TweetFollow Us on Twitter

Oct 01 Network Management

Volume Number: 17 (2001)
Issue Number: 10
Column Tag: Network Management

Networks 201 pt. 7

by John C. Welch

Layer 5: The Session Layer, and Layer 6: The Presentation Layer

Refresh

Well, we've made it into the last three layers, and things should go a bit faster now. We are also going to group Layers 5 and 6 in this one article, as they are quite thin, and often rolled into other layers. The first OSI layer we will talk about is Layer 5, the Session Layer. This Layer is quite similar in function to Layer 4, and in many situations, it is rolled into the Transport Layer, and is not even a separate part of the networking model. In fact, there is only one protocol that really makes specific use of the Session Layer, and that is AppleTalk. (Other protocols, notably IPX/SPX have aspects that use the Session Layer, but AppleTalk makes more specific use of it than most any other protocol.). In fact, TCP/IP almost completely ignores the Session Layer. So if I seem to talk about AppleTalk more than other protocols here, there's a reason for it.

The Session Layer

The basic function of the Session Layer is to manage communications flow during a session. Now, this is much of what Layer 4, the Transport Layer does, so why have a separate layer? Scope is the reason. The Session Layer is not concerned with things like error checking, reliability, etc. It is assuming that those things are being taken care of by the Transport Layer.

Instead, the Session Layer is concerned with managing the connections or sessions from a higher level point of view. It is concerned with the way traffic is flowing in the session, and making the session run smoother. It is dealing more with the application using the session, then the low - level mechanics of the session. So the Session Layer is said to control the dialog between two applications on a connection.

One of the aspects of this dialog control is the duplex level of the connection's data flow. That is, can the applications at both ends send and receive data simultaneously, (full duplex), or do they have to send and receive in separate actions, (half duplex). Deciding which to use, and handling it is a Session Layer function. An example of a protocol that deals with this type of functionality is the Apple Data Stream Protocol, or ADSP. ADSP's job is to handle connections between devices, and manage the bi-directional flow of data between them. It also allows the data to flow as a continuous stream from sender to receiver, and can handle things like out of order data problems. In that sense, ADSP is also performing some Transport Layer functions too.

Another function of the Session Layer is grouping. An example of this could be the AppleTalk Zone Information Protocol, or ZIP. This is the protocol that takes the information from the Name Binding Protocol, (NBP) and uses it to present the names of devices on an AppleTalk network in the correct logical groups, or zones. The Chooser is a classic use of ZIP and NBP.

Finally, the Session Layer deals with recovery, or what to do when the session fails for some reason. Now, there are two ways to handle this. The first is to just discard all the data on the receiving end, and retransmit all of it. For small pieces of data, this is not a big deal, but for say a five hundred page print job, that gets to be intolerable. So what the Session Layer does is allow for the insertion of checkpoints into the data. So now, if the connection dies for some reason, all that has to happen is for both ends to synchronize which checkpoint was last successfully transmitted, and proceed from there. An example of this functionality, although it occurs at a different layer is the ability to resume an interrupted FTP transfer. Another example of a protocol using this is the Printer Access Protocol, or PAP. When you have to stop and restart an AppleTalk print job, and it asks you "resume printing on page X", that's what we are talking about.

As you can see, there is not a lot to the Session Layer. The protocols that use the layer can be quite complex, but Layer 5 is, in and of itself, rather thin.

Layer 6: The Presentation Layer

This layer is different from all the previous layers, as it is the first layer that is not at all concerned with moving data from point A to point B. Instead, the Presentation Layer cares about how the data is sent up to the Application Layer, and how it is sent down to the Session Layer. Remember, so far, regardless of how we have moved the data, it has all been nothing but a series of bits. However, applications, and the users of those applications don't want ones and zeros. We want letters, numbers, colors, etc.

That's what the Presentation Layer is for. It handles things like ASCII encoding, EBCDIC encoding, converting between the two formats, etc. It also handles other things like dealing with different floating point formats and the representation of those formats. If you are doing things like data compression, or encryption via a protocol, then this is where you would handle that sort of thing.

In other words, the way data is handed off, and how it is encoded, or how it looks to the other layers is a Presentation Layer function. One example of a use for the Presentation Layer is Postscript printing. Although the actual data transfer would be handled by the Session Layer and below, when you send Postscript data to a printer, it has to be encoded correctly, it has to ‘look' right. Otherwise, the print job could fail.

Conclusion

Well, this was certainly a short article. Again, these two Layers really are light weight. They don't' really interact with the user or the user applications, as does the Application Layer. They also don't directly deal with the mechanics of data transfer. As we pointed out, the Presentation Layer doesn't deal with this aspect at all. Still, since AppleTalk makes use of the Session Layer, and almost all protocols deal with the Presentation Layer in one fashion or another, it's good to know why they are even there. Next time, it's the series conclusion, the end of the journey...Layer 7, the Application layer.

Bibliography and References

As already mentioned, you may wish to include a bibliography at the end of your article, especially if you wish to cite work in the body of the article. Here is some typical bibliography format, to serve as a model:

  • Tannenbaum, Andrew S. Computer Networks. Third Edition Prentice Hall, 1996.
  • Sportack, Mark. Networking Essentials Unleashed. SAMS Publishing, 1998.
  • Anders, James K. Live Wired A Guide to Networking Macs. Hayden Books, 1993.
  • Apple Computer, Inside Macintosh: Networking With Open Transport. Apple Computer, Inc. 1997.
  • Stallings, William, Local and Metropolitan Area Networks. Fifth Edition Prentice Hall, 1997.

