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Junxiao Shi, 08/15/2018 06:17 PM

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# Prefix Announcement Protocol
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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.
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## Prefix Announcement object
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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:
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* Name starts with the announced prefix, followed by a fixed `prefix-announcement` generic name component, followed by a version component and a segment component as defined by the NDN Naming Conventions. In this specification revision, segment number is always zero.
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* ContentType is "prefix announcement".
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* 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 will be ignored.
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**Validity period** of a prefix announcement object is given in ExpirationPeriod and ValidityPeriod elements in its Content.
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* 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.
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* 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.
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* When both ValidityPeriod and ExpirationPeriod are present, the most restricted constraint applies.
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Trust model of prefix announcement is to be defined by the application.
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### Example
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```
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Data
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  Name /net/example/prefix-announcement/%FD%01/%00%00
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  MetaInfo
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    ContentType "prefix announcement"
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  Content
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    ValidityPeriod
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      NotBefore 20181030T000000Z
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      NotAfter  20181124T235959Z
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    ExpirationPeriod 3600000
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  SignatureInfo
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  SignatureValue 
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```
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This prefix announcement object announces the prefix `/net/example`.
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It is valid for one hour since receipt, within the date range 30OCT2018 and 24NOV2018.
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## Command Verbs
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### 'announce'
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This command announces a prefix using a prefix announcement object.
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NFD-RIB accepts this command as a [[ndn-cxx:CommandInterest|Command Interest]] with the following syntax:
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```
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/localhost/nfd/rib/announce/<params-sha256>/<command-interest-components>
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/localhop/nfd/rib/announce/<params-sha256>/<command-interest-components>
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```
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The Interest's Parameters element carries the prefix announcement object.
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NFD-RIB converts this command into an equivalent `rib/register` command. Route *origin* is set to "prefix announcement" (number 129).
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Response is the same as `rib/register` command.
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### 'unannounce'
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There is no 'unannounce' command. Instead, use `rib/unregister` command.