Internet DRAFT - draft-ietf-6man-eh-limits
draft-ietf-6man-eh-limits
Network Working Group T. Herbert
Internet-Draft SiPanda
Intended status: Standards Track 28 February 2023
Expires: 1 September 2023
Limits on Sending and Processing IPv6 Extension Headers
draft-ietf-6man-eh-limits-02
Abstract
This specification defines various limits that may be applied to
receiving, sending, and otherwise processing packets that contain
IPv6 extension headers. The need for such limits is pragmatic to
facilitate interoperability amongst hosts and routers in the presence
of extension headers and thereby increasing the feasibility of
deployment of extension headers. The limits described herein
establish the minimum baseline of support for use of extension
headers in the Internet. If it is known that all communicating
parties for a particular communication, including end hosts and any
intermediate nodes in the path, are capable of supporting more than
the baseline then these default limits may be freely exceeded.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 1 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Herbert Expires 1 September 2023 [Page 1]
Internet-Draft Extension Header Limits February 2023
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Related work . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Adherence to the Robustness Principle . . . . . . . . . . 4
1.2.1. Be conservative in what you send . . . . . . . . . . 4
1.2.2. Be liberal in what you receive . . . . . . . . . . . 5
2. Overview and motivation of extension header limits . . . . . 5
2.1. Types of nodes . . . . . . . . . . . . . . . . . . . . . 6
2.2. Types of limits . . . . . . . . . . . . . . . . . . . . . 6
2.2.1. Limits on extension header length . . . . . . . . . . 6
2.2.2. Limits on option length . . . . . . . . . . . . . . . 7
2.2.3. Limits on number of extension headers . . . . . . . . 7
2.2.4. Limits on number of options . . . . . . . . . . . . . 7
2.2.5. Limits on padding options . . . . . . . . . . . . . . 8
2.2.6. Limit on IPv6 header chain length . . . . . . . . . . 9
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1. List of limits . . . . . . . . . . . . . . . . . . . . . 12
3.2. Host requirements . . . . . . . . . . . . . . . . . . . . 12
3.2.1. Sending extension headers . . . . . . . . . . . . . . 12
3.2.2. Receiving extension headers . . . . . . . . . . . . . 14
3.3. Intermediate node and intermediate destination
requirements . . . . . . . . . . . . . . . . . . . . . . 15
3.4. Intermediate destination requirements . . . . . . . . . . 17
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Normative References . . . . . . . . . . . . . . . . . . 18
4.2. Informative References . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Extension headers are a core component of the IPv6 protocol as
specified in [RFC8200]. IPv6 extension headers were originally
defined with few restrictions. For instance, there is no specified
limit on the number of extension headers a packet may have, nor is
there a limit on the length in bytes of extension headers in a packet
(other than being limited by the MTU). Similarly, variable length
extension headers typically do not have prescribed limits such as
limits on the number of Hop-by-Hop or Destination options in a
packet. The lack of limits essentially requires implementations to
handle every conceivable usage of the protocol, including a myriad of
use cases those are obviously outside the realm of ever being
Herbert Expires 1 September 2023 [Page 2]
Internet-Draft Extension Header Limits February 2023
realistic or useful in real world deployment.
The lack of limits and the requirements for supporting a virtually
open-ended protocol have led to a significant lack of support and
deployment of extension headers [RFC7872]. Instead of attempting to
satisfy the protocol requirements concerning extension headers, some
router and middlebox vendors have opted to either invent and apply
their own ad hoc limits, relegate packets with extension headers to
slow path processing, or have gone so far as to summarily discard all
packets with extension headers [RFC9098]. The net result of this
situation is that deployment and use of extension headers is
underwhelming to the extent that they are sometimes considered
unusable on the Internet, and hence IPv6 extension headers have not
lived up to their potential as the extensibility mechanism of IPv6.
As an example, consider that Hop-by-Hop Options and Destination
Options have no limit on how many options may be placed in a packet,
nor any limits as to how many options a receiver must process. A
single 1500 byte MTU sized packet could legally contain a Hop-by-Hop
Options extension header with over seven hundred two byte options.
There is no use case for this other than a Denial of Service attack
where an attacker simply creates packets with hundreds of small
unknown Hop-by-Hop Options with the two high order bits in the option
type set to 00 meaning to skip the unknown option. Any node in the
path that attempts to dutifully process all these options per the
requirements of [RFC2460] would be easily overwhelmed by the
processing needed to parse these options (this is true for both
hardware or software implementations).
