Sametime Unified Telephony 8
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`Session Initiation Protocol (SIP)
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`This chapter provides information on the following topics:
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`SIP Overview
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`Using SIP in the Media Server.
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`SIP Overview
`The SIP communication protocol was developed by the IETF as application layer protocol. It originally realized the
`signaling of IP-based voice transmission, but has in the meantime been expanded for applications in the areas video
`and instant messaging. Since SIP within this scope only realizes the signaling of connections, it uses various other
`protocols for the actual transmission of usable information. This includes e. g. the RTP protocol used for transmitting
`real-time data.
`In the voice area, SIP enables all basic call functions such as connection set-up and shutdown, call notification or dial
`tone. Beyond that, further features are already implemented in SIP. Among these are the features parking, call
`transfer or conference switch. Considering the supported features, SIP can by now be compared with H.323.
`Contrary to H.323, the SIP communication model realizes a big part of the functions in the terminal device. This
`significantly differentiates it from the traditional communication concepts in which the actual intelligence lies in the
`network nodes.
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`SIP Network Components
`An SIP network consists of different SIP-specific logic components. These intercommunicate in a defined way.
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`The logic components of an SIP network are:
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`User Agent (UA)
`The user agent represents the terminal device in an SIP network. It signals and manages SIP
`connections that it initiates with other user agents1 or proxy servers. Upon logging in at an SIP network a
`user agent registers with the registrar server in charge.
`The user agent is logically divided into the user agent client and the user agent server. The user agent
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`Petitioner Apple Inc. - Exhibit 1081, p. 1
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`client is in charge of initiating SIP connections. The user agent server replies to connection requests.
`User agents are e.g. SIP telephones or an SIP client software.
`Registrar Server
`Registrar servers are responsible for registering user agents. The information collected during the logging
`of the single user agents are transferred to the location server that administer these data for future
`requests.
`Registrar servers often operate on the same platform as the associated location server (see below).
`Location Server
`Each SIP domain has a location server. This server administers e.g. information on the bindings of logical
`and physical SIP addresses (cf. Section "Addressing an SIP subscriber").
`Location servers often operate on the same platform as the associated registrar server (see below).
`Proxy Server
`Each SIP domain has a proxy server. This server receives the connection requests by user agents or
`other proxy servers. The proxy server is logically divided into a client and a server component, which
`perform individual connection requests on behalf of user agents.
`With handling a connection request, two cases must be differentiated:
`The communication participants are in the same SIP domain
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`In this case only the proxy server of the domain concerned is involved in the connection
`establishment.
`The communication participant is in different SIP domains
`In this case the two proxy servers of the domains concerned are involved in the connection
`establishment.
`For the resolution of SIP addresses the proxy server communicates with the associating location and
`redirect server.
`Redirect Server
`A redirect server processes proxy server requests as regards configured call reroutings for user agents. It
`operates domain-spanning.
`For the resolution of SIP addresses the redirect server communicates with the associated location server.
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`Addressing an SIP subscriber
`SIP addressing is based on a logical and a physical2 SIP address. The logical address is assigned to the person
`"user", the physical address is allocated for the device, with which the user registers with the registrar server. The
`actual routing within an SIP network occurs based on the physical address.
`Example:
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`User Martin has in our example the logical address Martin@MyDomain.com. Under this address he is to be
`addressed in the entire SIP network. His physical address, according to which messages are routed for him in the
`network, depends on the terminal device with which he registers in the SIP network. In our example Martin registers
`with a laptop. This laptop has the physical addressLaptop31@OtherDomain.com.
`If another SIP subscriber wants to establish a communication relation to Martin, he/she addresses the connection
`setup with the logical destination address Martin@MyDomain.com. This makes sense since the SIP users do not
`know via which terminal device a subscriber registers. In the course of the connection setup this logical address is
`resolved by the location server into the current physical address.
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`Interaction between logical SIP Components
`The interactions between the logical SIP components are based on various transactions. These consist in each case
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`Petitioner Apple Inc. - Exhibit 1081, p. 2
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`a request sent by an SIP client to an SIP server
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`an associated reply sent by an SIP server to an SIP client
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`Let's take a look at two simple examples to describe the general interactions between the SIP components to
`establish a connection between two user agents. For reasons of simplification we discount the registrar and redirect
`server function.
`The first example describes the connection establishment between two agents that are in the same SIP domain. In
`the second case the user agents involved are in different SIP domains.
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`Connection between two Subscribers in one SIP Domain
`Let us see how a SIP connection is established between two SIP clients that are in the same SIP domain.
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`User-agent A initiates a connection to user-agent B. For this purpose he/she sends a corresponding
`connection request to the proxy server of the domain. The connection request contains the logical SIP
`address of the destination party - Agent_B@Domain.com.
`The proxy server requests the registered address information of the destination subscriber from the
`location server.
`The location server replies to the proxy server with the physical SIP address of the destination
`subscriber.
`Subsequently, the proxy server resolves the received SIP address into an IP address via the DNS server
`in charge.
`The proxy server sends an individual connection request to the IP address of user-agent B.
`User-agent B replies to this request and informs the proxy server that he/she accepts the requested
`connection.
`The proxy server correspondingly accepts the original connection request of user-agent A (see 1).
`User-agent A then establishes a direct RTP connection to user-agent B.
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`Connection between two Subscribers in different SIP Domains
`The following example shows the general structure of an SIP connection between two subscribers in different
`domains.
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`Petitioner Apple Inc. - Exhibit 1081, p. 3
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`User-agent A initiates a connection to user-agent B. For this purpose he/she sends a corresponding
`connection request to proxy server A of his/her own domain. The connection requests contains the logical
`SIP address of the destination party - Agent_B@Domain_B.com.
`By the domain name of the destination address Domain_B.com , proxy server A recognizes that user
`agent B does not belong to the own domain A. Subsequently, it queries the IP address of the proxy
`server responsible for domain B at the DNS server in charge. Thus the IP address of proxy server B.
`Proxy server A sends an individual connection request to proxy server B3.
`Proxy server B then requests the SIP address under which the destination subscriber is registered in the
`SIP network from the location server in charge.
`The location server replies with the physical address of the destination subscriber.
`Subsequently, the proxy server resolves the received address into an IP address via the DNS server in
`charge.
`Proxy server B now sends an individual connection request to user-agent B.
`User-agent B replies to this request and informs proxy server B that he/she accepts the requested
`connection.
`Proxy server B accepts the connection request by proxy server A (see 4).
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`10. Afterwards, proxy server A accepts the connection request by user-agent A (see 1).
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`11. User-agent A then establishes a direct RTP connection to user-agent B.
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`You find further information on SIP in the appropriate literature or in the relevant RFC standards of the IETF
`(www.ietf.org).
` 1A communication connection between two user agents is directly established via RTP.
`2The physical SIP address is not to be confused with an address of the physical OSI layer.
`3Consequently, proxy server A becomes the Outbound, proxy server B the Inbound proxy server.
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`Petitioner Apple Inc. - Exhibit 1081, p. 4