Network Working Group S. Deering, Xerox PARC
Request for Comments: 1883 R. Hinden, Ipsilon Networks
Category: Standards Track December 1995
Internet Protocol, Version 6 (IPv6)
Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies version 6 of the Internet Protocol (IPv6),
also sometimes referred to as IP Next Generation or IPng.
Table of Contents
1. IntrodUCtion..................................................3
2. Terminology...................................................4
3. IPv6 Header Format............................................5
4. IPv6 Extension Headers........................................6
4.1 Extension Header Order...................................8
4.2 Options..................................................9
4.3 Hop-by-Hop Options Header...............................11
4.4 Routing Header..........................................13
4.5 Fragment Header.........................................19
4.6 Destination Options Header..............................24
4.7 No Next Header..........................................25
5. Packet Size Issues...........................................26
6. Flow Labels..................................................28
7. Priority.....................................................30
8. Upper-Layer Protocol Issues..................................31
8.1 Upper-Layer Checksums...................................31
8.2 Maximum Packet Lifetime.................................32
8.3 Maximum Upper-Layer Payload Size........................32
Appendix A. Formatting Guidelines for Options...................33
Security Considerations.........................................36
Acknowledgments.................................................36
Authors" Addresses..............................................36
References......................................................37
1. Introduction
IP version 6 (IPv6) is a new version of the Internet Protocol,
designed as a successor to IP version 4 (IPv4) [RFC-791]. The
changes from IPv4 to IPv6 fall primarily into the following
categories:
o EXPanded Addressing Capabilities
IPv6 increases the IP address size from 32 bits to 128 bits, to
support more levels of addressing hierarchy, a much greater
number of addressable nodes, and simpler auto-configuration of
addresses. The scalability of multicast routing is improved by
adding a "scope" field to multicast addresses. And a new type
of address called an "anycast address" is defined, used to send
a packet to any one of a group of nodes.
o Header Format Simplification
Some IPv4 header fields have been dropped or made optional, to
reduce the common-case processing cost of packet handling and
to limit the bandwidth cost of the IPv6 header.
o Improved Support for Extensions and Options
Changes in the way IP header options are encoded allows for
more efficient forwarding, less stringent limits on the length
of options, and greater flexibility for introducing new options
in the future.
o Flow Labeling Capability
A new capability is added to enable the labeling of packets
belonging to particular traffic "flows" for which the sender
requests special handling, such as non-default quality of
service or "real-time" service.
o Authentication and Privacy Capabilities
Extensions to support authentication, data integrity, and
(optional) data confidentiality are specified for IPv6.
This document specifies the basic IPv6 header and the initially-
defined IPv6 extension headers and options. It also discusses packet
size issues, the semantics of flow labels and priority, and the
effects of IPv6 on upper-layer protocols. The format and semantics
of IPv6 addresses are specified separately in [RFC-1884]. The IPv6
version of ICMP, which all IPv6 implementations are required to
include, is specified in [RFC-1885].
2. Terminology
node - a device that implements IPv6.
router - a node that forwards IPv6 packets not explicitly
addressed to itself. [See Note below].
host - any node that is not a router. [See Note below].
upper layer - a protocol layer immediately above IPv6. Examples are
transport protocols such as TCP and UDP, control
protocols such as ICMP, routing protocols such as OSPF,
and internet or lower-layer protocols being "tunneled"
over (i.e., encapsulated in) IPv6 such as IPX,
AppleTalk, or IPv6 itself.
link - a communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer
immediately below IPv6. Examples are Ethernets (simple
or bridged); PPP links; X.25, Frame Relay, or ATM
networks; and internet (or higher) layer "tunnels",
such as tunnels over IPv4 or IPv6 itself.
neighbors - nodes attached to the same link.
interface - a node"s attachment to a link.
address - an IPv6-layer identifier for an interface or a set of
interfaces.
packet - an IPv6 header plus payload.
link MTU - the maximum transmission unit, i.e., maximum packet
size in octets, that can be conveyed in one piece over
a link.
path MTU - the minimum link MTU of all the links in a path between
a source node and a destination node.
Note: it is possible, though unusual, for a device with multiple
interfaces to be configured to forward non-self-destined packets
arriving from some set (fewer than all) of its interfaces, and to
discard non-self-destined packets arriving from its other interfaces.
Such a device must obey the protocol requirements for routers when
receiving packets from, and interacting with neighbors over, the
former (forwarding) interfaces. It must obey the protocol
requirements for hosts when receiving packets from, and interacting
with neighbors over, the latter (non-forwarding) interfaces.
3. IPv6 Header Format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version Prio. Flow Label
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload Length Next Header Hop Limit
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Source Address +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Destination Address +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version 4-bit Internet Protocol version number = 6.
Prio. 4-bit priority value. See section 7.
Flow Label 24-bit flow label. See section 6.
Payload Length 16-bit unsigned integer. Length of payload,
i.e., the rest of the packet following the
IPv6 header, in octets. If zero, indicates that
the payload length is carried in a Jumbo Payload
hop-by-hop option.
Next Header 8-bit selector. Identifies the type of header
immediately following the IPv6 header. Uses
the same values as the IPv4 Protocol field
[RFC-1700 et seq.].
Hop Limit 8-bit unsigned integer. Decremented by 1 by
each node that forwards the packet. The packet
is discarded if Hop Limit is decremented to
zero.
Source Address 128-bit address of the originator of the
packet. See [RFC-1884].
Destination Address 128-bit address of the intended recipient
of the packet (possibly not the ultimate
recipient, if a Routing header is present).
See [RFC-1884] and section 4.4.
