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  1. 1. COMP3013 CONFERENCE COMPUTING - INTRODUCTION TO VOIP TECHNOLOGY AND ITS SECURITY ISSUES Aikaterini Gkritsi Electronics & Computer Science Department The University of Southampton Southampton SO17 1BJ ABSTRACT computer to telephone connection where the communication is established via a Public Switched Over the past few years Voice over IP (VoIP) Telephone Network (PSTN) through a PSTN-to-IP technology has seen an impressive growth. The aim gateway (PIG) (Figure 2) and telephone to of this paper is to introduce the reader into VoIP by telephone connection. [1] presenting the evolution of the technology, the way that it works and overview the protocols that are used. It then looks at the security issues that might arise by its usage and the possible ways to protect against these threats. Finally, the paper focuses on the features of a good Quality of Service (QoS) in a digital network like this and closes with its future applications and trends. Keywords VoIP, Protocols, Security, QoS 1. INTRODUCTION Nowadays VoIP technology is becoming more and more popular because of the broadband availability Figure 2 [1] and the many exciting features that are offered. This This paper discusses briefly on section 2 the history emerging technology allows users to make free calls of VoIP; it gives a description of how this technology between computers and among devices that have works and it presents the most popular protocols access to the Internet and are equipped with the that this technology uses in order to establish a call. appropriate software, independently of the location Section 3 examines the possible threats and of their users. There is also the possibility of making security issues that might arise by the usage of such international or long-distance calls at very low rates a technology and examines the available options in unlike normal phone lines. order to protect against these potential threats. As Condie notes, some of the most common Further, section 4 introduces all the features that are applications of VoIP are computer to computer necessary for a good QoS in a technology like this connection where two or more computers are and it proposes possible ways to improve it. Section connected via an IP network (Figure 1), 5 gives some interesting future features of VoIP and finally section 6 concludes the paper. 2. OVERVIEW OF VOIP 2.1 History The history of VoIP starts around 1970’s where the Figure 1 [1] first technology and ideas about it where Permission to make digital or hard copies of all or part of this established. [2] The first company that launched work for personal or classroom use is granted without fee Internet phone software was Vocaltec on 1995. The provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the new software was called “Internet Phone” and it full citation on the first page. To copy otherwise, to republish, to required two users running the software on their post on servers or to redistribute to lists, requires prior specific computers. Internet phone was used like the PC permission. phones that are used today with microphone, 4th Annual Multimedia Systems, Electronics and Computer speakers and sound card. Because the connection Science, University of Southampton between computers was made via modem, the © 2003 Electronics and Computer Science, University of Southampton
  2. 2. quality of the voice was very poor. However, as networks can be connected. Gatekeepers, even broadband availability was getting bigger and while though they are not an essential part of the H.323 Internet and Web were growing, more and more standard, they are responsible for the bandwidth people started using VoIP. management and the authentication process between the terminals and the gateways. Also, 2.2 How It Works MCU allows simultaneous conferences between the In VoIP technology voice is transmitted digitally in terminals that are connected to it. (Figure 4) As data packets. Special codecs convert voice into bits Swale states, H.323 protocol is a very powerful and bytes. When a user tries to contact another standard. It has the ability to switch on and off many user using VoIP services, their voice is compressed of its features, so it can be tailored to each network. and converted into digitalised signal and broken into [5] packets (1500 bytes). The signal is transmitted into public or private IP networks over the Internet. Each of the packets contains information about the place the packet was sent, its destination and timestamp which is necessary in order to reconstruct the packet in the correct order. When packets reach their receiving source, they are decoded and the other user is able to hear what the first user said. (Figure 3) [3] Figure 4 [6] 2.3.2 SIP Protocol SIP protocol is again another application layer protocol which is responsible for creating, modifying and terminating VoIP applications among terminals. SIP makes use of User Agents (UA) which represents the end user and proxy servers. “The proxy server is the intermediate server that acts on behalf of UA to forward the SIP messages to its destination”. [7] As Swale says again, SIP is very Figure 3 [4] easy to implement, powerful and it offers control to 2.3 Protocols adjust it to big networks because of its simple like- HTTP request-respond format. [5] In order to function, VoIP uses a number of protocols and standards to create, establish, maintain and terminate conversations between 2.3.3 MGCP Protocol users. Some of the most important protocols are H.323 from the International Telecommunication Finally, MGCP protocol is similar to H.323 standard Union, Session Initiation Protocol (SIP) and Media and is capable of controlling the communication Gateway Control Protocol (MGCP) from a branch of between gateways and call agents. In this protocol, the Engineering Task Force. all the functionalities like call control and the management of the gateways have been assigned to the call agents and not to the endpoint. 