path: root/p2p/discover/udp.go

package discover

import (
    "bytes"
    "crypto/ecdsa"
    "errors"
    "fmt"
    "net"
    "time"

    "github.com/ethereum/go-ethereum/crypto"
    "github.com/ethereum/go-ethereum/logger"
    "github.com/ethereum/go-ethereum/rlp"
)

var log = logger.NewLogger("P2P Discovery")

// Errors
var (
    errPacketTooSmall = errors.New("too small")
    errBadHash        = errors.New("bad hash")
    errExpired        = errors.New("expired")
    errTimeout        = errors.New("RPC timeout")
    errClosed         = errors.New("socket closed")
)

// Timeouts
const (
    respTimeout = 300 * time.Millisecond
    sendTimeout = 300 * time.Millisecond
    expiration  = 3 * time.Second

    refreshInterval = 1 * time.Hour
)

// RPC packet types
const (
    pingPacket = iota + 1 // zero is 'reserved'
    pongPacket
    findnodePacket
    neighborsPacket
)

// RPC request structures
type (
    ping struct {
        IP         string // our IP
        Port       uint16 // our port
        Expiration uint64
    }

    // reply to Ping
    pong struct {
        ReplyTok   []byte
        Expiration uint64
    }

    findnode struct {
        // Id to look up. The responding node will send back nodes
        // closest to the target.
        Target     NodeID
        Expiration uint64
    }

    // reply to findnode
    neighbors struct {
        Nodes      []*Node
        Expiration uint64
    }
)

// udp implements the RPC protocol.
type udp struct {
    conn       *net.UDPConn
    priv       *ecdsa.PrivateKey
    addpending chan *pending
    replies    chan reply
    closing    chan struct{}

    *Table
}

// pending represents a pending reply.
//
// some implementations of the protocol wish to send more than one
// reply packet to findnode. in general, any neighbors packet cannot
// be matched up with a specific findnode packet.
//
// our implementation handles this by storing a callback function for
// each pending reply. incoming packets from a node are dispatched
// to all the callback functions for that node.
type pending struct {
    // these fields must match in the reply.
    from  NodeID
    ptype byte

    // time when the request must complete
    deadline time.Time

    // callback is called when a matching reply arrives. if it returns
    // true, the callback is removed from the pending reply queue.
    // if it returns false, the reply is considered incomplete and
    // the callback will be invoked again for the next matching reply.
    callback func(resp interface{}) (done bool)

    // errc receives nil when the callback indicates completion or an
    // error if no further reply is received within the timeout.
    errc chan<- error
}

type reply struct {
    from  NodeID
    ptype byte
    data  interface{}
}

// ListenUDP returns a new table that listens for UDP packets on laddr.
func ListenUDP(priv *ecdsa.PrivateKey, laddr string) (*Table, error) {
    net, realaddr, err := listen(priv, laddr)
    if err != nil {
        return nil, err
    }
    net.Table = newTable(net, PubkeyID(&priv.PublicKey), realaddr)
    log.Debugf("Listening on %v, my ID %x\n", realaddr, net.self.ID[:])
    return net.Table, nil
}

func listen(priv *ecdsa.PrivateKey, laddr string) (*udp, *net.UDPAddr, error) {
    addr, err := net.ResolveUDPAddr("udp", laddr)
    if err != nil {
        return nil, nil, err
    }
    conn, err := net.ListenUDP("udp", addr)
    if err != nil {
        return nil, nil, err
    }
    realaddr := conn.LocalAddr().(*net.UDPAddr)

    udp := &udp{
        conn:       conn,
        priv:       priv,
        closing:    make(chan struct{}),
        addpending: make(chan *pending),
        replies:    make(chan reply),
    }
    go udp.loop()
    go udp.readLoop()
    return udp, realaddr, nil
}

func (t *udp) close() {
    close(t.closing)
    t.conn.Close()
    // TODO: wait for the loops to end.
}

// ping sends a ping message to the given node and waits for a reply.
func (t *udp) ping(e *Node) error {
    // TODO: maybe check for ReplyTo field in callback to measure RTT
    errc := t.pending(e.ID, pongPacket, func(interface{}) bool { return true })
    t.send(e, pingPacket, ping{
        IP:         t.self.Addr.String(),
        Port:       uint16(t.self.Addr.Port),
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    return <-errc
}

// findnode sends a findnode request to the given node and waits until
// the node has sent up to k neighbors.
func (t *udp) findnode(to *Node, target NodeID) ([]*Node, error) {
    nodes := make([]*Node, 0, bucketSize)
    nreceived := 0
    errc := t.pending(to.ID, neighborsPacket, func(r interface{}) bool {
        reply := r.(*neighbors)
        for i := 0; i < len(reply.Nodes); i++ {
            nreceived++
            n := reply.Nodes[i]
            if validAddr(n.Addr) && n.ID != t.self.ID {
                nodes = append(nodes, n)
            }
        }
        return nreceived == bucketSize
    })

    t.send(to, findnodePacket, findnode{
        Target:     target,
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    err := <-errc
    return nodes, err
}

func validAddr(a *net.UDPAddr) bool {
    return !a.IP.IsMulticast() && !a.IP.IsUnspecified() && a.Port != 0
}

// pending adds a reply callback to the pending reply queue.
// see the documentation of type pending for a detailed explanation.
func (t *udp) pending(id NodeID, ptype byte, callback func(interface{}) bool) <-chan error {
    ch := make(chan error, 1)
    p := &pending{from: id, ptype: ptype, callback: callback, errc: ch}
    select {
    case t.addpending <- p:
        // loop will handle it
    case <-t.closing:
        ch <- errClosed
    }
    return ch
}

