layer 4 port assignment purpose

  Layer 4 Transport Layer

TCP; UDP; Transporation

The tasks of the transport layer (also end-to-end control, transport control) include the segmentation of the data stream and in relieving congestion.

A data segment is a Service Data Unit, which is used for encapsulation on the fourth layer (transport layer). It consists of protocol elements that contain Layer 4 information control. When addressing the data segment assigned a Layer 4 address, so a port. The data segment is encapsulated in the layer 3 in a data packet.

The transport layer provides the application-oriented layers 5 to 7 standardized access so that they do not need to consider the characteristics of the communications network.

Five different service classes of different grades are defined in layer 4 and may be used by the upper layers, from the simplest to the most comfortable service with multiplex mechanisms, error protection and troubleshooting procedures..

OSI Layer 4 - Transport Layer

In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet Protocol Suite and the Open Systems Interconnection (OSI). The protocols of the layer provide host-to-host communication services for applications.[1] It provides services such as connection-oriented data stream support, reliability, flow control, and multiplexing. The details of implementation and semantics of the Transport Layer of the TCP/IP model (RFC 1122), which is the foundation of the Internet, and the Open Systems Interconnection (OSI) model of general networking, are different. In the OSI model the transport layer is most often referred to as Layer 4 or L4, while numbered layers are not used in TCP/IP. The best-known transport protocol of TCP/IP is the Transmission Control Protocol (TCP), and lent its name to the title of the entire suite. It is used for connection-oriented transmissions, whereas the connectionless User Datagram Protocol (UDP) is used for simpler messaging transmissions. TCP is the more complex protocol, due to its stateful design incorporating reliable transmission and data stream services. Other prominent protocols in this group are the Datagram Congestion Control Protocol (DCCP) and the Stream Control Transmission Protocol (SCTP). Wikipedia

• Connection-oriented communication
• Same order delivery
• Reliability
• Flow control
• Congestion avoidance
• Port Multiplexing

Popular Transport Layer Protocols

Layer 7   application layer, layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

The OSI Model – The 7 Layers of Networking Explained in Plain English

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

• Basic familiarity with common networking terms (explained below)
• A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

• What the OSI model is
• The purpose of each of the 7 layers
• The problems that can happen at each of the 7 layers
• The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

• Please | Physical Layer
• Do | Data Link Layer
• Not | Network Layer
• Tell (the) | Transport Layer
• Secret | Session Layer
• Password (to) | Presentation Layer
• Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

• Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
• Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
• Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
• Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
• Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
• Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
• Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

• Defunct cables, for example damaged wires or broken connectors
• Broken hardware network devices, for example damaged circuits
• Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Flow control. How much data should be transmitted?
• Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

• Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
• Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

• Header: typically includes MAC addresses for the source and destination nodes.
• Body: consists of the bits being transmitted.
• Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

• All the problems that can occur on Layer 1
• Unsuccessful connections (sessions) between two nodes
• Sessions that are successfully established but intermittently fail
• Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

• All the problems that can crop up on previous layers :)
• Faulty or non-functional router or other node
• IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

• Blocked ports - check your Access Control Lists (ACL) & firewalls
• Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

• Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
• Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

• Servers are unavailable
• Servers are incorrectly configured, for example Apache or PHP configs
• Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

• American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
• Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
• Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

• Non-existent or corrupted drivers
• Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

• All issues on previous layers
• Incorrectly configured software applications
• User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

• What is the OSI model?
• What are each of the layers?
• How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
• Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
• The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
• OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

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Transport Layer of OSI Model (Layer-4)

Transport Layer is the fourth layer in 7 Layer OSI Model after Network Layer. Similar to Layer-2 and Layer-3, this layer also performs addressing & multiplexing but in different domain through TCP and UDP.

OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it. The seven Layer model starts from Physical till Application Layer & Transport layer is in the middle. The core concept behind Transport layer is the “support of Multitasking”. It allows same computer, browser & internet connection to work on multiple applications simultaneously & this is achieved through Port Numbers, Transport Layer Addressing & Multiplexing. Below figure shows the position of Data Link layer in the OSI Model :

Transport layer is in the middle of the OSI Model as in below figure. It is a part of both the lower and upper of layer groups.

Lower layers, because it is involves the transport of data,

Upper Layers, because its functions are also somewhat high-level.

PDU at Transport Layer is called Segment .