2.3.1 H.323 Standard Comparing these three protocols it is easy to H.323 standard is used in many different conclude that all of them are designed to establish applications and one of them is VoIP. H.323 and support multimedia communication but they are standard consists of four components: terminals, not always compatible with each other. H.323 is a gateways, gatekeepers and Multipoint Control Unit combination of individual protocols (other for data (MCU). [2] As terminals, it is possible to like G.729, video like H.263, audio and transport). It characterise any type of devices that support operates better with PSTN and it supports video multimedia applications and the H.323 standard. better since it was designed for video conferencing Gateways are the means by which two different voice and other additional services [1]. Unlike types of networks like PSTN and all circuit switched H.323, SIP is more lightweight protocol. It was designed specially to support VoIP applications. It is
  3. 3. easy to design and debug and it provides many conversations, stealing email or passwords etc functionalities to the users like call forwarding, call without the consent for the user. Last but not least, identification etc. MGCP focuses more on the VoIP systems are vulnerable against viruses. This is endpoint control (like call waiting) and it because all the devices that use VoIP applications incorporates functionalities of both H.323 and SIP. are connected to the Internet this means that they The protocols that are mostly used are H.323 and are at risk. [10] SIP but H.323 is considered to be better since it is 3.2 How to Protect more robust and independent of the transport Nowadays, there are several ways by which users protocols that it runs on. It can also support and systems can be protected against the threats multimedia conferences without the use of an MCU that were mentioned above. A method that is able to like SIP and it provides bandwidth management protect against several attacks (DoS, ensuring that video and audio traffic will have eavesdropping, toll fraud) is the usage of Virtual priority over other applications. Local Area Networks (VLAN). Voice and data 3. SECURITY ISSUES packets should be separated. This way it is much easier to prioritise voice traffic over data. The traffic VoIP applications when first designed, they were of the network is hidden so it cannot be created in order to provide the best service quality eavesdropped or be attacked from DoS. Another available to users. Because the first and most way to prevent attacks is by using encryption. important feature of the VoIP systems was that, the Secure Real Time Protocol (SRTP) and Transport vulnerabilities and the security issues that could Level Security (TLS) are the most common used occur by their usage played a secondary role. PSTN protocols for encryption. The first protocol encrypts was considered secure and because VoIP the communication between the end users and the applications relied on it, it was believed that VoIP second encrypts the entire call process. would be as secure as that. The fact is that because Furthermore, Virtual Private Networks (VPN) can be Internet is vulnerable against attacks and VoIP is effective because all the transactions are encrypted depending on the Internet, this makes VoIP and they provide secure access. Finally, IP Security technology vulnerable against these attacks as well. (IPSec) “supports network-level peer authentication, 3.1 Security Issues of VoIP data origin authentication, data integrity, data confidentiality (encryption) and replay protection.” VoIP packetizes phone calls through the same routes used by network and Internet traffic and is [11] consequently prone to the same cyber threats that 4. QUALITY OF SERVICE plague data networks today. These threats include denial of service attacks, worms, viruses, and The term Quality of Service (QoS) describes how hacker exploitation. [8] Apart from that, PSTN lines satisfying is the experience that the user has while can be wiretapped so the attacker can keep a log of using any VoIP application. It measures the quality the users’ calls. [9] of the conversation e.g. how natural it is, if there is In particular, one of the most common attacks is any delay, echo or corruption of the conversation by Man-In-The-Middle (MITM). By using this method, unexpected reasons. “Internet gives a very variable the attacker can hijack users’ emergency calls, Quality of Service (QoS) when this term is applied to access their voicemail and either accept voicemail speech performance.” [1] For this reason some messages or redirect the caller to another phone requirements have been set in order to measure the number. Then, as Blake states toll fraud is one of QoS. First, the performance of the call should have the five most common threats in VoIP networks. [10] low delay which means that the delay of the delivery In that, a company’s phone system is violated and it of the transmitted packet should be small enough so is used illegally from a third party from a long- it does not interfere with normal voice distance