De-duplicate TCP state in TCPEndpointState vs tcp.endpoint

This change replaces individual private members in tcp.endpoint with a single
private TCPEndpointState member.

Some internal substructures within endpoint (receiver, sender) have been broken
into a public substructure (which is then copied into the TCPEndpointState
returned from completeState()) alongside other private fields.

Fixes #4466

PiperOrigin-RevId: 369329514

4 files changed, 528 insertions, 301 deletions

diff --git a/pkg/tcpip/stack/BUILD b/pkg/tcpip/stack/BUILD
index d19b879dc..2bd6a67f5 100644
--- a/pkg/tcpip/stack/BUILD
+++ b/pkg/tcpip/stack/BUILD
@@ -67,6 +67,7 @@ go_library(
         "stack.go",
         "stack_global_state.go",
         "stack_options.go",
+        "tcp.go",
         "transport_demuxer.go",
         "tuple_list.go",
     ],
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index f23112410..8c8909acd 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -35,7 +35,6 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/ports"
-	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
 	"gvisor.dev/gvisor/pkg/waiter"
 )
 
@@ -56,306 +55,6 @@ type transportProtocolState struct {
 	defaultHandler func(id TransportEndpointID, pkt *PacketBuffer) bool
 }
 
-// TCPProbeFunc is the expected function type for a TCP probe function to be
-// passed to stack.AddTCPProbe.
-type TCPProbeFunc func(s TCPEndpointState)
-
-// TCPCubicState is used to hold a copy of the internal cubic state when the
-// TCPProbeFunc is invoked.
-type TCPCubicState struct {
-	WLastMax                float64
-	WMax                    float64
-	T                       time.Time
-	TimeSinceLastCongestion time.Duration
-	C                       float64
-	K                       float64
-	Beta                    float64
-	WC                      float64
-	WEst                    float64
-}
-
-// TCPRACKState is used to hold a copy of the internal RACK state when the
-// TCPProbeFunc is invoked.
-type TCPRACKState struct {
-	XmitTime      time.Time
-	EndSequence   seqnum.Value
-	FACK          seqnum.Value
-	RTT           time.Duration
-	Reord         bool
-	DSACKSeen     bool
-	ReoWnd        time.Duration
-	ReoWndIncr    uint8
-	ReoWndPersist int8
-	RTTSeq        seqnum.Value
-}
-
-// TCPEndpointID is the unique 4 tuple that identifies a given endpoint.
-type TCPEndpointID struct {
-	// LocalPort is the local port associated with the endpoint.
-	LocalPort uint16
-
-	// LocalAddress is the local [network layer] address associated with
-	// the endpoint.
-	LocalAddress tcpip.Address
-
-	// RemotePort is the remote port associated with the endpoint.
-	RemotePort uint16
-
-	// RemoteAddress it the remote [network layer] address associated with
-	// the endpoint.
-	RemoteAddress tcpip.Address
-}
-
-// TCPFastRecoveryState holds a copy of the internal fast recovery state of a
-// TCP endpoint.
-type TCPFastRecoveryState struct {
-	// Active if true indicates the endpoint is in fast recovery.
-	Active bool
-
-	// First is the first unacknowledged sequence number being recovered.
-	First seqnum.Value
-
-	// Last is the 'recover' sequence number that indicates the point at
-	// which we should exit recovery barring any timeouts etc.
-	Last seqnum.Value
-
-	// MaxCwnd is the maximum value we are permitted to grow the congestion
-	// window during recovery. This is set at the time we enter recovery.
-	MaxCwnd int
-
-	// HighRxt is the highest sequence number which has been retransmitted
-	// during the current loss recovery phase.
-	// See: RFC 6675 Section 2 for details.
-	HighRxt seqnum.Value
-
-	// RescueRxt is the highest sequence number which has been
-	// optimistically retransmitted to prevent stalling of the ACK clock
-	// when there is loss at the end of the window and no new data is
-	// available for transmission.
-	// See: RFC 6675 Section 2 for details.
-	RescueRxt seqnum.Value
-}
-
-// TCPReceiverState holds a copy of the internal state of the receiver for
-// a given TCP endpoint.
-type TCPReceiverState struct {
-	// RcvNxt is the TCP variable RCV.NXT.
-	RcvNxt seqnum.Value
-
-	// RcvAcc is the TCP variable RCV.ACC.