Functions/Duties of Transport Layer

Each Layer in OSI Model Performs some important duties. Important functions performed by Transport Layer are listed here:

• Sequencing: Sequencing is a connection-oriented service that takes TCP segments that are received out of order and place them in the right order
• Error Control: Is uses error control mechanisms to ensure reliable delivery of data. Due to this acknowledgement mechanisms, the receiver can detect how many bits have been corrupted during transmission. Receiver then requests the sender to send those bits again to ensure that no data is lost during transmission.
• Other Functions of Transport Layer include: End to End Connection Management, Transmission, Segmentation and Flow Control
• Transport Layer is responsible for Layer-4 Addressing which is also called Process Level Addressing. It allows a computer to use multiple network layer protocols simultaneously.

Transport Layer also performs Multiplexing and De-multiplexing of data to allow multiple programs to run on same computer using different Port numbers. In modern multi-tasking environments, many network applications need to run on a computer simultaneously. So, there should be some mechanism to identify which application should receive the incoming data.

To make this work correctly, incoming data from different applications is multiplexed at the Transport layer and sent to the Media layers. On the other side of the communication, the data received from the Media layers are de-multiplexed at the Transport layer and delivered to the correct application. This is achieved by using Layer4 “Port Numbers”.

The range of Transport Layer Port numbers is from 0-65,535. 65000 , because Port number is a 16-bit number & maximum range through which it can span is 65,535.

The port numbers are divided into three ranges:

These are some Famous & Well Known Port Numbers:

Transport Layer Protocols

The OSI Model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. Each layer on the OSI Model has some protocols associated with it. Some important protocols on Transport layer are listed in below:

• TCP (Transmission Control Protocol)
• UDP (User Datagram Protocol)
• Fiber Channel Protocol
• HSRP, VRRP, …

Network Equipment/Components at Transport Layer

• Load Balancers

It is the 4th Layer in OSI seven layered Model . It performs important functions like Layer-4 Addressing, Multiplexing, Demultiplexing, Sequencing, Error Control, End to End Connection Management, Transmission, Segmentation & Flow Control. Important Protocols at Transport Layer include TCP, UDP, SPX, BGP, ESP, Fiber Channel Protocol, iSCSI and SCTP. Equipment operating at Transport Layer include Firewalls, Gateways and Load Balancers. PDU is called Segment.

Now, test your knowledge on OSI Model using our free Quizzes & Cheat sheet resources for long term memory:

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The Transport Layer: Understanding layer 4 of the OSI Model

I like to think of the transport layer as the layer of the OSI Model that enables more interesting traffic. While we network engineers may love a lot of the simpler uses of the IP protocol and networks in general, we’d all be jobless without the transport layer.

Layer 4 provides for the transparent transfer of data for users, systems, and applications and reliable data transfer services to the upper levels. Since the vast majority of our network traffic is IP-based nowadays, it’s probably easiest to think about layer 4 as it relates to IP traffic specifically.

The transport layer controls the reliability of communications through flow control, segmentation, and error control. Two great examples of transport protocols are TCP (as in TCP/IP ) and UDP . Understanding the differences between TCP and UDP really helps when troubleshooting and when trying to understand the results from a packet capture. TCP, or the Transmission Control Protocol, is connection oriented. This means that when a TCP conversation occurs a session is established and that session is used to control and ensure the flow of data between. Once the conversation is finished the session is terminated. UDP, or the User Datagram Protocol, is not connection oriented. It’s a simpler and in some ways more elegant protocol and data is transferred in a “best effort” type of style vs. the guaranteed delivery with TCP.

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Darragh Delaney: Troubleshooting application problems by looking at network traffic

Oftentimes, layers 4-7 can be grouped together and thought of as the application layers. Because we work so much with TCP/IP nowadays, even though TCP/IP is a layer 4 stack I sometimes find myself thinking of it as the application layer. Maybe it’s because I tend to associate applications with TCP ports. As a matter of fact, when I started writing this post I titled it “Layer 4 of the OSI Model – understanding the application layer,” and then I had to go back and correct myself. Even though this is a common mistake and many folks tend to group these levels, understanding the differences between layers 4-7 of the OSI Model will help to enhance your troubleshooting and design skills.

Layer 4 is also sort of the “hot” layer right now. Years ago, layer 3 was talked about a lot as layer 3 switches were new on the market and in high demand. Today, layer 4 switches are available and application accelerators, WAN accelerators, load balancers, and firewalls all operate at the layer 4 level.  In the case of a WAN accelerator, most operate by first identifying the application via TCP port and then by breaking down, optimizing, and the rebuilding the TCP session as it passes through and between the WAN optimizers.

