As per Relevance of the word destination, we have this rfc below:
Network Working Group S. Deering, Xerox
Request for Comments: 1883 R. Hinden, Ipsilon
Category: Standards Track December 1995
Internet Protocol, Version 6 (IPv6)
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
This document specifies version 6 of the Internet Protocol (IPv6),
also sometimes referred to as IP Next Generation or IPng
Deering & Hinden Standards Track [Page 1]
RFC 1883 IPv6 Specification December 1995
Table of
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
Deering & Hinden Standards Track [Page 2]
RFC 1883 IPv6 Specification December 1995
1.
IP version 6 (IPv6) is a new version of the Internet Protocol
designed as a successor to IP version 4 (IPv4) [RFC-791].
changes from IPv4 to IPv6 fall primarily into the
categories
o Expanded Addressing
IPv6 increases the IP address size from 32 bits to 128 bits,
support more levels of addressing hierarchy, a much
number of addressable nodes, and simpler auto-configuration
addresses. The scalability of multicast routing is improved
adding a "scope" field to multicast addresses. And a new
of address called an "anycast address" is defined, used to
a packet to any one of a group of nodes
o Header Format
Some IPv4 header fields have been dropped or made optional,
reduce the common-case processing cost of packet handling
to limit the bandwidth cost of the IPv6 header
o Improved Support for Extensions and
Changes in the way IP header options are encoded allows
more efficient forwarding, less stringent limits on the
of options, and greater flexibility for introducing new
in the future
o Flow Labeling
A new capability is added to enable the labeling of
belonging to particular traffic "flows" for which the
requests special handling, such as non-default quality
service or "real-time" service
o Authentication and Privacy
Extensions to support authentication, data integrity,
(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
size issues, the semantics of flow labels and priority, and
effects of IPv6 on upper-layer protocols. The format and
of IPv6 addresses are specified separately in [RFC-1884]. The IPv
version of ICMP, which all IPv6 implementations are required
include, is specified in [RFC-1885].
Deering & Hinden Standards Track [Page 3]
RFC 1883 IPv6 Specification December 1995
2.
node - a device that implements IPv6.
router - a node that forwards IPv6 packets not
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
transport protocols such as TCP and UDP,
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
communicate at the link layer, i.e., the
immediately below IPv6. Examples are Ethernets (
or bridged); PPP links; X.25, Frame Relay, or
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
interfaces
packet - an IPv6 header plus payload
link MTU - the maximum transmission unit, i.e., maximum
size in octets, that can be conveyed in one piece
a link
path MTU - the minimum link MTU of all the links in a path
a source node and a destination node
Note: it is possible, though unusual, for a device with
interfaces to be configured to forward non-self-destined
arriving from some set (fewer than all) of its interfaces, and
discard non-self-destined packets arriving from its other interfaces
Such a device must obey the protocol requirements for routers
receiving packets from, and interacting with neighbors over,
former (forwarding) interfaces. It must obey the
requirements for hosts when receiving packets from, and
with neighbors over, the latter (non-forwarding) interfaces
Deering & Hinden Standards Track [Page 4]
RFC 1883 IPv6 Specification December 1995
3. IPv6 Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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
IPv6 header, in octets. If zero, indicates
the payload length is carried in a Jumbo
hop-by-hop option
Next Header 8-bit selector. Identifies the type of
immediately following the IPv6 header.
the same values as the IPv4 Protocol
[RFC-1700 et seq.].
Hop Limit 8-bit unsigned integer. Decremented by 1
each node that forwards the packet. The
is discarded if Hop Limit is decremented
zero
Source Address 128-bit address of the originator of
packet. See [RFC-1884].
Deering & Hinden Standards Track [Page 5]
RFC 1883 IPv6 Specification December 1995
Destination Address 128-bit address of the intended
of the packet (possibly not the
recipient, if a Routing header is present).
See [RFC-1884] and section 4.4.
4. IPv6 Extension
In IPv6, optional internet-layer information is encoded in
headers that may be placed between the IPv6 header and the upper
layer header in a packet. There are a small number of such
headers, each identified by a distinct Next Header value.
illustrated in these examples, an IPv6 packet may carry zero, one,
more extension headers, each identified by the Next Header field
the preceding header
+---------------+------------------------
| IPv6 header | TCP header +
| |
| Next Header = |
| TCP |
+---------------+------------------------
+---------------+----------------+------------------------
| IPv6 header | Routing header | TCP header +
| | |
| Next Header = | Next Header = |
| Routing | TCP |
+---------------+----------------+------------------------
+---------------+----------------+-----------------+-----------------
| IPv6 header | Routing header | Fragment header | fragment of
| | | | header +
| Next Header = | Next Header = | Next Header = |
| Routing | Fragment | TCP |
+---------------+----------------+-----------------+-----------------
With one exception, extension headers are not examined or
by any node along a packet's delivery path, until the packet
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 IPv
header invokes the module to process the first extension header,
the upper-layer header if no extension header is present.
