Module 3: The Transport and Application Layers

The first three layers of a network model
have helped us describe how individual nodes on a network can communicate with
other nodes on either their own network or others. But we haven’t discussed
how individual computer programs can communicate with each other. It’s time to dive into this because that’s
really the aim of computer networking. We network computers together, not just so
they can send data to each other, but because we want programs running
on those computers to be able to send data to each other. This is where the transport and application layers of our
networking model come into play. In short, the transport layer
allows traffic to be directed to specific network applications, and
the application layer allows these applications to communicate
in a way they understand. By the end of this module, you’ll be able
to describe TCP ports and sockets, and identify the different
components of a TCP header. You’ll also be able to show the difference
between connection oriented and connection list protocols, and explain
how TCP is used to ensure data integrity. Are you ready to be
transported to the next lesson? I hope so because the transport
layer is up next, see you there

Tutorial: Transport Layer Functions

Introduction:

Welcome to the Transport Layer! This layer sits at the heart of network communication, ensuring applications on different devices can reliably exchange data. In this tutorial, we’ll explore its key functions and how they contribute to seamless networking experiences.

Key Functions of the Transport Layer:

1. Multiplexing and Demultiplexing:
   • Multiplexing: Imagine a busy airport with multiple flights departing from a single runway. That’s like the transport layer directing traffic from one device to various applications using unique identifiers called ports.
   • Demultiplexing: On the receiving end, it’s like ground control guiding incoming planes to their designated gates. The transport layer ensures data reaches the intended application using the same port numbers.
2. Ports:
   • Think of ports as virtual doors for applications on a device. Each port is assigned a 16-bit number, acting as a specific address for incoming and outgoing traffic.
   • Common port examples:
     • Port 80 for HTTP (unencrypted web traffic)
     • Port 443 for HTTPS (encrypted web traffic)
     • Port 25 for email
     • Port 21 for FTP (file transfers)
3. Connection-Oriented vs. Connectionless Protocols:
   • TCP (Transmission Control Protocol): Like a phone call, TCP establishes a persistent connection between devices before exchanging data. It guarantees reliable delivery, error checking, and data sequencing.
   • UDP (User Datagram Protocol): Like sending letters, UDP is more efficient for quick, less critical data exchanges. It doesn’t guarantee delivery or order, but it’s faster for tasks like live streaming or gaming.
4. The Three-Way Handshake (TCP):
   • To initiate a TCP connection, a “handshake” occurs:
     1. Client sends a SYN (synchronize) packet to the server.
     2. Server responds with a SYN-ACK (synchronize-acknowledge) packet.
     3. Client confirms with an ACK (acknowledge) packet.
   • This ensures both devices are ready for communication.
5. TCP Flags:
   • TCP uses flags within its header to control communication flow:
     • SYN: Initiate a connection
     • ACK: Acknowledge data receipt
     • FIN: Terminate a connection
     • PSH: Push data immediately
     • RST: Reset a connection in case of errors
6. Firewalls:
   • Firewalls act as security guards for networks, inspecting incoming and outgoing traffic.
   • They often filter traffic based on ports and protocols, blocking unauthorized access and protecting devices from threats.

Conclusion:

By understanding these transport layer functions, you’ll gain a deeper appreciation for the complexities and mechanisms that enable smooth network communication. This knowledge is essential for network troubleshooting, security configuration, and overall comprehension of how data travels across the internet.

The transport layer is responsible for lots of important functions of
reliable computer networking. These include multiplexing and
demultiplexing traffic, establishing long running connections and ensuring data integrity through error
checking and data verification. By the end of this lesson, you should be
able to describe what multiplexing and demultiplexing are and how they work. You’ll be able to identify the differences
between TCP and UDP, explain the three way handshake and understand
how TCP flags are used in this process. Finally, you’ll be able to describe the
basics of how firewalls keep network safe. The transport layer has the ability
to multiplex and demultiplex, which sets this layer
apart from all others. Multiplexing in the transport layer
means that nodes on a network have the ability to direct traffic toward
many different receiving services. Demultiplexing is the same concept
just at the receiving end, it’s taking traffic that’s all
aimed at the same node and delivering it to the proper
receiving service. The transport layer handles multiplexing
and demultiplexing through ports. A ports is a 16-bit number
that’s used to direct traffic to specific services running
on a networked computer. Remember the concept of server and
clients. A server or service is a program running on
a computer waiting to be asked for data. A client is another program
that is requesting this data. Different network services run while
listening on specific ports for incoming requests. For example,
the traditional ports for http or unencrypted web traffic is ports 80. If we want to request a web page
from a web server running on a computer listening on I P 10.1.1.100, the traffic would be directed
to ports 80 on that computer. Ports are normally denoted with
a colon after the IP address. So the full IP and
ports in this scenario could be described as 10.1.1.100:80. When written this way, it’s known as
a socket address or socket number. The same device might also be running
an FTP or file transfer protocol server. Ftp is an older method used for
transferring files from one computer to another, but
you still see it in use today. FTP traditionally listens on port 21. So, if you wanted to establish
a connection to an FTP server running on the same IP that our example web
server was running on you direct traffic to 10.1.1.100 ports 21. You might find yourself working in
IT support at a small business. In these environments, a single server could host almost all of
the applications needed to run a business. The same computer might host an internal
website, the mail server for the company, file server for sharing files,
a print server for sharing network printers,
pretty much anything. This is all possible because of
multiplexing and demultiplexing. And the addition of ports
to our addressing scheme.

Tutorial: Dissecting a TCP Segment

Introduction:

Welcome to the heart of reliable network communication! In this tutorial, we’ll dissect a TCP segment, exploring its vital components and how they ensure seamless data transfer across networks. By understanding these inner workings, you’ll gain valuable insights for troubleshooting, security, and overall network comprehension.

