Quiz3: Layer5,6,7 May 7, 2008
QUIZ:
1. ________ provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue.
2. the major part of the session layer functionality is ________.
3. The Session layer does not exist as a separate protocol layer or process in the four or five layer TCP/IP model, but its functionality is partly provided by ________, and partly by ________.
4. The presentation layer is responsible for ________.
5. ________ deals with issues like how strings are represented – whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, i.e. “thisisastring“).
6. ________ may be done at the presentation layer to improve the throughput of data.
7. Some types of encryption (and decryption) are performed at the presentation layer. This ensures ________ as it travels down the protocol stack.
8. ________ interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.
9. ________ can (and does) use services directly at the application layer.
10. In the OSI model, the application layer provides services for user applications to employ.For example, when you use your web browser, that actual software is an application running on your PC. It doesn’t really “reside” at the application layer. Rather, it makes use of the services offered by a protocol that operates at the application layer, which is called ________.
ANSWERS:
1. The session layer
2. typically unused in Internet applications.
3. the TCP/IP model transport layer, some TCP/IP model application layer protocols
4. the delivery and formatting of information to the application layer for further processing or display
5. The presentation layer
6. Compression (and decompression)
7. the security of the data
8. The application layer
9. The operating system itself
10. the Hypertext Transfer Protocol (HTTP)
Application Layer May 1, 2008
The application layer is the seventh level of the seven-layer OSI model, and the fifth layer of the five-layer TCP/IP model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.
The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.
The application layer of the four-layer and five-layer TCP/IP models corresponds to the application layer, the presentation layer and session layer in the seven layer OSI model.
It’s important to understand that what the OSI model calls an “application” is not exactly the same as what we normally think of as an “application”. In the OSI model, the application layer provides services for user applications to employ. For example, when you use your web browser, that actual software is an application running on your PC. It doesn’t really “reside” at the application layer. Rather, it makes use of the services offered by a protocol that operates at the application layer, which is called the Hypertext Transfer Protocol (HTTP). The distinction between the browser and HTTP is subtle, but important.
The reason for pointing this out is because not all user applications use the application layer of the network in the same way. Sure, your Web browser does, and so does your e-mail client and your usenet news reader. But if you use a text editor to open a file on another machine on your network, that editor is not using the application layer. In fact, it has no clue that the file you are using is on the network: it just sees a file addressed with a name that has been mapped to a network somewhere else. The operating system takes care of redirecting what the editor does, over the network.
Similarly, not all uses of the application layer are by applications. The operating system itself can (and does) use services directly at the application layer.
Examples of the ‘Application Layer’
Don’t forget, the OSI model is relevant to all kinds of devices, not just computers. The pictures below show that the Application Layer is relevant in mobile phones, iPods, computers, scientific instruments even audio players and so on. These are examples of a ‘Visual’ interface’. Can you think of some others?
As we’ve discussed elsewhere, the distinctions between the top layers are not very clear, and this is largely because of the decision made to not separate out session, presentation and application layer functions in the important TCP/IP protocol suite. All of the protocols mentioned above are from the TCP/IP protocol family, and some may cover all three of the top three OSI layers, two of them, or one; in the TCP/IP model, they are all applications.
Presentation Layer May 1, 2008
The presentation layer is the sixth level of the seven layer OSI model. It responds to service requests from the application layer and issues service requests to the session layer.
The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.
The presentation layer is the first one where people start to care about what they are sending at a more advanced level than just a bunch of ones and zeros. This layer deals with issues like how strings are represented – whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, i.e. “thisisastring“). The idea is that the application layer should be able to point at the data to be moved, and the Presentation layer will deal with the rest.
Presentation Layer Functions
Here are some of the specific types of data handling issues that the presentation layer handles:
- Translation: Networks can connect very different types of computer together: PCs, Macintoshes, UNIX systems, AS/400 servers and mainframes can all exist on the same network. These systems have many distinct characteristics and represent data in different ways; they may use different character sets for example. The presentation layer handles the job of hiding these differences between machines.
