MP-BGP and Address Families

TCP 3-way Handshake Summary Cheatsheet

OSI Model

OSI Model (Open Systems Interconnection Model) is simply a reference tool, a standardised Reference Framework for conceptualising data communications between networks.OSI Model

OSI was the first conceptual model that characterized and standardized the communication functions of a Network system without regard to its underlying internal structure and technology.

OSI is a Vendor Neutral Model. Before OSI Model, each vendor used to use its own network model to manufacture Network equipment. So, one vendor’s equipment & network was not compatible with another vendor’s equipment. Buyers were bound to buy only from one vendor.

For example, if you were buying IBM Computers then you were now bound to buy all Network Equipment from IBM as well in the future. Because IBM Computers couldn’t be connected by other vendor Switches or Routers. You must buy IBM Switches to connect them. They will not work with HP Server or Cisco Switches.

OSI Model

To resolve this problem, the idea of a standardised Network Model emerged. International Organization for Standardization (ISO) started working on it, different network models were proposed and finally the OSI Model was chosen & released in 1984 by ISO. OSI provided brought in the flexibility along with standardization.

In simple words, OSI reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer.

*An important point worth of mentioning here is that: Although OSI is the standard today but we never find it on real equipment. The model in action today is TCP/IP. OSI is used only for standardization & reference purposes.

 

Layers in OSI Model

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 … which we will discuss in detail later. OSI Model has 7x Layers starting from Physical till Application level as in below:OSI Model

Physical Layer standardises the Physical medium. For example, signal level, voltage, radio, cabling standards & so on…. It simply, standardises everything which you can touch & feel. Physical layer is responsible for ultimate transmission of digital data bits in the form of electrical, optical or radio signals from the Physical layer of the source or sending device over network communications media to the Physical layer of the receiving of destination device.  Data Link Layer standardizes the Physical Addressing. It is responsible for Data Switching. Physical addressing includes MAC Address & Ethernet Frames. Network Layer deals with Logical Addressing. It is responsible for Data Routing. Transport Layer ensures the error free data transmission across networks. It mainly focuses on data delivery. Transport Layer mainly uses two protocols called TCP & UDP which we will discuss in detail in later sections.  Session Layer is responsible for session control. In simple words, it manages on how to start, control, and end conversations between two hosts or computers.  Presentation Layer is responsible for data organization & conversion so that it is readable by Application layer of all hosts/applications involved in the data communication.  Application Layer is responsible for end-user processes or applications. It supplies network services to end-user applications.

 

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Network Topology Types

“The layout pattern & connectivity scheme between the devices in a network is called network topology”. It is a virtual shape or structure of the network. Devices on the network are called as nodes. The most common nodes are computers and peripheral devices. Network topology is a virtual illustration of these nodes and their connections using physical cabling. There are a number of different types of network topologies which are broadly divided into two types:

  • Physical Network Topology: Actual Connectivity/Layout as per real cabling & connections
  • Logical Network Topology: Virtual view of the network that member devices see

Physical Network Topology Types

There are many types of Physical Network Topology, out of which, most common 7xtypes include Point-to-Point, Ring, Mesh, Star (Hub/Spoke), Bus, Tree and Hybrid.

Network Topology Types Atech Waqas Karim (0)_

Let’s discuss each type one by one.

1. Point-to-Point Network Topology

Point-to-point topology is the simplest of all network topologies. The network simply consists of a direct link between two computers for a faster and more reliable connection. Using circuit-switching or packet-switching technologies, a point-to-point circuit can be set up dynamically and dropped when no longer needed. Switched point-to-point topologies are the basic model of conventional telephony.

This is fastest network topology because there is a direct connection so no broadcast is required on such network types. p-p topology is the most reliable than all other types of the connections because there is a direct connection. But p-p topology can only be used for small areas where computers are in close proximity & short distance. Below figure shows a Point-to-Point Network Topology along with its Advantages & Disadvantages:

 

Point-to-Point Network Topology

2. Ring Network Topology

In Ring topology, the devices in the network are connected in a circle. Each device has two paths to send & receive data. If one link goes down, data could be sent from the other side. A ring topology is similar to a bus topology in a closed loop. In this type of topology, data travels around the ring in one direction. When one node sends data to another, the data passes through each intermediate node on the ring until it reaches its sink or destination. The intermediate nodes simply repeat the data to keep the signal strong.