location. Another common attack is brute conversations. Also, the number of packets that are force where an attacker is trying to overcome all the lost during transmission should be low so it will not security checks of a system by applying all the affect the quality of the call. Then, the time when a possible combinations of keywords. Also, by using user dials the last digit of the number and the Denial of Service (DoS) the attacker floods the confirmation that the connection was successful network with useless and malfunctioned packets should be short, as well as the delay from the time until it cannot process any more requests. This that the other user accepts the call and the voice situation results to resource exhaustion, long term path being cut through. busy calls and sessions that disconnect 4.1 QoS Problems unexpectedly. Spoofing is another method by which “QoS (quality of service) is an important issue for IP- attackers are able to use the IP address of an based multimedia services.” [12] Some of the most authorised user in order to gain access to the host common technical issues that occur in VoIP are of the system. Eavesdropping on a VoIP networks delay in the transmission of the voice and latency. means that a third party is monitoring users’ calls,
  4. 4. VoIP quality depends on many aspects like the time will have big impact on the quality of the call. quality of the audio peripherals that are used and Another effective way of improving the quality of the codecs that are used for the encoding and the voice is Resource Reservation Protocol (RSVP) and decoding of the voice signal. Quality is also affected IP version 6 (IPv6). With the combination of these by the software scheduling by the operating system protocols voice packets are prioritised over the IP that the device or computer uses, how much delay network. [14] Finally, improvements on PSTN-to-IP or jitter occurs in the network and finally the number connection and introduction of protocols and of packets that is lost during the transmission of the standards help the situation. signal from one endpoint to the other. [13] Latency during a VoIP session occurs because of 5. APPLICATIONS AND FUTURE TRENDS the traffic that exists in the network which leads to Because VoIP is an emerging technology that late delivery of the packets to their destination. Also, evolves all the time, there are many very promising another reason of latency is the route that a packet applications and there is plenty of space for further has to go through in order to reach its destination. development in different areas. The more routers, switches or gateways packets 5.1 Mobile VoIP have to go through, the more time it takes for their delivery. Lastly, significant role to latency plays the First and foremost, there has been a lot of size of the delivered packet. [2] discussion on mobile VoIP. This means that a mobile phone is capable of supporting VoIP calls Jitter occurs when voice and data packets are while connected on a wireless network. Nowadays, transmitted at the same time so they interfere. This there are many mobile devices that have this leads to a need of a buffer which will store technology embedded, but because wireless temporarily the packets and transmit them again network has very limited coverage, users are with some delay. [14] The case of packet loss restricted to use it only at places where they can during the transmission process will lead to delay in have access to wireless networks. As the wireless the delivery of the signal to the other end. If the coverage expands and the number of hotspots delay is more than 200 milliseconds, the quality of multiplies, more and more people tend to perform the voice is considered unacceptable and very poor. VoIP calls instead of regular ones because it is “Data packets travelling through the Internet may much easier and makes significant difference to the not be secure and may require encryption, adding final cost. To provide more freedom and better overhead by increasing the necessary bit rate network coverage to users, the issue of adopting a beyond 14Kbps, hence reducing the bit rate technology similar to Wi-Fi but with more advantage of VoIP over PSTN. Encryption also capabilities arises. For this reason WiMAX increases the end-to-end latency caused by the (Worldwide Interoperability for Microwave Access) processing delay for encryption and decryption.”[2] networks need to be used. WiMAX is again a Finally, another problem that affects the quality of a wireless network which “is intended for wireless call is the presence of echo. This phenomenon "metropolitan area networks". It provides broadband occurs by the reflection of the signal during the call. wireless access (BWA) up to 30 miles (50 km) for If the delay is some milliseconds long, it leads to fixed stations, and 3 - 10 miles (5 - 15 km) for echo. mobile stations. In contrast, the Wi-Fi wireless local 4.2 Improvements area network standard is limited in most cases to 1 only 100 - 300 feet (30 - 100m).” Businesses and There are many ways by which the QoS can be customers already benefit from the low cost improved. In particular, in order to overcome the services that VoIP technology offers. jitter occurrence, it is possible to perform data Characteristically it was estimated that nowadays packet fragmentation and voice packet prioritisation. more than 4 million UK citizens are VoIP users, The first technique will ensure that the packets that 2 excluding businesses. This figure shows how much are delivered are not of varied size. All the