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

Google Earth 7.3.0.3830 - View and contr...
Google Earth gives you a wealth of imagery and geographic information. Explore destinations like Maui and Paris, or browse content from Wikipedia, National Geographic, and more. Google Earth combines... Read more
Transmit 5.0 - Excellent FTP/SFTP client...
Transmit is an excellent FTP (file transfer protocol), SFTP, S3 (Amazon.com file hosting) and iDisk/WebDAV client that allows you to upload, download, and delete files over the internet. With the... Read more
iMazing 2.3.2 - Complete iOS device mana...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
Cocktail 10.4.1 - General maintenance an...
Cocktail is a general purpose utility for macOS that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Backup and Sync 3.35.5978.2967 - File ba...
Backup and Sync (was Google Drive) is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a... Read more
Adobe Lightroom 6.12 - Import, develop,...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
Apple iTunes 12.6.2 - Play Apple Music a...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Apple macOS Sierra 10.12.6 - The latest...
With Apple macOS Sierra, Siri makes its debut on Mac, with new features designed just for the desktop. Your Mac works with iCloud and your Apple devices in smart new ways, and intelligent... Read more
VMware Fusion 8.5.8 - Run Windows apps a...
VMware Fusion 8 and Fusion 8 Pro--the latest versions of its virtualization software for running Windows on a Mac without rebooting--include full support for Windows 10, OS X El Capitan, and the... Read more
Apple Safari 10.1.2 - Apple's Web b...
Note: The direct download link is currently unavailable. It is available in the OS X 10.12.6 release, as well as in the Apple Security Updates. Apple Safari is Apple's web browser that comes with OS... Read more

Latest Forum Discussions

See All

Realpolitiks Mobile (Games)
Realpolitiks Mobile 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: PLEASE NOTE: The game might not work properly on discontinued 1GB of RAM devices (iPhone 5s, iPhone 6, iPhone 6 Plus, iPad... | Read more »
Layton’s Mystery Journey (Games)
Layton’s Mystery Journey 1.0.0 Device: iOS Universal Category: Games Price: $15.99, Version: 1.0.0 (iTunes) Description: THE MUCH-LOVED LAYTON SERIES IS BACK WITH A 10TH ANNIVERSARY INSTALLMENT! Developed by LEVEL-5, LAYTON’S... | Read more »
Full Throttle Remastered (Games)
Full Throttle Remastered 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Originally released by LucasArts in 1995, Full Throttle is a classic graphic adventure game from industry legend Tim... | Read more »
Stunning shooter Morphite gets a new tra...
Morphite is officially landing on iOS in September. The game looks like the space shooter we've been needing on mobile, and we're going to see if it fits the bill quite shortly. The game's a collaborative effort between Blowfish Studios, We're Five... | Read more »
Layton's Mystery Journey arrives to...
As you might recall, Layton's Mystery Journey is headed to iOS and Android -- tomorrow! To celebrate the impending launch, Level-5's released a new trailer, complete with an adorable hamster. [Read more] | Read more »
Sidewords (Games)
Sidewords 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Grab a cup of coffee and relax with Sidewords. Sidewords is part logic puzzle, part word game, all original. No timers. No... | Read more »
Noodlecake Games' 'Leap On!...
Noodlecake Games is always good for some light-hearted arcade fun, and its latest project, Leap On! could carry on that tradition. It's a bit like high stakes tetherball in a way. Your job is to guide a cute little blob around a series of floating... | Read more »
RuneScape goes mobile later this year
Yes, RuneScape still exists. In fact, it's coming to iOS and Android in just a few short months. Jagex, creators of the hit fantasy MMORPG of yesteryear, is releasing RuneScape Mobile and Old School RuneScape for mobile devices, complete with... | Read more »
Crash of Cars wants you to capture the c...
Crash of Cars is going full on medieval in its latest update, introducing castles and all manner of new cars and skins fresh from the Dark Ages. The update introduces a new castle-themed map (complete with catapults) and a gladiator-style battle... | Read more »
Golf Clash guide: How to rise through th...
For a little game as cartoony and colorful as Golf Clash, competition can be quite fierce. You need to be on top of your game and have the best equipment possible if you want to get very far. True success is measured by both player skill and the... | Read more »

Price Scanner via MacPrices.net

DEVONthink To Go 2.1.7 For iOS Brings Usabili...
DEVONtechnologies has updated DEVONthink To Go, the iOS companion to DEVONthink for Mac, with enhancements and bug fixes. Version 2.1.7 adds an option to clear the Global Inbox and makes the grid... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more
13-inch 2.3GHz Silver MacBook Pro on sale for...
B&H Photo has the new 2017 13″ 2.3GHz/256GB Silver MacBook Pro (MPXU2LL/A) on sale for $1399 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. Read more
Apple Tackles Distracted Driving With iOS 11...
One of the most important new features coming in iOS 11 is Do Not Disturb while driving, intended to help drivers stay more focused on the road. With Do Not Disturb while driving, your iPhone can... Read more
iMazing Mini for Mac: Free Automatic and Priv...
Geneva, Switzerland-based indie developer DigiDNA has released iMazing Mini, their free macOS utility designed to automatically back up iOS devices over any local Wi-Fi network. The app offers users... Read more
Clearance 2016 13-inch MacBook Airs, Apple re...
Apple dropped prices recently on Certified Refurbished 2016 13″ MacBook Airs, with models now available starting at $809. An Apple one-year warranty is included with each MacBook, and shipping is... Read more
9.7-inch 2017 iPads available for $299, save...
B&H Photo has 2017 9.7″ 32GB WiFi iPads on sale for $30 off MSRP for a limited time. Shipping is free, and pay sales tax in NY & NJ only: – 32GB iPad WiFi: $299, $30 off Read more
Welcome to the new MacPrices.net!
Welcome to the newly redesigned MacPrices.net! MacPrices has been comprehensively redesigned from the ground up over the past couple of weeks. The new design is cleaner, with less clutter, minimal... Read more
Clearance iMacs available for up to $370 off...
B&H Photo has clearance 21″ and 27″ Apple iMacs available for up to $370 off original MSRP, each including free shipping plus NY & NJ sales tax only: – 27″ 3.3GHz iMac 5K: $1929 $370 off... Read more
Sale! 15-inch 2.8GHz MacBook Pros for $100 of...
B&H Photo has the new 2017 15″ 2.8GHz Touch Bar MacBook Pros on sale for $100 off MSRP. Shipping is free, and B&H charges sales tax in NY & NJ only: – 15″ 2.8GHz MacBook Pro Space Gray: $... Read more

Jobs Board

Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
*Apple* Customer Experience (ACE) Leader - A...
…management to deliver on business objectivesTraining partner store staff on Apple products, services, and merchandising guidelinesCoaching partner store staff on Read more
Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
*Apple* Solutions Consultant - Apple Inc. (U...
…about helping others on a team while also delighting customers? As an Apple Solutions Consultant (ASC), you will discover customers needs and help connect them Read more
Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.