[SR-Dev] [Serdev] AVPs
jan at ryngle.com
Fri Dec 12 16:22:32 CET 2008
On 12-12 00:25, Daniel-Constantin Mierla wrote:
> I think we need to decide about AVPs. Although not deeply investigated,
> ser seems to have couple of AVPs lists. In kamailio/openser it is one
> Can a ser developer describe a bit the AVP implementation as it is now
A long time ago we started using AVPs to store all kinds of configuration
information. We needed some sort of general configuration mechanism because
real-world scripts were getting too complex and at that time the AVPs were
We started by loading attribute names and value from a database table into
AVPs. This mechanism was/is same as in k. We would always load a set of
attributes this way for a particular user (identified by username and domain
from his/her SIP URI). With this mechanism in place, we were able to load a
generic set of configuration options for a particular user of the server, I
guess you have the same.
To make the AVPs easily accessible from the configuration script, we added
support for AVP identifiers in the config script, so that you can access AVP
with name foo as $foo.
As we migrated more and more configuration options to AVPs, it became clear
that having just one set of AVPs was not enough. Although we could load all
the config options for the caller (From URI) from the database into a set of
AVPs, we could not do the same for the callee (Request-URI) at the same
time. We could not do it because they would conflict with the AVPs of the
caller as both users could have AVPs with same names but different values.
To get around this issue we added another list of AVPs. The new AVP list
works the same way, it can contain AVPs with same names as AVPs in the
original list and they do not conflict. All functions that work with lists
of AVPs now take the list to work on as parameter.
To make both AVP lists available in the configuration script, we extended
the syntax of AVPs identifiers so the script write can choose the AVP list
to work with. SO instead of just $foo you can write either
$f.foo or $t.foo
$f refers to the original AVP list which is commonly associated with the
caller. The address/uid of the caller is taken from From header, hence the
'f' in the identifier. $t refers to the AVP list which contains
configuration settings of the callee. The address/uid of the calle can be
taken either from the Request-URI or To headers, hence the 't' in the
The original syntax without any list identifier is still available, in other
words you can still write $foo, this is defined as a shorthand for
$f.foo. If you do not specify the AVP list to be used then you are referring
to the list containing AVPs of the caller (From).
It also turned out that in some cases we would be having too many attributes
in the database table storing AVPs. This can happen in bigger setups, having
tens or hundreds of thousands users or serving multiple domains. This can
slow the database down and makes SER load too much data. Based on my
observations it is common that a large number of users have AVPs with same
values. If ten thousands of users have attribute named 'foo' with value
'bar' then the attribute will be stored in the database ten thousand times.
As a remedy for this problem, we introduced the concept of AVP levels. The
AVPs described so far and stored in user_attrs table are called user-level
attributes and they store configuration specific to particular users. Then
we added another two AVP lists to store so called domain-level AVPs.
Domain-level AVPs are used to store configuration information that is shared
by a group of users. Domain-level AVPs are stored in a separate database
table, the name of the table is domain_attrs and its contents is cached in
memory by 'domain' SER module. This is an important difference, while
user-level AVPs need to be loaded every time a SIP request is processed,
domain-level AVPs are only loaded when the contents of domain_attrs table
The domain-level AVPs are called 'domain' because they are tied to a
particular domain handled by the SIP server. That could be 'iptel.org',
'sip-router.org', and so on. This mainly useful for multi-domain
setups. There are two domain-level AVP lists because it is common that SER
works with two domains at the same time, the domain from From header
(caller's domain) and the domain from Request-URI or To (callee's domain).
Again, we extended the syntax of AVP identifiers in the configuration
file. So you can write:
$fu.foo -- this way you are asking for the value of the user-level foo AVP.
$fd.foo -- This will return the value of the domain-level foo AVP.
And similarly there is $tu and $td for callee's user-level and domain-level
AVPs. If you specify 'u' in the AVP identifiers then SER searches only the
list of user-level attributes. If you specify 'd' then SER searches only the
list of domain-level attributes.
This behavior changes if you do NOT specify the level to be searched. In
that case SER searches the user-level list first and if no match is found
then the domain-level list will be searched. Thus if you write:
then you are telling SER to search for the value of 'foo' AVP in the
user-level list and continue with the domain-level list if no match is
found. In other words, user-level AVPs have higher priority than
domain-level AVPs. With this system you can efficiently set a configuration
option for all users within a domain by creating a domain-level attribute
and yet you can override that option with user-level attributes for
particular users within that domain.
Then there are the global AVPs. Global AVPs can be used to store settings
applicable to the whole SIP server and all domains on that server. Global
AVPs are stored in global_attrs table and the contents of that table is
cached in memory by gflags module. There is only one list of global
AVPs. Global AVPs can be accessed with:
(note that there is no 'f' or 't' in the identifier). The list of global
AVPs is searched after the list of domain-level AVPs. So if you write:
Then SER searches user-level AVPs first, then domain-level and then the
And finally there are also so called URI-level AVPs. They work just
like user-level AVPs, but a single user can have multiple sets of
uri-level AVPs. They are tied to SIP URIs of that user. Uri-level AVPs are
denoted by 'r' in the AVP identifier, for example:
> In openser/kamailio we have more or less same architecture as for ser
> 0.9.6 with couple extensions, but I will detail here so we can have full
> - avps are kept in shared memory
In our case global and domain level AVPs are stored in shared memory.
> - they are bound to each sip message and moved to transaction if that
> message create a transaction
This is same.
> - avp can have two types of names
> - integer id, referred as $avp(i:number)
> - string id, referred as $avp(s:string)
> - there can be aliases for avp names, defined as "alias=[is]:id", so the
> script writer can use $avp(alias)
We've kind of dropped support for integer avps, we've been using string
names pretty much everywhere, although the original code still exists.
> - a value of an avp can be either integer or string
> - not related to avp core part, but important to mention here -- tm,
> controlled by a parameter, can make the avps from transaction (request)
> available to onreply_route.
In SER tm restores all AVPs stored in a transaction in all route blocks
called from tm, such as onreply_route, failure_route. At least I hope I
remember it correctly.
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