3.2.1.1.1 Virtual Connection, Virtual Channel Hierarchy, and Protocol Variables
Each role specified by this protocol need to maintain a hierarchical data structure where at most one virtual IN and at most one virtual OUT channel are associated with a virtual connection, where each role MAY execute on separate network nodes.
The virtual channels that are components of a given virtual connection are defined to belong to this virtual connection. Each virtual connection is identified uniquely among a client, any number of inbound proxies, any number of outbound proxies, and a serverby using an RTS cookie known as a virtual connection cookie. Multiple inbound and outbound proxies MAY be deployed in conjunction with a TCP layer or HTTP layer load balancer to improve scalability and reliability. All load balancing MUST be performed in a manner transparent to the client. All roles defined by this protocol maintain a protocol variable to store the virtual connection cookie for a specific virtual connection. The virtual connection cookie is generated by the client. Other parties acquire the cookie by exchanging one or more PDUs with the client.
Each virtual IN channel is composed of an IN channel between a client and an inbound proxy and a second IN channel between an inbound proxy and a server.
Each virtual OUT channel is composed of an OUT channel between a client and an outbound proxy and a second OUT channel between an outbound proxy and a server.
Both of these channels are defined to be components of the virtual channel and transitively to be components of the virtual connection. It is also said that they belong to the virtual channel and transitively to the virtual connection.
The relationship is illustrated by the following diagram.
Figure 16: Virtual connection hierarchy
Each IN channel and OUT channel instance is identified uniquely among a client, one or more inbound proxies, one or more outbound proxies, and a server using an RTS cookie known as a "channel cookie".
As specified in sections 2.1.2.1.7 and 2.1.2.1.8, both virtual IN channel and virtual OUT channel are limited to transmitting only a certain number of bytes. For a virtual connection to be capable of sending an unlimited number of bytes, it must be able to discard IN channels or OUT channels whose lifetime has expired and replace them with successor IN channels or OUT channels. The process of discarding a predecessor IN channel or OUT channel and establishing a successor IN channel or OUT channel while ensuring that the reliable, in-order, at-most-once delivery guarantee is maintained is called channel recycling. The successor IN channel or OUT channel is called a predecessor replacement channel. During the recycling process, there is a period of time when both a predecessor channel and a successor channel instance are available. One of these is called the default channel, and the other is called the nondefault channel. The protocol sequences and message processing rules throughout section 3 specify which channel is the default in each particular case.
Every instance of a role belonging to this protocol maintains common abstract data elements for the Virtual Connection, the Virtual Connection Cookie Table, the Sending Channel, the Receiving Channel, and the Ping Originator. The abstract data elements and model for these common elements are described in the immediately following sections.
For each role that this protocol can operate in, additional abstract data elements are required. They are described in the following sections: