1 Applied Networks & Security VoIP with Critical Analysis John Kristoff IT 263 Spring 2006/2007 John Kristoff - DePaul University 1
2 Critical analysis disclaimer Following this disclaimer are slides used in other versions of the course. We mark up some slides using strikethroughs and underlined red in comic sans ms 20pt font. This is not meant to slight other teachers or their material. Much of the material is good and helpful so we use it. We do this to explore complex issues, refresh dated material, correct inaccuracies and stimulate critical thinking. In some cases we are pedantic where it seems useful, but we are not exhaustive and try to avoid being overly tedious when it is unnecessary. IT 263 Spring 2006/2007 John Kristoff - DePaul University 2
3 IT 263 Applied Networks and Security VoIP
4 Lecture 15 Outline 1. VoIP overview a. Examples 2. Components a. Terminals b. Gateways and Gatekeepers c. Multipoint Control Units 3. Packetized Voice a. RTP b. Network Performance Issues 4. Signaling a. H.323 b. SIP
5 Why VoIP? There should be a why not slide More efficient use of existing networks (can you prove it?) Simpler cabling Unified administration New applications Click-through to Voice Call from Web Improved Call Center Applications Enhanced On-line Collaboration Integration of voice/data on same transport (ISP, Cable provider) Savings on toll call charges (toll bypass) Local phone service charges Long distance charges Standards set by international organizations
6 Traditional Data vs. Voice: Separate Cabling Workstation Phone Workstation Phone Workstation Phone File Server PBX LAN Switch PSTN Router Internet
7 Integrated Data & Voice: Unified Cabling Workstation IP Phone Workstation IP Phone Workstation IP Phone PSTN GateKeeper Trunks LAN Switch Gateway Router Internet
8 Mixed Environment with IP-PBX Workstation Phone Workstation IP Workstation Phone Phone Trunks IP PBX LAN Switch PSTN Router Internet
9 VoIP Devices Multimedia Terminals (IP phone, PC, etc.) Supports audio May support video Gateways Connects a VoIP network (such as a LAN) to a non-voip network, i.e. the Public Switch Telephone Network (PSTN) May have telephony and data interfaces
10 VoIP Devices Gatekeepers provides centralized services: Address translation telephone number IP address Private dialing plans Authorization and authentication of end terminals Billing Bandwidth management Admission Control Also may be called Telephony Server, Call
11 VoIP Scenarios and Devices Peer-to-Peer Calling Multimedia PCs with appropriate software (i.e. MS NetMeeting) make direct connection Skype IP Phone to IP Phone Requires VoIP Gatekeeper for telephone number lookup / translation to IP address and call management. IP Phone to PSTN Phone Requires VoIP Gateway for translation between packet-switched voice and PSTN circuit-switched
12 Peer-to-Peer Calling First, let s consider a simple packet voice call across the Internet What issues need to be addressed to make this work?
13 Peer-to-Peer VoIP Calling Router Internet Router LAN Sw itch LAN Sw itch Multim edia PC Multim edia PC
15 Packet Audio In general, UDP is the transport protocol for multimedia A/D Encoder Encoder A/D D/A Decoder Decoder D/A Jitter Buffer Jitter Buffer IP Stack IP Stack IP Network
16 Voice Coding Standards Standard Data Rate Encoding Delay Quality G.711 (PCM) 64 Kbps 0.1 ms Excellent G.727 (ADPCM) G.728 (CELP) G.729 (CS- ACELP) G (vocoder) 32 Kbps 16 Kbps 8 Kbps 5.3/6.3 Kbps 0.2 ms 5 ms 15 ms 38 ms Excellent Good Good Pretty Good
17 Voice Coding Voice bit rate Quality decreases as bit rate decreases Compressed channels cannot carry fax Lowest bit rates can even obscure touch tones Lower bit rate increases coding delay Be sure to include protocol overhead when calculating total bandwidth required Silence Detection can reduce bandwidth No bandwidth needed during silent periods Voice Activity Detection may need adjustment Comfort Noise Generation
18 Voice Packet Overhead Voice packets must be kept small to reduce packetization delay Typically bytes voice data in each packet Multiple packet headers required: Ethernet header (for LAN transmission) IP header (for routing to destination) UDP header (for port numbers) RTP header (for performance monitoring)
19 Voice Packet Overhead L2 Hdr IP UDP RTP Voice Samples L2 Trlr Rough Rule of Thumb: Pkt Voice Bandwidth = (Voice Bandwidth * 3)!!! Example: to send G.728 (16 Kbps) voice, you should allocate approximately 48
20 Sources of Voice Quality Degradation IP Network introduces packet loss, delay and jitter (delay variation) Jitter buffers influence end-to-end delay and packet loss The delay should not exceed the maximum Mouth to Ear(M2E) allowable limit 400 ms An ITU Standard
21 Network Delay Effects End-End Delay ms ms ms ms. Above 800 ms. Effect on User Conversation Fully interactive. No problems Slightly perceivable delays, but few problems, if any. Noticeable annoying delay, but somewhat normal conversation possible. Interactivity is difficult. Speakers interrupt each other and must repeat information frequently. Fully interactive conversation is not possible. No longer feels like a telephone call at all.