Key Concepts:

1. TCP Segment Structure:
   • A TCP segment consists of two main parts:
     • TCP Header: Contains essential information for control and delivery.
     • Data Section: Carries the actual application data being transmitted.
2. TCP Header Fields:
   • Source Port: Identifies the sending application (e.g., web browser).
   • Destination Port: Identifies the receiving application (e.g., web server).
   • Sequence Number: Tracks the segment’s order within a larger data stream.
   • Acknowledgment Number: Specifies the expected next sequence number.
   • Data Offset: Indicates the starting point of the data section within the segment.
   • TCP Control Flags: Six bits used for signaling actions like connection setup, termination, acknowledgments, etc.
   • TCP Window: Determines how much data can be sent before an acknowledgment is required.
   • Checksum: Verifies data integrity by detecting errors during transmission.
   • Other Fields: Urgent pointer (rarely used), options (for advanced flow control), and padding.
3. Ephemeral Ports:
   • Dynamically assigned ports for outgoing connections, distinct from well-known ports like 80 for HTTP.
4. Segment Chaining:
   • TCP often splits large data chunks into multiple segments, relying on sequence numbers to reassemble them in the correct order at the recipient’s end.

Applications in Networking:

• Troubleshooting: Understanding TCP segments is crucial for analyzing network traffic patterns, identifying bottlenecks, and resolving connectivity issues.
• Security: Firewalls and security policies often rely on port numbers and TCP flags to filter traffic and protect networks from unauthorized access.
• Network Optimization: Optimizing TCP parameters like window size can impact network performance and data transfer speeds.

Conclusion:

By mastering the TCP segment, you’ve unlocked a deeper understanding of how reliable network communication functions. This knowledge empowers you to troubleshoot effectively, configure secure networks, and appreciate the complexities of data transmission across the internet.

Heads up. In this video, we’re going to dissect
a TCP segment. In IT support, if network traffic isn’t
behaving as users expect it to, you might have to analyze
it closely to troubleshoot. Get ready to take a peek
at all the inner workings. Just like how an Ethernet frame encapsulates an IP datagram, an IP datagram encapsulates
a TCP segment. Remember that an Ethernet
frame has a payload section, which is really just
the entire contents of an IP datagram. Remember also that an IP
datagram has a payload section. This is made up of what’s
known as a TCP segment. A TCP segment is made up of a TCP header and
a data section. This data section,
as you might guess, is just another payload area for where the application
layer places its data. A TCP header itself is split into lots of fields containing
lots of information. First, we have the source port and the destination port fields. The destination port is the port of the service the
traffic is intended for, which we talked about
in the last video. A source port is a high
numbered port chosen from a special section of ports
known as ephemeral ports. We’ll cover ephemeral ports in more detail in a little bit. For now, it’s enough to
know that a source port is required to keep lots of
outgoing connections separate. You know how I destination port, say port 80, is needed to make sure traffic reaches a web server
running on a certain IP. Similarly, a source port is needed so that when the
web server replies, the computer making the
original request can send this data to the program that
was actually requesting it. It is in this way that
when a web server responds to your request
to view a webpage, that this response
gets received by your web browser and not
your word processor. Next up is a field known
as the sequence number. This is a 32-bit number that’s used to keep
track of where in a sequence of TCP segments
this one is expected to be. You might remember that
lower on our protocol stack, there are limits
to the total size of what we send across the wire. An Ethernet frame
is usually limited in size to 1,518 bytes, but we usually need to send
way more data than that. At the transport layer, TCP splits all of this data
up into many segments. The sequence number in a header
is used to keep track of which segment out of many this particular
segment might be. The next field, the
acknowledgment number is a lot like the
sequence number. The acknowledgment number is the number of the next
expected segment. In very simple language, a sequence number of one and
an acknowledgment number of two could be read as
this is segment 1, expect segment 2 next. The data offset
field comes next. This field is a four-bit
number that communicates how long the TCP header
for this segment is. This is so that the
receiving network device understands where the
actual data payload begins. Then we have six bits that are reserved for the six
TCP control flags. The next field is a 16-bit number known
as the TCP window. A TCP window specifies
the range of sequence numbers
that might be sent before an acknowledgment
is required. TCP is a protocol that’s super reliant on
acknowledgments. This is done in order
to make sure that all expected data is
actually being received, and that the sending
device doesn’t waste time sending data that
isn’t being received. The next field is
a 16 bit checksum. It operates just like the checksum fields at the
IP and Ethernet level. Once all of this segment has
been ingested by recipient, the checksum is
calculated across the entire segment
and is compared with the checksum in the header
to make sure that there was no data lost or
corrupted along the way. The urgent pointer field is used in conjunction
with one of the TCP control
flags to point out particular segments that might be more important than others. This is a feature of TCP that hasn’t really ever
seen adoption, and you’ll probably never
find it in modern networking. Even so, it’s important to know what all sections of
the TCP header are. Next up, we have
the options field. Like the urgent pointer field, this is rarely used
in the real-world, but it’s sometimes used for more complicated flow
control protocols. Finally, we have some padding, which is just a sequence
of zeros to ensure that the data payload section begins
at the expected location.

Tutorial: TCP Connection Establishment and Flags

Introduction:

Welcome to the fascinating world of TCP! In this tutorial, we’ll delve into the core of reliable network communication by exploring how TCP establishes connections and utilizes control flags for seamless data exchange. This knowledge equips you to troubleshoot network issues, analyze traffic behavior, and gain a deeper appreciation for the intricate dance of data transmission across networks.

Understanding the Need for Connections:

Unlike lower protocols like IP and Ethernet that simply send individual packets, TCP prioritizes reliability by building connections. Think of it as establishing a dedicated communication channel between two devices, allowing for long data streams to be transmitted efficiently and with guaranteed delivery.

The Mighty TCP Control Flags:

TCP relies on six crucial flags embedded within its header to control and monitor the connection:

• URG (Urgent): Rarely used, marks a segment containing urgent data requiring immediate processing.
• ACK (Acknowledge): Indicates the recipient received data up to the specified sequence number.
• PSH (Push): Instructs the receiver to immediately deliver buffered data to the application, ensuring quick response times for critical information.
• RST (Reset): Abruptly terminates the connection due to unrecoverable errors or communication breakdown.
• SYN (Synchronize): Initiates a connection by setting the initial sequence number for data transmission.
• FIN (Finish): Signals that the sender has no more data to send and initiates connection closure.

