NFD

# NDNLP TLV packet format

NDN packet format is shifting from CCNB to [NDN-TLV](http://named-data.net/doc/ndn-tlv/).

In this format, every packet, and every field within a packet, is represented by a tuple of Type-Length-Value.

[NDNLP: A Link Protocol for NDN](http://named-data.net/publications/techreports/trlinkprotocol/) provides two services to enable NDN communication directly on Ethernet:

* Fragmentation and Reassembly

* Acknowledgement and Retransmission

NDNLP is initially designed to work with CCNx, and therefore adopts a CCNB-based packet format.

Although it's possible to enclose NDN-TLV packets in CCNB-based NDNLP packets, this is awkward.

Therefore, a TLV-based packet format should be defined for NDNLP.

This document defines the TLV-based packet format for NDNLP.

In operation reality, acknowledgement and retransmission is rarely used. This version only defines packet type and fields necessary for fragmentation and reassembly.

Basic [protocol operations](http://named-data.net/publications/techreports/trlinkprotocol/) are unchanged.

Fragmentation and reassembly operations are [extended to multicast](https://github.com/NDN-Routing/NDNFD/blob/80c3e009106d7e16db85ee8afd647ba85b5b7c2d/docs/NDNLP.md).

## Link Data Packet

A Link Data packet carries a NDN-TLV packet or a fragment of it as the payload.

It has the following format:

	NdnlpData ::= NDNLP-DATA-TYPE TLV-LENGTH

		            NdnlpSequence

		            NdnlpFragIndex?

		            NdnlpFragCount?

		            NdnlpPayload

These elements MUST appear in the given order.

All elements are required unless otherwise stated.

### sequence number

	NdnlpSequence ::= NDNLP-SEQUENCE-TYPE TLV-LENGTH

		                nonNegativeInteger

A unique sequence number for each packet.

### fragment index

	NdnlpFragIndex ::= NDNLP-FRAG-INDEX-TYPE TLV-LENGTH

		                 nonNegativeInteger

The 0-based index of current fragment within the network layer packet.

Optional. Default to '0' if not present.

### fragment count

	NdnlpFragCount ::= NDNLP-FRAG-COUNT-TYPE TLV-LENGTH

		                 nonNegativeInteger

Total number of fragments of the network layer packet.

Optional. Default to '1' if not present.

### payload

	NdnlpPayload ::= NDNLP-PAYLOAD-TYPE TLV-LENGTH

		               Interest or Data

The payload.

## TLV-TYPE assignments

NDNLP-TLV temporarily adopts its [original DTAGs](http://redmine.ccnx.org/issues/100710) as TLV-TYPE numbers.

Type                                        | Assigned value    | Assigned value (hex)

------------------------------------------- | ----------------- | --------------------

NdnlpData                                   | 20653248          |               

NdnlpSequence                               | 20653249          |               

NdnlpFragIndex                              | 20653251          |               

NdnlpFragCount                              | 20653252          |               

NdnlpFragCount                              | 20653253          |               

`NdnlpData` should be assigned a 1-byte TLV-TYPE number that does not conflict with NDN-TLV, so that it can be efficiently used on the same connection with NDN-TLV.

There is no uniqueness requirements on the TLV-TYPE numbers for other elements.