contents and semantics of each extension header determine whether
Deering & Hinden Standards Track [Page 6]
RFC 1883 IPv6 Specification December 1995
not to proceed to the next header. Therefore, extension headers
be processed strictly in the order they appear in the packet;
receiver must not, for example, scan through a packet looking for
particular kind of extension header and process that header prior
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
and processed by every node along a packet's delivery path,
the source and destination nodes. The Hop-by-Hop Options header
when present, must immediately follow the IPv6 header. Its
is indicated by the value zero in the Next Header field of the IPv
header
If, as a result of processing a header, a node is required to
to the next header but the Next Header value in the current header
unrecognized by the node, it should discard the packet and send
ICMP Parameter Problem message to the source of the packet, with
ICMP Code value of 2 ("unrecognized Next Header type encountered")
and the ICMP Pointer field containing the offset of the
value within the original packet. The same action should be taken
a node encounters a Next Header value of zero in any header
than an IPv6 header
Each extension header is an integer multiple of 8 octets long,
order to retain 8-octet alignment for subsequent headers. Multi
octet fields within each extension header are aligned on
natural boundaries, i.e., fields of width n octets are placed at
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
following extension headers
Hop-by-Hop
Routing (Type 0)
Destination
Encapsulating Security
The first four are specified in this document; the last two
specified in [RFC-1826] and [RFC-1827], respectively
Deering & Hinden Standards Track [Page 7]
RFC 1883 IPv6 Specification December 1995
4.1 Extension Header
When more than one extension header is used in the same packet, it
recommended that those headers appear in the following order
IPv6
Hop-by-Hop Options
Destination Options header (note 1)
Routing
Fragment
Authentication header (note 2)
Encapsulating Security Payload header (note 2)
Destination Options header (note 3)
upper-layer
note 1: for options to be processed by the first
that appears in the IPv6 Destination Address
plus subsequent destinations listed in the
header
note 2: additional recommendations regarding the
order of the Authentication and
Security Payload headers are given in [RFC-1827].
note 3: for options to be processed only by the
destination of the packet
Each extension header should occur at most once, except for
Destination Options header which should occur at most twice (
before a Routing header and once before the upper-layer header).
If the upper-layer header is another IPv6 header (in the case of IPv
being tunneled over or encapsulated in IPv6), it may be followed
its own extensions headers, which are separately subject to the
ordering recommendations
If and when other extension headers are defined, their
constraints relative to the above listed headers must be specified
IPv6 nodes must accept and attempt to process extension headers
any order and occurring any number of times in the same packet
except for the Hop-by-Hop Options header which is restricted
appear immediately after an IPv6 header only. Nonetheless, it
strongly advised that sources of IPv6 packets adhere to the
recommended order until and unless subsequent specifications
that recommendation
Deering & Hinden Standards Track [Page 8]
RFC 1883 IPv6 Specification December 1995
4.2
Two of the currently-defined extension headers -- the Hop-by-
Options header and the Destination Options header -- carry a
number of type-length-value (TLV) encoded "options", of the
format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Option Type | Opt Data Len | Option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Option Type 8-bit identifier of the type of option
Opt Data Len 8-bit unsigned integer. Length of the
Data field of this option, in octets
Option Data Variable-length field. Option-Type-
data
The sequence of options within a header must be processed strictly
the order they appear in the header; a receiver must not,
example, scan through the header looking for a particular kind
option and process that option prior to processing all
ones
The Option Type identifiers are internally encoded such that
highest-order two bits specify the action that must be taken if
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
packets's Destination Address was a multicast address,
an ICMP Parameter Problem, Code 2, message to the packet'
Source Address, pointing to the unrecognized Option Type
11 - discard the packet and, only if the packet's
Address was not a multicast address, send an ICMP
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
not the Option Data of that option can change en-route to
packet's final destination. When an Authentication header is
in the packet, for any option whose data may change en-route,
entire Option Data field must be treated as zero-valued octets
computing or verifying the packet's authenticating value
Deering & Hinden Standards Track [Page 9]
RFC 1883 IPv6 Specification December 1995
0 - Option Data does not change en-
1 - Option Data may change en-
Individual options may have specific alignment requirements,
ensure that multi-octet values within Option Data fields fall
natural boundaries. The alignment requirement of an option
specified using the notation xn+y, meaning the Option Type
