Page 1 of 69SIP, NAT, and FirewallsMaster’s ThesisByFredrik TherneliusMay 2000DEPARTMENTOFTELEINFORMATICSEmail: SIP:Private: fredrik_thernelius@hotmail .com Private: sip:[email protected]: [email protected] Work: sip:[email protected] 2 of 691 Table of contents1 TABLE OF CONTENTS .................................................................................................................................22 ABSTRACT ......................................................................................................................................................43 SAMMANFATTNING.....................................................................................................................................44 INTRODUCTION ............................................................................................................................................54.1 ACKNOWLEDGEMENTS................................................................................................................................54.2 THE GOAL OF THE PROJECT..........................................................................................................................54.3 UNDERSTANDING THE PROBLEMS WITH SIP AND FIREWALLS.....................................................................55 EARLIER STUDIES........................................................................................................................................66 INTRODUCTION TO INTERNET TELEPHONY AND VOICE OVER IP.............................................66.1 STREAMING AUDIO – THE REAL-TIME PROTOCOL......................................................................................66.2 THE SESSION DESCRIPTION PROTOCOL.......................................................................................................96.2.1 Text based.........................................................................................................................................106.2.2 The Structure of SDP........................................................................................................................106.3 SIP OVERVIEW..........................................................................................................................................116.3.1 Call setup - 1.....................................................................................................................................126.3.2 Call setup – 2....................................................................................................................................136.3.3 Routing SIP messages.......................................................................................................................146.3.4 SIP addresses....................................................................................................................................146.3.5 Transportation protocol for SIP .......................................................................................................156.3.6 SIP Terminology...............................................................................................................................156.3.7 SIP message structure.......................................................................................................................156.3.8 Summary of SIP requests..................................................................................................................166.3.9 Summary of SIP response codes.......................................................................................................176.3.10 Summary of SIP headers...................................................................................................................177 INTERNET FIREWALLS.............................................................................................................................187.1 PACKET FILTERING GATEWAYS.................................................................................................................187.2 CIRCUIT-LEVEL GATEWAYS......................................................................................................................207.3 APPLICATION LEVEL GATEWAYS..............................................................................................................218 IP ADDRESSING ISSUES ............................................................................................................................218.1 INTRODUCTION TO ADDRESSING WITH IPV4 ..............................................................................................218.2 INTRODUCTION TO ADDRESSING IN IPV6...................................................................................................228.3 PRIVATE ADDRESSES.................................................................................................................................238.4 NAT..........................................................................................................................................................248.4.1 Flavors of NAT .................................................................................................................................258.4.2 Static NAT.........................................................................................................................................258.4.3 Dynamic NAT....................................................................................................................................268.4.4 NAPT – Network Address and Port Translation...............................................................................269 INTRODUCTION TO SECURITY IN SIP .................................................................................................279.1 ENCRYPTION..............................................................................................................................................279.2 AUTHENTICATION......................................................................................................................................289.3 HIDE ROUTE..............................................................................................................................................2910 IMPLEMENTATION OF AN APPLICATION LEVEL GATEWAY FOR SIP.................................2910.1 WHAT IS THE SIP ALG SUPPOSED TO ACHIEVE?