diff --git a/src/flow.cpp b/src/flow.cpp
index ed9ddb51..5a8f093d 100644
--- a/src/flow.cpp
+++ b/src/flow.cpp
@@ -42,6 +42,7 @@ void flow::usage()
     std::cout << "  %K - (connection_number / 1000)       % 1000\n";
     std::cout << "  %M - (connection_number / 1000000)    % 1000\n";
     std::cout << "  %G - (connection_number / 1000000000) % 1000\n";
+    std::cout << "  %S - session ID\n";
     std::cout << "  %% - Output a '%'\n";
     std::cout << "\n";
     std::cout << "Default value is: '"<< flow::filename_template <<"'\n";
@@ -135,6 +136,9 @@ std::string flow::filename(uint32_t connection_count, bool is_pcap)
 	    case 'c': // connection_count if connection_count >0
 		if(connection_count>0) ss << connection_count;
 		break;
+	    case 'S': // session ID
+		ss << std::setfill('0') << std::setw(20) << session_id;
+		break;
 	    case '#': // always output connection count
 		ss << connection_count;
 		break;
diff --git a/src/tcpdemux.cpp b/src/tcpdemux.cpp
index 071485f6..95534b0d 100644
--- a/src/tcpdemux.cpp
+++ b/src/tcpdemux.cpp
@@ -37,7 +37,7 @@ tcpdemux::tcpdemux():
     outdir("."),flow_counter(0),packet_counter(0),
     xreport(0),pwriter(0),max_open_flows(),max_fds(get_max_fds()-NUM_RESERVED_FDS),
     flow_map(),open_flows(),saved_flow_map(),flow_fd_cache_map(0),
-    saved_flows(),start_new_connections(false),opt(),fs()
+    saved_flows(),start_new_connections(false),opt(),fs(),unique_id(0)
 {
     tcp_processor = &tcpdemux::process_tcp;
 }
@@ -553,12 +553,28 @@ int tcpdemux::process_tcp(const ipaddr &src, const ipaddr &dst,sa_family_t famil
 
         /* Don't process if this is not a SYN and there is no data. */
         if(syn_set==false && tcp_datalen==0) return 0;
+	
+	/* Check if this is the server->client flow related to a client->server flow that is being demultiplexed */
+	flow_addr reverse_flow(dst,src,ntohs(tcp_header->th_dport),ntohs(tcp_header->th_sport),family);
+	tcpip   *reverse_tcp = find_tcpip(reverse_flow);
+	uint64_t uid;
+	if (reverse_tcp)
+	{
+	    /* We found a matching client->server flow. Copy its session ID */
+	    uid = reverse_tcp->myflow.session_id;
+	}
+	else
+	{
+	    /* Assign a new unique ID */
+	    uid = unique_id++;
+	}
 
 	/* Create a new connection.
 	 * delta will be 0, because it's a new connection!
 	 */
         be13::tcp_seq isn = syn_set ? seq : seq-1;
 	tcp = create_tcpip(this_flow, isn, pi);
+	tcp->myflow.session_id = uid;
     }
 
     /* Now tcp is valid */
diff --git a/src/tcpdemux.h b/src/tcpdemux.h
index 7d81d1c4..9afdc0d9 100644
--- a/src/tcpdemux.h
+++ b/src/tcpdemux.h
@@ -138,6 +138,7 @@ class tcpdemux {
     pcap_writer *pwriter;               // where we should write packets
     unsigned int max_open_flows;        // how large did it ever get?
     unsigned int max_fds;               // maximum number of file descriptors for this tcpdemux
+    uint64_t unique_id;                 // next unique id to assign
 
     flow_map_t  flow_map;               // db of open tcpip objects, indexed by flow
     intrusive_list<tcpip> open_flows; // the tcpip flows with open files in access order
diff --git a/src/tcpip.h b/src/tcpip.h
index b24d6437..de1d79a9 100644
--- a/src/tcpip.h
+++ b/src/tcpip.h
@@ -182,6 +182,7 @@ public:;
     uint64_t len;     		        // off-wire length
     uint64_t caplen;    		// captured length
     uint64_t packet_count;		// packet count
+    uint64_t session_id;      // session unique id (used to match client->server and server->client flows
 
     // return a filename for a flow based on the template and the connection count
     std::string filename(uint32_t connection_count, bool);