i2pd/libi2pd/SSU2.cpp

/*
* Copyright (c) 2022, The PurpleI2P Project
*
* This file is part of Purple i2pd project and licensed under BSD3
*
* See full license text in LICENSE file at top of project tree
*/

#include "Log.h"
#include "RouterContext.h"
#include "Transports.h"
#include "NetDb.hpp"
#include "Config.h"
#include "SSU2.h"

namespace i2p
{
namespace transport
{	
	SSU2Server::SSU2Server ():
		RunnableServiceWithWork ("SSU2"), m_ReceiveService ("SSU2r"),
		m_SocketV4 (m_ReceiveService.GetService ()), m_SocketV6 (m_ReceiveService.GetService ()),
		m_AddressV4 (boost::asio::ip::address_v4()), m_AddressV6 (boost::asio::ip::address_v6()),
		m_TerminationTimer (GetService ()), m_ResendTimer (GetService ())
	{
	}

	void SSU2Server::Start ()
	{
		if (!IsRunning ())
		{
			StartIOService ();
			bool found = false;
			auto& addresses = i2p::context.GetRouterInfo ().GetAddresses ();
			for (const auto& address: addresses)
			{
				if (!address) continue;
				if (address->transportStyle == i2p::data::RouterInfo::eTransportSSU2)
				{
					auto port = address->port;
					if (!port)
					{
						uint16_t ssu2Port; i2p::config::GetOption ("ssu2.port", ssu2Port);
						if (ssu2Port) port = ssu2Port;
						else
						{
							uint16_t p; i2p::config::GetOption ("port", p);
							if (p) port = p;
						}
					}
					if (port)
					{
						if (address->IsV4 ())
						{
							found = true;
							OpenSocket (boost::asio::ip::udp::endpoint (m_AddressV4, port));
							m_ReceiveService.GetService ().post(
								[this]()
								{
									Receive (m_SocketV4);
								});
						}
						if (address->IsV6 ())
						{
							found = true;
							OpenSocket (boost::asio::ip::udp::endpoint (m_AddressV6, port));
							m_ReceiveService.GetService ().post(
							[this]()
								{
									Receive (m_SocketV6);
								});
						}
					}
					else
						LogPrint (eLogError, "SSU2: Can't start server because port not specified");
				}
			}
			if (found)
				m_ReceiveService.Start ();
			ScheduleTermination ();
		}
	}

	void SSU2Server::Stop ()
	{
		for (auto& it: m_Sessions)
		{	
			it.second->RequestTermination (eSSU2TerminationReasonRouterShutdown);
			it.second->Done ();
		}	
		m_Sessions.clear ();
		m_SessionsByRouterHash.clear ();
		m_PendingOutgoingSessions.clear ();
		m_Relays.clear ();
		m_Introducers.clear ();
		m_IntroducersV6.clear ();
		
		if (context.SupportsV4 () || context.SupportsV6 ())
			m_ReceiveService.Stop ();

		m_SocketV4.close ();
		m_SocketV6.close ();
		if (IsRunning ())
			m_TerminationTimer.cancel ();
		
		StopIOService ();
	}

	void SSU2Server::SetLocalAddress (const boost::asio::ip::address& localAddress)
	{
		if (localAddress.is_unspecified ()) return;
		if (localAddress.is_v4 ())
			m_AddressV4 = localAddress;
		else if (localAddress.is_v6 ())
			m_AddressV6 = localAddress;
	}	

	bool SSU2Server::IsSupported (const boost::asio::ip::address& addr) const
	{
		if (addr.is_v4 ())
		{	
			if (m_SocketV4.is_open ()) 
				return true;
		}	
		else if (addr.is_v6 ())
		{	
			if (m_SocketV6.is_open ()) 
				return true;
		}
		return false;
	}	
		
