Add support for TCP_CONGESTION socket option.

This CL also cleans up the error returned for setting congestion
control which was incorrectly returning EINVAL instead of ENOENT.

PiperOrigin-RevId: 252889093

10 files changed, 250 insertions, 62 deletions

diff --git a/pkg/sentry/socket/epsocket/epsocket.go b/pkg/sentry/socket/epsocket/epsocket.go
index f67451179..a50798cb3 100644
--- a/pkg/sentry/socket/epsocket/epsocket.go
+++ b/pkg/sentry/socket/epsocket/epsocket.go
@@ -920,6 +920,30 @@ func getSockOptTCP(t *kernel.Task, ep commonEndpoint, name, outLen int) (interfa
 
 		t.Kernel().EmitUnimplementedEvent(t)
 
+	case linux.TCP_CONGESTION:
+		if outLen <= 0 {
+			return nil, syserr.ErrInvalidArgument
+		}
+
+		var v tcpip.CongestionControlOption
+		if err := ep.GetSockOpt(&v); err != nil {
+			return nil, syserr.TranslateNetstackError(err)
+		}
+
+		// We match linux behaviour here where it returns the lower of
+		// TCP_CA_NAME_MAX bytes or the value of the option length.
+		//
+		// This is Linux's net/tcp.h TCP_CA_NAME_MAX.
+		const tcpCANameMax = 16
+
+		toCopy := tcpCANameMax
+		if outLen < tcpCANameMax {
+			toCopy = outLen
+		}
+		b := make([]byte, toCopy)
+		copy(b, v)
+		return b, nil
+
 	default:
 		emitUnimplementedEventTCP(t, name)
 	}
@@ -1222,6 +1246,12 @@ func setSockOptTCP(t *kernel.Task, ep commonEndpoint, name int, optVal []byte) *
 		}
 		return syserr.TranslateNetstackError(ep.SetSockOpt(tcpip.KeepaliveIntervalOption(time.Second * time.Duration(v))))
 
+	case linux.TCP_CONGESTION:
+		v := tcpip.CongestionControlOption(optVal)
+		if err := ep.SetSockOpt(v); err != nil {
+			return syserr.TranslateNetstackError(err)
+		}
+		return nil
 	case linux.TCP_REPAIR_OPTIONS:
 		t.Kernel().EmitUnimplementedEvent(t)
 
diff --git a/pkg/tcpip/tcpip.go b/pkg/tcpip/tcpip.go
index 85ef014d0..04c776205 100644
--- a/pkg/tcpip/tcpip.go
+++ b/pkg/tcpip/tcpip.go
@@ -472,6 +472,14 @@ type KeepaliveIntervalOption time.Duration
 // closed.
 type KeepaliveCountOption int
 
+// CongestionControlOption is used by SetSockOpt/GetSockOpt to set/get
+// the current congestion control algorithm.
+type CongestionControlOption string
+
+// AvailableCongestionControlOption is used to query the supported congestion
+// control algorithms.
+type AvailableCongestionControlOption string
+
 // MulticastTTLOption is used by SetSockOpt/GetSockOpt to control the default
 // TTL value for multicast messages. The default is 1.
 type MulticastTTLOption uint8
diff --git a/pkg/tcpip/transport/tcp/BUILD b/pkg/tcpip/transport/tcp/BUILD
index 9db38196b..a9dbfb930 100644
--- a/pkg/tcpip/transport/tcp/BUILD
+++ b/pkg/tcpip/transport/tcp/BUILD
@@ -21,6 +21,7 @@ go_library(
         "accept.go",
         "connect.go",
         "cubic.go",
+        "cubic_state.go",
         "endpoint.go",
         "endpoint_state.go",
         "forwarder.go",
diff --git a/pkg/tcpip/transport/tcp/cubic.go b/pkg/tcpip/transport/tcp/cubic.go
index e618cd2b9..7b1f5e763 100644
--- a/pkg/tcpip/transport/tcp/cubic.go
+++ b/pkg/tcpip/transport/tcp/cubic.go
@@ -23,6 +23,7 @@ import (
 // control algorithm state.
 //
 // See: https://tools.ietf.org/html/rfc8312.
+// +stateify savable
 type cubicState struct {
 	// wLastMax is the previous wMax value.
 	wLastMax float64
@@ -33,7 +34,7 @@ type cubicState struct {
 
