[SIPForum-discussion] Outbound and inbound proxy

Paul Kyzivat pkyzivat at alum.mit.edu
Sat Dec 20 16:01:01 UTC 2014

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 and 
bob at biloxy.com using the typical trapezoid topology for sip calls where 
alice's UA has registered with atlanta.com and bob's has registered with 

alice at alanta.com ----- atlanta.com ----- biloxy.com ----- bob at biloxy.com

then any proxies between alice and atlanta.com can be considered in- 
and/or out-bound proxies for alice, and any proxies between biloxy.com 
and bob to be in- and/or out-bound proxies for bob.

	Does that help?

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