OSI Network Model
The basic roles of the Transport Layer are to establish end-to-end connections from one computer to another on the network and provide reliable "transport" of data between devices.
Basic Transport Layer Services:
Resource Utilization (multiplexing)
Connection Management (establishing)
Flow Control (Buffering / Windowing)
Reliable Transport (positive acknowledgment / error checking)
Flow Control Once the connection has occurred and transfer is in progress, congestion of the data flow can occur at a destination for a variety of reasons. Possible options include:
The destination can become overwhelmed if multiple devices are trying to send it data at the same time.
It may become overwhelmed if the source is sending faster than it can physically receive.
Congestion Prevention
The Transport Layer is responsible for providing flow control to alleviate the issue of congestion and provide reliability in the data transfer. Two main methods for flow control include
•Buffering
•Windowing
Buffering
Buffering is a form of data flow control regulated by the Transport Layer. It is responsible for ensuring that sufficient buffers are available in the destination for the processing of data and that is data transmitted at a rate that does not exceed what the buffer can handle.
Windowing
Windowing is a flow control scheme in which the source computer will monitor and make adjustments to the amount of information sent based on successful, reliable receipt of data segments by the destination computer. The size of the data transmission, called the "window size", is negotiated at the time of connection establishment. It is determined by the amount of memory or buffer that is available.
Given a window size of 3, the source (in this case a router) sends 3 data segments to the destination. The destination sends an acknowledgement asking for the next set of data segments.
If the destination does not receive all three of the negotiated data segments, for example, due to a buffer overflow, it sends no acknowledgment. Since the source does not receive an acknowledgment, it knows the data segments should be retransmitted
Network Layer
The Network Layer is the 3rd layer in the OSI model and is responsible for identifying computers on a network. This layer works closely with layer 2 to translate data packets from a logical address (similar to an IP address) into hardware based MAC addresses.
This layer is concerned with 2 functions:
•Routing
•Fragmentation / Reassembly
Two types of packets are used at the Network layer:
There are 7 layers in the OSI model. Each layer is responsible for a particular aspect of data communication. For example, one layer may be responsible for establishing connections between devices, while another layer may be responsible for error checking during transfer.
The layers of the OSI model are divided into two groups: the upper layer and lower layer. The upper layers focus on user applications and how files are represented on the computers prior to transport. For the most part, network engineers are more concerned with the lower layers. It's the lower layers that concentrate on how the communication across a network actually occurs.
ALL People Seem to Need Data Processing (Layer 7 to 1)
Please Do Not Take Sausage Pizzas Away (Layer 1 to 7)
The Application Layer
The Application Layer is the highest layer in the protocol stack and the layer responsible for introducing data into the OSI stack. In it resides the protocols for user applications that incorporate the components of network applications.
Classification of Applications
Computer applications
Network applications
Internetwork applications
Examples: Telnet, FTP, HTTP, WWW Browsers, NFS, SMTP, POP, TFTP .
Presentation LayerThe Presentation Layer manipulates the representation of data for transfer to applications on different devices.
The Presentation Layer is responsible for the following services:
•Data representation
•Data security
•Data compression
Data Representation
Session Layer
The Session Layer establishes, manages, and terminates sessions (different from connections) between applications as they interact on different hosts on a network.
Its main job is to coordinate the service requests and responses between different hosts for applications.
Examples: NFS, SQL, RPC, ASP
Three different communication modes exists for data transfer within a session connection:
•Single-duplex
•Half-duplex
•Full-duplex.
Transport Layer
The layers of the OSI model are divided into two groups: the upper layer and lower layer. The upper layers focus on user applications and how files are represented on the computers prior to transport. For the most part, network engineers are more concerned with the lower layers. It's the lower layers that concentrate on how the communication across a network actually occurs.
ALL People Seem to Need Data Processing (Layer 7 to 1)
Please Do Not Take Sausage Pizzas Away (Layer 1 to 7)
The Application Layer
The Application Layer is the highest layer in the protocol stack and the layer responsible for introducing data into the OSI stack. In it resides the protocols for user applications that incorporate the components of network applications.
