Application layer(7) Recognized as the top layer of the OSI model, this layer serves as the window for application services. home of email programs, FTP, Telnet, web browsers, and office productivity suites, a viruses, worms, Trojan horse programs,

Presentation layer(6) The Presentation layer is responsible for taking data that has been passed up from lower levels and putting it into a format that Application layer programs can understand. The most critical process handled at this layer is encryption and decryption.

Session layer(5) Its functionality is put to use when creating, controlling, or shutting down a TCP session. Items such as the TCP connection establishment and TCP connection occur here. Session-layer protocols include items such as Remote Procedure Call and SQLNet from Oracle. The Session layer is vulnerable to attacks such as session hijacking. A session hijack can occur when a legitimate user has his session stolen by a hacker.

Transport layer(4) ensures completeness by handling end-to-end error recovery and flow control. Transport-layer protocols include TCP, a connection-oriented protocol. TCP provides reliable communication through the use of handshaking, acknowledgments, error detection, and session teardown, as well as User Datagram Protocol (UDP), a connectionless protocol. UDP offers speed and low overhead as its primary advantage. Security concerns at the transport level include Synchronize (SYN) attacks, Denial of Service (DoS), and buffer overflows.

Network layer(3). logical addressing and routing. The Network layer is the home of the Internet Protocol (IP), which makes a best effort at delivery of datagrams from their source to their destination. Security concerns at the network level include route poisoning, DoS, spoofing, and fragmentation attacks. Fragmentation attacks occur when hackers manipulate datagram fragments to overlap in such a way to crash the victim’s computer. IPSec is a key security service that is available at this layer.

Data Link layer(2). is responsible for formatting and organizing the data before sending it to the Physical layer. The Data Link layer organizes the data into frames. A frame is a logical structure in which data can be placed; it’s a packet on the wire. When a frame reaches the target device, the Data Link layer is responsible for stripping off the data frame and passing the data packet up to the Network layer. The Data Link layer is made up of two sub layers, including the logical link control layer (LLC) and the media access control layer (MAC). You might be familiar with the MAC layer, as it shares its name with the MAC addressing scheme. These 6-byte (48-bit) addresses are used to uniquely identify each device on the local network. A major security concern of the Data Link layer is the Address Resolution Protocol (ARP) process. ARP is used to resolve known Network layer addresses to unknown MAC addresses. ARP is a trusting protocol and, as such, can be used by hackers for ARP poisoning, which can allow them access to traffic on switches they should not have.

Physical layer(1). bit-level communication takes place. The bits have no defined meaning on the wire, but the Physical layer defines how long each bit lasts and how it is transmitted and received. From a security standpoint, you must be concerned anytime a hacker can get physical access. By accessing a physical component of a computer network—such as a computer, switch, or cable—the attacker might be able to use a hardware or software packet sniffer to monitor traffic on that network. Sniffers enable attacks to capture and decode packets. If no encryption is being used, a great deal of sensitive information might be directly available to the hacker.

from CEH

Wall Street climbed again Friday, wrapping up its biggest five-day rally in more than 75 years, even as investors digested signs of a bleak holiday season for retailers and fears that a flurry of reports next week will show more economic distress.

On the short trading day, investors snapped up the battered shares of blue-chip stalwarts Citigroup Inc., General Motors Corp. and Ford Motor Co., fueling a rally that has surprised many market experts whipsawed by wild swings during the past three months.

The market got big boosts over the past week from President-elect Barack Obama naming his economic team, the government propping up Citigroup, and the Federal Reserve deciding to buy massive amounts of mortgage-backed securities. These efforts sent mortgage rates plunging, and reassured the market that broad efforts are still being made to fight the financial crisis that intensified in September with the bankruptcy of Lehman Brothers Holdings Inc.

Just last week, the S&P 500 index fell to its lowest point since 1997 while Citigroup and GM were trading at 15-year and 70-year lows, respectively - touching off worries about how far the market would slide.

While the stock market's strong rebound was certainly welcome, analysts were hesitant about getting too optimistic. Not only did were trading volumes very light on Friday, but investors will be digesting a slew of economic data next week ranging from a reading on the manufacturing sector to the all-important employment report from the Labor Department. Both are expected to be dismal.

"We're seeing some confidence come back into this stock market, but I don't think that's necessarily a reason to be dropping our guard," said Scott Fullman, director of derivatives investment strategy for WJB Capital Group in New York. "You still have to be cautious. There's opportunity, but you have to be extremely selective and defensive."