 	// t denotes the time when the current congestion avoidance
 	// was entered.
-	t time.Time
+	t time.Time `state:".(unixTime)"`
 
 	// numCongestionEvents tracks the number of congestion events since last
 	// RTO.
diff --git a/pkg/tcpip/transport/tcp/cubic_state.go b/pkg/tcpip/transport/tcp/cubic_state.go
new file mode 100644
index 000000000..d0f58cfaf
--- /dev/null
+++ b/pkg/tcpip/transport/tcp/cubic_state.go
@@ -0,0 +1,29 @@
+// Copyright 2019 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 tcp
+
+import (
+	"time"
+)
+
+// saveT is invoked by stateify.
+func (c *cubicState) saveT() unixTime {
+	return unixTime{c.t.Unix(), c.t.UnixNano()}
+}
+
+// loadT is invoked by stateify.
+func (c *cubicState) loadT(unix unixTime) {
+	c.t = time.Unix(unix.second, unix.nano)
+}
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 23422ca5e..1efe9d3fb 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -17,6 +17,7 @@ package tcp
 import (
 	"fmt"
 	"math"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -286,7 +287,7 @@ type endpoint struct {
 
 	// cc stores the name of the Congestion Control algorithm to use for
 	// this endpoint.
-	cc CongestionControlOption
+	cc tcpip.CongestionControlOption
 
 	// The following are used when a "packet too big" control packet is
 	// received. They are protected by sndBufMu. They are used to
@@ -394,7 +395,7 @@ func newEndpoint(stack *stack.Stack, netProto tcpip.NetworkProtocolNumber, waite
 		e.rcvBufSize = rs.Default
 	}
 
-	var cs CongestionControlOption
+	var cs tcpip.CongestionControlOption
 	if err := stack.TransportProtocolOption(ProtocolNumber, &cs); err == nil {
 		e.cc = cs
 	}
@@ -898,6 +899,40 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 		e.mu.Unlock()
 		return nil
 
+	case tcpip.CongestionControlOption:
+		// Query the available cc algorithms in the stack and
+		// validate that the specified algorithm is actually
+		// supported in the stack.
+		var avail tcpip.AvailableCongestionControlOption
+		if err := e.stack.TransportProtocolOption(ProtocolNumber, &avail); err != nil {
+			return err
+		}
+		availCC := strings.Split(string(avail), " ")
+		for _, cc := range availCC {
+			if v == tcpip.CongestionControlOption(cc) {
+				// Acquire the work mutex as we may need to
+				// reinitialize the congestion control state.
+				e.mu.Lock()
+				state := e.state
+				e.cc = v
+				e.mu.Unlock()
+				switch state {
+				case StateEstablished:
+					e.workMu.Lock()
+					e.mu.Lock()
+					if e.state == state {
+						e.snd.cc = e.snd.initCongestionControl(e.cc)
+					}
+					e.mu.Unlock()
+					e.workMu.Unlock()
+				}
+				return nil
+			}
+		}
+
+		// Linux returns ENOENT when an invalid congestion
+		// control algorithm is specified.
+		return tcpip.ErrNoSuchFile
 	default:
 		return nil
 	}
@@ -1067,6 +1102,12 @@ func (e *endpoint) GetSockOpt(opt interface{}) *tcpip.Error {
 		}
 		return nil
 
+	case *tcpip.CongestionControlOption:
+		e.mu.Lock()
+		*o = e.cc
+		e.mu.Unlock()
+		return nil
+
 	default:
 		return tcpip.ErrUnknownProtocolOption
 	}
diff --git a/pkg/tcpip/transport/tcp/protocol.go b/pkg/tcpip/transport/tcp/protocol.go
index b31bcccfa..59f4009a1 100644
--- a/pkg/tcpip/transport/tcp/protocol.go
+++ b/pkg/tcpip/transport/tcp/protocol.go
@@ -79,13 +79,6 @@ const (
 	ccCubic = "cubic"
 )
 