This specification describes various limits that hosts and
intermediate nodes may apply to the processing of extension headers.
The goal of establishing limits is to narrow the requirements to
better match reasonable use cases thereby facilitating practical
implementation. Subsequently, this increases the viability of
extension headers as the extensibility mechanism of IPv6.
1.1. Related work
Some of the problems of unlimited extension headers have been
addressed in certain aspects.
[RFC8200] relaxed the requirement that all nodes in the path must
process Hop-by-Hop Options to be:
NOTE: While [RFC2460] required that all nodes must examine and
process the Hop-by-Hop Options header, it is now expected that
nodes along a packet's delivery path only examine and process the
Hop-by-Hop Options header if explicitly configured to do so.
Herbert Expires 1 September 2023 [Page 3]
Internet-Draft Extension Header Limits February 2023
Section 5.3 of [RFC8504] defines a number of limits that hosts may
apply to processing extensions. For instance:
A host MAY set a limit on the maximum number of non-padding
options allowed in the destination options and Hop-by-Hop
extension headers. If this feature is supported, the maximum
number SHOULD be configurable, and the default value SHOULD be set
to 8.
[RFC8883] defines a set of ICMP errors that my be sent if a limit
concerning extension headers is exceeded and a node discards a packet
as a result. This RFC allows both hosts and routers to send such
messages (effectively acknowledging that some routers drop packets
with extension headers even though such behavior is non-conformant
with [RFC8200]).
[RFC7872] presents real-world data regarding the extent to which
packets with IPv6 Extension Headers (EHs) are dropped in the
Internet, and [RFC9098] summarizes the operational implications of
IPv6 extension headers, and attempts to analyze reasons why packets
with IPv6 extension headers are often dropped in the public Internet.
This document sets the upper bounds on the number of hop-by-hop
options that a node should process. The lower bound is discussed in
[I-D.ietf-6man-hbh-processing].
1.2. Adherence to the Robustness Principle
The robustness principle, or Postel's Law, can be stated as "Be
conservative in what you send, liberal in what you receive". This
section considers the limits defined in this specification with
respect to the robustness principle.
1.2.1. Be conservative in what you send
The limits on sending extension headers are well aligned with the
send clause of the robustness principle. A sender of extension
headers is generally constrained in its use of extension headers.
Most of these limits are assumed to be the default to apply in an
arbitrary environment such as the public Internet, that is they can
be considered "baseline limits". These limits may be relaxed if a
sender has a priori information that all possible nodes in path will
properly handle packets that exceed the baseline limits. In
particular, if a sender is sending in a limited domain, it might be
known that all nodes in the limited domain have sufficient
capabilities to handle packets exceeding the baseline limits.
Herbert Expires 1 September 2023 [Page 4]
Internet-Draft Extension Header Limits February 2023
Specific mechanisms for a host to determine that baseline limits for
extension headers may be exceeded are out of scope for this document.
Conceivably, this determination could be done by configuration,
capabilities probing, or applying historical knowledge that all
intermediate nodes in the path and the destination node are capable
of handling packets that exceed the default limits.
1.2.2. Be liberal in what you receive
Considering the receive clause of the robustness principle, this
specification recommends that receivers accept all packets with
extension headers, however they may ignore extension headers or
options within extension headers (in accordance with [RFC8200]). In
particular, the philosophy of this specification is that intermediate
nodes should not drop packets on the basis that they don't have
sufficient capabilities to process all the headers in a packet. As
such, intermediate nodes may define arbitrarily restrictive limits on
what they process with regards to extension headers as long as the
action taken when those limits are exceeded is to ignore items beyond
the limit. Hosts are more constrained in this regard since they
generally can't correctly process a packet without processing all the
headers, so when limits are exceeded on a host, packets should be
discarded. It should be noted that hosts stacks inherently have more
processing capabilities than intermediate nodes, so it is expected
that they should be able to support higher limits for processing
extension headers.
This specification does specify one hard requirement for receiving
nodes, namely nodes must be able to properly handle packets having an
IPv6 header chain length up to 104 bytes. This requirement
acknowledges that some intermediate nodes perform deep packet
inspection to extract information from transport layer headers
[RFC9098]. Often a node that requires parsing transport layer
information will have a fixed sized "parsing buffer" to contain
packet headers. If the transport layer headers within a packet are
beyond the extent of the parsing buffer then an implementation might
take some detrimental action such as arbitrarily dropping packets.