The Three-Way Handshake:

This elegant dance of flags orchestrates connection establishment:

1. Computer A sends a SYN flag: Requesting to initiate a connection and setting its sequence number.
2. Computer B responds with SYN-ACK: Acknowledging the request and setting its own sequence number.
3. Computer A sends an ACK: Confirming receipt of Computer B’s sequence number and completing the handshake.

Full Duplex Communication and Graceful Closure:

Once the handshake is complete, both computers can send and receive data simultaneously (full duplex). Data segments are acknowledged with ACK flags, ensuring reliable delivery. When either computer wants to close the connection, a four-way handshake takes place:

1. FIN flag from closing device: Announces no further data transmission.
2. ACK flag from receiving device: Acknowledges the FIN flag.
3. (Optional) FIN flag from receiving device: Indicates it also has no more data.
4. ACK flag from closing device: Confirms receipt of the second FIN flag and completes closure.

Applications of this Knowledge:

Understanding TCP connection establishment and flags empowers you in various ways:

• Troubleshooting network issues: Analyze traffic patterns, identify flag sequences indicating errors, and diagnose connectivity problems.
• Interpreting network logs: Decipher log entries containing flag information to understand network behavior and identify potential security threats.
• Configuring firewalls and security policies: Utilize port numbers and specific flags to filter traffic and enhance network security.

Conclusion:

By mastering the art of TCP connection establishment and flags, you gain a powerful tool for understanding the intricate workings of network communication. This knowledge equips you to troubleshoot effectively, secure your networks, and appreciate the elegance and reliability that TCP brings to the digital world.

Remember, this is just the tip of the iceberg! Further exploration can delve into advanced topics like TCP windowing, congestion control mechanisms, and specialized flag applications.

As a protocol TCP establishes
connections used to send long chains of segments of data. You can contrast this with the protocols
that are lower in the networking model. These include IP and ethernet which
just send individual packets of data. As an IT Support Specialist, you need
to understand exactly how that works, so you can troubleshoot issues where
network traffic may not be behaving in the expected manner. The way TCP establishes
a connection is through the use of different TCP control flags
used in a very specific order. Before we cover how connections
are established and closed, let’s first define the six
TCP control flags. We’ll look at them in the order
that they appear in a TCP header. Heads up, this isn’t necessarily in the
same order of how frequently they’re sent or how important they are. The first flag is known as URG. This is short for urgent. A value of one here indicates that
the segment is considered urgent and that the urgent pointer field
has more data about this. This feature of TCP has never
really had widespread adoption and isn’t normally seen. The second flag is ACK,
short for acknowledged. A value of one in this field means that
the acknowledgement number field should be examined. The third flag is PSH,
which is short for push. This means that the transmitting device
wants the receiving device to push currently-buffered data to the application
on the receiving end as soon as possible. A buffer is a computing technique
where a certain amount of data is held somewhere before being
sent somewhere else. This has lots of practical applications. In terms of TCP, it’s used to send
large chunks of data more efficiently. By keeping some amount
of data in a buffer, TCP can deliver more meaningful chunks
of data to the program waiting for it. But in some cases you might be
sending a very small amount of information that you need the listening
program to respond to immediately. This is what the push flag does. The fourth flag is RST, short for reset. This means that one of the sides in
a TCP connection hasn’t been able to properly recover from a series of
missing or malformed segments. It’s a way for one of the partners in
a TCP connection to basically say, wait, I can’t put together what you mean,
let’s start over from scratch. The fifth flag is SYN,
which stands for synchronize. It’s used when first establishing
a TCP connection and make sure the receiving end knows to
examine the sequence number field. And finally our sixth flag is FIN,
which is short for finish. When this flag is sent to one, it means
the transmitting computer doesn’t have any more data to send and
the connection can be closed. For a good example of how
TCP control flags are used, let’s check out how a TCP
connection is established. Computer A will be our
transmitting computer and computer B will be our receiving computer. To start the process off computer
A sends a TCP segment to Computer B, with a SYN flag sent. This is computer A’s way of saying,
let’s establish a connection and look at my sequence number field so
we know where this conversation starts. Computer B then responds with a TCP
segment where both the SYN and ACK flags are sent. This is Computer B’s way of saying,
sure, let’s establish a connection and I acknowledge your sequence number. Then Computer A responds again
with just the ACK flag sent, which is just saying I acknowledge your
acknowledgement, let’s start sending data. I love how polite they are to each other. This exchange involving segments
that have SYN, SYN/ACK and ACK sent happens every single time a TCP
connection is established anywhere and is so famous that it has a nickname. The three-way handshake. A handshake is a way for two devices to
ensure that they’re speaking the same protocol and
will be able to understand each other. Once the three-way handshake is complete,
the TCP connection is established. Now, Computer A is free to send whatever
data it wants to Computer B and vice versa. Since both sides have now sent
SYN/ACK pairs to each other, a TCP connection in this state
is operating in full duplex. Each segment sent in either
direction should be responded to by a TCP segment with the ACK field sent. This way the other side always
knows what has been received. Once one of the devices involved with
the TCP connection is ready to close the connection, something known
as a four-way handshake happens. The computer ready to close
the connection sends a FIN flag, which the other computer
acknowledges with an ACK flag. Then if this computer is also
ready to close the connection, which will almost always be the case,
it will send a FIN flag. This is again responded to by an ACK flag. Hypothetically, a TCP connection can stay
open in simplex mode with only one side closing the connection, but this isn’t
something you’ll run into very often.

TCP Socket States for IT Support Specialists: A Troubleshooting Guide

Understanding the different states of TCP sockets is crucial for IT support specialists troubleshooting network connectivity issues. This guide breaks down the most common states you’ll encounter, helping you interpret them and diagnose problems more effectively.

What are TCP Sockets?

Before diving into states, let’s clarify the difference between ports and sockets. A port is like a virtual address on a network device, while a socket is the actual endpoint or “plug” for a specific communication channel. Think of it as the physical outlet where you plug your phone charger, while the port number is like the room where that outlet is located.

TCP sockets can exist in various states, reflecting the current stage of a network connection. Understanding these states is like knowing the different modes on your phone charger (plugged in, charging, fully charged). Each state provides valuable clues about the health and progress of a connection.

