1. NFD forwarding pipelines
Junxiao Shi,

2. Overview Forwarding consists of pipelines and strategies
Pipeline: a series of steps that operate on a packet or a PIT entry
Strategy: a decision maker on whether, when, and where to forward an Interest

3. Pipelines incoming Interest Interest loop ContentStore miss
ContentStore hit
outgoing Interest
Interest reject
Interest unsatisfied
Interest finalize
incoming Data
Data unsolicited
outgoing Data
incoming Nack
outgoing Nack

4. Legend in diagrams pipeline building block strategy tables feature
face feature

5. Pipelines Overall Workflow
Interest loop
Interest reject
incoming Interest
after receive Interest
before expire Interest
outgoing Interest
ContentStore miss
unsatisfy timer
Interest unsatisfied
ContentStore hit
before satisfy Interest
incoming Data
straggler timer
outgoing Data
Data unsolicited
Interest finalize
incoming Nack
after receive Nack
outgoing Nack

6. incoming Interest pipeline
receive Interest
violates /localhost?
detect duplicate Nonce with Dead Nonce List N Y
Interest loop Y Y N
detect duplicate Nonce in PIT entry
(drop)
PIT insert N
is pending? Y
cancel unsatisfy & straggler timer
ContentStore miss N
ContentStore hit
CS lookup N Y

7. detect duplicate Nonce
If the Name+Nonce of the incoming Interest appear in Dead Nonce List, or any InRecord or OutRecord in PIT entry contains the same Nonce as the incoming Interest, a duplicate Nonce is detected.
If the duplicate Nonce is found in InRecord only, this is a multi-path arrival, and not a loop.
If the duplicate Nonce is found in OutRecord or Dead Nonce Table, this is either a multi-path arrival or a loop, and these two reasons are indistinguishable.
Nonce is later recorded on an InRecord.

8. Interest loop pipeline
is multi-access face? N
reason=Duplicate
send Nack Y
(drop)
Process an Interest that has been considered looped.
Keep this simple for now: unconditionally send Nack- Duplicate when duplicate Nonce is detected.
Don't enter outgoing Nack pipeline: in-record isn't inserted yet.
In the future, strategy could be invoked, because duplicate Nonce may be multi-path arrival instead of loop.

9. ContentStore miss pipeline
This pipeline is entered when an incoming Interest
is pending (so ContentStore lookup is unnecessary), or
is miss from ContentStore
This pipeline will start forwarding the Interest

10. ContentStore miss pipeline
insert InRecord
set PIT unsatisfy timer
timer event
Interest unsatisfied
has NextHopFaceId?
chosen NextHop face exists? Y N
(drop) N Y
invoke incoming Interest callback
outgoing Interest

11. set PIT unsatisfy timer
Given PIT entry, set an unsatisfy timer which fires when InterestLifetime expires for all unexpired InRecords
When the unsatisfy timer fires, Interest unsatisfied pipeline is entered

12. dispatch incoming Interest to strategy
Given PIT entry and incoming Interest, determine which strategy should process this Interest, and trigger that strategy

13. ContentStore hit pipeline
Interest finalize
timer event
set PIT straggler timer
outgoing Data

14. outgoing Interest pipeline
(drop)
(drop) Y Y
violates /localhost?
violates /localhop? N N
pick Interest
insert OutRecord
send Interest

15. pick outgoing Interest packet
Given PIT entry and nexthop, decide the guiders on the outgoing Interest
Nonce and InterestLifetime come from an InRecord with longest remaining lifetime, however InRecord with same Face as the nexthop cannot be used
InterestLifetime is carried from the original packet without deducting the time elapsed
Scope is the most relaxed among all unexpired InRecords
The last incoming Interest is picked
This is a simple choice until we understand the effect of guiders better

16. Interest reject pipeline
Process an Interest that has been decided that it has nowhere to go
cancel unsatisfy & straggler timer
set PIT straggler timer
timer event
Interest finalize

17. Interest unsatisfied pipeline
invoke PIT unsatisfied callback
Interest finalize

18. Interest finalize pipeline
need Dead Nonce List insert? Y
Dead Nonce List insert N
PIT delete

19. Dead Nonce List insert Dead Nonce List insertion is needed if:
Interest is unsatisified, OR
Interest has MustBeFresh=yes and Data FreshnessPeriod is shorter than 6 seconds
Nonces in OutRecords are inserted to Dead Nonce List.

20. incoming Data pipeline
receive Data
violates /localhost? Y
(drop) N
PIT match Y
CS insert
foreach
PIT entry
cancel unsatisfy & straggler timer N
Data unsolicited
need Dead Nonce List insert?
invoke PIT satisfy callback Y
Dead Nonce List insert N
foreach
pending
downstream
outgoing Data
mark PIT satisfied
set PIT straggler timer
timer event
Interest finalize

21. set PIT straggler timer
Given PIT entry, set a straggler timer which fires after a short time
Tstraggler = 100ms
When the straggler timer fires, PIT entry is deleted
The purpose of retaining PIT entry for a short time is to facilitate loop detection and to collect measurement for non-fastest upstreams

22. Data unsolicited pipeline
accept to cache? N
(drop) Y
CS insert

23. outgoing Data pipeline
violates /localhost? Y
(drop) N
traffic manager
send Data

24. Pass-through traffic manager
Provide a traffic manager that does nothing and merely passes Data packet to the next step

25. Incoming Nack pipeline
is multi-access face? Y
receive Nack
(drop) N
PIT match
has out-record with correct Nonce? Y Y
mark out-record as Nacked N N
trigger strategy: after receive Nack
(drop)
(drop)

26. Outgoing Nack pipeline
strategy action: send Nack
has in-record?
is multi-access face? N Y
erase in-record
send Nack N Y
(drop)
(drop)

27. FIB lookup (in strategy)
in producer region?
has Selected
Delegation? Y N N
FIB lookup with first Delegation Name
has Link object? Y Y
in default-free zone? N N Y
FIB lookup with Interest Name
FIB lookup with SelectedDelegation
done
choose and set SelectedDelegation

28. determine whether in producer region
Input:
regionNames: the router's region names, from configuration
interest
Algorithm:
foreach delegation in Link:
foreach regionName in regionNames:
if delegation Name is a prefix of regionName, return true and abort these steps
return false
Note:
We consider all delegations, not just SelectedDelegation, to maximize flexibility. This is cheap because there's no table lookup.

29. determine whether in default-free zone
Input:
interest, with Link but without SelectedDelegation
FIB lookup result for the first delegation
Algorithm:
if FIB lookup result is the root entry (ndn:/), and the entry has at least one nexthop record, return false
return true
Note:
FIB lookup will never return "no match", because the FIB always has a root entry, but this entry can have no nexthop record.
We consider only the first delegation, to minimize table lookup cost. Checking all delegations probably gives better results, but it requires too many lookups.

30. choose and set SelectedDelegation
Input:
interest, with Link but without SelectedDelegation
FIB
Algorithm:
foreach delegation in Link, sorted by increasing Preference
lookup FIB with delegation Name
if a match (with non-zero nexthop records) is found, insert SelectedDelegation field with the index of this delegation, return the match, and abort these steps
lookup FIB with Interest Name
Note:
This is the only operation that requires multiple table lookups, but it happens only once on an Interest's path at the first default-free router.