4. IPv6 Extension Headers
In IPv6, optional internet-layer information is encoded in separate
headers that may be placed between the IPv6 header and the upper-
layer header in a packet. There are a small number of such extension
headers, each identified by a distinct Next Header value. As
illustrated in these examples, an IPv6 packet may carry zero, one, or
more extension headers, each identified by the Next Header field of
the preceding header:
+---------------+------------------------
IPv6 header TCP header + data

Next Header =
TCP
+---------------+------------------------
+---------------+----------------+------------------------
IPv6 header Routing header TCP header + data

Next Header = Next Header =
Routing TCP
+---------------+----------------+------------------------
+---------------+----------------+-----------------+-----------------
IPv6 header Routing header Fragment header fragment of TCP
header + data
Next Header = Next Header = Next Header =
Routing Fragment TCP
+---------------+----------------+-----------------+-----------------
With one exception, extension headers are not examined or processed
by any node along a packet"s delivery path, until the packet reaches
the node (or each of the set of nodes, in the case of multicast)
identified in the Destination Address field of the IPv6 header.
There, normal demultiplexing on the Next Header field of the IPv6
header invokes the module to process the first extension header, or
the upper-layer header if no extension header is present. The
contents and semantics of each extension header determine whether or
not to proceed to the next header. Therefore, extension headers must
be processed strictly in the order they appear in the packet; a
receiver must not, for example, scan through a packet looking for a
particular kind of extension header and process that header prior to
processing all preceding ones.
The exception referred to in the preceding paragraph is the Hop-by-
Hop Options header, which carries information that must be examined
and processed by every node along a packet"s delivery path, including
the source and destination nodes. The Hop-by-Hop Options header,
when present, must immediately follow the IPv6 header. Its presence
is indicated by the value zero in the Next Header field of the IPv6
header.
If, as a result of processing a header, a node is required to proceed
to the next header but the Next Header value in the current header is
unrecognized by the node, it should discard the packet and send an
ICMP Parameter Problem message to the source of the packet, with an
ICMP Code value of 2 ("unrecognized Next Header type encountered")
and the ICMP Pointer field containing the offset of the unrecognized
value within the original packet. The same action should be taken if
a node encounters a Next Header value of zero in any header other
than an IPv6 header.
Each extension header is an integer multiple of 8 octets long, in
order to retain 8-octet alignment for subsequent headers. Multi-
octet fields within each extension header are aligned on their
natural boundaries, i.e., fields of width n octets are placed at an
integer multiple of n octets from the start of the header, for n = 1,
2, 4, or 8.
A full implementation of IPv6 includes implementation of the
following extension headers:
Hop-by-Hop Options
Routing (Type 0)
Fragment
Destination Options
Authentication
Encapsulating Security Payload
The first four are specified in this document; the last two are
specified in [RFC-1826] and [RFC-1827], respectively.
4.1 Extension Header Order
When more than one extension header is used in the same packet, it is
recommended that those headers appear in the following order:
IPv6 header
Hop-by-Hop Options header
Destination Options header (note 1)
Routing header
Fragment header
Authentication header (note 2)
Encapsulating Security Payload header (note 2)
Destination Options header (note 3)
upper-layer header
note 1: for options to be processed by the first destination
that appears in the IPv6 Destination Address field
plus subsequent destinations listed in the Routing
header.
note 2: additional recommendations regarding the relative
order of the Authentication and Encapsulating
Security Payload headers are given in [RFC-1827].
note 3: for options to be processed only by the final
destination of the packet.
Each extension header should occur at most once, except for the
Destination Options header which should occur at most twice (once
before a Routing header and once before the upper-layer header).
If the upper-layer header is another IPv6 header (in the case of IPv6
being tunneled over or encapsulated in IPv6), it may be followed by
its own extensions headers, which are separately subject to the same
ordering recommendations.
If and when other extension headers are defined, their ordering
constraints relative to the above listed headers must be specified.
IPv6 nodes must accept and attempt to process extension headers in
any order and occurring any number of times in the same packet,
except for the Hop-by-Hop Options header which is restricted to
appear immediately after an IPv6 header only. Nonetheless, it is
strongly advised that sources of IPv6 packets adhere to the above
recommended order until and unless subsequent specifications revise
that recommendation.
4.2 Options
Two of the currently-defined extension headers -- the Hop-by-Hop
Options header and the Destination Options header -- carry a variable
number of type-length-value (TLV) encoded "options", of the following
format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type Opt Data Len Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type 8-bit identifier of the type of option.
Opt Data Len 8-bit unsigned integer. Length of the Option
Data field of this option, in octets.
Option Data Variable-length field. Option-Type-specific
data.
The sequence of options within a header must be processed strictly in
the order they appear in the header; a receiver must not, for
example, scan through the header looking for a particular kind of
option and process that option prior to processing all preceding
ones.
The Option Type identifiers are internally encoded such that their
highest-order two bits specify the action that must be taken if the
processing IPv6 node does not recognize the Option Type:
00 - skip over this option and continue processing the header.
01 - discard the packet.
10 - discard the packet and, regardless of whether or not the
packets"s Destination Address was a multicast address, send
an ICMP Parameter Problem, Code 2, message to the packet"s
Source Address, pointing to the unrecognized Option Type.
11 - discard the packet and, only if the packet"s Destination
Address was not a multicast address, send an ICMP Parameter
Problem, Code 2, message to the packet"s Source Address,
pointing to the unrecognized Option Type.
The third-highest-order bit of the Option Type specifies whether or
not the Option Data of that option can change en-route to the
packet"s final destination. When an Authentication header is present
in the packet, for any option whose data may change en-route, its
entire Option Data field must be treated as zero-valued octets when
computing or verifying the packet"s authenticating value.