appear at an integer multiple of x octets from the start of
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
subsequent options and to pad out the containing header to a
of 8 octets in length. These padding options must be recognized
all IPv6 implementations
Pad1 option (alignment requirement: none
+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
NOTE! the format of the Pad1 option is a special case -- it
not have length and value fields
The Pad1 option is used to insert one octet of padding into
Options area of a header. If more than one octet of padding
required, the PadN option, described next, should be used
rather than multiple Pad1 options
PadN option (alignment requirement: none
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| 1 | Opt Data Len | Option
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
The PadN option is used to insert two or more octets of
into the Options area of a header. For N octets of padding
the Opt Data Len field contains the value N-2, and the
Data consists of N-2 zero-valued octets
Appendix A contains formatting guidelines for designing new options
Deering & Hinden Standards Track [Page 10]
RFC 1883 IPv6 Specification December 1995
4.3 Hop-by-Hop Options
The Hop-by-Hop Options header is used to carry optional
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
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
immediately following the Hop-by-Hop
header. Uses the same values as the IPv
Protocol field [RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of
Hop-by-Hop Options header in 8-octet units
not including the first 8 octets
Options Variable-length field, of length such that
complete Hop-by-Hop Options header is an
multiple of 8 octets long. Contains one
more TLV-encoded options, as described
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
payloads longer than 65,535 octets. The Jumbo Payload Length
the length of the packet in octets, excluding the IPv6 header
including the Hop-by-Hop Options header; it must be greater
65,535. If a packet is received with a Jumbo Payload
containing a Jumbo Payload Length less than or equal to 65,535,
Deering & Hinden Standards Track [Page 11]
RFC 1883 IPv6 Specification December 1995
an ICMP Parameter Problem message, Code 0, should be sent to
packet's source, pointing to the high-order octet of the
Jumbo Payload Length field
The Payload Length field in the IPv6 header must be set to
in every packet that carries the Jumbo Payload option. If
packet is received with a valid Jumbo Payload option present
a non-zero IPv6 Payload Length field, an ICMP Parameter
message, Code 0, should be sent to the packet's source,
to the Option Type field of the Jumbo Payload option
The Jumbo Payload option must not be used in a packet
carries a Fragment header. If a Fragment header is
in a packet that contains a valid Jumbo Payload option, an
Parameter Problem message, Code 0, should be sent to the packet'
source, pointing to the first octet of the Fragment header
An implementation that does not support the Jumbo Payload
cannot have interfaces to links whose link MTU is greater
65,575 (40 octets of IPv6 header plus 65,535 octets of payload).
Deering & Hinden Standards Track [Page 12]
RFC 1883 IPv6 Specification December 1995
4.4 Routing
The Routing header is used by an IPv6 source to list one or
intermediate nodes to be "visited" on the way to a packet'
destination. This function is very similar to IPv4's Source
options. The Routing header is identified by a Next Header value
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
immediately following the Routing header
Uses the same values as the IPv4 Protocol
[RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of
Routing header in 8-octet units, not
the first 8 octets
Routing Type 8-bit identifier of a particular
header variant
Segments Left 8-bit unsigned integer. Number of
segments remaining, i.e., number of
listed intermediate nodes still to be
before reaching the final destination
type-specific data Variable-length field, of format determined
the Routing Type, and of length such that
complete Routing header is an integer
of 8 octets long
Deering & Hinden Standards Track [Page 13]
RFC 1883 IPv6 Specification December 1995
If, while processing a received packet, a node encounters a
header with an unrecognized Routing Type value, the required
of the node depends on the value of the Segments Left field,
follows
If Segments Left is zero, the node must ignore the Routing
and proceed to process the next header in the packet, whose
is identified by the Next Header field in the Routing header
If Segments Left is non-zero, the node must discard the packet
send an ICMP Parameter Problem, Code 0, message to the packet'
Source Address, pointing to the unrecognized Routing Type
Deering & Hinden Standards Track [Page 14]
RFC 1883 IPv6 Specification December 1995
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
immediately following the Routing header
Uses the same values as the IPv4 Protocol
[RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of
Routing header in 8-octet units, not
the first 8 octets. For the Type 0
header, Hdr Ext Len is equal to two times
number of addresses in the header, and
be an even number less than or equal to 46.
Routing Type 0.
Deering & Hinden Standards Track [Page 15]
RFC 1883 IPv6 Specification December 1995
Segments Left 8-bit unsigned integer. Number of
segments remaining, i.e., number of
listed intermediate nodes still to be
before reaching the final destination
Maximum legal value = 23.