22 Internet Packet Loss varies considerably Figure 1: packet loss ratio - G U.S. pkt size:720 bytes, #pkts sent:833, duration:5 mins a.psc.edu beach.utmb.edu caltech.edu ftp.cse.buffalo.edu ftp.gatech.edu packet loss ratio Day-1 12:00 AM Day-1 8:00 AM Day-5 8:00 AM Day-1 4:00 PM Day-2 12:00 AM Day-2 8:00 AM Day-2 4:00 PM Day-3 12:00 AM Day-3 8:00 AM Day-3 4:00 PM Day-4 12:00 AM Day-4 8:00 AM Day-4 4:00 PM Day-5 12:00 AM date time Day-5 4:00 PM Day-6 12:00 AM Day-6 8:00 AM Day-6 4:00 PM Day-7 12:00 AM Day-7 8:00 AM Day-7 4:00 PM Day-8 12:00 AM Day-8 8:00 AM Day-8 4:00 PM
23 Loss Effect on Audio Quality Speech quality is described as a subjective score MOS (Mean Opinion Score an ITU standard) Quality of an audio communication is highly sensitive to packet loss in particular MOS Quality 5 Excellent 4 Good 3 Fair 2 Poor 1 Bad MOS score G audio quality vs. loss percentage MOS Loss percentage
24 Real Time Protocol (RTP) RTP header includes the following: Timestamp Sequence Number Coding Information RTP provides the following services Monitors end-to-end delay and jitter Provides resequencing of out-of-order data Detects lost packets Payload identification Passes performance feedback from receiver back to sender (using Real-Time Control
25 VoIP Error Handling RTP allows us to detect lost packets. But what do we do about it? Might be able to request and get retransmission from sender within jitter buffer playout time (but probably not) Sender can transmit multiple copies of each packet Receiver can t just ignore packet loss receiver must play out some sound White noise
26 Forward Error Correction Packet-retransmission may not be a viable option for real-time audio Packet loss for audio can be rectified using Forward Error Correction (FEC) D3 D2 D1 D2 D1 P2 P1 D3 D2 D1 D3 D2 D1 XOR P2 P1 P2 XOR D3 FEC Encoder FEC decoder Fig. Operation of FEC
27 FEC Overhead But FEC has bandwidth and processing overhead Therefore, FEC should be used to maintain audio quality with optimal consumption of network bandwidth
28 VoIP Delay/Jitter Objectives For toll-quality VoIP (as good as wired telephone voice) the end-to-end packet delay on VoIP networks should be 150 ms. or less Delay is tolerated to 400ms for low bitrate codecs The network jitter should be 10% or less (i.e. no more than 10% variation in transmission delays)
29 Jitter effect on Voice Quality A network with large delay and low jitter is perceived as providing much better voice quality than a network with low/medium delay but large jitter. Reason: large jitter causes packet drops / disruptions in conversation.
30 VoIP Playout No jitter = perfect playout ms. of Audio Capture per packet Sender Clock Time Receiver Clock Time (packetization delay = 30 ms) VoIP packet #1 (assume transfer delay = 12 ms) Avg. End-to- End Delay: D = 42 ms VoIP packet #2 VoIP packet #3 (assume transfer delay = 12 ms) (assume transfer delay = 12 ms) Start playout of Packet #1 at Time = 42 ms Start playout of Packet #2 at Time = 72 ms VoIP packet #4 (assume transfer delay = 12 ms) Start playout of Packet #3 at Time = 102 ms
31 VoIP Playout 6 ms. jitter but no receiver delay = Gap ms. of Audio Capture per packet Sender Clock Time Receiver Clock Time (packetization delay = 30 ms) VoIP packet #1 (assume transfer delay = 12 ms) Avg. End-to- End Delay: D = 42 ms VoIP packet #2 VoIP packet #3 (assume transfer delay = 18 ms) (assume transfer delay = 12 ms) Start playout of Packet #1 at Time = 42 ms. End playout : 72 ms. GAP!! Start playout of Packet #2 at Time = 78 ms VoIP packet #4 (assume transfer delay = 6 ms) Start playout of Packet #3 at Time = 108 ms.