Ring Network Topology is more secure because if one link is broken in the Ring, data can travel from the side. It is easy to Troubleshoot and easy to Install. However, in Ring Network Topology, Scalability and expansion in existing network is a bit difficult & requires more skill. Below figure shows a Ring Network Topology along with its Advantages & Disadvantages:

Ring Network Topology

 

3. Mesh Network Topology

In Mesh network topology, every device is interconnected to every other device in the network. Nodes are connected to each other in a redundant fashion with multiple connections. There are two types of mesh topologies, Partial Mesh and Full Mesh. Mesh Network Topology is the most stable & fault tolerant. It is cable fault & device fault tolerant. However, it is the most expensive as it requires more cabling & infrastructure. Below figure shows a Mesh Network Topology along with its Advantages & Disadvantages:

Mesh Network Topology

 

4. Star Network Topology

In a Star Topology every device is connected to a central device such as a switch. Star topology requires more cable as compared to other topologies but it mode robust as a failure in one cable will only disconnect the specific connected computer via that cable to the central device. Star Network Topology is easier to manage. It is easier to locate node and cable problems in it and it is well suited for expansion into high-speed networking. However, it contains single point of failure and requires more network cable.

Below figure shows a Star Network Topology along with its Advantages & Disadvantages:

Star Network Topology

 

5. Bus Network Topology

It is an old topology. In this type of topology, one main trunk is chosen a backbone & all devices connect to it using T connectors or taps. It is cheap as it requires less cabling. Bus Network Topology doesn’t need any special equipment and it is less complex. However, Network down time is required whenever devices are added/removed. It is more prone to problems. If once cable goes down then whole network is disturbed. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the intended recipient. If the machine address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches the machine address, the data is accepted. Below figure shows a Bus Network Topology along with its Advantages & Disadvantages:

Bus Network Topology

 

6. Tree Network Topology

It is a topology in which network devices are connected in a tree manner. A tree network is also called star-bus network. Tree network topology is basically a combination of Star & Bus. It is a hybrid network topology in which star networks are interconnected via bus networks. Tree networks are hierarchical in nature and each node can have multiple no. of child nodes.

*Note

An important point to not here is that: in a tree topology, there can be only one connection between any two connected nodes

Tree Network Topology is a loop free topology & is used mostly in Layer2 Networks. It is an easy to scale network. We can simply continue adding devices on branches with no down time required. However, there is a Single Point of Failure in case the Backbone goes down. Below figure shows a Tree Network Topology along with its Advantages & Disadvantages:

Tree Network Topology

 

7. Hybrid Network Topology

Hybrid network is simply a combination of more than one network topology types. e.g. Ring & bus combine together. With Hybrid Network Topology, we can achieve combined advantage of individual member topologies if properly designed. However, it becomes complex if the network is no properly designed. Below figure shows a Hybrid Network Topology along with its Advantages & Disadvantages:

Hybrid Network Topology

 

 

TCP 3-way Handshake Process

TCP (Transmission Control Protocol) is “a Connection Oriented Transport Layer (Layer4) protocol for reliable communication between network devices”

TCPv4 was published in RFC-793 in 1981 and is still the current version. TCP is a connection oriented full-featured transport layer protocol that provides all the functions needed by a typical application for the reliable transportation of data across an internetwork. TCP is a host-to-host protocol. TCP provides reliable communication with a mechanism called Positive Acknowledgement with Re-transmission(PAR) employing the 3-way Handshake.

 

3-way Handshake Process

TCP uses a process called 3-way Handshake for reliable communication. In 3-way Handshake process, three TCP segments are exchanged between the sender and the receiver to establish a reliable connection. These are called SYN, SYN-ACK and ACK.

TCP 3-way handshake process - ATech Waqas Karim

Step1 (SYN): In the first step, client wants to establish a connection with server, so it sends a segment with SYN (Synchronize Sequence Number). This informs server that client is likely to start communication and with what sequence number it starts segments with.

Step2 (SYN + ACK): In the second step, Server responds to the client request with SYN-ACK signal bits set. Acknowledgement(ACK) signifies the response of segment it received and SYN signifies with what sequence number it is likely to start the segments with.

Step3 (ACK): In the last step, client acknowledges the response of server by ACK message. Status on both sides change to ESTABLISHED and both get ready to start the actual data transfer.

The steps 1 and 2 establish the connection parameter (seq nos.) for one direction and it is acknowledged. The steps 2 and 3 establish the connection parameter (seq nos.) for the other direction and it is acknowledged. After the whole process, a full-duplex communication is established. ISN (Initial Sequence Numbers) are randomly selected while establishing connections between client and server. Below is the simplified view of TCP 3-way handshake process:

Any device using this 3-way Handshake process resends the data unit until it receives an acknowledgement. Transport Layer checks the data with checksum functionality for Error Detection at the receiver side. If the data unit received at the receiver’s end is damaged, then receiver discards the segment. So, the sender has to resend the data unit for which positive acknowledgement is not received.

TCP allows one side/device to establish a connection. The other side/device may either accept the connection or refuse it. The device that is establishing/requesting the connection is called client and the side waiting for a connection is called server. TCP uses virtual ports to create a virtual end-to-end connection that can reuse the physical connections between two computers. TCP is encapsulated within the data field of IP datagrams and TCP encapsulates higher level protocol data such as Network Mgmt (SNM), HTTP (web), SMTP (email) and many other protocols.

 

TCP Open Call Types

TCP identifies two types of OPEN calls which are Active OPEN and Passive OPEN calls as in below:

  1. Active OPEN: In an Active Open call a device using TCP takes the active role and initiates the connection by sending a TCP SYN message to start the connection. The Device in Active OPEN state is called Client
  2. Passive OPEN: In passive OPEN state, device is waiting for an active OPEN from other. It does not generate any TCP message segment. The Device in Passive OPEN state is called Server

 

TCP Message Types

There are four types of message that control transitions between states during TCP 3-way Handshake process. SYNchronize and ACKnowledge messages are indicated by a either the SYN bit, or the ACK bit inside the TCP header, and the SYN-ACK message has both the SYN and the ACK bits turned on (set to 1) in the TCP header.

TCP/IP Model

 

TCP States

There are 6 types of TCP states during the 3-way handshake process which are called CLOSED, LISTEN, SYN-SENT, SYN-ACK SENT, SYN RCVD and ESTABLISHED. Below table lists the detail of each state:

TCP/IP Model

After the Three-way handshake, the connection is open and the participant computers start sending data using the agreed sequence and acknowledge numbers

 

TCP Complete Data Transfer Process

TCP Data Transfer process completes in three phases as in below:

Phase1: In first Phase, Three-way handshake is completed so that sender & receiver can know each other & build the trust relationship.

Phase2: After the first phase, the connection is opened and the participant devices start sending data using the agreed sequence and acknowledge numbers that they have agreed upon in phase1

Phase3: Once all Data transfer is completed, the connection is terminated using the FIN flags from both sides.

This whole setup and teardown of a TCP socket connection is part of what qualifies TCP a reliable protocol. TCP also acknowledges that data is successfully received and guarantees the data is reassembled in the correct order. Below is the detailed overview of TCP processes showing all three Phases.

TCP 3-way handshake process - ATech Waqas Karim

 

TCP 3-way Handshake Example with Packet Tracer

Below is a Wireshark capture which shows detailed packets exchanges that happen between a Client and Server during TCP handshake, Data Transfer & Connection Termination phases:

TCP 3-way handshake process - ATech Waqas Karim

 

TCP/IP Model

“ TCP/IP Model is a standard reference Model for conceptualising data communications between networks ”

The TCP/IP is a practical Model that provides E2E data communication specifying how data should be packetized, addressed, transmitted, routed, and received. It characterizes and standardizes the communication functions of a Network system without regard to its underlying internal structure and technology. It is also called the Internet Model or the DoD Model. TCP/IP is a Vendor Neutral Model. It resolved many problems as described in below.

TCP IP Model - ATech Waqas Karim (2)Problem: Before TCP/IP Model, each vendor used to use its own network model to manufacture Network equipment. So, one vendor’s equipment & network was not compatible with another vendor’s equipment. Buyers were bound to buy only from one vendor. For example, if you were buying IBM Computers then you were now bound to buy all Network Equipment from IBM as well in the future. Because IBM Computers couldn’t be connected by other vendor Switches or Routers. You must buy IBM Switches to connect them. They will not work with HP Server or Cisco Switches.

