Side_1_360

A number of options can be included, contained

in the Optionsfield. These options are for exam-

ple used for testing and fault detection and local-

isation. Each option consists of one octet code

field, one octet length field and a set of data

octets. Options can for example be used for

recording the route (each router along the path

adds its address), specifying the route a packet

should follow (in strict or loose sense) and for

recording the time a packet is handled by a

router (time stamp inserted). The options are

commonly given according to a type-length-

value format, see Box B.

The Paddingfield represents bytes containing

zeros that may be used to ensure that the packet

header is a multiple of 32 bits. The Data field

contains the higher level information, like trans-

port protocol and user data.

2.2 IP version 6

In recognition of a growing need for upgrading

capabilities of IP, work was initiated to devise a

“new” protocol. The major goals of this protocol

were to (ref. [Tane96]):

• support more hosts, even with inefficient
  address space allocation;


• reduce the size of routing tables;


• simplify the protocol, to allow routers to pro-
  cess packets faster;


• provide better security (authentication and
  privacy) than IPv4;


• pay more attention to type of service, particu-
  lar for real time data;


• aid multicasting by allowing scopes to be
  specified;


• make it possible for a host to roam without
  changing its address;


• allow the protocol to evolve in the future;


• permit the old and new protocols to coexist
  for years.

This resulted in version 6 of IP, IPv6, as de-

scribed in [RFC2460]. The main motivations for

specifying IPv6 compared to version 4 were:

• More addresses and addressing capabilities.
  IPv6 has 128 bit addressing (compared to 32
  bit for IPv4). Addressing hierarchy and other
  grouping of addresses can also be extended.


• Simplified header format. Some of the fields
  in the IPv4 header have been made optional.


• Better support for extensions. Further options
  can be introduced.


• Flow labelling. By introducing the flow field,
  packets belonging to a traffic flow can be
  explicitly identified, e.g. when they request
  special handling.


• Improved security capabilities. Extensions are
  added to support authentication, integrity and
  confidentiality.

In addition to unicast and multicast addresses,

IPv6 also supports anycast. Anycast is like mul-

ticast, except that rather than trying to deliver

the packet to all destinations specified, it is only

delivered to one of them, like the “nearest” one.

This could be used for co-operating file servers

or any other service where one out of a set of

servers may be selected.

Unlike IPv4, only the source may fragment

packets when IPv6 is used. In case a router then

receives a packet that is too large, it discards the

packet and returns an ICMP packet, see Section

Figure 3 Header format
of IPv6

Type Length Value

Box B Type Length Value-Formatting

A type-length-value (TLV) format is commonly applied in protocols for each of

the fields/attributes. This is applied when a number of optional attributes can

be included in a packet. As the name suggests, three fields are then given as

depicted in Figure Box B-1.

Figure Box B-1 TLV formatting

The first field gives the type (of optional) attribute, that is identifying the attribute.

Then, the next field tells how long the attribute is, e.g. measured in number of

octets. The last field contains the value of the attribute itself. Naturally, when

only a fixed length is allowed for an attribute, the Length field could be omitted.

In case an attribute is mandatory and its position given, the Type field is fre-

quently omitted.

This way of formatting allows for flexibility when including attributes in each

of the packets, as well as potential further enhancements by defining new

attributes.

Version Flow label

Payload length Hop limit

Source address

Destination address

0 4 12 16 24 31

Traffic class

Next header