[SIPForum-discussion] Outbound and inbound proxy

Paul Kyzivat pkyzivat at alum.mit.edu
Mon Dec 22 20:39:27 UTC 2014

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.


> 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

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