0 - Option Data does not change en-route
1 - Option Data may change en-route
Individual options may have specific alignment requirements, to
ensure that multi-octet values within Option Data fields fall on
natural boundaries. The alignment requirement of an option is
specified using the notation xn+y, meaning the Option Type must
appear at an integer multiple of x octets from the start of the
header, plus y octets. For example:
2n means any 2-octet offset from the start of the header.
8n+2 means any 8-octet offset from the start of the header,
plus 2 octets.
There are two padding options which are used when necessary to align
subsequent options and to pad out the containing header to a multiple
of 8 octets in length. These padding options must be recognized by
all IPv6 implementations:
Pad1 option (alignment requirement: none)
+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+
NOTE! the format of the Pad1 option is a special case -- it does
not have length and value fields.
The Pad1 option is used to insert one octet of padding into the
Options area of a header. If more than one octet of padding is
required, the PadN option, described next, should be used,
rather than multiple Pad1 options.
PadN option (alignment requirement: none)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
1 Opt Data Len Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
The PadN option is used to insert two or more octets of padding
into the Options area of a header. For N octets of padding,
the Opt Data Len field contains the value N-2, and the Option
Data consists of N-2 zero-valued octets.
Appendix A contains formatting guidelines for designing new options.
4.3 Hop-by-Hop Options Header
The Hop-by-Hop Options header is used to carry optional information
that must be examined by every node along a packet"s delivery path.
The Hop-by-Hop Options header is identified by a Next Header value of
0 in the IPv6 header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

. .
. Options .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header 8-bit selector. Identifies the type of header
immediately following the Hop-by-Hop Options
header. Uses the same values as the IPv4
Protocol field [RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of the
Hop-by-Hop Options header in 8-octet units,
not including the first 8 octets.
Options Variable-length field, of length such that the
complete Hop-by-Hop Options header is an integer
multiple of 8 octets long. Contains one or
more TLV-encoded options, as described in
section 4.2.
In addition to the Pad1 and PadN options specified in section 4.2,
the following hop-by-hop option is defined:
Jumbo Payload option (alignment requirement: 4n + 2)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
194 Opt Data Len=4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Jumbo Payload Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Jumbo Payload option is used to send IPv6 packets with
payloads longer than 65,535 octets. The Jumbo Payload Length is
the length of the packet in octets, excluding the IPv6 header but
including the Hop-by-Hop Options header; it must be greater than
65,535. If a packet is received with a Jumbo Payload option
containing a Jumbo Payload Length less than or equal to 65,535,
an ICMP Parameter Problem message, Code 0, should be sent to the
packet"s source, pointing to the high-order octet of the invalid
Jumbo Payload Length field.
The Payload Length field in the IPv6 header must be set to zero
in every packet that carries the Jumbo Payload option. If a
packet is received with a valid Jumbo Payload option present and
a non-zero IPv6 Payload Length field, an ICMP Parameter Problem
message, Code 0, should be sent to the packet"s source, pointing
to the Option Type field of the Jumbo Payload option.
The Jumbo Payload option must not be used in a packet that
carries a Fragment header. If a Fragment header is encountered
in a packet that contains a valid Jumbo Payload option, an ICMP
Parameter Problem message, Code 0, should be sent to the packet"s
source, pointing to the first octet of the Fragment header.
An implementation that does not support the Jumbo Payload option
cannot have interfaces to links whose link MTU is greater than
65,575 (40 octets of IPv6 header plus 65,535 octets of payload).
4.4 Routing Header
The Routing header is used by an IPv6 source to list one or more
intermediate nodes to be "visited" on the way to a packet"s
destination. This function is very similar to IPv4"s Source Route
options. The Routing header is identified by a Next Header value of
43 in the immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len Routing Type Segments Left
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

. .
. type-specific data .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header 8-bit selector. Identifies the type of header
immediately following the Routing header.
Uses the same values as the IPv4 Protocol field
[RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of the
Routing header in 8-octet units, not including
the first 8 octets.
Routing Type 8-bit identifier of a particular Routing
header variant.
Segments Left 8-bit unsigned integer. Number of route
segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited
before reaching the final destination.
type-specific data Variable-length field, of format determined by
the Routing Type, and of length such that the
complete Routing header is an integer multiple
of 8 octets long.
If, while processing a received packet, a node encounters a Routing
header with an unrecognized Routing Type value, the required behavior
of the node depends on the value of the Segments Left field, as
follows:
If Segments Left is zero, the node must ignore the Routing header
and proceed to process the next header in the packet, whose type
is identified by the Next Header field in the Routing header.
If Segments Left is non-zero, the node must discard the packet and
send an ICMP Parameter Problem, Code 0, message to the packet"s
Source Address, pointing to the unrecognized Routing Type.
The Type 0 Routing header has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len Routing Type=0 Segments Left
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved Strict/Loose Bit Map
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Address[1] +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Address[2] +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . .
. . .
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Address[n] +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header 8-bit selector. Identifies the type of header
immediately following the Routing header.
Uses the same values as the IPv4 Protocol field
[RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of the
Routing header in 8-octet units, not including
the first 8 octets. For the Type 0 Routing
header, Hdr Ext Len is equal to two times the
number of addresses in the header, and must
be an even number less than or equal to 46.
Routing Type 0.
Segments Left 8-bit unsigned integer. Number of route
segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited
before reaching the final destination.
Maximum legal value = 23.
Reserved 8-bit reserved field. Initialized to zero for
transmission; ignored on reception.
