Challenges of Voice-over-IP – The Second Quarter Century

1. Challenges of Voice-over-IP – The Second Quarter Century Henning Schulzrinne Dept. of Computer Science Columbia University

2. Outline • A brief history • Challenges: • QoS • Security • NATs • Service creation • Scaling • Interworking • Emergency calls • CINEMA project at Columbia

3. A brief history • August 1974 • Realtime packet voice between USC/ISI and MIT/LL, using CVSD and NVP. • December 1974 • Packet voice between CHI and MIT/LL, using LPC and NVP • January 1976 • Live packet voice conferencing between USC/ISI, MIT/LL, SRI, using LPC and NVCP • Approximately 1976 • First packetized speech over SATNET between Lincoln Labs and NTA (Norway) and UCL (UK) • 1990 • ITU recommendation G.764 (Voice packetization – packetized voice protocols)

4. A brief history • February 1991 • DARTnet voice experiments • August 1991 • LBL's audio tool vat released for DARTnet use • March 1992 • First IETF MBONE broadcast (San Diego) • January 1996 • RTP standardized (RFC 1889/1890) • November 1996 • H.323v1 published • February/March 1999 • SIP standardized (RFC 2543)

5. VoIP applications • Trunk replacements between PBXs • Ethernet trunk cards for PBXs • T1/E1 gateways • IP centrex – outsourcing the gateway • Denwa, Worldcom • Enterprise telephony • Cisco Avvid, 3Com, Mitel, ... • Consumer calling cards (phone-to-phone) • net2phone, iConnectHere (deltathree), ... • PC-to-phone, PC-to-PC • net2phone, dialpad, iConnectHere, mediaring, ...

6. Where are we? • Variety of robust SIP phones (and lots of proprietary ones) • not yet in Wal-Mart • SIP carriers terminate LAN VoIP • number portability? • 911 • 50+ vendors at SIPit • Building blocks: media servers, unified messaging, conferencing, VoiceXML, …

7. Status in 2002 • 2000: 6b wholesale, 15b minutes retail • 2001: 10b worldwide – 6% of traffic (only phone-to-phone) • e.g., net2phone: 341m min/quarter

8. Where are we? • Not quite what we had in mind • initially, SIP for initiating multicast conferencing • in progress since 1992 • still small niche • even the IAB and IESG meet by POTS conference… • then VoIP • written-off equipment (circuit-switched) vs. new equipment (VoIP) • bandwidth is (mostly) not the problem • “can’t get new services if other end is POTS’’  “why use VoIP if I can’t get new services”

9. Where are we? • VoIP: avoiding the installed base issue • cable modems – lifeline service • 3GPP – vaporware? • Finally, IM/presence and events • probably, first major application • offers real advantage: interoperable IM • also, new service

10. VoIP at Home • Lifeline (power) • Multiple phones per household • expensive to do over PNA or 802.11 • BlueTooth range too short • need wireless SIP base station + handsets • PDAs with 802.11 and GSM? (Treo++) • Incentives • SMS & IM services

11. SIP phones • Hard to build really basic phones • need real multitasking OS • need large set of protocols: • IP, DNS, DHCP, maybe IPsec, SNTP and SNMP • UDP, TCP, maybe TLS • HTTP (configuration), RTP, SIP • user-interface for entering URLs is a pain • see “success” of Internet appliances • “PCs with handset” cost $500 and still have a Palm-size display

12. VoIP protocol components • RTP for data transmission • ROHC, CRTP for header compression • SIP or H.323 for call setup (signaling) • sometimes, H.248 (Megaco) for control of gateways • ENUM for mapping E.164 numbers to (SIP) URIs • TRIP for large gateway clouds

13. Challenges: QoS • Bottlenecks: access and interchanges • Backbones: e.g., Worldcom Jan. 2002 • 50 ms US, 79 ms transatlantic RTT • 0.067% US, 0.042% transatlantic packet loss • Keynote 2/2002: “almost all had error rates less then 0.25%” (but some up to 1%) • LANs: generally, less than 0.1% loss, but beware of hubs

15. Challenges: QoS • Not lack of protocols – RSVP, diff-serv • Lack of policy mechanisms and complexity • which traffic is more important? • how to authenticate users? • cross-domain authentication • may need for access only – bidirectional traffic • DiffServ: need agreed-upon code points • NSIS WG in IETF – currently, requirements only