	boost::asio::ip::udp::socket& SSU2Server::OpenSocket (const boost::asio::ip::udp::endpoint& localEndpoint)
	{
		boost::asio::ip::udp::socket& socket = localEndpoint.address ().is_v6 () ? m_SocketV6 : m_SocketV4;
		try
		{
			socket.open (localEndpoint.protocol ());
			if (localEndpoint.address ().is_v6 ())
				socket.set_option (boost::asio::ip::v6_only (true));
			socket.set_option (boost::asio::socket_base::receive_buffer_size (SSU2_SOCKET_RECEIVE_BUFFER_SIZE));
			socket.set_option (boost::asio::socket_base::send_buffer_size (SSU2_SOCKET_SEND_BUFFER_SIZE));
			socket.bind (localEndpoint);
			LogPrint (eLogInfo, "SSU2: Start listening on ", localEndpoint);
		}
		catch (std::exception& ex )
		{
			LogPrint (eLogError, "SSU2: Failed to bind to ", localEndpoint, ": ", ex.what());
			ThrowFatal ("Unable to start SSU2 transport on ", localEndpoint, ": ", ex.what ());
		}
		return socket;
	}

	void SSU2Server::Receive (boost::asio::ip::udp::socket& socket)
	{
		Packet * packet = m_PacketsPool.AcquireMt ();
		socket.async_receive_from (boost::asio::buffer (packet->buf, SSU2_MAX_PACKET_SIZE), packet->from,
			std::bind (&SSU2Server::HandleReceivedFrom, this, std::placeholders::_1, std::placeholders::_2, packet, std::ref (socket)));
	}

	void SSU2Server::HandleReceivedFrom (const boost::system::error_code& ecode, size_t bytes_transferred,
		Packet * packet, boost::asio::ip::udp::socket& socket)
	{
		if (!ecode)
		{
			i2p::transport::transports.UpdateReceivedBytes (bytes_transferred);
			packet->len = bytes_transferred;

			boost::system::error_code ec;
			size_t moreBytes = socket.available (ec);
			if (!ec && moreBytes)
			{
				std::vector<Packet *> packets;
				packets.push_back (packet);
				while (moreBytes && packets.size () < 32)
				{
					packet = m_PacketsPool.AcquireMt ();
					packet->len = socket.receive_from (boost::asio::buffer (packet->buf, SSU2_MTU), packet->from, 0, ec);
					if (!ec)
					{
						i2p::transport::transports.UpdateReceivedBytes (packet->len);
						packets.push_back (packet);
						moreBytes = socket.available(ec);
						if (ec) break;
					}
					else
					{
						LogPrint (eLogError, "SSU2: receive_from error: code ", ec.value(), ": ", ec.message ());
						m_PacketsPool.ReleaseMt (packet);
						break;
					}
				}
				GetService ().post (std::bind (&SSU2Server::HandleReceivedPackets, this, packets));
			}
			else
				GetService ().post (std::bind (&SSU2Server::HandleReceivedPacket, this, packet));
			Receive (socket);
		}
		else
		{
			m_PacketsPool.ReleaseMt (packet);
			if (ecode != boost::asio::error::operation_aborted)
			{
				LogPrint (eLogError, "SSU2: Receive error: code ", ecode.value(), ": ", ecode.message ());
				auto ep = socket.local_endpoint ();
				socket.close ();
				OpenSocket (ep);
				Receive (socket);
			}
		}
	}

	void SSU2Server::HandleReceivedPacket (Packet * packet)
	{
		if (packet)
		{
			ProcessNextPacket (packet->buf, packet->len, packet->from);
			m_PacketsPool.ReleaseMt (packet);
			if (m_LastSession) m_LastSession->FlushData ();
		}
	}

	void SSU2Server::HandleReceivedPackets (std::vector<Packet *> packets)
	{
		for (auto& packet: packets)
			ProcessNextPacket (packet->buf, packet->len, packet->from);
		m_PacketsPool.ReleaseMt (packets);
		if (m_LastSession) m_LastSession->FlushData ();
	}

	void SSU2Server::AddSession (std::shared_ptr<SSU2Session> session)
	{
		if (session)
		{
			m_Sessions.emplace (session->GetConnID (), session);
			AddSessionByRouterHash (session);
		}
	}