Strict/Loose Bit Map
24-bit bit-map, numbered 0 to 23, left-to-right.
Indicates, for each segment of the route, whether
or not the next destination address must be a
neighbor of the preceding address: 1 means strict
(must be a neighbor), 0 means loose (need not be
a neighbor).
Address[1..n] Vector of 128-bit addresses, numbered 1 to n.
Multicast addresses must not appear in a Routing header of Type 0, or
in the IPv6 Destination Address field of a packet carrying a Routing
header of Type 0.
If bit number 0 of the Strict/Loose Bit Map has value 1, the
Destination Address field of the IPv6 header in the original packet
must identify a neighbor of the originating node. If bit number 0
has value 0, the originator may use any legal, non-multicast address
as the initial Destination Address.
Bits numbered greater than n, where n is the number of addresses in
the Routing header, must be set to 0 by the originator and ignored by
receivers.
A Routing header is not examined or processed until it reaches the
node identified in the Destination Address field of the IPv6 header.
In that node, dispatching on the Next Header field of the immediately
preceding header causes the Routing header module to be invoked,
which, in the case of Routing Type 0, performs the following
algorithm:
if Segments Left = 0 {
proceed to process the next header in the packet, whose type is
identified by the Next Header field in the Routing header
}
else if Hdr Ext Len is odd or greater than 46 {
send an ICMP Parameter Problem, Code 0, message to the Source
Address, pointing to the Hdr Ext Len field, and discard the
packet
}
else {
compute n, the number of addresses in the Routing header, by
dividing Hdr Ext Len by 2
if Segments Left is greater than n {
send an ICMP Parameter Problem, Code 0, message to the Source
Address, pointing to the Segments Left field, and discard the
packet
}
else {
decrement Segments Left by 1;
compute i, the index of the next address to be visited in
the address vector, by suBTracting Segments Left from n
if Address [i] or the IPv6 Destination Address is multicast {
discard the packet
}
else {
swap the IPv6 Destination Address and Address[i]
if bit i of the Strict/Loose Bit map has value 1 and the
new Destination Address is not the address of a neighbor
of this node {
send an ICMP Destination Unreachable -- Not a Neighbor
message to the Source Address and discard the packet
}
else if the IPv6 Hop Limit is less than or equal to 1 {
send an ICMP Time Exceeded -- Hop Limit Exceeded in
Transit message to the Source Address and discard the
packet
}
else {
decrement the Hop Limit by 1
resubmit the packet to the IPv6 module for transmission
to the new destination
}
}
}
}
As an example of the effects of the above algorithm, consider the
case of a source node S sending a packet to destination node D, using
a Routing header to cause the packet to be routed via intermediate
nodes I1, I2, and I3. The values of the relevant IPv6 header and
Routing header fields on each segment of the delivery path would be
as follows:
As the packet travels from S to I1:
Source Address = S Hdr Ext Len = 6
Destination Address = I1 Segments Left = 3
Address[1] = I2
(if bit 0 of the Bit Map is 1, Address[2] = I3
S and I1 must be neighbors; Address[3] = D
this is checked by S)
As the packet travels from I1 to I2:
Source Address = S Hdr Ext Len = 6
Destination Address = I2 Segments Left = 2
Address[1] = I1
(if bit 1 of the Bit Map is 1, Address[2] = I3
I1 and I2 must be neighbors; Address[3] = D
this is checked by I1)
As the packet travels from I2 to I3:
Source Address = S Hdr Ext Len = 6
Destination Address = I3 Segments Left = 1
Address[1] = I1
(if bit 2 of the Bit Map is 1, Address[2] = I2
I2 and I3 must be neighbors; Address[3] = D
this is checked by I2)
As the packet travels from I3 to D:
Source Address = S Hdr Ext Len = 6
Destination Address = D Segments Left = 0
Address[1] = I1
(if bit 3 of the Bit Map is 1, Address[2] = I2
I3 and D must be neighbors; Address[3] = I3
this is checked by I3)
4.5 Fragment Header
The Fragment header is used by an IPv6 source to send packets larger
than would fit in the path MTU to their destinations. (Note: unlike
IPv4, fragmentation in IPv6 is performed only by source nodes, not by
routers along a packet"s delivery path -- see section 5.) The
Fragment header is identified by a Next Header value of 44 in the
immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Reserved Fragment Offset ResM
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Identification
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header 8-bit selector. Identifies the initial header
type of the Fragmentable Part of the original
packet (defined below). Uses the same values
as the IPv4 Protocol field [RFC-1700 et seq.].
Reserved 8-bit reserved field. Initialized to zero for
transmission; ignored on reception.
Fragment Offset 13-bit unsigned integer. The offset, in 8-octet
units, of the data following this header,
relative to the start of the Fragmentable Part
of the original packet.
Res 2-bit reserved field. Initialized to zero for
transmission; ignored on reception.
M flag 1 = more fragments; 0 = last fragment.
Identification 32 bits. See description below.
In order to send a packet that is too large to fit in the MTU of the
path to its destination, a source node may divide the packet into
fragments and send each fragment as a separate packet, to be
reassembled at the receiver.
For every packet that is to be fragmented, the source node generates
an Identification value. The Identification must be different than
that of any other fragmented packet sent recently* with the same
Source Address and Destination Address. If a Routing header is
present, the Destination Address of concern is that of the final
destination.
* "recently" means within the maximum likely lifetime of a packet,
including transit time from source to destination and time spent
awaiting reassembly with other fragments of the same packet.
