Conference Calling 2.0 (aka SFT)¶
Previously, Wire group calls were implemented as a mesh, where each participant was connected to each other in a peer-to-peer fashion. This meant that a client would have to upload their video and audio feeds separately for each participant. This in practice meant that the amount of participants was limited by the upload bandwidth of the clients.
Wire now has a signalling-forwarding unit called SFT which allows clients to upload once and then the SFT fans it out to the other clients. Because connections are not end-to-end anymore now, dTLS encryption offered by WebRTC is not enough anymore as the encryption is terminated at the server-side. To avoid Wire from seeing the contents of calls SFT utilises WebRTC InsertibleStreams to encrypt the packets a second time with a group key that is not known to the server.
With SFT it is thus possible to have conference calls with many participants without compromising end-to-end security.
The following diagram is centered around SFT and its role within a calling setup. Restund is seen as a mere client proxy and its relation to and interaction with a client is explained here. The diagram shows that a call resides on a single SFT instance and that the instance allocates at least one port for media transport per participant in the call.
Establishing a call¶
Client A wants to initiate a call. It contacts all the known SFT servers via HTTPS. The SFT server that is quickest to respond is the one that will be used by the client. (Request 1:
Client A gathers connection candidates (own public IP, public IP of the network the client is in with the help of STUN, through TURN servers) 1 for the SFT server to establish a media connection to Client A. These information are then being send again from Client A to the chosen SFT server via HTTPS request. (Request 2:
The SFT server tests which of the connection candidates actually work. Meaning, it goes through all the candidates until one leads to a successful media connection between itself and client A
Client A sends an OTR 2 message
CONFSTART(to all members of chat), which contains the IP address of the SFT server that is being used for the call.
Any other client that wants to join the call, does 1. + 2. with the exception of only contacting one SFT server i.e. the one that client A chose and told all other potential participants about via
At that point a media connection between client A and the SFT server has been established, and they continue talking to each other by using the data-channel, which uses the media connection (i.e. no more HTTPS at that point). There are just 2 HTTPS request/response sequences per participant.
For Conference Calling to function properly, clients need to be able to reach the HTTPS interface of the SFT server(s) - either directly or through a load balancer sitting in front of the servers. This is only needed for the call initiation/joining part. Additionally, for the media connection, clients and SFT servers should be able to reach each other via UDP (see Firewall rules). If that is not possible, then at least SFT servers and Restund servers should be able to reach each other via UDP - and clients may connect via UDP and/or TCP to Restund servers (see Protocols and open ports), which in turn will connect to SFT server. In the unlikely scenario where no UDP is allowed whatsoever or SFT servers may not be able to reach the Restund servers that clients are using to make themselves reachable, an SFT server itself can also choose to proxy itself by a Restund server, which could be different from the Restund servers used by clients (see TURN discovery flag).
The SFT may need to receive and send traffic over UDP and TCP on a wide range of ports. Due to the fact that Kubernetes services do not support setting port ranges, and Kubernetes pods not being publicly routable (at least in IPv4) we require the SFT pods to run in hostNetwork mode and the pod will bind directly to the default interface of the node.
Due to this hostNetwork limitation only one SFT instance can run per node so if you want to scale up your SFT deployment you will need to increase the amount of kubernetes nodes in your cluster.
As a rule of thumb you will need 1vCPU of compute per 50 participants. SFT will utilise multiple cores. You can use this rule of thumb to decide how many kubernetes nodes you need to provision.
For more information about capacity planning and networking please refer to the technical documentation