diff --git a/plugin/index.js b/plugin/index.js
index feeaec5..ea66b2c 100644
--- a/plugin/index.js
+++ b/plugin/index.js
@@ -2,6 +2,33 @@
 const express = require('express')
 const router = express.Router()
 
+// The following endpoints are supported.
+//
+// /before_receive
+// /route
+// /before_route (only for PROTOCOL=ROUTE connectors)
+// /before_send
+// /after_send
+// /before_dlr (gets dlrinfo)
+// /after_dlr
+
+// The req.body contains the following fields.
+//
+// ccarg - from PLUGIN=pluginname:ccarg of the current connector
+// qe - message or delivery report, with field names in uppercase
+// result - for after_send and after_dlr
+// dlrinfo - custom options for incoming delivery reports
+// route - as returned by the previously called plugin
+
+// Output is a list of key=value entries, one per line.
+// The key can be as follows.
+//
+// result - for before_receive, where non-zero means the message will be rejected
+// route - the name of the outgoing connector, used for /route
+// qe.OPTION_NAME - new value for a message option called "OPTION_NAME"
+// dlrinfo.OPTION_NAME - for before_dlr, to return HLR lookup results:
+// - OPTION_NAME can be mnc, mcc, msc, and imsi
+
 router.post('/before_receive', (req, res) => {
 	console.log('in before_receive') //, body:', req.body)
 	const qe = req.body.qe