However, it is not required that a source node know the maximum
packet lifetime. Rather, it is assumed that the requirement can
be met by maintaining the Identification value as a simple, 32-
bit, "wrap-around" counter, incremented each time a packet must
be fragmented. It is an implementation choice whether to
maintain a single counter for the node or multiple counters,
e.g., one for each of the node"s possible source addresses, or
one for each active (source address, destination address)
combination.
The initial, large, unfragmented packet is referred to as the
"original packet", and it is considered to consist of two parts, as
illustrated:
original packet:
+------------------+----------------------//-----------------------+
Unfragmentable Fragmentable
Part Part
+------------------+----------------------//-----------------------+
The Unfragmentable Part consists of the IPv6 header plus any
extension headers that must be processed by nodes en route to the
destination, that is, all headers up to and including the Routing
header if present, else the Hop-by-Hop Options header if present,
else no extension headers.
The Fragmentable Part consists of the rest of the packet, that is,
any extension headers that need be processed only by the final
destination node(s), plus the upper-layer header and data.
The Fragmentable Part of the original packet is divided into
fragments, each, except possibly the last ("rightmost") one, being an
integer multiple of 8 octets long. The fragments are transmitted in
separate "fragment packets" as illustrated:
original packet:
+------------------+--------------+--------------+--//--+----------+
Unfragmentable first second last
Part fragment fragment .... fragment
+------------------+--------------+--------------+--//--+----------+
fragment packets:
+------------------+--------+--------------+
Unfragmentable Fragment first
Part Header fragment
+------------------+--------+--------------+
+------------------+--------+--------------+
Unfragmentable Fragment second
Part Header fragment
+------------------+--------+--------------+
o
o
o
+------------------+--------+----------+
Unfragmentable Fragment last
Part Header fragment
+------------------+--------+----------+
Each fragment packet is composed of:
(1) The Unfragmentable Part of the original packet, with the
Payload Length of the original IPv6 header changed to contain
the length of this fragment packet only (excluding the length
of the IPv6 header itself), and the Next Header field of the
last header of the Unfragmentable Part changed to 44.
(2) A Fragment header containing:
The Next Header value that identifies the first header of
the Fragmentable Part of the original packet.
A Fragment Offset containing the offset of the fragment,
in 8-octet units, relative to the start of the
Fragmentable Part of the original packet. The Fragment
Offset of the first ("leftmost") fragment is 0.
An M flag value of 0 if the fragment is the last
("rightmost") one, else an M flag value of 1.
The Identification value generated for the original
packet.
(3) The fragment itself.
The lengths of the fragments must be chosen such that the resulting
fragment packets fit within the MTU of the path to the packets"
destination(s).
At the destination, fragment packets are reassembled into their
original, unfragmented form, as illustrated:
reassembled original packet:
+------------------+----------------------//------------------------+
Unfragmentable Fragmentable
Part Part
+------------------+----------------------//------------------------+
The following rules govern reassembly:
An original packet is reassembled only from fragment packets that
have the same Source Address, Destination Address, and Fragment
Identification.
The Unfragmentable Part of the reassembled packet consists of all
headers up to, but not including, the Fragment header of the first
fragment packet (that is, the packet whose Fragment Offset is
zero), with the following two changes:
The Next Header field of the last header of the Unfragmentable
Part is obtained from the Next Header field of the first
fragment"s Fragment header.
The Payload Length of the reassembled packet is computed from
the length of the Unfragmentable Part and the length and offset
of the last fragment. For example, a formula for computing the
Payload Length of the reassembled original packet is:
PL.orig = PL.first - FL.first - 8 + (8 * FO.last) + FL.last
where
PL.orig = Payload Length field of reassembled packet.
PL.first = Payload Length field of first fragment packet.
FL.first = length of fragment following Fragment header of
first fragment packet.
FO.last = Fragment Offset field of Fragment header of
last fragment packet.
FL.last = length of fragment following Fragment header of
last fragment packet.
The Fragmentable Part of the reassembled packet is constructed
from the fragments following the Fragment headers in each of the
fragment packets. The length of each fragment is computed by
subtracting from the packet"s Payload Length the length of the
headers between the IPv6 header and fragment itself; its relative
position in Fragmentable Part is computed from its Fragment Offset
value.
The Fragment header is not present in the final, reassembled
packet.
The following error conditions may arise when reassembling fragmented
packets:
If insufficient fragments are received to complete reassembly of a
packet within 60 seconds of the reception of the first-arriving
fragment of that packet, reassembly of that packet must be
abandoned and all the fragments that have been received for that
packet must be discarded. If the first fragment (i.e., the one
with a Fragment Offset of zero) has been received, an ICMP Time
Exceeded -- Fragment Reassembly Time Exceeded message should be
sent to the source of that fragment.
If the length of a fragment, as derived from the fragment packet"s
Payload Length field, is not a multiple of 8 octets and the M flag
of that fragment is 1, then that fragment must be discarded and an
ICMP Parameter Problem, Code 0, message should be sent to the
source of the fragment, pointing to the Payload Length field of
the fragment packet.
If the length and offset of a fragment are such that the Payload
Length of the packet reassembled from that fragment would exceed
65,535 octets, then that fragment must be discarded and an ICMP
Parameter Problem, Code 0, message should be sent to the source of
the fragment, pointing to the Fragment Offset field of the
fragment packet.
The following conditions are not expected to occur, but are not
considered errors if they do:
The number and content of the headers preceding the Fragment
header of different fragments of the same original packet may
differ. Whatever headers are present, preceding the Fragment
header in each fragment packet, are processed when the packets
arrive, prior to queueing the fragments for reassembly. Only
those headers in the Offset zero fragment packet are retained in
the reassembled packet.