	void SSU2Server::RemoveSession (uint64_t connID)
	{
		auto it = m_Sessions.find (connID);
		if (it != m_Sessions.end ())
		{
			auto ident = it->second->GetRemoteIdentity ();
			if (ident)
				m_SessionsByRouterHash.erase (ident->GetIdentHash ());
			if (m_LastSession == it->second)
				m_LastSession = nullptr;
			m_Sessions.erase (it);
		}
	}
		
	void SSU2Server::AddSessionByRouterHash (std::shared_ptr<SSU2Session> session)
	{
		if (session)
		{
			auto ident = session->GetRemoteIdentity ();
			if (ident)
			{
				auto ret = m_SessionsByRouterHash.emplace (ident->GetIdentHash (), session);
				if (!ret.second)
				{
					// session already exists
					LogPrint (eLogWarning, "SSU2: Session to ", ident->GetIdentHash ().ToBase64 (), " aready exists");
					// terminate existing
					GetService ().post (std::bind (&SSU2Session::Terminate, ret.first->second));
					// update session
					ret.first->second = session;
				}
			}
		}
	}

	bool SSU2Server::AddPendingOutgoingSession (std::shared_ptr<SSU2Session> session)
	{
		if (!session) return false;
		return m_PendingOutgoingSessions.emplace (session->GetRemoteEndpoint (), session).second;
	}

	std::shared_ptr<SSU2Session> SSU2Server::FindSession (const i2p::data::IdentHash& ident) const
	{
		auto it = m_SessionsByRouterHash.find (ident);
		if (it != m_SessionsByRouterHash.end ())
			return it->second;
		return nullptr;
	}	

	std::shared_ptr<SSU2Session> SSU2Server::FindPendingOutgoingSession (const boost::asio::ip::udp::endpoint& ep) const
	{		
		auto it = m_PendingOutgoingSessions.find (ep);
		if (it != m_PendingOutgoingSessions.end ())
			return it->second;
		return nullptr;
	}

	void SSU2Server::RemovePendingOutgoingSession (const boost::asio::ip::udp::endpoint& ep)
	{
		m_PendingOutgoingSessions.erase (ep);
	}	
		
	std::shared_ptr<SSU2Session> SSU2Server::GetRandomSession (
		i2p::data::RouterInfo::CompatibleTransports remoteTransports, const i2p::data::IdentHash& excluded) const
	{
		if (m_Sessions.empty ()) return nullptr;
		uint16_t ind;
		RAND_bytes ((uint8_t *)&ind, sizeof (ind));
		ind %= m_Sessions.size ();
		auto it = m_Sessions.begin ();
		std::advance (it, ind);
		while (it != m_Sessions.end ())
		{
			if ((it->second->GetRemoteTransports () & remoteTransports) && 
			    it->second->GetRemoteIdentity ()->GetIdentHash () != excluded)
				return it->second;
			it++;
		}
		// not found, try from begining
		it = m_Sessions.begin ();
		while (it != m_Sessions.end () && ind)
		{
			if ((it->second->GetRemoteTransports () & remoteTransports) && 
			    it->second->GetRemoteIdentity ()->GetIdentHash () != excluded)
				return it->second;
			it++; ind--;
		}	
		return nullptr;
	}	
		
	void SSU2Server::AddRelay (uint32_t tag, std::shared_ptr<SSU2Session> relay)
	{
		m_Relays.emplace (tag, relay);
	}

	void SSU2Server::RemoveRelay (uint32_t tag)
	{
		m_Relays.erase (tag);
	}

	std::shared_ptr<SSU2Session> SSU2Server::FindRelaySession (uint32_t tag)
	{
		auto it = m_Relays.find (tag);
		if (it != m_Relays.end ())
		{
			if (it->second->IsEstablished ())
				return it->second;
			else
				m_Relays.erase (it);
		}
		return nullptr;
	}