packets impact the evolution of this technology will have in should not contain large data in order to avoid everyday life. queuing delays. The other feature prioritises the delivery of voice packets over the data packets. This 5.2 IP PBX way latency can be eliminated by sending As mentioned earlier, VoIP also plays a significant messages to routers over the networks to find out role on businesses. In particular, these days which is the fastest route and with less traffic, which companies tend to replace their common phone will lead to better quality of the voice. To this contributes the fact that nowadays the routers and 1 gateways are much faster than before; which means, Last accessed: that more packets can be transmitted December 2009 2 simultaneously. Also, a choice of a codec that has the ability to encode and decode the voice digitally future-hold-for-voip.html, Last accessed: December in high quality and with short encoding-decoding 2009
  5. 5. lines with VoIP compatible PBX (Private Branch frequencies. In order for this feature to work, end- Exchange). PBX is a private branch telephone users need to have devices that support this kind of 4 network that enterprises use. With this, employees technology. use a certain number of external land lines. This way of connection is very cost effective because 6. CONCLUSION there is not the need of mapping an external phone This paper covered the fundamental issues line to every employee. The advantage of IP PBX concerning VoIP and its aim was to inspire readers over conventional PBX is the fact that it supports look further in this very interesting technology. services like video or data but over data networks Judging from the architecture and the variety of rather than PSTN. possible applications of VoIP technology, it is easy 5.3 Hosted IP communications to conclude that this technology will play a significant role in many aspects of everyday life. Hosted IP communications from a service provider Despite the fact that there are many security issues is a voice service targeted for all kinds and sizes of with this technology, there are plenty of ways to enterprises. This service is available to users prevent and eliminate them so VoIP continues its independently of their location at the time. This growth. service gives companies the possibility to add new users and new locations or even to move users 7. REFERENCES between locations and implement business plans. [1] S J T Condie et al., ‘VoIP applied – lab bench to As David Perry, the senior manager of Nortel’s multimedia service’, BT Technology Journal, Carrier VoIP & Applications Solutions says, because Volume 19, Number 2 / April, 2001, Springer of the global recession this solution is very effective Netherlands, 2004 for businesses. This is because with this service enterprises will not need to buy, manage, maintain [2] Upkar Varshney et al., ‘Voice over IP’, and upgrade their PBX. Instead, a service provider Communications of the ACM , Volume 45 Issue will be responsible for offering a service package for 1, ICST (Institute for Computer Sciences, per-user, per-month fee. These packages will Social-Informatics and Telecommunications include services concerning call diversion, Engineering), January 2002 voicemail, instant messaging, video conferencing [3] Vesselin Iossifov et al., ‘Experiences in VoIP 3 and collaboration etc. telephone network security policy at the University of Applied Sciences (FHTW) Berlin’, 5.4 IPTV CompSysTech '07: Proceedings of the 2007 A very promising future application of VoIP international conference on Computer systems technology is IPTV. This feature is an ambition from and technologies, ACM, June 2007 Nortel. With this service users will be able to receive [4] Federal Communications Commission: calls at home. The TV will automatically pause live, broadcasting in order to show to users the details of last accessed: November 2009 the caller and allow them to accept the call or send it to voicemail only by pressing a button on the [5] Swale, R.P. (2001) ‘VoIP – panacea or PIG’s remote control. 3 ear?’ BT Technology Journal, Volume 19, Number 2 / April, 2001, Springer Netherlands 5.5 Voice-to-text transcription [6] International Engineering Consortium: Another very promising application of VoIP is the conversion of voicemail messages into text so users p, last accessed November 2009 are able to read their voicemail in a form of text message instead. This way users are able to avoid [7] Ruishan Zhang, et al., (2009) ‘An Empirical listen to all their messages in sequence and Investigation into the Security of Phone pressing buttons to interact with the operator. 4 Features in SIP-Based VoIP Systems’, vol. 5451/2009, Lecture Notes in Computer Science, 5.6 HD Voice Springer Link, pp. 59-70 Finally, a very interesting future feature of VoIP is [8] Insu, K. and Keecheon, K. (2007) ‘Secure the incorporation of HD (High Definition) Voice Session Management Mechanism in VoIP capability into IP phones. HD voice in contrast with Service’ Book Series Lecture Notes in simple voice on phones is that audio is twice the Computer Science, vol. 4743/2007, pp. 96- rate and it transmits a much wider range of sound 104, Springer Berlin / Heidelberg [9] Jill Slay, Matthew Simon, ‘Voice over IP 3 Forensics’, e-Forensics '08: Proceedings of the can-it-take-you, Last accessed: December 2009 1st international conference on Forensic 4 applications and techniques in 010509/, Last accessed: December 2009 telecommunications, information, and
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