Practical TCP/IP and Ethernet Networking- P24 potx

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IPv6 includes an improved option mechanism over IPv4. Instead of placing extra options
bytes within the main header, IPv6 options are placed in separate extension headers that
are located between the IPv6 header and the transport layer header in a packet.
Most IPv6 extension headers are not examined or processed by routers along a packet’s
path until it arrives at its final destination. This leads to a major improvement in router
performance for packets containing options. In IPv4 the presence of any options requires
the router to examine all options.
IPv6 extension headers can be of arbitrary length and the total amount of options
carried in a packet is not limited to 40 bytes as with IPv4. They are also not carried within
the main header, as with IPv4, but are only used when needed, and are carried behind the
main header. This feature plus the manner in which they are processed, permits IPv6
options to be used for functions, which were not practical in IPv4. Good examples of this
are the IPv6 authentication and security encapsulation options.
In order to improve the performance when handling subsequent option headers and the
transport protocol which follows, IPv6 options are always an integer multiple of 8 octets
long, in order to retain this alignment for subsequent headers.
The IPv6 extension headers currently defined are:
• Routing header (for extended routing, similar to the IPv4 loose source route).
• Fragment header (for fragmentation and reassembly).
• Authentication header (for integrity and authentication).
• Encrypted security payload (for confidentiality).
• Hop-by-hop options header (for special options that require hop-by-hop
processing).
• Destination options header (for optional information to be examined by the
destination node).

Figure 6.19
Carrying IPv6 extension headers
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IPv6 addresses are 128 bits long and are identifiers for individual interfaces or sets of
interfaces. IPv6 Addresses of all types are assigned to interfaces (i.e. network interface
Cards) and NOT to nodes i.e. hosts. Since each interface belongs to a single node, any of
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that node’s interface’s unicast addresses may be used as an identifier for the node. A
single interface may be assigned multiple IPv6 addresses of any type.
There are three types of IPv6 addresses. These are unicast, anycast, and multicast.
• Unicast addresses identify a single interface
• Anycast addresses identify a set of interfaces such that a packet sent to an
anycast address will be delivered to one member of the set
• Multicast addresses identify a group of interfaces, such that a packet sent to
a multicast address is delivered to all of the interfaces in the group. There are
no broadcast addresses in IPv6, their function being superseded by multicast
addresses

The IPv6 address is four times the length of IPv4 addresses (128 vs 32). This is 4
billion times 4 billion (2
96
) times the size of the IPv4 address space (2
32
). This works out
to be 340 282 366 920 938 463 463 374 607 431 768 211 456. Theoretically this is
approximately 665 570 793 348 866 943 898 599 addresses per square meter of the
surface of the planet Earth (assuming the Earth surface is 511 263 971 197 990 square

meters). In more practical terms, considering that the creation of addressing hierarchies,
which reduces the efficiency of the usage of the address space, IPv6 is still expected to
support between 8×10
17
to 2×10
33
nodes. Even the most pessimistic estimate provides
around 1500 addresses per square meter of the surface of planet Earth.
The leading bits in the address indicate the specific type of IPv6 address. The variable-
length field comprising these leading bits is called the format prefix (FP). The current
allocation of these prefixes is as follows:

Figure 6.20
IPv6 address ranges
This allocation supports the direct allocation of global unicast addresses, local use
addresses, and multicast addresses. Space is reserved for NSAP addresses, IPX addresses,
and geographic-based unicast addresses. The remainder of the address space is
unassigned for future use. This can be used for expansion of existing use (e.g., additional
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provider addresses, etc) or new uses (e.g., separate locators and identifiers). Note that
anycast addresses are not shown here because they are allocated out of the unicast address
space.
Approximately fifteen per cent of the address space is initially allocated. The remaining
85% is reserved for future use.
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There are several forms of unicast address assignment in IPv6. These are:
• Global unicast addresses
• Unspecified addresses

• Loopback addresses
• IPv4-based addresses
• Site local addresses
• Link local addresses
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These addresses are used for global communication. They are similar in function to IPv4
addresses under CIDR. Their format is:


Figure 6.21

Address format: Global unicast address

The first 3 bits identify the address as a global unicast address.
The next, 13-bit, field (TLA) identifies the top level aggregator. This number will be
used to identify the relevant Internet ‘exchange point’, or long-haul (‘backbone’)
provider. These numbers (8192 of them) will be issued by IANA, to be further distributed
via the three regional registries (ARIN, RIPE and APNIC), who could possibly further
delegate the allocation of sub-ranges to national or regional registries such as the French
NIC managed by INRIA for French networks.
The third, 32-bit, field (NLA) identifies the next level aggregator. This will probably
be structured by long-haul providers to identify a second-tier provider by means of the
first n bits, and to identify a subscriber to that second-tier provider by means of the
remaining 32–n bits.
The fourth, 16-bit, field is the SLA or site local aggregator. This will be allocated to a
link within a site, and is not associated with a registry or service provider. In other words,
it will remain unchanged despite a change of service provider. Its closest equivalent in
IPv4 would be the ‘NetID’.
The last field is the 64-bit interface ID. This is the equivalent of the ‘HostID’ in IPv4.
However, instead of an arbitrary number it would consist of the hardware address of the

interface, e.g. the Ethernet MAC address.
• All identifiers will be 64 bits long even if there are only a few devices on the
network
• Where possible these identifiers will be based on the IEEE EUI-64 format

Existing 48-bit MAC addresses are converted to EUI-64 format by splitting them in the
middle and inserting the string FF-FE in between the two halves.
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Figure 6.22
Converting a 48-bit MAC address to EUI-64 format
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This can be written as 0:0:0:0:0:0:0:0, or simply ‘::’ (double colon). This address can be
used as a source address by a station that has not yet been configured with an IP address.
It can never be used as a destination address. This is similar to 0.0.0.0 in IPv4.
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The loopback address 0:0:0:0:0:0:0:1 can be used by a node to send a datagram to itself.
It is similar to the 127.0.0.1 of IPv4.
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It is possible to construct an IPv6 address out of an existing IPv4 address. This is done by
prepending 96 zero bits to a 32-bit IPv4 address. The result is written as
0:0:0:0:0:0:192.100.100.3, or simply ::192.100.100.3.
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Site local addresses are partially equivalent of the IPv4 private addresses. The site local
addressing prefix 111 1110 11 has been reserved for this purpose. A typical site local
address will consist of this prefix, a set of 38 zeros, a subnet ID, and the interface
identifier. Site local addresses cannot be routed in the Internet, but only between two

stations on a single site.
The last 80 bits of a site local address are identical to the last 80 bits of a global unicast
address. This allows for easy renumbering where a site has to be connected to the
Internet.

Figure 6.23

Site-local unicast addresses
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Stations that are not yet configured with either a provider-based address or a site local
address may use link local addresses. Theses are composed of the link local prefix, 1111
1110 10, a set of 0s, and an interface identifier. These addresses can only be used by
stations connected to the same local network and packets addressed in this way cannot
traverse a router.
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Figure 6.24
Link-local unicast addresses
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An IPv6 anycast address is an address that is assigned to more than one interface
(typically belonging to different nodes), with the property that a packet sent to an anycast
address is routed to the ‘nearest’ interface having that address, according to the routing
protocols’ measure of distance. Anycast addresses, when used as part of a route
sequence, permits a node to select which of several internet service providers it wants to
carry its traffic. This capability is sometimes called ‘source selected policies’. This
would be implemented by configuring anycast addresses to identify the set of routers
belonging to Internet service providers (e.g. one anycast address per Internet service
provider). These anycast addresses can be used as intermediate addresses in an IPv6

routing header, to cause a packet to be delivered via a particular provider or sequence of
providers.
Other possible uses of anycast addresses are to identify the set of routers attached to a
particular subnet, or the set of routers providing entry into a particular routing domain.
Anycast addresses are allocated from the unicast address space, using any of the defined
unicast address formats. Thus, anycast addresses are syntactically indistinguishable from
unicast addresses. When a unicast address is assigned to more than one interface, thus
turning it into an anycast address, the nodes to which the address is assigned must be
explicitly configured to know that it is an anycast address.
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An IPv6 multicast address is an identifier for a group of interfaces. An interface may
belong to any number of multicast groups. Multicast addresses have the following format:

Figure 6.25
Address format: IPv6 multicast
The 11111111 (0xFF) at the start of the address identify the address as being a multicast
address.
• FLGS. Four bits are reserved for flags. The first 3 bits are currently
reserved, and set to 0. The last bit (the one on the right) is called T for
‘transient’. T = 0 indicates a permanently assigned (‘well-known’) multicast
address, assigned by IANA, while T = 1 indicates a non-permanently
assigned (‘transient’) multicast address