122 lines
3.4 KiB
Go
122 lines
3.4 KiB
Go
/*
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This is the local node discovery protocol. In principle we might be better
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served by something more standardized, such as mDNS / DNS-SD. In practice, this
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was much easier and quicker to get up and running.
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The mode of operation is to periodically (currently once every 30 seconds)
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transmit a broadcast UDP packet to the well known port number 21025. The packet
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has the following format:
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Magic Number |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Port Number | Length of NodeID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ NodeID (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The sending node's address is not encoded -- it is taken to be the source
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address of the announcement. Every time such a packet is received, a local
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table that maps NodeID to Address is updated. When the local node wants to
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connect to another node with the address specification 'dynamic', this table is
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consulted.
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*/
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package discover
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import (
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"encoding/binary"
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"fmt"
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"net"
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"sync"
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"time"
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)
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type Discoverer struct {
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MyID string
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ListenPort int
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BroadcastIntv time.Duration
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conn *net.UDPConn
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registry map[string]string
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registryLock sync.RWMutex
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}
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func NewDiscoverer(id string, port int) (*Discoverer, error) {
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local4 := &net.UDPAddr{IP: net.IP{0, 0, 0, 0}, Port: 21025}
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conn, err := net.ListenUDP("udp4", local4)
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if err != nil {
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return nil, err
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}
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disc := &Discoverer{
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MyID: id,
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ListenPort: port,
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BroadcastIntv: 30 * time.Second,
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conn: conn,
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registry: make(map[string]string),
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}
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go disc.sendAnnouncements()
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go disc.recvAnnouncements()
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return disc, nil
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}
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func (d *Discoverer) sendAnnouncements() {
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remote4 := &net.UDPAddr{IP: net.IP{255, 255, 255, 255}, Port: 21025}
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idbs := []byte(d.MyID)
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buf := make([]byte, 4+4+4+len(idbs))
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binary.BigEndian.PutUint32(buf, uint32(0x121025))
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binary.BigEndian.PutUint16(buf[4:], uint16(d.ListenPort))
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binary.BigEndian.PutUint16(buf[6:], uint16(len(idbs)))
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copy(buf[8:], idbs)
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for {
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_, _, err := d.conn.WriteMsgUDP(buf, nil, remote4)
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if err != nil {
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panic(err)
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}
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time.Sleep(d.BroadcastIntv)
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}
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}
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func (d *Discoverer) recvAnnouncements() {
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var buf = make([]byte, 1024)
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for {
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_, addr, err := d.conn.ReadFromUDP(buf)
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if err != nil {
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panic(err)
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}
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magic := binary.BigEndian.Uint32(buf)
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if magic != 0x121025 {
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continue
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}
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port := binary.BigEndian.Uint16(buf[4:])
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l := binary.BigEndian.Uint16(buf[6:])
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idbs := buf[8 : l+8]
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id := string(idbs)
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if id != d.MyID {
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nodeAddr := fmt.Sprintf("%s:%d", addr.IP.String(), port)
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d.registryLock.Lock()
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if d.registry[id] != nodeAddr {
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d.registry[id] = nodeAddr
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}
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d.registryLock.Unlock()
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}
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}
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}
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func (d *Discoverer) Lookup(node string) (string, bool) {
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d.registryLock.Lock()
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defer d.registryLock.Unlock()
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addr, ok := d.registry[node]
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return addr, ok
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}
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