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# Prefix Announcement Protocol
The **prefix announcement protocol** allows registering prefixes using a *prefix announcement object*. It is an alternative to NFD [[RibMgmt|RIB Management protocol]]'s `rib/register` verb.
As of November 2024, this protocol is not yet implemented in NFD.
## Prefix Announcement Object
The **prefix announcement object** is a Data packet that represents an application's intent of registering a prefix toward itself. A Data packet that satisfies the following requirements is a prefix announcement object:
* Name starts with the announced prefix, followed by a fixed `32=PA` keyword name component, followed by a version component and a segment component as defined by the [NDN Naming Conventions](https://named-data.net/publications/techreports/ndn-tr-22-ndn-memo-naming-conventions/). In the current revision of this specification, the segment number MUST be zero.
* ContentType MUST be 5 (prefix announcement).
* Content contains a sequence of TLV elements, including at least an ExpirationPeriod element. Ordering of these TLV elements is insignificant. Unrecognized non-critical TLV elements are permitted and MUST be ignored.
The **validity period** of a prefix announcement object is given in the ExpirationPeriod and ValidityPeriod elements in its Content.
* ExpirationPeriod gives the duration for which the prefix announcement remains valid. The duration begins when a node or program receives the prefix announcement. This element is required.
* ValidityPeriod gives the absolute time range in which the prefix announcement remains valid. It is ignored if the receiving node does not have a UnixTime clock. This element is optional.
* When both ExpirationPeriod and ValidityPeriod are present, the most restrictive constraint applies.
The trust model of a prefix announcement is to be defined by the application.
### Example
```
Data
Name /net/example/32=PA/%FD%01/%00%00
MetaInfo
ContentType 5 (prefix announcement)
Content
ExpirationPeriod 3600000
ValidityPeriod
NotBefore 20181030T000000
NotAfter 20181124T235959
SignatureInfo
SignatureValue
```
This prefix announcement object announces the prefix `/net/example`. It is valid for one hour from its receipt, between the dates 2018-10-30 and 2018-11-24 (included).
## RIB Command Verb: `announce`
This command announces a prefix using a prefix announcement object. NFD-RIB accepts this command as a *signed Interest* with the following name:
```
/localhost/nfd/rib/announce/<params-sha256>
/localhop/nfd/rib/announce/<params-sha256>
```
The Interest's ApplicationParameters element carries the prefix announcement object.
NFD-RIB converts this command into an equivalent `rib/register` command. The route's *origin* is set to "prefix announcement" (=129).
The response is the same as the `rib/register` command.
Note: There is no `unannounce` command. Use the `rib/unregister` command instead.
### Example
```
Interest
Name /localhop/nfd/rib/announce/params-sha256=607a2bc7653eea18b47e92b84edc58bab5aaa321d4efb39ceeccc6cafbcbb3d1
MustBeFresh
ApplicationParameters
PrefixAnnouncement
InterestSignatureInfo
InterestSignatureValue
```
Assuming the enclosed PrefixAnnouncement is the one shown in the previous example, NFD-RIB should interpret this command same as `rib/register` command with the following ControlParameters:
* Name: `/net/example`
* Origin: 129
* ExpirationPeriod: 3600000; optionally, constrained by the ValidityPeriod of the PrefixAnnouncement