NFD » NDN Specifications

# Realtime Data Retrieval (RDR) protocol

[Real-Time Data Retrieval in Named Data Networking](https://named-data.net/publications/hoticn18realtime-retrieval/) published in HotICN'2018 introduces Real-time Data Retrieval (RDR) protocol for discovering latest version number of real-time streaming data.

This document defines a recommended packet format of RDR protocol.

RDR is an application level protocol, and its usage could be application specific.

Therefore, it is RECOMMENDED that each application defines its own instance of RDR protocol based on this document, rather than directly referencing this specification.

## Metadata Packet

In RDR protocol, the *metadata packet* is a Data packet that carries the latest version number of a data stream.

### Name Suffix

The name of a metadata packet MUST end with these three name components:

1. The keyword component `32=metadata`, indicating this Data packet is a metadata packet under this specification.

2. A version number component, whose format is defined in Naming Conventions.

3. A segment number component, whose format is defined in Naming Conventions. Normally the metadata is not segmented and thus the segment number is zero, but this component is included for future extension.

### Name Prefix

The name prefix of a metadata packet before these three name components SHOULD same as the name prefix of versioned data before the version number component. For example, an application can have the following namespace:

```

# data stream

/net/example/www/frontpage/%FD%97%47%1E%6C/%00%00

/net/example/www/frontpage/%FD%97%47%1E%6C/%00%01

/net/example/www/frontpage/%FD%97%47%1E%6D/%00%00

/net/example/www/frontpage/%FD%97%47%1E%6D/%00%01

# metadata packet

/net/example/www/frontpage/32=metadata/%FD%7E%5F%45%08/%00%00

```

### FreshnessPeriod

FreshnessPeriod of a metadata packet MUST NOT be zero.

See the RDR paper for guidelines on an appropriate value for this field.

### Content Payload

The Content of a metadata packet MUST be a sequence of TLV elements, at least one of which is a *Name* element.

The first *Name* element indicates the versioned name of the latest version in the data stream.

For example:

```

<Content>

  <ArbitraryElement>this is ignored</ArbitraryElement>

  <Name>/net/example/www/frontpage/%FD%97%47%1E%6D</Name>

  <Name>this is ignored</Name>

</Content>

```

## RDR Discovery Interest

In RDR protocol, consumers express RDR Discovery Interests to retrieve metadata packets, in order to discover latest version number in the stream.

An RDR Discovery Interest MUST:

* end its name with `32=metadata`;

* set CanBePrefix;

* set MustBeFresh.