Solution: To resolve this problem, the idea of a standardised Network Model emerged and TCP/IP Model was formalised. In simple words, TCP/IP reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer.

TCP and IP

TCP (Transmission Control Protocol) & IP (Internet Protocol) work together to form the foundations of TCP/IP Protocol suite. They form a team that work together to make it possible for applications to easily run over an internetwork.

TCP is “a Connection Oriented Transport Layer (Layer4) protocol for reliable communication b/w network devices”. TCPv4 was standardised in RFC-793 in1981. It is also called Host-to-Host protocol. It provides to applications a method of easily use the IP. It also fills in the capabilities that IP lacks. It allows TCP/IP devices to establish and manage connections and send data reliably and takes care of handling all the potential issues that can occur during transmission so that each application doesn’t need to worry about these details. It provides Reliable communication through PAR (Positive Acknowledgement with Re-transmission) using 3-way handshake process which is discussed in next section.

IP is related with classic Network Layer tasks (Logical Addressing, Routing) for basic internetworking capabilities.

 

TCP/IP Model Layers

TCP/IP Model divides the network communication processes into Four Layers in order to simplify it. Each layer performs specific functions to support the layers above it. The layers of the protocol suite near the top are logically closer to the user application, while those near the bottom are logically closer to the physical transmission of the data. TCP/IP Model has 4x Layers starting from Physical Network Interface till Application level as in below:

 

TCP IP Model - ATech Waqas Karim (3)

Physical Network Interface Layer  standardises the Physical medium & is responsible for Physical Addressing. For example, signal level, voltage, radio, cabling standards & so on,… It is responsible for ultimate transmission of digital data bits in the form of electrical, optical or radio signals from the Network Interface layer of the sending device over network communications media to the Physical Network Interface Layer of the receiving or destination device. It also standardizes the Physical Addressing. It is responsible for Data Switching. Physical addressing includes MAC Address & Ethernet Frames.  Internet Layer  deals with Logical Addressing (IP Addressing). It is responsible for Data Routing.  Transport Layer ensures the error free data transmission across networks. It is also responsible for Sessions Management in TCP/IP Model (in contrast to OSI Model, where sessions are managed by Session Layer). It mainly focuses on data delivery. Transport Layer mainly uses two protocols called TCP & UDP which we will discuss in detail in later sections.   Application Layer is responsible for end-user processes or applications. It supplies network services to end-user applications. It is also responsible for data organization & conversion so that it is readable by Application layer of all hosts/applications involved in the data communication.

 

 

TCP/IP Model vs OSI Model

OSI (Open System Interconnection) is another model of the same kind. It also has same layered structure. The TCP/IP model is a condensed version of the OSI reference model consisting of only the 4xLayers. The functions of these four layers are comparable to the functions of the seven layers of the OSI model. Below figure shows the comparison between the layers of the two models:

TCP IP Model - ATech Waqas Karim (4)

OSI is used as standard only but we never find it on real equipment. The model in action today is TCP/IP. OSI is used only for standardization & reference purposes. Below are the detailed differences between OSI Model & TCP/IP Model:

TCP IP Model - ATech Waqas Karim (5)

 

TCP 3-way Handshake Process

TCP uses a process called 3-way Handshake for reliable communication. In 3-way Handshake process, three TCP segments are exchanged between the sender and the receiver to establish a reliable connection. These are called SYN, SYN-ACK and ACK.

Step1 (SYN): In the first step, client wants to establish a connection with server, so it sends a segment with SYN (Synchronize Sequence Number). This informs server that client is likely to start communication and with what sequence number it starts segments with.

Step2 (SYN + ACK): In the second step, Server responds to the client request with SYN-ACK signal bits set. Acknowledgement(ACK) signifies the response of segment it received and SYN signifies with what sequence number it is likely to start the segments with.

Step3 (ACK): In the last step, client acknowledges the response of server by ACK message. Status on both sides change to ESTABLISHED and both get ready to start the actual data transfer.

The steps 1 and 2 establish the connection parameter (seq nos.) for one direction and it is acknowledged. The steps 2 and 3 establish the connection parameter (seq nos.) for the other direction and it is acknowledged. After the whole process, a full-duplex communication is established. ISN’s (Initial Sequence Numbers) are randomly selected while establishing connections between client and server. Below is the simplified view of TCP 3-way handshake process:

TCP IP Model - ATech Waqas Karim (6)

TCP allows one side/device to establish a connection. The other side/device may either accept the connection or refuse it. The device that is establishing/requesting the connection is called client and the side waiting for a connection is called server.