-	RcvAcc seqnum.Value
-
-	// RcvWndScale is the window scaling to use for inbound segments.
-	RcvWndScale uint8
-
-	// PendingBufUsed is the number of bytes pending in the receive
-	// queue.
-	PendingBufUsed int
-}
-
-// TCPSenderState holds a copy of the internal state of the sender for
-// a given TCP Endpoint.
-type TCPSenderState struct {
-	// LastSendTime is the time at which we sent the last segment.
-	LastSendTime time.Time
-
-	// DupAckCount is the number of Duplicate ACK's received.
-	DupAckCount int
-
-	// SndCwnd is the size of the sending congestion window in packets.
-	SndCwnd int
-
-	// Ssthresh is the slow start threshold in packets.
-	Ssthresh int
-
-	// SndCAAckCount is the number of packets consumed in congestion
-	// avoidance mode.
-	SndCAAckCount int
-
-	// Outstanding is the number of packets in flight.
-	Outstanding int
-
-	// SackedOut is the number of packets which have been selectively acked.
-	SackedOut int
-
-	// SndWnd is the send window size in bytes.
-	SndWnd seqnum.Size
-
-	// SndUna is the next unacknowledged sequence number.
-	SndUna seqnum.Value
-
-	// SndNxt is the sequence number of the next segment to be sent.
-	SndNxt seqnum.Value
-
-	// RTTMeasureSeqNum is the sequence number being used for the latest RTT
-	// measurement.
-	RTTMeasureSeqNum seqnum.Value
-
-	// RTTMeasureTime is the time when the RTTMeasureSeqNum was sent.
-	RTTMeasureTime time.Time
-
-	// Closed indicates that the caller has closed the endpoint for sending.
-	Closed bool
-
-	// SRTT is the smoothed round-trip time as defined in section 2 of
-	// RFC 6298.
-	SRTT time.Duration
-
-	// RTO is the retransmit timeout as defined in section of 2 of RFC 6298.
-	RTO time.Duration
-
-	// RTTVar is the round-trip time variation as defined in section 2 of
-	// RFC 6298.
-	RTTVar time.Duration
-
-	// SRTTInited if true indicates take a valid RTT measurement has been
-	// completed.
-	SRTTInited bool
-
-	// MaxPayloadSize is the maximum size of the payload of a given segment.
-	// It is initialized on demand.
-	MaxPayloadSize int
-
-	// SndWndScale is the number of bits to shift left when reading the send
-	// window size from a segment.
-	SndWndScale uint8
-
-	// MaxSentAck is the highest acknowledgement number sent till now.
-	MaxSentAck seqnum.Value
-
-	// FastRecovery holds the fast recovery state for the endpoint.
-	FastRecovery TCPFastRecoveryState
-
-	// Cubic holds the state related to CUBIC congestion control.
-	Cubic TCPCubicState
-
-	// RACKState holds the state related to RACK loss detection algorithm.
-	RACKState TCPRACKState
-}
-
-// TCPSACKInfo holds TCP SACK related information for a given TCP endpoint.
-type TCPSACKInfo struct {
-	// Blocks is the list of SACK Blocks that identify the out of order segments
-	// held by a given TCP endpoint.
-	Blocks []header.SACKBlock
-
-	// ReceivedBlocks are the SACK blocks received by this endpoint
-	// from the peer endpoint.
-	ReceivedBlocks []header.SACKBlock
-
-	// MaxSACKED is the highest sequence number that has been SACKED
-	// by the peer.
-	MaxSACKED seqnum.Value
-}
-
-// RcvBufAutoTuneParams holds state related to TCP receive buffer auto-tuning.
-type RcvBufAutoTuneParams struct {
-	// MeasureTime is the time at which the current measurement
-	// was started.
-	MeasureTime time.Time
-
-	// CopiedBytes is the number of bytes copied to user space since
-	// this measure began.
-	CopiedBytes int
-
-	// PrevCopiedBytes is the number of bytes copied to userspace in
-	// the previous RTT period.
-	PrevCopiedBytes int
-
-	// RcvBufSize is the auto tuned receive buffer size.
-	RcvBufSize int
-
-	// RTT is the smoothed RTT as measured by observing the time between
-	// when a byte is first acknowledged and the receipt of data that is at
-	// least one window beyond the sequence number that was acknowledged.
-	RTT time.Duration
-
-	// RTTVar is the "round-trip time variation" as defined in section 2
-	// of RFC6298.