- Compression: Compression (and decompression) may be done at the presentation layer to improve the throughput of data. (There are some who believe this is not, strictly speaking, a function of the presentation layer.)
- Encryption: Some types of encryption (and decryption) are performed at the presentation layer. This ensures the security of the data as it travels down the protocol stack. For example, one of the most popular encryption schemes that is usually associated with the presentation layer is the secure sockets layer (SSL) protocol. Not all encryption is done at layer 6, however; some encryption is often done at lower layers in the protocol stack, in technologies such as IPSec.
Session Layer May 1, 2008
The session layer is level five of the seven level OSI model. It responds to service requests from the presentation layer and issues service requests to the transport layer.
The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications. Session layers are commonly used in application environments that make use of remote procedure calls (RPCs).
An example of a session layer protocol is the OSI protocol suite Session Layer Protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session layer protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages.
The Session layer does not exist as a separate protocol layer or process in the four or five layer TCP/IP model, but its functionality is partly provided by the TCP/IP model transport layer, and partly by some TCP/IP model application layer protocols. However, a major part of its functionality is typically unused in Internet applications.
For example, you may have a Word session open and wish to save your work, at the same time you may have a web browser open and are flicking across the web. Each application wants the commands and data it is using to go to the right place i.e. your Word file ends up on the file server and the Web pages are displayed in your browser.
Session Layer Functions
As I have mentioned in a few places in this Guide, the boundaries between layers start to get very fuzzy once you get to the session layer, which makes it hard to categorize what exactly belongs at layer 5. Some technologies really span layers 5 through 7, and especially in the world of TCP/IP, it is not common to identify protocols that are specific to the OSI session layer.
The term “session” is somewhat vague, and this means that there is sometimes disagreement on the specific functions that belong at the session layer, or even whether certain protocols belong at the session layer or not. To add to this potential confusion, there is the matter of differentiating between a “connection” and a “session”. Connections are normally the province of layer four and layer three, yet a Transmission Control Protocol (TCP) connection, for example, can persist for a long time. The longevity of TCP connections makes them hard to distinguish from “sessions” (and in fact there are some people who feel that the TCP/IP host-to-host transport layer really straddles OSI layers four and five).
Quiz2:Layer1,2,3,4 March 12, 2008
QUIZ:
1. When considering network interconnection devices, the simplest ones operate at the physical layer are: ______, ______and ______.
2. No packet headers nor trailers are consequently added to the data by the ______ layer.
3. Physical Layer Functions includes: Definition of Hardware Specifications,______,Data Transmission and Reception,______.
4. OSI Model stands for______.
5. A layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the ______layer to send and receive packets that make up the contents of the path.
6. ______is the layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment.
7. At layer ______, data does not need to be able to go everywhere, just able to go somewhere else.
8. Data Link Layer Sublayers are ______.
9. ______refers to the procedures used by devices to control access to the network medium.
10. A cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if ______.
11. In the TCP/IP reference model the network layer is called ______.
12. In all of the models, the network layer responds to service requests from the ______ layer and issues service requests to the ______layer.
13. In essence, the network layer is responsible for ______ packet delivery, whereas the data link layer is responsible for ______ frame delivery.
14. The OSI Network Layer protocol can be either connection-oriented, or connectionless. The TCP/IP Internet Layer (equivalent to OSI’s Network Layer) supports only the ______ Internet Protocol (IP).
15. If the packet that the network layer wants to send is too large, the ______ layer must split the packet up, send each piece to the ______ layer, and then have pieces reassembled once they arrive at the ______ layer on the destination machine.
16. The physical layer handles ______; the data link layer deals with ______ and the network layer handles ______.
17. Due to this role, the transport layer’s overall job is to provide the necessary functions to enable communication between ______ on different computers.
18. the transport layer is sometimes said to be responsible for ______transport.
