diff --git a/peer-conn-msg-writer.go b/peer-conn-msg-writer.go
new file mode 100644
index 00000000..809d85fc
--- /dev/null
+++ b/peer-conn-msg-writer.go
@@ -0,0 +1,133 @@
+package torrent
+
+import (
+	"bytes"
+	"io"
+	"time"
+
+	"github.com/anacrolix/log"
+	"github.com/anacrolix/sync"
+
+	"github.com/anacrolix/torrent/internal/chansync"
+	pp "github.com/anacrolix/torrent/peer_protocol"
+)
+
+func (pc *PeerConn) startWriter() {
+	w := &pc.messageWriter
+	*w = peerConnMsgWriter{
+		fillWriteBuffer: func() {
+			pc.locker().Lock()
+			defer pc.locker().Unlock()
+			if pc.closed.IsSet() {
+				return
+			}
+			pc.fillWriteBuffer()
+		},
+		closed: &pc.closed,
+		logger: pc.logger,
+		w:      pc.w,
+		keepAlive: func() bool {
+			pc.locker().Lock()
+			defer pc.locker().Unlock()
+			return pc.useful()
+		},
+		writeBuffer: new(bytes.Buffer),
+	}
+	go func() {
+		defer pc.locker().Unlock()
+		defer pc.close()
+		defer pc.locker().Lock()
+		pc.messageWriter.run(time.Minute)
+	}()
+}
+
+type peerConnMsgWriter struct {
+	// Must not be called with the local mutex held, as it will call back into the write method.
+	fillWriteBuffer func()
+	closed          *chansync.SetOnce
+	logger          log.Logger
+	w               io.Writer
+	keepAlive       func() bool
+
+	mu        sync.Mutex
+	writeCond chansync.BroadcastCond
+	// Pointer so we can swap with the "front buffer".
+	writeBuffer *bytes.Buffer
+}
+
+// Routine that writes to the peer. Some of what to write is buffered by
+// activity elsewhere in the Client, and some is determined locally when the
+// connection is writable.
+func (cn *peerConnMsgWriter) run(keepAliveTimeout time.Duration) {
+	var (
+		lastWrite      time.Time = time.Now()
+		keepAliveTimer *time.Timer
+	)
+	keepAliveTimer = time.AfterFunc(keepAliveTimeout, func() {
+		cn.mu.Lock()
+		defer cn.mu.Unlock()
+		if time.Since(lastWrite) >= keepAliveTimeout {
+			cn.writeCond.Broadcast()
+		}
+		keepAliveTimer.Reset(keepAliveTimeout)
+	})
+	cn.mu.Lock()
+	defer cn.mu.Unlock()
+	defer keepAliveTimer.Stop()
+	frontBuf := new(bytes.Buffer)
+	for {
+		if cn.closed.IsSet() {
+			return
+		}
+		if cn.writeBuffer.Len() == 0 {
+			func() {
+				cn.mu.Unlock()
+				defer cn.mu.Lock()
+				cn.fillWriteBuffer()
+			}()
+		}
+		if cn.writeBuffer.Len() == 0 && time.Since(lastWrite) >= keepAliveTimeout && cn.keepAlive() {
+			cn.writeBuffer.Write(pp.Message{Keepalive: true}.MustMarshalBinary())
+			torrent.Add("written keepalives", 1)
+		}
+		if cn.writeBuffer.Len() == 0 {
+			writeCond := cn.writeCond.Signaled()
+			cn.mu.Unlock()
+			select {
+			case <-cn.closed.Done():
+			case <-writeCond:
+			}
+			cn.mu.Lock()
+			continue
+		}
+		// Flip the buffers.
+		frontBuf, cn.writeBuffer = cn.writeBuffer, frontBuf
+		cn.mu.Unlock()
+		n, err := cn.w.Write(frontBuf.Bytes())
+		cn.mu.Lock()
+		if n != 0 {
+			lastWrite = time.Now()
+			keepAliveTimer.Reset(keepAliveTimeout)
+		}
+		if err != nil {
+			cn.logger.WithDefaultLevel(log.Debug).Printf("error writing: %v", err)
+			return
+		}
+		if n != frontBuf.Len() {
+			panic("short write")
+		}
+		frontBuf.Reset()
+	}
+}
+
+func (cn *peerConnMsgWriter) write(msg pp.Message) bool {
+	cn.mu.Lock()
+	defer cn.mu.Unlock()
+	cn.writeBuffer.Write(msg.MustMarshalBinary())
+	cn.writeCond.Broadcast()
+	return !cn.writeBufferFull()
+}
+
+func (cn *peerConnMsgWriter) writeBufferFull() bool {
+	return cn.writeBuffer.Len() >= writeBufferHighWaterLen
+}