The Session Layer: Understanding layer 5 of the OSI Model

According to the OSI Model, the session layer is where connections are established, managed, and torn down. For connection-oriented network protocols, understanding how the session layer works, and what symptoms would help you identify when it's not working, is an important part of your job as a network administrator. However, because TCP doesn't respect the OSI model, we have to sort of carve TCP/IP out of this conversation, which really limits its practical application in most environments.

When you think about session-oriented communications vs. connectionless conversations, you might compare them to a telephone conversation vs. using a walkie-talkie. With a telephone, you call the person who you want to talk to, establish a connection -- or session -- and then you hang up, severing the connection once the conversation is completed. With a walkie-talkie, you simply speak into the device and hope that the person on the other end is listening and that they respond in kind. There's no session established.

Two session-oriented protocols that you are still likely to see on production networks are Netbios and RPC. These protocols are commonly used within Microsoft-based LAN environments. However, problems with these protocols are seldom seen and when they are, since they're used on the LAN, it's unlikely that the problem is network related. More likely, it's an application problem.

Discussing the session layer can be a bit confusing as when most people think about sessions nowadays they're thinking about TCP sessions. It's important to understand that the TCP Reference Model doesn't really concern itself with the OSI Model. With TCP traffic, the connections are established at the transport layer.

Of the seven layers of the OSI Model, layers 5 and 6, the session and presentation layers, are the two least relevant layers for network engineers. Disagree? Got a great example of how expert knowledge of these two layers came in handy for a network troubleshooting scenario? If so, please share it here as I'm always looking to learn something new.

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Josh Stephens is Head Geek and VP of Technology at SolarWinds, an IT management software company based in Austin, Texas. He shares network management best practices on SolarWinds’ GeekSpeak and thwack. Follow Josh on Twitter@sw_headgeek and SolarWinds @solarwinds_inc.

Copyright © 2011 IDG Communications, Inc.

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