Soft-State Protocol for Accessing Multimedia Archives

One-line summary: Soft-state lightweight session protcol, based on announce/listen and refreshable soft state, for serving multimedia streaming content to implicit or explicit subscription groups using mcast.

Overview/Main Points

• Functionality goals: maintenance of shared state; group formation/mgt; fault detection and recovery.
• Initial handshake launches an SSAC "archive agent". As soon as it's launched, it starts reflecting the control state in heartbeats.
• Client and server each periodically announce state info. State info contains complete info necessary to resume a session in case of failure, even if the session has already timed out.
• Asymptotically reliable "Set": to set a value, client does this in its heartbeat, and waits for new value to be reflected in server heartbeat. Value is assumed "set" when the first server heartbeat containing the new value is multicast. For explcit groups (subscription), server must manage race conditions if multiple clients set a parameter to different values. State change latency is therefore about 2 RTT.
• Alternative: Group fork-- a client param change can cause client to leave a group and start its own new session. This is useful under implicit group management, where server attempts to collect "similar" clients into a group automatically but clients can leave the group once they're no longer similar.
• Client annoucnement frequency should be kept constant as number of clients increases. Not a problem since all clients benefit from every client's annoucnements.
• Related work: RTSP doesn't explicitly address fault discovery/recovery; MBone VCR-on-demand can only have 1 client controlling a session (others are listen-only).

Relevance

Flaws