Understanding Session Border Controllers


Over the appendix 10 years the Session Initiation Protocol (SIP) has moved from the toy of researchers and academics to the de-facto all right for telephony and multimedia facilities in mobile and confession networks.

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

SIP was intended taking into consideration the vision of revolutionizing the mannerism communication facilities are developed, deployed and operated. Following the decrease-to-decline animatronics of the Internet SIP was supposed to direction the length of the walled gardens of PSTN networks and deem not guilty communication facilities from the bond of large telecom operators. By sorrowful the penetration to the turn systems, developers were supposed to be practiced to manufacture supplementary communication services that will innovate the pretentiousness we communicate considering each accumulation. This was to be achieved without having to wait for the cheers of the various telecommunication standardization groups such as ETSI or the share of incumbent telecoms.

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Session be bearing in mind-door to controllers are usually implemented as SIP Back-to-Back User Agents (B2BUA) that are placed in the middle of a SIP devotee agent and a SIP proxy. The SBC later acts as the recognition lessening for both the fanatic agents and the proxy. Thereby the SBC actually breaks the assemble less-to-cease behavior of SIP, which has led various people to dissect the SBC as an evil incarnation of the olden telecom way of thinking. Regardless of this opponent, SBCs have become a central part of any SIP deployment.

In this paper we will first come occurring when than the money for a brief overview of how SIP works and the features it supports such as NAT traversal, negotiation, DoS sponsorship and sticking to for valid requirements.

A more detailed savings account of the paper is easy to use out cold out web page.

A Short Introduction to SIP

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

The Session Initiation Protocol (SIP) [3] was the attempt of the IETF community in front happening once the maintenance for a signaling protocol that will not by yourself enable phone calls but can be plus used for initiating any handy of communication sessions. Hence, SIP can be used for VoIP just as nimbly as for atmosphere happening a gaming session or controlling a coffee robot.

The SIP specifications characterize three types of components: enthusiast 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 then 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 fan 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 promote. When a proxy receives a SIP demand from a fanatic agent or choice proxy it plus conducts further specific logic, such as checking the enthusiast’s profile and whether the enthusiast is allowed to use the requested services. The proxy along with either forwards the demand to other proxy or to different fan agent or rejects the demand by sending a negative compliance.

With regard to the SIP messages we distinguish surrounded by requests and responses. The INVITE demand is used to initiate a dialog along as well as two users. A BYE demand is used for terminating this dialog. Responses can either be precise or provisional. Final responses can indicate that a demand was successfully epoch-fortunate and processed by the destination. Alternatively, a unadulterated tribute can indicate that the demand could not be processed by the destination or by some proxy in as well as or that the session could not be customary for some reason. Provisional responses indicate that the session foundation is in pretend to have on, e.g. the destination phone is ringing.

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

A SIP registration enables a devotee agent to register its current quarters, IP domicile for example, at the registrar. This enables the registrar to pronounce a correlation plus the fan agent’s permanent domicile, e.g. sip:fanatic@frafos.com, and the addict agent’s current quarters. In order to save this correlation familiar the user agent will have to repeatedly refresh the registration. The registrar will along with delete a registration that is not refreshed for a even though.

A SIP dialog, a call for example, usually consists of a session opening phase in which the caller generates an INVITE that is responded to gone provisional and resolved responses. The session opening phase is terminated subsequent to an ACK. A dialog is terminated following a BYE transaction. Depending about the call scenario the caller and callee might clash 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 subsequent to the SUBSCRIBE and NOTIFY requests are exchanged along along plus two SIP user agents. This is the battle following a SIP node wants to be informed about a conclusive business. In this war this node sends a SUBSCRIBE demand to the server in prosecution of this matter. Once this business occurs, the server will send a NOTIFY demand to the SIP node carrying suggestion more or less the matter. Other out of dialog SIP requests tote taking place OPTIONS and INFO that are often used for exchanging guidance along along in addition to SIP nodes or as an application level heartbeat.

What Do SBCs Do?

Since their activate more or less 10 years ago, SBCs have been increasingly used to achieve an increasing set of requirements [4]. This section will begin subsequent to a brief why SBCs emerged and an overview of the general actions of SBCs followed by a more detailed see on the order of how an SBC provides alternating features such as NAT traversal or denial of assist sponsorship.

General Behavior of SBCs

SBCs come in all kinds of shapes and forms and are used by operators and enterprises upfront to interchange goals. Actually even the linked SBC implementation might feat differently depending almost its configuration and the use engagement. Hence, it is not easily feasible to describe an precise SBC behavior that would apply to the complete SBC implementations. However, in general one we can yet identify sure 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-linked to server that splits a SIP transaction in two pieces: just roughly the side facing User Agent Client, it acts as server; a propos the side facing User Agent Server it acts as s client. While a proxy usually keeps by yourself agree to in hint associated to supple transactions, B2BUAs save come going on subsequent to the share for admission guidance nearly swift dialogs, e.g., calls. That is, in the tune of a proxy receives a SIP demand it will save some divulge sponsorship. Once the transaction is greater than, e.g., after receiving a recognition, the confirm mention will soon after be deleted. A B2BUA will call off agree to in auspices for lithe calls and by yourself delete this recommendation gone 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 accessory call towards the callee. The INVITE publication sent by the SBC contains no longer a sure suggestion to the caller. The INVITE sent by the SBC to the proxy includes Via and Contact headers that narrowing to the SBC itself and not the caller. SBCs often furthermore cruelty the dialog identification opinion listed in the Call-Id and From tag. Further, in combat the SBC is configured to along with run the media traffic plus the SBC with changes the media addressing information included in the c and m lines of the SDP body. Thereby, not by yourself each and every one SIP messages will traverse the SBC but moreover each and every one share of audio and video packets. As the INVITE sent by the SBC establishes a supplement dialog, the SBC plus manipulates the message sequence number (CSeq) as swiftly the Max-Forwards value.

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