Describing IPv6 Addressing Architecture

This topic provides an introduction to the IPv6 frame format.

The IP version 4 (IPv4) header contains 12 basic header fields, followed by an options field and a data portion (usually the transport layer segment). The basic IPv4 header has a fixed size of 20 octets. The variable-length options field increases the size of the total IP header. IPv6 contains 5 of the 12 IPv4 basic header fields. The IPv6 header does not require the other seven fields.

Routers handle fragmentation in IPv4, which causes a variety of processing issues. IPv6 routers no longer perform fragmentation. Instead, a discovery process is used to determine the optimum maximum transmission unit (MTU) to use during a given session.

In the discovery process, the source IPv6 device attempts to send a packet at the size that is specified by the upper IP layers, for example, the transport and application layers.

If the device receives an "ICMP packet too big" message, it retransmits the MTU discover packet with a smaller MTU and repeats the process until it gets a response that the discover packet arrived intact. Then it sets the MTU for the session.

The "ICMP packet too big" message contains the proper MTU size for the pathway. Each source device needs to track the MTU size for each session. Generally, the tracking is done by creating a cache that is based on the destination address; however, it can also be done by using the flow label. If source-based routing is performed, the tracking of the MTU size can be done by using the source address.

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The discovery process is beneficial because, as routing pathways change, a new MTU might be more appropriate. When a device receives an "ICMP packet too big" message, it decreases its MTU size if the Internet Control Message Protocol (ICMP) message contains a recommended MTU that is less than the current MTU of the device.

A device performs an MTU discovery every 5 minutes to see whether the MTU has increased along the pathway. Application and transport layers for IPv6 accept MTU reduction notifications from the IPv6 layer.

If they do not accept the notifications, IPv6 has a mechanism to fragment packets that are too large; however, upper layers are encouraged to avoid sending messages that require fragmentation.

Link-layer technologies already perform checksum and error control. Because link-layer technologies are relatively reliable, an IP header checksum is considered to be redundant. Without the IP header checksum, the upper-layer optional checksums, such as User Datagram Protocol (UDP), are now mandatory.

© 2006 Cisco Systems, Inc. Implementing IPv6 8-13

IPv4 Header

IPv6 Header

Source Address Destination Address

Source Address

Source Address Destination Address

I I - Field name kept from IPv4 to IPv6

- Fields not kept in IPv6

- Name and position changed in IPv6 ■ - New field in IPv6

Source Address

I I - Field name kept from IPv4 to IPv6

- Fields not kept in IPv6

- Name and position changed in IPv6 ■ - New field in IPv6

Destination Address

The IPv6 header has 40 octets in contrast to the 20 octets in IPv4. IPv6 has a smaller number of fields, and the header is 64-bit aligned to enable fast processing by current processors. Address fields are four times larger than in IPv4.

The IPv6 header contains these fields:

■ Version: A 4-bit field, the same as in IPv4. It contains the number 6 instead of the number 4 for IPv4.

■ Traffic Class: An 8-bit field similar to the type of service (ToS) field in IPv4. It tags the packet with a traffic class that it uses in differentiated services (DiffServ). These functionalities are the same for IPv6 and IPv4.

■ Flow Label: A completely new 20-bit field. It tags a flow for the IP packets. It can be used for multilayer switching techniques and faster packet-switching performance.

■ Payload Length: Similar to the Total Length field of IPv4.

■ Next Header: The value of this field determines the type of information that follows the basic IPv6 header. It can be a transport-layer packet, such as TCP or UDP, or it can be an extension header. The next header field is similar to the Protocol field of IPv4.

■ Hop Limit: This field specifies the maximum number of hops that an IP packet can traverse. Each hop or router decreases this field by one (similar to the Time to Live [TTL] field in IPv4). Because there is no checksum in the IPv6 header, the router can decrease the field without recomputing the checksum. On IPv4 routers the recomputation costs processing time.

■ Source Address: This field has 16 octets or 128 bits. It identifies the source of the packet.

■ Destination Address: This field has 16 octets or 128 bits. It identifies the destination of the

■ Extension Headers: The extension headers, if any, and the data portion of the packet follow the eight fields. The number of extension headers is not fixed, so the total length of the extension header chain is variable.

packet.

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© 2006 Cisco Systems, Inc.

Simpler and more efficient header means:

• IPv6 has extension headers.

• It handles the options more efficiently.

• It enables faster forwarding rate and end nodes processing.

There are many types of extension headers. When multiple extension headers are used in the same packet, the order of the headers should be as follows:

1. IPv6 header: This header is the basic header described in the previous figure.

2. Hop-by-hop options header: When this header is used for the router alert (Resource Reservation Protocol [RSVP] and Multicast Listener Discovery version 1 [MLDv1]) and the jumbogram, this header (value = 0) is processed by all hops in the path of a packet. When present, the hop-by-hop options header always follows immediately after the basic IPv6 packet header.

3. Destination options header (when the routing header is used): This header (value = 60) can follow any hop-by-hop options header, in which case the destination options header is processed at the final destination and also at each visited address specified by a routing header. Alternatively, the destination options header can follow any Encapsulating Security Payload (ESP) header, in which case the destination options header is processed only at the final destination. For example, mobile IP uses this header.

4. Routing header: This header (value = 43) is used for source routing and mobile IPv6.

5. Fragment header: This header is used when a source must fragment a packet that is larger than the MTU for the path between itself and a destination device. The fragment header is used in each fragmented packet.

6. Authentication header and Encapsulating Security Payload header: The authentication header (value = 51) and the ESP header (value = 50) are used within IPsec to provide authentication, integrity, and confidentiality of a packet. These headers are identical for both IPv4 and IPv6.

7. Upper-layer header: The upper-layer (transport) headers are the typical headers used inside a packet to transport the data. The two main transport protocols are TCP (value = 6) and UDP (value = 17).

© 2006 Cisco Systems, Inc.

Implementing IPv6

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Continue reading here: Pv6 Address Types

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