torrent/dht/msg.go

package dht

import (
	"fmt"

	"github.com/anacrolix/torrent/util"
)

// Msg represents messages that nodes in the network send to each other as specified by the protocol.
// They are also refered to as the KRPC messages.
// There are three types of messages: QUERY, RESPONSE, ERROR
// The message is a dictonary that is then
// "bencoded" (serialization & compression format adopted by the BitTorrent)
// and sent via the UDP connection to peers.
//
// A KRPC message is a single dictionary with two keys common to every message and additional keys depending on the type of message.
// Every message has a key "t" with a string value representing a transaction ID.
// This transaction ID is generated by the querying node and is echoed in the response, so responses
// may be correlated with multiple queries to the same node. The transaction ID should be encoded as a short string of binary numbers, typically 2 characters are enough as they cover 2^16 outstanding queries. The other key contained in every KRPC message is "y" with a single character value describing the type of message. The value of the "y" key is one of "q" for query, "r" for response, or "e" for error.
// 3 message types:  QUERY, RESPONSE, ERROR
type Msg struct {
	Q string `bencode:"q,omitempty"` //  method name of the query (on of 4: "ping", "find_node", "get_peers", "announce_peer")
	A *struct {
		ID       string `bencode:"id"`        // ID of the quirying Node
		InfoHash string `bencode:"info_hash"` // InfoHash of the torrent
		Target   string `bencode:"target"`    // ID of the node sought
	} `bencode:"a,omitempty"` // named arguments sent with a query
	T string     `bencode:"t"`           // required: transaction ID
	Y string     `bencode:"y"`           // required: type of the message: q for QUERY, r for RESPONSE, e for ERROR
	R *Return    `bencode:"r,omitempty"` // RESPONSE type only
	E *KRPCError `bencode:"e,omitempty"` // ERROR type only
}

type Return struct {
	ID     string              `bencode:"id"` // ID of the querying node
	Nodes  CompactIPv4NodeInfo `bencode:"nodes,omitempty"`
	Token  string              `bencode:"token"`
	Values []util.CompactPeer  `bencode:"values,omitempty"`
}

var _ fmt.Stringer = Msg{}

func (m Msg) String() string {
	return fmt.Sprintf("%#v", m)
}

// The node ID of the source of this Msg.
func (m Msg) SenderID() string {
	switch m.Y {
	case "q":
		return m.A.ID
	case "r":
		return m.R.ID
	}
	return ""
}

func (m Msg) Error() *KRPCError {
	if m.Y != "e" {
		return nil
	}
	return m.E
}