-	RTTVar time.Duration
-
-	// RTTMeasureSeqNumber is the highest acceptable sequence number at the
-	// time this RTT measurement period began.
-	RTTMeasureSeqNumber seqnum.Value
-
-	// RTTMeasureTime is the absolute time at which the current RTT
-	// measurement period began.
-	RTTMeasureTime time.Time
-
-	// Disabled is true if an explicit receive buffer is set for the
-	// endpoint.
-	Disabled bool
-}
-
-// TCPEndpointState is a copy of the internal state of a TCP endpoint.
-type TCPEndpointState struct {
-	// ID is a copy of the TransportEndpointID for the endpoint.
-	ID TCPEndpointID
-
-	// SegTime denotes the absolute time when this segment was received.
-	SegTime time.Time
-
-	// RcvBufSize is the size of the receive socket buffer for the endpoint.
-	RcvBufSize int
-
-	// RcvBufUsed is the amount of bytes actually held in the receive socket
-	// buffer for the endpoint.
-	RcvBufUsed int
-
-	// RcvBufAutoTuneParams is used to hold state variables to compute
-	// the auto tuned receive buffer size.
-	RcvAutoParams RcvBufAutoTuneParams
-
-	// RcvClosed if true, indicates the endpoint has been closed for reading.
-	RcvClosed bool
-
-	// SendTSOk is used to indicate when the TS Option has been negotiated.
-	// When sendTSOk is true every non-RST segment should carry a TS as per
-	// RFC7323#section-1.1.
-	SendTSOk bool
-
-	// RecentTS is the timestamp that should be sent in the TSEcr field of
-	// the timestamp for future segments sent by the endpoint. This field is
-	// updated if required when a new segment is received by this endpoint.
-	RecentTS uint32
-
-	// TSOffset is a randomized offset added to the value of the TSVal field
-	// in the timestamp option.
-	TSOffset uint32
-
-	// SACKPermitted is set to true if the peer sends the TCPSACKPermitted
-	// option in the SYN/SYN-ACK.
-	SACKPermitted bool
-
-	// SACK holds TCP SACK related information for this endpoint.
-	SACK TCPSACKInfo
-
-	// SndBufSize is the size of the socket send buffer.
-	SndBufSize int
-
-	// SndBufUsed is the number of bytes held in the socket send buffer.
-	SndBufUsed int
-
-	// SndClosed indicates that the endpoint has been closed for sends.
-	SndClosed bool
-
-	// SndBufInQueue is the number of bytes in the send queue.
-	SndBufInQueue seqnum.Size
-
-	// PacketTooBigCount is used to notify the main protocol routine how
-	// many times a "packet too big" control packet is received.
-	PacketTooBigCount int
-
-	// SndMTU is the smallest MTU seen in the control packets received.
-	SndMTU int
-
-	// Receiver holds variables related to the TCP receiver for the endpoint.
-	Receiver TCPReceiverState
-
-	// Sender holds state related to the TCP Sender for the endpoint.
-	Sender TCPSenderState
-}
-
 // ResumableEndpoint is an endpoint that needs to be resumed after restore.
 type ResumableEndpoint interface {
 	// Resume resumes an endpoint after restore. This can be used to restart
diff --git a/pkg/tcpip/stack/stack_global_state.go b/pkg/tcpip/stack/stack_global_state.go
index dfec4258a..33824afd0 100644
--- a/pkg/tcpip/stack/stack_global_state.go
+++ b/pkg/tcpip/stack/stack_global_state.go
@@ -14,6 +14,78 @@
 
 package stack
 
+import "time"
+
 // StackFromEnv is the global stack created in restore run.
 // FIXME(b/36201077)
 var StackFromEnv *Stack
+
+// saveT is invoked by stateify.
+func (t *TCPCubicState) saveT() unixTime {
+	return unixTime{t.T.Unix(), t.T.UnixNano()}
+}
+
+// loadT is invoked by stateify.
+func (t *TCPCubicState) loadT(unix unixTime) {
+	t.T = time.Unix(unix.second, unix.nano)
+}
+
+// saveXmitTime is invoked by stateify.
+func (t *TCPRACKState) saveXmitTime() unixTime {
+	return unixTime{t.XmitTime.Unix(), t.XmitTime.UnixNano()}
+}
+
+// loadXmitTime is invoked by stateify.
+func (t *TCPRACKState) loadXmitTime(unix unixTime) {
+	t.XmitTime = time.Unix(unix.second, unix.nano)
+}
+
+// saveLastSendTime is invoked by stateify.
