Skype For Business Online - Signal and Media Traffic Flow - LAN Users

Skype For Business Online - Signal and Media Traffic Flow

Hope you have already gone through the brief about types of traffic and protocols used in Skype for business.

If not, here is the link to have a look https://sjohnonline.blogspot.com/2018/12/skype-for-business-online-types-of.html.

Here we are going to see about how the traffic flows when you initiate a Skype call/sharing session in a LAN network.

Skype signalling and media traffic flow between LAN users

Skype uses ICE protocol to establish the communication between peers, let me take you through the  process happening in the background.

Step 1 SIP Registration and Candidate discovery

SIP Signalling initiate user authentication and registration with MRAS server in O365 cloud and identify the IP addresses, ports and protocols used by caller, in our case caller is user01

Step 2 invitation with SDP Candidates of caller.

User01 invites user02 by sharing the details of IP addresses, ports and protocols used by user01.

Step 3 Invitation acceptance with SDP Candidates of callee

User02 accept the invitation and share the details of IP addresses, ports and protocols used by user02 with user01.

During this time caller can hear the ring tone which indicates that the connection establishment is in progress.

Step 4 Candidate exchange and connectivity check (Candidate validation)

Caller and callee exchange their IP addresses and ports details with each other and try to check connectivity between the users via all possible routes available.

In our case the details of Candidate is as follows (Candidate - IP addresses, and ports).

 User01 

Local IP address - 192.168.10.44

NAT IP address/Public IP - 106.23.110.116  (Also called as reflex IP address)

Relay server IP address - 52.112.132.73 (Also called as relay IP address)

User02

Local IP address - 192.168.1-.125

NAT IP address/Public IP - 106.23.110.116 Also called as reflex IP address)

Relay server IP address - 52.112.132.65 (Also called as relay IP address)

Here the reflex IP addresses (NAT IP's) are same for both users as they both belongs to same LAN network

Connectivity checks are performed to find out the most direct media path possible between endpoints using STUN protocol.

As UDP is preferred for audio/video communication, the connectivity checks start with UDP 

a) First try to check connectivity between local IP's 192.168.10.44 & 192.168.10.125 over UDP

b) Then try to check connectivity between reflex IP's 106.23.110.116 over UDP

c) Finally try to check connectivity between relay IP's  52.112.132.65 & 52.112.132.73 over UDP

d) If all the above checks failed then start the connectivity check using TCP.

Step 5 Connection establishment 

After the connectivity checks, STUN protocol will evaluate the routes identified and find out most direct route to establish connection between the users.

In this scenario connection establishment between users will be via local IP's 192.168.10.44 & 192.168.10.125 as both belongs to same LAN networks, so the shortest route and NAT IP's are same.

If both the users are on same LAN network and using different internet connections, then the connection may establish via reflex/NAT IP's.

Since the clients are behind a NAT device direct communication may fails and connection established using Reflex IP addresses 52.112.132.65 & 52.112.132.73.

As mentioned if none of the above connectivity method is possible, connection will be established using the relay server in O365 cloud and which may effect quality of communication.

So if you are calling your colleague who belongs to the same network, the voice, video and sharing session traffic will be exchanged between them through the local LAN network, that is via local cables and switches placed inside the office.

But always remember you need a internet connection for signalling traffic but it won't eat up the internet bandwidth.

Always follow and implement Skype network requirement best practices in your network and open necessary ports and protocols in the firewalls to get the good quality communication.

Hope this is informative for you.

Cheers 😊