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The implementation of these applications is complicated by the practices of participants: users may move between endpoints, they may be addressable by multiple names, and they may communicate in several different media - sometimes simultaneously. Numerous protocols have been authored that carry various forms of real-time multimedia session data such as voice, video, or text messages. For locating prospective session participants, and for other functions, SIP enables the creation of an infrastructure of network hosts called proxy servers to which user agents can send registrations, invitations to sessions, and other requests.
SIP is an agile, general-purpose tool for creating, modifying, and terminating sessions that works independently of underlying transport protocols and without dependency on the type of session that is being established.
SIP can also invite participants to already existing sessions, such as multicast conferences. Media can be added to and removed from an existing session. SIP transparently supports name mapping and redirection services, which supports personal mobility [ 27 ] - users can maintain a single externally visible identifier regardless of their network location. SIP supports five facets of establishing and terminating multimedia communications: User location: determination of the end system to be used for communication; User availability: determination of the willingness of the called party to engage in communications; User capabilities: determination of the media and media parameters to be used; Session setup: "ringing", establishment of session parameters at both called and calling party; Session management: including transfer and termination of sessions, modifying session parameters, and invoking services.
SIP is not a vertically integrated communications system. Therefore, SIP should be used in conjunction with other protocols in order to provide complete services to the users. However, the basic functionality and operation of SIP does not depend on any of these protocols. SIP does not provide services. Rather, SIP provides primitives that can be used to implement different services. For example, SIP can locate a user and deliver an opaque object to his current location. If this primitive is used to deliver a session description written in SDP, for instance, the endpoints can agree on the parameters of a session.
If the same primitive is used to deliver a photo of the caller as well as the session description, a "caller ID" service can be easily implemented. As this example shows, a single primitive is typically used to provide several different services. SIP does not offer conference control services such as floor control or voting and does not prescribe how a conference is to be managed.
SIP can be used to initiate a session that uses some other conference control protocol. Since SIP messages and the sessions they establish can pass through entirely different networks, SIP cannot, and does not, provide any kind of network resource reservation capabilities. The nature of the services provided make security particularly important. To that end, SIP provides a suite of security services, which include denial-of-service prevention, authentication both user to user and proxy to user , integrity protection, and encryption and privacy services.
This section is tutorial in nature and does not contain any normative statements. Rosenberg, et. Standards Track [Page 10] RFC SIP: Session Initiation Protocol June The first example shows the basic functions of SIP: location of an end point, signal of a desire to communicate, negotiation of session parameters to establish the session, and teardown of the session once established.
Each message is labeled with the letter "F" and a number for reference by the text. Also shown are two SIP proxy servers that act on behalf of Alice and Bob to facilitate the session establishment. This typical arrangement is often referred to as the "SIP trapezoid" as shown by the geometric shape of the dotted lines in Figure 1. It has a similar form to an email address, typically containing a username and a host name.
In this case, it is sip:bob biloxi. An example would be sips:bob biloxi. From there, the request is sent securely to the callee, but with security mechanisms that depend on the policy of the domain of the callee.
Each transaction consists of a request that invokes a particular method, or function, on the server and at least one response. Header fields are named attributes that provide additional information about a message. The lines that follow are a list of header fields.
This example contains a minimum required set. The header fields are briefly described below: Rosenberg, et. It also contains a branch parameter that identifies this transaction. Display names are described in RFC [ 3 ]. This header field also has a tag parameter containing a random string that was added to the URI by the softphone. It is used for identification purposes. CSeq or Command Sequence contains an integer and a method name.
The CSeq number is incremented for each new request within a dialog and is a traditional sequence number. While the Via header field tells other elements where to send the response, the Contact header field tells other elements where to send future requests. Max-Forwards serves to limit the number of hops a request can make on the way to its destination.
It consists of an integer that is decremented by one at each hop. Content-Type contains a description of the message body not shown. Content-Length contains an octet byte count of the message body. The complete set of SIP header fields is defined in Section The details of the session, such as the type of media, codec, or sampling rate, are not described using SIP. Rather, the body of a SIP message contains a description of the session, encoded in some other protocol format.
