From bd77bb41df51d635db435cd6e74072359dc3a818 Mon Sep 17 00:00:00 2001
From: Martin Stein <martin.stein@genode-labs.com>
Date: Thu, 12 Jul 2018 00:56:14 +0200
Subject: [PATCH] nic_router: improve handling of TCP termination

Like suggested by RFC 2663, reprogram the dissolve timeout of a TCP link
state to 2 times the maximum segment lifetime (by default 1 minute) when
receiving a matching packet with the FIN flag set, or with the ACK flag
set to acknowledge a FIN of the remote side.

Mark a link state as closed (no further reprogramming of the dissolve
timeout) and set the dissolve timeout to 2 times the maximum segment
lifetime when receiving a packet with the RESET flag set.

Issue #2953
---
 repos/os/src/server/nic_router/link.cc | 58 ++++++++++----------------
 repos/os/src/server/nic_router/link.h  | 24 +++++++----
 2 files changed, 39 insertions(+), 43 deletions(-)

diff --git a/repos/os/src/server/nic_router/link.cc b/repos/os/src/server/nic_router/link.cc
index 7c11bb2971..707d039f3b 100644
--- a/repos/os/src/server/nic_router/link.cc
+++ b/repos/os/src/server/nic_router/link.cc
@@ -218,46 +218,34 @@ Tcp_link::Tcp_link(Interface                     &cln_interface,
 { }
 
 
-void Tcp_link::_fin_acked()
+void Tcp_link::_tcp_packet(Tcp_packet &tcp,
+                           Peer       &sender,
+                           Peer       &receiver)
 {
-	if (_server_fin_acked && _client_fin_acked) {
-		_dissolve_timeout.schedule(Microseconds(_config().tcp_max_segm_lifetime().value << 1));
-		_closed = true;
-	}
-}
-
-
-void Tcp_link::server_packet(Tcp_packet &tcp)
-{
-	if (_closed) {
+	if (_state == State::CLOSED) {
 		return; }
 
-	if (tcp.fin()) {
-		_server_fin  = true; }
-
-	if (tcp.ack() && _client_fin) {
-		_client_fin_acked = true;
-		_fin_acked();
+	if (tcp.rst()) {
+		_state = State::CLOSED;
+	} else {
+		if (tcp.fin()) {
+			sender.fin = true;
+			_state = State::CLOSING;
+		}
+		if (receiver.fin && tcp.ack()) {
+			receiver.fin_acked = true;
+			if (sender.fin_acked) {
+				_state = State::CLOSED; }
+			else {
+				_state = State::CLOSING; }
+		}
 	}
-	if (!_closed) {
-		_packet(); }
-}
-
-
-void Tcp_link::client_packet(Tcp_packet &tcp)
-{
-	if (_closed) {
-		return; }
-
-	if (tcp.fin()) {
-		_client_fin = true; }
-
-	if (tcp.ack() && _server_fin) {
-		_server_fin_acked = true;
-		_fin_acked();
+	if (_state == State::OPEN) {
+		_packet();
+	} else {
+		_dissolve_timeout.schedule(
+			Microseconds(_config().tcp_max_segm_lifetime().value << 1));
 	}
-	if (!_closed) {
-		_packet(); }
 }
 
 
diff --git a/repos/os/src/server/nic_router/link.h b/repos/os/src/server/nic_router/link.h
index 079a742037..e531200068 100644
--- a/repos/os/src/server/nic_router/link.h
+++ b/repos/os/src/server/nic_router/link.h
@@ -200,13 +200,21 @@ class Net::Tcp_link : public Link
 {
 	private:
 
-		bool _client_fin       { false };
-		bool _server_fin       { false };
-		bool _client_fin_acked { false };
-		bool _server_fin_acked { false };
-		bool _closed           { false };
+		enum class State : Genode::uint8_t { OPEN, CLOSING, CLOSED, };
 
-		void _fin_acked();
+		struct Peer
+		{
+			bool fin       { false };
+			bool fin_acked { false };
+		};
+
+		State _state  { State::OPEN };
+		Peer  _client { };
+		Peer  _server { };
+
+		void _tcp_packet(Tcp_packet &tcp,
+		                 Peer       &sender,
+		                 Peer       &receiver);
 
 	public:
 
@@ -219,9 +227,9 @@ class Net::Tcp_link : public Link
 		         Configuration                 &config,
 		         L3_protocol             const  protocol);
 
-		void client_packet(Tcp_packet &tcp);
+		void client_packet(Tcp_packet &tcp) { _tcp_packet(tcp, _client, _server); }
 
-		void server_packet(Tcp_packet &tcp);
+		void server_packet(Tcp_packet &tcp) { _tcp_packet(tcp, _server, _client); }
 };