	void SSU2Server::ProcessNextPacket (uint8_t * buf, size_t len, const boost::asio::ip::udp::endpoint& senderEndpoint)
	{
		if (len < 24) return;
		uint64_t connID;
		memcpy (&connID, buf, 8);
		connID ^= CreateHeaderMask (i2p::context.GetSSU2IntroKey (), buf + (len - 24));
		if (!m_LastSession || m_LastSession->GetConnID () != connID)
		{
			if (m_LastSession) m_LastSession->FlushData ();
			auto it = m_Sessions.find (connID);
			if (it != m_Sessions.end ())
				m_LastSession = it->second;
			else
				m_LastSession = nullptr;
		}
		if (m_LastSession)
		{
			switch (m_LastSession->GetState ())
			{
				case eSSU2SessionStateEstablished:
				case eSSU2SessionStateSessionConfirmedSent:	
					m_LastSession->ProcessData (buf, len);
				break;
				case eSSU2SessionStateSessionCreatedSent:
					m_LastSession->ProcessSessionConfirmed (buf, len);
				break;
				case eSSU2SessionStateIntroduced:
					if (m_LastSession->GetRemoteEndpoint ().address ().is_unspecified ())	
						m_LastSession->SetRemoteEndpoint (senderEndpoint);
					if (m_LastSession->GetRemoteEndpoint () == senderEndpoint)
						m_LastSession->ProcessHolePunch (buf, len);
					else
					{
						LogPrint (eLogWarning, "SSU2: HolePunch endpoint ", senderEndpoint,
							" doesn't match RelayResponse ", m_LastSession->GetRemoteEndpoint ());
						m_LastSession->Terminate ();
						m_LastSession = nullptr;	
					}		
				break;
				case eSSU2SessionStatePeerTest:
					m_LastSession->SetRemoteEndpoint (senderEndpoint);
					m_LastSession->ProcessPeerTest (buf, len);
				break;
				case eSSU2SessionStateClosing:
					m_LastSession->RequestTermination (eSSU2TerminationReasonIdleTimeout); // send termination again
				break;	
				case eSSU2SessionStateTerminated:
					m_LastSession = nullptr;
				break;	
				default:
					LogPrint (eLogWarning, "SSU2: Invalid session state ", (int)m_LastSession->GetState ());
			}
		}
		else
		{
			// check pending sessions if it's SessionCreated or Retry
			auto it1 = m_PendingOutgoingSessions.find (senderEndpoint);
			if (it1 != m_PendingOutgoingSessions.end ())
			{
				if (it1->second->GetState () == eSSU2SessionStateSessionRequestSent &&
					it1->second->ProcessSessionCreated (buf, len))
					m_PendingOutgoingSessions.erase (it1); // we are done with that endpoint			
				else
					it1->second->ProcessRetry (buf, len);
			}
			else
			{
				// assume new incoming session
				auto session = std::make_shared<SSU2Session> (*this);
				session->SetRemoteEndpoint (senderEndpoint);
				session->ProcessFirstIncomingMessage (connID, buf, len);
			}
		}
	}

	void SSU2Server::Send (const uint8_t * header, size_t headerLen, const uint8_t * payload, size_t payloadLen,
		const boost::asio::ip::udp::endpoint& to)
	{
		std::vector<boost::asio::const_buffer> bufs
		{
			boost::asio::buffer (header, headerLen),
			boost::asio::buffer (payload, payloadLen)
		};
		boost::system::error_code ec;
		if (to.address ().is_v6 ())
			m_SocketV6.send_to (bufs, to, 0, ec);
		else
			m_SocketV4.send_to (bufs, to, 0, ec);
		if (!ec)
			i2p::transport::transports.UpdateSentBytes (headerLen + payloadLen);
		else
			LogPrint (eLogError, "SSU2: Send exception: ", ec.message (), " to ", to);
	}

