code: func (e *endpoint) HandleControlPacket(id stack.TransportEndpointID, typ stack.ControlType, extra uint32, vv buffer.VectorisedView) {
  	switch typ {
  	case stack.ControlPacketTooBig:
  		e.sndBufMu.Lock()
  		e.packetTooBigCount++
  		if v := int(extra); v < e.sndMTU {
  			e.sndMTU = v
  		}
  		e.sndBufMu.Unlock()
  
  		e.notifyProtocolGoroutine(notifyMTUChanged)
  	}
  }
docs: // HandleControlPacket implements stack.TransportEndpoint.HandleControlPacket.