path.go

     1  // Copyright 2024 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package quic
     6  
     7  import "time"
     8  
     9  type pathState struct {
    10  	// Response to a peer's PATH_CHALLENGE.
    11  	// This is not a sentVal, because we don't resend lost PATH_RESPONSE frames.
    12  	// We only track the most recent PATH_CHALLENGE.
    13  	// If the peer sends a second PATH_CHALLENGE before we respond to the first,
    14  	// we'll drop the first response.
    15  	sendPathResponse pathResponseType
    16  	data             pathChallengeData
    17  }
    18  
    19  // pathChallengeData is data carried in a PATH_CHALLENGE or PATH_RESPONSE frame.
    20  type pathChallengeData [64 / 8]byte
    21  
    22  type pathResponseType uint8
    23  
    24  const (
    25  	pathResponseNotNeeded = pathResponseType(iota)
    26  	pathResponseSmall     // send PATH_RESPONSE, do not expand datagram
    27  	pathResponseExpanded  // send PATH_RESPONSE, expand datagram to 1200 bytes
    28  )
    29  
    30  func (c *Conn) handlePathChallenge(_ time.Time, dgram *datagram, data pathChallengeData) {
    31  	// A PATH_RESPONSE is sent in a datagram expanded to 1200 bytes,
    32  	// except when this would exceed the anti-amplification limit.
    33  	//
    34  	// Rather than maintaining anti-amplification state for each path
    35  	// we may be sending a PATH_RESPONSE on, follow the following heuristic:
    36  	//
    37  	// If we receive a PATH_CHALLENGE in an expanded datagram,
    38  	// respond with an expanded datagram.
    39  	//
    40  	// If we receive a PATH_CHALLENGE in a non-expanded datagram,
    41  	// then the peer is presumably blocked by its own anti-amplification limit.
    42  	// Respond with a non-expanded datagram. Receiving this PATH_RESPONSE
    43  	// will validate the path to the peer, remove its anti-amplification limit,
    44  	// and permit it to send a followup PATH_CHALLENGE in an expanded datagram.
    45  	// https://www.rfc-editor.org/rfc/rfc9000.html#section-8.2.1
    46  	if len(dgram.b) >= smallestMaxDatagramSize {
    47  		c.path.sendPathResponse = pathResponseExpanded
    48  	} else {
    49  		c.path.sendPathResponse = pathResponseSmall
    50  	}
    51  	c.path.data = data
    52  }
    53  
    54  func (c *Conn) handlePathResponse(now time.Time, _ pathChallengeData) {
    55  	// "If the content of a PATH_RESPONSE frame does not match the content of
    56  	// a PATH_CHALLENGE frame previously sent by the endpoint,
    57  	// the endpoint MAY generate a connection error of type PROTOCOL_VIOLATION."
    58  	// https://www.rfc-editor.org/rfc/rfc9000.html#section-19.18-4
    59  	//
    60  	// We never send PATH_CHALLENGE frames.
    61  	c.abort(now, localTransportError{
    62  		code:   errProtocolViolation,
    63  		reason: "PATH_RESPONSE received when no PATH_CHALLENGE sent",
    64  	})
    65  }
    66  
    67  // appendPathFrames appends path validation related frames to the current packet.
    68  // If the return value pad is true, then the packet should be padded to 1200 bytes.
    69  func (c *Conn) appendPathFrames() (pad, ok bool) {
    70  	if c.path.sendPathResponse == pathResponseNotNeeded {
    71  		return pad, true
    72  	}
    73  	// We're required to send the PATH_RESPONSE on the path where the
    74  	// PATH_CHALLENGE was received (RFC 9000, Section 8.2.2).
    75  	//
    76  	// At the moment, we don't support path migration and reject packets if
    77  	// the peer changes its source address, so just sending the PATH_RESPONSE
    78  	// in a regular datagram is fine.
    79  	if !c.w.appendPathResponseFrame(c.path.data) {
    80  		return pad, false
    81  	}
    82  	if c.path.sendPathResponse == pathResponseExpanded {
    83  		pad = true
    84  	}
    85  	c.path.sendPathResponse = pathResponseNotNeeded
    86  	return pad, true
    87  }
    88
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