quic/conn_send.go - net - Git at Google

 // Copyright 2023 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 //go:build go1.21

 package quic

 import (
 	"crypto/tls"
 	"errors"
 	"time"
 )

 // maybeSend sends datagrams, if possible.
 //
 // If sending is blocked by pacing, it returns the next time
 // a datagram may be sent.
 //
 // If sending is blocked indefinitely, it returns the zero Time.
 func (c *Conn) maybeSend(now time.Time) (next time.Time) {
 	// Assumption: The congestion window is not underutilized.
 	// If congestion control, pacing, and anti-amplification all permit sending,
 	// but we have no packet to send, then we will declare the window underutilized.
 	underutilized := false
 	defer func() {
 		c.loss.cc.setUnderutilized(c.log, underutilized)
 	}()

 	// Send one datagram on each iteration of this loop,
 	// until we hit a limit or run out of data to send.
 	//
 	// For each number space where we have write keys,
 	// attempt to construct a packet in that space.
 	// If the packet contains no frames (we have no data in need of sending),
 	// abandon the packet.
 	//
 	// Speculatively constructing packets means we don't need
 	// separate code paths for "do we have data to send?" and
 	// "send the data" that need to be kept in sync.
 	for {
 		limit, next := c.loss.sendLimit(now)
 		if limit == ccBlocked {
 			// If anti-amplification blocks sending, then no packet can be sent.
 			return next
 		}
 		if !c.sendOK(now) {
 			return time.Time{}
 		}
 		// We may still send ACKs, even if congestion control or pacing limit sending.

 		// Prepare to write a datagram of at most maxSendSize bytes.
 		c.w.reset(c.loss.maxSendSize())

 		dstConnID, ok := c.connIDState.dstConnID()
 		if !ok {
 			// It is currently not possible for us to end up without a connection ID,
 			// but handle the case anyway.
 			return time.Time{}
 		}

 		// Initial packet.
 		pad := false
 		var sentInitial *sentPacket
 		if c.keysInitial.canWrite() {
 			pnumMaxAcked := c.loss.spaces[initialSpace].maxAcked
 			pnum := c.loss.nextNumber(initialSpace)
 			p := longPacket{
 				ptype:     packetTypeInitial,
 				version:   quicVersion1,
 				num:       pnum,
 				dstConnID: dstConnID,
 				srcConnID: c.connIDState.srcConnID(),
 				extra:     c.retryToken,
 			}
 			c.w.startProtectedLongHeaderPacket(pnumMaxAcked, p)
 			c.appendFrames(now, initialSpace, pnum, limit)
 			if logPackets {
 				logSentPacket(c, packetTypeInitial, pnum, p.srcConnID, p.dstConnID, c.w.payload())
 			}
 			if c.logEnabled(QLogLevelPacket) && len(c.w.payload()) > 0 {
 				c.logPacketSent(packetTypeInitial, pnum, p.srcConnID, p.dstConnID, c.w.packetLen(), c.w.payload())
 			}
 			sentInitial = c.w.finishProtectedLongHeaderPacket(pnumMaxAcked, c.keysInitial.w, p)
 			if sentInitial != nil {
 				// Client initial packets and ack-eliciting server initial packaets
 				// need to be sent in a datagram padded to at least 1200 bytes.
 				// We can't add the padding yet, however, since we may want to
 				// coalesce additional packets with this one.
 				if c.side == clientSide || sentInitial.ackEliciting {
 					pad = true
 				}
 			}
 		}

