fix: encode 0 in observe option with zero byte length

lib/agent.ts

@@ -512,15 +512,7 @@ class Agent extends EventEmitter {
             }, multicastTimeout)
         }
 
-        if (typeof (url.observe) === 'number') {
-            req.setOption('Observe', url.observe)
-        } else if (typeof (url.observe) === 'string') {
-            req.setOption('Observe', parseInt(url.observe))
-        } else if (url.observe === true) {
-            req.setOption('Observe', 0)
-        } else {
-            req.on('response', this._cleanUp.bind(this))
-        }
+        this._setObserveOption(req, req.url)
 
         this._requests++
 
@@ -529,6 +521,34 @@ class Agent extends EventEmitter {
         return req
     }
 
+    _setObserveOption (req: OutgoingMessage, requestParameters: CoapRequestParams): void {
+        const observeParameter = requestParameters.observe
+
+        if (observeParameter == null) {
+            req.on('response', this._cleanUp.bind(this))
+            return
+        }
+
+        // `null` indicates an option value of zero here, encoded with zero length.
+        // Using `null` avoids issues with some devices that cannot process an
+        // option value of 0 that is encoded as a byte containing eight zeros.
+        let observeValue: null | number
+
+        if (typeof observeParameter === 'number') {
+            observeValue = observeParameter === 0 ? null : observeParameter
+        } else if (typeof observeParameter === 'string') {
+            observeValue = parseInt(observeParameter)
+        } else if (observeParameter) {
+            observeValue = null
+        } else {
+            return
+        }
+
+        if (observeValue == null || !isNaN(observeValue)) {
+            req.setOption('Observe', observeValue)
+        }
+    }
+
     abort (req: OutgoingMessage): void {
         req.sender.removeAllListeners()
         req.sender.reset()

models/models.ts

@@ -16,7 +16,7 @@ import CoAPServer from '../lib/server'
 
 export declare function requestListener (req: IncomingMessage, res: OutgoingMessage): void
 
-export type OptionValue = string | number | Buffer | Buffer[]
+export type OptionValue = null | string | number | Buffer | Buffer[]
 export type BlockCacheMap<T> = Map<string, {payload: T, timeoutId: NodeJS.Timeout}>
 export type CoapOptions = Partial<Record<OptionName, OptionValue>>
 

test/request.ts

@@ -1422,7 +1422,7 @@ describe('request', function () {
             server.on('message', (msg, rsinfo) => {
                 const packet = parse(msg)
                 expect(packet.options[0].name).to.eql('Observe')
-                expect(packet.options[0].value).to.eql(Buffer.of(0))
+                expect(packet.options[0].value).to.eql(Buffer.alloc(0))
                 done()
             })
         })

test/server.ts

@@ -1320,7 +1320,7 @@ describe('Client Identifier', function () {
                     authenticationHeader = request._packet.options.find(o => o.name === 2109)
                 }
 
-                if (typeof authenticationHeader !== 'undefined') {
+                if (authenticationHeader?.value != null) {
                     return `auth:${authenticationHeader.value.toString()}`
                 }
                 return `unauth:${request.rsinfo.address}:${request.rsinfo.port}`