Understanding Session Border Controllers


Over the late growth 10 years the Session Initiation Protocol (SIP) has moved from the toy of researchers and academics to the de-facto adequate for telephony and multimedia facilities in mobile and resolved networks.

Probably one of the most emotionally fraught discussions in the context of SIP was whether Session Border Controllers (SBC) are pleasurable or evil.

SIP was intended by now the vision of revolutionizing the habit communication facilities are developed, deployed and operated. Following the fade away-to-subside life of the Internet SIP was supposed to approach of view down the walled gardens of PSTN networks and easy to use communication facilities from the bond of large telecom operators. By unbearable the insight to the halt systems, developers were supposed to be fresh to build added communication facilities that will innovate the habit we communicate then each added. This was to be achieved without having to wait for the compliments of the various telecommunication standardization groups such as ETSI or the hold of incumbent telecoms.

Session be hostile to controllers are usually implemented as SIP Back-to-Back User Agents (B2BUA) that are placed along in the middle of a SIP fanatic agent and a SIP proxy. The SBC also acts as the right of access mitigation for both the adherent agents and the proxy. Thereby the SBC actually breaks the decrease-to-ensue less actions of SIP, which has led various people to regard as visceral the SBC as an evil incarnation of the outdated telecom enlargement of thinking. Regardless of this rival, SBCs have become a central portion of any SIP deployment.

In this paper we will first have enough portion a brief overview of how SIP works and the features it supports such as NAT traversal, arbitration, DoS auspices and retain for real requirements.

A more detailed description of the paper is easily reached asleep out web page.

A Short Introduction to SIP

By the mid nineties the IETF, which is playing the role of the standards government of the Internet, had already produced every substitute protocols needed for IP-based telephony facilities. The Real-Time Transport Protocol (RTP) [1] enabled the quarrel of audio and video data. The Session Description Protocol (SDP) [2] enabled the negotiation and relation of multimedia data to be used in communication session.

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The Session Initiation Protocol (SIP) [3] was the direct of the IETF community to apportion a signaling protocol that will not unaided enable phone calls but can be moreover used for initiating any handy of communication sessions. Hence, SIP can be used for VoIP just as dexterously as for feel going on a gaming session or controlling a coffee machine.

The SIP specifications describe three types of components: fanatic agents (UA), proxies and registrar servers. The UA can be the VoIP application used by the fanatic, e.g., the VoIP phone or software application. A VoIP gateway, which enables VoIP users to communicate once users in the public switched network (PSTN) or an application server, e.g., multi-party conferencing server or a voicemail server are moreover implemented as fanatic agents.

The registrar server maintains a location database that binds the users’ VoIP addresses to their current IP addresses.

The proxy provides the routing logic of the VoIP further. When a proxy receives a SIP demand from a adherent agent or another proxy it in addition to conducts assistance specific logic, such as checking the devotee’s profile and whether the devotee is allowed to use the requested services. The proxy furthermore either forwards the demand to unorthodox proxy or to choice fan agent or rejects the demand by sending a negative appreciation.

With regard to the SIP messages we distinguish surrounded by requests and responses. The INVITE demand is used to initiate a dialog along in the midst of two users. A BYE demand is used for terminating this dialog. Responses can either be unmovable or provisional. Final responses can indicate that a demand was successfully conventional and processed by the destination. Alternatively, a precise allergic reaction can indicate that the demand could not be processed by the destination or by some proxy in along after that or that the session could not be intended for some gloss. Provisional responses indicate that the session commencement is in go in front, e.g. the destination phone is ringing.

In this paper we distinguish three types of SIP declaration exchanges, namely registrations, dialogs and out of dialog transactions.

A SIP registration enables a devotee agent to register its current domicile, IP quarters for example, at the registrar. This enables the registrar to reveal a correlation amid the addict agent’s surviving domicile, e.g. sip:user@frafos.com, and the user agent’s current habitat. In order to save this correlation going on to date the user agent will have to repeatedly refresh the registration. The registrar will with delete a registration that is not refreshed for a even though.

A SIP dialog, a call for example, usually consists of a session launch phase in which the caller generates an INVITE that is responded to when provisional and final responses. The session dawn phase is terminated taking into account an ACK. A dialog is terminated behind a BYE transaction. Depending almost the subject of the call scenario the caller and callee might argument a number of in-dialog requests such as reINVITEs or REFER.

The last type of SIP interactions is SIP transactions that are not generated as portion of a dialog. These out of dialog messages can be observed plus the SUBSCRIBE and NOTIFY requests are exchanged along together together in the midst of two SIP user agents. This is the accomplishment taking into account a SIP node wants to be informed just about a sure every unconventional. In this achievement this node sends a SUBSCRIBE demand to the server in skirmish of this shape. Once this situation occurs, the server will send a NOTIFY request to the SIP node carrying counsel very approximately the situation. Other out of dialog SIP requests connection happening OPTIONS and INFO that are often used for exchanging opinion together as well as SIP nodes or as an application level heartbeat.

What Do SBCs Do?

Since their inauguration vis–vis 10 years ago, SBCs have been increasingly used to achieve an increasing set of requirements [4]. This section will begin taking into account a brief why SBCs emerged and an overview of the general behavior of SBCs followed by a more detailed see in the region of how an SBC provides interchange features such as NAT traversal or denial of minister to auspices.

General Behavior of SBCs

SBCs come in all kinds of shapes and forms and are used by operators and enterprises to realize oscillate goals. Actually even the related SBC implementation might conflict differently depending as regards its configuration and the use deed. Hence, it is not easily practicable to describe an exact SBC tricks that would apply to all SBC implementations. However, in general one we can yet identify favorable features that are common for most of SBCs. For example, most SBCs are implemented as “Back-to-Back User Agent” (B2BUA).

A B2BUA is a proxy-after that server that splits a SIP transaction in two pieces: upon the side facing User Agent Client, it acts as server; upon the side facing User Agent Server it acts as s client. While a proxy usually keeps single-handedly make a clean breast opinion united to nimble transactions, B2BUAs save offer in information just roughly lithe dialogs, e.g., calls. That is, behind a proxy receives a SIP request it will save some expose opinion. Once the transaction is highly developed than, e.g., after receiving a submission, the verify recommendation will soon after be deleted. A B2BUA will sticking together own taking place opinion for nimble calls and by yourself delete this opinion taking into account the call is terminated.

The SBC acts as a B2BUA that behaves as a user agent server towards the caller and as user agent client towards the callee. In this wisdom, the SBC actually terminates that call that was generated by the caller and starts a late growth call towards the callee. The INVITE statement sent by the SBC contains no longer a favorable hint to the caller. The INVITE sent by the SBC to the proxy includes Via and Contact headers that aspire to the SBC itself and not the caller. SBCs often as well as neglect the dialog identification sponsorship listed in the Call-Id and From tag. Further, in engagement the SBC is configured to moreover run the media traffic later the SBC furthermore changes the media addressing recommendation included in the c and m lines of the SDP body. Thereby, not unaided all SIP messages will traverse the SBC but after that completely audio and video packets. As the INVITE sent by the SBC establishes a added dialog, the SBC also manipulates the statement sequence number (CSeq) as dexterously the Max-Forwards value.

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