The OSI Model - Open System InterConnection reference model
Before you get into the OSI model -- bear in mind this very basic premise:"What goes down the model from a source application (client email message) must come up through the model to the destination client e-mail application."
Also, be aware that the e-mail message encounters peer-to-peer relationships between each source and destination layer like so:
You have an image file in JPEG format - that's handled at the Presentation Layer within the source model, right? The assumption is that if you e-mailed the file to a destination address -- the destination's Presentation Layer will be able to identify the particular format, uncompress it if necessary and then send the file upward to the destination e-mail client within the application layer. That's a peer-to-peer relationship and at the heart of standardization.
Think of the model outline below as beginning at a source application (Microsoft Outlook, for example).
| Layer 7 | Application |
User Interface: - Identifies & establishes the availability of intended communications partners. - Synchronizes cooperating applications. - Establishes agreement on procedure for error recovery. - Controls data integrity |
Direct Network Applications: - Client / Server relationship: -- FTP,TFTP, HTTP, SMTP, Web browsers, Email / Web Server / Email Server. Indirect Network Support: - Redirectors work with operating systems and network clients. Protocols that use redirectors: -- Apple Talk Filing Protocol. -- NetBIOS Extended User Interface. -- Network File System (NFS) of the TCP/IP suite. - DNS - Domain Name System: -- .edu / .com /.gov /.org /.net / .biz / .info -- http://www.celticrover.com --- becomes: -- http://64.224.39.150 -Network applications: - Telnet, HTTP, POP3, FTP. - Clients: -- Outlook, Internet Explorer, Eudora, Netscape Navigator. |
Data: Email message - web page - download / upload file. |
| Layer 6 | Presentation |
- How data is presented to the Application Layer. - Format, syntax, compression, encryption. |
-ASCII / EBCDIC / JPEG / GIF / AVI / TIFF / MP3 / MIDI / HTML | Data: Specific format - ASCII ..... |
| Layer 5 | Session |
Interhost Communication: - Establishes, manages, and terminates sessions. - Mini-conversations occur between applications located on different networks. - Decides whether to use full-duplex (2-way simultaneous communication or half-duplex (two-way alternate) communication using Dialog Control. - Dialog separation is used to initial, terminate and manage communication in an orderly fashion. |
- Remote Procedure Call - RPC - X-Window System - Apple Talk Session Protocol (ASP) - Zone Information Protocol (ZIP) - Digital Network Architecture Session Control Protocol (DNA SCP) - SQL |
Data |
| Layer 4 | Transport |
- Reliable transport of data between source and destination. - Logical connection between end-points on a network. - End-to-end control is provided by sliding windows and reliability in sequencing numbers and acknowledgements. - TCP and UDP use port numbers to keep track of different conversation that cross the network at the same time. - Error correction before retransmission. Flow control: - Avoids overflowing buffers. - Ensures the segments delivered will be acknowledged. - Provides for retransmission of any segments that are not acknowledged. - Puts segments back into their correct sequence at the destination. - Provides congestion avoidance and control. |
TCP, UDP, SPX | PDU: Segments |
| Layer 3 | Network |
- Logical addresses associated with the source and destination stations. - Paths through the network to reach desired destination. - Encapsulates data & attaches header. The packets or datagrams include the logical addresses Routers operate on Layer 3: - Tie together, or interconnect, network segments or entire networks. - Make logical decision regarding best path for the delivery. - Forward packets through the network. IP (logical) Addresses: - Class A, Class B, Class C, Class D, Class E. - 32 bit (4-octets) binary number. - Static - Dynamic Addressing. Subnetting: - Network, Subnet, Host. - Subnet Mask - Broadcast Address. |
IP (routed protocol), IPX, RARP, ARP, RIP, DHCP. | PDU: Packets |
| Layer 2 | Data Link |
- Physical (Hexadecimal) addresses used by smart devices: -- Bridges and Switches. - Access to media using MAC (physical) addresses. - Combines bits into frames. - Error detection -- not correction. Important Note: A NIC (Network Interface Card) is considered both a Layer 1 & Layer 2 interface. (serial communication) |
- LLC Sublayer* - Logical Link Control (IEEE 802.2) -- Identifies different protocols. -- Supports connectionless and connection-oriented services. -- Encapsulates the network information into frames which includes physical addresses. -- Ethernet (Non-Deterministic) * IEEE defined - MAC Sublayer - Media Access Control: -- Defines how to xmit 802.3 frames on physical media. -- Physical addressing, network topology. -- Line discipline. -- Ethernet (Non-Deterministic - first come, first served) (spans both sublayers) - CSMA/CD -- IEEE 802.3 (spans Layer 1 and 2) -- CSMA/CD -- 10BaseT -- Token Ring (Deterministic - taking turns)/IEEE 802.5 (spans Layer 1 and 2) -- FDDI - Logical Ring / physical dual ring. |
PDU: Frames |
| Layer 1 | Physical |
- Moves bits between devices. - Specifies voltage, wire speed and pin-out cables. - Combines bits into frames. - Error detection -- not correction. - Passive devices: Modem/CSU/Hub/Repeater. Important Note: A NIC (Network Interface Card) is considered both a Layer 1 & Layer 2 interface. |
EIA/TIA-232 - 449 IEEE 802.3 (spans Layer 1 and 2) - IEEE 802.5 (spans Layer 1 and 2) |
PDU: Bits 0s / 1s |
ARP - Address Resolution Protocol: | Top
A protocol that is used to map an IP (logical-binary) address to a MAC (physical-hexadecimal) address.
