Details on SIP Protocol extensions in IMS

Source: Internet
Author: User

I believe that you have mastered the basics of IMS and SIP protocols and some precautions. These contents are also described in two articles, "briefly describing the SIP protocol in IMS" and "Problems Existing in IMS and SIP. Next we will explain the SIP Protocol extensions in IMS.

1. SIP Compression

The session creation time may be extended because of the time required to transmit the SIP message over a limited bandwidth channel. therefore, there must be a mechanism to effectively transmit SIP signaling packets through wireless interfaces by compressing the SIP messages between UA and SIP exit proxy servers and between the SIP exit proxy server and UA. you must also compress the IP address and the transport layer protocol header that sends these SIP messages.

1) compression and decompression of the SIP request and the response to the P-CSCF

The compression of SIP messages is an execution option. however, compression is strongly recommended. because the compression support is mandatory, the UE can send or even be the first message to be compressed. sigComp provides a mechanism for the UE to know whether the State has been created in the P-CSCF.

UE must also extract SIP requests and response messages received from the P-CSCF. if UE detects a decompression failure in the P-CSCF, the repair mechanism will be executed and the algorithm can be changed.

The compression rules of SIP in the P-CSCF are the same as those of UE. the exchange of bytecode during registration will prevent unnecessary delays during session creation. the SIP request and the response to the UE also need to be compressed, and the response received from the UE also needs to be decompressed, they follow the same rules as the above P-CSCF.

2) compression operations are independent

The selected solution must be applicable to computation rules that cannot be compressed.

3) scalability of SIP Compression

The selected solution results must be scalable. When they are available, they are used in reverse compatibility to promote the merger of new and improved compression operations.

4) The minimal effect of SIP compression on the network

The impact of specific application compression on the existing 3GPP access network must be minimized. On the other hand, the compression mechanism must be independent of access. For example, compression must be defined on UA and exit SIP protocol proxy servers.

5) Availability of SIP Compression

It is necessary to make the use of SIP signaling compression optional. to make it easier for mobile terminals to roam over a compressed network, mobile terminals must always support SIP signaling compression. if compression is not supported, the communication can continue without compression, which depends on the local network's local policies.

6) compression Reliability

The compression mechanism should be reliable and can automatically fix Errors generated during decompression.

2. Private header of the SIP protocol

1) correlated URI (P-Associated-URI): transmits all Associated Uris of the mobile terminal registration address. It is used in the 200OK response to the REGISTER request.

2) Called Party identifier (P-Called-Party-ID): transfer the Called identity. when a mobile terminal needs to register multiple Uris, P-Called-Party-ID can be used to identify the real Called URI.

3) Visiting Network identifier (P-Visited-Network-ID): identifies the globally unique visiting Network. generally, a network identification is required for the scope of a P-CSCF, and the network identification code must be uniformly allocated by the network operator.

4) Access Network Information (P-Access-Network-Info): transmits the wireless Access technology and Network information used by mobile terminals.

5) billing Address (P-Charging-Function-Address): IMS has two types of billing functions: the physical billing collection Function (CCF) and the event billing Function (ECF ). this message header field indicates the physical address information of the billing function used. with this header field, IMS can achieve redundant storage of billing information.

6) billing Vector (P-Charging-Vector): transmits the billing information in IMS, such as the billing collection point id, IP address, caller network identifier, and callee network identifier.

3. Security

The security authentication function of the mobile terminal is implemented by using the WWW-authenticate and Authorization header fields of the SIP protocol. when the UE sends a registration or call request to CSCF, it must provide security parameters such as the Protocol identity and password in the Authorization header of the REGISTER message. When the UE does not contain security parameters, CSCF will send a 401 response (unauthorized) to the UE, including the WWW-authenticate field. The WWW-authenticate field carries the necessary security parameters (such as the authentication method) for UE authentication ).

4 Pre-processing Precondition

In IMS, the availability of all necessary resources is the prerequisite for session establishment. therefore, the SDP-based provision/response mechanism and related extensions of SIP and SDPPrecondition are introduced. precondition extended usage leads to a specific SIP call process. IMS uses Go interface between GGSN and P-CSCF to control media resources.

5. Release a network-initiated call

In a mobile network, an ongoing call is required to be released because the signal is not covered or the battery is powered off. this problem can be solved by sending a BYE request to UE from the network side. but this does not comply with the SIP principle, that is, the proxy server cannot send BYE messages. however, due to the lack of a better solution, IETF accepts the requirements of 3GPP and this solution.

Parameters are extended for some SIP headers. For example, parameters are extended for the WWW-authenticate header, and a new auth-param parameter field is defined, this field is used in the 401 (Unauthorized) response to the REGISTER request. This field also includes two specific parameters: integrityOkey and cipher-key.

Added the "application/3gpp-ims + xml" type to the message body MIME type in the SIP protocol, that is, 3GPPIP multimedia core subsystem extensible Language Body version 1, it is also agreed that this type of content cannot be sent out of the 3GPP network.


Although the current IMS architecture only supports mobile service access and does not support fixed access methods, the proposal of IMS conforms to the development trend of network convergence. currently, many standard entities such as ETSI/TISPAN (for fixed network applications) and ITU-TFGNGN have been involved. it is foreseeable by the three major trends of communication development (IP-based information organization, wireless information transmission, and multi-media information content). Driven by this development trend, h.323 and SIP will coexist in the future, and because the SIP and IP methods are more compatible, with the gradual improvement of the SIP interoperability function, SIP may eventually become a global protocol.

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