 		// Handshake packet.
 		if c.keysHandshake.canWrite() {
 			pnumMaxAcked := c.loss.spaces[handshakeSpace].maxAcked
 			pnum := c.loss.nextNumber(handshakeSpace)
 			p := longPacket{
 				ptype:     packetTypeHandshake,
 				version:   quicVersion1,
 				num:       pnum,
 				dstConnID: dstConnID,
 				srcConnID: c.connIDState.srcConnID(),
 			}
 			c.w.startProtectedLongHeaderPacket(pnumMaxAcked, p)
 			c.appendFrames(now, handshakeSpace, pnum, limit)
 			if logPackets {
 				logSentPacket(c, packetTypeHandshake, pnum, p.srcConnID, p.dstConnID, c.w.payload())
 			}
 			if c.logEnabled(QLogLevelPacket) && len(c.w.payload()) > 0 {
 				c.logPacketSent(packetTypeHandshake, pnum, p.srcConnID, p.dstConnID, c.w.packetLen(), c.w.payload())
 			}
 			if sent := c.w.finishProtectedLongHeaderPacket(pnumMaxAcked, c.keysHandshake.w, p); sent != nil {
 				c.packetSent(now, handshakeSpace, sent)
 				if c.side == clientSide {
 					// "[...] a client MUST discard Initial keys when it first
 					// sends a Handshake packet [...]"
 					// https://www.rfc-editor.org/rfc/rfc9001.html#section-4.9.1-2
 					c.discardKeys(now, initialSpace)
 				}
 			}
 		}

 		// 1-RTT packet.
 		if c.keysAppData.canWrite() {
 			pnumMaxAcked := c.loss.spaces[appDataSpace].maxAcked
 			pnum := c.loss.nextNumber(appDataSpace)
 			c.w.start1RTTPacket(pnum, pnumMaxAcked, dstConnID)
 			c.appendFrames(now, appDataSpace, pnum, limit)
 			if pad && len(c.w.payload()) > 0 {
 				// 1-RTT packets have no length field and extend to the end
 				// of the datagram, so if we're sending a datagram that needs
 				// padding we need to add it inside the 1-RTT packet.
 				c.w.appendPaddingTo(paddedInitialDatagramSize)
 				pad = false
 			}
 			if logPackets {
 				logSentPacket(c, packetType1RTT, pnum, nil, dstConnID, c.w.payload())
 			}
 			if c.logEnabled(QLogLevelPacket) && len(c.w.payload()) > 0 {
 				c.logPacketSent(packetType1RTT, pnum, nil, dstConnID, c.w.packetLen(), c.w.payload())
 			}
 			if sent := c.w.finish1RTTPacket(pnum, pnumMaxAcked, dstConnID, &c.keysAppData); sent != nil {
 				c.packetSent(now, appDataSpace, sent)
 			}
 		}

 		buf := c.w.datagram()
 		if len(buf) == 0 {
 			if limit == ccOK {
 				// We have nothing to send, and congestion control does not
 				// block sending. The congestion window is underutilized.
 				underutilized = true
 			}
 			return next
 		}

 		if sentInitial != nil {
 			if pad {
 				// Pad out the datagram with zeros, coalescing the Initial
 				// packet with invalid packets that will be ignored by the peer.
 				// https://www.rfc-editor.org/rfc/rfc9000.html#section-14.1-1
 				for len(buf) < paddedInitialDatagramSize {
 					buf = append(buf, 0)
 					// Technically this padding isn't in any packet, but
 					// account it to the Initial packet in this datagram
 					// for purposes of flow control and loss recovery.
 					sentInitial.size++
 					sentInitial.inFlight = true
 				}
 			}
 			// If we're a client and this Initial packet is coalesced
 			// with a Handshake packet, then we've discarded Initial keys
 			// since constructing the packet and shouldn't record it as in-flight.
 			if c.keysInitial.canWrite() {
 				c.packetSent(now, initialSpace, sentInitial)
 			}
 		}

 		c.endpoint.sendDatagram(datagram{
 			b:        buf,
 			peerAddr: c.peerAddr,
 		})
 	}
 }

 func (c *Conn) packetSent(now time.Time, space numberSpace, sent *sentPacket) {
 	c.idleHandlePacketSent(now, sent)
 	c.loss.packetSent(now, c.log, space, sent)
 }

 func (c *Conn) appendFrames(now time.Time, space numberSpace, pnum packetNumber, limit ccLimit) {
 	if c.lifetime.localErr != nil {
 		c.appendConnectionCloseFrame(now, space, c.lifetime.localErr)
 		return
 	}

 	shouldSendAck := c.acks[space].shouldSendAck(now)
 	if limit != ccOK {
 		// ACKs are not limited by congestion control.
 		if shouldSendAck && c.appendAckFrame(now, space) {
 			c.acks[space].sentAck()
 		}
 		return
 	}
 	// We want to send an ACK frame if the ack controller wants to send a frame now,
 	// OR if we are sending a packet anyway and have ack-eliciting packets which we
 	// have not yet acked.
 	//
 	// We speculatively add ACK frames here, to put them at the front of the packet
 	// to avoid truncation.
 	//
 	// After adding all frames, if we don't need to send an ACK frame and have not
 	// added any other frames, we abandon the packet.
 	if c.appendAckFrame(now, space) {
 		defer func() {
 			// All frames other than ACK and PADDING are ack-eliciting,
 			// so if the packet is ack-eliciting we've added additional
 			// frames to it.
 			if !shouldSendAck && !c.w.sent.ackEliciting {
 				// There's nothing in this packet but ACK frames, and
 				// we don't want to send an ACK-only packet at this time.
 				// Abandoning the packet means we wrote an ACK frame for
 				// nothing, but constructing the frame is cheap.
 				c.w.abandonPacket()
 				return
 			}
 			// Either we are willing to send an ACK-only packet,
 			// or we've added additional frames.
 			c.acks[space].sentAck()
 			if !c.w.sent.ackEliciting && c.shouldMakePacketAckEliciting() {
 				c.w.appendPingFrame()
 			}
 		}()
 	}
 	if limit != ccOK {
 		return
 	}
 	pto := c.loss.ptoExpired

 	// TODO: Add all the other frames we can send.

 	// CRYPTO
 	c.crypto[space].dataToSend(pto, func(off, size int64) int64 {
 		b, _ := c.w.appendCryptoFrame(off, int(size))
 		c.crypto[space].sendData(off, b)
 		return int64(len(b))
 	})

 	// Test-only PING frames.
 	if space == c.testSendPingSpace && c.testSendPing.shouldSendPTO(pto) {
 		if !c.w.appendPingFrame() {
 			return
 		}
 		c.testSendPing.setSent(pnum)
 	}

 	if space == appDataSpace {
 		// HANDSHAKE_DONE
 		if c.handshakeConfirmed.shouldSendPTO(pto) {
 			if !c.w.appendHandshakeDoneFrame() {
 				return
 			}
 			c.handshakeConfirmed.setSent(pnum)
 		}

 		// NEW_CONNECTION_ID, RETIRE_CONNECTION_ID
 		if !c.connIDState.appendFrames(c, pnum, pto) {
 			return
 		}

 		// PATH_RESPONSE
 		if pad, ok := c.appendPathFrames(); !ok {
 			return
 		} else if pad {
 			defer c.w.appendPaddingTo(smallestMaxDatagramSize)
 		}

 		// All stream-related frames. This should come last in the packet,
 		// so large amounts of STREAM data don't crowd out other frames
 		// we may need to send.
 		if !c.appendStreamFrames(&c.w, pnum, pto) {
 			return
 		}

