Definition
MPEG is the famous four-letter word which stands for the "Moving Pictures Experts Groups.
To the real word, MPEG is a generic means of compactly representing digital video and audio signals for consumer distributionThe essence of MPEG is its syntax: the little tokens that make up the bitstream. MPEG's semantics then tell you (if you happen to be a decoder, that is) how to inverse represent the compact tokens back into something resembling the original stream of samples.
These semantics are merely a collection of rules (which people like to called algorithms, but that would imply there is a mathematical coherency to a scheme cooked up by trial and error….). These rules are highly reactive to combinations of bitstream elements set in headers and so forth.
MPEG is an institution unto itself as seen from within its own universe. When (unadvisedly) placed in the same room, its inhabitants a blood-letting debate can spontaneously erupt among, triggered by mere anxiety over the most subtle juxtaposition of words buried in the most obscure documents. Such stimulus comes readily from transparencies flashed on an overhead projector. Yet at the same time, this gestalt will appear to remain totally indifferent to critical issues set before them for many months.
It should therefore be no surprise that MPEG's dualistic chemistry reflects the extreme contrasts of its two founding fathers: the fiery Leonardo Chairiglione (CSELT, Italy) and the peaceful Hiroshi Yasuda (JVC, Japan). The excellent byproduct of the successful MPEG Processes became an International Standards document safely administered to the public in three parts: Systems (Part), Video (Part 2), and Audio (Part 3).
Pre MPEG
Before providence gave us MPEG, there was the looming threat of world domination by proprietary standards cloaked in syntactic mystery. With lossy compression being such an inexact science (which always boils down to visual tweaking and implementation tradeoffs), you never know what's really behind any such scheme (other than a lot of the marketing hype).
Seeing this threat… that is, need for world interoperability, the Fathers of MPEG sought help of their colleagues to form a committee to standardize a common means of representing video and audio (a la DVI) onto compact discs…. and maybe it would be useful for other things too.
MPEG borrowed a significantly from JPEG and, more directly, H.261. By the end of the third year (1990), a syntax emerged, which when applied to represent SIF-rate video and compact disc-rate audio at a combined bitrate of 1.5 Mbit/sec, approximated the pleasure-filled viewing experience offered by the standard VHS format.
After demonstrations proved that the syntax was generic enough to be applied to bit rates and sample rates far higher than the original primary target application ("Hey, it actually works!"), a second phase (MPEG-2) was initiated within the committee to define a syntax for efficient representation of broadcast video, or SDTV as it is now known (Standard Definition Television), not to mention the side benefits: frequent flier miles
Definition
Sockets are interfaces that can "plug into" each other over a network. Once so "plugged in", the programs so connected communicate. A "server" program is exposed via a socket connected to a certain /etc/services port number. A "client" program can then connect its own socket to the server's socket, at which time the client program's writes to the socket are read as stdin to the server program, and stdout from the server program are read from the client's socket reads.
Before a user process can perform I/O operations, it calls Open to specify and obtain permissions for the file or device to be used. Once an object has been opened, the user process makes one or more calls to Read or Write data. Read reads data from the object and transfers it to the user process, while Write transfers data from the user process to the object. After all transfer operations are complete, the user process calls Close to inform the operating system that it has finished using that object.
When facilities for InterProcess Communication (IPC) and networking were added, the idea was to make the interface to IPC similar to that of file I/O. In Unix, a process has a set of I/O descriptors that one reads from and writes to. These descriptors may refer to files, devices, or communication channels (sockets). The lifetime of a descriptor is made up of three phases: creation (open socket), reading and writing (receive and send to socket), and destruction (close socket).
History
Sockets are used nearly everywhere, but are one of the most severely misunderstood technologies around. This is a 10,000 foot overview of sockets. It's not really a tutorial - you'll still have work to do in getting things working. It doesn't cover the fine points (and there are a lot of them), but I hope it will give you enough background to begin using them decently.I'm only going to talk about INET sockets, but they account for at least 99% of the sockets in use. And I'll only talk about STREAM sockets - unless you really know what you're doing (in which case this HOWTO isn't for you!), you'll get better behavior and performance from a STREAM socket than anything else. I will try to clear up the mystery of what a socket is, as well as some hints on how to work with blocking and non-blocking sockets. But I'll start by talking about blocking sockets. You'll need to know how they work before dealing with non-blocking sockets.
Part of the trouble with understanding these things is that "socket" can mean a number of subtly different things, depending on context. So first, let's make a distinction between a "client" socket - an endpoint of a conversation, and a "server" socket, which is more like a switchboard operator. The client application (your browser, for example) uses "client" sockets exclusively; the web server it's talking to uses both "server" sockets and "client" sockets.
Of the various forms of IPC (Inter Process Communication), sockets are by far the most popular. On any given platform, there are likely to be other forms of IPC that are faster, but for cross-platform communication, sockets are about the only game in town.
They were invented in Berkeley as part of the BSD flavor of Unix. They spread like wildfire with the Internet. With good reason -- the combination of sockets with INET makes talking to arbitrary machines around the world unbelievably easy (at least compared to other schemes).
Definition
The Cisco IOS Firewall, provides robust, integrated firewall and intrusion detection functionality for every perimeter of the network. Available for a wide range of Cisco IOS software-based routers, the Cisco IOS Firewall offers sophisticated security and policy enforcement for connections within an organization (intranet) and between partner networks (extranets), as well as for securing Internet connectivity for remote and branch offices.
A security-specific, value-add option for Cisco IOS Software, the Cisco IOS Firewall enhances existing Cisco IOS security capabilities, such as authentication, encryption, and failover, with state-of-the-art security features, such as stateful, application-based filtering (context-based access control), defense against network attacks, per user authentication and authorization, and real-time alerts.
The Cisco IOS Firewall is configurable via Cisco ConfigMaker software, an easy-to-use Microsoft Windows 95, 98, NT 4.0 based software tool.
A Firewall is a network security device that ensures that all communications attempting to cross it meet an organization's security policy. Firewalls track and control communications deciding whether to allow ,reject or encrypt communications.Firewalls are used to connect a corporate local network to the Internet and also within networks. In other words they stand in between the trusted network and the untrusted network.
The first and most important decision reflects the policy of how your company or organization wants to operate the system. Is the firewall in place to explicitly deny all services except those critical to the mission of connecting to the net, or is the firewall is in place to provide a metered and audited method of 'Queuing' access in a non-threatening manner. The second is what level of monitoring, reducing and control do you want? Having established the acceptable risk level you can form a checklist of what should be monitored, permitted and denied. The third issue is financial.
Implementation methods
Two basic methods to implement a firewall are
1.As a Screening Router:
A screening router is a special computer or an electronic device that screens (filters out) specific packets based on the criteria that is defined. Almost all current screening routers operate in the following manner.
a. Packet Filter criteria must be stored for the ports of the packet filter device. The packet filter criteria are called packet filter ruler.
b. When the packets arrive at the port, the packet header is parsed. Most packet filters examine the fields in only the IP, TCP and UDP headers.
c. The packet filter rules are stored in a specific order. Each rule is applied to the packet in the order in which the packet filter is stored.
d. If the rule blocks the transmission or reception of a packet the packet is not allowed.
e. If the rule allows the transmission or reception of a packet the packet is allowed.
f. If a packet does not satisfy any rule it is blocked.