This SDP message not shown in the Rosenberg, et. Since the softphone does not know the location of Bob or the SIP server in the biloxi. The address of the atlanta. The atlanta. A proxy server receives SIP requests and forwards them on behalf of the requestor. Responses in SIP use a three-digit code followed by a descriptive phrase.
This is described in [ 4 ]. As a result, it obtains the IP address of the biloxi. Before forwarding the request, the atlanta.
The biloxi. The proxy server consults a database, generically called a location service, that contains the current IP address of Bob.
We shall see in the next section how this database can be populated. Each proxy uses the Via header field to determine where to send the response and removes its own address from the top.
As a result, although DNS and location service lookups were required to route the initial INVITE, the Ringing response can be returned to the caller without lookups or without state being Rosenberg, et. In this example, Bob decides to answer the call. When he picks up the handset, his SIP phone sends a OK response to indicate that the call has been answered.
If Bob did not wish to answer the call or was busy on another call, an error response would have been sent instead of the OK , which would have resulted in no media session being established. The complete list of SIP response codes is in Section The remaining lines contain header fields. This tag will be incorporated by both endpoints into the dialog and will be included in all future Rosenberg, et.
In addition to DNS and location service lookups shown in this example, proxy servers can make flexible "routing decisions" to decide where to send a request.
This type of parallel search is known as forking. The lookups performed by the two proxies are no longer needed, so the proxies drop out of the call flow. Full details on session setup are in Section In general, the end-to-end media packets take a different path from the SIP signaling messages. During the session, either Alice or Bob may decide to change the characteristics of the media session. This re- INVITE references the existing dialog so that the other party knows that it is to modify an existing session instead of establishing a new session.
The other party sends a OK to accept the change. If the other party does not accept the change, he sends an error response such as Not Acceptable Here , which also receives an ACK.
However, the failure of the re-INVITE does not cause the existing call to fail - the session continues using the previously negotiated characteristics. Full details on session modification are in Section The reasons for this special handling for INVITE will be discussed later, but relate to the reliability mechanisms in SIP, the length of time it can take for a ringing phone to be answered, and forking.
Full details on session termination are in Section Section In some cases, it may be useful for proxies in the SIP signaling path to see all the messaging between the endpoints for the duration of the session.
For example, if the biloxi. Each proxy can independently decide to receive subsequent messages, and those messages will pass through all proxies that elect to receive it. This capability is frequently used for proxies that are providing mid-call features. Registration is another common operation in SIP. Registration is one way that the biloxi. The registrar writes this association, also called a binding, to a database, called the location service, where it can be used by the proxy in the biloxi.
Often, a registrar server for a domain is co-located with the proxy for that domain. It is an important concept that the distinction between types of SIP servers is logical, not physical. Bob is not limited to registering from a single device.
For example, both his SIP phone at home and the one in the office could send registrations. This information is stored together in the location Rosenberg, et. Similarly, more than one user can be registered on a single device at the same time. The location service is just an abstract concept.
Timestamping Authority (TSA)
The implementation of these applications is complicated by the practices of participants: users may move between endpoints, they may be addressable by multiple names, and they may communicate in several different media - sometimes simultaneously. Numerous protocols have been authored that carry various forms of real-time multimedia session data such as voice, video, or text messages. For locating prospective session participants, and for other functions, SIP enables the creation of an infrastructure of network hosts called proxy servers to which user agents can send registrations, invitations to sessions, and other requests. SIP is an agile, general-purpose tool for creating, modifying, and terminating sessions that works independently of underlying transport protocols and without dependency on the type of session that is being established. SIP can also invite participants to already existing sessions, such as multicast conferences.
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Google Network Working Group C. Cain BBN D. Pinkas Integris R. Zuccherato Entrust August Internet X. Please refer to the current edition of the "Internet Official Protocol Standards" STD 1 for the standardization state and status of this protocol.