The stock market closed three hours early the day after Thanksgiving and locked in gains of 16.9 percent for the Dow since the rally began Nov. 21, 19.1 percent for the S&P 500; and 16.7 percent for the Nasdaq.

It was the first time the Dow rose for five consecutive sessions since July 2007, and the biggest five-day percentage gain over five sessions since Aug. 8, 1932. For the S&P 500, it was the first five-day string of gains since July 2007, and the largest five-day percentage gain since March 16, 1933.

The month of November wiped out $1 trillion of shareholder wealth, but the last five days gained $1.2 trillion, according to the Dow Jones Wilshire 5000 Composite Index, which reflects the value of nearly all U.S. stocks.

What could stymie the rally, however, is if the holiday shopping period, which began in earnest Friday, turns out even worse than expected. Wall Street already anticipates that retailers will suffer as consumers, nervous about a difficult job market, lower home values and a jittery stock market, grow more restrained in their spending this year.

"You've seen all sorts of numbers that point to the fact that discretionary spending in the economy has come to an absolute halt," said David Reilly, director of portfolio strategy at Rydex Investments.

Some retail stocks rose Friday as some investors hoped the predictions have been overly dour. Macy's Inc. added 5.6 percent, though some discounters, like Wal-Mart Stores Inc., slipped.

A rare drop in year-over-year holiday spending would be troubling, as it is the most important period of the year for most retailers and because consumer purchases account for more than two-thirds of U.S. economic activity. But while some stores around the nation appeared busy Friday as shoppers looked for bargains, the early evidence was anecdotal and Wall Street would have to wait for cash register tallies.

"The discounting appears to be unbelievable," said Reilly. "The retail sector is going to do whatever it can to get people through the door."

On Friday, the Dow rose 102.43, or 1.17 percent, to 8,829.04.

Broader stock indicators also rose. The S&P 500 index advanced 8.56, or 0.96 percent, to 896.24, while the Nasdaq composite index rose 3.47, or 0.23 percent, to 1,535.57 after spending much of the session lower.

The Russell 2000 index of smaller companies rose 4.28, or 0.91 percent, to 473.14.

Government bonds rose. The yield on the benchmark 10-year Treasury note, which moves opposite its price, tumbled to 2.92 percent from 2.99 percent late Wednesday. The yield on the three-month T-bill, considered one of the safest investments, edged up to 0.05 percent from 0.03 percent Wednesday.

The average overnight rate on a 30-year fixed mortgage was 5.76 percent Friday, according to Bankrate.com, down from 6.00 percent a week ago. The average overnight rate on a 15-year fixed mortgage was 5.50 percent, down from 5.64 percent.

Citigroup was by far the biggest gainer Friday among the 30 stocks that make up the Dow industrials, rising $1.24, or 17.6 percent, to $8.29. Just a week ago, the bank's stock was selling off precipitously, before the government put together a plan to backstop more than $300 billion of the bank's assets.

Ryan Detrick, senior technical strategist at Schaeffer's Investment Research, noted that the day after Thanksgiving is historically a winning day for the market, and that the recent bounce resembles those seen in October when the market stormed higher on relatively light volume only to retreat in the face of gloomy economic readings. Market advances on light volume can indicate that there are simply fewer sellers rather than a strong number of buyers snapping up stocks with conviction.

"We're looking at this like not much more than a light-volume, bear market bounce," Detrick said. "They go away just as quickly as they happen, unfortunately."

In addition to next week's economic data, investors will be waiting to see if Detroit's major automakers can secure federal loans after sending restructuring plans to Capitol Hill by Tuesday. General Motors Corp. rose 43 cents, or 8.9 percent, to $5.24 Friday, while Ford Motor Co. rose 54 cents, or 25 percent, to $2.69. Chrysler LLC isn't publicly traded.

The dollar mostly rose against other major currencies, while gold prices also advanced.

Light, sweet crude fell a penny to settle at $54.43 per barrel on the New York Mercantile Exchange.

Advancing issues outpaced decliners by more than 2 to 1 on the New York Stock Exchange, where consolidated volume came to 2.63 billion shares, down from 5.71 billion shares on Wednesday.

Overseas, Japan's Nikkei stock average fell 0.23 percent. Stocks in India rose a day after trading was suspended because of the terrorist attacks in Mumbai, the country's financial capital. The Sensex Index ended the day with an advance of 0.7 percent.

