Turns out internet MTU must be honored, otherwise packets are dropped/not reassembled. Setting to 1500 bytes for transfer messages.

protocol/Message Encoding Transfer.go

@@ -76,7 +76,7 @@ func DecodeTransfer(msg *MessageRaw) (result *MessageTransfer, err error) {
 }
 
 // TransferMaxEmbedSize is the maximum size of embedded data inside the Transfer message.
-const TransferMaxEmbedSize = udpMaxPacketSize - PacketLengthMin - transferPayloadHeaderSize
+const TransferMaxEmbedSize = internetSafeMTU - PacketLengthMin - transferPayloadHeaderSize
 
 // EncodeTransfer encodes a transfer message. The embedded packet size must be smaller than TransferMaxEmbedSize.
 func EncodeTransfer(senderPrivateKey *btcec.PrivateKey, data []byte, control, transferProtocol uint8, hash []byte, offset, limit uint64) (packetRaw []byte, err error) {

protocol/Message Encoding.go

@@ -26,6 +26,9 @@ type MessageRaw struct {
 // However, due to the MTU soft limit and fragmentation, packets should be as small as possible.
 const udpMaxPacketSize = 65507
 
+// internetSafeMTU is a value relatively safe to use for transmitting over the internet
+const internetSafeMTU = 1500 - 20 - 8
+
 // isPacketSizeExceed checks if the max packet size would be exceeded with the payload
 func isPacketSizeExceed(currentSize int, testSize int) bool {
 	return currentSize+testSize > udpMaxPacketSize-PacketLengthMin