-// CongestionControlOption sets the current congestion control algorithm.
-type CongestionControlOption string
-
-// AvailableCongestionControlOption returns the supported congestion control
-// algorithms.
-type AvailableCongestionControlOption string
-
 type protocol struct {
 	mu                         sync.Mutex
 	sackEnabled                bool
@@ -93,7 +86,6 @@ type protocol struct {
 	recvBufferSize             ReceiveBufferSizeOption
 	congestionControl          string
 	availableCongestionControl []string
-	allowedCongestionControl   []string
 }
 
 // Number returns the tcp protocol number.
@@ -188,7 +180,7 @@ func (p *protocol) SetOption(option interface{}) *tcpip.Error {
 		p.mu.Unlock()
 		return nil
 
-	case CongestionControlOption:
+	case tcpip.CongestionControlOption:
 		for _, c := range p.availableCongestionControl {
 			if string(v) == c {
 				p.mu.Lock()
@@ -197,7 +189,9 @@ func (p *protocol) SetOption(option interface{}) *tcpip.Error {
 				return nil
 			}
 		}
-		return tcpip.ErrInvalidOptionValue
+		// linux returns ENOENT when an invalid congestion control
+		// is specified.
+		return tcpip.ErrNoSuchFile
 	default:
 		return tcpip.ErrUnknownProtocolOption
 	}
@@ -223,14 +217,14 @@ func (p *protocol) Option(option interface{}) *tcpip.Error {
 		*v = p.recvBufferSize
 		p.mu.Unlock()
 		return nil
-	case *CongestionControlOption:
+	case *tcpip.CongestionControlOption:
 		p.mu.Lock()
-		*v = CongestionControlOption(p.congestionControl)
+		*v = tcpip.CongestionControlOption(p.congestionControl)
 		p.mu.Unlock()
 		return nil
-	case *AvailableCongestionControlOption:
+	case *tcpip.AvailableCongestionControlOption:
 		p.mu.Lock()
-		*v = AvailableCongestionControlOption(strings.Join(p.availableCongestionControl, " "))
+		*v = tcpip.AvailableCongestionControlOption(strings.Join(p.availableCongestionControl, " "))
 		p.mu.Unlock()
 		return nil
 	default:
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index b236d7af2..daa3e8341 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -194,8 +194,6 @@ func newSender(ep *endpoint, iss, irs seqnum.Value, sndWnd seqnum.Size, mss uint
 
 	s := &sender{
 		ep:               ep,
-		sndCwnd:          InitialCwnd,
-		sndSsthresh:      math.MaxInt64,
 		sndWnd:           sndWnd,
 		sndUna:           iss + 1,
 		sndNxt:           iss + 1,
@@ -238,7 +236,13 @@ func newSender(ep *endpoint, iss, irs seqnum.Value, sndWnd seqnum.Size, mss uint
 	return s
 }
 
-func (s *sender) initCongestionControl(congestionControlName CongestionControlOption) congestionControl {
+// initCongestionControl initializes the specified congestion control module and
+// returns a handle to it. It also initializes the sndCwnd and sndSsThresh to
+// their initial values.
+func (s *sender) initCongestionControl(congestionControlName tcpip.CongestionControlOption) congestionControl {
+	s.sndCwnd = InitialCwnd
+	s.sndSsthresh = math.MaxInt64
+
 	switch congestionControlName {
 	case ccCubic:
 		return newCubicCC(s)
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 779ca8b76..7d8987219 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -3205,13 +3205,14 @@ func TestTCPEndpointProbe(t *testing.T) {
 	}
 }
 