Britain's FTSE index rose 1.46 percent, Germany's DAX index rose 0.09 percent, and France's CAC-40 advanced 0.38 percent.

The Dow Jones industrial average ended the week up 782.62, or 9.73 percent, at 8,829.04. The Standard & Poor's 500 index finished up 96.21, or 12.03 percent, at 896.24. The Nasdaq composite index ended the week up 151.22, or 10.92 percent, at 1,384.35.

The Russell 2000 index finished the week up 66.60, or 16.38 percent, at 473.14.

The Dow Jones Wilshire 5000 Composite Index - a free-float weighted index that measures 5,000 U.S. based companies - ended at 8,945.20, up 1,019.14 points, or 12.86 percent, for the week. A year ago, the index was at 15,581.48.

Copyright 2008 Associated Press. All rights reserved. This material may not be published broadcast, rewritten, or redistributed

A disheartened Wall Street fell for the third straight session Wednesday as investors absorbed another series of dismal corporate reports and news that the government won't buy banks' soured mortgage assets after all. The Dow Jones industrials skidded 410 points, and all the major indexes dropped more than 4 percent.

The market started the day falling on more signs that companies are being hurt by a severe pullback in consumer spending. Macy's Inc. said it lost $44 million in the third quarter as sales at the department store retailer fell more than 7 percent. And consumer electronics retailer Best Buy Co. slashed its fiscal 2009 guidance on fears that consumer spending will erode even further.

Meanwhile, Morgan Stanley, suffering from the ongoing losses on Wall Street, outlined plans to cut 10 percent of staff in its institutional securities group - its biggest business that covers everything from investment banking to stock trading.

The bleak reports, which followed disappointing news from coffee retailer Starbucks Corp. and homebuilder Toll Brothers Inc. earlier in the week, made it increasingly clear to investors that companies across the economy are suffering from the aftermath of the housing and credit crises.

"There just doesn't appear to be an end in sight to the bad news," said Anton Schutz, portfolio manager of the Burnham Financial Industries Fund and the Burnham Financial Services Fund. "The selling is relentless."

There was more pain at mid-morning, when Treasury Secretary Henry Paulson said the government's $700 billion financial rescue package won't purchase troubled assets from banks. He said that plan would have taken too much time, and that the Treasury instead will rely on buying stakes in banks and encouraging them to resume more normal lending.

While the market had been pleased by the government's decision weeks ago to buy banks' stock, investors still hoped to see the financial industry relieved of the burden of the mortgage assets whose decline in value helped set off the nation's financial crisis. His comments, which underscored the anxiety that remains about the health of the financial system, sent stocks falling further.

Analysts believe the market is in the process of retesting the intraday low hit on Oct. 10, when the blue chips fell to 7,882.50.

"We're just going through the typical process of testing and retesting," said Matt King, chief investment officer of Bell Investment Advisors. "If we can continue to build higher and higher lows, that's definitely a positive. If the Dow can build a base above 8,100 and bounce off that, we see that as a definite technical positive."

The selling accelerated in the last hour of the day, as it has done in most sessions over the past two months.

"When there is a lot of volatility, especially on a big down day, people just decide they don't want to own stocks overnight," said Ryan Detrick, senior technical strategist at Schaeffer's Investment Research. "News doesn't drive this lower, fear does. Investors will back the next morning after they see where things settled."

According to preliminary calculations, The Dow shed 411.30, or 4.73 percent, to 8,282.66.

The broader Standard & Poor's 500 index dropped 46.65, or 5.19 percent, to 852.30, and the Nasdaq composite index stumbled 81.69, or 5.17 percent, to 1,499.21.

Potential grows for lame-duck session of Congress

(CNN) -- A rapidly deteriorating situation in the U.S. auto industry may serve as the backdrop for a classic contest of political wills between the outgoing Bush administration on one hand and both President-elect Obama and the newly strengthened Democratic congressional majority on the other.

Congress may reconvene next week to tackle the troubled auto industry's financial woes.

If President Bush refuses to help bail out the struggling Big Three automakers, the Democratic leadership is promising that it will do so, most likely in the form of a lame-duck session convened as early as next week.

House Speaker Nancy Pelosi, in a statement released Tuesday afternoon, called on key congressional leaders to work with the Bush administration "to craft legislation to provide emergency and limited financial assistance to the automobile industry under the Emergency Economic Stabilization Act."

