Type-Length-Value (TLV) Encoding

Each NDN packet is encoded in a Type-Length-Value (TLV) format. NDN Interest and Data packets are distinguished by the type value in the first and outmost TLV0.

An NDN packet is mainly a collection of TLVs inside TLV0. Some TLVs may contain sub-TLVs, and each sub-TLV may also be further nested. A guiding design principle is to keep the order of TLVis deterministic, and keep the level of nesting as small as possible to minimize both processing overhead and chances for errors.

Note that NDN packet format does not have a fixed packet header nor does it encode a protocol version number. Instead the design uses the TLV format to provide the flexibility of adding new types and phasing out old types as the protocol evolves over time. The absence of a fixed header makes it possible to support packets of very small sizes efficiently, without the header overhead. There is also no packet fragmentation support at network level. Whenever needed, NDN packets may be fragmented and reassembled hop-by-hop. [1]

[1]“Packet Fragmentation in NDN: Why NDN Uses Hop-By-Hop Fragmentation (NDN Memo)” by A. Afanasyev, J. Shi, L. Wang, B. Zhang, and L. Zhang., NDN Memo, Technical Report NDN-0032

Variable Size Encoding for type (T) and length (L)

(Both the text below and that in TLV encoding section are adopted from an earlier packet specification draft by Mark Stapp)

To minimize the overhead during early deployment and to allow flexibility of future protocol extensions to meet unforeseeable needs, both type (T) and length (L) take a variable size format. For implementation simplicity, both type and length take the same encoding format.

We define a variable-length encoding for numbers in NDN as follows:

VAR-NUMBER := BYTE+

The first octet of the number either carries the actual numeric value, or signals that a multi-octet encoding is present, as defined below:

  • if the first octet is < 253, the number is encoded in that octet;
  • if the first octet == 253, the number is encoded in the following 2 octets, in net byte-order;
  • if the first octet == 254, the number is encoded in the following 4 octets, in net byte-order;
  • if the first octet == 255, the number is encoded in the following 8 octets, in net byte-order.

One-octet value:

 0 1 2 3 4 5 6 7
+---------------+
| < 253 = VALUE |
+---------------+

Two-octet value:

                     1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+---------------+---------------+---------------+
|      253      |  VALUE (MSB)     VALUE (LSB)  |
+---------------+---------------+---------------+

Four-octet value:

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+----------------+--------------+
|      254      |  VALUE (MSB)                                  /
+---------------+---------------+----------------+--------------+
|  VALUE (LSB)  |
+---------------+

Eight-octet value:

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+----------------+--------------+
|      255      |  VALUE (MSB)                                  /
+---------------+                                               +
|                                                               /
+               +---------------+----------------+--------------+
|  VALUE (LSB)  |
+---------------+

TLV Encoding

TLV encoding for NDN packets is defined as follows:

NDN-TLV := TLV-TYPE TLV-LENGTH TLV-VALUE?
TLV-TYPE := VAR-NUMBER
TLV-LENGTH := VAR-NUMBER
TLV-VALUE := BYTE+

TLV-TYPE SHOULD be unique at all nested levels. The TLV Type number space and initial assignments listed in Section Type value assignment of this document.

The TLV-LENGTH value represents number of bytes that TLV-VALUE uses. It does not include number of bytes that TLV-TYPE and TLV-LENGTH fields themselves occupy. In particular, empty payload TLV will carry TLV-LENGTH equal to 0.

This encoding offers a reasonable balance between compactness and flexibility. Most common, standardized Type codes will be allocated from a small-integer number-space, and these common types will be able to use the compact, single-byte encoding.

Non Negative Integer Encoding

A number of TLV elements in NDN packet format take a non-negative integer as their value, with the following definition:

nonNegativeInteger ::= BYTE+

Length value of the TLV element MUST be either 1, 2, 4, or 8. Depending on the length value, a nonNegativeInteger is encoded as follows:

  • if the length is 1 (i.e. the value length is 1 octet), the nonNegativeInteger is encoded in one octet;
  • if the length is 2 (= value length is 2 octets), the nonNegativeInteger is encoded in 2 octets, in net byte-order;
  • if the length is 4 (= value length is 4 octets), the nonNegativeInteger is encoded in 4 octets, in net byte-order;
  • if the length is 8 (= value length is 8 octets), the nonNegativeInteger is encoded in 8 octets, in net byte-order.

The following shows a few examples of TLVs that have nonNegativeInteger as their value component in hexadecimal format (where TT represents TLV-TYPE, followed by the TLV-LENGTH, then TLV-VALUE):

0     => TT0100
1     => TT0101
255   => TT01FF
256   => TT020100
65535 => TT02FFFF
65536 => TT0400010000