| VoIP has taken the world by storm. There is an | | | | very feature (along with other VoIP protocols). SIP |
| accented probability that you might already be using it, | | | | registrar keeps track of user presence and |
| without even being aware of it. It was not very far | | | | whereabouts. Another important element is the |
| when you called up your friend in Karachi and you | | | | 'redirect server', which redirects the calls to other |
| were actually owning 700 miles of odd copper. The | | | | servers. It is not necessary that these elements |
| world has changed; technology has gone even finer, to | | | | (registrar and redirect server) are physically distinctive. |
| the extent that it has influenced the social inclinations | | | | They are logically working together, in concert with a |
| and doings of the 'beau monde'. VoIP has been an | | | | 'SIP proxy server'. |
| instrument of excellence for multinational corporate, | | | | Having come across the features and elements of |
| banking institutions, mammoth businesses and an | | | | SIP, knowing what signaling does and how is it different |
| absolute charm of the research fields. VoIP has | | | | from the orthodox telephony system, lets make our |
| particularly impressed telecom operators and service | | | | first SIP call and see how closely these elements are |
| providers for its smarter and chic use of bandwidth, | | | | knitted together. The first step is informing the SIP |
| crunching CODECS and immense flexibility to nest | | | | registrar/proxy about the whereabouts of both the |
| contents other then voice. VoIP protocols are | | | | caller and the callee. All the user agent clients initiate a |
| differentiated principally on their signaling schemes, | | | | SIP request using the method of "REGISTER", |
| media streams and message formats. Some widely | | | | informing their current location to the SIP registrar. This |
| popular VoIP protocols are SIP, H.323 (protocol suite), | | | | feature allows the mobility of SIP clients over the |
| MGCP and IAX. SIP is (for some reason) is the most | | | | internet cloud. For initiating a call, an "INVITE" request is |
| bragged about signaling protocol by the VoIP vendors. | | | | sent to the SIP proxy, tagged with callee's identification. |
| What does SIP do? Who are the wizards behind it? | | | | The proxy looks up the location of callee (by means |
| What makes SIP so popular? Is the SIP gospel (RFC | | | | of the identification tagged by the caller) and relay the |
| 3261) perfect? How far can the tin man walk? Let me | | | | handshake request to the callee's end point. Callee |
| knit it up for you. SIP is a 'Session Initiation Protocol' | | | | considers the request and reply with an "OK 200" |
| drafted under the RFC 3261. SIP is, put simply, a | | | | message to the proxy, which relays it back to the |
| signaling protocol that is being used to initiate, establish, | | | | caller. Once it receives the confirmation of callee's |
| exchange information and tearing down sessions for | | | | availability, caller sends back an acknowledgment |
| communication. Before you get snarled and lost in | | | | "ACK" message to the callee for establishing the |
| abbreviations, think of SIP as two individuals shaking | | | | media stream for communication. During the |
| hands before getting into a conversation. Shaking | | | | handshake, both parties have also agreed to the |
| hands is not just a social gesture, but an implicit | | | | content and format of the communication. Now the |
| exchange of information that both individuals are not | | | | session is established and both parties can converse, |
| only available for conversation, but they also know the | | | | until the session is torn out with a "BYE" message. If |
| possible medium of communication and somewhat the | | | | the callee is not available or busy, a "400 BAD |
| content of the conversation as well. Some may argue | | | | REQUEST" response is sent to the caller (smells like |
| that they can start a conversation without shaking | | | | HTTP? yes, it surely does). |
| hands (which is actually possible between individuals), | | | | In the strength of SIP, lies its weakness. SIP is criticized |
| but in the world of bits and bytes, no two nodes can | | | | for the simplicity and clear text transmission of |
| start conversing without a hand shake (or shall we say | | | | information, which can not only sabotage a subscriber's |
| signaling). A typical SIP handshake includes handshake | | | | privacy but also leaves them vulnerable to several |
| method (INVITE), user availability, response code, | | | | attacks. These attacks can include registration |
| transport medium, message format and other | | | | hijacking, possible eavesdropping, rerouting of transport |
| supporting protocol information. | | | | streams and session replay attacks. There are several |
| SIP was initially designed by Henning Schulzrinne | | | | tools available in the wild, to audit the SIP |
| (Columbia University) and Mark Handley (UCL) in 1996. | | | | infrastructures which can also be used maliciously. |
| Now it is being maintained by "Internet Engineering | | | | Since the voice has changed its nature from the |
| Task Force" (IETF), which is a forum of network | | | | fringing frequency to hoard of data packets, it is a lot |
| operators, vendors, researchers and developers | | | | easier for a frustrated college drop out to make |
| interested in the evolution of internet. SIP is designed in | | | | headlines in the press. However, if you are a system |
| such a way that it can house a variety of multimedia | | | | administrator (for a VoIP infrastructure) and don't like |
| content that can include images, video or even a URL. | | | | emergency calls at night, you still can go an extra mile |
| SIP is light weight with only six methods to manipulate | | | | to secure both the signaling and the transport medium |
| a session, transport independent since it can either use | | | | as your first line of defense. Careful and well |
| UDP or TCP as its transport mechanism and | | | | measured implementation of SIP proxies can ensure a |
| text-based, thus resulting in lower overheads. | | | | peaceful slumber. Keep track of the advisories related |
| SIP operates by distributing the intelligence over the | | | | to your VoIP product, scroll through the vulnerabilities |
| edge of the network. While some of the comparative | | | | published at BugTraq mailing list and ensure patch |
| technologies have a far more complex and centralized | | | | management. All these practices still cannot save you |
| intelligence. SIP end points are not dumb like the | | | | from zero-day vulnerabilities or against a skilled hacker |
| orthodox telephony architecture. Instead these end | | | | (who is financially motivated), but it surely is a |
| points have abilities to parse and understand a | | | | pragmatic approach towards security and does |
| message, decode the media stream and enforce the | | | | provide you a comfort zone. |
| authentication and integrity checks. SIP end points are | | | | From the invention of paper in China to the press |
| usually IP telephones or soft phone clients running on | | | | machine in Europe, it has only been the means and |
| top of some ordinary computing hardware (referred | | | | course of communication that has evolved human |
| as user agent clients in the RFC). SIP facilitates its | | | | knowledge, revolutionized societies and overturned |
| tremendous mobility by means of a 'SIP registrar', | | | | economies. From circuit voice to packet-ised voice, |
| which makes it possible for the user agent client to | | | | from television broadcast to video conferences, the |
| move transparently over the internet cloud. Can you | | | | means are only getting better and there is no |
| imagine carrying your home phone and tariff along, on | | | | conclusion to it. |
| a vacation to the Bahama Islands? SIP promises the | | | | |