16. Challenges: Security • Classical model of restricted access systems -> cryptographic security • Objectives: • identification for access control & billing • phone/IM spam control (black/white lists) • call routing • privacy

17. SIP security • Bar is higher than for email – telephone expectations (albeit wrong) • SIP carries media encryption keys • Potential for nuisance – phone spam at 2 am • Safety – prevent emergency calls

18. System model outbound proxy SIP trapezoid a@foo.com: 128.59.16.1 registrar

19. Threats • Bogus requests (e.g., fake From) • Modification of content • REGISTER Contact • SDP to redirect media • Insertion of requests into existing dialogs: BYE, re-INVITE • Denial of service (DoS) attacks • Privacy: SDP may include media session keys • Inside vs. outside threats • Trust domains – can proxies be trusted?

20. Threats • third-party • not on path • can generate requests • passive man-in-middle (MIM) • listen, but not modify • active man-in-middle • replay • cut-and-paste

21. L3/L4 security options • IPsec • Provides keying mechanism • but IKE is complex and has interop problems • works for all transport protocol (TCP, SCTP, UDP, …) • no credential-fetching API • TLS • provides keying mechanism • good credential binding mechanism • no support for UDP; SCTP in progress

22. Hop-by-hop security: TLS • Server certificates well-established for web servers • Per-user certificates less so • email return-address (class 1) certificate not difficult (Thawte, Verisign) • Server can challenge client for certificate  last-hop challenge

23. HTTP Digest authentication • Allows user-to-user (registrar) authentication • mostly client-to-server • but also server-to-client (Authentication-Info) • Also, Proxy-Authenticate and Proxy-Authorization • May be stacked for multiple proxies on path

24. HTTP Digest authentication REGISTER To: sip:alice@example.com 401 Unauthorized WWW-Authenticate: Digest realm="alice@example.com", qop=auth, nonce="dcd9" REGISTER To: sip:alice@example.com Authorization: Digest username="alice", nc=00000001, cnonce="defg", response="9f01" REGISTER To: sip:alice@example.com Authorization: Digest username="alice", nc=00000002, cnonce="abcd", response="6629"

25. End-to-end authentication • What do we need to prove? • Person sending BYE is same as sending INVITE • Person calling today is same as yesterday • Person is indeed "Alice Wonder, working for Deutsche Bank" • Person is somebody with account at MCI Worldcom

26. End-to-end authentication • Why end-to-end authentication? • prevent phone/IM spam • nuisance callers • trust: is this really somebody from my company asking about the new widget? • Problem: generic identities are cheap • filtering bozo@aol.com doesn't prevent calls from jerk@yahoo.com (new day, sam person)

27. End-to-end authentication and confidentiality • Shared secrets • only scales (N2) to very small groups • OpenPGP chain of trust • S/MIME-like encapsulation • CA-signed (Verisign, Thawte) • every end point needs to have list of Cas • need CRL checking • ssh-style

28. Ssh-style authentication • Self-signed (or unsigned) certificate • Allows active man-in-middle to replace with own certificate • always need secure (against modification) way to convey public key • However, safe once established

29. DOS attacks • CPU complexity: get SIP entity to perform work • Memory exhaustion: SIP entity keeps state (TCP SYN flood) • Amplification: single message triggers group of message to target • even easier in SIP, since Via not subject to address filtering

30. DOS attacks: amplification • Normal SIP UDP operation: • one INVITE with fake Via • retransmit 401/407 (to target) 8 times • Modified procedure: • only send one 401/407 for each INVITE • Suggestion: have null authentication • prevents amplification of other responses • E.g., user "anonymous", password empty

31. DOS attacks: memory • SIP vulnerable if state kept after INVITE • Same solution: challenge with 401 • Server does not need to keep challenge nonce, but needs to check nonce freshness

32. Challenges: NATs and firewalls • NATs and firewalls reduce Internet to web and email service • firewall, NAT: no inbound connections • NAT: no externally usable address • NAT: many different versions -> binding duration • lack of permanent address (e.g., DHCP) not a problem -> SIP address binding • misperception: NAT = security

33. Challenges: NAT and firewalls • Solutions: • longer term: IPv6 • longer term: MIDCOM for firewall control? • control by border proxy? • short term: • NAT: STUN and SHIPWORM • send packet to external server • server returns external address, port • use that address for inbound UDP packets

34. Challenges: service creation • Can’t win by (just) recreating PSTN services • Programmable services: • equipment vendors, operators: JAIN • local sysadmin, vertical markets: sip-cgi • proxy-based call routing: CPL • voice-based control: VoiceXML

