[Nfd-dev] NFD RIB Daemon - trigger FIB updates from RIB

[Junxiao Shi]

This is a design proposal for NFD RIB Daemon.

Background
RIB Management functionality is implemented as a separate process: NFD RIB
Daemon.
RIB Daemon processes RIB updates from local applications and routing
protocols, and then synchronizes the RIB to the FIB in main NFD process via
FIB Management protocol.
Currently, RIB Daemon has a RibManager component that is responsible for
processing RIB updates, and a RIB data structure which is exclusively
accessed by the RibManager.
Historically, before RIB is implemented, RIB Daemon is a pass-through
protocol translator that translates each RIB update into one FIB update,
ignoring route inheritance flags.
In the protocol translator, FIB updates are sent from RibManager, and a RIB
update command is answered after the corresponding FIB update is complete.

Problem
Task 1326 implements Route expiration in RIB: when a Route (prefix
registration) expires, the FaceEntry is removed from the RIB, which shall
trigger FIB updates.
During the code review http://gerrit.named-data.net/911 (patchset 4
rib-manager.cpp), a question is raised: should RibManager set the timer to
remove the FaceEntry and trigger FIB updates, or should the RIB set the
timer and notify RibManager?
Reviewer opinion is: expiration period is an attribute of a Route, so the
timer should be set in RIB.
This leads to another problem: after removing the FaceEntry, if the FIB
update(s) fails, RIB and FIB will be out-of-sync.
A related problem is: if a RIB update triggers multiple FIB updates, if
some FIB updates succeed but others fail, RIB and FIB will also be
out-of-sync.

Proposed Design
RibManager updates the RIB only; it does not directly update the FIB.

RIB updates are batched. A batch of RIB updates is packaged into a
RibUpdateBatch object. All RIB updates in the same RibUpdateBatch object
must refer to the same face.
When RibManager receives one or more RIB updates, it generates a
RibUpdateBatch, and passes it to Rib::beginApplyUpdate() to begin the RIB
update procedure.
When one or more Route expires, the RIB itself generates a RibUpdateBatch,
and begin the RIB update procedure.

The RIB update procedure has admission control: if a previous
RibUpdateBatch is in progress, the new batch will be put in a queue until
the previous batch is complete.
To apply a batch of RIB updates, the current collection of RIB entries and
the RibUpdateBatch are passed to FibUpdater component to compute the
optimal set of FIB updates, and the FibUpdater component shall send these
FIB updates.
If any FIB update fails, (a) in case of a non-recoverable error (eg.
signing key of RIB Daemon is not trusted), the RIB Daemon shall crash; (b)
in case of a non-existent face error, the RibUpdateBatch is abandoned; (c)
in other cases, the FIB update is retried.
If the RibUpdateBatch is abandoned, the RIB is unchanged; otherwise, after
all FIB updates are successful, the RIB entries are updated.

For a prefix registration request from application, the RibManager should
pass a callback to Rib::beginApplyUpdate, which is called after all FIB
updates are successful and the RIB is updated. The called function can
respond to the prefix registration request.
For routing updates, the RibManager can respond with "100 continue" or
similar code right away without waiting for FIB updates to complete.

Benefits
The proposed design decouples FIB updates from RIB updates.
A RIB update, regardless of source, can trigger FIB updates.
If a RIB update *command* needs to wait for the RIB update and FIB updates
to complete, RibManager can wait on a callback from the RIB.



Yours, Junxiao
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