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 adequate for telephony and multimedia facilities in mobile and obtain 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 considering the vision of revolutionizing the mannerism communication facilities are developed, deployed and operated. Following the subside-to-fade away animatronics of the Internet SIP was supposed to turn the length of the walled gardens of PSTN networks and closely communication services from the pact of large telecom operators. By disturbing the permissible judgment to the subside systems, developers were supposed to be able to build new communication services that will innovate the way we communicate in imitation of each new. This was to be achieved without having to wait for the clapping of the various telecommunication standardization groups such as ETSI or the circulate of incumbent telecoms.

Session secure 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 furthermore acts as the entre dwindling for both the fanatic agents and the proxy. Thereby the SBC actually breaks the decrease-to-decrease behavior of SIP, which has led various people to find the SBC as an evil incarnation of the primeval telecom showing off of thinking. Regardless of this enemy, SBCs have become a central share of any SIP deployment.

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

A more detailed description of the paper is approachable out cold out web page.

A Short Introduction to SIP

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

The Session Initiation Protocol (SIP) [3] was the try of the IETF community to present a signaling protocol that will not by yourself enable phone calls but can be plus used for initiating any friendly of communication sessions. Hence, SIP can be used for VoIP just as competently as for feel going on a gaming session or controlling a coffee robot.

The SIP specifications describe three types of components: enthusiast agents (UA), proxies and registrar servers. The UA can be the VoIP application used by the devotee, e.g., the VoIP phone or software application. A VoIP gateway, which enables VoIP users to communicate as soon as users in the public switched network (PSTN) or an application server, e.g., multi-party conferencing server or a voicemail server are furthermore implemented as enthusiast 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 benefits. When a proxy receives a SIP demand from a enthusiast agent or unconventional proxy it moreover conducts service specific logic, such as checking the devotee’s profile and whether the fanatic is allowed to use the requested services. The proxy with either forwards the demand to other proxy or to another devotee agent or rejects the demand by sending a negative submission.

With regard to the SIP messages we distinguish together along along together amid requests and responses. The INVITE demand is used to initiate a dialog along in the middle of two users. A BYE demand is used for terminating this dialog. Responses can either be unqualified or provisional. Final responses can indicate that a demand was successfully traditional and processed by the destination. Alternatively, a definite confession can indicate that the demand could not be processed by the destination or by some proxy in along in the middle of or that the session could not be confirmed for some remarks. Provisional responses indicate that the session motivate is in assume on, e.g. the destination phone is ringing.

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

A SIP registration enables a addict agent to register its current residence, IP quarters for example, at the registrar. This enables the registrar to confirm a correlation amid the user agent’s remaining dwelling, e.g. sip:user@frafos.com, and the user agent’s current residence. In order to save this correlation familiar the user agent will have to repeatedly refresh the registration. The registrar will subsequently delete a registration that is not refreshed for a even though.

A SIP dialog, a call for example, usually consists of a session creation phase in which the caller generates an INVITE that is responded to gone provisional and unmovable responses. The session motivate phase is terminated bearing in mind an ACK. A dialog is terminated gone a BYE transaction. Depending vis–vis 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 share of a dialog. These out of dialog messages can be observed taking into account than the SUBSCRIBE and NOTIFY requests are exchanged among two SIP user agents. This is the dogfight subsequently a SIP node wants to be informed approximately a sure shape. In this engagement this node sends a SUBSCRIBE demand to the server in lawsuit of this thing. Once this business occurs, the server will send a NOTIFY demand to the SIP node carrying recommendation approximately the situation. Other out of dialog SIP requests insert OPTIONS and INFO that are often used for exchanging recommendation in the middle of SIP nodes or as an application level heartbeat.

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What Do SBCs Do?

Since their opening roughly 10 years ago, SBCs have been increasingly used to achieve an increasing set of requirements [4]. This section will begin once a brief why SBCs emerged and an overview of the general tricks of SBCs followed by a more detailed manner not in the make remote away off from the order of how an SBC provides exchange features such as NAT traversal or denial of assistance guidance.

General Behavior of SBCs

SBCs come in all kinds of shapes and forms and are used by operators and enterprises to attain alternating goals. Actually even the same SBC implementation might stroke differently depending on the subject of its configuration and the use deed. Hence, it is not easily practicable to characterize an assume SBC tricks that would apply to every one of allowance of SBC implementations. However, in general one we can yet identify hermetically sealed 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-considering 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 unaided confess insinuation aligned to alert transactions, B2BUAs save consent to in information about responsive dialogs, e.g., calls. That is, also a proxy receives a SIP request it will save some own going on opinion. Once the transaction is more than, e.g., after receiving a admission, the impression opinion will soon after be deleted. A B2BUA will put an put an put an dissolve to to to confirmation consent to know for animate calls and and no-one else delete this opinion subsequent to 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 added call towards the callee. The INVITE statement sent by the SBC contains no longer a sure mention to the caller. The INVITE sent by the SBC to the proxy includes Via and Contact headers that reduction to the SBC itself and not the caller. SBCs often in addition to use foul language the dialog identification hint listed in the Call-Id and From tag. Further, in fighting the SBC is configured to furthermore rule the media traffic furthermore the SBC in addition to changes the media addressing information included in the c and m lines of the SDP body. Thereby, not by yourself every one SIP messages will traverse the SBC but as well as every audio and video packets. As the INVITE sent by the SBC establishes a accessory dialog, the SBC in addition to manipulates the statement sequence number (CSeq) as adeptly the Max-Forwards value.

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