	void SSU2Server::Send (const uint8_t * header, size_t headerLen, const uint8_t * headerX, size_t headerXLen,
		const uint8_t * payload, size_t payloadLen, const boost::asio::ip::udp::endpoint& to)
	{
		std::vector<boost::asio::const_buffer> bufs
		{
			boost::asio::buffer (header, headerLen),
			boost::asio::buffer (headerX, headerXLen),
			boost::asio::buffer (payload, payloadLen)
		};
		boost::system::error_code ec;
		if (to.address ().is_v6 ())
			m_SocketV6.send_to (bufs, to, 0, ec);
		else
			m_SocketV4.send_to (bufs, to, 0, ec);

		if (!ec)
			i2p::transport::transports.UpdateSentBytes (headerLen + headerXLen + payloadLen);
		else
			LogPrint (eLogError, "SSU2: Send exception: ", ec.message (), " to ", to);
	}

	bool SSU2Server::CreateSession (std::shared_ptr<const i2p::data::RouterInfo> router,
		std::shared_ptr<const i2p::data::RouterInfo::Address> address, bool peerTest)
	{
		if (router && address)
		{
			// check is no peding session
			bool isValidEndpoint = !address->host.is_unspecified () && address->port;
			if (isValidEndpoint)
			{	
				if (i2p::util::net::IsInReservedRange(address->host)) return false;
				auto s = FindPendingOutgoingSession (boost::asio::ip::udp::endpoint (address->host, address->port));
				if (s)
				{	
					if (peerTest)
					{
						// if peer test requested add it to the list for pending session
						auto onEstablished = s->GetOnEstablished ();
						if (onEstablished)
							s->SetOnEstablished ([s, onEstablished]() 
								{ 
									onEstablished (); 
									s->SendPeerTest (); 
								 });
						else	
							s->SetOnEstablished ([s]() { s->SendPeerTest (); });
					}	
					return false;
				}	
			}	
			
			auto session = std::make_shared<SSU2Session> (*this, router, address);
			if (peerTest)
				session->SetOnEstablished ([session]() {session->SendPeerTest (); });

			if (address->UsesIntroducer ())
				GetService ().post (std::bind (&SSU2Server::ConnectThroughIntroducer, this, session));
			else if (isValidEndpoint) // we can't connect without endpoint				
				GetService ().post ([session]() { session->Connect (); });
			else
				return false;
		}
		else
			return false;
		return true;
	}

	void SSU2Server::ConnectThroughIntroducer (std::shared_ptr<SSU2Session> session)
	{
		if (!session) return;
		auto address = session->GetAddress ();
		if (!address) return;
		session->WaitForIntroduction ();
		// try to find existing session first
		for (auto& it: address->ssu->introducers)
		{
			auto it1 = m_SessionsByRouterHash.find (it.iKey);
			if (it1 != m_SessionsByRouterHash.end ())
			{
				it1->second->Introduce (session, it.iTag);
				return;
			}
		}
		// we have to start a new session to an introducer
		auto ts = i2p::util::GetSecondsSinceEpoch ();
		std::shared_ptr<i2p::data::RouterInfo> r;
		uint32_t relayTag = 0;
		for (auto& it: address->ssu->introducers)
		{
			if (it.iTag && ts < it.iExp)
			{	
				r = i2p::data::netdb.FindRouter (it.iKey);
				if (r && r->IsReachableFrom (i2p::context.GetRouterInfo ()))
				{
					relayTag = it.iTag;
					if (relayTag) break;
				}
			}	
		}
		if (r)
		{
			if (relayTag)
			{
				// introducer and tag found connect to it through SSU2
				auto addr = address->IsV6 () ? r->GetSSU2V6Address () : r->GetSSU2V4Address ();
				if (addr)
				{
					bool isValidEndpoint = !addr->host.is_unspecified () && addr->port;
					if (isValidEndpoint)
					{	
						auto s = FindPendingOutgoingSession (boost::asio::ip::udp::endpoint (addr->host, addr->port));
						if (!s)
						{	
							s = std::make_shared<SSU2Session> (*this, r, addr);
							s->SetOnEstablished ([session, s, relayTag]() { s->Introduce (session, relayTag); });
							s->Connect ();
						}
						else
						{
							auto onEstablished = s->GetOnEstablished ();
							if (onEstablished)
								s->SetOnEstablished ([session, s, relayTag, onEstablished]()
									{
										onEstablished ();
										s->Introduce (session, relayTag);
									});
							else	
								s->SetOnEstablished ([session, s, relayTag]() {s->Introduce (session, relayTag); });
						}	
					}	
				}
			}
		}
		else
		{
			// introducers not found, try to request them
			for (auto& it: address->ssu->introducers)
				if (it.iTag && ts < it.iExp)
					i2p::data::netdb.RequestDestination (it.iKey);
		}
	}