To this end, this specification requires that any intermediate node
that requires access to to transport layer header must minimally be
able to parse at least 128 bytes of headers, from which the 104 byte
limit for the IP header chain is derived.
2. Overview and motivation of extension header limits
This specification considers extension header limits in three
dimensions: 1) The types of nodes that may process extension headers
and the requirements specific to each type, 2) The types of limits
that may be applied, 3) The action taken when a limit is exceeded.
Herbert Expires 1 September 2023 [Page 5]
Internet-Draft Extension Header Limits February 2023
2.1. Types of nodes
For the purposes of describing handling of extension headers this
specification considers three types of node in an IPv6 network:
* Hosts: The source of an IPv6 packet, as addressed by the source
address, or the final destination node of a packet as addressed by
the destination address in a packet with no Routing header or as
addressed by last segment in a Routing header
* Intermediate destination: An intermediate destination node in a
Routing header as addressed by the destination address of a packet
with a Routing header where the address is not the address of the
last segment in the Routing header
* Intermediate nodes: A router on the path that is not addressed by
the packet's destination address
2.2. Types of limits
The limits and requirements for handling extension headers defined in
this specification fall in the following categories:
* Limits on extension header length
* Limits on option length
* Limits on number of extension headers
* Limits on number of options
* Limits on padding for extension headers with options
* Limits on the length of the IPv6 header chain
2.2.1. Limits on extension header length
[RFC8504] defines limits that may be defined for the length of an
extension header. Those limits are extended to be applicable to
intermediate nodes. [RFC8883] defines ICMP Parameter Problem codes
that may be sent when an extension header is exceeded.
Herbert Expires 1 September 2023 [Page 6]
Internet-Draft Extension Header Limits February 2023
2.2.2. Limits on option length
A node may establish a limit on the size Hop-by-Hop or Destination
options. Conceivably, such a limit could apply to all option types,
or length limits may be specific to individual options. [RFC8883]
defines ICMP Parameter Problem codes that may be sent when an option
length limit is exceeded.
2.2.3. Limits on number of extension headers
A node may define a limit on the number of extension headers it will
process. Although [RFC8200] only defines four types of extension
headers, it does not preclude the same type of extension header being
present multiple times. A limit on the number of extension headers
could be useful to disallow packets that contain multiple instances
of the same extension header.
2.2.4. Limits on number of options
Limits may be established for the number of options sent or received
(specifically applicable to Hop-by-Hop options and Destination
options). The need for this limit arises from the fact that
[RFC8200] does not specify a limit. Requiring nodes to process
packets with tens or hundreds of options has no foreseeable use cases
in deployment except as a denial of service attack. [RFC8504] has
proposed such a limit for host processing of Hop-by-Hop and
Destination options with a default of eight options. This
specification extends that limit to be applicable to intermediate
nodes. Specific limits may be established for the number of non-
padding options or the number of all options including padding.
To derive a limit for the total number options in an extension
header, one can assume that at most one padding option is used
between two non-padding options (an explicit limit on consecutive
padding options is described below). With this assumption, we can
extrapolate a reasonable limit on the number of all options that
should be twice the limit of the number of non-padding options. Per
[RFC8504], the recommended default limit for the number of non-
padding options is eight, so this specification establishes a maximum
default limit of sixteen options including padding options. The
choice of sixteen options as a default limit attempts to strikes a
balance between allowing extensibility and maintaining reasonable
expectations for node processing requirements.
With regards to extensibility, it is observed that in the almost
thirty year history of IPv6 there are only thirteen defined non-
deprecated Destination options and Hop-by-Hop options and three
temporary assigned options. Current evidence suggests that having
Herbert Expires 1 September 2023 [Page 7]
Internet-Draft Extension Header Limits February 2023
more than one Destination option or Hop-by-Hop option in a packet is
rare, and extrapolating that point with the rate of new options being
defined suggests a limit of eight non-padding options allows for
sufficient extensibility in the foreseeable future.
With regards to processing requirements, TLVs, such as Hop-by-Hop
options and Destination options, have historically been considered
difficult to process efficiently due to their serial processing
requirements and combinatorial nature. TLV processing has been a
particularly acute problem for ASIC based hardware devices.