+func (t *TCPSenderState) saveLastSendTime() unixTime {
+	return unixTime{t.LastSendTime.Unix(), t.LastSendTime.UnixNano()}
+}
+
+// loadLastSendTime is invoked by stateify.
+func (t *TCPSenderState) loadLastSendTime(unix unixTime) {
+	t.LastSendTime = time.Unix(unix.second, unix.nano)
+}
+
+// saveRTTMeasureTime is invoked by stateify.
+func (t *TCPSenderState) saveRTTMeasureTime() unixTime {
+	return unixTime{t.RTTMeasureTime.Unix(), t.RTTMeasureTime.UnixNano()}
+}
+
+// loadRTTMeasureTime is invoked by stateify.
+func (t *TCPSenderState) loadRTTMeasureTime(unix unixTime) {
+	t.RTTMeasureTime = time.Unix(unix.second, unix.nano)
+}
+
+// saveMeasureTime is invoked by stateify.
+func (r *RcvBufAutoTuneParams) saveMeasureTime() unixTime {
+	return unixTime{r.MeasureTime.Unix(), r.MeasureTime.UnixNano()}
+}
+
+// loadMeasureTime is invoked by stateify.
+func (r *RcvBufAutoTuneParams) loadMeasureTime(unix unixTime) {
+	r.MeasureTime = time.Unix(unix.second, unix.nano)
+}
+
+// saveRTTMeasureTime is invoked by stateify.
+func (r *RcvBufAutoTuneParams) saveRTTMeasureTime() unixTime {
+	return unixTime{r.RTTMeasureTime.Unix(), r.RTTMeasureTime.UnixNano()}
+}
+
+// loadRTTMeasureTime is invoked by stateify.
+func (r *RcvBufAutoTuneParams) loadRTTMeasureTime(unix unixTime) {
+	r.RTTMeasureTime = time.Unix(unix.second, unix.nano)
+}
+
+// saveSegTime is invoked by stateify.
+func (t *TCPEndpointState) saveSegTime() unixTime {
+	return unixTime{t.SegTime.Unix(), t.SegTime.UnixNano()}
+}
+
+// loadSegTime is invoked by stateify.
+func (t *TCPEndpointState) loadSegTime(unix unixTime) {
+	t.SegTime = time.Unix(unix.second, unix.nano)
+}
diff --git a/pkg/tcpip/stack/tcp.go b/pkg/tcpip/stack/tcp.go
new file mode 100644
index 000000000..2e32b8a8e
--- /dev/null
+++ b/pkg/tcpip/stack/tcp.go
@@ -0,0 +1,455 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
+)
+
+// TCPProbeFunc is the expected function type for a TCP probe function to be
+// passed to stack.AddTCPProbe.
+type TCPProbeFunc func(s TCPEndpointState)
+
+// TCPCubicState is used to hold a copy of the internal cubic state when the
+// TCPProbeFunc is invoked.
+//
+// +stateify savable
+type TCPCubicState struct {
+	// WLastMax is the previous wMax value.
+	WLastMax float64
+
+	// WMax is the value of the congestion window at the time of the last
+	// congestion event.
+	WMax float64
+
+	// T is the time when the current congestion avoidance was entered.
+	T time.Time `state:".(unixTime)"`
+
+	// TimeSinceLastCongestion denotes the time since the current
+	// congestion avoidance was entered.
+	TimeSinceLastCongestion time.Duration
+
+	// C is the cubic constant as specified in RFC8312, page 11.
+	C float64
+
+	// K is the time period (in seconds) that the above function takes to
+	// increase the current window size to WMax if there are no further
+	// congestion events and is calculated using the following equation:
+	//
+	// K = cubic_root(WMax*(1-beta_cubic)/C) (Eq. 2, page 5)
+	K float64
+
+	// Beta is the CUBIC multiplication decrease factor. That is, when a
+	// congestion event is detected, CUBIC reduces its cwnd to
+	// WC(0)=WMax*beta_cubic.
+	Beta float64
+
+	// WC is window computed by CUBIC at time TimeSinceLastCongestion. It's
+	// calculated using the formula:
+	//
+	//  WC(TimeSinceLastCongestion) = C*(t-K)^3 + WMax (Eq. 1)
+	WC float64
+
+	// WEst is the window computed by CUBIC at time
+	// TimeSinceLastCongestion+RTT i.e WC(TimeSinceLastCongestion+RTT).
