ErrDatagramResponseMsgTooLargeDatagram

ErrDatagramResponseMsgTooLargeDatagram

Error message

datagram response error message length exceeds the length of the provided datagram

What it means

ErrDatagramResponseMsgTooLargeDatagram is returned when unmarshaling a UDPSessionRegistrationResponseDatagram whose declared error-message length (read at bytes 18:20) exceeds the bytes actually remaining after the fixed 20-byte header (data[20:]). Unlike the maximum-size variant, the length is protocol-legal but the datagram is truncated: the message body does not fit in the provided buffer. UnmarshalBinary wraps it via wrapUnmarshalErr; match with errors.Is.

Source

Thrown at quic/v3/datagram_errors.go:17

package v3

import (
	"errors"
	"fmt"
)

var (
	ErrInvalidDatagramType                 error = errors.New("invalid datagram type expected")
	ErrDatagramHeaderTooSmall              error = fmt.Errorf("datagram should have at least %d byte", datagramTypeLen)
	ErrDatagramPayloadTooLarge             error = errors.New("payload length is too large to be bundled in datagram")
	ErrDatagramPayloadHeaderTooSmall       error = errors.New("payload length is too small to fit the datagram header")
	ErrDatagramPayloadInvalidSize          error = errors.New("datagram provided is an invalid size")
	ErrDatagramResponseMsgInvalidSize      error = errors.New("datagram response message is an invalid size")
	ErrDatagramResponseInvalidSize         error = errors.New("datagram response is an invalid size")
	ErrDatagramResponseMsgTooLargeMaximum  error = fmt.Errorf("datagram response error message length exceeds the length of the datagram maximum: %d", maxResponseErrorMessageLen)
	ErrDatagramResponseMsgTooLargeDatagram error = fmt.Errorf("datagram response error message length exceeds the length of the provided datagram")
	ErrDatagramICMPPayloadTooLarge         error = fmt.Errorf("datagram icmp payload exceeds %d bytes", maxICMPPayloadLen)
	ErrDatagramICMPPayloadMissing          error = errors.New("datagram icmp payload is missing")
)

func wrapMarshalErr(err error) error {
	return fmt.Errorf("datagram marshal error: %w", err)
}

func wrapUnmarshalErr(err error) error {
	return fmt.Errorf("datagram unmarshal error: %w", err)
}

View on GitHub (pinned to 2253eeeb25)

Solutions

Before unmarshaling, check len(data) >= 20 + declared message length and reject truncated buffers.
Use errors.Is(err, v3.ErrDatagramResponseMsgTooLargeDatagram) to detect truncated datagrams and request retransmission or drop.
Audit the read path that produced the buffer for off-by-one slicing that dropped trailing bytes.
Check for network equipment (MTU, UDP fragmentation) truncating datagrams.

Example fix

// before
err := resp.UnmarshalBinary(buf)
// after
if len(buf) >= 20 {
    msgLen := int(binary.BigEndian.Uint16(buf[18:20]))
    if len(buf[20:]) < msgLen {
        return fmt.Errorf("truncated response datagram: need %d, have %d", msgLen, len(buf)-20)
    }
}
err := resp.UnmarshalBinary(buf)

Defensive patterns

Strategy: validation

Validate before calling

func isCompleteResponseDatagram(data []byte) bool {
    if len(data) < 20 {
        return false
    }
    msgLen := int(binary.BigEndian.Uint16(data[18:20]))
    return len(data[20:]) >= msgLen
}

Try / catch

if errors.Is(err, v3.ErrDatagramResponseMsgTooLargeDatagram) {
    logger.Warn().Msg("truncated response datagram; discarding")
    return nil
}

Prevention

Verify datagram completeness (header + declared message length) before unmarshaling.
Audit buffer slicing for off-by-one errors that drop trailing bytes.
Monitor for MTU/fragmentation issues if truncation recurs in production.

When it happens

Trigger: Calling UnmarshalBinary on a UDPSessionRegistrationResponseDatagram where int(errMsgLen) > len(data[20:]) — the datagram was cut short after the header, or the length field was corrupted upward.

Common situations: MTU/truncation issues chopping datagrams in transit; buffer-slicing bugs upstream passing a partial payload; version mismatch where the peer writes a longer message format than expected; test fixtures with deliberately truncated payloads.

Related errors

AI-assisted analysis of cloudflare/cloudflared@2253eeeb25 (2026-09-06). Data as JSON: /api/errors/14e52dde5adca4ad. Report an issue: GitHub.