 		if !c.appendKeepAlive(now) {
 			return
 		}
 	}

 	// If this is a PTO probe and we haven't added an ack-eliciting frame yet,
 	// add a PING to make this an ack-eliciting probe.
 	//
 	// Technically, there are separate PTO timers for each number space.
 	// When a PTO timer expires, we MUST send an ack-eliciting packet in the
 	// timer's space. We SHOULD send ack-eliciting packets in every other space
 	// with in-flight data. (RFC 9002, section 6.2.4)
 	//
 	// What we actually do is send a single datagram containing an ack-eliciting packet
 	// for every space for which we have keys.
 	//
 	// We fill the PTO probe packets with new or unacknowledged data. For example,
 	// a PTO probe sent for the Initial space will generally retransmit previously
 	// sent but unacknowledged CRYPTO data.
 	//
 	// When sending a PTO probe datagram containing multiple packets, it is
 	// possible that an earlier packet will fill up the datagram, leaving no
 	// space for the remaining probe packet(s). This is not a problem in practice.
 	//
 	// A client discards Initial keys when it first sends a Handshake packet
 	// (RFC 9001 Section 4.9.1). Handshake keys are discarded when the handshake
 	// is confirmed (RFC 9001 Section  4.9.2). The PTO timer is not set for the
 	// Application Data packet number space until the handshake is confirmed
 	// (RFC 9002 Section 6.2.1). Therefore, the only times a PTO probe can fire
 	// while data for multiple spaces is in flight are:
 	//
 	// - a server's Initial or Handshake timers can fire while Initial and Handshake
 	//   data is in flight; and
 	//
 	// - a client's Handshake timer can fire while Handshake and Application Data
 	//   data is in flight.
 	//
 	// It is theoretically possible for a server's Initial CRYPTO data to overflow
 	// the maximum datagram size, but unlikely in practice; this space contains
 	// only the ServerHello TLS message, which is small. It's also unlikely that
 	// the Handshake PTO probe will fire while Initial data is in flight (this
 	// requires not just that the Initial CRYPTO data completely fill a datagram,
 	// but a quite specific arrangement of lost and retransmitted packets.)
 	// We don't bother worrying about this case here, since the worst case is
 	// that we send a PTO probe for the in-flight Initial data and drop the
 	// Handshake probe.
 	//
 	// If a client's Handshake PTO timer fires while Application Data data is in
 	// flight, it is possible that the resent Handshake CRYPTO data will crowd
 	// out the probe for the Application Data space. However, since this probe is
 	// optional (recall that the Application Data PTO timer is never set until
 	// after Handshake keys have been discarded), dropping it is acceptable.
 	if pto && !c.w.sent.ackEliciting {
 		c.w.appendPingFrame()
 	}
 }

 // shouldMakePacketAckEliciting is called when sending a packet containing nothing but an ACK frame.
 // It reports whether we should add a PING frame to the packet to make it ack-eliciting.
 func (c *Conn) shouldMakePacketAckEliciting() bool {
 	if c.keysAppData.needAckEliciting() {
 		// The peer has initiated a key update.
 		// We haven't sent them any packets yet in the new phase.
 		// Make this an ack-eliciting packet.
 		// Their ack of this packet will complete the key update.
 		return true
 	}
 	if c.loss.consecutiveNonAckElicitingPackets >= 19 {
 		// We've sent a run of non-ack-eliciting packets.
 		// Add in an ack-eliciting one every once in a while so the peer
 		// lets us know which ones have arrived.
 		//
 		// Google QUICHE injects a PING after sending 19 packets. We do the same.
 		//
 		// https://www.rfc-editor.org/rfc/rfc9000#section-13.2.4-2
 		return true
 	}
 	// TODO: Consider making every packet sent when in PTO ack-eliciting to speed up recovery.
 	return false
 }

 func (c *Conn) appendAckFrame(now time.Time, space numberSpace) bool {
 	seen, delay := c.acks[space].acksToSend(now)
 	if len(seen) == 0 {
 		return false
 	}
 	d := unscaledAckDelayFromDuration(delay, ackDelayExponent)
 	return c.w.appendAckFrame(seen, d)
 }

 func (c *Conn) appendConnectionCloseFrame(now time.Time, space numberSpace, err error) {
 	c.sentConnectionClose(now)
 	switch e := err.(type) {
 	case localTransportError:
 		c.w.appendConnectionCloseTransportFrame(e.code, 0, e.reason)
 	case *ApplicationError:
 		if space != appDataSpace {
 			// "CONNECTION_CLOSE frames signaling application errors (type 0x1d)
 			// MUST only appear in the application data packet number space."
 			// https://www.rfc-editor.org/rfc/rfc9000#section-12.5-2.2
 			c.w.appendConnectionCloseTransportFrame(errApplicationError, 0, "")
 		} else {
 			c.w.appendConnectionCloseApplicationFrame(e.Code, e.Reason)
 		}
 	default:
 		// TLS alerts are sent using error codes [0x0100,0x01ff).
 		// https://www.rfc-editor.org/rfc/rfc9000#section-20.1-2.36.1
 		var alert tls.AlertError
 		switch {
 		case errors.As(err, &alert):
 			// tls.AlertError is a uint8, so this can't exceed 0x01ff.
 			code := errTLSBase + transportError(alert)
 			c.w.appendConnectionCloseTransportFrame(code, 0, "")
 		default:
 			c.w.appendConnectionCloseTransportFrame(errInternal, 0, "")
 		}
 	}
 }
	// Copyright 2023 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	//go:build go1.21