Recently, there is a strong trend in programmable implementation,
even in high performance routers, of using emerging programming
frameworks such as PANDA and P4. Programmable implementations are
better equipped to handle TLVs, at least for a reasonably small
number. It might also be pointed out that the need to efficiently
process TLVs exists in other protocols, for instance processing TCP
requires processing of TLVs in the form of TCP options which are an
intrinsic part of the protocol.
2.2.5. Limits on padding options
[RFC8200] defines PAD1 and PADN options that respectively provide one
byte or N bytes of padding in an extension header. The purpose of
padding is to properly align the following non-padding option to its
expected alignment, or to add padding after the last Destination or
Hop-by-Hop option so that the length of the extension header is a
multiple of eight bytes as required by [RFC8200]. [RFC8504] defines
limits on number of bytes used for consecutive padding where the
amount of padding between options or at the end of the extension
header is no more than seven bytes; this limit is sufficient to align
any following data after the padding to eight bytes. These limits
are extended to be applicable to intermediate nodes.
This specification allows a receiving node to set a requirement that
consecutive padding options are not present in a packet; which in
turn requires a sender to not place consecutive padding options in a
packet. The rationale for this limit is that a PAD1 or PADN option
is able to provide one to 257 bytes of padding, so a single padding
option is sufficient for any expected use case of padding. When the
sender creates options, it can compute the amount of padding
necessary to satisfy the alignment requirements of the following
data. If one byte of padding is needed a PAD1 option is used, if
more than one byte of padding is needed then an appropriate PADN
option is used.
Herbert Expires 1 September 2023 [Page 8]
Internet-Draft Extension Header Limits February 2023
2.2.6. Limit on IPv6 header chain length
Intermediate nodes often perform deep packet inspection (DPI) in
order to implement various functions in the network. Routers perform
DPI when they inspect packets beyond the IPv6 header or beyond Hop-
by-Hop options if they are present. Some router implementations must
inspect the transport layer headers in order to process and forward
the packet, and if the transport layer headers are not readable a
packet might be dropped. Even if a transport layer header is in
plain text within a packet, some devices may not be capable of
reading it if the header is too deep in the packet.
Hardware devices often have constraints on how much of the headers in
a packet can be parsed for DPI. A typical design is that some
portion of the beginning of a received packet is loaded into a memory
buffer for header parsing (i.e. the parsing buffer). The size of
this parsing buffer is often fixed per device.
To derive a size limit for the IPv6 header chain, we need to take
into account headers in a packet that might be subject to DPI which
include the link layer header through at least the pertinent fields
of the transport layer header. The most common required information
is the transport layer port numbers which typically occupy the first
four bytes of the transport headers (in TCP, UDP, SCTP, DCCP, etc.).
Inspection of port numbers may be needed for stateless load balancing
as well as port filtering. There are middleboxes that may need to
inspect more of transport layer headers or the transport payload,
however those can be considered specialized devices that perform work
beyond simple packet forwarding and filtering and hence should have
more capabilities for DPI.
In addition to limits on the length of the IP header chain, it is
conceivable that there could be a limit on the length of the whole
header chain. The whole header chain would comprise the IPv6 header
chain as well as any headers that are part of network encapsulation
that precede the innermost transport layer. The definition of such a
limit is out of scope for this document, however [RFC8883] defines an
ICMP error to send when a limit on size of an aggregate header chain
is exceeded.
This document specifies that the minimum supported limit for IPv6
header chains is 104 bytes. The value is derived by assuming that
nodes have the ability to process at least the first 128 bytes of a
packet (that is they have a parsing buffer that can contain at least
128 bytes). The 128 byte parsing buffer would be expected to at
least contain:
* 16 bytes for a Layer 2 header (for instance an Ethernet header)
Herbert Expires 1 September 2023 [Page 9]
Internet-Draft Extension Header Limits February 2023
* 40 bytes for the IPv6 header
* 64 bytes for the extension headers
* 8 bytes for the transport layer (i.e the first eight bytes of the
transport layer header
This scheme thus establishes a requirement that all Internet devices
are capable of correctly processing packets with up to sixty-four
bytes of extension headers, and subsequently it establishes a
requirement that a host shouldn't send packets with more than sixty-
four bytes of extension headers. Note that this establishes a global
baseline requirement across the Internet, within a limited domain
higher limits could be applied.
