# Passive Observation

The passive observation modules (sometimes referred to as “observer chains”) are used by the active measurement plugins, but can also be used independently. At this time, it is not possible to create new passive observation modules as plugins unless they are part of an active measurement plugin.

A number of modules ship with the PATHspider distribution. You can find their documentation here:

- Basic Chain
- DNS Chain
- DSCP Chain
- ECN Chain
- Evil Bit Chain
- ICMP Chain
- TCP Maximum Segment Size Chain
- TCP Chain
- TCP Fast Open Chain
- UDP Chain