Classification of Applications
Computer applications
Network applications
Internetwork applications
Examples: Telnet, FTP, HTTP, WWW Browsers, NFS, SMTP, POP, TFTP .
Presentation LayerThe Presentation Layer manipulates the representation of data for transfer to applications on different devices.
The Presentation Layer is responsible for the following services:
•Data representation
•Data security
•Data compression
Data Representation
Session Layer
The Session Layer establishes, manages, and terminates sessions (different from connections) between applications as they interact on different hosts on a network.
Its main job is to coordinate the service requests and responses between different hosts for applications.
Examples: NFS, SQL, RPC, ASP
Three different communication modes exists for data transfer within a session connection:
•Single-duplex
•Half-duplex
•Full-duplex.
Transport Layer
The basic roles of the Transport Layer are to establish end-to-end connections from one computer to another on the network and provide reliable "transport" of data between devices.
Basic Transport Layer Services:
Resource Utilization (multiplexing)
Connection Management (establishing)
Flow Control (Buffering / Windowing)
Reliable Transport (positive acknowledgment / error checking)
Flow Control Once the connection has occurred and transfer is in progress, congestion of the data flow can occur at a destination for a variety of reasons. Possible options include:
The destination can become overwhelmed if multiple devices are trying to send it data at the same time.
It may become overwhelmed if the source is sending faster than it can physically receive.
Congestion Prevention
The Transport Layer is responsible for providing flow control to alleviate the issue of congestion and provide reliability in the data transfer. Two main methods for flow control include
•Buffering
•Windowing
Buffering
Buffering is a form of data flow control regulated by the Transport Layer. It is responsible for ensuring that sufficient buffers are available in the destination for the processing of data and that is data transmitted at a rate that does not exceed what the buffer can handle.
Windowing
Windowing is a flow control scheme in which the source computer will monitor and make adjustments to the amount of information sent based on successful, reliable receipt of data segments by the destination computer. The size of the data transmission, called the "window size", is negotiated at the time of connection establishment. It is determined by the amount of memory or buffer that is available.
Given a window size of 3, the source (in this case a router) sends 3 data segments to the destination. The destination sends an acknowledgement asking for the next set of data segments.
If the destination does not receive all three of the negotiated data segments, for example, due to a buffer overflow, it sends no acknowledgment. Since the source does not receive an acknowledgment, it knows the data segments should be retransmitted
Network Layer
The Network Layer is the 3rd layer in the OSI model and is responsible for identifying computers on a network. This layer works closely with layer 2 to translate data packets from a logical address (similar to an IP address) into hardware based MAC addresses.
This layer is concerned with 2 functions:
•Routing
•Fragmentation / Reassembly
Two types of packets are used at the Network layer:
Data packets: Used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols. Eg. IP and IPX.
Route update packets: Used to update neighboring routers about the network connected to all routers within the internetwork. Protocols that send route updates are called routing protocols. Eg. RIP, EIGRP, OSPF
Data Link / Physical Layer
LAN and WAN protocols occupy the bottom two layers of the OSI model. These two layers, Physical Layer and Data Link Layer, work very closely together to ensure data transfer across the physical network. Examples: HDLC, Frame Relay, PPP, ATM, FDDI, IEEE 802.3/802.2
To accomplish accurate delivery, the Data Link Layer provides the following services:
1. Machine address determination of both sending and receiving machines
2. Formatting of Network Layer "packets" into frames with machine addresses attached
3. Sequencing and resequencing of frames transmitted out of sequence
Data Link SublayersLogical Link Control (LLC) responsible for identifying Network layer protocols and encapsulating them.
Media Access Control (MAC) defines how packets are placed on media
Physical Layer The Physical Layer is the lowest layer in the OSI model and is concerned with how the physical structure of the network enables transmission of data. It is responsible for defining the mechanical and electrical specifications for the transmission medium within a connection, as well as the transformation or encoding of data into “bits”.
