Quality of Service and performance characterization of IPv6 relative to IPv4

II.5.3 Header Simplified

The IP header in IPv6 has been streamlined and defined to be of a fixed length (40 bytes). In designing the IPv6 Header, fields from the IPv4 Header have been removed, renamed (but still providing the same type of functionality) or moved to the new optional IPv6 Extension Headers.

II.5.3.1 Changes in the general header structure:

Five fields have been removed from the IPv4 header:

1. Header Length (fixed length header doesn't need it)

2. Identification

3. Flags

4. Fragment Offset

5. Header Checksum (checked at the transport layer)

Three fields have been renamed and modified from the IPv4 header:

1. Traffic Class (type of service in IPv4)

2. Next Header (protocol type in IPv4)

3. Hop Limit (Time to live in IPv4)

One field has been added:

1. Flow Label.^23(*)

As shown above in the IPv4 Header (Figure 1) and the IPv6 Header format (Figure 2), here are changes in the general headers:

· The «IHL» or header length field is no longer needed since the IPv6 Header is now a fixed length.

· The IPv4 «Type of Service» is equivalent to the IPv6 «Traffic class» field.

· The «Total Length» field has been replaced with the «Payload Length» field.

· Since IPv6 only allows for fragmentation to be performed by the IPv6 source and destination nodes, and not individual routers, the IPv4 segment control fields (Identification, Flags, and Fragment Offset fields) have been moved to similar fields within the Fragment Extension Header.

· The functionality provided by the «Time to Live» field has been replaced with the «Hop Limit» field.

· The «Protocol» field has been replaced with the «Next Header Type» field.

· The «Header Checksum» field was removed, which has the main advantage of not having each relay spend time processing the checksum. This, however, also introduces the risk of undetected errors, which is seen as minimal due to checksums being used in most of the encapsulating procedures.

· The «Options» field is no longer part of the header as it was in IPv4. Options are specified in the optional IPv6 Extension Headers. The removal of the options field from the header provides for more efficient routing; only the information that is needed by a router needs to be processed.

* ²³ http://searchnetworking.techtarget.com/originalContent/0,289142,sid7_gci870277,00.html (October 08, 2006)