Broadcast Address: | Top
A special address reserved for sending a message to all stations. Generally, a broadcast address is a MAC destination address of all 1s.
The Class A IP address: | Top
Supports very large networks. When expressed in binary form the left most bit of the first octet (8-bits) is always Zero - like so:
1st address: 00000001.00000000.00000000.00000000
2nd address: 01111111.00000000.00000000.00000000
Mathematically (using 8-bit binary calculations) - this offers a range of: 1-127. However, it is important to note that low of 0 is not used while high of 127 is reserved. This offers 1 through 126 as a valid network address range within a Class A address.
A Class A network address is created by using the first octet (8-Bits) of the 32-bit IP address. The remaining 24 bits (32 - 8 = 24) may be used for the host portion of the IP address up to 16,777,214 ( (2 to the power of 24) - 2 ) possible IP addresses.
The Class B IP address: | Top
Supports very large networks. When expressed in binary form - the first two bits of first octet (8-bits) are always 10 (1 and 0) - like so:
1st address: 10000000.00000000.00000000.00000000
2nd address: 10111111.11111111.00000000.00000000
Mathematically - the first octet offers a network address range of: 128.0.0.0 - 191.255.0.0
Class B network addresses are created by using the first two octets (16-Bits) of the 32-bit IP address.
The remaining 16 bits (32 - 16 = 16) may be used for the host portion of the IP address up to 65,534 ( (2 to the power of 16) - 2 ) possible IP addresses.
The Class C IP address: | Top
Supports a small network. When expressed in binary form the first two three bits of the first octet (8-bits) are always 110 (1,1 and 0) - like so:
1st address: 11000000.00000000.00000000.00000000
2nd address: 11011111.11111111.11111111.00000000
Mathematically (using 8-bit binary calculations) - this offers a network address range of: 192.0.0.0 to 223.255.255.0.
Critical point: A Class C network address is created by using the first three octets (24-Bits) of the 32-bit IP address. The remaining 8 bits (32 - 24 = 8) may be used for the host portion of the IP address up to 254 ([2 to the power of 8] - 2)possible IP addresses.
CSMA/CD - Carrier Sense Multiple Access with Collision Detect: | Top
A media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If not carrier is sensed for a specific period of time, a device can transmit. It two devices transmit at once, a collision occurs and is detected by all colliding devices. The collision subsequently delays retransmission from those devices for a random length of time. CSMA/CD access is used by Ethernet and IEEE802.3.
DHCP - Dynamic Host Configuration Protocol: | Top
A protocol that provides a mechanism for allocating IP addresses dynamically so that address automatically can be reused when hosts no longer need them.
DNS - Domain Name System: | Top
The system used in the Internet for translating names of network nodes into addresses.
Encapsulation: | Top
Wrapping of data in a particular protocol header. For example, upper-layer data is wrapped in a specific Ethernet header before network
transit. Also, when bridging dissimilar networks, the entire frame from one network can simply be placed in the header used by the data link
layer protocol of the other network.
Ethernet: | Top
A baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet
networks use CSMA/CD and run over a variety of cable types at 10,100, and 1000 Mbps. Ethernet is similar to the IEEE 802.3 series of
standards. (Deterministic - first come, first served.)
File Transfer Protocol: | Top
An application layer protocol used for transferring files between network nodes.
Flow Control: | Top
A technique for ensuring that a transmitting entity does not overwhelm a receiving entity with data. When buffers on the receiving device are
full, a message is sent to the sending device to suspend the transmission until the data in the buffers has been processed.