Examples:EIA/TIA-232, V.35, EIA/TIA-449, RJ-45, Ethernet, 802.3
Protocols defined at the Physical Layer standardize physical connections. Specifications include voltage levels, maximum transmission distances, data rates, and physical connectors.
Each layer depends on the service function of the ISO/OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then it can add whatever headers and trailers the layer will use to perform its function.
As networks perform services for users, the flow and packaging of the information changes. In this example of internetworking, five conversion steps occur:
What do the 7 layers really do?
TCP/IP
The Transmission Control Protocol/Internet Protocol (TCP/IP) suite of protocols was developed as part of the research done by the Defense Advanced Research Projects Agency (DARPA).
Route update packets: Used to update neighboring routers about the network connected to all routers within the internetwork. Protocols that send route updates are called routing protocols. Eg. RIP, EIGRP, OSPF
Data Link / Physical Layer
LAN and WAN protocols occupy the bottom two layers of the OSI model. These two layers, Physical Layer and Data Link Layer, work very closely together to ensure data transfer across the physical network. Examples: HDLC, Frame Relay, PPP, ATM, FDDI, IEEE 802.3/802.2
To accomplish accurate delivery, the Data Link Layer provides the following services:
1. Machine address determination of both sending and receiving machines
2. Formatting of Network Layer "packets" into frames with machine addresses attached
3. Sequencing and resequencing of frames transmitted out of sequence
Data Link SublayersLogical Link Control (LLC) responsible for identifying Network layer protocols and encapsulating them.
Media Access Control (MAC) defines how packets are placed on media
Physical Layer The Physical Layer is the lowest layer in the OSI model and is concerned with how the physical structure of the network enables transmission of data. It is responsible for defining the mechanical and electrical specifications for the transmission medium within a connection, as well as the transformation or encoding of data into “bits”.
Examples:EIA/TIA-232, V.35, EIA/TIA-449, RJ-45, Ethernet, 802.3
Protocols defined at the Physical Layer standardize physical connections. Specifications include voltage levels, maximum transmission distances, data rates, and physical connectors.
Each layer depends on the service function of the ISO/OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then it can add whatever headers and trailers the layer will use to perform its function.
As networks perform services for users, the flow and packaging of the information changes. In this example of internetworking, five conversion steps occur:
What do the 7 layers really do?
TCP/IP
The Transmission Control Protocol/Internet Protocol (TCP/IP) suite of protocols was developed as part of the research done by the Defense Advanced Research Projects Agency (DARPA).
TCP/IP Protocol Layers•Process/Application Layer
•Transport Layer or Host-to-Host Layer
•Internet Layer
•Network Access Layer
•Transport Layer or Host-to-Host Layer
•Internet Layer
•Network Access Layer
Application protocols exist for file transfer, e-mail, and remote login. Network management is also supported at the application layer.
Transport services allow users to segment and reassemble several upper-layer applications onto the same transport-layer data stream.
TCP Segment
UDP Segment
IP provides connectionless, best-effort delivery routing of datagrams. It is not concerned with the content of the datagrams. Instead, it looks for a way to move the datagrams to their destination.
IP Datagram
Version - Version number (4 bits)
Header Length - Header length in 32-bit words (4 bits)
Priority and Type of Service - How the datagram should be handled. The first 3 bits are priority bits (8 bits).
IP Options - Network testing, debugging, security, and others (0 or 32 bits if any)
ICMP
The Internet Control Message Protocol (ICMP) is implemented by all TCP/IP hosts. ICMP messages are carried in IP datagrams and are used to send error and control messages.
ICMP uses the following types of defined messages:
1. Destination Unreachable
2. Time Exceeded
3. Parameter Problem
4. Subnet Mask Request
5. Redirect
6. Echo
7. Echo Reply
8. Information Request
9. Information Reply
10.Address Request
11.Address Reply
Transport services allow users to segment and reassemble several upper-layer applications onto the same transport-layer data stream.
TCP Segment
UDP Segment
IP provides connectionless, best-effort delivery routing of datagrams. It is not concerned with the content of the datagrams. Instead, it looks for a way to move the datagrams to their destination.