Last but not least, network management technologies that leverage flow-based network traffic analysis like NetFlow and IPFix leverage the transport layer information including TCP port numbers and session start/stop/duration to identify and measure application traffic.

In a nutshell, while in the old days many network administrators would consider anything above layer 3 to beyond their demarc of responsibility – sagacious network engineers today treat layer 4 and above as well.

Flame on…

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Josh Stephens is Head Geek and VP of Technology at  SolarWinds , an IT management software company based in Austin, Texas. He shares network management best practices on SolarWinds’  GeekSpeak  and  thwack . Follow Josh on Twitter @sw_headgeek  and SolarWinds  @solarwinds_inc .  

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Unlike layer 3, there are really only two protocols of note found in layer 4: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Returning to our postal mail protocol analogy, layer 3 is preoccupied with ensuring that the address on the envelope could be located and that the envelope could ultimately be delivered. Layer 4 shifts the focus to the process of the actual delivery of the envelope.

• Transmission Control Protocol (TCP) TCP is the "registered mail" protocol of internets.
• User Datagram Protocol (UDP) When postal mail is not registered, there is a small chance that the letter we send might never be seen again. The letter is addressed and sent and no more effort is spent on it. It is a matter of the mail carrier doing its job correctly. It is the same case with UDP.
• Sockets The term socket in a TCP or UDP context fully describes the endpoint of a connection. The socket is consequently a combination of an IP address, a port number, and the protocol being used.

Home | TCP/IP Essentials | Transport Layer | TCP and UDP

Transport Layer (Layer 4) - Reserved TCP and UDP Port Numbers

* Real Audio's rtsp (real-time streaming protocol) via port 554 is a good example.

Layer 4 server ports module

The Layer 4 server port statistics include traffic counters for each TCP and UDP server port on the network.

Web interface

The layer 4 server ports are seperated in a TCP and UDP tab. For each protocol a list with all seen server ports are shown. The statistics include the total amount of bytes and packets transfered to and from the specific server port as well as the current throughput. A history graph is available to show sent and received bytes and packets over time. As usual, the resolution can be changed using the top buttons to any interval. It is also possible to capture traffic for a specific TCP or UDP server port by clicking on the corresponding PCAP button in the last column. For TCP there is also a separate graph for TCP traffic that has not been accounted for any server port. This includes traffic to ports where there is no server running or the device did not see the connection establishment to determine the server.

Statistics for port

By clicking on a port the detail page will be displayed. The detail page shows an overview about traffic counters for that specific port as well as graphs for a view over time. On the bottom of the detail page there are tabs to choose between the list of IP addresses, layer 3 QoS and layer 2 QoS classes seen for that specific port.

In this tab there is a table listing all IP addresses that have been seen with the specific port as a server. The table contains the following information:

• The IPv4 or IPv6 address (linked to the respective Per IP statistics view of the IP module )
• Alternative names for the IP address like DNS name or DHCP names
• Number of total packets for that IP and port
• Number of total bytes for that IP and port (counting full ethernet frames)
• A button to trigger a packet capture including only traffic for that IP and port (see Capture module )
• A graph showing throughput history for that IP and port

When multiple pages of IPs are available, there will be a control field for switching table pages. The IP search bars allow for entering IP addresses or names to see only those element for which the entered string is part of the IP or dns/dhcp name. The table is sortable by most of the columns. IPs are linked to the respective Per IP statistics view of the IP module .

Layer 3 QoS

For layer 3 IP differentiated services codepoint (DSCP) are displayed in a table with traffic counters, a history graph of traffic over time and a PCAP button for that certain DSCP value. More information about QoS can be found in QoS module .

Layer 2 QoS

For layer 2 VLAN priority code points and MPLS traffic classes are analysed and displayed in a table with traffic counters, a history graph of traffic over time and a PCAP button that certain QoS tag. More information about QoS can be found in QoS module .

Navigation menu

Layer 4 port assignment

A rational number is any number that can be made by dividing oneinteger by another.0.5 is a rational number as it can be made by dividing the number 1by the number 22 is a rational number because it can be made by dividing 2 by 1-6.6 is a rational number because it can be made by dividing -66 by10---------------------------------------------------------Note there are number that are called Irrational Numbers .Irrational numbers are all "real" numbers (numbers with a decimalpoint) that cannot be written as a simple fraction - the decimalgoes on forever without repeating.For instance the number Pi is an irrational number.A rational number is a real number that can be expressed as a ratio of two integers. Another way to think about it is this: if you can write a number as a fraction then it's a rational number.

