eXtensible Session Protocol
A key insight in Phoebus is that an articulated, session-based connection can often outperform a single transport protocol and connection, therefore managing a number of separate transport features allows us to take advantage of a network path that may present a variety of network links all with unique characteristics.
To enable negotiation of features and maintain session state between gateways, Phoebus utilizes the Extensible Session Protocol. Within XSP message headers, a variable size option block defines what transport-layer capabilities are requested as well as what is currently supported at each gateway in the network. This exchange takes place as a negotiation between two data transfer services at the end points in determining the features to be used during an actual data transfer. Phoebus gateways may also use XSP to negotiate a number of other path-specific parameters (multi-pathing, failover routes, protocol settings) based on external monitoring and attached network topology services. One of the key features in XSP is the extensible nature of the protocol, as the name implies. Additional features may be easily defined as option blocks and communicated across enabled gateways.
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