Reserved 8-bit reserved field. Initialized to zero
transmission; ignored on reception
Strict/Loose Bit
24-bit bit-map, numbered 0 to 23, left-to-right
Indicates, for each segment of the route,
or not the next destination address must be
neighbor of the preceding address: 1 means
(must be a neighbor), 0 means loose (need not
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,
in the IPv6 Destination Address field of a packet carrying a
header of Type 0.
If bit number 0 of the Strict/Loose Bit Map has value 1,
Destination Address field of the IPv6 header in the original
must identify a neighbor of the originating node. If bit number 0
has value 0, the originator may use any legal, non-multicast
as the initial Destination Address
Bits numbered greater than n, where n is the number of addresses
the Routing header, must be set to 0 by the originator and ignored
receivers
A Routing header is not examined or processed until it reaches
node identified in the Destination Address field of the IPv6 header
In that node, dispatching on the Next Header field of the
preceding header causes the Routing header module to be invoked
which, in the case of Routing Type 0, performs the
algorithm
Deering & Hinden Standards Track [Page 16]
RFC 1883 IPv6 Specification December 1995
if Segments Left = 0 {
proceed to process the next header in the packet, whose type
identified by the Next Header field in the Routing
}
else if Hdr Ext Len is odd or greater than 46 {
send an ICMP Parameter Problem, Code 0, message to the
Address, pointing to the Hdr Ext Len field, and discard
}
else {
compute n, the number of addresses in the Routing header,
dividing Hdr Ext Len by 2
if Segments Left is greater than n {
send an ICMP Parameter Problem, Code 0, message to the
Address, pointing to the Segments Left field, and discard
}
else {
decrement Segments Left by 1;
compute i, the index of the next address to be visited
the address vector, by subtracting Segments Left from
if Address [i] or the IPv6 Destination Address is multicast {
discard the
}
else {
swap the IPv6 Destination Address and Address[i
if bit i of the Strict/Loose Bit map has value 1 and
new Destination Address is not the address of a
of this node {
send an ICMP Destination Unreachable -- Not a
message to the Source Address and discard the
}
else if the IPv6 Hop Limit is less than or equal to 1 {
send an ICMP Time Exceeded -- Hop Limit Exceeded
Transit message to the Source Address and discard
}
else {
decrement the Hop Limit by 1
resubmit the packet to the IPv6 module for
to the new
}
}
}
}
Deering & Hinden Standards Track [Page 17]
RFC 1883 IPv6 Specification December 1995
As an example of the effects of the above algorithm, consider
case of a source node S sending a packet to destination node D,
a Routing header to cause the packet to be routed via
nodes I1, I2, and I3. The values of the relevant IPv6 header
Routing header fields on each segment of the delivery path would
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] = I
(if bit 0 of the Bit Map is 1, Address[2] = I
S and I1 must be neighbors; Address[3] =
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] = I
(if bit 1 of the Bit Map is 1, Address[2] = I
I1 and I2 must be neighbors; Address[3] =
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] = I
(if bit 2 of the Bit Map is 1, Address[2] = I
I2 and I3 must be neighbors; Address[3] =
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] = I
(if bit 3 of the Bit Map is 1, Address[2] = I
I3 and D must be neighbors; Address[3] = I
this is checked by I3)
Deering & Hinden Standards Track [Page 18]
RFC 1883 IPv6 Specification December 1995
4.5 Fragment
The Fragment header is used by an IPv6 source to send packets
than would fit in the path MTU to their destinations. (Note:
IPv4, fragmentation in IPv6 is performed only by source nodes, not
routers along a packet's delivery path -- see section 5.)
Fragment header is identified by a Next Header value of 44 in
immediately preceding header, and has the following format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Reserved | Fragment Offset |Res|M
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header 8-bit selector. Identifies the initial
type of the Fragmentable Part of the
packet (defined below). Uses the same
as the IPv4 Protocol field [RFC-1700 et seq.].
