This article documents VPN Lync connection problems. The research troubleshoots Skype for Business clients on a VPN, that connect to external Lync clients. The full article explains traffic flow for both Lync servers and Lync clients.
*Update 12/6/2016: Bypass the VPN using NRTP and Firewall GPOs.
Skype for Business audio and video (AV) does not work between an internal-to-external call between the branch office VPN and staff working from home.
- Instant messaging (IM) works between the VPN branch office and external users.
- VPN Lync clients are unable to establish conference calls to external users.
- The problem only occurs between the VPN branch office and external users.
The communication problem between VPN clients and external clients result from a missing persistent route to the branch office’s subnet. The Access Edge server resides between the DMZ and the Perimeter network. The Access Edge has a gateway on the DMZ interface. The gateway is not configured on the Perimeter interface for traffic restriction purposes. The Access Edge server is configured to communicate with the data center subnet. There is not a persistent route that forwards traffic to the VPN branch office subnet.
- Add a persistent route that forwards to the branch office subnet on the Access Edge server’s local route table. This is a quick fix (see next solution).
- VPN tunnels add unnecessary packet overhead that introduce jitter and network latency. Pushing external AV traffic can also saturate available VPN bandwidth. Microsoft recommends branch offices bypass the VPN tunnel and instead connect through the outside Access Edge server as external clients.
- Deploy an additional Reverse Proxy and AV edge server at remote site. There is no additional Lync licensing for this solution. The servers require additional Windows licenses and may operate on physical or virtual servers.
- Branch office VPN is split-tunnel. All Internet traffic exits local gateway. All company traffic uses the VPN.
- Branch office subnet is not restricted. VPN provides access to all internal resources.
Lync clients send and receive instant messages using Instant Messaging Multipoint control unit (IMMCU). IMMCU is a Lync Front End service that provides instant message communication between Lync clients. Peer-to-peer communication (and multi-party IM) is only possible when the Lync server and IMMCU is active and available.
*The missing persistent route on the Access Edge prevents AV communication on the internal interface to the VPN branch office. IMs continue to work between the VPN and external clients because IMs are sent to-and-from the Front End server; not the Access Edge.
Lync clients communicate differently from IM when using AV services. Clients prefer to communicate via peer-to-peer. Internal and external clients use the Interactive Connectivity Establishment (ICE) RFC 5245 framework to determine the most direct path between endpoints. Benefits of peer-to-peer transmission include:
1. UDP Direct. UPD direct occurs when ICE determines both Lync clients communicate using only host (or local candidate) IP addresses.
· UDP Direct assumes no firewall or Natting is used between Lync clients.
· Clients that reside within the same broadcast domain or reside on the same subnet.
· Example may be two Lync clients used at an office or at the same hotel.
- For clients that cannot communicate directly peer-to-peer. Routing or firewall problem may prevent communications.
- Both clients can individually communicate to the Access Edge server.
Distinction worth noting is that internally connected Lync clients will ignore the TURN (or server relay) candidate provided by the ICE server. This IP address is used only by external (e.g. foreign) endpoints like federated Lync clients. A Lync client will be made aware of its assigned media relay server via in-band provisioning details provided during initial registration and sign-in to Lync and it will always connect to this FQDN for media relay functionality. http://blog.schertz.name/2012/10/lync-edge-stun-turn/
- UDP is preferred but TCP is always considered as a candidate.
- There are always 2 potential candidates per network path; UDP and TCP.