CoAP Block-Wise Transfer Fails — Complete Guide
DodaTech
Updated 2026-06-26
1 min read
In this tutorial, you'll learn about CoAP Block. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.
The Problem
Large CoAP payloads fail to transfer completely using Block-wise transfer.
Quick Fix
Wrong
res.end(largePayload) # Exceeds UDP datagram size (64 KB)
Response truncated at the UDP datagram boundary.
Right
const coap = require('coap')
// Server handles block-wise
const server = coap.createServer()
server.on('request', (req, res) => {
// Large response (e.g., firmware image)
const firmware = getFirmwareBinary()
// CoAP library handles Block2 automatically
// Client MUST request with Block2 option
res.setOption('Block2', [new Buffer([0x00])])
res.code = '2.05'
res.end(firmware)
})
server.listen(5683)
// Client requests with block-wise
const req = coap.request({
pathname: '/firmware',
block2: { szx: 2 } // Request 1024-byte blocks
})
let received = Buffer.alloc(0)
req.on('response', (res) => {
received = Buffer.concat([received, res.payload])
if (res.code === '2.05' && res.headers['block2'].more) {
// Request next block
req.emit('request', req)
} else {
console.log('Firmware received:', received.length, 'bytes')
}
})
Large firmware (e.g., 100 KB) transferred in 1 KB blocks.
Prevention
CoAP Block-wise transfer (RFC 7959) splits large payloads into 2^SZX byte blocks (16, 32, 64, 128, 256, 512, 1024). Block1: request body (POST/PUT). Block2: response body (GET). The MORE flag indicates more blocks follow. MAX_BLOCK_SZX is typically 6 (1024 bytes).
DodaTech engineers apply these same patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro for production IoT reliability.
FAQ
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