Reserved 8-bit reserved field. Initialized to zero
transmission; ignored on reception
Fragment Offset 13-bit unsigned integer. The offset, in 8-
units, of the data following this header
relative to the start of the Fragmentable
of the original packet
Res 2-bit reserved field. Initialized to zero
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
path to its destination, a source node may divide the packet
fragments and send each fragment as a separate packet, to
reassembled at the receiver
For every packet that is to be fragmented, the source node
an Identification value. The Identification must be different
that of any other fragmented packet sent recently* with the
Source Address and Destination Address. If a Routing header
present, the Destination Address of concern is that of the
destination
* "recently" means within the maximum likely lifetime of a packet
including transit time from source to destination and time
Deering & Hinden Standards Track [Page 19]
RFC 1883 IPv6 Specification December 1995
awaiting reassembly with other fragments of the same packet
However, it is not required that a source node know the
packet lifetime. Rather, it is assumed that the requirement
be met by maintaining the Identification value as a simple, 32-
bit, "wrap-around" counter, incremented each time a packet
be fragmented. It is an implementation choice whether
maintain a single counter for the node or multiple counters
e.g., one for each of the node's possible source addresses,
one for each active (source address, destination address
combination
The initial, large, unfragmented packet is referred to as
"original packet", and it is considered to consist of two parts,
illustrated
original packet
+------------------+----------------------//-----------------------+
| Unfragmentable | Fragmentable |
| Part | Part |
+------------------+----------------------//-----------------------+
The Unfragmentable Part consists of the IPv6 header plus
extension headers that must be processed by nodes en route to
destination, that is, all headers up to and including the
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
destination node(s), plus the upper-layer header and data
The Fragmentable Part of the original packet is divided
fragments, each, except possibly the last ("rightmost") one, being
integer multiple of 8 octets long. The fragments are transmitted
separate "fragment packets" as illustrated
original packet
+------------------+--------------+--------------+--//--+----------+
| Unfragmentable | first | second | | last |
| Part | fragment | fragment | .... | fragment |
+------------------+--------------+--------------+--//--+----------+
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RFC 1883 IPv6 Specification December 1995
fragment packets
+------------------+--------+--------------+
| Unfragmentable |Fragment| first |
| Part | Header | fragment |
+------------------+--------+--------------+
+------------------+--------+--------------+
| Unfragmentable |Fragment| second |
| Part | Header | fragment |
+------------------+--------+--------------+
+------------------+--------+----------+
| Unfragmentable |Fragment| last |
| Part | Header | fragment |
+------------------+--------+----------+
Each fragment packet is composed of
(1) The Unfragmentable Part of the original packet, with
Payload Length of the original IPv6 header changed to
the length of this fragment packet only (excluding the
of the IPv6 header itself), and the Next Header field of
last header of the Unfragmentable Part changed to 44.
(2) A Fragment header containing
The Next Header value that identifies the first header
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
Fragmentable Part of the original packet. The
Offset of the first ("leftmost") fragment is 0.
An M flag value of 0 if the fragment is the
("rightmost") one, else an M flag value of 1.
The Identification value generated for the
packet
(3) The fragment itself
The lengths of the fragments must be chosen such that the
fragment packets fit within the MTU of the path to the packets
destination(s).
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RFC 1883 IPv6 Specification December 1995
At the destination, fragment packets are reassembled into
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
have the same Source Address, Destination Address, and
Identification
The Unfragmentable Part of the reassembled packet consists of
headers up to, but not including, the Fragment header of the
fragment packet (that is, the packet whose Fragment Offset
zero), with the following two changes
The Next Header field of the last header of the
Part is obtained from the Next Header field of the
fragment's Fragment header
The Payload Length of the reassembled packet is computed
the length of the Unfragmentable Part and the length and
of the last fragment. For example, a formula for computing
Payload Length of the reassembled original packet is
PL.orig = PL.first - FL.first - 8 + (8 * FO.last) + FL.
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
first fragment packet
FO.last = Fragment Offset field of Fragment header
last fragment packet
FL.last = length of fragment following Fragment header
last fragment packet
The Fragmentable Part of the reassembled packet is
from the fragments following the Fragment headers in each of
fragment packets. The length of each fragment is computed
subtracting from the packet's Payload Length the length of
headers between the IPv6 header and fragment itself; its
position in Fragmentable Part is computed from its Fragment
value
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RFC 1883 IPv6 Specification December 1995
The Fragment header is not present in the final,
packet
The following error conditions may arise when reassembling
packets
If insufficient fragments are received to complete reassembly of
packet within 60 seconds of the reception of the first-
fragment of that packet, reassembly of that packet must
abandoned and all the fragments that have been received for
packet must be discarded. If the first fragment (i.e., the
with a Fragment Offset of zero) has been received, an ICMP
Exceeded -- Fragment Reassembly Time Exceeded message should
sent to the source of that fragment
If the length of a fragment, as derived from the fragment packet'
Payload Length field, is not a multiple of 8 octets and the M
of that fragment is 1, then that fragment must be discarded and
ICMP Parameter Problem, Code 0, message should be sent to
source of the fragment, pointing to the Payload Length field
the fragment packet
If the length and offset of a fragment are such that the
Length of the packet reassembled from that fragment would
65,535 octets, then that fragment must be discarded and an
Parameter Problem, Code 0, message should be sent to the source
the fragment, pointing to the Fragment Offset field of
fragment packet
The following conditions are not expected to occur, but are
considered errors if they do
The number and content of the headers preceding the
header of different fragments of the same original packet
differ. Whatever headers are present, preceding the
header in each fragment packet, are processed when the
arrive, prior to queueing the fragments for reassembly.
those headers in the Offset zero fragment packet are retained
the reassembled packet
The Next Header values in the Fragment headers of
fragments of the same original packet may differ. Only the
from the Offset zero fragment packet is used for reassembly
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RFC 1883 IPv6 Specification December 1995
4.6 Destination Options
The Destination Options header is used to carry optional
that need be examined only by a packet's destination node(s).
