Response timeout for block transfer

Hey, thanks for your work!

I am running this code (taken from blockwise_put.js) to send a chunk of data:

const coap = require('../')

const bufferSize = 250000
const testBuffer = Buffer.alloc(bufferSize)
const containedData = 'This is a test buffer with a lot of nothing and a bit of something'
testBuffer.fill('X', 'utf-8')
testBuffer.write(containedData, 'utf-8')
testBuffer.write(containedData, testBuffer.length - containedData.length, containedData.length, 'utf-8')

const request = coap.request({
    hostname: 'fdd2:3ab9:44ef:78c5:3b48:3e87:df48:678e',
    port: 5683,
    pathname: '/test',
    method: 'PUT'
})
request.setOption('Block1', Buffer.from([6]))

request.on('response', (res) => {
    console.log('Client Received Response: ' + res.payload.toString('utf-8'))
    console.log('Client Received Response: ' + res.code)
    process.exit(0)
})
console.log('Sending large data from client...')
request.end(testBuffer)
console.log('Sent to server')

And this code (same source) to receive:

const coap = require('coap')

coap.createServer({ type: 'udp6' }, (req, res) => {
    setTimeout(() => {
        console.log('Server Received ' + req.payload.length + ' bytes')
        console.log('Sending back pleasantries')
        res.statusCode = '2.04'
        res.end('Congratulations!')
        console.log('Sent back')
    }, 500)
}).listen(() => {
    console.log('listening...')
})

Now, I am observing this behavior in wireshark:

And this error on the receiving side:

RetrySendError: No reply in 247 seconds.
at Timeout._onTimeout (/home/admin/coap/node_modules/coap/dist/lib/retry_send.js:56:25)
at listOnTimeout (node:internal/timers:581:17)
at process.processTimers (node:internal/timers:519:7)
Emitted 'error' event on OutMessage instance at:
RetrySend.emit (node:events:518:28)
at Timeout._onTimeout (/home/admin/coap/node_modules/coap/dist/lib/retry_send.js:58:22)
at listOnTimeout (node:internal/timers:581:17)
at process.processTimers (node:internal/timers:519:7) {
retransmitTimeout: 247
        inet6 fdd2:3ab9:44ef:78c5:0:ff:fe00:9c00  prefixlen 64  scopeid 0x0<global>                                                                                                       │}

While this is the stdout of the client:

Sending large data from client...
Sent to server
Client Received Response: Congratulations!
Client Received Response: 2.04

So, the reponse from the server seems to be parsed.

Do you have an idea why the packet is being retransmitted?

Start by running the client and server examples based on blockwise_put.js and compare the packet exchange with the retry reported at retry_send.js:56. Trace why the server response is parsed by the client while a retransmission remains pending. Done means the retransmission cause is explained and the block-transfer exchange no longer produces the reported timeout.