This is a question that has plagued me for several years, in that I have never been able to establish a consistent level of Skype quality in a corporate environment, despite having lots of bandwidth and obtained the consultancy services of CCIE level network experts.
The answer to the question is ultimately, no.
Let me explain by running through the questions.
1. How does Skype work at a network level?
Skype is a “Peer To Peer” (P2P) application. That means that when 2 people are having a Skype conversation, their computers *should* be directly connected, rather than connected via a 3rd computer. For the sake of comparison, Google Hangouts is not a P2P application. Google Hangout participants connect to each other via Google Conference Servers.
2. Does Skype work with UDP or TCP?
Skype’s preference is for UDP, and when Skype can establish a direct P2P connection using UDP, which is typically the case for residential users, call quality is very good. This is because UDP is a much faster protocol than TCP when used for streaming audio and video.
3. What’s the difference between residential and corporate users?
Residential internet connections are typically allocated a temporary fixed public ip address. This IP gets registered to a Skype user on Skype’s servers, so when someone needs to contact that user, Skype knows where to direct the call, and can use UDP to establish a call between the participating users.
In corporate environments, where there are lots of users using the same internet connection, sharing of a a single public IP address between those users has to occur (Port Address Translation). That means that the Skype servers will have registered the same public ip address for all the users in that organisation. This means that Skype is not able to establish a direct UDP P2P connection between a user on the outside of that organisation and a user in that organisation, and has to use other means to make that connection.
4. What are those other means?
When direct connectivity between clients is not possible, Skype uses a process called “UDP hole punching”. In this mechanism, 2 computers that cannot communicate directly with each other communicate with one or more third party computers that can communicate with both computers.
Connection information is passed between the computers in order to try and establish a direct connection between the 2 computers participating in the Skype call.
If ultimately a direct connection cannot be established, Skype will use the intermediary computers to relay the connection between the 2 computers participating in the conversation.
In Skype terminology, these are known as “relay nodes”, which are basically just computers running Skype than have direct UDP P2P capability (typically residential users with good broadband speeds).
From the Skype Administrators Manual:
If a Skype client can’t communicate directly with another client, it will find the appropriate relays for the connection and call traffic. The nodes will then try connecting directly to the relays. They distribute media and signalling information between multiple relays for fault tolerance purposes. The relay nodes forward traffic between the ordinary nodes. Skype communication (IM, voice, video, file transfer) maintains its encryption end-to-end between the two nodes, even with relay nodes inserted.
As with supernodes, most business users are rarely relays, as relays must be reachable directly from the internet. Skype software minimizes disruption to the relay node’s performance by limiting the amount of bandwidth transferred per relay session.
5. Does that mean that corporate Skype traffic is being relayed via anonymous third party computers?
Yes. The traffic is encrypted, but it is still relayed through other unknown hosts if a direct connection between 2 Skype users is not possible.
6. Is this why performance in corporate environments is sometimes not good?
Yes. If a Skype conversation is dependent on one of more relay nodes, and one of these nodes experiences congestion, this will impact on the quality of the call.
7. Surely, there is some solution to this?
A corporate network can deploy a proxy server, which is directly mapped to a dedicated public ip address. Ideally, this should be a UDP-enabled SOCKS5 server, but a TCP HTTP Proxy server can also be used. If all Skype connections are relayed through this server, Skype does not have to use relay nodes, as Port Address Translation is not in use.
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8. So what’s the catch?
The problem with this solution is that it is not generally possible to force the Skype client to use a Proxy Server. When the client is configured to use a Proxy Server, it will only use it if there is no other way to connect to the Internet. So, if you have a direct Internet connection, even one based on Port Address Translation, which impacts on Skype quality, Skype will continue to use this, even if a better solution is available via a Proxy Server.
9. Why would Skype do this?
Skype is owned by Microsoft. Skype have a business product that attaches to Microsoft Active Directory that allows you do force a Proxy connection. So if you invest in a Microsoft network, Microsoft will give you a solution to enable better Skype performance in corporate networks. If you don’t want to invest in a Microsoft network, you’re stuck, and your only option is to block all outbound Internet access from your network and divert it via your Proxy server.
For a lot of companies, particularly software development companies who depend on 3rd party web services, this is not a practical option.
10. What is the solution?
At this time the primary options for desktop Audio/Video conferencing are either Skype or Google Hangouts.
When Skype can be used in an environment where P2P UDP connectivity is “always on”, it provides a superior audio/video experience to Google Hangouts, which is not P2P, and which communicates via central Google Servers.
Where an environment uses Port Address Translation, Skype performance will depend on the ability of Skype client to establish connections via relays, which means Skype performance becomes dependent on the resources available to those relays.
In this instance, Google Hangout may be a better choice where consistent quality is required, as quality can be guaranteed by providing sufficient bandwidth between the corporate network and Google.