The routing variables _tag and _tag_relay are used to implement a special behaviour called Tagging in PSYC.

It is mostly used to map a response back to its request and gives the sender a way to easily figure out which replies were triggered by packets it sent and thus handle responses asynchronously.

When an entity receives a packet and intends to send a response to it, it MUST copy the contents of the _tag variable to the _tag_relay variable of the response.

The contents of _tag is opaque and can be an arbitrary value. It is strongly recommended to generate globally unique ids for tags.

Should the routing layer of any involved router encounter an error while delivering a tagged message and intend to generate an error message, it SHOULD copy the tag to a _tag_relay variable in the error report.