WAN Connectivity

Moving on from layer 3 protocols to layer 2, we look into (mainly) two different implementations: Cisco's proprietary HDLC and the industry standard PPP.

Of course, there is a Cisco proprietary L2 protocol. Cisco didn't get to be the world's largest internetworking company by giving its intellectual property away. If your network is all Cisco, HDLC (High-level Data Link Control) started out as an open standard, but lacked multi protocol support. Cisco took the standard and added the features it thought necessary, and made that the default encapsulation for serial interfaces on all Cisco equipment.

The nice thing about HDLC is that it is extremely simple. If you have the physical layer connected properly and both ends of the circuit are using HDLC there is nothing to get in the way. It is very efficient because it lacks any configurable options. If it was changed from HDLC on a router, you only enter "encapsulation hdlc" on the interface and it is done.

What HDLC lacks in configurability, PPP (Point-to-Point Protocol) offers. PPP gives the option of configuring authentication, call-back, compression and multi-link features to enhance your WAN network computing needs. Topping it all off, it is an open standard and allows communication between any vendor's equipment, making it the de facto standard for WAN communication.

PPP authentication comes in two varieties, either PAP (Password Authentication Protocol) or CHAP (Challenge-Handshake Authentication Protocol). PAP is only used if you are using very old equipment, because the password is sent unencrypted in clear text, and also because the client controls the sending of the credentials. Essentially, the client makes the connection and sends the password when it is darn good and ready, after which it does not require continued authentication. This makes it vulnerable to playback attacks, where an attacker captures the data stream and sends the credentials to take over the session.

CHAP is inherently more secure because it uses MD5 hashes to secure the password, and the server (in our case, router) requests the credentials at connection and then again randomly throughout the remainder of the session. If a client device doesn't offer up the password hash when the router asks for it, the router terminates the session immediately. This prevents the playback attack, because predicting when that call for authentication iwll take place would be nearly impossible.

Call-back is just as it sounds. You set the router up to call back a remote user at a predefined number, so that when a client attempts to make a connection the router hangs up on them and dials a predefined phone number where the user with that username and password should be. If it doesn't answer, the connection is not established (obviously).

Compression is used to increase WAN bandwidth at the expense of router CPU and memory. There is the Stacker algorithm, which is a straight-forward dictionary type compression. It reads the data stream, replaces the data with a code and moves on to the next bit of data. The Predictor algorithm tries to predict the next character based on cached data that it has already compressed. This is good for connection types where the protocol does not change often. Then there is the Microsoft-proprietary MPPC which is only good for connecting Microsoft devices (blah).

Finally we have Multilink capabilities. Multilink allows PPP to combine multiple WAN links into a single, logical link. This allows you to manage and monitor a single interface for throughput, to combine anything from a couple 33.6 bps links to several T1 links for increased bandwidth, and exact-bit load balancing by chopping packets into exact-same-size fragments and sending them out across the MPPP link. TO gain this functionality you will sacrifice router CPU and memory - but what do you want for nothing? A rubber biscuit?

These features (authentication, callback, compression and multilink) are provided by one of PPP's sub-protocols. This is the LCP, or Link Control Protocol. Beneath that in the Layer 2 implementation of PPP are the NCP (Network Control Protocol) and the OSI implementation of HDLC. The OSI implementation of HDLC, like I said already, lacks support for multiple protocols, but it acts as in interface to Layer 1. From that, the NCP allows multiple network protocols to "plug in" to it by providing a standard interface. So HDLC gives the ability to support multiple devices, NCP gives PPP the ability to use multiple network protocols (think of it as a standard "jack" into which you plug in your network protocol), and LCP provides added functionality.

Later we go on to Frame-relay and ATM. What fun!

Hi Geoff...