Common TCP Socket States:

1. LISTEN:
   • This state signifies a server-side socket ready to accept incoming connections. Imagine a waiter standing at the entrance of a restaurant, ready to greet and seat arriving guests.
2. SYN_SENT:
   • This state indicates a client-side socket that has sent a connection request (SYN packet) to a server but hasn’t yet received a response. It’s like the waiter approaching a restaurant and knocking on the door, waiting for someone to answer.
3. SYN_RECEIVED:
   • This state signifies a server-side socket that has received a connection request (SYN packet) from a client and sent a response (SYN/ACK packet) back. It’s like the waiter receiving the knock, opening the door a crack, and saying, “Coming!” before fully opening the door.
4. ESTABLISHED:
   • This is the happy state! Both sides are connected and can freely send and receive data. It’s like the waiter finally leading the guest to their table and taking their order.
5. FIN_WAIT:
   • This state indicates that one side has sent a FIN packet to initiate closing the connection but is waiting for the other side’s confirmation (ACK packet). Imagine the waiter bringing the bill and waiting for the guest to pay before clearing the table.
6. CLOSE_WAIT:
   • This state signifies that the connection has been closed at the TCP level, but the application that opened the socket on one side hasn’t yet released its hold on the socket. It’s like the waiter clearing the table but leaving the chairs empty until the restaurant closes.
7. CLOSED:
   • This state marks the final stage, where the connection is fully terminated, and no further communication is possible. It’s like the waiter turning off the lights and locking the restaurant door for the night.

Additional Notes:

• Remember, these are just the most common states. Other less frequent states exist.
• Socket state names and definitions may vary slightly between operating systems. Always refer to the specific system documentation for precise details.
• When troubleshooting network connectivity issues, understanding the states of sockets involved can provide valuable insights into the problem’s location and nature.

By mastering the language of TCP socket states, you’ll be better equipped to diagnose and resolve network problems, keeping your users connected and productive. So, the next time you encounter a connectivity issue, remember these states and start sleuthing your way to a solution!

A socket is the instantiation of
an endpoint in a potential TCP connection. An instantiation is the actual
implementation of something defined elsewhere. TCP sockets require actual
programs to instantiate them. You can contrast this with a port which
is more of a virtual descriptive thing. In other words, you can send
traffic to any ports you want, but you’re only going to get a response if a
program has opened a socket on that port. TCP sockets can exist in lots of states. And being able to understand what those
mean will help you troubleshoot network connectivity issues as
an IT support specialist. We’ll cover the most common ones here. LISTEN, listen means that
a TCP socket is ready and listening for incoming connections. You’d see this on the server side only. SYN_SENT, this means that
a synchronization request has been sent, but the connection hasn’t
been established yet. You’d see this on the client side only. SYN_RECEIVED, this means that
a socket previously in a LISTEN state has received a synchronization request and
sent a SYN/ACK back, but it hasn’t received the final
ACK from the client yet. You’d see this on the server side only. ESTABLISHED, this means that the TCP
connection is in working order and both sides are free to
send each other data. You’d see this state on both the client
and server side of a connection. This will be true of all
the following socket states too. So keep that in mind. FIN_WAIT, this means that
a FIN has been sent but the corresponding ACK from the other
end hasn’t been received yet. CLOSE_WAIT, this means that the connection
has been closed at the TCP layer, but that the application that
opened the socket hasn’t released its hold on the socket yet. CLOSED, this means that the connection
has been fully terminated and that no further communication is possible. There are other TCP socket
states that exists. Additionally, socket states and their names can vary from operating
system to operating system. That’s because they exist outside of
the scope of the definition of TCP itself. TCP, as a protocol, is universal in
how it’s used since every device speaking the TCP protocol has to
do this in the exact same way for communications to be successful. Choosing how to describe
the state of a socket at the operating system level
isn’t quite as universal. When troubleshooting issues at the TCP
layer, make sure you check out the exact socket state definitions for
the systems you’re working with.

Navigating the Network: A Guide to TCP vs. UDP Protocols

Ever wondered how your favorite streaming service delivers seamless video, while important files require meticulous transfer? The answer lies in the fundamental differences between two key data transmission protocols: TCP and UDP. This tutorial unpacks their contrasting approaches, empowering you to make informed choices in your next network adventure.

Connection Champions: The TCP Approach

Imagine a cautious traveler meticulously planning their route, seeking confirmation at every step. That’s TCP: a connection-oriented protocol that establishes a reliable two-way street for data to flow. Before exchanging information, TCP handshakes, setting up a virtual tunnel with sequence numbers and acknowledgments. Each data packet sent receives a confirmation, ensuring complete delivery. Like a meticulous postman, TCP retries if a packet goes astray, guaranteeing accurate arrival.

Benefits of TCP:

• Reliability: Ensures all data arrives complete and in order, making it ideal for critical tasks like file transfers and secure communication.
• Error checking: Built-in checksums and acknowledgments detect and rectify transmission errors.
• Flow control: Adjusts data sending rate based on receiver capacity, preventing overload.

Drawbacks of TCP:

• Overhead: Handshakes and acknowledgments create additional traffic, slightly impacting speed.
• Latency: Establishing connections and retries take time, affecting real-time applications.

Speed Demons: The UDP Approach

Picture a daredevil cyclist zipping through traffic, trusting fate for delivery. That’s UDP: a connectionless protocol that fires data packets like arrows flung into the digital ether. Without handshakes or confirmations, UDP prioritizes speed and efficiency, making it perfect for time-sensitive applications like live video streaming and online gaming.

Benefits of UDP:

• Speed: No connection overhead means faster data transfer, crucial for real-time applications.
• Efficiency: Less traffic on the network, enabling smooth streaming and responsive gaming.
• Simplicity: Lightweight protocol with minimal complexity, making it adaptable to various applications.

Drawbacks of UDP:

• Unreliable: No error checking or retries means lost packets are unrecoverable, potentially impacting data integrity.
• Out-of-order delivery: Packets may arrive in scrambled order, requiring application-level reassembly.
• Congestion sensitivity: Flooding networks with UDP packets can cause congestion and lag.

