fragment_buffer.go

// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
// SPDX-License-Identifier: MIT

package dtls

import (
	"github.com/pion/dtls/v3/pkg/protocol"
	"github.com/pion/dtls/v3/pkg/protocol/handshake"
	"github.com/pion/dtls/v3/pkg/protocol/recordlayer"
)

// 2 megabytes
const fragmentBufferMaxSize = 2000000

type fragment struct {
	recordLayerHeader recordlayer.Header
	handshakeHeader   handshake.Header
	data              []byte
}

type fragmentBuffer struct {
	// map of MessageSequenceNumbers that hold slices of fragments
	cache map[uint16][]*fragment

	currentMessageSequenceNumber uint16
}

func newFragmentBuffer() *fragmentBuffer {
	return &fragmentBuffer{cache: map[uint16][]*fragment{}}
}

// current total size of buffer
func (f *fragmentBuffer) size() int {
	size := 0
	for i := range f.cache {
		for j := range f.cache[i] {
			size += len(f.cache[i][j].data)
		}
	}
	return size
}

// Attempts to push a DTLS packet to the fragmentBuffer
// when it returns true it means the fragmentBuffer has inserted and the buffer shouldn't be handled
// when an error returns it is fatal, and the DTLS connection should be stopped
func (f *fragmentBuffer) push(buf []byte) (isHandshake, isRetransmit bool, err error) {
	if f.size()+len(buf) >= fragmentBufferMaxSize {
		return false, false, errFragmentBufferOverflow
	}

	frag := new(fragment)
	if err := frag.recordLayerHeader.Unmarshal(buf); err != nil {
		return false, false, err
	}

	// fragment isn't a handshake, we don't need to handle it
	if frag.recordLayerHeader.ContentType != protocol.ContentTypeHandshake {
		return false, false, nil
	}

	for buf = buf[recordlayer.FixedHeaderSize:]; len(buf) != 0; frag = new(fragment) {
		if err := frag.handshakeHeader.Unmarshal(buf); err != nil {
			return false, false, err
		}

		// Fragment is a retransmission. We have already assembled it before successfully
		isRetransmit = frag.handshakeHeader.FragmentOffset == 0 && frag.handshakeHeader.MessageSequence < f.currentMessageSequenceNumber

		if _, ok := f.cache[frag.handshakeHeader.MessageSequence]; !ok {
			f.cache[frag.handshakeHeader.MessageSequence] = []*fragment{}
		}

		// end index should be the length of handshake header but if the handshake
		// was fragmented, we should keep them all
		end := int(handshake.HeaderLength + frag.handshakeHeader.Length)
		if size := len(buf); end > size {
			end = size
		}

		// Discard all headers, when rebuilding the packet we will re-build
		frag.data = append([]byte{}, buf[handshake.HeaderLength:end]...)
		f.cache[frag.handshakeHeader.MessageSequence] = append(f.cache[frag.handshakeHeader.MessageSequence], frag)
		buf = buf[end:]
	}

	return true, isRetransmit, nil
}

func (f *fragmentBuffer) pop() (content []byte, epoch uint16) {
	frags, ok := f.cache[f.currentMessageSequenceNumber]
	if !ok {
		return nil, 0
	}

	// Go doesn't support recursive lambdas
	var appendMessage func(targetOffset uint32) bool

	rawMessage := []byte{}
	appendMessage = func(targetOffset uint32) bool {
		for _, f := range frags {
			if f.handshakeHeader.FragmentOffset == targetOffset {
				fragmentEnd := (f.handshakeHeader.FragmentOffset + f.handshakeHeader.FragmentLength)
				if fragmentEnd != f.handshakeHeader.Length && f.handshakeHeader.FragmentLength != 0 {
					if !appendMessage(fragmentEnd) {
						return false
					}
				}

				rawMessage = append(f.data, rawMessage...)
				return true
			}
		}
		return false
	}

	// Recursively collect up
	if !appendMessage(0) {
		return nil, 0
	}

	firstHeader := frags[0].handshakeHeader
	firstHeader.FragmentOffset = 0
	firstHeader.FragmentLength = firstHeader.Length

	rawHeader, err := firstHeader.Marshal()
	if err != nil {
		return nil, 0
	}

	messageEpoch := frags[0].recordLayerHeader.Epoch

	delete(f.cache, f.currentMessageSequenceNumber)
	f.currentMessageSequenceNumber++
	return append(rawHeader, rawMessage...), messageEpoch
}