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
immediately following the Destination
header. Uses the same values as the IPv
Protocol field [RFC-1700 et seq.].
Hdr Ext Len 8-bit unsigned integer. Length of
Destination Options header in 8-octet units
not including the first 8 octets
Options Variable-length field, of length such that
complete Destination Options header is
integer multiple of 8 octets long.
one or more TLV-encoded options, as
in section 4.2.
The only destination options defined in this document are the Pad
and PadN options specified in section 4.2.
Note that there are two possible ways to encode optional
information in an IPv6 packet: either as an option in the
Options header, or as a separate extension header. The
header and the Authentication header are examples of the
approach. Which approach can be used depends on what action
desired of a destination node that does not understand the
information
o if the desired action is for the destination node to
the packet and, only if the packet's Destination Address is
a multicast address, send an ICMP Unrecognized Type message
the packet's Source Address, then the information may
encoded either as a separate header or as an option in
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RFC 1883 IPv6 Specification December 1995
Destination Options header whose Option Type has the value 11
in its highest-order two bits. The choice may depend on
factors as which takes fewer octets, or which yields
alignment or more efficient parsing
o if any other action is desired, the information must be
as an option in the Destination Options header whose
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
The value 59 in the Next Header field of an IPv6 header or
extension header indicates that there is nothing following
header. If the Payload Length field of the IPv6 header indicates
presence of octets past the end of a header whose Next Header
contains 59, those octets must be ignored, and passed on unchanged
the packet is forwarded
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RFC 1883 IPv6 Specification December 1995
5. Packet Size
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
in one piece, link-specific fragmentation and reassembly must
provided at a layer below IPv6.
From each link to which a node is directly attached, the node
be able to accept packets as large as that link's MTU. Links
have a configurable MTU (for example, PPP links [RFC-1661]) must
configured to have an MTU of at least 576 octets; it is
that a larger MTU be configured, to accommodate
encapsulations (i.e., tunneling) without incurring fragmentation
It is strongly recommended that IPv6 nodes implement Path
Discovery [RFC-1191], in order to discover and take advantage
paths with MTU greater than 576 octets. However, a minimal IPv
implementation (e.g., in a boot ROM) may simply restrict itself
sending packets no larger than 576 octets, and omit implementation
Path MTU Discovery
In order to send a packet larger than a path's MTU, a node may
the IPv6 Fragment header to fragment the packet at the source
have it reassembled at the destination(s). However, the use of
fragmentation is discouraged in any application that is able
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,
reassembly, is as large as 1500 octets, including the IPv6 header.
node is permitted to accept fragmented packets that reassemble
more than 1500 octets. However, a node must not send fragments
reassemble to a size greater than 1500 octets unless it has
knowledge that the destination(s) can reassemble a packet of
size
In response to an IPv6 packet that is sent to an IPv4
(i.e., a packet that undergoes translation from IPv6 to IPv4),
originating IPv6 node may receive an ICMP Packet Too Big
reporting a Next-Hop MTU less than 576. In that case, the IPv6
is not required to reduce the size of subsequent packets to less
576, but must include a Fragment header in those packets so that
IPv6-to-IPv4 translating router can obtain a suitable
value to use in resulting IPv4 fragments. Note that this means
payload may have to be reduced to 528 octets (576 minus 40 for
IPv6 header and 8 for the Fragment header), and smaller still
additional extension headers are used
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RFC 1883 IPv6 Specification December 1995
Note: Path MTU Discovery must be performed even in cases where
host "thinks" a destination is attached to the same link
itself
Note: Unlike IPv4, it is unnecessary in IPv6 to set a "Don'
Fragment" flag in the packet header in order to perform Path
Discovery; that is an implicit attribute of every IPv6 packet
Also, those parts of the RFC-1191 procedures that involve use
a table of MTU "plateaus" do not apply to IPv6, because the IPv
version of the "Datagram Too Big" message always identifies
exact MTU to be used
Deering & Hinden Standards Track [Page 27]
RFC 1883 IPv6 Specification December 1995
6. Flow
The 24-bit Flow Label field in the IPv6 header may be used by
source to label those packets for which it requests special
by the IPv6 routers, such as non-default quality of service
"real-time" service. This aspect of IPv6 is, at the time of writing
still experimental and subject to change as the requirements for
support in the Internet become clearer. Hosts or routers that do
support the functions of the Flow Label field are required to set
field to zero when originating a packet, pass the field on
when forwarding a packet, and ignore the field when receiving
packet
A flow is a sequence of packets sent from a particular source to
particular (unicast or multicast) destination for which the
desires special handling by the intervening routers. The nature
that special handling might be conveyed to the routers by a
protocol, such as a resource reservation protocol, or by
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
of this document
There may be multiple active flows from a source to a destination,
well as traffic that is not associated with any flow. A flow
uniquely identified by the combination of a source address and
non-zero flow label. Packets that do not belong to a flow carry
flow label of zero
A flow label is assigned to a flow by the flow's source node.
