Structured Overlay support for TTL limited messages:

When the [overlay] configuration key "expire" is set to 1, proposals
and validations will include a hops field. The hops is incremented with
each relay. Messages with a hop count will be dropped when they exceed
the TTL (Time to Live). Messages containing a hops field will not be
relayed or broadcast to older versions of rippled that don't understand
the field.

This change will not affect normal operation of the network or rippled
instances that do not set "expire" to 1.

src/ripple/overlay/impl/OverlayImpl.cpp

@@ -18,6 +18,7 @@
 //==============================================================================
 
 #include <BeastConfig.h>
+#include <ripple/app/misc/IHashRouter.h>
 #include <ripple/basics/Log.h>
 #include <ripple/basics/make_SSLContext.h>
 #include <ripple/protocol/JsonFields.h>
@@ -693,6 +694,75 @@ OverlayImpl::findPeerByShortID (Peer::id_t const& id)
     return Peer::ptr();
 }
 
+void
+OverlayImpl::send (protocol::TMProposeSet& m)
+{
+    if (setup_.expire)
+        m.set_hops(0);
+    auto const sm = std::make_shared<Message>(
+        m, protocol::mtPROPOSE_LEDGER);
+    for_each([&](std::shared_ptr<PeerImp> const& p)
+    {
+        if (! m.has_hops() || p->hopsAware())
+            p->send(sm);
+    });
+}
+void
+OverlayImpl::send (protocol::TMValidation& m)
+{
+    if (setup_.expire)
+        m.set_hops(0);
+    auto const sm = std::make_shared<Message>(
+        m, protocol::mtVALIDATION);
+    for_each([&](std::shared_ptr<PeerImp> const& p)
+    {
+        if (! m.has_hops() || p->hopsAware())
+            p->send(sm);
+    });
+}
+
+void
+OverlayImpl::relay (protocol::TMProposeSet& m,
+    uint256 const& uid)
+{
+    if (m.has_hops() && m.hops() >= maxTTL)
+        return;
+    std::set<Peer::id_t> skip;
+    if (! getApp().getHashRouter().swapSet (
+            uid, skip, SF_RELAYED))
+        return;
+    auto const sm = std::make_shared<Message>(
+        m, protocol::mtPROPOSE_LEDGER);
+    for_each([&](std::shared_ptr<PeerImp> const& p)
+    {
+        if (skip.find(p->id()) != skip.end())
+            return;
+        if (! m.has_hops() || p->hopsAware())
+            p->send(sm);
+    });
+}
+
+void
+OverlayImpl::relay (protocol::TMValidation& m,
+    uint256 const& uid)
+{
+    if (m.has_hops() && m.hops() >= maxTTL)
+        return;
+    std::set<Peer::id_t> skip;
+    if (! getApp().getHashRouter().swapSet (
+            uid, skip, SF_RELAYED))
+        return;
+    auto const sm = std::make_shared<Message>(
+        m, protocol::mtVALIDATION);
+    for_each([&](std::shared_ptr<PeerImp> const& p)
+    {
+        if (skip.find(p->id()) != skip.end())
+            return;
+        if (! m.has_hops() || p->hopsAware())
+            p->send(sm);
+    });
+}
+
 //------------------------------------------------------------------------------
 
 void
@@ -777,6 +847,7 @@ setup_Overlay (BasicConfig const& config)
     else
         setup.promote = Overlay::Promote::automatic;
     setup.context = make_SSLContext();
+    setup.expire = get<bool>(section, "expire", false);
     return setup;
 }