[asterisk-dev] Feature discussion: extract REFER headers into dialplan.

[Kirill Katsnelson]

There were users asking for this feature for as long as I have been 
using Asterisk, albeit the requests are infrequent. Basically, when the 
transferrer initiates transfer with a SIP REFER message, we want access 
to some interesting (normally additional) headers in this message from 
the dialplan.

I currently have an implementation that extracts headers into the parent 
channel's dialplan hash with the fixed name TRANSFER_DATA. Header name 
becomes the key, value the data under this key. Duplicate header names 
not supported (later in the message wins its spot in the hash). This 
part looks good to me. I am dismissing any need to support duplicates, 
as probably there is about 0 users in the world who would want that.

I want to submit a patch for this feature. What I do not know, is the 
best way to specify which headers the user wants in the hash. In my 
current code, the parent channel than wants the data sets a variable 
TRANSFER_SIPHEADER_PREFIX to the prefix matching the interesting header 
names. In my case the prefix is "X-" for all "extended" headers. I 
imagine other users may want to set that to a specific name (e. g. 
"X-Altitude-Data", see 
https://community.asterisk.org/t/how-to-send-variable-information-via-sip-header/70178/7). 
If not set, no headers are put in the hash (unchanged behavior).

I am not sure if this is a good approach. I would appreciate suggestions 
to make the interface as good as it should be. Basically, what I am 
undecided on is:

1. Is variable a good way to specify the headers? I cannot find a place 
to document it. Options here: a setting in sip.conf (inflexible); a 
function that sets the variable (better, self-documenting).

2. Is prefix a good way to specify the pattern? Options: regex (more 
flexible but computationally expensive); wildcards (but we have no 
functions to readily support them).

Somewhat related to the question 2, another thing I want to add as we 
are at it is function SIP_HEADERS([pattern]) to complement SIP_HEADER(), 
that would return an array of header names matching the pattern from the 
incoming INVITE. Whether the function itself a matter of additional 
discussion or not, I want the syntax of the pattern be same in both 
places, so this is a related part.

  -kkm