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SIP Proxies

SIP Proxies. Jonathan Rosenberg Chief Scientist. Presentation Agenda. SIP Overview Functions of a Proxy Features to Look for in Proxies. Developed in mmusic Group in IETF Proposed standard RFC2543, February 1999 Work began 1995 Part of Internet Multimedia Conferencing Suite

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SIP Proxies

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  1. SIP Proxies Jonathan Rosenberg Chief Scientist

  2. Presentation Agenda • SIP Overview • Functions of a Proxy • Features to Look for in Proxies

  3. Developed in mmusic Group in IETF Proposed standard RFC2543, February 1999 Work began 1995 Part of Internet Multimedia Conferencing Suite Main Functions Invite users to sessions Find the user’s current location, match with their capabilities and preferencesin order to deliver invitation Carry opaque session descriptions Modification of sessions Termination of sessions Session Initiation Protocol (SIP)

  4. Main Features Personal mobility services Wide area operation Session flexibility Voice; video; games; chat; virtual reality; etc. Leverages other Internet protocols Session Initiation Protocol (SIP) cont.

  5. User Agent Client (UAC) End systems Send SIP requests User Agent Server (UAS) Listens for call requests Prompts user or executes program to determine response User Agent UAC plus UAS Protocol Components

  6. Redirect Server Network server - redirects users to try other server Proxy Server Network server - a proxy request to another server can “fork” request to multiple servers, creating a search tree Registrar Receives registrations regarding current user locations Protocol Components cont.

  7. Request Response SIP Redirect Server Media Location Service 2 3 5 4 6 1 7 11 12 10 SIP Proxy SIP Proxy 13 8 SIP Client 14 9 SIP Client (User Agent Server) SIP Architecture

  8. Main Functions of SIP Proxy Server • Routing Services • Authentication and Authorization • Logging/Billing • Firewall/NAT Traversal • Load Balancing

  9. Routing • Problem Definition • To determine the next hop server(s) which can better handle a SIP request • Next hop can be UAS, proxy or redirect • Type of next hop not known to server • Routing Can Be Based on Arbitrarily Complex Logic and Inputs • Primary Routing Techniques in Proxies • Registration database • Telephone routing prefixes • TRIP and TRIP-GW • Caller preferences • External databases

  10. On Startup, SIP UA Sends REGISTER to Registrar Registration Data Provides Addresses to Reach User Registration Database Forms a Dynamic Routing Database of Users Centralized Store is Desired for Scalability DB Registrar SQL/LDAP/? Proxy Farm REGISTER INVITE Registration Database

  11. SIP INVITE Can Contain Phone Numbers sip:17325551212@domain.com tel:17325551212 Do Not Correspond to Users on IP Network, but PSTN Terminals Call Must Be Routed to Gateway Gateways Often Arranged Through Peering Which One to Use Based on Prefixes (Domestic = gw1, Europe = gw2) Route Table is Mapping From Prefixes to Next Hop IP address/port/transport Plus URL Rewrite Rules Telephone Routing Prefixes sip:19735551212@ longdistance.com tel:19735551212

  12. Inter-domain Protocol for Gateway Route Exchange Completed February 2000 TRIP Supports Various Models Bilateral agreements Centralized settlements provider Wholesaler service TRIP Based on Scalable IP Routing Technology Uses BGP4 as a basis Supports aggregation Uses proven algorithms Proxy = TRIP LS Allows proxy to build routing table dynamically Telephony Routing Over IP (TRIP) Gateways Location Server ISP B TRIP End Users ISP A Front End

  13. Normal TRIP Runs Interdomain TRIP-GW: Lightweight Version That Runs Between LS and Local Gateways Provides Gateway Information Exported to Other Domains Via TRIP Provides Gateway Management Capabilities Load balancing based on available ports/codecs Liveness detection Failover TRIP and Gateways INVITE TRIP-GW

  14. SIP Extensions for Specifying Caller Preferences and Callee State Presence Preferences Carried in INVITE Setup Message Preferences for Reaching callee at home or work Fax, video, audio call Mobile or landline Secretary or voicemail Priority locations Caller Can Specify Proxy Routing SIP Caller Preferences Proxy Server Preference Video

  15. Routing Information Can Also Be Located in External Databases LDAP SQL whois++ Static or Dynamic Several Standards External Databases DB Query INVITE

