and how the Internet functions.
[P3]
2. LO 6.2.1 Explain characteristics of the
Internet and the systems built on it. [P5]
3. LO 6.2.2 Explain how the characteristics of
the Internet influence the systems built on
it. [P4]

12. Essential Knowledge:
• EK 6.1.1B An end-to-end architectures facilitates
connecting new devices and networks on the
Internet.
• EK 6.1.1C Devices and networks that make up the
Internet are connected and communicate using
addresses and protocols.
• EK 6.1.1D The Internet and the systems built on it
facilitate collaboration.
• EK 6.1.1E Connecting new devices to the Internet is
enabled by assignment of an Internet protocol (IP)
address.

13. Essential Knowledge:
• EK 6.1.1F The Internet is built on evolving standards,
including those for addresses and names.
• EK 6.1.1H The number of devices that could use an IP
address has grown so fast that a new protocol (IPv6)
has been established to handle routing of many more
devices.
• EK 6.1.1I Standards such as hypertext transfer
protocol (HTTP), IP, and simple mail transfer protocol
(SMTP) are developed and overseen by the Internet
Engineering Task Force (IETF).
• EK 6.2.1A The Internet and the systems built on it are
hierarchical and redundant.

14. Essential Knowledge:
• EK 6.2.1D Routing on the Internet is fault tolerant
and redundant.
• EK 6.2.2B The redundancy of routing (i.e., more
than one way to route data) between two points
on the Internet increases the reliability of the
Internet and helps it scale to more devices and
more people.
• EK 6.2.2D Interfaces and protocols enable
widespread use of the Internet.
• EK 6.2.2E Open standards fuel the growth of the
Internet.

15. Essential Knowledge:
• EK 6.2.2F The Internet is a packet-switched
system through which digital data is sent by
breaking the data into blocks of bits called
packets, which contain both the data being
transmitted and control information for
routing the data.
• EK 6.2.2G Standards for packets and routing
include transmission control
protocol/Internet protocol (TCP/IP).

16. Essential Knowledge:
• EK 6.2.2H Standards for sharing information
and communicating between browsers and
servers on the Web include HTTP and secure
sockets layer/transport layer security
(SSL/TLS).
• EK 6.3.1M Certificate authorities (CAs) issue
digital certificates that validate the
ownership of encrypted keys used in secured
communication and are based on a trust
model.