+	WEst float64
+}
+
+// TCPRACKState is used to hold a copy of the internal RACK state when the
+// TCPProbeFunc is invoked.
+//
+// +stateify savable
+type TCPRACKState struct {
+	// XmitTime is the transmission timestamp of the most recent
+	// acknowledged segment.
+	XmitTime time.Time `state:".(unixTime)"`
+
+	// EndSequence is the ending TCP sequence number of the most recent
+	// acknowledged segment.
+	EndSequence seqnum.Value
+
+	// FACK is the highest selectively or cumulatively acknowledged
+	// sequence.
+	FACK seqnum.Value
+
+	// RTT is the round trip time of the most recently delivered packet on
+	// the connection (either cumulatively acknowledged or selectively
+	// acknowledged) that was not marked invalid as a possible spurious
+	// retransmission.
+	RTT time.Duration
+
+	// Reord is true iff reordering has been detected on this connection.
+	Reord bool
+
+	// DSACKSeen is true iff the connection has seen a DSACK.
+	DSACKSeen bool
+
+	// ReoWnd is the reordering window time used for recording packet
+	// transmission times. It is used to defer the moment at which RACK
+	// marks a packet lost.
+	ReoWnd time.Duration
+
+	// ReoWndIncr is the multiplier applied to adjust reorder window.
+	ReoWndIncr uint8
+
+	// ReoWndPersist is the number of loss recoveries before resetting
+	// reorder window.
+	ReoWndPersist int8
+
+	// RTTSeq is the SND.NXT when RTT is updated.
+	RTTSeq seqnum.Value
+}
+
+// TCPEndpointID is the unique 4 tuple that identifies a given endpoint.
+//
+// +stateify savable
+type TCPEndpointID struct {
+	// LocalPort is the local port associated with the endpoint.
+	LocalPort uint16
+
+	// LocalAddress is the local [network layer] address associated with
+	// the endpoint.
+	LocalAddress tcpip.Address
+
+	// RemotePort is the remote port associated with the endpoint.
+	RemotePort uint16
+
+	// RemoteAddress it the remote [network layer] address associated with
+	// the endpoint.
+	RemoteAddress tcpip.Address
+}
+
+// TCPFastRecoveryState holds a copy of the internal fast recovery state of a
+// TCP endpoint.
+//
+// +stateify savable
+type TCPFastRecoveryState struct {
+	// Active if true indicates the endpoint is in fast recovery. The
+	// following fields are only meaningful when Active is true.
+	Active bool
+
+	// First is the first unacknowledged sequence number being recovered.
+	First seqnum.Value
+
+	// Last is the 'recover' sequence number that indicates the point at
+	// which we should exit recovery barring any timeouts etc.
+	Last seqnum.Value
+
+	// MaxCwnd is the maximum value we are permitted to grow the congestion
+	// window during recovery. This is set at the time we enter recovery.
+	// It exists to avoid attacks where the receiver intentionally sends
+	// duplicate acks to artificially inflate the sender's cwnd.
+	MaxCwnd int
+
+	// HighRxt is the highest sequence number which has been retransmitted
+	// during the current loss recovery phase.  See: RFC 6675 Section 2 for
+	// details.
+	HighRxt seqnum.Value
+
+	// RescueRxt is the highest sequence number which has been
+	// optimistically retransmitted to prevent stalling of the ACK clock
+	// when there is loss at the end of the window and no new data is
+	// available for transmission.  See: RFC 6675 Section 2 for details.
+	RescueRxt seqnum.Value
+}
+
+// TCPReceiverState holds a copy of the internal state of the receiver for a
+// given TCP endpoint.
+//
+// +stateify savable
+type TCPReceiverState struct {
+	// RcvNxt is the TCP variable RCV.NXT.
+	RcvNxt seqnum.Value
+
+	// RcvAcc is one beyond the last acceptable sequence number. That is,
+	// the "largest" sequence value that the receiver has announced to its
+	// peer that it's willing to accept. This may be different than RcvNxt
+	// + (last advertised receive window) 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 is the window scaling to use for inbound segments.
+	RcvWndScale uint8
+
+	// PendingBufUsed is the number of bytes pending in the receive queue.
+	PendingBufUsed int
+}
+
+// TCPRTTState holds a copy of information about the endpoint's round trip
+// time.
