Foundations of Networking | OSPF Network Design Solutions

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Upper Layers (Layers 5, 6, 7 Handle Application Issues)

The upper layers of the OSI reference model are concerned with application issues. They are generally implemented only in software. The Application layer is the highest layer and is closest to the end user. Both users and Application layer processes interact with software applications containing a communications component.

Notes:
The term upper layer is often used to refer to any higher layer, relative to a given layer.

Layer 7 Application

Essentially, the Application layer acts as the end-user interface. This is the layer where interaction between the mail application (cc:Mail, MS Outlook, and so forth) or communications package and the user occurs. For example, when a user desires to send an e-mail message or access a file on the server, this is where the process starts. Another example of the processes going on at this layer are things like Network File System (NFS) use and the mapping of drives through Windows NT.

Layer 6 Presentation

The Presentation layer is responsible for the agreement of the communication format (syntax) between applications. For example, the Presentation layer enables Microsoft Exchange to correctly interpret a message from Lotus Notes. Another example of the actions occurring in this layer is the encryption and decryption of data in PGP (Pretty Good Privacy).

Layer 5 Session

The Session layer is responsible for the Application Layer s management of information transfer, to the Data Transport portion of the OSI reference model. An example is Sun s or Novell s Remote Procedure Call (RPC), this functionality uses Layer 5.

Lower Layers (Layers 1, 2, 3, 4 Handle Data Transport Issues)

The lower layers of the OSI reference model handle data transport issues. The Physical and Data Link layers are implemented in hardware and software. The other lower layers are generally implemented only in software.

Layer 4 Transport

The Transport layer is responsible for the logical transport mechanism, which includes functions conforming to the mechanism s characteristics. For example, the Transmission Control Protocol (TCP), a logical transport mechanism, provides a level of error checking and reliability to the transmission of user data to the lower layers of the OSI reference model. This layer is the only layer that provides true source-to-destination end-to-end connectivity. This layer also supports multiple connections based upon port as found in TCP or UDP.

Layer 3 Network

The Network layer determines physical interface address locations. Routing decisions are made based upon the locations of the Internet Protocol (IP) address in question. For example, IP addresses establish logical topologies known as subnets. Applying this definition to a LAN workstation environment, the workstation determines the location of a particular IP address and where its associated subnet resides through the Network layer. Therefore, a packet sent to IP address A.B.C.D will be forwarded through the workstation s Ethernet card and out onto the network.

Notes:
At this time it would be beneficial to give a brief high-level overview of the ARP process. Address Resolution Protocol (ARP) picks up where the IP address and the routing table fall short. As data travels across a network, it must obey the Physical layer protocols that are in use; however, the Physical layer protocols do not understand IP addressing. The most common example of the Network layer translation function is the conversion from IP address to Ethernet address. The protocol responsible for this is ARP, which has been defined in RFC 826. ARP maintains a dynamic table of translations between IP addresses and Ethernet addresses. When ARP receives a request to translate an IP address it checks this table; if it is found, the Ethernet address is returned to the requestor. If it is not found, ARP broadcasts a packet to every host on the Ethernet segment. This packet contains the IP address in question. If the host is found, it responds back with its Ethernet address, which is entered into the ARP table.
The opposite of this is Reverse Address Resolution Protocol (RARP). RARP translates addresses in the opposite direction as defined in RFC 903. RARP is used to enable a diskless workstation to learn its IP address because it has no disk from which to read its TCP/IP configuration. Nevertheless, every system knows its Ethernet address because it is burned in on its Ethernet card. So the diskless workstation uses the Ethernet broadcast ability to request its IP address from a server that looks it up by comparing the Ethernet address to a table that can match it to the appropriate IP address. It is important to note that RARP has nothing to do with routing data from one system to another, and it is often confused with ARP.

Layer 2 Data Link

The Data Link layer provides framing, error, and flow control across the network media being used. An important characteristic of this layer is that the information that is applied to it is used by devices to determine if the packet needs to be acted upon by this layer (that is, proceed to Layer 3 or discard). The Data Link layer also assigns a Media Access Control (MAC) address to every LAN interface on a device. For example, on an Ethernet LAN segment, all packets are broadcast and received by every device on the segment. Only the device whose MAC address is contained within this layer s frame acts upon the packet; all others do not. It is important to note at this point that serial interfaces do not normally require MAC addresses unless it is necessary to identify the receiving end.

TIPS:
It is important to note that MAC addresses are 48-bits in size, three of which are dedicated for vendor identification and another three of which are for unique identification. Additional information on this subject can be found at: http://www.Cisco.com/warp/public/701/33.html.

Layer 1 Physical

The Physical layer is the lowest layer and is closest to the physical network medium (the network cabling connecting various pieces of network equipment, for example). It is responsible for actually placing information on the physical media in the correct electrical format (that is, raw bits). For example, an RJ45 cable is wired very differently from an Attachment Unit Interface (AUI); this means that the Physical layer must place the information slightly differently for each media type. Figure 1-5 shows the actual relationship (peering) between the seven layers.

Figure 1-5 Detailed OSI layer relationships.

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