Side_1_360

• Encapsulating security payload header, see
  [RFC2406].


• Destination options header to be processed
  only by the final destination.


• Upper-layer header (e.g. TCP, UDP, etc.).

IPv6 requires that all links have an MTU greater

than or equal to 1280 octets, although a value of

around 1500 octets is recommended. In order to

find the largest packets that can be transferred,

the path MTU discovery mechanisms as de-

scribed in [RFC1981] can be implemented.

For various reasons IPv6 can be used for tun-

nelling other IP packets. One motivation could

be to ease the transition from IPv4 to IPv6 by

utilising tunnelling during the transition phase.

IPv6 tunnelling is a technique of forwarding a

packet encapsulated within an IPv6 packet. The

forwarding path between the source and the des-

tination of the tunnel packet is called an IPv6

tunnel. For the encapsulated packet, such a tun-

nel can be seen as a virtual link, as depicted in

Figure 6. Tunnels looking like virtual point-to-

multipoint links may also be composed.

A tunnel is unidirectional. Two such tunnels can

be combined to have a bidirectional tunnelling

between the same two end-nodes.

When encapsulating a packet, an IPv6 header

(and additional optional extension headers) is

prepended to the original packet. This can be

nested, that is having several “levels” of tunnels

as shown in Figure 6. The source address is the

tunnel entry point and the destination address is

the tunnel exit point.

Naturally, tunnelling may also be applied for

IPv4.

3 IP Error and

Control Messages

The Internet Control Message Protocol (ICMP)

is seen as a mandatory part of IP. An ICMP

message is transferred in the data field of an IP

packet. However, ICMP is not considered as a

higher-level protocol. This protocol allows

hosts/terminals and routers to exchange informa-

tion for operational and maintenance purposes.

Each ICMP message begins with three fields: a

one octet type field, a one octet code field that

provides further information about the message

type, and a two octet checksum field.

In case the ICMP message was initiated as a

result of an error occurring when handling an IP

packet, the IP packet header of that packet and

first 8 octets of the packet causing the error are

included. Some uses of ICMP are: testing desti-

nation status, reporting on unreachable destina-

tions, flow control (to tell a source to reduce its

sending rate), requesting change of routing (e.g.

sent by a router detecting an inefficient routing

being applied), detecting circular or excessive

long routes (e.g. due to the Timefield in the IP

packet header being decreased to zero), detecting

incorrect IP packet headers, synchronising

clocks and estimating transfer delays (using time

stamping functions in the identified router/host),

and obtaining a network address and subnet

address mask.

IPv6 uses an updated version of the Internet

Control Message Protocol, referred to as ICMPv6

(see [RC2463]). ICMPv6 must be implemented

by every IPv6 node (being an integral part of

IPv6). Similar to above, IPv6 nodes uses ICMPv6

to various tasks, like reporting errors and ping-

ing. Two classes are defined: error messages

Figure 4 Example of changing the routing header information along a packet’s path

Figure 5 Example of fragmenting IPv6 packets

Figure 6 Terms related to
IPv6 tunnelling

S N1 N2 D

Source address = S

Destination address = N1

Routing header:

Header extension length = 4

Segments left = 2

Address [1] = N2

Address [2] = D

Source address = S

Destination address = N2

Routing header:

Header extension length = 4

Segments left = 1

Address [1] = N1

Address [2] = D

Source address = S

Destination address = D

Routing header:

Header extension length = 4

Segments left = 0

Address [1] = N1

Address [2] = N2

Unfragmentable

part

Fragmentable part

Unfragmentable

part

Fragment Last fragment

header

Unfragmentable

part

Fragment Last fragment

header

•

•

tunnel

tunnel end-node

tunnel entry-point

original

packet

inner

tunnel

outer

tunnel

tunnel end-node

tunnel exit-point

tunnel

packet

nested

tunnel

packet

tunnel

header

original

header

nested tunnel

header