+//
+// +stateify savable
+type TCPRTTState struct {
+	// SRTT is the smoothed round trip time defined in section 2 of RFC
+	// 6298.
+	SRTT time.Duration
+
+	// RTTVar is the round-trip time variation as defined in section 2 of
+	// RFC 6298.
+	RTTVar time.Duration
+
+	// SRTTInited if true indicates that a valid RTT measurement has been
+	// completed.
+	SRTTInited bool
+}
+
+// TCPSenderState holds a copy of the internal state of the sender for a given
+// TCP Endpoint.
+//
+// +stateify savable
+type TCPSenderState struct {
+	// LastSendTime is the timestamp at which we sent the last segment.
+	LastSendTime time.Time `state:".(unixTime)"`
+
+	// DupAckCount is the number of Duplicate ACKs received. It is used for
+	// fast retransmit.
+	DupAckCount int
+
+	// SndCwnd is the size of the sending congestion window in packets.
+	SndCwnd int
+
+	// Ssthresh is the threshold between slow start and congestion
+	// avoidance.
+	Ssthresh int
+
+	// SndCAAckCount is the number of packets acknowledged during
+	// congestion avoidance. When enough packets have been ack'd (typically
+	// cwnd packets), the congestion window is incremented by one.
+	SndCAAckCount int
+
+	// Outstanding is the number of packets that have been sent but not yet
+	// acknowledged.
+	Outstanding int
+
+	// SackedOut is the number of packets which have been selectively
+	// acked.
+	SackedOut int
+
+	// SndWnd is the send window size in bytes.
+	SndWnd seqnum.Size
+
+	// SndUna is the next unacknowledged sequence number.
+	SndUna seqnum.Value
+
+	// SndNxt is the sequence number of the next segment to be sent.
+	SndNxt seqnum.Value
+
+	// RTTMeasureSeqNum is the sequence number being used for the latest
+	// RTT measurement.
+	RTTMeasureSeqNum seqnum.Value
+
+	// RTTMeasureTime is the time when the RTTMeasureSeqNum was sent.
+	RTTMeasureTime time.Time `state:".(unixTime)"`
+
+	// Closed indicates that the caller has closed the endpoint for
+	// sending.
+	Closed bool
+
+	// RTO is the retransmit timeout as defined in section of 2 of RFC
+	// 6298.
+	RTO time.Duration
+
+	// RTTState holds information about the endpoint's round trip time.
+	RTTState TCPRTTState
+
+	// MaxPayloadSize is the maximum size of the payload of a given
+	// segment.  It is initialized on demand.
+	MaxPayloadSize int
+
+	// SndWndScale is the number of bits to shift left when reading the
+	// send window size from a segment.
+	SndWndScale uint8
+
+	// MaxSentAck is the highest acknowledgement number sent till now.
+	MaxSentAck seqnum.Value
+
+	// FastRecovery holds the fast recovery state for the endpoint.
+	FastRecovery TCPFastRecoveryState
+
+	// Cubic holds the state related to CUBIC congestion control.
+	Cubic TCPCubicState
+
+	// RACKState holds the state related to RACK loss detection algorithm.
+	RACKState TCPRACKState
+}
+
+// TCPSACKInfo holds TCP SACK related information for a given TCP endpoint.
+//
+// +stateify savable
+type TCPSACKInfo struct {
+	// Blocks is the list of SACK Blocks that identify the out of order
+	// segments held by a given TCP endpoint.
+	Blocks []header.SACKBlock
+
+	// ReceivedBlocks are the SACK blocks received by this endpoint from
+	// the peer endpoint.
+	ReceivedBlocks []header.SACKBlock
+
+	// MaxSACKED is the highest sequence number that has been SACKED by the
+	// peer.
+	MaxSACKED seqnum.Value
+}
+
+// RcvBufAutoTuneParams holds state related to TCP receive buffer auto-tuning.
+//
+// +stateify savable
+type RcvBufAutoTuneParams struct {
+	// MeasureTime is the time at which the current measurement was
+	// started.
+	MeasureTime time.Time `state:".(unixTime)"`
+
+	// CopiedBytes is the number of bytes copied to user space since this
+	// measure began.
+	CopiedBytes int
+
+	// PrevCopiedBytes is the number of bytes copied to userspace in the
+	// previous RTT period.
+	PrevCopiedBytes int
+
+	// RcvBufSize is the auto tuned receive buffer size.