32 The Jitter Problems Problem #1: If there s too much jitter, you may have gaps in audio playout at times when a packet arrives late. Results can be improved by trying to intelligently compensate for the gap at the receiver by replaying/interpolating sounds. Problem #2: Jitter makes it hard to estimate how long you should wait for a packet at receiver before you assume it is lost. If you wait too long, you delay the audio playout. If you don t wait long enough, you miss packets.
33 The Playout Buffer Solution Receiver equipment assumes some maximum acceptable jitter time call it B. First packet at receiver is put into a buffer and playout is delayed for time B. The second, third, etc. packets are added to buffer when they arrive and buffer plays out continuously. If packet is more than B ms. late in arrival, then it is assumed lost and the receiver inserts one packet time (for example: 30 ms.) worth of compensatory audio for listener. Compensatory audio = repeat previous sound or
34 VoIP Playout 6 ms. jitter and B = 10 ms ms. of Audio Capture per packet Sender Clock Time Receiver Clock Time (packetization delay = 30 ms) VoIP packet #1 (assume transfer delay = 12 ms) Avg. End-to- End Delay: D = 52 ms VoIP packet #2 VoIP packet #3 VoIP packet #4 (assume transfer delay = 18 ms) (assume transfer delay = 12 ms) (assume transfer delay = 6 ms) Packet #1 arrives at Time = 42 ms. Put into buffer for 10 ms Packet #2 arrives at Time = 78 ms. Put Into buffer Packet #3 arrives at Time = 102 ms. Put Into buffer Packet #4 arrives at Time = 126 ms. Put Into buffer Start playout of Packet #1 at Time = 52 ms. End playout #1: 82 ms. Start playout of Packet #2 at Time = 82 ms. End playout #2: 112 ms. Start playout of Packet #3 at Time = 112 ms. End playout #3: 142 ms. Start playout of Packet #4 at Time = 142 ms.
35 Playout Buffer Ads/Disads Advantage of Playout Buffer Eliminates gaps: Every packet plays out correctly (no gaps) if all jitter is less than B. Disadvantage of Playout Buffer Increases the end-to-end delay: End-to-end delay is increased by time B.
36 How to reduce jitter? Provide higher priority to VoIP traffic compared with other traffic on the network. How to implement priorities? Ethernet priorities - IEEE 802.1p and 802.1q IP precedence field Multi-Protocol Label Switching (MPLS) DiffServ point codes Discussing these specific protocols is beyond the scope of this class.
37 VoIP Signaling OK, so now we have discussed the issues in just sending packetized voice across a network. Let s move on to signaling.
38 VoIP Signaling What do we need signaling for? Setting up calls Destination address lookup Telephone number IP address translation Indicating terminal capabilities Call Routing Offering call (ringing) Accepting calls Calling features conference, transfer, etc. Tearing down calls Administration Billing and auditing Admissions control
39 VoIP Signaling Packet Flows Terminal Gatekeeper Signaling packets exchanged to do destination address lookup, admission control, and call administration Terminal Gateway Signaling packets exchanged to set up, manage and tear down calls to non-voip devices Terminal Terminal Signaling packets may be exchanged directly to set up, manage and tear down
40 H.323 suite Dueling Standards Developed by ITU Most mature and most widely implemented Session Initiation Protocol (SIP) Developed by IETF More efficient and IP-friendly
41 Signaling Packets L 2 H d r I P T C P H o r S I P S i g n a l i n g I n f o L 2 T r l r Signaling packets are typically sent over TCP connections for reliability, but UDP is also an option.
42 Let s talk SIP Session Initiation Protocol is one of a set of protocols defined by IETF since 1999 to do VoIP call session signaling and management. SIP protocols: Designed to be simple and efficient Based on simple text commands and responses as with HTTP SIP model: places most call intelligence in enduser terminal rather than in the gatekeeper.