When data is sent across a network it is encapsulated at different layers of the OSI model. Mainly layer 2, 3, and 4. Layers 2 and 3 are intra and inter network data respectively. At layer 2 data is a frame and has a header that tells which type of media the frame will be transmitted over and a trailer that tells the receiving device if data has been corrupted. At layer 3 you have your IP addresses and network routing information, data here is called a packet. There is only a header at layer 3. Layer 4 is transport which encapsulates data as a Segment. This is where port assignments are important. Layer 4 ports are not to be confused with Layer 1 interfaces which are often also called ports, they are virtual ports that are used by the NIC to determine which protocol will handle the incoming data. From port 1 - 1023 are your well known ports usually used by servers, from 1024 to 49151 are registered ports and are opened by your machine as needed for the applicable services. They aren't as static as the Layer 3 ports but it makes it more difficult to know exactly which parts of your computer are open by assigning them custom assignments. The last set of ports, 49151 and up are your dynamic ports, which I don't know all that much about. The important thing to know about Layer 4 is the difference between TCP and UDP. Whether or not there is a connection between the two end devices and what kind of recovery the machines will perform in transmitting data determines whether or not the port is a UDP or TCP port. Data in forms of web pages and e-mails need to be delivered in full and without error in order for users to have the full experience and so that important information isn't lost in translation. These types of transmissions are handled with TCP or Transmission Control Protocol. In this case a connection is established and each frame is counted and reassembled in the correct order. In cases where data is corrupted at layer 2 and dropped the receiving device will tell the other device that it is missing important information and the sending device will retransmit. Because of the time that this takes segments in the form of videos and phone calls which experience a constant stream of data and are continuously open to segment loss TCP is not desirable. This is where UDP comes into play. Unreliable Delivery Protocol ports simply take data in as it goes and send it to the appropriate programming in what is called "Best effort" transmission. This means that missing sequence numbers are not retransmitted and there is no logical connection between the server sending the information and host receiving. The reason why assigning ports is important is because each port coordinates with its own protocol, or set of rules for handling the segments. You don't want e-mails which are handled and reassembled by TCP being pushed through a UDP port to a protocol that simply handles video streaming, nothing would happen. So assigning port numbers is important for a number of reasons, the most important being telling your computer how to handle the data that it is receiving. On your personal machine and company servers you may assign port numbers that are not well known to other devices, you may not want HTTP to run on just port 80, and you may not want mail going through the common 110 or 25 because you don't want intruders to know which ports they will have direct access to by default. Ultimately your computer will know which ports to assign, and in client mode (asking for data) it will automatically open a random port for the server to send data to. The server will usually have well known ports running because it's easier for client devices to figure out where to go to ask for the data that you need.

to identify the processes or services that are communicating within the end devices

deine mutter

Add your answer:

What layer 4 protocol is use for a telnet connection?

TCP port 23

What TCPIP model layer does DHCP work at?

DHCP is a layer-4 protocol, most commonly transported over UDP. UDP port number 67 is the destination port of a server, and UDP port number 68 is used by the client

What two options represents layer four addressing in the osi model?

Layer 4 usually uses segments, layer 3 are packets, layer 2 are frames. Layer 4 uses either tcp (SYN and ACKS) and UDP (connectionless) with port numbers

What layer 4 protocol does RIP use?

RIP uses UDP protocol with port number 520

The layer 4 header contains which type of information to aid in the delivery of data?

service port number

Which protocol uses TCP port 443 at layer 4?

HTTPS ( Hypertext Transfer Protocol over SSL/TLS)

Name the highest layer on which each one what entities operates 1. Four Port Token Ring Bridge 2. Modem 3. FTP Server 4. FTP Client 5. Netmeeting Software 6. Network Interface Card?

Four Port Token Ring layer 2

What are the different position assign in a housekeeping department?

1- Routine assignment. 2- days offs cover assignment. 3- leaves cover assignment. 4- Project work assignment.

Which OSI layer uses a connection-oriented protocol to ensure reliable delivery of data?

transport layer

What are the release dates for Benelli on Assignment - 2009 Texas Whitetail 2 4-4?

Benelli on Assignment - 2009 Texas Whitetail 2 4-4 was released on: USA: 1 July 2011

The 4 phases of development are predeployment fort to port port to port and?

What is whitney port's full name.

Whitney Port was born on March 4, 1985

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CCNA 1 Ch 2

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