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I've done my own research in the UccAPI logs as well as opening a ticket with Office 365 support but the results are still muddled and confusing. I have yet to do full on packet captures to really nail it down...simply for lack of time.

My question is: does the Lync client really talk straight peer-to-peer for its IM and video call communications when not in a "Lync Meeting"? (Note: for me it's Lync Online/Office 365, but it should function very similar to an on-premise deployment)

Massimo's question here: How can two Lync client talk when there is no route between them? states:

Now, as everyone knows, Lync clients are only managed by the Lync servers, but talk directly between themselves for actual communications; unless of course a conference is involved, in which case they connect all to the front-end servers which acts as a MCU.

However, after working with MS support on a desktop sharing issue within Lync, the tech stated that the Lync server is always the MCU, even during basic IM messages. He said that even though 2 clients may be on the same LAN they will still relay their traffic through the Lync Online server, in essence acting as the proxy for the call/IM.

Looking at the UCCAPI.log on my client I see a lot of activity back and forth to the Lync server itself, but never really direct peer-to-peer traffic:

From: "Bob Smith"<sip:bob@foo.com>;tag=e7833a30;epid=1275117fecd
To: <sip:mary@foo.com>;epid=1d2a1221b4;tag=173235356c1
Call-ID: 07ec7f7a031111111111121100
CSeq: 1 INVITE
Record-Route: <sip:sippoolbl20a04.infra.lync.com:443;transport=tls;ms-fe=BLES03.infra.lync.com;opaque=state:Ci.Rfea2c00:Ti.W85ERLQ6ZEmmt2GgmW1EdwAA;lr;ms-route-sig=cf2qT3Y_wo-oN53Ui0Fro_lRokEb9YLApu_iBPn0fgV_0m71111AAA>;ms-rrsig=cfZAEgWED1u9111111RclA3N488uz7fXX7CSIQATO_8m7DA1rdtBTAAA;tag=F4A2EFB01111111111114FB97F116
Record-Route: <sip:sippoolbl20a04.infra.lync.com:5061;transport=tls;ms-fe=BL20AS04.infra.lync.com;opaque=state:F:Ti.W85ERLQ6ZEmmt2Ggm1111wAA;lr>;tag=4A2AA729141D0111177C6EA4C8AC6E7
Record-Route: <sip:sippoolbl20a04.infra.lync.com:5061;transport=tls;ms-fe=BL20AS02.infra.lync.com;opaque=state:T:Ti.W85ERLQ6ZEmmt2Ggm1111wAA;lr>;tag=0C9E9993ECA011111111893C255B4855
User-Agent: UCCAPI/15.0.4517.1004 OC/15.0.4517.1004 (Microsoft Lync)
Proxy-Authorization: TLS-DSK qop="auth", realm="SIP Communications Service", opaque="C09B2801", targetname="BL2003.infra.lync.com", crand="f2258f89", cnum="684", response="9b515b67187e3c1111ca712d797771e449df"
Content-Length: 0

So, am I interpreting the logs and what the MS tech said incorrectly or not?

The reason I ask the question in bold is because we are trying to decipher how much Lync client traffic is "lan client to lan client" vs. "lan->wan->O365 Lync server->wan->lan". I want to determine if the connections are routing over the fastest links available to them for Lync client communications.

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Lync IM traffic always passes through the server, using the SIP protocol. Media will pass P2P if possible, otherwise it will go through (a different component) of the server as well. One key factor is whether the client device is logged in remotely via an edge server, since this affects the address space that the device can access. Note that routing of SIP in Lync Online is slightly different to on-premise Lync.

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Thanks, when you say "slightly different to on-prem" what do you mean? –  TheCleaner Sep 26 '13 at 18:20
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