detect MTU for some known ipv6 tunnel brokers

libi2pd/RouterContext.cpp

@@ -428,13 +428,13 @@ namespace i2p
 					if (mtu)
 					{
 						LogPrint (eLogDebug, "Router: Our v6 MTU=", mtu);
-						int maxMTU = address->IsSSU2 () ? 1500 : 1488; // must be multiple of 16 for SSU1
+						int maxMTU = i2p::util::net::GetMaxMTU (host.to_v6 ());
 						if (mtu > maxMTU) 
 						{ 
 							mtu = maxMTU;
 							LogPrint(eLogWarning, "Router: MTU dropped to upper limit of ", maxMTU, " bytes");
 						}
-						else if (mtu && !address->IsSSU2 ()) // SSU1
+						if (mtu && !address->IsSSU2 ()) // SSU1
 							mtu = (mtu >> 4) << 4; // round to multiple of 16
 						if (address->ssu) address->ssu->mtu = mtu;
 					}

libi2pd/util.cpp

@@ -12,6 +12,7 @@
 
 #include "util.h"
 #include "Log.h"
+#include "I2PEndian.h"
 
 #if not defined (__FreeBSD__)
 #include <pthread.h>
@@ -423,6 +424,25 @@ namespace net
 #endif
 	}
 
+	int GetMaxMTU (const boost::asio::ip::address_v6& localAddress)
+	{
+		uint32_t prefix = bufbe32toh (localAddress.to_bytes ().data ());
+		switch (prefix)
+		{
+			case 0x20010470:
+			case 0x260070ff:
+			// Hurricane Electric	
+				return 1480;
+			break;	
+			case 0x2a06a004:
+			//  route48	
+				return 1420;
+			break;	
+			default: ;	
+		}	
+		return 1500;
+	}	
+	
 	static bool IsYggdrasilAddress (const uint8_t addr[16])
 	{
 		return addr[0] == 0x02 || addr[0] == 0x03;

libi2pd/util.h

@@ -1,5 +1,5 @@
 /*
-* Copyright (c) 2013-2021, The PurpleI2P Project
+* Copyright (c) 2013-2022, The PurpleI2P Project
 *
 * This file is part of Purple i2pd project and licensed under BSD3
 *
@@ -218,6 +218,7 @@ namespace util
 	namespace net
 	{
 		int GetMTU (const boost::asio::ip::address& localAddress);
+		int GetMaxMTU (const boost::asio::ip::address_v6& localAddress); // check tunnel broker for ipv6 address
 		const boost::asio::ip::address GetInterfaceAddress (const std::string & ifname, bool ipv6=false);
 		boost::asio::ip::address_v6 GetYggdrasilAddress ();
 		bool IsLocalAddress (const boost::asio::ip::address& addr);