-func TestSetCongestionControl(t *testing.T) {
+func TestStackSetCongestionControl(t *testing.T) {
 	testCases := []struct {
-		cc       tcp.CongestionControlOption
-		mustPass bool
+		cc  tcpip.CongestionControlOption
+		err *tcpip.Error
 	}{
-		{"reno", true},
-		{"cubic", true},
+		{"reno", nil},
+		{"cubic", nil},
+		{"blahblah", tcpip.ErrNoSuchFile},
 	}
 
 	for _, tc := range testCases {
@@ -3221,62 +3222,135 @@ func TestSetCongestionControl(t *testing.T) {
 
 			s := c.Stack()
 
-			if err := s.SetTransportProtocolOption(tcp.ProtocolNumber, tc.cc); err != nil && tc.mustPass {
-				t.Fatalf("s.SetTransportProtocolOption(%v, %v) = %v, want not-nil", tcp.ProtocolNumber, tc.cc, err)
+			var oldCC tcpip.CongestionControlOption
+			if err := s.TransportProtocolOption(tcp.ProtocolNumber, &oldCC); err != nil {
+				t.Fatalf("s.TransportProtocolOption(%v, %v) = %v", tcp.ProtocolNumber, &oldCC, err)
 			}
 
-			var cc tcp.CongestionControlOption
+			if err := s.SetTransportProtocolOption(tcp.ProtocolNumber, tc.cc); err != tc.err {
+				t.Fatalf("s.SetTransportProtocolOption(%v, %v) = %v, want %v", tcp.ProtocolNumber, tc.cc, err, tc.err)
+			}
+
+			var cc tcpip.CongestionControlOption
 			if err := s.TransportProtocolOption(tcp.ProtocolNumber, &cc); err != nil {
 				t.Fatalf("s.TransportProtocolOption(%v, %v) = %v", tcp.ProtocolNumber, &cc, err)
 			}
-			if got, want := cc, tc.cc; got != want {
+
+			got, want := cc, oldCC
+			// If SetTransportProtocolOption is expected to succeed
+			// then the returned value for congestion control should
+			// match the one specified in the
+			// SetTransportProtocolOption call above, else it should
+			// be what it was before the call to
+			// SetTransportProtocolOption.
+			if tc.err == nil {
+				want = tc.cc
+			}
+			if got != want {
 				t.Fatalf("got congestion control: %v, want: %v", got, want)
 			}
 		})
 	}
 }
 
-func TestAvailableCongestionControl(t *testing.T) {
+func TestStackAvailableCongestionControl(t *testing.T) {
 	c := context.New(t, 1500)
 	defer c.Cleanup()
 
 	s := c.Stack()
 
 	// Query permitted congestion control algorithms.
-	var aCC tcp.AvailableCongestionControlOption
+	var aCC tcpip.AvailableCongestionControlOption
 	if err := s.TransportProtocolOption(tcp.ProtocolNumber, &aCC); err != nil {
 		t.Fatalf("s.TransportProtocolOption(%v, %v) = %v", tcp.ProtocolNumber, &aCC, err)
 	}
-	if got, want := aCC, tcp.AvailableCongestionControlOption("reno cubic"); got != want {
-		t.Fatalf("got tcp.AvailableCongestionControlOption: %v, want: %v", got, want)
+	if got, want := aCC, tcpip.AvailableCongestionControlOption("reno cubic"); got != want {
+		t.Fatalf("got tcpip.AvailableCongestionControlOption: %v, want: %v", got, want)
 	}
 }
 
-func TestSetAvailableCongestionControl(t *testing.T) {
+func TestStackSetAvailableCongestionControl(t *testing.T) {
 	c := context.New(t, 1500)
 	defer c.Cleanup()
 
 	s := c.Stack()
 
 	// Setting AvailableCongestionControlOption should fail.
-	aCC := tcp.AvailableCongestionControlOption("xyz")
+	aCC := tcpip.AvailableCongestionControlOption("xyz")
 	if err := s.SetTransportProtocolOption(tcp.ProtocolNumber, &aCC); err == nil {
 		t.Fatalf("s.TransportProtocolOption(%v, %v) = nil, want non-nil", tcp.ProtocolNumber, &aCC)
 	}
 
