NFD

# ccnd Nonce handling

This document describes how ccnd 0.7.2 handles Nonce.

## Global Nonce table

`nonce_ok` function performs lookup and insertion on the global Nonce table `ccnd_handle.nonce_tab`.

It has four possible return values beside error condition:

* 0: Nonce is duplicate, and this Nonce first comes from a different face

* 1: Nonce is new

* 2: Nonce is duplicate, and this Nonce first comes from the same face

* 3: Interest has no Nonce

An entry in the global Nonce table expires after 6 seconds (hard-coded).

If a duplicate Nonce is detected, its expiration time is reset to be 6 seconds after current time, but the incoming face is unchanged.

Having a global Nonce table implies that Nonces can be compared across all Interest passing through ccnd, instead of just those Interests with same Name and Selectors.

## Loop Detection

Loop detection is based solely on the global Nonce table.

`process_incoming_interest` calls `nonce_ok` with the Interest packet.

This implies that the global Nonce table contains Nonces from incoming Interests.

An incoming Interest is considered to be looped if `nonce_ok` returns 0.

The return value 1 (duplicate from same face) is considered a refresh and not a loop. This is potentially unsafe, but [[CcndStrategy|ccnd's strategy]] only forwards new Interests, and won't forward the refresh Interest again.

## Nonces in Outgoing Interests

In `propagate_interest`, if the outgoing Interest has no Nonce field, a random Nonce is generated, and `nonce_ok` is called to record this Nonce.

Therefore, the global Nonce list contains not only Nonces from incoming Interests, but also Nonces from outgoing Interests.