  16. Restrict/Grant User Access to Proxy Services Types of Services Make incoming and outgoing calls (careful!!) Selection of routing tables Ability to have CPL execution Ability to register Inbound and Outbound Directions Nearly Impossible to Authenticate Inbound Requests Authentication and Authorization Outgoing proxy Incoming proxy

  17. Authentication Mechanisms Basic Digest PGP All are Challenge-Response Basic and Digest Are Shared Secret - Assume Trust Relationship Between UA and Proxy Only for outgoing requests SIP Can Also Authenticate Responses Not widely used SIP Authentication Request Challenge (nonce, realm) ACK Request w/credentials

  18. DoS Attacks Flooding of packets Malicious content Proxy Acts as DMZ Machine Sole point of entry for calls to network (requires firewall) Filtering Functions Absorbs bursts Blocks large messages Removes content with viruses String parsing checks and validations DoS Attack Protection DMZ

  19. Importance of Logging Customer care Troubleshooting Intrusion detection DoS attack detection Billing Billing Issues Must bill for a real service Gateways MCUs Proxy “fronts” gateway Need secure association to gateway Session timer Logging to Remote Logging Server is Key Benefit Logging and Billing Log Server Billing Server Remote logging SA Gateways

  20. Getting SIP Through Firewalls • Firewalls Typically Statically Configured to Let Traffic in/out of Specific Ports/Addresses • SIP Itself Can Easily Be Let in/out • Static port 5060 opened • But SIP Signals Media Sessions, Usually RTP • RTP Difficult to Isolate • Uses dynamic UDP ports • Not its own protocol • No way to statelessly identify • Therefore, Media Sessions Will Not Flow Through Firewall

  21. Network Address Translation (NAT) Modifies IP Addresses/Ports in Packets Benefits Avoids network renumbering on change of provider Allows multiplexing of multiple private addresses into a single public address ($$ savings) Maintains privacy of internal addresses Getting SIP Through NATs

  22. Issues If a host includes its IP address inside of an application packet, it is wrong to the outside SIP fundamentally handles this Addresses inside of SIP must be rewritten Where Can IP Addresses Be? SDP From field To field Contact Record-route Via Getting SIP Through NATs cont.

  23. Continuing Challenges • Other Application Protocols Have Trouble With Firewalls and NAT • ftp • H.323 • Solution is to Embed Application Layer Gateway (ALG) into Firewall/NAT • Actually goes into packet and modifies addresses • Requires understanding of protocol • Embedding ALG in NAT is Not Ideal Solution • Scaling • Separation of function • Expertise issue

  24. Separate Application Layer NAT/Firewall from IP Layer NAT/Firewall Similar to megaco decomposition MG analagous to packet filter MGC analagous to ALG (proxy) Same benefits Better scaling Faster Lower Cost Expertise problem solved Deployment paths for new apps Load balancing Proposed Solution Decomposed Firewall/NAT Proxy Server/ALG Firewall/NATPacket Filter Control SIP RTP

  25. Control Protocol Between SIP ALG and IP NAT/Firewall Main Requirements Binding request: give a private address, obtain a public address Binding release Open hole (firewall) Close hole (firewall) Group bindings INVITE BIND REQ BINDING INVITE 200 OK 200 OK OPEN ACK ACK Proxy Server Firewall The Missing Piece

  26. IETF Efforts on Firewall Traversal • SIP Working Group • Informational RFC will be developed • Summarizes SIP operations needed in firewall controlling proxy • Addresses problems and issues • Call flows and examples • FOGLAMPS BoF Session • Pre-Working Group • May develop protocol between firewall-controlling proxy and firewall • Currently working on protocol requirements • J. Kuthan, J. Rosenberg, “Firewall Control Protocol Framework and Requirements”, draft-kuthan-fcp-01.txt

  27. Goal is To Deploy Clusters of Proxy Servers Scaling up the network Any One Can Handle Request Load Balancing Goal is to balance work across cluster Requests for same call to same server Failover Cease using machine in cluster upon failure detection Load Balancing and Failover

  28. Configuration and Management Command line interface web SNMP Stateless and Stateful Modes Performance vs. services Record-routing Needed for billing/logging Separate Registration Database Scaling, centralized management of subscribers Checklist of Other Desired Features

  29. Subscriber Management Add users to system Define services and capabilities CPL or not? Authorize services against subscriber lists Dynamic Reconfiguration Change parameters/routing table entries on the fly Customized Logging Outputs XML, apache, etc. Checklist of Other Desired Features cont.

  30. Information Resource • Jonathan Rosenberg • jdrosen@dynamicsoft.com • +1 973.952.5000

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