The Next Header values in the Fragment headers of different
fragments of the same original packet may differ. Only the value
from the Offset zero fragment packet is used for reassembly.
4.6 Destination Options Header
The Destination Options header is used to carry optional information
that need be examined only by a packet"s destination node(s). The
Destination Options header is identified by a Next Header value of 60
in the immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

. .
. Options .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header 8-bit selector. Identifies the type of header
immediately following the Destination Options
header. Uses the same values as the IPv4
Protocol field [RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of the
Destination Options header in 8-octet units,
not including the first 8 octets.
Options Variable-length field, of length such that the
complete Destination Options header is an
integer multiple of 8 octets long. Contains
one or more TLV-encoded options, as described
in section 4.2.
The only destination options defined in this document are the Pad1
and PadN options specified in section 4.2.
Note that there are two possible ways to encode optional destination
information in an IPv6 packet: either as an option in the Destination
Options header, or as a separate extension header. The Fragment
header and the Authentication header are examples of the latter
approach. Which approach can be used depends on what action is
desired of a destination node that does not understand the optional
information:
o if the desired action is for the destination node to discard
the packet and, only if the packet"s Destination Address is not
a multicast address, send an ICMP Unrecognized Type message to
the packet"s Source Address, then the information may be
encoded either as a separate header or as an option in the
Destination Options header whose Option Type has the value 11
in its highest-order two bits. The choice may depend on such
factors as which takes fewer octets, or which yields better
alignment or more efficient parsing.
o if any other action is desired, the information must be encoded
as an option in the Destination Options header whose Option
Type has the value 00, 01, or 10 in its highest-order two bits,
specifying the desired action (see section 4.2).
4.7 No Next Header
The value 59 in the Next Header field of an IPv6 header or any
extension header indicates that there is nothing following that
header. If the Payload Length field of the IPv6 header indicates the
presence of octets past the end of a header whose Next Header field
contains 59, those octets must be ignored, and passed on unchanged if
the packet is forwarded.
5. Packet Size Issues
IPv6 requires that every link in the internet have an MTU of 576
octets or greater. On any link that cannot convey a 576-octet packet
in one piece, link-specific fragmentation and reassembly must be
provided at a layer below IPv6.
From each link to which a node is directly attached, the node must
be able to accept packets as large as that link"s MTU. Links that
have a configurable MTU (for example, PPP links [RFC-1661]) must be
configured to have an MTU of at least 576 octets; it is recommended
that a larger MTU be configured, to accommodate possible
encapsulations (i.e., tunneling) without incurring fragmentation.
It is strongly recommended that IPv6 nodes implement Path MTU
Discovery [RFC-1191], in order to discover and take advantage of
paths with MTU greater than 576 octets. However, a minimal IPv6
implementation (e.g., in a boot ROM) may simply restrict itself to
sending packets no larger than 576 octets, and omit implementation of
Path MTU Discovery.
In order to send a packet larger than a path"s MTU, a node may use
the IPv6 Fragment header to fragment the packet at the source and
have it reassembled at the destination(s). However, the use of such
fragmentation is discouraged in any application that is able to
adjust its packets to fit the measured path MTU (i.e., down to 576
octets).
A node must be able to accept a fragmented packet that, after
reassembly, is as large as 1500 octets, including the IPv6 header. A
node is permitted to accept fragmented packets that reassemble to
more than 1500 octets. However, a node must not send fragments that
reassemble to a size greater than 1500 octets unless it has explicit
knowledge that the destination(s) can reassemble a packet of that
size.
In response to an IPv6 packet that is sent to an IPv4 destination
(i.e., a packet that undergoes translation from IPv6 to IPv4), the
originating IPv6 node may receive an ICMP Packet Too Big message
reporting a Next-Hop MTU less than 576. In that case, the IPv6 node
is not required to reduce the size of subsequent packets to less than
576, but must include a Fragment header in those packets so that the
IPv6-to-IPv4 translating router can obtain a suitable Identification
value to use in resulting IPv4 fragments. Note that this means the
payload may have to be reduced to 528 octets (576 minus 40 for the
IPv6 header and 8 for the Fragment header), and smaller still if
additional extension headers are used.
Note: Path MTU Discovery must be performed even in cases where a
host "thinks" a destination is attached to the same link as
itself.
Note: Unlike IPv4, it is unnecessary in IPv6 to set a "Don"t
Fragment" flag in the packet header in order to perform Path MTU
Discovery; that is an implicit attribute of every IPv6 packet.
Also, those parts of the RFC-1191 procedures that involve use of
a table of MTU "plateaus" do not apply to IPv6, because the IPv6
version of the "Datagram Too Big" message always identifies the
exact MTU to be used.
6. Flow Labels
The 24-bit Flow Label field in the IPv6 header may be used by a
source to label those packets for which it requests special handling
by the IPv6 routers, such as non-default quality of service or
"real-time" service. This ASPect of IPv6 is, at the time of writing,
still experimental and subject to change as the requirements for flow
support in the Internet become clearer. Hosts or routers that do not
support the functions of the Flow Label field are required to set the
field to zero when originating a packet, pass the field on unchanged
when forwarding a packet, and ignore the field when receiving a
packet.
A flow is a sequence of packets sent from a particular source to a
particular (unicast or multicast) destination for which the source
desires special handling by the intervening routers. The nature of
that special handling might be conveyed to the routers by a control
protocol, such as a resource reservation protocol, or by information
within the flow"s packets themselves, e.g., in a hop-by-hop option.
The details of such control protocols or options are beyond the scope
of this document.
There may be multiple active flows from a source to a destination, as
well as traffic that is not associated with any flow. A flow is
uniquely identified by the combination of a source address and a
non-zero flow label. Packets that do not belong to a flow carry a
flow label of zero.
