Tcp buffer bug fix (#79)

* start working on tracking down tcp buffer bug

* mtu refactor

* modularize test

* more precise testing

* tests fail, but is it the failure we are looking for?

* fix typo in espradio link (#76)

* implement a new backoff abstraction (#75)

* rewrite backoff api

* rewrite tcp.Conn.Write

* keep fixing small things

* much better Conn.Read implementation

* fix critical overflow bug in internal.ConnRWBackoff

---------

Co-authored-by: Joel Wetzell <jwetzell@yahoo.com>

internet/definitions.go

@@ -159,6 +159,8 @@ func (h *handlers) encapsulateNode(node *node, buf []byte, offsetIP, offsetThisF
 		err = nil  // CLOSE error handled gracefully by deleting node.
 		node = nil // Node is destroyed in tryHandleError and invalidated.
 	}
+	// TODO(soypat): We have fuzz tests in place, maybe we can start returning the error up the chain to catch invalid settings at application level so that users don't have to have logs in place to understand invalid config/buffer size.
+	// Encapsulate should only fail with error on programmer errors.
 	if n > 0 {
 		return n, err
 	} else if err != nil {

internet/pcap/capture_test.go

@@ -35,7 +35,7 @@ func makeHttpPayload(body string) ([]byte, error) {
 }
 
 func TestCap(t *testing.T) {
-	const mtu = 1500
+	const mtu = ethernet.MaxMTU
 	const httpBody = "{200,ok}"
 	var buf [mtu]byte
 	var gen ltesto.PacketGen

internet/stack-ethernet.go

@@ -79,7 +79,7 @@ func (ls *StackEthernet) Reset6(mac, gateway [6]byte, mtu, maxNodes int) error {
 // It validates the configuration parameters and resets internal state.
 // The connection ID is incremented on each call to invalidate existing connections.
 func (ls *StackEthernet) Configure(cfg StackEthernetConfig) error {
-	if cfg.MTU > (math.MaxUint16-ethernet.MaxOverheadSize) || cfg.MTU < 256 {
+	if cfg.MTU > ethernet.MaxMTU || cfg.MTU < ethernet.MinimumMTU {
 		return lneto.ErrInvalidConfig
 	} else if cfg.MaxNodes <= 0 {
 		return lneto.ErrInvalidConfig

internet/stack-ip.go

@@ -149,7 +149,7 @@ func (sb *StackIP) Demux(carrierData []byte, offset int) error {
 
 func (sb *StackIP) Encapsulate(carrierData []byte, offsetToIP, offsetToFrame int) (int, error) {
 	frame := carrierData[offsetToFrame:]
-	if len(frame) < 256 {
+	if len(frame) < ipv4.MinimumMTU {
 		return 0, io.ErrShortBuffer
 	}
 	ifrm, _ := ipv4.NewFrame(frame)