The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Feature Protocols

PPP Feature Protocols 1 2 PPP Compression Control Protocol (CCP) and Compression Algorithms

LQR Counters

Assuming that the negotiation is successful, LQR will be enabled. A number of counters are set up that keep track of various link statistics, and a timer used to regulate the sending of quality reports over the link. Each time the timer expires a special link quality report is generated and sent in a PPP frame over the link. These are sent using the special PPP Protocol field hexadecimal value 0xC025.

Each counter holds information about a different statistic regarding the use of the link. Each of these counters is reset to zero when LQR is set up and then incremented each time a transmission is made or an event occurs that is relevant to the counter. The statistics tracked include the following:

• The number of frames sent/received.
  
  
• The number of octets (bytes) in all frames sent/received.
  
  
• The number of errors that have occurred.
  
  
• The number of frames that had to be discarded.
  
  
• The number of link quality reports generated.

These counters are only reset at the start of the link, so they contain figures kept cumulatively over the life of the connection. The counters can be used in the absolute sense, meaning that the counter value itself is reported. Alternately, they can be expressed as relative (or delta) values, which represent the change since the last report. This is done when a report is received simply by subtracting the previous report's numbers from those in the current report.

LQR specifies the quality reporting mechanism, but not specific standards for link quality, since these are so implementation-dependent. Based on the numbers in these reports, a device can decide for itself what conclusions to draw about link quality, and in turn what action to take, if any. For example:

• Some devices might decide to shut down a link if the absolute number of errors seen in any report reaches a certain threshold.
  
  
• Some might look at the trend in successive reporting periods and take action if they detect certain trends, such as an increase in the rate of discarded frames.
  
  
• Some devices might just log the information and take no action at all.

Note that LQR aggregates its statistics for all higher-layer protocols transmitted over a particular link. It doesn't keep track of statistics for different higher-layer protocols separately (which makes sense, since the quality of the link shouldn't vary from one higher layer protocol to the next).

PPP Feature Protocols 1 2 PPP Compression Control Protocol (CCP) and Compression Algorithms