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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 (
"container/heap"
"gvisor.googlesource.com/gvisor/pkg/tcpip/seqnum"
)
// receiver holds the state necessary to receive TCP segments and turn them
// into a stream of bytes.
type receiver struct {
ep *endpoint
rcvNxt seqnum.Value
// rcvAcc is one beyond the last acceptable sequence number. That is,
// the "largest" sequence value that the receiver has announced to the
// its peer that it's willing to accept. This may be different than
// rcvNxt + rcvWnd if the receive window is reduced; in that case we
// have to reduce the window as we receive more data instead of
// shrinking it.
rcvAcc seqnum.Value
rcvWndScale uint8
closed bool
pendingRcvdSegments segmentHeap
pendingBufUsed seqnum.Size
pendingBufSize seqnum.Size
}
func newReceiver(ep *endpoint, irs seqnum.Value, rcvWnd seqnum.Size, rcvWndScale uint8) *receiver {
return &receiver{
ep: ep,
rcvNxt: irs + 1,
rcvAcc: irs.Add(rcvWnd + 1),
rcvWndScale: rcvWndScale,
pendingBufSize: rcvWnd,
}
}
// acceptable checks if the segment sequence number range is acceptable
// according to the table on page 26 of RFC 793.
func (r *receiver) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
rcvWnd := r.rcvNxt.Size(r.rcvAcc)
if rcvWnd == 0 {
return segLen == 0 && segSeq == r.rcvNxt
}
return segSeq.InWindow(r.rcvNxt, rcvWnd) ||
seqnum.Overlap(r.rcvNxt, rcvWnd, segSeq, segLen)
}
// getSendParams returns the parameters needed by the sender when building
// segments to send.
func (r *receiver) getSendParams() (rcvNxt seqnum.Value, rcvWnd seqnum.Size) {
// Calculate the window size based on the current buffer size.
n := r.ep.receiveBufferAvailable()
acc := r.rcvNxt.Add(seqnum.Size(n))
if r.rcvAcc.LessThan(acc) {
r.rcvAcc = acc
}
return r.rcvNxt, r.rcvNxt.Size(r.rcvAcc) >> r.rcvWndScale
}
// nonZeroWindow is called when the receive window grows from zero to nonzero;
// in such cases we may need to send an ack to indicate to our peer that it can
// resume sending data.
func (r *receiver) nonZeroWindow() {
if (r.rcvAcc-r.rcvNxt)>>r.rcvWndScale != 0 {
// We never got around to announcing a zero window size, so we
// don't need to immediately announce a nonzero one.
return
}
// Immediately send an ack.
r.ep.snd.sendAck()
}
// consumeSegment attempts to consume a segment that was received by r. The
// segment may have just been received or may have been received earlier but
// wasn't ready to be consumed then.
//
// Returns true if the segment was consumed, false if it cannot be consumed
// yet because of a missing segment.
func (r *receiver) consumeSegment(s *segment, segSeq seqnum.Value, segLen seqnum.Size) bool {
if segLen > 0 {
// If the segment doesn't include the seqnum we're expecting to
// consume now, we're missing a segment. We cannot proceed until
// we receive that segment though.
if !r.rcvNxt.InWindow(segSeq, segLen) {
return false
}
// Trim segment to eliminate already acknowledged data.
if segSeq.LessThan(r.rcvNxt) {
diff := segSeq.Size(r.rcvNxt)
segLen -= diff
segSeq.UpdateForward(diff)
s.sequenceNumber.UpdateForward(diff)
s.data.TrimFront(int(diff))
}
// Move segment to ready-to-deliver list. Wakeup any waiters.
r.ep.readyToRead(s)
} else if segSeq != r.rcvNxt {
return false
}
// Update the segment that we're expecting to consume.
r.rcvNxt = segSeq.Add(segLen)
// Trim SACK Blocks to remove any SACK information that covers
// sequence numbers that have been consumed.
TrimSACKBlockList(&r.ep.sack, r.rcvNxt)
if s.flagIsSet(flagFin) {
r.rcvNxt++
// Send ACK immediately.
r.ep.snd.sendAck()
// Tell any readers that no more data will come.
r.closed = true
r.ep.readyToRead(nil)
// Flush out any pending segments, except the very first one if
// it happens to be the one we're handling now because the
// caller is using it.
first := 0
if len(r.pendingRcvdSegments) != 0 && r.pendingRcvdSegments[0] == s {
first = 1
}
for i := first; i < len(r.pendingRcvdSegments); i++ {
r.pendingRcvdSegments[i].decRef()
}
r.pendingRcvdSegments = r.pendingRcvdSegments[:first]
}
return true
}
// handleRcvdSegment handles TCP segments directed at the connection managed by
// r as they arrive. It is called by the protocol main loop.
func (r *receiver) handleRcvdSegment(s *segment) {
// We don't care about receive processing anymore if the receive side
// is closed.
if r.closed {
return
}
segLen := seqnum.Size(s.data.Size())
segSeq := s.sequenceNumber
// If the sequence number range is outside the acceptable range, just
// send an ACK. This is according to RFC 793, page 37.
if !r.acceptable(segSeq, segLen) {
r.ep.snd.sendAck()
return
}
// Defer segment processing if it can't be consumed now.
if !r.consumeSegment(s, segSeq, segLen) {
if segLen > 0 || s.flagIsSet(flagFin) {
// We only store the segment if it's within our buffer
// size limit.
if r.pendingBufUsed < r.pendingBufSize {
r.pendingBufUsed += s.logicalLen()
s.incRef()
heap.Push(&r.pendingRcvdSegments, s)
}
UpdateSACKBlocks(&r.ep.sack, segSeq, segSeq.Add(segLen), r.rcvNxt)
// Immediately send an ack so that the peer knows it may
// have to retransmit.
r.ep.snd.sendAck()
}
return
}
// By consuming the current segment, we may have filled a gap in the
// sequence number domain that allows pending segments to be consumed
// now. So try to do it.
for !r.closed && r.pendingRcvdSegments.Len() > 0 {
s := r.pendingRcvdSegments[0]
segLen := seqnum.Size(s.data.Size())
segSeq := s.sequenceNumber
// Skip segment altogether if it has already been acknowledged.
if !segSeq.Add(segLen-1).LessThan(r.rcvNxt) &&
!r.consumeSegment(s, segSeq, segLen) {
break
}
heap.Pop(&r.pendingRcvdSegments)
r.pendingBufUsed -= s.logicalLen()
s.decRef()
}
}
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