	bool SSU2Server::StartPeerTest (std::shared_ptr<const i2p::data::RouterInfo> router, bool v4)
	{
		if (!router) return false;
		auto addr = v4 ? router->GetSSU2V4Address () : router->GetSSU2V6Address ();
		if (!addr) return false;
		auto it = m_SessionsByRouterHash.find (router->GetIdentHash ());
		if (it != m_SessionsByRouterHash.end ())
		{
			auto s = it->second;
			if (it->second->IsEstablished ())
				GetService ().post ([s]() { s->SendPeerTest (); });
			else	
				s->SetOnEstablished ([s]() { s->SendPeerTest (); });	
			return true;	
		}	
		CreateSession (router, addr, true);
		return true;
	}	
		
	void SSU2Server::ScheduleTermination ()
	{
		m_TerminationTimer.expires_from_now (boost::posix_time::seconds(SSU2_TERMINATION_CHECK_TIMEOUT));
		m_TerminationTimer.async_wait (std::bind (&SSU2Server::HandleTerminationTimer,
			this, std::placeholders::_1));
	}

	void SSU2Server::HandleTerminationTimer (const boost::system::error_code& ecode)
	{
		if (ecode != boost::asio::error::operation_aborted)
		{
			auto ts = i2p::util::GetSecondsSinceEpoch ();
			for (auto it = m_PendingOutgoingSessions.begin (); it != m_PendingOutgoingSessions.end ();)
			{
				if (it->second->IsTerminationTimeoutExpired (ts))
				{
					//it->second->Terminate ();
					it = m_PendingOutgoingSessions.erase (it);
				}
				else
					it++;
			}

			for (auto it: m_Sessions)
			{
				auto state = it.second->GetState ();
				if (state == eSSU2SessionStateTerminated || state == eSSU2SessionStateClosing)
					GetService ().post (std::bind (&SSU2Session::Terminate, it.second));
				else if (it.second->IsTerminationTimeoutExpired (ts))
				{
					if (it.second->IsEstablished ())
						it.second->RequestTermination (eSSU2TerminationReasonIdleTimeout);
					else
						GetService ().post (std::bind (&SSU2Session::Terminate, it.second));
				}
				else
					it.second->CleanUp (ts);
			}

			for (auto it = m_IncomingTokens.begin (); it != m_IncomingTokens.end (); )
			{
				if (ts > it->second.second)
					it = m_IncomingTokens.erase (it);
				else
					it++;
			}

			for (auto it = m_OutgoingTokens.begin (); it != m_OutgoingTokens.end (); )
			{
				if (ts > it->second.second)
					it = m_OutgoingTokens.erase (it);
				else
					it++;
			}

			ScheduleTermination ();
		}
	}

	void SSU2Server::ScheduleResend ()
	{
		m_ResendTimer.expires_from_now (boost::posix_time::seconds(SSU2_RESEND_INTERVAL));
		m_ResendTimer.async_wait (std::bind (&SSU2Server::HandleResendTimer,
			this, std::placeholders::_1));
	}

	void SSU2Server::HandleResendTimer (const boost::system::error_code& ecode)
	{
		if (ecode != boost::asio::error::operation_aborted)
		{
			auto ts = i2p::util::GetSecondsSinceEpoch ();
			for (auto it: m_Sessions)
				it.second->Resend (ts);
			for (auto it: m_PendingOutgoingSessions)
				it.second->Resend (ts);
			ScheduleResend ();
		}
	}