35. Emergency calls • Opportunity for enhanced services: • video, biometrics, IM • Finding the right emergency call center (PSAP) • VoIP admin domain may span multiple 911 calling areas • Common emergency address • User location • GPS doesn’t work indoors • phones can move easily – IP address does not help

36. Emergency calls common emergency identifier: sos@domain EPAD REGISTER sip:sos Location: 07605 302 Moved Contact: sip:sos@psap.leonia.nj.us Contact: tel:+1-201-911-1234 SIP proxy INVITE sip:sos Location: 07605 INVITE sip:sos@psap.leonia.nj.us Location: 07605

37. Scaling and redundancy • Single host can handle 10-100 calls + registrations/second  18,000-180,000 users • 1 call, 1 registration/hour • Conference server: about 50 small conferences or large conference with 100 users • For larger system and redundancy, replicate proxy server

38. Scaling and redundancy • DNS SRV records allow static load balancing and fail-over • but failed systems increase call setup delay • can also use IP address “stealing” to mask failed systems, as long as load < 50% • Still need common database • can separate REGISTER • make rest read-only

39. Large system stateless proxies sip1.example.com a1.example.com a2.example.com sip2.example.com sip:bob@example.com b1.example.com sip:bob@b.example.com sip3.example.com b2.example.com _sip._udp SRV 0 0 b1.example.com 0 0 b2.example.com _sip._udp SRV 0 0 sip1.example.com 0 0 sip2.example.com 0 0 sip3.example.com

40. Enterprise VoIP • Allow migration of enterprises to IP multimedia communication • Add capacity to existing PBX, without upgrade • Allow both • IP centrex: hosted by carrier • “PBX”-style: locally hosted • Unlike classical centrex, transition can be done transparently

41. Motivation • Not cheaper phone calls • Single number, follow-me – even for analog phone users • Integration of presence • person already busy – better than callback • physical environment (IR sensors) • Integration of IM • no need to look up IM address • missed calls become IMs • move immediately to voice if IM too tedious

42. Migration strategy • Add IP phones to existing PBX or Centrex system – PBX as gateway • Initial investment: $2k for gateway • Add multimedia capabilities: PCs, dedicated video servers • “Reverse” PBX: replace PSTN connection with SIP/IP connection to carrier • Retire PSTN phones

43. Example: Columbia Dept. of CS • About 100 analog phones on small PBX • DID • no voicemail • T1 to local carrier • Added small gateway and T1 trunk • Call to 7134 becomes sip:7134@cs • Ethernet phones, soft phones and conference room • CINEMA set of servers, running on 1U rackmount server

44. CINEMA components Cisco 7960 MySQL sipconf rtspd user database LDAP server plug'n'sip RTSP conferencing media server server (MCU) wireless sipd 802.11b RTSP proxy/redirect server unified messaging server Pingtel sipum Nortel Cisco Meridian 2600 VoiceXML PBX server T1 T1 SIP sipvxml PhoneJack interface sipc SIP-H.323 converter sip-h323

45. Experiences • Need flexible name mapping • Alice.Cueba@cs alice@cs • sources: database, LDAP, sendmail aliases, … • Automatic import of user accounts: • In university, thousands each September • /etc/passwd • LDAP, ActiveDirectory, … • much easier than most closed PBXs • Integrate with Ethernet phone configuration • often, bunch of tftp files • Integrate with RADIUS accounting

46. Experiences • Password integration difficult • Digest needs plain-text, not hashed • Different user classes: students, faculty, admin, guests, … • Who pays if call is forwarded/proxied? • authentication and billing behavior of PBX and SIP system may differ • but much better real-time rating

47. SIP doesn’t have to be in a phone

48. Event notification • Missing new service in the Internet • Existing services: • get & put data, remote procedure call: HTTP/SOAP (ftp) • asynchronous delivery with delayed pick-up: SMTP (+ POP, IMAP) • Do not address asynchronous (triggered) + immediate

49. Event notification • Very common: • operating systems (interrupts, signals, event loop) • SNMP trap • some research prototypes (e.g., Siena) • attempted, but ugly: • periodic web-page reload • reverse HTTP

50. SIP event notification • Uses beyond SIP and IM/presence: • Alarms (“fire on Elm Street”) • Web page has changed • cooperative web browsing • state update without Java applets • Network management • Distributed games