Cisco Unified CM is a Business to Business User Application (B2BUA). Therefore, any SIP to SIP call consists of 2 SIP dialogs. It is often useful to pass information from one dialog to the other during the life of the dialogs. This includes call setup, mid call, and end of call messaging. Using the pass through object described earlier, it is possible to trigger transparent pass through of information on from one SIP dialog (representing 1 of the call legs) to the other.
Currently, transparency is limited to INVITE dialogs between 2 SIP Trunks. If SIP Line is involved, transparency is not supported. SUBSCRIBE dialogs, PUBLISH, out-of-dialog REFER, out-of-dialog unsolicited NOTIFY are not supported, and MESSAGE are not supported.
The following messages support transparency:
•Initial INVITE and associated responses
– INVITE response
– 180 response
– 183 response
– 200 response
– 4XX, 5XX, 6XX responses
•reINVITE and associated responses
• UPDATE message (transparency for responses to UPDATE is not supported)
•INFO message (transparency for responses to INFO is not supported)
•BYE message (transparency for response to BYE is not supported)
The following messages do not support Transparency:
•PRACK and associated responses
•INVITE with replaces and associated responses
•REFER and associated responses
Typically Cisco Unified CM processes the following information (i.e. parameters, headers, and content bodies) locally. That is relative to a particular call leg. Hence, the SIP information that is understood is consumed and not passed across to the other call leg (which may not even be SIP anyway). This allows Cisco Unified CM to support various protocol inter-workings such as SIP to H.323, SIP to MGCP, etc. SIP information which is not understood by Cisco Unified CM is typically ignored.
In the following section, information which is understood and consumed by Cisco Unified CM is said to be known and the information not understood and consumed by Cisco Unifiecd CM is said to unknown.
The following information can be passed through transparently:
The following is the list of known headers:
If the script attempts to pass through a known header or content-body, it will trigger an execution error.
A script writer will quickly figure out that there is a way to pass known data through without it being consumed by or interfering with Cisco Unified CM processing. Effectively, the script can get the value for a known header and place into an unknown header. The same can be done with content bodies. The 181 Transparency example below does just that with the Reason header. It gets the Reason header value and passes it through as an X-Reason header. Of course, if there is no script on the other side to consume the X-Reason header and remove it, the header will be sent to the network.
Example for 181 Transparency
Without transparency, if Cisco Unified CM receives a 181 on the outbound trunk leg, Cisco Unified CM's native behavior is to send a 180 back on the inbound trunk leg. To achieve 181 transparency, a script is required for both the inbound 181 (received on the B side) and for the would-be outbound 180 (sent on the A side).
Since the 181 is received from the PBX-B first, consider doing the following first:
•Get the Reason header value
•Pass through the Reason header value— Since the Reason header is a known header, the script will bypass the known header check by passing through the value using the header name X-Reason.
Cisco Unified CM will automatically merge the pass through data with the outbound message it would have sent. As mentioned previously, it would natively send a 180. The auto merge functionality therefore, places the X-Reason header into an outbound 180.
Next, one must consider what the A side needs to do:
•Get the X-Reason header value and see if contains something about 181
•Add a Reason header with the X-Reason header value
•Remove the X-Reason header
•Convert the response code and phrase to 181 Call is Being Forwarded.
These steps are depicted in the following callflow diagram:
The B-side and A-side scripts are shown below:
local pt = msg:getPassThrough()
local reason = msg:getHeader("Reason")
local reason = msg:getHeader("X-Reason")
if string.find(reason, "cause=181")
msg:setResponseCode(181,"Call is being forwarded")
Example for INFO Transparrency
Without transparency, Cisco Unified CM ignores the inbound INFO message and content body. Using transparency, Cisco Unified CM extracts the proprietary content body sent by a Nortel PBX, extract the DTMF digits from that content body, create a new dtmf-relay content body and pass that through to the other call leg.
local body = msg:getContentBody("application/vnd.nortelnetworks.digits")
local digits = string.match(body, "d=(%d+)")
pt = msg:getPassThrough()
body = string.format("Signal=%d\r\nDuration=100\r\n", digits)
INFO sip: email@example.com SIP/2.0
Via: SIP/2.0/UDP 10.10.10.57:5060
INFO sip: firstname.lastname@example.org SIP/2.0
Via: SIP/2.0/UDP 10.10.10.1:5060