Choosing the Right Protocol:

The choice between TCP and UDP depends on your needs:

• For reliable, complete data delivery, choose TCP. Think file transfers, online banking, and secure communication.
• For real-time applications where speed and low latency are paramount, choose UDP. Think video streaming, online gaming, and real-time voice communication.

Remember, network protocols are tools and their effectiveness depends on how you wield them. Understanding the strengths and weaknesses of TCP and UDP empowers you to navigate the digital landscape with confidence, ensuring your data journeys reach their destination, whether seamlessly or with a touch of adventurous spirit.

So far, we’ve mostly focused on TCP
which is a connection-oriented protocol. A connection-oriented protocol is one
that establishes a connection, and uses this to ensure that all data
has been properly transmitted. A connection at the transport layer
implies that every segment of data sent is acknowledged, this way both ends of the
connection always know which bits of data have definitely been delivered to
the other side and which haven’t. Connection-oriented protocols
are important because the internet is a vast and busy place and lots of things could go wrong while trying
to get data from point a to point b. If even a single bit doesn’t
get transmitted properly, the resulting data is often
incomprehensible by the receiving end. And remember that at the lowest level, a bit is just an electrical signal
within a certain voltage range. But there are plenty of other reasons
why traffic might not reach its destination beyond lineaires. It could be anything, pure congestion
might cause a router to drop your traffic in favor of forwarding
more important traffic. Or a construction company could cut
a fiber cable connecting to ISPs, anything’s possible. Connection-oriented protocols like TCP,
protect against this by forming connections and through
the constant stream of acknowledgments. Our protocols at lower levels of our
network model like IP and Ethernet, do use check sums to ensure that all
the data they received was correct. But did you notice that we
never discussed any attempts at resending data that
doesn’t pass this check, that’s because that’s entirely up
to the transport layer protocol. At the IP or Ethernet level, if a checksum
doesn’t compute all of that data is just discarded, it’s up to TCP to
determine when to resend this data. Since TCP expects an ACK for
every bit of data it sends, it’s in the best position to know what
data successfully got delivered and can make the decision to
resend a segment if needed. This is another reason why
sequence numbers are so important. While TCP will generally send all
segments in sequential order, they may not always arrive in that order. If some of the segments had to be
resent due to errors at lower layers, it doesn’t matter if they
arrive slightly out of order. This is because sequence numbers allow for all of the data to be put back
together in the right order. It’s pretty handy. Now, as you might have picked up on, there’s a lot of overhead with
connection-oriented protocols like TCP. You have to establish the connection, you have to send a stream of
constant streams of acknowledgments. You have to tear the connection down
at the end, that all accounts for a lot of extra traffic. Well this is important traffic,
it’s really only useful if you absolutely, positively have to be sure your
data reaches its destination. You can contrast this with
connectionless protocols, the most common of these is known
as UDP or User Datagram Protocol. Unlike TCP,
UDP doesn’t rely on connections and it doesn’t even support
the concept of an acknowledgement. With UDP, you just set a destination
port and send the packet. This is useful for
messages that aren’t super important, a great example of UDP is streaming video. Let’s imagine that each UDP Datagram
is a single frame of a video, for the best viewing experience, you might
hope that every single frame makes it to the viewer, but it doesn’t really
matter if a few get lost along the way. A video will still be pretty watchable
unless it’s missing a lot of its frames. By getting rid of all the overhead of TCP, you might actually be able to send
higher quality video with UDP. That’s because you’ll be saving
more of the available bandwidth for actual data transfer instead of the
overhead of establishing connections and acknowledging delivered data segments.

Firewall Fundamentals: Building Your Network’s Wall of Defense

In the digital world, where data flows like electricity, staying secure requires more than just locking your doors. You need a firewall, the vigilant gatekeeper standing guard against unwelcome traffic. This tutorial delves into the essential concepts of firewalls, empowering you to build a fortified network with a clear understanding of their function and configuration.

What is a Firewall?

Imagine a bouncer at a bustling nightclub, checking IDs and credentials before granting entry. That’s like a firewall, a security system that filters incoming and outgoing network traffic based on predefined rules. Essentially, it acts as a wall, separating your trusted internal network from the potentially dangerous outside world.

Firewall Layers:

Think of your network as a layered cake. Firewalls can operate at different layers, each offering specific advantages:

• Packet Filtering: The simplest layer, like a basic bouncer, checks packet headers for information like source and destination addresses, blocking those that don’t meet the criteria.
• Stateful Inspection: A more sophisticated approach, analyzing the entire data packet and its connection state, making it adept at detecting advanced threats.
• Application Layer Inspection: The deepest analysis, examining the actual content of the data for malicious elements, ideal for protecting against application-specific attacks.

Port Control: The Firewall’s Toolbox

Imagine each port on your network as a specific entrance to your digital building. A firewall allows you to:

• Open Ports: Grant access to specific ports like port 80 for web servers or port 22 for secure shell connections.
• Close Ports: Block unwanted traffic by shutting down access to non-essential ports.
• Filter Traffic: Define rules based on source and destination IP addresses, protocols, and even specific data patterns to further refine incoming and outgoing traffic.

Types of Firewalls:

Just like bouncers come in different styles, firewalls have their own variations:

• Packet-Filtering Firewalls: Basic and efficient, ideal for home users or small networks.
• Stateful Firewalls: Offer more advanced protection with enhanced threat detection capabilities.
• Application-Level Firewalls: Provide the highest level of security by inspecting traffic content, but with increased processing demands.

Firewall Deployment:

The wall that protects your network can be:

• Hardware Firewalls: Dedicated devices installed at network entry points.
• Software Firewalls: Programs built into operating systems or running on individual devices.
• Combined Solutions: Many routers integrate firewall functionality with other network features.

Building Your Firewall Strategy:

Securing your network with a firewall requires careful planning:

• Identify Your Needs: Assess your network’s vulnerabilities and the level of protection needed.
• Choose the Right Type: Select a firewall that aligns with your budget, technical expertise, and security requirements.
• Configure Effectively: Define clear rules and access controls based on your specific needs.
• Maintain Vigilance: Keep your firewall software and rules updated to adapt to evolving threats.

Remember, firewalls are a vital layer of your network’s security infrastructure. By understanding their principles and applying them effectively, you can create a robust digital fortress, keeping your data safe and secure in the ever-evolving online landscape.

This tutorial is just the beginning! Feel free to delve deeper into specific firewall types, configuration details, and threat management strategies to build an impenetrable defense for your network.

You know what network
device we haven’t mentioned that you’re
probably super familiar with? A firewall. A firewall is just a device that blocks traffic that meets
certain criteria. Firewalls are a critical
concept to keeping a network secure since they’re the primary way you
can stop traffic, you don’t want from
entering the network. Firewalls can actually operate at lots of different
layers of the network. There are firewalls
that can perform inspection of application
layer traffic and firewalls that
primarily deal with blocking ranges
of IP addresses. The reason we cover firewalls
here is that they’re most commonly used at the
transportation layer. Firewalls that operate at the transportation layer will generally have a configuration that enables them
to block traffic to certain ports while allowing
traffic to other ports. Let’s imagine a simple
small business network. The small business might have one server which hosts
multiple network services. The server might have a
web server that hosts the company’s website
while also serving as the file server for a
confidential internal document. A firewall placed at the perimeter of the network
could be configured to allow anyone to send traffic to port 80 in order to
view the web page. At the same time, it could block all access for external IPs to any other port so that
no one outside of the local area network could
access the file server. Firewalls are sometimes
independent network devices but it’s really better
to think of them as a program that can run anywhere. For many companies and
almost all home users, the functionality
of a router and a firewall is performed
by the same device. Firewalls can run on individual hosts instead
of being a network device. All major modern
operating systems have firewall
functionality built in. That way, blocking or allowing traffic to
various ports and therefore to specific services can be performed at the
host level as well.

Here’s a tutorial on the Application Layer in Networking Model:

Welcome to the Top of the Network!

In this tutorial, we’ll explore the application layer, the topmost layer in the networking model that brings applications to life on the network.

Imagine a bustling marketplace with vendors speaking different languages. That’s the application layer—a diverse space where applications communicate using various protocols.

Here’s what we’ll cover:

1. What’s the Application Layer’s Role?
   • It’s where applications like web browsers, email clients, and streaming services exchange data.
   • It’s the layer that directly interacts with user applications.
2. Protocols for Every Occasion
   • Unlike lower layers with standardized protocols, the application layer boasts a wide array of protocols tailored to specific application needs.
   • Common examples include:
     • HTTP for web browsing
     • FTP for file transfers
     • SMTP for email
     • DNS for domain name resolution
     • And many more!
3. Standardization Within the Chaos
   • Despite the diversity, there’s standardization within application types.
   • For example, all web browsers and servers use the HTTP protocol to ensure seamless communication, regardless of vendor.
4. How It Works:
   • Applications generate data (e.g., emails, web page requests).
   • The application layer passes data to the transport layer, which adds its own information (e.g., port numbers).
   • Lower layers handle routing and delivery until data reaches its destination.
   • The receiving application layer extracts data and presents it to the user-facing application.

Key Points to Remember:

• The application layer is where the magic happens—it enables applications to interact over networks.
• It’s characterized by a diverse set of protocols, each serving specific application needs.
• Standardization within application types ensures interoperability across different vendors.

Stay Curious!

• Explore common application layer protocols in detail.
• Investigate how firewalls and security measures operate at this layer.
• Discover the exciting world of application development and networking!

By understanding the application layer, you’ll gain a deeper appreciation for the seamless communication that powers our digital world. Happy exploring!

Welcome to our lesson about
the application layer. We’re almost done covering all aspects of our
networking model, which means you’ve already
learned how computers process electrical or optical
signals to send communication across a cable
at the physical layer. We’ve also covered how
individual computers can address each other and send each other data using Ethernet at
the data link layer. We’ve discussed how the
network layer is used by computers and routers to communicate between
different networks using IP. In our last lesson, we covered how the
transportation layer ensures that data
is received and sent by the proper applications. You’re chock-full of
layers of new information. Now, we can finally
talk about how those actual applications send and receive data using
the application layer. Just like with
every other layer, TCP segments have a generic
data section to them. As you might have guessed, this payload section is
actually the entire contents of whatever data applications wants to send to each other. It can be contents of a webpage. If a web browser is
connecting to a web server, this could be the
streaming video content of your Netflix app on
your PlayStation connecting to the
Netflix servers. It could be the
contents of a document your word processor
is sending to a printer, and many more things. There are a lot of protocols used at the application layer, and they are numerous
and diverse. At the data link layer, the most common
protocol is Ethernet. I should call out that
wireless technologies do use other protocols
at this layer. At the network layer, use of
IP is everywhere you look. At the transport layer, TCP and UDP cover most
of the use cases. But at the application layer, there are just so many
different protocols in use, it wouldn’t make sense
for us to cover them. Even so, one concept
you can take away about application layer
protocols is that there’s still standardized across
application types. Let’s dive a little deeper into web servers and web
browsers for an example. There are lots of
different web browsers. You can be using Chrome, Safari, you name it. They’ll need to
speak the protocol. The same thing is
true for web servers. In this case, the
web browser would be the client and the web
server would be the server. The most popular web
servers are Microsoft IS, Apache, NGINX, but they also need to all
speak the same protocol. This way, you ensure that no matter which browser
you’re using, you’d still be able to
speak to any server. For web traffic, the
application layer protocol is known as HTTP. All of these different
web browsers and web servers have to
communicate using the same HTTP protocol
specification in order to ensure
interoperability. The same is true for most
other classes of application. You might have dozens of
choices for an FTP client, but they all need to speak the FTP protocol
in the same way.

Here’s a tutorial on Network Layer Models:

Welcome to the World of Network Architecture!

In this tutorial, we’ll explore different ways of conceptualizing network communication: Network Layer Models. They’re like blueprints, revealing how data travels through networks in an organized manner.

Let’s dive into two common models:

1. The 5-Layer Model:

• Physical Layer: Manages physical connections (cables, signals).
• Data Link Layer: Handles local device addressing and error correction.
• Network Layer: Routes data between networks using IP addresses.
• Transport Layer: Manages data transfer between applications, ensuring reliable delivery.
• Application Layer: Interacts with user applications (web browsers, email clients, etc.).

