OSI Model and its Layers - 2

The Internet Message Access Protocol (commonly known as IMAP) is an application layer internet protocol that allows an e-mail client to access e-mail on a remote mail server.

A fourth generation firewall cannot understand FTP as FTP operates in application layer, which resides higher than transport layer. A fourth-generation firewall can understand protocols upto layer 4 only.

255.255.255.255 is reserved for Broadcast.

IPv4 is a connectionless protocol for use on packet-switched networks. It operates on a best effort delivery model, in that it does not guarantee delivery, nor does it assure proper sequencing or avoidance of duplicate delivery. These aspects, including data integrity, are addressed by an upper layer transport protocol, such as the Transmission Control Protocol (TCP).

The 16-bit checksum field is used for error-checking of the header. When a packet arrives at a router, the router calculates the checksum of the header and compares it to the checksum field. If the values do not match, the router discards the packet. Errors in the data field must be handled by the encapsulated protocol. Both UDP and TCP have checksum fields.

Automatic Repeat reQuest (ARQ), is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a data frame or packet) and timeouts (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve reliable data transmission over an unreliable service. If the sender does not receive an acknowledgment before the timeout, it usually re-transmits the frame/packet until the sender receives an acknowledgment or exceeds a predefined number of re-transmissions.

The Network Configuration Protocol (NETCONF), is an IETF network management protocol.NETCONF provides mechanisms to install, manipulate, and delete the configuration of network devices. Its operations are realized on top of a simple Remote Procedure Call (RPC) layer.

ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins.These virtual circuits may be “permanent”, i.e. dedicated connections that are usually preconfigured by the service provider, or “switched”, i.e. set up on a per-call basis using signalling and disconnected when the call is terminated.

The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged ethernet local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them.

Since each connection has exactly two end points, therefore, multicasting is not possible with this connection.

In the seven-layer OSI model of computer networking, the presentation layer is layer 6 and serves as the data translator for the network. It is sometimes called the syntax layer.

The data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. It connects hosts within the same network.

In computer networking, promiscuous mode or promisc mode is a mode for a wired network interface controller (NIC) or wireless network interface controller (WNIC) that causes the controller to pass all traffic it receives to the central processing unit (CPU) rather than passing only the frames that the controller is intended to receive.

An eight-bit time to live field helps prevent datagrams from persisting (e.g. going in circles) on an internet. This field limits a datagram's lifetime. It is specified in seconds, but time intervals less than 1 second are rounded up to 1. In practice, the field has become a hop count—when the datagram arrives at a router, the router decrements the TTL field by one. When the TTL field hits zero, the router discards the packet and typically sends an ICMP Time Exceeded message to the sender.

Computer networks use a tunneling protocol when one network protocol (the delivery protocol) encapsulates a different payload protocol. By using tunneling one can (for example) carry a payload over an incompatible delivery-network, or provide a secure path through an untrusted network.