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// Copyright 2016 The Netstack Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package tcp
import (
"sync/atomic"
"gvisor.googlesource.com/gvisor/pkg/tcpip/buffer"
"gvisor.googlesource.com/gvisor/pkg/tcpip/header"
"gvisor.googlesource.com/gvisor/pkg/tcpip/seqnum"
"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
)
// Flags that may be set in a TCP segment.
const (
flagFin = 1 << iota
flagSyn
flagRst
flagPsh
flagAck
flagUrg
)
// segment represents a TCP segment. It holds the payload and parsed TCP segment
// information, and can be added to intrusive lists.
// segment is mostly immutable, the only field allowed to change is viewToDeliver.
type segment struct {
segmentEntry
refCnt int32
id stack.TransportEndpointID
route stack.Route
data buffer.VectorisedView
// views is used as buffer for data when its length is large
// enough to store a VectorisedView.
views [8]buffer.View
// viewToDeliver keeps track of the next View that should be
// delivered by the Read endpoint.
viewToDeliver int
sequenceNumber seqnum.Value
ackNumber seqnum.Value
flags uint8
window seqnum.Size
// parsedOptions stores the parsed values from the options in the segment.
parsedOptions header.TCPOptions
options []byte
}
func newSegment(r *stack.Route, id stack.TransportEndpointID, vv *buffer.VectorisedView) *segment {
s := &segment{
refCnt: 1,
id: id,
route: r.Clone(),
}
s.data = vv.Clone(s.views[:])
return s
}
func newSegmentFromView(r *stack.Route, id stack.TransportEndpointID, v buffer.View) *segment {
s := &segment{
refCnt: 1,
id: id,
route: r.Clone(),
}
s.views[0] = v
s.data = buffer.NewVectorisedView(len(v), s.views[:1])
return s
}
func (s *segment) clone() *segment {
t := &segment{
refCnt: 1,
id: s.id,
sequenceNumber: s.sequenceNumber,
ackNumber: s.ackNumber,
flags: s.flags,
window: s.window,
route: s.route.Clone(),
viewToDeliver: s.viewToDeliver,
}
t.data = s.data.Clone(t.views[:])
return t
}
func (s *segment) flagIsSet(flag uint8) bool {
return (s.flags & flag) != 0
}
func (s *segment) decRef() {
if atomic.AddInt32(&s.refCnt, -1) == 0 {
s.route.Release()
}
}
func (s *segment) incRef() {
atomic.AddInt32(&s.refCnt, 1)
}
// logicalLen is the segment length in the sequence number space. It's defined
// as the data length plus one for each of the SYN and FIN bits set.
func (s *segment) logicalLen() seqnum.Size {
l := seqnum.Size(s.data.Size())
if s.flagIsSet(flagSyn) {
l++
}
if s.flagIsSet(flagFin) {
l++
}
return l
}
// parse populates the sequence & ack numbers, flags, and window fields of the
// segment from the TCP header stored in the data. It then updates the view to
// skip the data. Returns boolean indicating if the parsing was successful.
func (s *segment) parse() bool {
h := header.TCP(s.data.First())
// h is the header followed by the payload. We check that the offset to
// the data respects the following constraints:
// 1. That it's at least the minimum header size; if we don't do this
// then part of the header would be delivered to user.
// 2. That the header fits within the buffer; if we don't do this, we
// would panic when we tried to access data beyond the buffer.
//
// N.B. The segment has already been validated as having at least the
// minimum TCP size before reaching here, so it's safe to read the
// fields.
offset := int(h.DataOffset())
if offset < header.TCPMinimumSize || offset > len(h) {
return false
}
s.options = []byte(h[header.TCPMinimumSize:offset])
s.parsedOptions = header.ParseTCPOptions(s.options)
s.data.TrimFront(offset)
s.sequenceNumber = seqnum.Value(h.SequenceNumber())
s.ackNumber = seqnum.Value(h.AckNumber())
s.flags = h.Flags()
s.window = seqnum.Size(h.WindowSize())
return true
}
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