2. The 7-Layer OSI Model:

• Includes all 5 layers from the 5-layer model, plus:
  • Session Layer: Coordinates communication between applications and transport layer.
  • Presentation Layer: Prepares data for application-level understanding (encryption, compression).

Key Differences:

• The 5-layer model is often preferred for practicality and troubleshooting in IT support.
• The 7-layer OSI model offers a more rigorous theoretical framework, often used in academia and certifications.

Which to Use When:

• For daily network troubleshooting and understanding, the 5-layer model is often sufficient.
• For deeper conceptual understanding and academic discussions, the 7-layer OSI model provides more granular detail.

Key Takeaways:

• Network layer models visualize how data travels through networks.
• Different models exist, each with its own strengths and use cases.
• Understanding both the 5-layer and 7-layer OSI models provides a comprehensive understanding of network architecture.

Stay Curious!

• Explore how different protocols function at each layer.
• Research how network layer models inform network design and troubleshooting.
• Discover the fascinating world of network engineering!

By mastering network layer models, you’ll gain a powerful tool for understanding and navigating the complexities of modern networks. Happy exploring!

In our opening module, we talked about how
there are lots of competing network layer models. We’ve been working from
a five layer model, but you’ll probably run into various other models during your career as an IT
support specialist. Some models might combine the physical and
data link layers into one and only talk
about four layers. But you might remember a
certain model we called out specifically in a
reading section back in the first module. This is the OSI or Open
Systems Interconnection model. This model is important
to understand with our five layer model because it’s the most
rigorously defined. That means it’s often used in academic settings or by various network
certification organizations. The OSI model has seven
layers and introduces two additional layers between our transport layer and
our application layer. The fifth layer in the OSI
model is the session layer. The concept of the session layer is that it’s responsible
for things like facilitating the
communication between actual applications and
the transport layer. It’s the part of the
operating system that takes the application layer data that’s been unencapsulated from all the layers below
it and hands it off to the next layer in the OSI
model, the presentation layer. The presentation layer is responsible for making sure that the unencapsulated
application layer data is actually able to be understood by the
application in question. This is the part of an
operating system that might handle encryption
or compression of data. While these are important
concepts to keep in mind, you’ll notice that there isn’t any encapsulation going on. That’s why in our model, we lump all of these functions into
the application layer. We believe a five layer model
is the most useful when it comes to the day-to-day business of understanding networking, but the seven-layer OSI
model is also prevalent. No networking education would be complete without
understanding its basics.

Unraveling the Magic: A Tutorial on Computer Network Communication Across Layers

Have you ever wondered how a simple click opens a web page from another continent? It’s not magic, it’s the intricate dance of network layers! This tutorial will demystify this multi-layered journey, taking you from your browser to your destination website.

Imagine This: You search for “cat videos” on your computer. Behind the scenes, a fascinating series of events unfolds:

1. Application Layer:

• Your web browser (application) sends a request to the DNS server to translate the website URL (e.g., “catvideos.com”) into its numerical IP address (e.g., 172.16.100).

2. Transport Layer:

• Once the IP address is known, your computer establishes a TCP connection with the web server. This ensures reliable data transfer, like numbered and acknowledged packets.

3. Network Layer:

• Your computer packages the data into IP datagrams, each containing the source and destination IP addresses, and sends them out.
• Routers on the network use these addresses to route the datagrams through the maze of interconnected networks, hopping from one router to another based on routing tables.

4. Data Link Layer:

• At each hop, the datagrams are wrapped in Ethernet frames containing the router’s MAC address (hardware address) for further delivery.
• Switches use these MAC addresses to efficiently direct the frames to the next destination on the same network segment.

5. Physical Layer:

• Finally, the frames are converted into electrical or optical signals and sent through cables, wifi signals, or other physical mediums.

On the Receiving End:

• The web server’s computer performs the journey in reverse, receiving and validating each layer’s information.
• It then sends you the requested cat videos (data) back through the same layered path.

Putting it All Together:

• Each layer has its own specialized function, like packing, addressing, routing, and transmitting data.
• They work together seamlessly, like cogs in a machine, to ensure efficient and reliable network communication.

Want to Go Deeper?

• Explore specific protocols like TCP, IP, and Ethernet to understand their intricate rules.
• Learn about different network devices like routers, switches, and firewalls.
• Discover the challenges and security concerns in modern networks.

By understanding the layers of network communication, you gain a deeper appreciation for the invisible infrastructure that connects us all. So, the next time you watch a cat video, remember the incredible journey it took to reach your screen!

This is just a starting point. Feel free to explore and delve deeper into the fascinating world of network communication!