 

TCP/IP Protocols

TCP/IP Model provides a practical 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 TCP/IP Model has some protocols associated with it. Some important protocols on each layer are as in below:

TCP/IP Model - ATech Waqas Karim (7)

TCP/IP Devices

Similar to protocols, each layer has associated equipment with it. For example, Hubs, Repeaters, Media Converters, L2 Switches & Bridges operate at Physical Network Interface Layer, Routers & Layer3 Switches operate on Internet Layer, Firewalls work on Transport Layer4. End Devices like PC’s/Server/Phones, Firewalls, IDS’s operate on Application Layer of TCP/IP Model as in below:

TCP/IP Model - ATech Waqas Karim (8)

 

07. Application Layer of OSI Model (Layer7)

The last layer of OSI Model, the Application Layer is responsible for End user Network Applications. It is the 7th & highest Layer in the 7Layer OSI Model. This is the layer where real traffic data is generated in most of the cases. This could be a web request generated from HTTP protocol, a command from telnet protocol, a file download request from FTP protocol etc.

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 7x Layer model starts from Physical till Application Layer as in below fig:

OSI Model Layers Waqas Karim

 

Functions/Duties of Application Layer

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

  • Application Layer manages applications software running on a computer or terminal. This is the most important function of Applications Layer.
  • Application Layer supplies Network Services to End User It works like an interface between Network Services & Applications.
  • Application Layer is also responsible for End User Processes Management.
  • Application Layer also provides miscellaneous other services including Naming Services (e.g. DNS), Mail Services (e.g. SMTP/POP3), Remote Login Services (e.g. Telnet/SSH), Remote Host Mgmt (e.g. SNMP) ,…

 

Application 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 Physical layer are listed in below:

OSI Model Application Layer Waqas Karim Pic1

 

Network Equipment/Components at Application Layer

Similar to protocols, each layer has associated equipment with it. Some important Equipment that operate at Physical Layer of OSI Model are listed in below:

OSI Model Application Layer Waqas Karim

 

Summary

  • 7th Layer in OSI 7 Layer Model
  • It performs important functions like Applications Management, End User Processes Mgmt & Miscellaneous other services like DNS, DHCP, SMTP, Telnet, …
  • Important Protocols at Application Layer include HTTP, DNS, DHCP, FTP, Telnet, POP3/IMAP, BOOTP, Bitcoin …
  • Equipment operating at Application Layer include End Devices, Firewalls, Gateways, Load Balancers.

 

06. Presentation Layer of OSI Model (Layer6)

As the name points, Presentation Layer is responsible for representation & formatting of data for session Layer in Encapsulation Process. It is the 6th Layer in the 7Layer OSI Model after Session Layer. It serves like a translator & takes care that the data is sent in such a way that the receiver will understand the information/data and will be able to use the data.

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 7x Layer model starts from Physical till Application Layer as in below figure:

OSI Model Layers Waqas Karim

 

Functions/Duties of Presentation Layer

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

  • The first & most important is, of course Data Formatting & Representation. When the presentation layer receives data from the application layer, to be sent over the network, it makes sure that the data is in the proper format. If it is not, the presentation layer converts the data to the proper format. On the other side of communication, when the presentation layer receives network data from the Presentation layer, it makes sure that the data is in the proper format and once again converts it if it is not.
  • It is also responsible for Data Encryption/Decryption: Presentation Layer carries out encryption at the transmitter end and decryption at the receiver end to keep data secure during transmission.
  • Data Compression/De-compression also falls under the responsibility matrix of Presentation Layer. Presentation Layer compresses data to a small size to reduce resource usage such as data storage space or transmission capacity.

*Encryption is typically done at this layer as well, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages.