 	// Verify that we still get the expected list of congestion control options.
-	var cc tcp.AvailableCongestionControlOption
+	var cc tcpip.AvailableCongestionControlOption
 	if err := s.TransportProtocolOption(tcp.ProtocolNumber, &cc); err != nil {
 		t.Fatalf("s.TransportProtocolOption(%v, %v) = %v", tcp.ProtocolNumber, &cc, err)
 	}
-	if got, want := cc, tcp.AvailableCongestionControlOption("reno cubic"); got != want {
-		t.Fatalf("got tcp.AvailableCongestionControlOption: %v, want: %v", got, want)
+	if got, want := cc, tcpip.AvailableCongestionControlOption("reno cubic"); got != want {
+		t.Fatalf("got tcpip.AvailableCongestionControlOption: %v, want: %v", got, want)
+	}
+}
+
+func TestEndpointSetCongestionControl(t *testing.T) {
+	testCases := []struct {
+		cc  tcpip.CongestionControlOption
+		err *tcpip.Error
+	}{
+		{"reno", nil},
+		{"cubic", nil},
+		{"blahblah", tcpip.ErrNoSuchFile},
+	}
+
+	for _, connected := range []bool{false, true} {
+		for _, tc := range testCases {
+			t.Run(fmt.Sprintf("SetSockOpt(.., %v) w/ connected = %v", tc.cc, connected), func(t *testing.T) {
+				c := context.New(t, 1500)
+				defer c.Cleanup()
+
+				// Create TCP endpoint.
+				var err *tcpip.Error
+				c.EP, err = c.Stack().NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, &c.WQ)
+				if err != nil {
+					t.Fatalf("NewEndpoint failed: %v", err)
+				}
+
+				var oldCC tcpip.CongestionControlOption
+				if err := c.EP.GetSockOpt(&oldCC); err != nil {
+					t.Fatalf("c.EP.SockOpt(%v) = %v", &oldCC, err)
+				}
+
+				if connected {
+					c.Connect(789 /* iss */, 32768 /* rcvWnd */, nil)
+				}
+
+				if err := c.EP.SetSockOpt(tc.cc); err != tc.err {
+					t.Fatalf("c.EP.SetSockOpt(%v) = %v, want %v", tc.cc, err, tc.err)
+				}
+
+				var cc tcpip.CongestionControlOption
+				if err := c.EP.GetSockOpt(&cc); err != nil {
+					t.Fatalf("c.EP.SockOpt(%v) = %v", &cc, err)
+				}
+
+				got, want := cc, oldCC
+				// If SetSockOpt is expected to succeed then the
+				// returned value for congestion control should match
+				// the one specified in the SetSockOpt above, else it
+				// should be what it was before the call to SetSockOpt.
+				if tc.err == nil {
+					want = tc.cc
+				}
+				if got != want {
+					t.Fatalf("got congestion control: %v, want: %v", got, want)
+				}
+			})
+		}
 	}
 }
 
 func enableCUBIC(t *testing.T, c *context.Context) {
 	t.Helper()
-	opt := tcp.CongestionControlOption("cubic")
+	opt := tcpip.CongestionControlOption("cubic")
 	if err := c.Stack().SetTransportProtocolOption(tcp.ProtocolNumber, opt); err != nil {
 		t.Fatalf("c.s.SetTransportProtocolOption(tcp.ProtocolNumber, %v = %v", opt, err)
 	}
diff --git a/pkg/tcpip/transport/tcp/testing/context/context.go b/pkg/tcpip/transport/tcp/testing/context/context.go
index 69a43b6f4..a4d89e24d 100644
--- a/pkg/tcpip/transport/tcp/testing/context/context.go
+++ b/pkg/tcpip/transport/tcp/testing/context/context.go
@@ -520,35 +520,21 @@ func (c *Context) CreateConnected(iss seqnum.Value, rcvWnd seqnum.Size, epRcvBuf
 	c.CreateConnectedWithRawOptions(iss, rcvWnd, epRcvBuf, nil)
 }
 
