IPv6: add sendRedirects parameter to suppress ICMPv6 Redirects on ad-hoc links

RFC 4861 Section 8.2 has a router send an ICMPv6 Redirect when it forwards a
packet back out the interface it arrived on. On a wireless multi-hop / ad-hoc
network (e.g. MANET routing over a single shared link) the "better" next hop is
a geographic relay the source cannot reach directly, so the Redirect is useless;
worse, the Redirects are themselves forwarded multi-hop and trigger further
Redirects, causing a storm that congests the medium.

Add a bool sendRedirects parameter to Ipv6 (default true, preserving existing
behavior) and gate the redirect-on-forward block on it, so it can be disabled on
such networks. IPv4 has no redirect-on-forward, so this is IPv6-only.

Author: avarga

src/inet/networklayer/ipv6/Ipv6.cc

@@ -88,6 +88,7 @@ void Ipv6::initialize(int stage)
 
         curFragmentId = 0;
         lastCheckTime = SIMTIME_ZERO;
+        sendRedirects = par("sendRedirects");
         fragbuf.init(icmp);
 
         // NetFilter:
@@ -526,8 +527,10 @@ void Ipv6::routePacket(Packet *packet, const NetworkInterface *destIE, const Net
     // don't raise error if sent to ND or ICMP!
 
     // RFC 4861 Section 8.2: a router sends a Redirect if it is forwarding a
-    // packet back out on the same interface it arrived on, to a different next-hop
-    if (rt->isRouter() && !fromHL && fromIE && interfaceId == fromIE->getInterfaceId()) {
+    // packet back out on the same interface it arrived on, to a different next-hop.
+    // Disabled (sendRedirects=false) on wireless multi-hop/ad-hoc networks, where the
+    // next hop is not directly reachable by the source and redirects cause a storm.
+    if (sendRedirects && rt->isRouter() && !fromHL && fromIE && interfaceId == fromIE->getInterfaceId()) {
         Ipv6Address pktSrcAddr = ipv6Header->getSrcAddress();
         if (nextHop != pktSrcAddr && !pktSrcAddr.isUnspecified() && !pktSrcAddr.isMulticast()) {
             NetworkInterface *outIe = ift->getInterfaceById(interfaceId);

src/inet/networklayer/ipv6/Ipv6.h

@@ -69,6 +69,7 @@ class INET_API Ipv6 : public OperationalBase, public NetfilterBase, public INetw
     ModuleRefByPar<Icmpv6> icmp;
 
     // working vars
+    bool sendRedirects = true; // whether to send ICMPv6 Redirects when forwarding back out the arrival interface (RFC 4861 8.2)
     unsigned int curFragmentId = -1; // counter, used to assign unique fragmentIds to datagrams
     Ipv6FragBuf fragbuf; // fragmentation reassembly buffer
     simtime_t lastCheckTime; // when fragbuf was last checked for state fragments

src/inet/networklayer/ipv6/Ipv6.ned

@@ -67,6 +67,7 @@ simple Ipv6 extends SimpleModule
         string ipv6NeighbourDiscoveryModule;
         string icmpv6Module;
         double procDelay @unit(s) = default(0s);
+        bool sendRedirects = default(true); // RFC 4861 Section 8.2: when a router forwards a packet back out the interface it arrived on, send the source an ICMPv6 Redirect. Disable on wireless multi-hop / ad-hoc networks (e.g. MANET routing), where the geographic next hop is not directly reachable by the source, so redirects are useless and trigger a storm.
         displayStringTextFormat = default("{ipv6StatusText}");
         @display("i=block/network2");
         @signal[packetDropped](type=inet::Packet);