 

Presentation 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 Presentation layer are listed in below:

OSI Model Presentation Layer Waqas Karim Pic1

 

Network Equipment/Components at Presentation Layer

Similar to protocols, each layer has associated equipment with it. Some important Equipment that operate at Presentation Layer of OSI Model are listed in below:

OSI Model Presentation Layer Waqas Karim Pic2

 

Summary

  • It is the 6th Layer in OSI 7 Layer Model
  • It performs important functions like Data Formatting, Data Representation, Data Encryption/Decryption, Data Compression/De-compression …
  • Important Protocols at Presentation Layer include ASCII, EBCDIC, JPEG, MPEG, GIF, PNG, TIFF, SSL, TLS, …
  • Equipment operating at Presentation Layer include Firewalls, Gateways, Load Balancers, Computers,….

 

 

03. Network Layer of OSI Model (Layer3)

Network Layer is the 3rd layer in the 7Layer OSI Model after Data Link Layer. This is the place where most Routing and network stuff resides. It controls the Logical boundaries of Network. Most of the students confuse Layer2 & Layer3. There is a basic difference b/w Network Layer & Data Link Layer. Network Layer is concerned with data transfer from one computer to another computer even if it is on a remote network, while the data link layer only deals with devices that are local to each other & are within one network. PDU at Data Link Layer is called Packet. 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 7x Layer model starts from Physical till Application Layer & Network Layer the 3rd in this architecture as in below figure:

 

OSI Model Layers Waqas Karim

 

Functions/Duties of Network Layer

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

  • It is responsible for Logical Addressing. It assigns a unique ID to each device on L3 Network which we call IP Address
  • Network Layer is also responsible for It processes the incoming packets, filters them & sends them to their desired destination
  • Encapsulation also falls under the responsibility matrix of Network Layer. It encapsulates Transport Layer segments into Layer3 Packets & forwards to Data Link Layer for further processing
  • It is also responsible for Fragmentation of packets to match the MTU size of L2 Frames
  • Error Control also falls under the responsibilities & functions of Network Layer. Is uses error control mechanisms to ensure reliable delivery of data. Due to these acknowledgement mechanisms, the receiver can detect how many Layer3 Packets have been corrupted during transmission. Receiver then requests the sender to send those Layer3 Packets again to ensure that no data is lost during transmission

 

Network 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. The most common protocol at Network Layer is of course the Internet Protocol which we know as IP.

Other important Protocols at Network Layer include:

OSI Model Network Layer Waqas Karim Pic2

 

Network Equipment/Components at Network Layer

Similar to protocols, each layer has associated equipment with it. Some important Equipment that operate at Physical Layer of OSI Model are listed in below:

OSI Model Network Layer Waqas Karim Pic1

 

Summary

  • It is the 3rd Layer in OSI 7 Layer Model
  • It performs important functions like Logical Addressing, Routing, Encapsulation, Fragmentation, Error Control, …
  • Important Protocols at Network Layer include IPv4, IPv6, RIP, OSPF, BGP, ICMP, EIGRP, IPSec, IPX, NAT, GRE, HSRP, VRRP, …
  • Equipment operating at Network Layer include Routers, L3 Switches, …
  • PDU is called Packet

 

04. Transport Layer of OSI Model (Layer4)

Transport Layer is the fourth layer in 7Layer OSI Model after Network Layer. Similar to L2 & L3, this layer also performs addressing & multiplexing … but at Transport Layer. 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 7x Layer model starts from Physical till Application Layer. The core concept behind Transport is “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.

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.

OSI Model Layers Waqas Karim

 

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: E2E Connection Mgmt, Transmission, Segmentation, Flow Control
  • Transport Layer is responsible for Layer4 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:

OSI Model Transport Layer Waqas Karim Pic1

These are some Famous & Well Known Port Numbers:

OSI Model Transport Layer Waqas Karim Pic2

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:

OSI Model Transport Layer Waqas Karim Pic4

Network Equipment/Components at Transport Layer

Similar to protocols, each layer has associated equipment with it. Some important Equipment that operate at Transport Layer of OSI Model are listed in below:

OSI Model Transport Layer Waqas Karim Pic5

 

Summary

  • It is the 4th Layer in OSI 7 Layer Model
  • It performs important functions like L4 Addressing, Multiplexing/De-multiplexing, Sequencing, Error Control, E2E Connection Mgmt, Transmission,  Segmentation, Flow Control …
  • Important Protocols at Transport Layer include TCP, UDP, SPX, BGP, ESP, Fiber Channel Protocol, iSCSI, SCTP, …
  • Equipment operating at Transport Layer include Firewalls, Gateways, Load Balancers.
  • PDU is called Segment