	package quic

	import (
	"crypto/tls"
	"errors"
	"time"
	)

	// maybeSend sends datagrams, if possible.
	//
	// If sending is blocked by pacing, it returns the next time
	// a datagram may be sent.
	//
	// If sending is blocked indefinitely, it returns the zero Time.
	func (c *Conn) maybeSend(now time.Time) (next time.Time) {
	// Assumption: The congestion window is not underutilized.
	// If congestion control, pacing, and anti-amplification all permit sending,
	// but we have no packet to send, then we will declare the window underutilized.
	underutilized := false
	defer func() {
	c.loss.cc.setUnderutilized(c.log, underutilized)
	}()

	// Send one datagram on each iteration of this loop,
	// until we hit a limit or run out of data to send.
	//
	// For each number space where we have write keys,
	// attempt to construct a packet in that space.
	// If the packet contains no frames (we have no data in need of sending),
	// abandon the packet.
	//
	// Speculatively constructing packets means we don't need
	// separate code paths for "do we have data to send?" and
	// "send the data" that need to be kept in sync.
	for {
	limit, next := c.loss.sendLimit(now)
	if limit == ccBlocked {
	// If anti-amplification blocks sending, then no packet can be sent.
	return next
	}
	if !c.sendOK(now) {
	return time.Time{}
	}
	// We may still send ACKs, even if congestion control or pacing limit sending.

	// Prepare to write a datagram of at most maxSendSize bytes.
	c.w.reset(c.loss.maxSendSize())

	dstConnID, ok := c.connIDState.dstConnID()
	if !ok {
	// It is currently not possible for us to end up without a connection ID,
	// but handle the case anyway.
	return time.Time{}
	}

	// Initial packet.
	pad := false
	var sentInitial *sentPacket
	if c.keysInitial.canWrite() {
	pnumMaxAcked := c.loss.spaces[initialSpace].maxAcked
	pnum := c.loss.nextNumber(initialSpace)
	p := longPacket{
	ptype: packetTypeInitial,
	version: quicVersion1,
	num: pnum,
	dstConnID: dstConnID,
	srcConnID: c.connIDState.srcConnID(),
	extra: c.retryToken,
	}
	c.w.startProtectedLongHeaderPacket(pnumMaxAcked, p)
	c.appendFrames(now, initialSpace, pnum, limit)
	if logPackets {
	logSentPacket(c, packetTypeInitial, pnum, p.srcConnID, p.dstConnID, c.w.payload())
	}
	if c.logEnabled(QLogLevelPacket) && len(c.w.payload()) > 0 {
	c.logPacketSent(packetTypeInitial, pnum, p.srcConnID, p.dstConnID, c.w.packetLen(), c.w.payload())
	}
	sentInitial = c.w.finishProtectedLongHeaderPacket(pnumMaxAcked, c.keysInitial.w, p)
	if sentInitial != nil {
	// Client initial packets and ack-eliciting server initial packaets
	// need to be sent in a datagram padded to at least 1200 bytes.
	// We can't add the padding yet, however, since we may want to
	// coalesce additional packets with this one.
	if c.side == clientSide \|\| sentInitial.ackEliciting {
	pad = true
	}
	}
	}

