set MTU if local address is specified explicitly. update MTU for ipv6 if not set

libi2pd/RouterContext.cpp

@@ -420,8 +420,12 @@ namespace i2p
 			if (address->host != host && address->IsCompatible (host) &&
 				!i2p::util::net::IsYggdrasilAddress (address->host))
 			{
+				// update host
 				address->host = host;
-				if (host.is_v6 () && (address->transportStyle == i2p::data::RouterInfo::eTransportSSU || address->IsSSU2 ()))
+				updated = true;
+			}
+			if (host.is_v6 () && address->IsV6 () && address->ssu && 
+			    (!address->ssu->mtu || updated))
 			{
 				// update MTU
 				auto mtu = i2p::util::net::GetMTU (host);
@@ -436,12 +440,11 @@ namespace i2p
 					}
 					if (mtu && !address->IsSSU2 ()) // SSU1
 						mtu = (mtu >> 4) << 4; // round to multiple of 16
-						if (address->ssu) address->ssu->mtu = mtu;
-					}
-				}
+					address->ssu->mtu = mtu;
 					updated = true;
 				}
 			}
+		}
 		auto ts = i2p::util::GetSecondsSinceEpoch ();
 		if (updated || ts > m_LastUpdateTime + ROUTER_INFO_UPDATE_INTERVAL)
 			UpdateRouterInfo ();
@@ -866,6 +869,43 @@ namespace i2p
 		UpdateRouterInfo ();
 	}
 
+	void RouterContext::SetMTU (int mtu, bool v4)
+	{
+		if (mtu < 1280 || mtu > 1500) return;
+		auto& addresses = m_RouterInfo.GetAddresses ();
+		for (auto& addr: addresses)
+		{
+			if (addr->ssu && ((v4 && addr->IsV4 ()) || (!v4 && addr->IsV6 ())))
+			{
+				if (!addr->IsSSU2 ()) // SSU1
+				{	
+					// round to multiple of 16
+					if (v4)
+					{	
+						if (mtu > 1484) mtu = 1484;
+						else
+						{	
+							mtu -= 12;
+							mtu = (mtu >> 4) << 4;
+							mtu += 12;
+						}	
+					}		
+					else	
+					{
+						if (mtu > 1488) mtu = 1488;
+						else
+							mtu = (mtu >> 4) << 4;
+					}	
+				}	
+				if (mtu)
+				{	
+					addr->ssu->mtu = mtu;
+					LogPrint (eLogDebug, "Router: MTU for ", v4 ? "ipv4" : "ipv6", " address is set to ", mtu);
+				}	
+			}	
+		}	
+	}	
+		
 	void RouterContext::UpdateNTCP2V6Address (const boost::asio::ip::address& host)
 	{
 		bool isYgg = i2p::util::net::IsYggdrasilAddress (host);

libi2pd/RouterContext.h

@@ -144,6 +144,7 @@ namespace garlic
 			void SetSupportsV6 (bool supportsV6);
 			void SetSupportsV4 (bool supportsV4);
 			void SetSupportsMesh (bool supportsmesh, const boost::asio::ip::address_v6& host);
+			void SetMTU (int mtu, bool v4);
 			i2p::crypto::NoiseSymmetricState& GetCurrentNoiseState () { return m_CurrentNoiseState; };
 
 			void UpdateNTCP2V6Address (const boost::asio::ip::address& host); // called from Daemon. TODO: remove

libi2pd/SSU2.cpp

@@ -115,9 +115,22 @@ namespace transport
 	{
 		if (localAddress.is_unspecified ()) return;
 		if (localAddress.is_v4 ())
+		{	
 			m_AddressV4 = localAddress;
+			int mtu = i2p::util::net::GetMTU (localAddress);
+			if (mtu < (int)SSU2_MIN_PACKET_SIZE) mtu = SSU2_MIN_PACKET_SIZE;
+			if (mtu > (int)SSU2_MAX_PACKET_SIZE) mtu = SSU2_MAX_PACKET_SIZE;
+			i2p::context.SetMTU (mtu, true);
+		}	
 		else if (localAddress.is_v6 ())
+		{	
 			m_AddressV6 = localAddress;
+			int maxMTU = i2p::util::net::GetMaxMTU (localAddress.to_v6 ());
+			int mtu = i2p::util::net::GetMTU (localAddress);
+			if (mtu > maxMTU) mtu = maxMTU;
+			if (mtu < (int)SSU2_MIN_PACKET_SIZE) mtu = SSU2_MIN_PACKET_SIZE;
+			i2p::context.SetMTU (mtu, false);
+		}	
 	}	
 
 	bool SSU2Server::IsSupported (const boost::asio::ip::address& addr) const