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

Latest Forum Discussions

See All

Tokkun Studio unveils alpha trailer for...
We are back on the MMORPG news train, and this time it comes from the sort of international developers Tokkun Studio. They are based in France and Japan, so it counts. Anyway, semantics aside, they have released an alpha trailer for the upcoming... | Read more »
Win a host of exclusive in-game Honor of...
To celebrate its latest Jujutsu Kaisen crossover event, Honor of Kings is offering a bounty of login and achievement rewards kicking off the holiday season early. [Read more] | Read more »
Miraibo GO comes out swinging hard as it...
Having just launched what feels like yesterday, Dreamcube Studio is wasting no time adding events to their open-world survival Miraibo GO. Abyssal Souls arrives relatively in time for the spooky season and brings with it horrifying new partners to... | Read more »
Ditch the heavy binders and high price t...
As fun as the real-world equivalent and the very old Game Boy version are, the Pokemon Trading Card games have historically been received poorly on mobile. It is a very strange and confusing trend, but one that The Pokemon Company is determined to... | Read more »
Peace amongst mobile gamers is now shatt...
Some of the crazy folk tales from gaming have undoubtedly come from the EVE universe. Stories of spying, betrayal, and epic battles have entered history, and now the franchise expands as CCP Games launches EVE Galaxy Conquest, a free-to-play 4x... | Read more »
Lord of Nazarick, the turn-based RPG bas...
Crunchyroll and A PLUS JAPAN have just confirmed that Lord of Nazarick, their turn-based RPG based on the popular OVERLORD anime, is now available for iOS and Android. Starting today at 2PM CET, fans can download the game from Google Play and the... | Read more »
Digital Extremes' recent Devstream...
If you are anything like me you are impatiently waiting for Warframe: 1999 whilst simultaneously cursing the fact Excalibur Prime is permanently Vault locked. To keep us fed during our wait, Digital Extremes hosted a Double Devstream to dish out a... | Read more »
The Frozen Canvas adds a splash of colou...
It is time to grab your gloves and layer up, as Torchlight: Infinite is diving into the frozen tundra in its sixth season. The Frozen Canvas is a colourful new update that brings a stylish flair to the Netherrealm and puts creativity in the... | Read more »
Back When AOL WAS the Internet – The Tou...
In Episode 606 of The TouchArcade Show we kick things off talking about my plans for this weekend, which has resulted in this week’s show being a bit shorter than normal. We also go over some more updates on our Patreon situation, which has been... | Read more »
Creative Assembly's latest mobile p...
The Total War series has been slowly trickling onto mobile, which is a fantastic thing because most, if not all, of them are incredibly great fun. Creative Assembly's latest to get the Feral Interactive treatment into portable form is Total War:... | Read more »

Price Scanner via MacPrices.net

Early Black Friday Deal: Apple’s newly upgrad...
Amazon has Apple 13″ MacBook Airs with M2 CPUs and 16GB of RAM on early Black Friday sale for $200 off MSRP, only $799. Their prices are the lowest currently available for these newly upgraded 13″ M2... Read more
13-inch 8GB M2 MacBook Airs for $749, $250 of...
Best Buy has Apple 13″ MacBook Airs with M2 CPUs and 8GB of RAM in stock and on sale on their online store for $250 off MSRP. Prices start at $749. Their prices are the lowest currently available for... Read more
Amazon is offering an early Black Friday $100...
Amazon is offering early Black Friday discounts on Apple’s new 2024 WiFi iPad minis ranging up to $100 off MSRP, each with free shipping. These are the lowest prices available for new minis anywhere... Read more
Price Drop! Clearance 14-inch M3 MacBook Pros...
Best Buy is offering a $500 discount on clearance 14″ M3 MacBook Pros on their online store this week with prices available starting at only $1099. Prices valid for online orders only, in-store... Read more
Apple AirPods Pro with USB-C on early Black F...
A couple of Apple retailers are offering $70 (28%) discounts on Apple’s AirPods Pro with USB-C (and hearing aid capabilities) this weekend. These are early AirPods Black Friday discounts if you’re... Read more
Price drop! 13-inch M3 MacBook Airs now avail...
With yesterday’s across-the-board MacBook Air upgrade to 16GB of RAM standard, Apple has dropped prices on clearance 13″ 8GB M3 MacBook Airs, Certified Refurbished, to a new low starting at only $829... Read more
Price drop! Apple 15-inch M3 MacBook Airs now...
With yesterday’s release of 15-inch M3 MacBook Airs with 16GB of RAM standard, Apple has dropped prices on clearance Certified Refurbished 15″ 8GB M3 MacBook Airs to a new low starting at only $999.... Read more
Apple has clearance 15-inch M2 MacBook Airs a...
Apple has clearance, Certified Refurbished, 15″ M2 MacBook Airs now available starting at $929 and ranging up to $410 off original MSRP. These are the cheapest 15″ MacBook Airs for sale today at... Read more
Apple drops prices on 13-inch M2 MacBook Airs...
Apple has dropped prices on 13″ M2 MacBook Airs to a new low of only $749 in their Certified Refurbished store. These are the cheapest M2-powered MacBooks for sale at Apple. Apple’s one-year warranty... Read more
Clearance 13-inch M1 MacBook Airs available a...
Apple has clearance 13″ M1 MacBook Airs, Certified Refurbished, now available for $679 for 8-Core CPU/7-Core GPU/256GB models. Apple’s one-year warranty is included, shipping is free, and each... Read more

Jobs Board

Seasonal Cashier - *Apple* Blossom Mall - J...
Seasonal Cashier - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
Seasonal Fine Jewelry Commission Associate -...
…Fine Jewelry Commission Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) Read more
Seasonal Operations Associate - *Apple* Blo...
Seasonal Operations Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Read more
Hair Stylist - *Apple* Blossom Mall - JCPen...
Hair Stylist - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Blossom Read more
Cashier - *Apple* Blossom Mall - JCPenney (...
Cashier - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Blossom Mall Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.