A flow label is assigned to a flow by the flow"s source node. New
flow labels must be chosen (pseudo-)randomly and uniformly from the
range 1 to FFFFFF hex. The purpose of the random allocation is to
make any set of bits within the Flow Label field suitable for use as
a hash key by routers, for looking up the state associated with the
flow.
All packets belonging to the same flow must be sent with the same
source address, destination address, priority, and flow label. If
any of those packets includes a Hop-by-Hop Options header, then they
all must be originated with the same Hop-by-Hop Options header
contents (excluding the Next Header field of the Hop-by-Hop Options
header). If any of those packets includes a Routing header, then
they all must be originated with the same contents in all extension
headers up to and including the Routing header (excluding the Next
Header field in the Routing header). The routers or destinations are
permitted, but not required, to verify that these conditions are
satisfied. If a violation is detected, it should be reported to the
source by an ICMP Parameter Problem message, Code 0, pointing to the
high-order octet of the Flow Label field (i.e., offset 1 within the
IPv6 packet).
Routers are free to "opportunistically" set up flow-handling state
for any flow, even when no explicit flow establishment information
has been provided to them via a control protocol, a hop-by-hop
option, or other means. For example, upon receiving a packet from a
particular source with an unknown, non-zero flow label, a router may
process its IPv6 header and any necessary extension headers as if the
flow label were zero. That processing would include determining the
next-hop interface, and possibly other actions, such as updating a
hop-by-hop option, advancing the pointer and addresses in a Routing
header, or deciding on how to queue the packet based on its Priority
field. The router may then choose to "remember" the results of those
processing steps and cache that information, using the source address
plus the flow label as the cache key. Subsequent packets with the
same source address and flow label may then be handled by referring
to the cached information rather than examining all those fields
that, according to the requirements of the previous paragraph, can be
assumed unchanged from the first packet seen in the flow.
Cached flow-handling state that is set up opportunistically, as
discussed in the preceding paragraph, must be discarded no more than
6 seconds after it is established, regardless of whether or not
packets of the same flow continue to arrive. If another packet with
the same source address and flow label arrives after the cached state
has been discarded, the packet undergoes full, normal processing (as
if its flow label were zero), which may result in the re-creation of
cached flow state for that flow.
The lifetime of flow-handling state that is set up explicitly, for
example by a control protocol or a hop-by-hop option, must be
specified as part of the specification of the explicit set-up
mechanism; it may exceed 6 seconds.
A source must not re-use a flow label for a new flow within the
lifetime of any flow-handling state that might have been established
for the prior use of that flow label. Since flow-handling state with
a lifetime of 6 seconds may be established opportunistically for any
flow, the minimum interval between the last packet of one flow and
the first packet of a new flow using the same flow label is 6
seconds. Flow labels used for explicitly set-up flows with longer
flow-state lifetimes must remain unused for those longer lifetimes
before being re-used for new flows.
When a node stops and restarts (e.g., as a result of a "crash"), it
must be careful not to use a flow label that it might have used for
an earlier flow whose lifetime may not have expired yet. This may be
accomplished by recording flow label usage on stable storage so that
it can be remembered across crashes, or by refraining from using any
flow labels until the maximum lifetime of any possible previously
established flows has expired (at least 6 seconds; more if explicit
flow set-up mechanisms with longer lifetimes might have been used).
If the minimum time for rebooting the node is known (often more than
6 seconds), that time can be deducted from the necessary waiting
period before starting to allocate flow labels.
There is no requirement that all, or even most, packets belong to
flows, i.e., carry non-zero flow labels. This observation is placed
here to remind protocol designers and implementors not to assume
otherwise. For example, it would be unwise to design a router whose
performance would be adequate only if most packets belonged to flows,
or to design a header compression scheme that only worked on packets
that belonged to flows.
7. Priority
The 4-bit Priority field in the IPv6 header enables a source to
identify the desired delivery priority of its packets, relative to
other packets from the same source. The Priority values are divided
into two ranges: Values 0 through 7 are used to specify the priority
of traffic for which the source is providing congestion control,
i.e., traffic that "backs off" in response to congestion, such as TCP
traffic. Values 8 through 15 are used to specify the priority of
traffic that does not back off in response to congestion, e.g.,
"real-time" packets being sent at a constant rate.
For congestion-controlled traffic, the following Priority values are
recommended for particular application categories:
0 - uncharacterized traffic
1 - "filler" traffic (e.g., netnews)
2 - unattended data transfer (e.g., email)
3 - (reserved)
4 - attended bulk transfer (e.g., FTP, NFS)
5 - (reserved)
6 - interactive traffic (e.g., telnet, X)
7 - internet control traffic (e.g., routing protocols, SNMP)
For non-congestion-controlled traffic, the lowest Priority value (8)
should be used for those packets that the sender is most willing to
have discarded under conditions of congestion (e.g., high-fidelity
video traffic), and the highest value (15) should be used for those
packets that the sender is least willing to have discarded (e.g.,
low-fidelity audio traffic). There is no relative ordering implied
between the congestion-controlled priorities and the non-congestion-
controlled priorities.
