SIP Message Manipulation

Overview

Session Initiation Protocol (SIP) is a signaling communications protocol, widely used for controlling multimedia communication sessions such as voice and video calls over Internet Protocol (IP) networks. The protocol defines the messages that are sent between peers and govern establishment, termination and other essential elements of a call. SIP can be used for creating, modifying and terminating two-party (unicast) or multiparty (multicast) sessions consisting of one or several media streams.

A SIP message consists of headers, each of which has a name (before the colon) and value (after the colon).  The value can consist other specific parts, such as tokens and parameters.

Example SIP message:

INVITE sip:4448451148@10.34.20.100:5060 SIP/2.0
Via: SIP/2.0/UDP 10.34.13.56:23012;branch=z9hG4bK0cB4182841436;maddr=10.34.20.80
Max-Forwards: 70
From: <sip:13032220008@10.34.13.56:23012>;tag=4182841436
To: <sip:4448451148@10.34.20.100:5060>
Call-ID: 4182841436@10.34.13.56
CSeq: 10 INVITE
Contact: <sip:13032220008@10.34.13.56:23012;transport=udp>
Content-Type: application/sdp
Content-Length: 153
Expires: 90
User-Agent: ST Simulation
Accept: application/sdp
Accept-Encoding: gzip
Accept-Language: da, en-gb;q=0.8, en;q=0.7
Alert-Info: <http://www.example.com/sounds/moo.wav>
Allow: INVITE, ACK, OPTIONS, CANCEL, BYE
server: East

Anatomy of a SIP Packet

The following diagram identifies the various segments of a SIP packet:

SMM in SBC

SIP Message Manipulation (SMM) allows you to modify SIP messages as they are processed by the SBC Core in both inbound and outbound directions. SMM involves defining a set of criteria and actions in a SIP Adapter Profile against select SIP messages in order to modify their headers and/or parameters. SMM is applied at the zone or trunk group level. In the case where SMM is applied at both levels, the trunk group SMM takes higher precedence.

A SIP adaptor profile contains a set of rules for how a SIP message should be manipulated. Each SIP adaptor profile is attached to a trunk group (or zone), and assigned to act on incoming or outgoing SIP messages. When a message comes into or out of a trunk group that has a SIP adaptor profile assigned to it, the message is evaluated based on a set of criteria in the profile. If it meets those criteria, it is modified according to actions defined in the profile.

A SIP Adaptor Profile contains one or more rules. Each rule contains one or more criteria, for example:

• Check if the message is a request message of the type Invite
• Check if a header name “From” is present in the message.

Each rule contains one or more actions, for example:

• Delete the “From” header.
• Add a “To” header
• Modify the message body.

Here is a simple example of SMM rule:

Check all Request messages of the type Invite to see if they have “Supported” header with the value 100rel.  If so, delete the “Supported” header.

The SBC provides a set of powerful mechanisms to manage the processing of SIP messages between ingress and egress SIP Trunk groups using the SIP Adaptor Profile. For example, in Figure 1 a SIP Adaptor Profile is configured on ingress and egress trunk groups to apply rules to messages sequentially as the messages traverse the route.

The SIP Adaptor Profile object is the main construct for SIP Message Manipulation (SMM) functionality. A SIP Adaptor Profile is associated with a SIP Trunk Group or Zone in order to act upon SIP messages passing into or out of that group. A SIP Trunk Group can have two SIP Adaptor Profiles, an input adaptor profile for manipulation of inbound messages, and an output adaptor profile for manipulation of outbound messages. Similarly, a Zone can have two SIP Adaptor Profiles.

SBC SMM Mechanisms

Additional topics in this section: