sync does a handshake and then measures the latency on the network in coordination with the other side.
()
| 162 | // sync does a handshake and then measures the latency on the network in |
| 163 | // coordination with the other side. |
| 164 | func (c *conn) sync() error { |
| 165 | const ( |
| 166 | pingMsg = "syncPing" |
| 167 | warmup = 10 // minimum number of iterations to measure latency |
| 168 | giveUp = 50 // maximum number of iterations to measure latency |
| 169 | accuracy = time.Millisecond // req'd accuracy to stop early |
| 170 | goodRun = 3 // stop early if latency within accuracy this many times |
| 171 | ) |
| 172 | |
| 173 | type syncMsg struct { |
| 174 | SendT int64 // Time sent. If zero, stop. |
| 175 | RecvT int64 // Time received. If zero, fill in and respond. |
| 176 | } |
| 177 | |
| 178 | // A trivial handshake |
| 179 | if err := binary.Write(c.Conn, binary.BigEndian, []byte(pingMsg)); err != nil { |
| 180 | return err |
| 181 | } |
| 182 | var ping [8]byte |
| 183 | if err := binary.Read(c.Conn, binary.BigEndian, &ping); err != nil { |
| 184 | return err |
| 185 | } else if string(ping[:]) != pingMsg { |
| 186 | return fmt.Errorf("malformed handshake message: %v (want %q)", ping, pingMsg) |
| 187 | } |
| 188 | |
| 189 | // Both sides are alive and syncing. Calculate network delay / clock skew. |
| 190 | att := 0 |
| 191 | good := 0 |
| 192 | var latency time.Duration |
| 193 | localDone, remoteDone := false, false |
| 194 | send := true |
| 195 | for !localDone || !remoteDone { |
| 196 | if send { |
| 197 | if err := binary.Write(c.Conn, binary.BigEndian, syncMsg{SendT: now().UnixNano()}); err != nil { |
| 198 | return err |
| 199 | } |
| 200 | att++ |
| 201 | send = false |
| 202 | } |
| 203 | |
| 204 | // Block until we get a syncMsg |
| 205 | m := syncMsg{} |
| 206 | if err := binary.Read(c.Conn, binary.BigEndian, &m); err != nil { |
| 207 | return err |
| 208 | } |
| 209 | |
| 210 | if m.RecvT == 0 { |
| 211 | // Message initiated from other side. |
| 212 | if m.SendT == 0 { |
| 213 | remoteDone = true |
| 214 | continue |
| 215 | } |
| 216 | // Send response. |
| 217 | m.RecvT = now().UnixNano() |
| 218 | if err := binary.Write(c.Conn, binary.BigEndian, m); err != nil { |
| 219 | return err |
| 220 | } |
| 221 | continue |