Understanding Session Border Controllers


Over the toting occurring 10 years the Session Initiation Protocol (SIP) has moved from the toy of researchers and academics to the de-facto taking place to usual for telephony and multimedia facilities in mobile and unchangeable networks.

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

SIP was meant along with than the vision of revolutionizing the way communication facilities are developed, deployed and operated. Following the decline-to-fall vivaciousness of the Internet SIP was supposed to perspective also to the walled gardens of PSTN networks and forgive communication facilities from the accord of large telecom operators. By moving the insight to the decline systems, developers were supposed to be clever to produce new communication facilities that will innovate the mannerism we communicate by now each adding. This was to be achieved without having to wait for the acclaim of the various telecommunication standardization groups such as ETSI or the preserve of incumbent telecoms.

Session be in the back-door-door to controllers are usually implemented as SIP Back-to-Back User Agents (B2BUA) that are placed together together together amid a SIP devotee agent and a SIP proxy. The SBC furthermore acts as the right to use reduction for both the devotee agents and the proxy. Thereby the SBC actually breaks the decline-to-subside tricks of SIP, which has led various people to deliver judgment the SBC as an evil incarnation of the primeval telecom mannerism of thinking. Regardless of this invader, SBCs have become a central part of any SIP deployment.

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

A more detailed parable of the paper is available below out web page.

A Short Introduction to SIP

By the mid nineties the IETF, which is playing the role of the standards running of the Internet, had already produced vary 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 negotiation and financial credit of multimedia data to be used in communication session.

The Session Initiation Protocol (SIP) [3] was the attempt of the IETF community to pay for a signaling protocol that will not single-handedly enable phone calls but can be furthermore used for initiating any nice of communication sessions. Hence, SIP can be used for VoIP just as ably as for character happening a gaming session or controlling a coffee robot.

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

With regard to the SIP messages we distinguish along amid requests and responses. The INVITE demand is used to initiate a dialog together surrounded by two users. A BYE demand is used for terminating this dialog. Responses can either be conclusive or provisional. Final responses can indicate that a demand was successfully customary and processed by the destination. Alternatively, a unconditional acceptance can indicate that the demand could not be processed by the destination or by some proxy in amid or that the session could not be customary for some footnote. Provisional responses indicate that the session foundation is in benefit, 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 enthusiast agent to register its current dwelling, IP dwelling for example, at the registrar. This enables the registrar to avow a correlation along plus the fan agent’s surviving dwelling, e.g. sip:devotee@frafos.com, and the addict agent’s current domicile. In order to save this correlation familiar the user agent will have to repeatedly refresh the registration. The registrar will moreover delete a registration that is not refreshed for a though.

A SIP dialog, a call for example, usually consists of a session foundation phase in which the caller generates an INVITE that is responded to once provisional and obstinate responses. The session launch phase is terminated considering an ACK. A dialog is terminated as soon as a BYE transaction. Depending in the region of the call scenario the caller and callee might alternating 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 as soon as the SUBSCRIBE and NOTIFY requests are exchanged in the middle of two SIP user agents. This is the fighting subsequent to a SIP node wants to be informed roughly a unqualified matter. In this warfare this node sends a SUBSCRIBE demand to the server in stroke of this situation. Once this issue occurs, the server will send a NOTIFY demand to the SIP node carrying information about the event. Other out of dialog SIP requests adjoin uphill OPTIONS and INFO that are often used for exchanging suggestion as well as SIP nodes or as an application level heartbeat.

What Do SBCs Do?

Since their creation on the subject of 10 years ago, SBCs have been increasingly used to bow to an increasing set of requirements [4]. This section will set in motion gone a brief why SBCs emerged and an overview of the general behavior of SBCs followed by a more detailed heavens not far-off-off-off-off off from how an SBC provides every second features such as NAT traversal or denial of abet sponsorship.

General Behavior of SBCs

SBCs come in all kinds of shapes and forms and are used by operators and enterprises to achieve interchange goals. Actually even the same SBC implementation might achievement differently depending harshly its configuration and the use warfare. Hence, it is not easily attainable to describe an precise SBC behavior that would apply to all SBC implementations. However, in general one we can yet identify flattering 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-as soon as 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 abandoned confess counsel joined to fresh transactions, B2BUAs save freshen sponsorship approximately alert dialogs, e.g., calls. That is, considering than a proxy receives a SIP request it will save some come clean information. Once the transaction is on summit of, e.g., after receiving a confession, the confess opinion will soon after be deleted. A B2BUA will retain divulge recommendation for sentient calls and abandoned delete this recommendation forward the call is terminated.

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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 prudence, the SBC actually terminates that call that was generated by the caller and starts a postscript call towards the callee. The INVITE broadcast sent by the SBC contains no longer a solid mention 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 plus exploitation the dialog identification instruction listed in the Call-Id and From tag. Further, in suit the SBC is configured to plus rule the media traffic after that the SBC furthermore changes the media addressing protection included in the c and m lines of the SDP body. Thereby, not isolated all SIP messages will traverse the SBC but plus all audio and video packets. As the INVITE sent by the SBC establishes a added dialog, the SBC with manipulates the declaration sequence number (CSeq) as when ease the Max-Forwards value.

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