[PATCH 4 of 4] Stream: added MPTCP support

[Anthony Doeraene]

# HG changeset patch
# User Anthony Doeraene <anthony.doeraene.dev at gmail.com>
# Date 1723532563 -7200
#      Tue Aug 13 09:02:43 2024 +0200
# Node ID 3bc05b0a87ba68fc436444294f7ec1529a13c10e
# Parent  19780d9b772a8b7192c20fc0fe7890dd381f0cce
Stream: added MPTCP support.

Multipath TCP (MPTCP), standardized in RFC8684 [1], is a TCP extension
that enables a TCP connection to use different paths.

Multipath TCP has been used for several use cases. On smartphones, MPTCP
enables seamless handovers between cellular and Wi-Fi networks while
preserving Established connections. This use-case is what pushed Apple
to use MPTCP since 2013 in multiple applications [2]. On dual-stack
hosts, Multipath TCP enables the TCP connection to automatically use the
best performing path, either IPv4 or IPv6. If one path fails, MPTCP
automatically uses the other path.

The benefit from MPTCP, both the client and the server have to support
it. Multipath TCP is a backward-compatible TCP extension that is enabled
by default on recent Linux distributions (Debian, Ubuntu, Redhat, ...).
Multipath TCP is included in the Linux kernel since version 5.6 [3].
To use it on Linux, an application must explicitly enable it when
creating the socket. No need to change anything else in the application.

Even if MPTCP is supported by different OS, only Linux supports the
`IPPROTO_MPTCP` protocol, which is why this feature is currently
limited to Linux only.

This patch adds a new parameter 'multipath' to the 'listen' directive
in the Stream module. This new parameter is only compatible with TCP if
IPPROTO_MPTCP is defined, not with QUIC so far.

Co-developed-by: Maxime Dourov <mux99 at live.be>

Link: https://www.rfc-editor.org/rfc/rfc8684.html [1]
Link: https://www.tessares.net/apples-mptcp-story-so-far/ [2]
Link: https://www.mptcp.dev [3]

diff -r 19780d9b772a -r 3bc05b0a87ba src/stream/ngx_stream.c
--- a/src/stream/ngx_stream.c	Tue Aug 13 08:59:41 2024 +0200
+++ b/src/stream/ngx_stream.c	Tue Aug 13 09:02:43 2024 +0200
@@ -486,6 +486,7 @@
             ls->handler = ngx_stream_init_connection;
             ls->pool_size = 256;
             ls->type = addr[i].opt.type;
+            ls->protocol = addr[i].opt.protocol;
 
             cscf = addr->opt.ctx->srv_conf[ngx_stream_core_module.ctx_index];
 
diff -r 19780d9b772a -r 3bc05b0a87ba src/stream/ngx_stream.h
--- a/src/stream/ngx_stream.h	Tue Aug 13 08:59:41 2024 +0200
+++ b/src/stream/ngx_stream.h	Tue Aug 13 09:02:43 2024 +0200
@@ -69,6 +69,7 @@
     int                            fastopen;
 #endif
     int                            type;
+    int                            protocol;
 } ngx_stream_listen_t;
 
 
diff -r 19780d9b772a -r 3bc05b0a87ba src/stream/ngx_stream_core_module.c
--- a/src/stream/ngx_stream_core_module.c	Tue Aug 13 08:59:41 2024 +0200
+++ b/src/stream/ngx_stream_core_module.c	Tue Aug 13 09:02:43 2024 +0200
@@ -654,6 +654,13 @@
         }
 #endif
 
+#ifdef IPPROTO_MPTCP
+        if (ngx_strcmp(value[i].data, "multipath") == 0) {
+            ls->protocol = IPPROTO_MPTCP;
+            continue;
+        }
+#endif
+
         if (ngx_strncmp(value[i].data, "backlog=", 8) == 0) {
             ls->backlog = ngx_atoi(value[i].data + 8, value[i].len - 8);
             ls->bind = 1;
@@ -886,6 +893,12 @@
 #endif
     }
 
+#ifdef IPPROTO_MPTCP
+        if (ls->protocol == IPPROTO_MPTCP) {
+             return "\"multipath\" parameter is incompatible with \"udp\"";
+        }
+#endif
+
     for (n = 0; n < u.naddrs; n++) {
 
         for (i = 0; i < n; i++) {