	// Handshake packet.
	if c.keysHandshake.canWrite() {
	pnumMaxAcked := c.loss.spaces[handshakeSpace].maxAcked
	pnum := c.loss.nextNumber(handshakeSpace)
	p := longPacket{
	ptype: packetTypeHandshake,
	version: quicVersion1,
	num: pnum,
	dstConnID: dstConnID,
	srcConnID: c.connIDState.srcConnID(),
	}
	c.w.startProtectedLongHeaderPacket(pnumMaxAcked, p)
	c.appendFrames(now, handshakeSpace, pnum, limit)
	if logPackets {
	logSentPacket(c, packetTypeHandshake, pnum, p.srcConnID, p.dstConnID, c.w.payload())
	}
	if c.logEnabled(QLogLevelPacket) && len(c.w.payload()) > 0 {
	c.logPacketSent(packetTypeHandshake, pnum, p.srcConnID, p.dstConnID, c.w.packetLen(), c.w.payload())
	}
	if sent := c.w.finishProtectedLongHeaderPacket(pnumMaxAcked, c.keysHandshake.w, p); sent != nil {
	c.packetSent(now, handshakeSpace, sent)
	if c.side == clientSide {
	// "[...] a client MUST discard Initial keys when it first
	// sends a Handshake packet [...]"
	// https://www.rfc-editor.org/rfc/rfc9001.html#section-4.9.1-2
	c.discardKeys(now, initialSpace)
	}
	}
	}

	// 1-RTT packet.
	if c.keysAppData.canWrite() {
	pnumMaxAcked := c.loss.spaces[appDataSpace].maxAcked
	pnum := c.loss.nextNumber(appDataSpace)
	c.w.start1RTTPacket(pnum, pnumMaxAcked, dstConnID)
	c.appendFrames(now, appDataSpace, pnum, limit)
	if pad && len(c.w.payload()) > 0 {
	// 1-RTT packets have no length field and extend to the end
	// of the datagram, so if we're sending a datagram that needs
	// padding we need to add it inside the 1-RTT packet.
	c.w.appendPaddingTo(paddedInitialDatagramSize)
	pad = false
	}
	if logPackets {
	logSentPacket(c, packetType1RTT, pnum, nil, dstConnID, c.w.payload())
	}
	if c.logEnabled(QLogLevelPacket) && len(c.w.payload()) > 0 {
	c.logPacketSent(packetType1RTT, pnum, nil, dstConnID, c.w.packetLen(), c.w.payload())
	}
	if sent := c.w.finish1RTTPacket(pnum, pnumMaxAcked, dstConnID, &c.keysAppData); sent != nil {
	c.packetSent(now, appDataSpace, sent)
	}
	}

	buf := c.w.datagram()
	if len(buf) == 0 {
	if limit == ccOK {
	// We have nothing to send, and congestion control does not
	// block sending. The congestion window is underutilized.
	underutilized = true
	}
	return next
	}

	if sentInitial != nil {
	if pad {
	// Pad out the datagram with zeros, coalescing the Initial
	// packet with invalid packets that will be ignored by the peer.
	// https://www.rfc-editor.org/rfc/rfc9000.html#section-14.1-1
	for len(buf) < paddedInitialDatagramSize {
	buf = append(buf, 0)
	// Technically this padding isn't in any packet, but
	// account it to the Initial packet in this datagram
	// for purposes of flow control and loss recovery.
	sentInitial.size++
	sentInitial.inFlight = true
	}
	}
	// If we're a client and this Initial packet is coalesced
	// with a Handshake packet, then we've discarded Initial keys
	// since constructing the packet and shouldn't record it as in-flight.
	if c.keysInitial.canWrite() {
	c.packetSent(now, initialSpace, sentInitial)
	}
	}

	c.endpoint.sendDatagram(datagram{
	b: buf,
	peerAddr: c.peerAddr,
	})
	}
	}

	func (c Conn) packetSent(now time.Time, space numberSpace, sent sentPacket) {
	c.idleHandlePacketSent(now, sent)
	c.loss.packetSent(now, c.log, space, sent)
	}

	func (c *Conn) appendFrames(now time.Time, space numberSpace, pnum packetNumber, limit ccLimit) {
	if c.lifetime.localErr != nil {
	c.appendConnectionCloseFrame(now, space, c.lifetime.localErr)
	return
	}