According to spokesman Brendan Daly, Pelosi believes that the administration can assist the auto industry under its existing authority, but "in case [the administration doesn't], it needs to be done one way or another."

In her statement, Pelosi directed House Financial Services Committee Chairman Barney Frank to draft legislation extending the Treasury Department's authority to use bailout funds for the auto companies.

If the bill is needed, according to Daly, Congress can come back and vote as soon as next week, regardless of whether there is a deal on the larger economic stimulus package.

"We'll have to see what Speaker Pelosi is proposing. There are no details, so there's nothing to react to," White House spokesman Tony Fratto said in a statement Tuesday. "If Congress wants to change the law, we'll see how they intend to do it. Of course, it's strange that congressional Democrats would choose to ignore the $25 billion program they actually created to assist the automakers; that would be a better place to start."

Senate Majority Leader Harry Reid released a statement Tuesday night saying Democrats are "determined to pass legislation" to help the auto industry.

"Senate Democrats are committed to doing all we can to help the auto industry. ... They deserve no less," he said. "But until next year, we still have the slimmest of majorities in Senate; this will only get done if President Bush and Senate Republicans work with us in a bipartisan fashion, and I am confident they will do what is right for our economy."

The question of a possible bailout of the automotive sector -- and whether it might be tied to policy objectives more strongly favored by the Bush administration -- has been the subject of intense debate this week.

Both the White House and a senior aide to President-elect Barack Obama emphatically denied Tuesday that there had been any attempt on the part of President Bush, while meeting with Obama on Monday, to link a federal bailout of the auto industry or a second stimulus package to passage of a Colombia free trade deal.

Those two financial packages are favored by many Democrats, including Obama, and the free trade deal remains a top priority for the outgoing Republican administration.

"The president does support free trade but did not suggest a quid pro quo" with Obama, White House spokeswoman Dana Perino said Tuesday. "He did discuss the merits of free trade, but there was no linkage between Colombia free trade and a second stimulus package."

The denial of a proposed quid pro quo from Bush runs counter to reports in the New York Times and Washington Post suggesting otherwise. Those reports were based in large part on a leak from the Obama camp, which was adamant in denying any potential deal Tuesday.

There was no "wheeling or dealing" between Obama and Bush during their private Oval Office meeting Monday, the Obama aide said; the president and the president-elect each listed his top priorities but did not attempt to reach any agreements.

Obama is not "under any great illusion" that Bush will support a second economic stimulus plan, the aide said.

Obama did, however, strongly urge Bush to support billions of dollars in aid for the struggling auto industry during the increasingly likely lame-duck session of Congress, according to three officials briefed on the meeting.

The officials said Bush expressed skepticism about giving taxpayer money to automakers on the heels of a string of government bailouts for other industries. In addition, they said, the president urged Obama to help push through the free trade pact with Colombia, a key legacy item for the outgoing administration that is facing stiff resistance from Democrats on Capitol Hill.

But a senior Bush administration official downplayed suggestions that Bush was offering a tradeoff, saying the White House still believes that the trade deal "deserves to pass on its own merits" without being linked to anything else.

The officials familiar with the meeting said Obama, pushing the auto industry aid, made the case that dramatic action needs to be taken this year -- rather than after he is sworn into office -- because the Big Three U.S. automakers are bleeding cash at an alarming rate.

One of the officials noted that about one in 10 jobs in America is tied to the auto industry and that if one of the companies goes bankrupt, it could have a massive spillover effect in the credit industry and other sectors.

The senior Bush administration official said the White House is "open to ideas from Congress to accelerate funds they've already appropriated" to help the auto industry.

But the administration official said support for the auto industry would come only "as long as funding will continue to go to viable firms and with strong taxpayer protections."

"Congress created a loan program for the auto industry," Perino noted. "As we read it, we don't see anything in there that would give us the authority to help individual industries, but we are willing to listen to Congress as to how they might choose or not choose to provide additional authority so that we could accelerate those loans to viable companies. We understand that they're going through a very difficult time."

An official in the auto industry said that bringing the Colombian pact into the negotiations could be a poison pill that would prevent passage of an auto industry aid package.

But a senior Democratic aide suggested that even if Bush attempted to link the aid package to the trade deal, Congress may be likely to stand up to him and pass the aid package separately.

The senior aide said Democrats do not think "this president wants to add the demise of GM to his legacy list.