John Welch jwelch@aer.com is a Mac/PC/Unix administrator and Mac writer living in the wilds of that part of Massachusetts that is not Boston. He has over fifteen years of experience at making computers work. His specialties are figuring out ways to make the Mac do what nobody thinks it can, and showing that the Mac is the superior administrative platform.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Ellipsis - Touch. Explore. Survive. (...
Ellipsis - Touch. Explore. Survive. 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Ys Chronicles II (Games)
Ys Chronicles II 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: After a hard fight to recover the six sacred books in Ys Chronicles I, Adol is back for a sequel! | Read more »
FINAL FANTASY Ⅸ (Games)
FINAL FANTASY Ⅸ 1.0.4 Device: iOS Universal Category: Games Price: $16.99, Version: 1.0.4 (iTunes) Description: ==========●Special sale price for the FINAL FANTASY IX release! ●20% off from February 10 to February 21, 2016... | Read more »
Tennis Club Story (Games)
Tennis Club Story 1.03 Device: iOS Universal Category: Games Price: $4.99, Version: 1.03 (iTunes) Description: Aim for the ace position of tennis club prestige in this simulation! Your leadership decides if players make it to the big... | Read more »
Juggernaut Wars guide - How to use skill...
Juggernaut Warsis a brand new auto-RPG on iOS and Android that challenges you to build a team of heroes, send them out into various different missions to defeat waves of heroes, and level them up to increase their power. The actual combat itself... | Read more »
Check out the new Pirate Attack update i...
Love pirates and board games? Well, you'll love the new Pirate Attack themed update that just launched in Game of Dice. It adds a bunch of new content themed around pirates, like an all new event map based on a pirate ship which revamps the toll... | Read more »
Splash Cars guide - How to paint the tow...
Splash Cars is an arcade driving game that feels like a hybrid between Dawn of the Plow and Splatoon. In it, you'll need to drive a car around to repaint areas of a town that have lost all of their color. Check out these tips to help you perform... | Read more »
The best video player on mobile
We all know the stock video player on iOS is not particularly convenient, primarily because it asks us to hook a device up to iTunes to sync video in a world that has things like Netflix. [Read more] | Read more »
Four apps to help improve your Super Bow...
Super Bowl Sunday is upon us, and whether you’re a Panthers or a Broncos fan you’re no doubt gearing up for it. [Read more] | Read more »
LooperSonic (Music)
LooperSonic 1.0 Device: iOS Universal Category: Music Price: $4.99, Version: 1.0 (iTunes) Description: LooperSonic is a multi-track audio looper and recorder that will take your loops to the next level. Use it like a loop pedal to... | Read more »

Price Scanner via MacPrices.net

App Annie Reveals Future of the App Economy:...
App Annie, a San Francisco based mobile app data and insights platform, has launched its first comprehensive app economy forecast. This new offering will provide brands, agencies, investors and app... Read more
Apple restocks Certified Refurbished Mac mini...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more
What iPad Pro Still Needs To Make It Truly Pr...
I love my iPad Air 2. So much that I’m grudgingly willing to put up with its compromises and limitations as a production tool in order to take advantage of its virtues. However, since a computer for... Read more
21-inch 3.1GHz 4K on sale for $1399, $100 off...
B&H Photo has the 21″ 3.1GHz 4K iMac on sale $1399 for a limited time. Shipping is free, and B&H charges NY sales tax only. Their price is $100 off MSRP: - 21″ 3.1GHz 4K iMac (MK452LL/A): $... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Save up to $240 with Apple Certified Refurbis...
Apple is now offering Certified Refurbished 12″ Retina MacBooks for up to $240 off the cost of new models. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more
Apple refurbished Time Capsules available for...
Apple has certified refurbished Time Capsules available for $120 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $179, $120 off - 3TB... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more
Apple refurbished 15-inch Retina MacBook Pros...
Apple has Certified Refurbished 2015 15″ Retina MacBook Pros available for up to $380 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2... Read more

Jobs Board

Lead Engineer - *Apple* OSX & Hardware...
Lead Engineer - Apple OSX & Hardware **Job ID:** 3125919 **Full/Part\-Time:** Full\-time **Regular/Temporary:** Regular **Listed:** 2016\-02\-10 **Location:** Cary, Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
Infrastructure Engineer - *Apple* /Mac - Rem...
…part of a team Requires proven problem solving skills Preferred Additional: Apple Certified System Administrator (ACSA) Apple Certified Technical Coordinator (ACTC) Read more
Lead Engineer - *Apple* OSX & Hardware...
Lead Engineer - Apple OSX & Hardware **Job ID:** 3125919 **Full/Part\-Time:** Full\-time **Regular/Temporary:** Regular **Listed:** 2016\-02\-10 **Location:** Cary, Read more
*Apple* System Analyst - ATOS IT Services...
Apple System AnalystReference no.198783CountryUSARegionUS - CALIFORNIACityUS - CALIFORNIA - BURBANKPosition TypeProfessionalJob AreaIT SupportJob TypeFull Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.