	shouldSendAck := c.acks[space].shouldSendAck(now)
	if limit != ccOK {
	// ACKs are not limited by congestion control.
	if shouldSendAck && c.appendAckFrame(now, space) {
	c.acks[space].sentAck()
	}
	return
	}
	// We want to send an ACK frame if the ack controller wants to send a frame now,
	// OR if we are sending a packet anyway and have ack-eliciting packets which we
	// have not yet acked.
	//
	// We speculatively add ACK frames here, to put them at the front of the packet
	// to avoid truncation.
	//
	// After adding all frames, if we don't need to send an ACK frame and have not
	// added any other frames, we abandon the packet.
	if c.appendAckFrame(now, space) {
	defer func() {
	// All frames other than ACK and PADDING are ack-eliciting,
	// so if the packet is ack-eliciting we've added additional
	// frames to it.
	if !shouldSendAck && !c.w.sent.ackEliciting {
	// There's nothing in this packet but ACK frames, and
	// we don't want to send an ACK-only packet at this time.
	// Abandoning the packet means we wrote an ACK frame for
	// nothing, but constructing the frame is cheap.
	c.w.abandonPacket()
	return
	}
	// Either we are willing to send an ACK-only packet,
	// or we've added additional frames.
	c.acks[space].sentAck()
	if !c.w.sent.ackEliciting && c.shouldMakePacketAckEliciting() {
	c.w.appendPingFrame()
	}
	}()
	}
	if limit != ccOK {
	return
	}
	pto := c.loss.ptoExpired

	// TODO: Add all the other frames we can send.

	// CRYPTO
	c.crypto[space].dataToSend(pto, func(off, size int64) int64 {
	b, _ := c.w.appendCryptoFrame(off, int(size))
	c.crypto[space].sendData(off, b)
	return int64(len(b))
	})

	// Test-only PING frames.
	if space == c.testSendPingSpace && c.testSendPing.shouldSendPTO(pto) {
	if !c.w.appendPingFrame() {
	return
	}
	c.testSendPing.setSent(pnum)
	}

	if space == appDataSpace {
	// HANDSHAKE_DONE
	if c.handshakeConfirmed.shouldSendPTO(pto) {
	if !c.w.appendHandshakeDoneFrame() {
	return
	}
	c.handshakeConfirmed.setSent(pnum)
	}

	// NEW_CONNECTION_ID, RETIRE_CONNECTION_ID
	if !c.connIDState.appendFrames(c, pnum, pto) {
	return
	}

	// PATH_RESPONSE
	if pad, ok := c.appendPathFrames(); !ok {
	return
	} else if pad {
	defer c.w.appendPaddingTo(smallestMaxDatagramSize)
	}

	// All stream-related frames. This should come last in the packet,
	// so large amounts of STREAM data don't crowd out other frames
	// we may need to send.
	if !c.appendStreamFrames(&c.w, pnum, pto) {
	return
	}