Now that you know
the basics of how every layer of our
network model works, let’s go through an
exercise to look at how everything works at
every step of the way. Spoiler alert, things
are about to get a little geeky in a good way. Imagine three networks. Network A will contain
address space 10.1.1.0/24, network B will contain
address space 192.168.1.0/24, and network C will
be 172.16.1.0/24. Router A sits between
network A and network B, with an interface configured
with an IP of 10.1.1.1 on network A and an interface at
192.168.1.254 on network B. There’s a second
router, router B, which connects
networks B and C. It has an interface on network
B with an IP address of 192.168.1.1 and an interface on network C with an IP
address of 172.16.1.1. Now, let’s put a computer
on one of the networks. Imagine it’s a desktop sitting on someone’s
desk at their workplace. It’ll be our client
in this scenario, and we’ll refer to
it as Computer 1. It’s part of network A and has been assigned
an IP address of 10.1.1.100 Now, let’s put another computer on
one of our other networks. This one is a server
in a data center. It will act as our server in this scenario and we’ll
refer to it as Computer 2. Its part of network C and has been assigned
an IP address of 172.16.100 and has a web
server listening on Port 80. In end-user sitting
at Computer 1, opens up a web
browser and enters 172.16.1.100 into
the address bar. Let’s see what happens.
The web browser running on Computer 1 knows it’s been ordered to retrieve a
webpage from 172.16.1.100. The web browser communicates with the local networking stack, which is the part of
the operating system responsible for handling
networking functions. The web browser explains that it’s going to want to establish a TCP connection to
172.16.1.100 Port 80. The networking stack will
now examine its own subnet. It sees that it lives on
the network 10.1.1.0/24, which means that the destination 172.16.1.100 is on
another network. At this point, Computer 1 knows that it will have
to send any data to its gateway for routing
to a remote network and it’s been configured
with a gateway of 10.1.1.1. Next, Computer 1 looks
at its ARP table to determine what mac
address of 10.1.1.1 is, but it doesn’t find any
corresponding entry. Oh, it’s okay. Computer a crafts in ARP request for an IP
address of 10.1.1.1, which it sends to the hardware
broadcast address of all S. This ARP discovery request is sent to every node
on the local network. When router A receives
this ARP message, it sees that it’s the
computer currently assigned the IP
address of 10.1.1.1. It responds to Computer
1 to let it know about its own MAC address of
00: 11:22:33:44:55. Computer 1 receives
this response and now knows the hardware
address of its gateway. This means that it’s ready to start constructing
the outbound packet. Computer 1 knows that
it’s being asked by the web browser to form an
outbound TCP connection, which means it will need
an outbound TCP port. The operating system identifies the ephemeral port of 50,000 as being available
and opens a socket connecting the web
browser to this port. Since this is a TCP connection, the networking stack knows
that before it can actually transmit any of the data the
web browser wants it to, it’ll need to establish
a connection. The networking stack starts
to build a TCP segment. It fills in all the appropriate
fields in the header, including a source port of 50,000 and a
destination port of 80. A sequence number is chosen and is used to fill in the
sequence number field. Finally, the SYN flag is set and a checksum for the segment is calculated and written
to the checksum field. Our newly constructed
TCP segment is now passed along to the IP layer
of the networking stack. This layer constructs
an IP header. This header is filled
in with the source IP, the destination IP,
and a TTL of 64, which is a pretty standard
value for this field. Next, the TCP segment is
inserted as the data payload for the IP datagram and a checksum is calculated
for the whole thing. Now that the IP datagram
has been constructed, Computer 1 needs to get
this to its gateway, which it now knows has a MAC address of
00:11:22:33:44:55. An Ethernet frame
is constructed. All the relevant fields are filled in with the
appropriate data, most notably the source and
destination MAC addresses. Finally, the IP
datagram is inserted as the data payload of the Ethernet frame and another
checksum is calculated. Now we have an entire
Ethernet frame ready to be sent across
the physical layer. The network interface
connected to Computer 1 sends this binary data
as modulations of the voltage of an electrical
current running across a Cat 6 cable that’s connected between it and a network switch. This switch receives the frame and inspects the
destination MAC address. The switch knows which of its interfaces this MAC
address is attached to and forwards the frame across only the cable connected
to this interface. At the other end of
this link is Router A, which receives the frame and recognizes its own hardware
address as the destination. Router A knows that this
frame is intended for itself, so it now takes the entirety of the frame and calculates
a checksum against it. Router A compares
this checksum with the one in the
Ethernet frame header and sees that they match, meaning all of the data
has made it in one piece. Next, Router A strips
away the Ethernet frame, leaving it with just
the IP datagram. Again, it performs a checksum calculation against the entire datagram and again, it finds that it matches, meaning all the data is correct. It inspects the
destination IP address and performs a lookup of this destination in
its routing table. Router A sees that
in order to get data to the 172.16.1.0/24 network. The quickest path is one
hop away via Router B, which has an IP of 192.168.1.1. Router A looks at all the
data in the IP datagram, decrements the TTL by one, calculates a new
checksum reflecting the new TTL value and makes a new IP datagram
with this data. Router A knows that it needs to get this
datagram to Router B, which has an IP address
of 192.168.1.1. It looks at its ARP table
and sees that it has an entry for 192.168.1.1. Now Router A can
begin to construct an Ethernet frame with the
MAC address of its interface on Network B as the source
and the MAC address of Router B’s interface on
Network B as the destination. Once the values for all fields in this frame
have been filled out, Router A places the newly
constructed IP datagram into the data payload field, calculates a checksum and
places this checksum into place and sends the
frame out to Network B. Just like before, this frame makes it across
Network B and is received by Router B. Router B performs
all the same checks, removes the Ethernet
frame encapsulation and performs a checksum
against the IP datagram. It then examines the
destination IP address. Looking at its routing table, Router B sees that the destination
address of Computer 2, or 172.16.1.100 is on a
locally connected network, so it decrements the
TTL by one again, calculates a new checksum and
creates a new IP datagram. This new IP datagram is again encapsulated by a
new Ethernet frame. This one with the
source and destination MAC address of Router
B and Computer 2. The whole process is
repeated one last time. The frame is sent
out onto Network C, a switch ensures it
gets sent out of the interface that Computer
2 is connected to. Computer 2 receives
the frame frame, identifies its own
MAC address as the destination and knows that
it’s intended for itself. Computer 2 then strips
away the Ethernet frame, leaving it with the IP datagram. It performs a CRC and recognizes that the data
has been delivered intact. It then examines the
destination IP address and recognizes that as its own. Next, Computer 2 strips
away the IP datagram, leaving it with just
the TCP segment. Again, the checksum for this layer is examined and
everything checks out. Next, Computer 2 examines the destination
port, which is 80. The networking stack on
Computer 2 checks to ensure that there’s
an open socket on Port 80, which there is. It’s in the listen
state and held open by running
Apache web server. Computer 2 then sees that this packet has
the SYN flag set. It examines the sequence
number and stores that. Since it’ll need to put
this sequence number in the acknowledgment field
once it crafts the response. After all of that, all we’ve done is get
a single TCP segment containing a SYN flag from
one computer to a second one. Everything would have to
happen all over again for Computer 2 to send a SYN
ACK response to Computer 1. Then everything would have
to happen all over again for Computer 1 to send an
ACK back to Computer 2, and so on and so on. Looking at all of this end-to-end hopefully
helps show how all the different layers of our networking
model have to work together to get the job done. I hope it also gives you some perspective
in understanding how remarkable computer
networking truly is.

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