Quality of Service and performance characterization of IPv6 relative to IPv4

II.5 IP HEADER

II.5.1 IPv4 Header

Legend:

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Figure 1: IPv4 Header

Source: http://en.wikipedia.org/wiki/IPv4 (August 13, 2066)

The IPv4 header fields are discussed below:^21(*)

· Version: The first field in the IP header is the version field. This is used to identify which version of IP was used to create the header. It always set to the value 0100 (4 -bit). For IPv4, this has a value of 4 (hence the name IPv4).

· Internet Header Length (IHL): The second field is a 4-bit Internet Header Length (IHL) telling the number of 32-bit words in the header. It provides the length of the header itself.

· Type of Service (ToS): Now known as Differentiated Services Code Point (DSCP) (usually set to 0, but may indicate particular Quality of Service needs from the network, the DSCP defines the way routers should queue packets while they are waiting to be forwarded). This filed has 8 bits were allocated to a Type of Service (ToS) field.

· Total Length: This field defines the entire datagram size, including header and data, in bytes.

· Identification: This field is an identification field and is primarily used for uniquely identifying fragments of an original IP datagram.

· Flags: This filed is used to control whether routers are allowed to fragment a packet, and to indicate the parts of a packet to the receiver.

· Fragment Offset: a byte count from the start of the original sent packet, set by any router which performs IP router fragmentation

· Time to Live (TTL): An 8-bit time to live (TTL) field helps prevent number of hops /links which the packet may be routed over.

· Protocol: This filed indicates the type of transport packet being carried.

· Header Checksum: This field is a hop counter. The 16-bit checksum field is used for error-checking of the header. Each time the data unit traverses through a router, the router decrements this field by 1. When the time-to-live field reaches a value of zero, the data unit is discarded.

· Source Address: The field of IP address of the original sender of the packet.

· Destination Address: The filed of the IP address of the final destination of the packet.

· Options: Additional header fields (called options) may follow the destination address field, but these are not often used, but when used the IP header length will be greater than five 32-bit words to indicate the size of the options field

II.5.2 IPv6 Header

The general header structure of IPv6 has a fixed-length of 40 bytes:

· 16 bytes for source address

· 16 bytes for destination address

· 8 bytes for general header information (Version 4-bits, Traffic Class 1-byte, Flow label 20-bits, Payload Length 2-byte, Next Header 1-byte, Hop limit 1-byte)

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Legend:

Figure 2: IPv6 Header

Source: http://en.wikipedia.org/wiki/Image:IPv6_header_rv1.png (September 02, 2006)

The IPv6 header fields are discussed below:^22(*)

· Version: Internet Protocol Version number (IPv6 is 6). This has a value of 0110 (6-bits), it identifies its version.

· Traffic Class (also known as priority field): Enables a source to identify the desired delivery priority of the packets. Priority values are divided into ranges: traffic where the source provides congestion control and non-congestion control traffic.

· Flow label: Used by a source to label those products for which it requests special handling by the IPv6 router. The flow is uniquely identified by the combination of a source address and a non-zero flow label.

· Payload length: Length of payload (in octets).

· Next header: Identifies the type of header immediately following the IPv6 header.

· Hop limit: 8-bit integer that is decremented by one by each node that forwards the packet. The packet is discarded if the Hop Limit is decremented to zero.

· Source address: 128-bit address of the originator of the packet.

· Destination address: 128-bit address of the intended recipient of the packet.

* ²¹ http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/ip-packet.html (September 15, 2006 )

* ²² http://www.protocols.com/pbook/tcpip2.htm (October 08, 2006)