	if !c.appendKeepAlive(now) {
	return
	}
	}

	// If this is a PTO probe and we haven't added an ack-eliciting frame yet,
	// add a PING to make this an ack-eliciting probe.
	//
	// Technically, there are separate PTO timers for each number space.
	// When a PTO timer expires, we MUST send an ack-eliciting packet in the
	// timer's space. We SHOULD send ack-eliciting packets in every other space
	// with in-flight data. (RFC 9002, section 6.2.4)
	//
	// What we actually do is send a single datagram containing an ack-eliciting packet
	// for every space for which we have keys.
	//
	// We fill the PTO probe packets with new or unacknowledged data. For example,
	// a PTO probe sent for the Initial space will generally retransmit previously
	// sent but unacknowledged CRYPTO data.
	//
	// When sending a PTO probe datagram containing multiple packets, it is
	// possible that an earlier packet will fill up the datagram, leaving no
	// space for the remaining probe packet(s). This is not a problem in practice.
	//
	// A client discards Initial keys when it first sends a Handshake packet
	// (RFC 9001 Section 4.9.1). Handshake keys are discarded when the handshake
	// is confirmed (RFC 9001 Section 4.9.2). The PTO timer is not set for the
	// Application Data packet number space until the handshake is confirmed
	// (RFC 9002 Section 6.2.1). Therefore, the only times a PTO probe can fire
	// while data for multiple spaces is in flight are:
	//
	// - a server's Initial or Handshake timers can fire while Initial and Handshake
	// data is in flight; and
	//
	// - a client's Handshake timer can fire while Handshake and Application Data
	// data is in flight.
	//
	// It is theoretically possible for a server's Initial CRYPTO data to overflow
	// the maximum datagram size, but unlikely in practice; this space contains
	// only the ServerHello TLS message, which is small. It's also unlikely that
	// the Handshake PTO probe will fire while Initial data is in flight (this
	// requires not just that the Initial CRYPTO data completely fill a datagram,
	// but a quite specific arrangement of lost and retransmitted packets.)
	// We don't bother worrying about this case here, since the worst case is
	// that we send a PTO probe for the in-flight Initial data and drop the
	// Handshake probe.
	//
	// If a client's Handshake PTO timer fires while Application Data data is in
	// flight, it is possible that the resent Handshake CRYPTO data will crowd
	// out the probe for the Application Data space. However, since this probe is
	// optional (recall that the Application Data PTO timer is never set until
	// after Handshake keys have been discarded), dropping it is acceptable.
	if pto && !c.w.sent.ackEliciting {
	c.w.appendPingFrame()
	}
	}

	// shouldMakePacketAckEliciting is called when sending a packet containing nothing but an ACK frame.
	// It reports whether we should add a PING frame to the packet to make it ack-eliciting.
	func (c *Conn) shouldMakePacketAckEliciting() bool {
	if c.keysAppData.needAckEliciting() {
	// The peer has initiated a key update.
	// We haven't sent them any packets yet in the new phase.
	// Make this an ack-eliciting packet.
	// Their ack of this packet will complete the key update.
	return true
	}
	if c.loss.consecutiveNonAckElicitingPackets >= 19 {
	// We've sent a run of non-ack-eliciting packets.
	// Add in an ack-eliciting one every once in a while so the peer
	// lets us know which ones have arrived.
	//
	// Google QUICHE injects a PING after sending 19 packets. We do the same.
	//
	// https://www.rfc-editor.org/rfc/rfc9000#section-13.2.4-2
	return true
	}
	// TODO: Consider making every packet sent when in PTO ack-eliciting to speed up recovery.
	return false
	}

	func (c *Conn) appendAckFrame(now time.Time, space numberSpace) bool {
	seen, delay := c.acks[space].acksToSend(now)
	if len(seen) == 0 {
	return false
	}
	d := unscaledAckDelayFromDuration(delay, ackDelayExponent)
	return c.w.appendAckFrame(seen, d)
	}

	func (c *Conn) appendConnectionCloseFrame(now time.Time, space numberSpace, err error) {
	c.sentConnectionClose(now)
	switch e := err.(type) {
	case localTransportError:
	c.w.appendConnectionCloseTransportFrame(e.code, 0, e.reason)
	case *ApplicationError:
	if space != appDataSpace {
	// "CONNECTION_CLOSE frames signaling application errors (type 0x1d)
	// MUST only appear in the application data packet number space."
	// https://www.rfc-editor.org/rfc/rfc9000#section-12.5-2.2
	c.w.appendConnectionCloseTransportFrame(errApplicationError, 0, "")
	} else {
	c.w.appendConnectionCloseApplicationFrame(e.Code, e.Reason)
	}
	default:
	// TLS alerts are sent using error codes [0x0100,0x01ff).
	// https://www.rfc-editor.org/rfc/rfc9000#section-20.1-2.36.1
	var alert tls.AlertError
	switch {
	case errors.As(err, &alert):
	// tls.AlertError is a uint8, so this can't exceed 0x01ff.
	code := errTLSBase + transportError(alert)
	c.w.appendConnectionCloseTransportFrame(code, 0, "")
	default:
	c.w.appendConnectionCloseTransportFrame(errInternal, 0, "")
	}
	}
	}