	void SSU2Server::UpdateOutgoingToken (const boost::asio::ip::udp::endpoint& ep, uint64_t token, uint32_t exp)
	{
		m_OutgoingTokens[ep] = {token, exp};
	}

	uint64_t SSU2Server::FindOutgoingToken (const boost::asio::ip::udp::endpoint& ep) const
	{
		auto it = m_OutgoingTokens.find (ep);
		if (it != m_OutgoingTokens.end ())
		{
			if (i2p::util::GetSecondsSinceEpoch () + SSU2_TOKEN_EXPIRATION_THRESHOLD > it->second.second)
				return 0; // token expired
			return it->second.first;
		}	
		return 0;
	}

	uint64_t SSU2Server::GetIncomingToken (const boost::asio::ip::udp::endpoint& ep)
	{
		auto it = m_IncomingTokens.find (ep);
		if (it != m_IncomingTokens.end ())
			return it->second.first;
		uint64_t token;
		RAND_bytes ((uint8_t *)&token, 8);
		m_IncomingTokens.emplace (ep, std::make_pair (token, i2p::util::GetSecondsSinceEpoch () + SSU2_TOKEN_EXPIRATION_TIMEOUT));
		return token;
	}

	std::pair<uint64_t, uint32_t> SSU2Server::NewIncomingToken (const boost::asio::ip::udp::endpoint& ep)
	{
		m_IncomingTokens.erase (ep); // drop previous
		uint64_t token;
		RAND_bytes ((uint8_t *)&token, 8);
		auto ret = std::make_pair (token, i2p::util::GetSecondsSinceEpoch () + SSU2_NEXT_TOKEN_EXPIRATION_TIMEOUT); 
		m_IncomingTokens.emplace (ep, ret);
		return ret;
	}	

	std::list<std::shared_ptr<SSU2Session> > SSU2Server::FindIntroducers (int maxNumIntroducers, 
		bool v4, const std::set<i2p::data::IdentHash>& excluded) const
	{
		std::list<std::shared_ptr<SSU2Session> > ret;
		for (const auto& s : m_Sessions)
		{
			if (s.second->IsEstablished () && (s.second->GetRelayTag () && !s.second->IsOutgoing ()) &&	
			    !excluded.count (s.second->GetRemoteIdentity ()->GetIdentHash ()) &&
			    ((v4 && (s.second->GetRemoteTransports () & i2p::data::RouterInfo::eSSU2V4)) ||
			     (!v4 && (s.second->GetRemoteTransports () & i2p::data::RouterInfo::eSSU2V6))))
				ret.push_back (s.second);
		}	
		if ((int)ret.size () > maxNumIntroducers)
		{
			// shink ret randomly
			int sz = ret.size () - maxNumIntroducers;
			for (int i = 0; i < sz; i++)
			{
				auto ind = rand () % ret.size ();
				auto it = ret.begin ();
				std::advance (it, ind);
				ret.erase (it);
			}
		}
		return ret;
	}	

	void SSU2Server::UpdateIntroducers (bool v4)
	{
		std::list<std::shared_ptr<SSU2Session>> newList;
		auto& introducers = v4 ? m_Introducers : m_IntroducersV6;
		for (const auto& it : introducers)
		{
			if (it->IsEstablished ())
			{
				it->SendKeepAlive ();
				newList.push_back (it);
			}	
		}	
		if (newList.size () < SSU2_MAX_NUM_INTRODUCERS)
		{
			std::set<i2p::data::IdentHash> excluded;
			for (auto& it1: newList)
				excluded.insert (it1->GetRemoteIdentity ()->GetIdentHash ());
			auto sessions = FindIntroducers (SSU_MAX_NUM_INTRODUCERS - newList.size (), v4, excluded);
			for (const auto& it : sessions)
			{
				newList.push_back (it);
			}	
		}	
		introducers = newList;
	}	
}
}