Frame - Protocol Data Unit: | Top
A logical grouping of information sent a as data link-layer unit over transmission media Often refers to header and trailer, and used for
synchronization and error control, that surround the data contained in the unit. The terms cell, datagram message, packet, and segment are
also used to describe logical information groupings within the various layers of the OSI model.
Hop: | Top
The passage of a data packet from one network node, typically a router, to another.
HTML - Hypertext Markup Language: | Top
A simple hypertext document formatting language that uses tags to indicate how a given part of a document should be interpreted by a viewing
application, such as a web browser.
HTTP - Hypertext Transfer Protocol: | Top
Used by Web browsers and Web servers to transfer files, such as text and graphics files.
LLC - Logical Link Control: | Top
The higher of two data link-layer sublayers defined by IEEE. LLC handles error control, flow control, frame and MAC sublayer addressing. The
most common LLC protocol is IEEE 802.2 which incorporates both connectionless and connection-orientated variants.
IEEE - Institute of Electrical and Electronic Engineers (I-triple E) 802.2: | Top
A LAN protocol that specifies an implementation of the Logical Link Control sublayer of the data link layer (Layer 2). 802.2 handles errors,
framing, flow control and the network layer (Layer 3) service interface.
IEEE (I-triple E) 802.3: | Top
A LAN protocol that specifies an implementation of the physical layer and the MAC sublayer of the data link layer. 802.3 uses CSMA/CD access
at a variety of speeds over a variety of physical media. Extensions of the IEEE 802.3 specification include 10Base2, 10Base5,10BaseF,
10BaseT, and 10Broad36. Physical variations for Fast Ethernet include 100BaseTX and 100BaseFX.
IEEE (I-triple E) 802.5: | Top
A LAN protocol that specifies an implementation of the physical layer and MAC sublayer of the data link layer. 802.5 users token passing
access at 4 or 16 Mbps over Shielded Twisted-Pair or Unshielded Twisted-Pair cabling and is functionally and operationally equivalent to IBM
Token Ring.
PDU - Protocol Data Unit: | Top
- Layer 1 = Bits - 0s / 1s - on / off
- Layer 2 = Frames - encapsulates network layer information and adds physical addresses.
- Layer 3 = Packets - encapsulates data & attaches header. The packets or datagrams include the logical addresses.
- Layer 4 = Segments - grouping of data into more manageable parts.
RIP uses hop count as a routing metric -- distance vector. Be careful with this term. In context - a single transport-layer unit of information.
RARP - Reverse Address Resolution Protocol: | Top
A protocol that provides a method for finding IP (logical-binary) addresses based a MAC (physical-hexadecimal) addresses.
Segmenting Data: | Top
As each application creates a stream data to be sent to a remote application, this data must be prepared to be sent across the media in manageable pieces. The Transport layer protocols describe services that segment this data from the Application layer. This includes the encapsulation required on each piece of data. Each piece of application data requires headers to be added at the Transport layer to indicate to which communication it is associated.
Reassembling Segments: At the receiving host, each piece of data may be directed to the appropriate application. Additionally, these individual pieces of data must also be reconstructed into a complete data stream that is useful to the Application layer. The protocols at the Transport layer describe the how the Transport layer header information is used to reassemble the data pieces into streams to be passed to the Application layer.
SPX - Sequenced Packet Exchange (Novell): | Top
A reliable, connection-orientated protocol that supplements the datagram service provided by network layer protocols.
Subnet Mask: | Top
A 32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address.
TCP - Transmission Control Protocol: | Top
A connection-orientated transport-layer protocol that provides reliable full-duplex data transmission.
Packet - Protocol Data Unit | Top
A logical grouping of information that includes a header containing control information and usually user data. Packets are most often used to refer to network-layer units of data.
Post Office Protocol (POP3): | Top
Is an Internet standard for storing e-mail on a mail server until you can access it and download it to your computer. It enables users to recive mail from their in-boxes using various levels of security.
Protocol: | Top
A formal description of a set of rules and conventions that govern how devices on a network share information.
Telnet: | Top
Is a standard terminal emulation protocol. It is used for remote connections, enabling users to log in to remote systems to use resources as if they were connected to a local system.
Token Ring: | Top
A "token" passing LAN developed and supported by IBM. Token Ring runs at 4 or 16 Mbps over a ring topology. Similar to 802.5. (Deterministic - taking turns)
UDP - User Datagram Protocol: | Top
A connectionless transport-layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgements or guaranteed deliver, requiring that errors processing and retransmission be handled by other protocols.