IP Datagram
Version - Version number (4 bits)
Header Length - Header length in 32-bit words (4 bits)
Priority and Type of Service - How the datagram should be handled. The first 3 bits are priority bits (8 bits).
IP Options - Network testing, debugging, security, and others (0 or 32 bits if any)
ICMP
The Internet Control Message Protocol (ICMP) is implemented by all TCP/IP hosts. ICMP messages are carried in IP datagrams and are used to send error and control messages.
ICMP uses the following types of defined messages:
1. Destination Unreachable
2. Time Exceeded
3. Parameter Problem
4. Subnet Mask Request
5. Redirect
6. Echo
7. Echo Reply
8. Information Request
9. Information Reply
10.Address Request
11.Address Reply
Address Resolution Protocol
Address Resolution Protocol (ARP) is used to resolve or map a known IP address to a MAC sublayer address to allow communication on a multi-access medium such as Ethernet.
The term local ARP is used to describe resolving an address when both the requesting host and the destination host share the same media or wire.
Reverse ARP
Address Resolution Protocol (ARP) is used to resolve or map a known IP address to a MAC sublayer address to allow communication on a multi-access medium such as Ethernet.
The term local ARP is used to describe resolving an address when both the requesting host and the destination host share the same media or wire.
Reverse ARP
Reverse Address Resolution Protocol (RARP) relies on the presence of a RARP server with a table entry or other means to respond to these requests.
ARP and RARP are implemented directly on top of the data link layer
IP Address
In a TCP/IP environment, end stations communicate seamlessly with servers or other end stations. This communication occurs because each node using the TCP/IP protocol suite has a unique 32-bit logical IP address.
Each IP datagram includes the source IP address and destination IP address that identifies the source and destination network and host.
When IP was first developed, there were no classes of addresses. Now, for ease of administration, the IP addresses are broken up into classes.
The bits in the first octet identify the address class. The router uses the first bits to identify how many bits it must match to interpret the network portion of the address
Class A addresses include the following:
•The first bit is 0.
•Range of network numbers: 1.0.0.0 to 126.0.0.0
•Number of possible networks: 127 (1- 126 usable, 127 is reserved)
•Number of possible values in the host portion: 16,777,216.
Class B addresses include the following:
•The first two bits are 10.
•Range of network numbers: 128.0.0.0 to 191.255.0.0
•Number of possible networks: 16,384
•Number of possible values in the host portion: 65,536
Class C addresses include the following:
•The first three bits are 110.
•Range of network numbers: 192.0.0.0 to 223.255.255.0
•Number of possible networks: 2,097,152
•Number of possible values in the host portion: 256
Class D addresses include the following:
•Range of network numbers:
224.0.0.0 to 239.255.255.255
ARP and RARP are implemented directly on top of the data link layer
IP Address
In a TCP/IP environment, end stations communicate seamlessly with servers or other end stations. This communication occurs because each node using the TCP/IP protocol suite has a unique 32-bit logical IP address.
Each IP datagram includes the source IP address and destination IP address that identifies the source and destination network and host.
When IP was first developed, there were no classes of addresses. Now, for ease of administration, the IP addresses are broken up into classes.
The bits in the first octet identify the address class. The router uses the first bits to identify how many bits it must match to interpret the network portion of the address
Class A addresses include the following:
•The first bit is 0.
•Range of network numbers: 1.0.0.0 to 126.0.0.0
•Number of possible networks: 127 (1- 126 usable, 127 is reserved)
•Number of possible values in the host portion: 16,777,216.
Class B addresses include the following:
•The first two bits are 10.
•Range of network numbers: 128.0.0.0 to 191.255.0.0
•Number of possible networks: 16,384
•Number of possible values in the host portion: 65,536
Class C addresses include the following:
•The first three bits are 110.
•Range of network numbers: 192.0.0.0 to 223.255.255.0
•Number of possible networks: 2,097,152
•Number of possible values in the host portion: 256
Class D addresses include the following:
•Range of network numbers:
224.0.0.0 to 239.255.255.255
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