flow labels must be chosen (pseudo-)randomly and uniformly from
range 1 to FFFFFF hex. The purpose of the random allocation is
make any set of bits within the Flow Label field suitable for use
a hash key by routers, for looking up the state associated with
flow
All packets belonging to the same flow must be sent with the
source address, destination address, priority, and flow label.
any of those packets includes a Hop-by-Hop Options header, then
all must be originated with the same Hop-by-Hop Options
contents (excluding the Next Header field of the Hop-by-Hop
header). If any of those packets includes a Routing header,
they all must be originated with the same contents in all
headers up to and including the Routing header (excluding the
Header field in the Routing header). The routers or destinations
permitted, but not required, to verify that these conditions
satisfied. If a violation is detected, it should be reported to
source by an ICMP Parameter Problem message, Code 0, pointing to
high-order octet of the Flow Label field (i.e., offset 1 within
IPv6 packet).
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RFC 1883 IPv6 Specification December 1995
Routers are free to "opportunistically" set up flow-handling
for any flow, even when no explicit flow establishment
has been provided to them via a control protocol, a hop-by-
option, or other means. For example, upon receiving a packet from
particular source with an unknown, non-zero flow label, a router
process its IPv6 header and any necessary extension headers as if
flow label were zero. That processing would include determining
next-hop interface, and possibly other actions, such as updating
hop-by-hop option, advancing the pointer and addresses in a
header, or deciding on how to queue the packet based on its
field. The router may then choose to "remember" the results of
processing steps and cache that information, using the source
plus the flow label as the cache key. Subsequent packets with
same source address and flow label may then be handled by
to the cached information rather than examining all those
that, according to the requirements of the previous paragraph, can
assumed unchanged from the first packet seen in the flow
Cached flow-handling state that is set up opportunistically,
discussed in the preceding paragraph, must be discarded no more
6 seconds after it is established, regardless of whether or
packets of the same flow continue to arrive. If another packet
the same source address and flow label arrives after the cached
has been discarded, the packet undergoes full, normal processing (
if its flow label were zero), which may result in the re-creation
cached flow state for that flow
The lifetime of flow-handling state that is set up explicitly,
example by a control protocol or a hop-by-hop option, must
specified as part of the specification of the explicit set-
mechanism; it may exceed 6 seconds
A source must not re-use a flow label for a new flow within
lifetime of any flow-handling state that might have been
for the prior use of that flow label. Since flow-handling state
a lifetime of 6 seconds may be established opportunistically for
flow, the minimum interval between the last packet of one flow
the first packet of a new flow using the same flow label is 6
seconds. Flow labels used for explicitly set-up flows with
flow-state lifetimes must remain unused for those longer
before being re-used for new flows
When a node stops and restarts (e.g., as a result of a "crash"),
must be careful not to use a flow label that it might have used
an earlier flow whose lifetime may not have expired yet. This may
accomplished by recording flow label usage on stable storage so
it can be remembered across crashes, or by refraining from using
flow labels until the maximum lifetime of any possible
established flows has expired (at least 6 seconds; more if
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RFC 1883 IPv6 Specification December 1995
flow set-up mechanisms with longer lifetimes might have been used).
If the minimum time for rebooting the node is known (often more
6 seconds), that time can be deducted from the necessary
period before starting to allocate flow labels
There is no requirement that all, or even most, packets belong
flows, i.e., carry non-zero flow labels. This observation is
here to remind protocol designers and implementors not to
otherwise. For example, it would be unwise to design a router
performance would be adequate only if most packets belonged to flows
or to design a header compression scheme that only worked on
that belonged to flows
7.
