Challenges of Voice-over-IP

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Challenges of Voice-over-IP The Second Quarter
Century

• Henning Schulzrinne
• Dept. of Computer Science
• Columbia University

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Outline

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

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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)

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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)

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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, ...

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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,

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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

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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
• cant get new services if other end is POTS ??
  why use VoIP if I cant get new services

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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

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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

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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

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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

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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

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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

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Challenges Security

• Classical model of restricted access systems -gt
  cryptographic security
• Objectives
• identification for access control billing
• phone/IM spam control (black/white lists)
• call routing
• privacy

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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

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System model
outbound proxy
SIP trapezoid
a_at_foo.com 128.59.16.1
registrar
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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?

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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

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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

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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

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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

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HTTP Digest authentication
REGISTER To sipalice_at_example.com
401 Unauthorized WWW-Authenticate Digest
realm"alice_at_example.com", qopauth,
nonce"dcd9"
REGISTER To sipalice_at_example.com Authorization
Digest username"alice", nc00000001,
cnonce"defg", response"9f01"
REGISTER To sipalice_at_example.com Authorization
Digest username"alice", nc00000002,
cnonce"abcd", response"6629"
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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

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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_at_aol.com doesn't prevent calls from
  jerk_at_yahoo.com (new day, sam person)

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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

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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

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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

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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

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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

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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 -gt binding duration
• lack of permanent address (e.g., DHCP) not a
  problem -gt SIP address binding
• misperception NAT security

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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

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Challenges service creation

• Cant 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

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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 doesnt work indoors
• phones can move easily IP address does not help

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Emergency calls
common emergency identifier sos_at_domain
EPAD
REGISTER sipsos Location 07605
302 Moved Contact sipsos_at_psap.leonia.nj.us Conta
ct tel[phone]
SIP proxy
INVITE sipsos Location 07605
INVITE sipsos_at_psap.leonia.nj.us Location 07605
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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

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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 lt 50
• Still need common database
• can separate REGISTER
• make rest read-only

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Large system
stateless proxies
a1.example.com
sip1.example.com
a2.example.com
sip2.example.com
sipbob_at_example.com
b1.example.com
sipbob_at_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
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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

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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

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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

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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 sip7134_at_cs
• Ethernet phones, soft phones and conference room
• CINEMA set of servers, running on 1U rackmount
  server

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CINEMA components
Cisco 7960
MySQL
rtspd
sipconf
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
Cisco
Nortel
2600
Meridian
VoiceXML
PBX
server
T1
T1
SIP
sipvxml
PhoneJack interface
sipc
SIP-H.323
converter
sip-h323
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Experiences

• Need flexible name mapping
• Alice.Cueba_at_cs ? alice_at_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

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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

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SIP doesnt have to be in a phone
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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

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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

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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

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Conclusion

• Transition to VoIP will take much longer than
  anticipated ? replacement service
• digital telephone took 20 years...
• 3G (UMTS R5) as driver?
• combination with IM, presence, event notification
• Emphasis protocols ?operational infrastructure
• security
• service creation
• PSTN interworking