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// Copyright 2024 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 "time"
type pathState struct {
// Response to a peer's PATH_CHALLENGE.
// This is not a sentVal, because we don't resend lost PATH_RESPONSE frames.
// We only track the most recent PATH_CHALLENGE.
// If the peer sends a second PATH_CHALLENGE before we respond to the first,
// we'll drop the first response.
sendPathResponse pathResponseType
data pathChallengeData
// pathChallengeData is data carried in a PATH_CHALLENGE or PATH_RESPONSE frame.
type pathChallengeData [64 / 8]byte
type pathResponseType uint8
const (
pathResponseNotNeeded = pathResponseType(iota)
pathResponseSmall // send PATH_RESPONSE, do not expand datagram
pathResponseExpanded // send PATH_RESPONSE, expand datagram to 1200 bytes
func (c *Conn) handlePathChallenge(_ time.Time, dgram *datagram, data pathChallengeData) {
// A PATH_RESPONSE is sent in a datagram expanded to 1200 bytes,
// except when this would exceed the anti-amplification limit.
// Rather than maintaining anti-amplification state for each path
// we may be sending a PATH_RESPONSE on, follow the following heuristic:
// If we receive a PATH_CHALLENGE in an expanded datagram,
// respond with an expanded datagram.
// If we receive a PATH_CHALLENGE in a non-expanded datagram,
// then the peer is presumably blocked by its own anti-amplification limit.
// Respond with a non-expanded datagram. Receiving this PATH_RESPONSE
// will validate the path to the peer, remove its anti-amplification limit,
// and permit it to send a followup PATH_CHALLENGE in an expanded datagram.
if len(dgram.b) >= smallestMaxDatagramSize {
c.path.sendPathResponse = pathResponseExpanded
} else {
c.path.sendPathResponse = pathResponseSmall
} = data
func (c *Conn) handlePathResponse(now time.Time, _ pathChallengeData) {
// "If the content of a PATH_RESPONSE frame does not match the content of
// a PATH_CHALLENGE frame previously sent by the endpoint,
// the endpoint MAY generate a connection error of type PROTOCOL_VIOLATION."
// We never send PATH_CHALLENGE frames.
c.abort(now, localTransportError{
code: errProtocolViolation,
reason: "PATH_RESPONSE received when no PATH_CHALLENGE sent",
// appendPathFrames appends path validation related frames to the current packet.
// If the return value pad is true, then the packet should be padded to 1200 bytes.
func (c *Conn) appendPathFrames() (pad, ok bool) {
if c.path.sendPathResponse == pathResponseNotNeeded {
return pad, true
// We're required to send the PATH_RESPONSE on the path where the
// PATH_CHALLENGE was received (RFC 9000, Section 8.2.2).
// At the moment, we don't support path migration and reject packets if
// the peer changes its source address, so just sending the PATH_RESPONSE
// in a regular datagram is fine.
if !c.w.appendPathResponseFrame( {
return pad, false
if c.path.sendPathResponse == pathResponseExpanded {
pad = true
c.path.sendPathResponse = pathResponseNotNeeded
return pad, true