-// CreateConnectedWithRawOptions creates a connected TCP endpoint and sends
-// the specified option bytes as the Option field in the initial SYN packet.
+// Connect performs the 3-way handshake for c.EP with the provided Initial
+// Sequence Number (iss) and receive window(rcvWnd) and any options if
+// specified.
 //
 // It also sets the receive buffer for the endpoint to the specified
 // value in epRcvBuf.
-func (c *Context) CreateConnectedWithRawOptions(iss seqnum.Value, rcvWnd seqnum.Size, epRcvBuf *tcpip.ReceiveBufferSizeOption, options []byte) {
-	// Create TCP endpoint.
-	var err *tcpip.Error
-	c.EP, err = c.s.NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, &c.WQ)
-	if err != nil {
-		c.t.Fatalf("NewEndpoint failed: %v", err)
-	}
-	if got, want := tcp.EndpointState(c.EP.State()), tcp.StateInitial; got != want {
-		c.t.Errorf("Unexpected endpoint state: want %v, got %v", want, got)
-	}
-
-	if epRcvBuf != nil {
-		if err := c.EP.SetSockOpt(*epRcvBuf); err != nil {
-			c.t.Fatalf("SetSockOpt failed failed: %v", err)
-		}
-	}
-
+//
+// PreCondition: c.EP must already be created.
+func (c *Context) Connect(iss seqnum.Value, rcvWnd seqnum.Size, options []byte) {
 	// Start connection attempt.
 	waitEntry, notifyCh := waiter.NewChannelEntry(nil)
 	c.WQ.EventRegister(&waitEntry, waiter.EventOut)
 	defer c.WQ.EventUnregister(&waitEntry)
 
-	err = c.EP.Connect(tcpip.FullAddress{Addr: TestAddr, Port: TestPort})
-	if err != tcpip.ErrConnectStarted {
+	if err := c.EP.Connect(tcpip.FullAddress{Addr: TestAddr, Port: TestPort}); err != tcpip.ErrConnectStarted {
 		c.t.Fatalf("Unexpected return value from Connect: %v", err)
 	}
 
@@ -590,8 +576,7 @@ func (c *Context) CreateConnectedWithRawOptions(iss seqnum.Value, rcvWnd seqnum.
 	// Wait for connection to be established.
 	select {
 	case <-notifyCh:
-		err = c.EP.GetSockOpt(tcpip.ErrorOption{})
-		if err != nil {
+		if err := c.EP.GetSockOpt(tcpip.ErrorOption{}); err != nil {
 			c.t.Fatalf("Unexpected error when connecting: %v", err)
 		}
 	case <-time.After(1 * time.Second):
@@ -604,6 +589,27 @@ func (c *Context) CreateConnectedWithRawOptions(iss seqnum.Value, rcvWnd seqnum.
 	c.Port = tcpHdr.SourcePort()
 }
 
+// CreateConnectedWithRawOptions creates a connected TCP endpoint and sends
+// the specified option bytes as the Option field in the initial SYN packet.
+//
+// It also sets the receive buffer for the endpoint to the specified
+// value in epRcvBuf.
+func (c *Context) CreateConnectedWithRawOptions(iss seqnum.Value, rcvWnd seqnum.Size, epRcvBuf *tcpip.ReceiveBufferSizeOption, options []byte) {
+	// Create TCP endpoint.
+	var err *tcpip.Error
+	c.EP, err = c.s.NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, &c.WQ)
+	if err != nil {
+		c.t.Fatalf("NewEndpoint failed: %v", err)
+	}
+
+	if epRcvBuf != nil {
+		if err := c.EP.SetSockOpt(*epRcvBuf); err != nil {
+			c.t.Fatalf("SetSockOpt failed failed: %v", err)
+		}
+	}
+	c.Connect(iss, rcvWnd, options)
+}
+
 // RawEndpoint is just a small wrapper around a TCP endpoint's state to make
 // sending data and ACK packets easy while being able to manipulate the sequence
 // numbers and timestamp values as needed.