[SIPForum-discussion] Outbound and inbound proxy

Dipan Kumar m.dipan1987 at gmail.com
Tue Dec 23 15:59:01 UTC 2014

Hi Paul,

If I see the following sip trapezoid which tell about outbound and inbound

Then from my previous mail the P2, P3,P4,P5 proxies, if they will provide
such functionality,Can they be termed as Outbound/inbound proxy?

If I am wrong please rectify me .

*SIP Trapezoid*
[image: Basic SIP Trapezoid]
Fine, till now you know the basics of the above SIP Entities. You know what
is a Proxy and what is its role. So you must be now wondering what is an
outbound/inbound proxy, as depicted in the SIP Trapezoid. Here it goes
        Now suppose you want to make a call. Since the *call* is going out,
it can be called as an outbound call. This call is sent to a Proxy which
looks up the Request URI (its the address of the callee) in the DNS and
then after address resolution sends the call to its destination. Since this
Proxy is providing "outbound service" of DNS resolution it is known as
an *Outbound
        The destination we got above after address resolution is actually
the address of another Proxy. This proxy is a home proxy serving the
callee's domain. Its purpose is to contact the Location Service to find
where and on which device the callee is registered. After it gets the info
from the Location Service it dispatches the call to the callee's registered
device. This is an "inbound service" and is thus called an *Inbound Proxy.*

*Few Points:*
a)  User Agent is always associated with a domain.
b)  Proxies are associated with domains or serve one/more domains.
c)  A single proxy can have the logic to act as an Outbound/Inbound proxy
for a A to B call.

I think from point C ,I believe SBC or any stateful proxy can act as
Am I right here ?
Please clarify !!!!!!!!

Dipan Kumar

On Tue, Dec 23, 2014 at 2:09 AM, Paul Kyzivat <pkyzivat at alum.mit.edu> wrote:

> On 12/22/14 11:49 AM, Dipan Kumar wrote:
>> Thanx to all for your earliest response.
>> But If think of a scenario where 6 proxies in between
>> A  ------>P1->P2->P3->P4->P5->P6-------->B
>> Then according to all of you P1 is outbound proxy and P6 is inbound
>> proxy if call is from A to B.
>> Then how can we define outbound/inbound proxy for  P2 ,P3,P4,P5 ?
>> On what  basis we define a proxy as outbound and inbound?
> Understand that this is not stuff that is formally defined, so you are
> probably going to get different opinions, and there is no way to judge
> which one is "correct".
> One way to approach this is that the definitions are relative to a
> particular device. So B might consider P6 to be its inbound and outbound
> proxy, while A considers P1 to fill that role. I don't find this especially
> helpful.
> Another way, which is what I described, is to first identify the "home"
> proxy for each UA. The home proxy is the one that handles requests
> addressed to the domain portion of a UA's AOR. Then any proxies between the
> UA and its home proxy are inbound and/or outbound proxies depending on
> which paths they participate in.
> There can be many proxies, handling a whole range of things. We don't have
> special names for all of them.
>         Thanks,
>         Paul
>  On Sat, Dec 20, 2014 at 9:31 PM, Paul Kyzivat <pkyzivat at alum.mit.edu
>> <mailto:pkyzivat at alum.mit.edu>> wrote:
>>     On 12/18/14 6:35 AM, Dipan Kumar wrote:
>>      > Hi All
>>      >
>>      > 1.Can anybody clarify what is the diffrence between outbound and
>>     inbound
>>      > proxy
>>      > 2.If there are multiple proxy in between then how to define which
>>     one is
>>      > inbound and which one is outbound
>>      > 3.How many outbound and how many inbound proxy are possible
>>     between two
>>      > endpoints in case of multiple proxy scenario.
>>     These things aren't formally defined (with the exception of rfc5626) -
>>     they are terms for things that are observed in deployed systems.
>>     Loosely, an outbound proxy is one that a UAC uses when sending out of
>>     dialog requests.
>>     And an inbound proxy is one that is inserted in front of the UAS for
>>     out-of-dialog requests coming to a UAS.
>>     Quite often a single proxy will serve both purposes.
>>     Once you establish a dialog, the set of servers on the path is largely
>>     determined by the Record-Route of the dialog establishing request. If
>>     there were outbound and inbound proxies during the dialog establishing
>>     request, and they Record-Route themselves, then they will be there for
>>     subsequent requests within the dialog. And they can still be
>> considered
>>     to be inbound and outbound proxies in that case.
>>     It is also possible for a UAC to insert a proxy in in-dialog requests
>>     even though it isn't in the route set for the dialog. If so, that
>> would
>>     also be an outbound proxy.
>>     If you consider a consider a call between alice at atlanta.com
>>     <mailto:alice at atlanta.com> and
>>     bob at biloxy.com <mailto:bob at biloxy.com> using the typical trapezoid
>>     topology for sip calls where
>>     alice's UA has registered with atlanta.com <http://atlanta.com> and
>>     bob's has registered with
>>     biloxy.com <http://biloxy.com>:
>>     alice at alanta.com <mailto:alice at alanta.com> ----- atlanta.com
>>     <http://atlanta.com> ----- biloxy.com <http://biloxy.com> -----
>>     bob at biloxy.com <mailto:bob at biloxy.com>
>>     then any proxies between alice and atlanta.com <http://atlanta.com>
>>     can be considered in-
>>     and/or out-bound proxies for alice, and any proxies between
>>     biloxy.com <http://biloxy.com>
>>     and bob to be in- and/or out-bound proxies for bob.
>>              Does that help?
>>              Paul
>>     _______________________________________________
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>> --
>> Regards,
>> Dipan Kumar
>> Sr.Software Engineer


Dipan Kumar
Sr.Software Engineer
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