// loop runs in its own goroutin. it keeps track of
// the refresh timer and the pending reply queue.
func (t *udp) loop() {
    var (
        pending      []*pending
        nextDeadline time.Time
        timeout      = time.NewTimer(0)
        refresh      = time.NewTicker(refreshInterval)
    )
    <-timeout.C // ignore first timeout
    defer refresh.Stop()
    defer timeout.Stop()

    rearmTimeout := func() {
        if len(pending) == 0 || nextDeadline == pending[0].deadline {
            return
        }
        nextDeadline = pending[0].deadline
        timeout.Reset(nextDeadline.Sub(time.Now()))
    }

    for {
        select {
        case <-refresh.C:
            go t.refresh()

        case <-t.closing:
            for _, p := range pending {
                p.errc <- errClosed
            }
            return

        case p := <-t.addpending:
            p.deadline = time.Now().Add(respTimeout)
            pending = append(pending, p)
            rearmTimeout()

        case reply := <-t.replies:
            // run matching callbacks, remove if they return false.
            for i, p := range pending {
                if reply.from == p.from && reply.ptype == p.ptype && p.callback(reply.data) {
                    p.errc <- nil
                    copy(pending[i:], pending[i+1:])
                    pending = pending[:len(pending)-1]
                    i--
                }
            }
            rearmTimeout()

        case now := <-timeout.C:
            // notify and remove callbacks whose deadline is in the past.
            i := 0
            for ; i < len(pending) && now.After(pending[i].deadline); i++ {
                pending[i].errc <- errTimeout
            }
            if i > 0 {
                copy(pending, pending[i:])
                pending = pending[:len(pending)-i]
            }
            rearmTimeout()
        }
    }
}

const (
    macSize  = 256 / 8
    sigSize  = 520 / 8
    headSize = macSize + sigSize // space of packet frame data
)

var headSpace = make([]byte, headSize)

func (t *udp) send(to *Node, ptype byte, req interface{}) error {
    b := new(bytes.Buffer)
    b.Write(headSpace)
    b.WriteByte(ptype)
    if err := rlp.Encode(b, req); err != nil {
        log.Errorln("error encoding packet:", err)
        return err
    }

    packet := b.Bytes()
    sig, err := crypto.Sign(crypto.Sha3(packet[headSize:]), t.priv)
    if err != nil {
        log.Errorln("could not sign packet:", err)
        return err
    }
    copy(packet[macSize:], sig)
    // add the hash to the front. Note: this doesn't protect the
    // packet in any way. Our public key will be part of this hash in
    // the future.
    copy(packet, crypto.Sha3(packet[macSize:]))

    log.DebugDetailf(">>> %v %T %v\n", to.Addr, req, req)
    if _, err = t.conn.WriteToUDP(packet, to.Addr); err != nil {
        log.DebugDetailln("UDP send failed:", err)
    }
    return err
}

// readLoop runs in its own goroutine. it handles incoming UDP packets.
func (t *udp) readLoop() {
    defer t.conn.Close()
    buf := make([]byte, 4096) // TODO: good buffer size
    for {
        nbytes, from, err := t.conn.ReadFromUDP(buf)
        if err != nil {
            return
        }
        if err := t.packetIn(from, buf[:nbytes]); err != nil {
            log.Debugf("Bad packet from %v: %v\n", from, err)
        }
    }
}

func (t *udp) packetIn(from *net.UDPAddr, buf []byte) error {
    if len(buf) < headSize+1 {
        return errPacketTooSmall
    }
    hash, sig, sigdata := buf[:macSize], buf[macSize:headSize], buf[headSize:]
    shouldhash := crypto.Sha3(buf[macSize:])
    if !bytes.Equal(hash, shouldhash) {
        return errBadHash
    }
    fromID, err := recoverNodeID(crypto.Sha3(buf[headSize:]), sig)
    if err != nil {
        return err
    }

    var req interface {
        handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error
    }
    switch ptype := sigdata[0]; ptype {
    case pingPacket:
        req = new(ping)
    case pongPacket:
        req = new(pong)
    case findnodePacket:
        req = new(findnode)
    case neighborsPacket:
        req = new(neighbors)
    default:
        return fmt.Errorf("unknown type: %d", ptype)
    }
    if err := rlp.Decode(bytes.NewReader(sigdata[1:]), req); err != nil {
        return err
    }
    log.DebugDetailf("<<< %v %T %v\n", from, req, req)
    return req.handle(t, from, fromID, hash)
}

func (req *ping) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    t.mutex.Lock()
    // Note: we're ignoring the provided IP/Port right now.
    e := t.bumpOrAdd(fromID, from)
    t.mutex.Unlock()

    t.send(e, pongPacket, pong{
        ReplyTok:   mac,
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    return nil
}

func (req *pong) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    t.mutex.Lock()
    t.bump(fromID)
    t.mutex.Unlock()

    t.replies <- reply{fromID, pongPacket, req}
    return nil
}

func (req *findnode) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    t.mutex.Lock()
    e := t.bumpOrAdd(fromID, from)
    closest := t.closest(req.Target, bucketSize).entries
    t.mutex.Unlock()

    t.send(e, neighborsPacket, neighbors{
        Nodes:      closest,
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    return nil
}

func (req *neighbors) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    t.mutex.Lock()
    t.bump(fromID)
    t.add(req.Nodes)
    t.mutex.Unlock()

    t.replies <- reply{fromID, neighborsPacket, req}
    return nil
}

func expired(ts uint64) bool {
    return time.Unix(int64(ts), 0).Before(time.Now())
}