43 SIP Advantages SIP generally uses fewer messages and runs over UDP so it is fast and lightweight SIP operations can easily be extended (new call features, etc.) by any competent programmer SIP servers keep no state information about calls. End user devices responsible for all call status. SIP designed to fully utilize existing Internet infrastructure (DHCP, DNS, IP routing, IP multicasting)
44 SIP Protocols Session Initiation Protocol (RFC 2543) Signaling to set up call, transfer call, conference call, etc. Session Description Protocol (RFC 2327) Signaling to determine what features are supported by another device. Session Announcement Protocol (RFC 2974) Signaling to register presence of particular device/session on the network.
45 SIP Components User Agent is the user telephony application User Agent Client (UAC) can initiate calls. User Agent Server (UAS) can receive calls. SIP Server acts as central point to relay signaling information SIP Registrar is server that accepts registration messages Location Server stores user device
46 SIP Components WorkstationSIP Phone Workstation SIP Phone WorkstationSIP Phone PSTN SIP Server Trunks LAN Switch PSTN Gateway Router Internet
47 SIP Registration When SIP device first powers up, it registers itself with Registration Server. This information can then be used for many services: VoIP call Videoconferencing Instant messaging
48 SIP Addressing SIP users are addressed by a SIP URL, in the format This is very flexible Examples:
49 SIP Calling If SIP User Agents already know each other s addresses, they just communicate with each other directly (via INVITE message peer-to-peer). Otherwise, INVITE message is sent to a SIP Server, which may operate in one of two modes: Proxy Server mode: looks up destination and forwards INVITE Redirect Server mode: looks up destination and
50 SIP Signaling Messages SIP Requests: INVITE Initiates a call by inviting user to participate in session. ACK - Confirms that the client has received a final response to an INVITE request. BYE - Indicates termination of the call. CANCEL - Cancels a pending request. REGISTER Registers the user agent. OPTIONS Used to query the capabilities of a server. INFO Used to carry out-of- SIP Responses: 1xx - Informational Messages. 2xx - Successful Responses. 3xx - Redirection Responses. 4xx - Request Failure Responses. 5xx - Server Failure Responses. 6xx - Global Failures Responses.
52 SIP Call Proxy Server SIP UAC INVITE 200 OK SIP Server INVITE 200 OK SIP UAS ACK RTP stream RTP stream RTCP stream
53 Related IETF Working Groups Internet Telephony WG Audio / Video Transport WG Firewall Traversal WG Instant Messaging WG Quality of Service WG PSTN Interconnection WG
54 SIP Web Pages IETF SIP Charter SIP Forum SIP Pages Many SIP product manufacturers have good information as well.
55 VoIP Security VoIP security concerns: The usual PC problems (spam, viruses, worms, etc.) can now attack your telephone. Spam phone calls Denial of service attacks Confidentiality can someone tap your phone? Communications Assistance for Law Enforcement Act (CALEA) law requires that law enforcement be able to tap your phone how to comply? Integrity can someone modify your message? Can be resolved by digital signatures
56 VoIP Security VoIP security concerns: Availability needs to match 5-9s (99.999%) availability provided by PSTN Denial of Service attacks on VoIP may be easy Authenticity how can you be sure who you are talking to? E911 services VoIP services cannot always locate user and/or appropriate E911 service center for 911 calls. Currently, VoIP providers are required by law to tell subscribers NOT to use VoIP phones for 911 calls.
57 VoIP and Network Security VoIP is hard to configure with NAT and firewalls VoIP software chooses random port numbers to set up new calls. Which ports should firewall open up? IP addresses and port numbers are buried deep within VoIP packets. NAT or Firewall may not find them all. Even if they can what if the user is encrypting the packet?
58 VoIP Server Compromise If a VoIP Call Manager (Gatekeeper) or other VoIP server is compromised by a hacker, then security is lost. If a VoIP Gateway is compromised, then security is lost. You should authenticate: The person calling you The gatekeeper registering your VoIP identity Other VoIP servers along the way
59 Summary VoIP is an emerging technology to use IP as the carrier for voice communication (we've been saying that for years! literally!) May use the Internet Many issues still being ironed out, but growing in popularity (been saying that too) Many security issues missing, for example, ever wonder about potential for VoIP-based spam? Hmm...
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