CHAN

SPECIFICATION
S. O'Malley and L. Peterson. A Dynamic Network Architecture. ACM Transactions on Computer Systems 10, 2 (May 1992), 110-143.

B. Welch. The Sprite remote procedure call. University of California at Berkeley, Tech Report UCB/CSD 86/302, June 1986.

SYNOPSIS
CHAN is a single protocol version of Sprite RPC's reliable request-reply channel. The algorithm was extracted from Sprite and made into a stand-alone protocol. Each CHAN session supports the Birrell-Nelson implicit acking RPC algorithm between two hosts.

CHAN provides ``at most once'' RPC semantics. When a CHAN call returns successfully, the protocol guarantees that the request has been processed exactly once by the server. If CHAN returns unsuccessfully (XK_FAILURE), the server may have processed the request once, or it may not have seen the request at all.

Channel numbers are entirely internal to the CHAN protocol. When a new client channel session is created, a new host-host channel number is selected internally by CHAN. When protocols openEnable CHAN, they will receive connections from any channel number on any remote host. Each open of CHAN by a client session will result in the passive creation of a corresponding session on the server.

Each channel session will accept only a single outstanding request. Sending additional requests on a channel before the first request has returned is not allowed.

CHAN relies on the BIDCTL and BID protocols to determine when a peer has rebooted. When notified of a peer's reboot, CHAN will disable all active channels to that host. Outstanding calls will return XK_FAILURE, as will all subsequent calls on that channel session. Replies sent through disabled server channels will be discarded.

CHAN must know several things about the transport protocol used to actually send the message. This information is represented in the following structure:

typedef struct {
    XObj transport;
    int  ticket;
    int  reliable;
    int  expensive;
    unsigned int timeout;
} chan_info_t;

This structure is defined in the CHAN session state and a pointer to it is attached as an attribute to each outgoing message. Before the message is send CHAN zero's out all fields of the structure. When xPush returns CHAN assumes that some lower level protocol may have filled in the fields.

If transport has been defined CHAN will perform a FREERESOURCES control operation on transport when the current message has been acked. If the lower level protocol is reliable CHAN will never retransmit the entire message and will not start a timer. If the lower level protocol is expensive CHAN will not retransmit the entire message when it times out. It simply requests an ACK. The timeout field is ignored for the moment. If transport has been defined CHAN will invoke a CHAN_RETRANSMITcontrol operation on it before retransmitting. If this control operation returns 0 CHAN will not retransmit the body of the message. This allows a lower level protocol like BLAST to discourage CHAN from retransmitting while the message is still being sent.

REALM
CHAN lies on the boundary between the ASYNC realm and the RPC realm. That is, it looks like an ASYNC protocol to protocols below it, and an RPC protocol to protocols above it.

CONTROL OPERATIONS

CHAN_ABORT_CALL:

When invoked on a channel session, it causes the current call (if one is outstanding) to abort and return XK_FAILURE.

Input:

none

Output:

none

PARTICIPANTS
CHAN neither removes from nor adds to the participant stacks, passing the participants untouched to the transport protocol on an open and ignoring the participant structure on an openenable.

CONFIGURATION
CHAN requires its lower transport protocol configured as the first lower protocol and BIDCTL configured as the second lower protocol. CHAN requires that it's transport protocol will deliver incoming messages from different hosts through different lower sessions and that all CHAN messages from the same host come from the same lower session.

CHAN is a realm boundary protocol which assumes its transport protocol is symmetric (in the ASYNC realm.)

Because CHAN affixes a pointer to the outgoing message it must be in the same address space as any transport protocol which will attempt to set the structure passed in the attribute.

AUTHOR
Sean O'Malley