8. Upper-Layer Protocol Issues
8.1 Upper-Layer Checksums
Any transport or other upper-layer protocol that includes the
addresses from the IP header in its checksum computation must be
modified for use over IPv6, to include the 128-bit IPv6 addresses
instead of 32-bit IPv4 addresses. In particular, the following
illustration shows the TCP and UDP "pseudo-header" for IPv6:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Source Address +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

+ Destination Address +

+ +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
zero Next Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o If the packet contains a Routing header, the Destination
Address used in the pseudo-header is that of the final
destination. At the originating node, that address will be in
the last element of the Routing header; at the recipient(s),
that address will be in the Destination Address field of the
IPv6 header.
o The Next Header value in the pseudo-header identifies the
upper-layer protocol (e.g., 6 for TCP, or 17 for UDP). It will
differ from the Next Header value in the IPv6 header if there
are extension headers between the IPv6 header and the upper-
layer header.
o The Payload Length used in the pseudo-header is the length of
the upper-layer packet, including the upper-layer header. It
will be less than the Payload Length in the IPv6 header (or in
the Jumbo Payload option) if there are extension headers
between the IPv6 header and the upper-layer header.
o Unlike IPv4, when UDP packets are originated by an IPv6 node,
the UDP checksum is not optional. That is, whenever
originating a UDP packet, an IPv6 node must compute a UDP
checksum over the packet and the pseudo-header, and, if that
computation yields a result of zero, it must be changed to hex
FFFF for placement in the UDP header. IPv6 receivers must
discard UDP packets containing a zero checksum, and should log
the error.
The IPv6 version of ICMP [RFC-1885] includes the above pseudo-header
in its checksum computation; this is a change from the IPv4 version
of ICMP, which does not include a pseudo-header in its checksum. The
reason for the change is to protect ICMP from misdelivery or
corruption of those fields of the IPv6 header on which it depends,
which, unlike IPv4, are not covered by an internet-layer checksum.
The Next Header field in the pseudo-header for ICMP contains the
value 58, which identifies the IPv6 version of ICMP.
8.2 Maximum Packet Lifetime
Unlike IPv4, IPv6 nodes are not required to enforce maximum packet
lifetime. That is the reason the IPv4 "Time to Live" field was
renamed "Hop Limit" in IPv6. In practice, very few, if any, IPv4
implementations conform to the requirement that they limit packet
lifetime, so this is not a change in practice. Any upper-layer
protocol that relies on the internet layer (whether IPv4 or IPv6) to
limit packet lifetime ought to be upgraded to provide its own
mechanisms for detecting and discarding obsolete packets.
8.3 Maximum Upper-Layer Payload Size
When computing the maximum payload size available for upper-layer
data, an upper-layer protocol must take into account the larger size
of the IPv6 header relative to the IPv4 header. For example, in
IPv4, TCP"s MSS option is computed as the maximum packet size (a
default value or a value learned through Path MTU Discovery) minus 40
octets (20 octets for the minimum-length IPv4 header and 20 octets
for the minimum-length TCP header). When using TCP over IPv6, the
MSS must be computed as the maximum packet size minus 60 octets,
because the minimum-length IPv6 header (i.e., an IPv6 header with no
extension headers) is 20 octets longer than a minimum-length IPv4
header.
Appendix A. Formatting Guidelines for Options
This appendix gives some advice on how to lay out the fields when
designing new options to be used in the Hop-by-Hop Options header or
the Destination Options header, as described in section 4.2. These
guidelines are based on the following assumptions:
o One desirable feature is that any multi-octet fields within the
Option Data area of an option be aligned on their natural
boundaries, i.e., fields of width n octets should be placed at
an integer multiple of n octets from the start of the Hop-by-
Hop or Destination Options header, for n = 1, 2, 4, or 8.
o Another desirable feature is that the Hop-by-Hop or Destination
Options header take up as little space as possible, subject to
the requirement that the header be an integer multiple of 8
octets long.
o It may be assumed that, when either of the option-bearing
headers are present, they carry a very small number of options,
usually only one.
These assumptions suggest the following approach to laying out the
fields of an option: order the fields from smallest to largest, with
no interior padding, then derive the alignment requirement for the
entire option based on the alignment requirement of the largest field
(up to a maximum alignment of 8 octets). This approach is
illustrated in the following examples:
Example 1
If an option X required two data fields, one of length 8 octets and
one of length 4 octets, it would be laid out as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type=X Opt Data Len=12
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ 8-octet field +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Its alignment requirement is 8n+2, to ensure that the 8-octet field
starts at a multiple-of-8 offset from the start of the enclosing
header. A complete Hop-by-Hop or Destination Options header
containing this one option would look as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len=1 Option Type=X Opt Data Len=12
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ 8-octet field +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example 2
If an option Y required three data fields, one of length 4 octets,
one of length 2 octets, and one of length 1 octet, it would be laid
out as follows:
+-+-+-+-+-+-+-+-+
Option Type=Y
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Opt Data Len=7 1-octet field 2-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Its alignment requirement is 4n+3, to ensure that the 4-octet field
starts at a multiple-of-4 offset from the start of the enclosing
header. A complete Hop-by-Hop or Destination Options header
containing this one option would look as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len=1 Pad1 Option=0 Option Type=Y
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Opt Data Len=7 1-octet field 2-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PadN Option=1 Opt Data Len=2 0 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example 3
A Hop-by-Hop or Destination Options header containing both options X
and Y from Examples 1 and 2 would have one of the two following
formats, depending on which option appeared first:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len=3 Option Type=X Opt Data Len=12
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ 8-octet field +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PadN Option=1 Opt Data Len=1 0 Option Type=Y
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Opt Data Len=7 1-octet field 2-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PadN Option=1 Opt Data Len=2 0 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Hdr Ext Len=3 Pad1 Option=0 Option Type=Y
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Opt Data Len=7 1-octet field 2-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PadN Option=1 Opt Data Len=4 0 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 0 Option Type=X Opt Data Len=12
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4-octet field
+-+-+-+-+-+-+-+-+-+-