The 4-bit Priority field in the IPv6 header enables a source
identify the desired delivery priority of its packets, relative
other packets from the same source. The Priority values are
into two ranges: Values 0 through 7 are used to specify the
of traffic for which the source is providing congestion control
i.e., traffic that "backs off" in response to congestion, such as
traffic. Values 8 through 15 are used to specify the priority
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
recommended for particular application categories
0 - uncharacterized
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
have discarded under conditions of congestion (e.g., high-
video traffic), and the highest value (15) should be used for
packets that the sender is least willing to have discarded (e.g.,
low-fidelity audio traffic). There is no relative ordering
between the congestion-controlled priorities and the non-congestion
controlled priorities
Deering & Hinden Standards Track [Page 30]
RFC 1883 IPv6 Specification December 1995
8. Upper-Layer Protocol
8.1 Upper-Layer
Any transport or other upper-layer protocol that includes
addresses from the IP header in its checksum computation must
modified for use over IPv6, to include the 128-bit IPv6
instead of 32-bit IPv4 addresses. In particular, the
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
Address used in the pseudo-header is that of the
destination. At the originating node, that address will be
the last element of the Routing header; at the recipient(s),
that address will be in the Destination Address field of
IPv6 header
o The Next Header value in the pseudo-header identifies
upper-layer protocol (e.g., 6 for TCP, or 17 for UDP). It
differ from the Next Header value in the IPv6 header if
are extension headers between the IPv6 header and the upper
layer header
o The Payload Length used in the pseudo-header is the length
the upper-layer packet, including the upper-layer header.
will be less than the Payload Length in the IPv6 header (or
Deering & Hinden Standards Track [Page 31]
RFC 1883 IPv6 Specification December 1995
the Jumbo Payload option) if there are extension
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,
originating a UDP packet, an IPv6 node must compute a
checksum over the packet and the pseudo-header, and, if
computation yields a result of zero, it must be changed to
FFFF for placement in the UDP header. IPv6 receivers
discard UDP packets containing a zero checksum, and should
the error
The IPv6 version of ICMP [RFC-1885] includes the above pseudo-
in its checksum computation; this is a change from the IPv4
of ICMP, which does not include a pseudo-header in its checksum.
reason for the change is to protect ICMP from misdelivery
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
value 58, which identifies the IPv6 version of ICMP
8.2 Maximum Packet
Unlike IPv4, IPv6 nodes are not required to enforce maximum
lifetime. That is the reason the IPv4 "Time to Live" field
renamed "Hop Limit" in IPv6. In practice, very few, if any, IPv
implementations conform to the requirement that they limit
lifetime, so this is not a change in practice. Any upper-
protocol that relies on the internet layer (whether IPv4 or IPv6)
limit packet lifetime ought to be upgraded to provide its
mechanisms for detecting and discarding obsolete packets
8.3 Maximum Upper-Layer Payload
When computing the maximum payload size available for upper-
data, an upper-layer protocol must take into account the larger
of the IPv6 header relative to the IPv4 header. For example,
IPv4, TCP's MSS option is computed as the maximum packet size (
default value or a value learned through Path MTU Discovery) minus 40
octets (20 octets for the minimum-length IPv4 header and 20
for the minimum-length TCP header). When using TCP over IPv6,
MSS must be computed as the maximum packet size minus 60 octets
because the minimum-length IPv6 header (i.e., an IPv6 header with
extension headers) is 20 octets longer than a minimum-length IPv
header
Deering & Hinden Standards Track [Page 32]
RFC 1883 IPv6 Specification December 1995
Appendix A. Formatting Guidelines for
This appendix gives some advice on how to lay out the fields
designing new options to be used in the Hop-by-Hop Options header
the Destination Options header, as described in section 4.2.
guidelines are based on the following assumptions
o One desirable feature is that any multi-octet fields within
Option Data area of an option be aligned on their
boundaries, i.e., fields of width n octets should be placed
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
Options header take up as little space as possible, subject
the requirement that the header be an integer multiple of 8
octets long
o It may be assumed that, when either of the option-
headers are present, they carry a very small number of options
usually only one
These assumptions suggest the following approach to laying out
fields of an option: order the fields from smallest to largest,
no interior padding, then derive the alignment requirement for
entire option based on the alignment requirement of the largest
(up to a maximum alignment of 8 octets). This approach
illustrated in the following examples
Example 1
If an option X required two data fields, one of length 8 octets
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
starts at a multiple-of-8 offset from the start of the
Deering & Hinden Standards Track [Page 33]
RFC 1883 IPv6 Specification December 1995
header. A complete Hop-by-Hop or Destination Options
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,