ANSWERS:
1. repeaters, conventional hubs and transceivers
2. physical
3. Encoding and Signaling, Topology and Physical Network Design
4. Open System Interconnection Basic Reference
5. the next lower
6. The data link layer
7. The data link layer
8. Logical Link Control(LLC) and Media Access Control(MAC)
9. MAC
10. if it was received correctly
11. the Internet Layer
12. the transport layer, the data link layer
13. end to end. node to node
14. the connectionless
15. network, data link, network
16. the bits, local networks, routing between networks
17. software application process
18. end-to-end or host-to-host transport
Quiz1:Terms March 12, 2008
QUIZ:
1. ______ is a computer network covering a small geographic area, like a home, office, or group of buildings e.g. a school.
2. ______ is a network that connects two or more Local Area Networks or Campus Area Networks together but does not extend beyond the boundaries of the immediate town, city, or metropolitan area.
3. The largest and most well-known example of a WAN is______.
4. A network card, network adapter or NIC (network interface card) is ______.
5. The signal in ______ (an electronic device)can cover longer distances without degradation.
6. ______work at the physical layer (layer 1) of the OSI model, and the term layer 1 switch is often used interchangeably with this device.The device is thus a form of multiport repeater.
7. ______do not promiscuously copy traffic to all ports, as hubs do, but learns which MAC addresses are reachable through specific ports.
8. Routers are the networking device that forward data packets along networks by using headers and forwarding tables to determine______to forward the packets.
9. The goal of ______is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.
10. Ethernet is a family of ______for local area networks (LANs).
11. IEEE stands for______.
12. TCP/IP stands for______.
ANSWERS:
1. A local area network
2. A Metropolitan Area Network
3. the Internet
4. a piece of computer hardware designed to allow computers to communicate over a computer network
5. a repeater
6. Hubs
7. Bridges
8. the best path
9. modem
10. frame-based computer networking technologies
11. Institute of Electrical and Electronics Engineers
12. Transmission Control Protocal and Internet Protocol
Transport Layer March 11, 2008
Transport layer
The fourth and “middle” layer of the OSI Reference Model protocol stack is the transport layer. I consider the transport layer in some ways to be part of both the lower and upper “groups” of layers in the OSI model. It is more often associated with the lower layers, because it concerns itself with the transport of data, but its functions are also somewhat high-level, resulting in the layer having a fair bit in common with layers 5 through 7 as well.
Recall that layers 1, 2 and 3 are concerned with the actual packaging, addressing, routing and delivery of data; the physical layer handles the bits; the data link layer deals with local networks and the network layer handles routing between networks. The transport layer, in contrast, is sufficiently conceptual that it no longer concerns itself with these “nuts and bolts” matters. It relies on the lower layers to handle the process of moving data between devices.
The transport layer really acts as a “liaison” of sorts between the abstract world of applications at the higher layers, and the concrete functions of layers one to three. Due to this role, the transport layer’s overall job is to provide the necessary functions to enable communication between software application processes on different computers. This encompasses a number of different but related duties
Modern computers are multitasking, and at any given time may have many different software applications all trying to send and receive data. The transport layer is charged with providing a means by which these applications can all send and receive data using the same lower-layer protocol implementation. Thus, the transport layer is sometimes said to be responsible for end-to-end or host-to-host transport (in fact, the equivalent layer in the TCP/IP model is called the “host-to-host transport layer”).
Network Layer March 11, 2008
Network layer
The network layer is the third layer out of seven in the OSI model and the third layer out of five in the TCP/IP model. In the TCP/IP reference model it is called the Internet layer. In all of the models, the network layer responds to service requests from the transport layer and issues service requests to the data link layer.
In essence, the network layer is responsible for end to end (source to destination) packet delivery, whereas the data link layer is responsible for node to node (hop to hop) frame delivery.
The network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service, and error control functions.