+	RcvBufSize int
+
+	// RTT is the smoothed RTT as measured by observing the time between
+	// when a byte is first acknowledged and the receipt of data that is at
+	// least one window beyond the sequence number that was acknowledged.
+	RTT time.Duration
+
+	// RTTVar is the "round-trip time variation" as defined in section 2 of
+	// RFC6298.
+	RTTVar time.Duration
+
+	// RTTMeasureSeqNumber is the highest acceptable sequence number at the
+	// time this RTT measurement period began.
+	RTTMeasureSeqNumber seqnum.Value
+
+	// RTTMeasureTime is the absolute time at which the current RTT
+	// measurement period began.
+	RTTMeasureTime time.Time `state:".(unixTime)"`
+
+	// Disabled is true if an explicit receive buffer is set for the
+	// endpoint.
+	Disabled bool
+}
+
+// TCPRcvBufState contains information about the state of an endpoint's receive
+// socket buffer.
+//
+// +stateify savable
+type TCPRcvBufState struct {
+	// RcvBufSize is the size of the receive socket buffer for the
+	// endpoint.
+	RcvBufSize int
+
+	// RcvBufUsed is the amount of bytes actually held in the receive
+	// socket buffer for the endpoint.
+	RcvBufUsed int
+
+	// RcvBufAutoTuneParams is used to hold state variables to compute the
+	// auto tuned receive buffer size.
+	RcvAutoParams RcvBufAutoTuneParams
+
+	// RcvClosed if true, indicates the endpoint has been closed for
+	// reading.
+	RcvClosed bool
+}
+
+// TCPSndBufState contains information about the state of an endpoint's send
+// socket buffer.
+//
+// +stateify savable
+type TCPSndBufState struct {
+	// SndBufSize is the size of the socket send buffer.
+	SndBufSize int
+
+	// SndBufUsed is the number of bytes held in the socket send buffer.
+	SndBufUsed int
+
+	// SndClosed indicates that the endpoint has been closed for sends.
+	SndClosed bool
+
+	// SndBufInQueue is the number of bytes in the send queue.
+	SndBufInQueue seqnum.Size
+
+	// PacketTooBigCount is used to notify the main protocol routine how
+	// many times a "packet too big" control packet is received.
+	PacketTooBigCount int
+
+	// SndMTU is the smallest MTU seen in the control packets received.
+	SndMTU int
+}
+
+// TCPEndpointStateInner contains the members of TCPEndpointState used directly
+// (that is, not within another containing struct) within the endpoint's
+// internal implementation.
+//
+// +stateify savable
+type TCPEndpointStateInner struct {
+	// TSOffset is a randomized offset added to the value of the TSVal
+	// field in the timestamp option.
+	TSOffset uint32
+
+	// SACKPermitted is set to true if the peer sends the TCPSACKPermitted
+	// option in the SYN/SYN-ACK.
+	SACKPermitted bool
+
+	// SendTSOk is used to indicate when the TS Option has been negotiated.
+	// When sendTSOk is true every non-RST segment should carry a TS as per
+	// RFC7323#section-1.1.
+	SendTSOk bool
+
+	// RecentTS is the timestamp that should be sent in the TSEcr field of
+	// the timestamp for future segments sent by the endpoint. This field
+	// is updated if required when a new segment is received by this
+	// endpoint.
+	RecentTS uint32
+}
+
+// TCPEndpointState is a copy of the internal state of a TCP endpoint.
+//
+// +stateify savable
+type TCPEndpointState struct {
+	// TCPEndpointStateInner contains the members of TCPEndpointState used
+	// by the endpoint's internal implementation.
+	TCPEndpointStateInner
+
+	// ID is a copy of the TransportEndpointID for the endpoint.
+	ID TCPEndpointID
+
+	// SegTime denotes the absolute time when this segment was received.
+	SegTime time.Time `state:".(unixTime)"`
+
+	// RcvBufState contains information about the state of the endpoint's
+	// receive socket buffer.
+	RcvBufState TCPRcvBufState
+
+	// SndBufState contains information about the state of the endpoint's
+	// send socket buffer.
+	SndBufState TCPSndBufState
+
+	// SACK holds TCP SACK related information for this endpoint.
+	SACK TCPSACKInfo
+
+	// Receiver holds variables related to the TCP receiver for the
+	// endpoint.
+	Receiver TCPReceiverState
+
+	// Sender holds state related to the TCP Sender for the endpoint.
+	Sender TCPSenderState
+}