The network layer deals with transmitting information all the way from its source to its destination – transmitting from anywhere, to anywhere. Here are some things that the network layer needs to address:
- Is the network connection-oriented or connectionless?
- For example, snail mail is connectionless, because you can send a letter to someone without them doing anything and they will receive the letter. On the other hand, the telephone system is connection-oriented, because the other party is required to pick up the phone before communication can be established. The OSI Network Layer protocol can be either connection-oriented, or connectionless. The TCP/IP Internet Layer (equivalent to OSI’s Network Layer) supports only the connectionless Internet Protocol (IP).
- What are the Global Addresses?
- Everybody in the network needs to have a unique address which determines who they are. This address will normally be hierarchical, so you can be “Fred Murphy” to Dubliners, or “Fred Murphy, Dublin” to people in Ireland, or “Fred Murphy, Dublin, Ireland” to people anywhere in the world. On the internet, these addresses are known as IP Addresses.
- How do you forward a message?
- This is of particular interest to mobile applications, where a user may rapidly move from place to place, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) doesn’t really allow for this, though it has been hacked somewhat since its inception. Fortunately, the forthcoming IPv6 has a much better designed solution, which should make this type of application much smoother.
Network Layer Functions
Some of the specific jobs normally performed by the network layer include:
- Logical Addressing: Every device that communicates over a network has associated with it a logical address, sometimes called a layer three address. For example, on the Internet, the Internet Protocol (IP) is the network layer protocol and every machine has an IP address. Note that addressing is done at the data link layer as well, but those addresses refer to local physical devices. In contrast, logical addresses are independent of particular hardware and must be unique across an entire internetwork.
- Routing: Moving data across a series of interconnected networks is probably the defining function of the network layer. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go. I discuss routing in the OSI model more completely in this topic on the topic on indirect device connection, and show how it works by way of an OSI model analogy.
- Datagram Encapsulation: The network layer normally encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header.
- Fragmentation and Reassembly: The network layer must send messages down to the data link layer for transmission. Some data link layer technologies have limits on the length of any message that can be sent. If the packet that the network layer wants to send is too large, the network layer must split the packet up, send each piece to the data link layer, and then have pieces reassembled once they arrive at the network layer on the destination machine. A good example is how this is done by the Internet Protocol.
- Error Handling and Diagnostics: Special protocols are used at the network layer to allow devices that are logically connected, or that are trying to route traffic, to exchange information about the status of hosts on the network or the devices themselves.
Other Related Terms March 11, 2008
Ethernet
Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media Access Control (MAC)/Data Link Layer, and a common addressing format.
Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from around 1980 to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET. In recent years, Wi-Fi, the wireless LAN standardized by IEEE 802.11, is prevalent in home and small office networks and augmenting Ethernet in larger installations.
IEEE 802.3
IEEE 802.3 is a collection of IEEE(Institute of Electrical and Electronics Engineers) standards defining the physical layer, and the media access control (MAC) sublayer of the data link layer, of wired Ethernet. This is generally a LAN technology with some WAN applications. Physical connections are made between nodes and/or infrastructure devices (hubs, switches, routers) by various types of copper or fiber cable.
802.3 is a technology that can support the IEEE 802.1 network architecture.
The maximum packet size is 1518 bytes, although to allow the Q-tag for Virtual LAN and priority data in 802.3ac it is extended to 1522 bytes. If the upper layer protocol submits a protocol data unit (PDU) less than 64 bytes, 802.3 will pad the data field to achieve the minimum 64 bytes.
Internet protocol suite
The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet and most commercial networks run. It has also been referred to as the TCP/IP protocol suite, which is named after two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were also the first two networking protocols defined. Today’s IP networking represents a synthesis of two developments that began to evolve in the 1960s and 1970s, namely LANs (Local Area Networks) and the Internet, which, together with the invention of the World Wide Web by Tim Berners-Lee in 1989, have revolutionized computing.
The Internet Protocol suite—like many protocol suites—can be viewed as a set of layers. Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted.