128 bytes is likely the minimal useful parsing buffer size in
deployment today. Devices performing a very narrow DPI could
conceptually use a smaller parsing buffer, for instance that could be
as small as sixty-four bytes which accommodates an L2 header, IPv6
header, and eight bytes of transport header; however, such a device
would be extremely limited in capabilities and if they do exist they
are likely legacy devices that will eventually be decommissioned.
Many routers now have the capability to perform DPI into
encapsulation headers which implies they already have a larger
parsing buffer than this baseline minimum.
Similar to limiting the number of options allowing in a packet,
setting a limit for IP header length chain is a tradeoff between
extensibility and feasible implementation.
For extensibility, the pertinent extension headers contributing to
the sixty-four byte limit are mostly Hop-by-Hop and Destination
options. The Routing Header extension header is really intended for
limited domains and not the Internet (for instance, the SRv6 Routing
Header is confined to a Segment Routing Domain) and therefore would
be subject to a domain specific limit for IP header chain length.
Encryption Header may be used on the Internet, however encryption
obfuscates the encapsulated transport headers such that such that
intermediate nodes can't inspect them regardless of their position in
a packet. Fragmentation may be used in the Internet, however only
the first fragment of a fragmented packet might contain transport
layer headers that could be read by an Intermediate node. In any
case, the Fragment Header is only four bytes so that would not be a
particularly large portion of a sixty-four byte limit.
The Authentication Header is usable on the Internet and does allow
the transport layer headers to be in readable in plain text. The
Authentication Header is relatively large, typically thirty-two bytes
Herbert Expires 1 September 2023 [Page 10]
Internet-Draft Extension Header Limits February 2023
or more, so it would contribute significantly to a limit on IP header
chain length; however, the use of Authentication Header without
encryption is currently rare on the Internet.
Individual Hop-by-Hop Destination Options may also be categorized as
being intended for use over the Internet or just in limited domains.
For instance, the IOAM Hop-by-Hop option is intended for use in
limited domains.
Paring this down, the types extension headers and Destination and
Hop-by-Hop options that might be used outside of limited domains are
fairly limited. Options that are intended for use over the public
Internet could be defined to be small and compact to promote not
exceeding a sixty-four byte limit on extension headers, whereas
options constrained to a limited domain could be larger since larger
limits might be assumed.
2.2.6.1. Action when limit is exceeded
For each limit that is defined, an action is specified for when the
limit is exceeded. The appropriate action depends on whether the
processing node is the destination host, an intermediate destination,
or an intermediate node. For a destination host, the typical action
to take when a limit is exceeded is to discard the packet. This is
appropriate since the destination host is required to process all of
the headers in a packet, and if a limit is exceeded then it cannot
process the packet so there is no other alternative but to discard.
For intermediate nodes, the typical action to take when a limit is
exceeded is to stop processing headers at the point the limit is
reached and to forward the packet on. [RFC8200] allows that an
intermediate node may not process the Hop-by-Hop Options extension
headers, therefore an intermediate node may ignore all of the Hop-by-
Hop options in a packet. This specification expands on that
requirement to allow an intermediate node to process some arbitrary
subset of consecutive Hop-by-Hop options in the TLV list and to
ignore the following ones. In the case of an egregious violation of
a limit, for instance an attacker sends three hundred options in a
packet, the destination host can decide if the appropriate response
is to drop (the destination host must process all options). Note
that this provision motivates the sender to place Hop-by-Hop Options
in the packet so that those considered more important are placed
first. It should also be noted that [RFC8504] sets a default limit
of eight; this specification adds a counterpart for sending hosts
that they shouldn't send more than eight Hop-by-Hop options by
default.
Herbert Expires 1 September 2023 [Page 11]
Internet-Draft Extension Header Limits February 2023
Intermediate destinations have characteristics of both hosts and
intermediate modes. If a limit is exceeded related to Hop-by-Hop
options then the suggested action in this specification is to assume
the same processing of limits as intermediate nodes. If limits are
exceeded that affect the processing specific to an intermediate
destination, such as limits on Destination options before the Routing
header, then the action should be to discard packet.
3. Requirements
This section lists the normative requirements related to sending and
processing extension headers.
3.1. List of limits
The set of limits that a node may apply when processing extension
headers include:
* Too many non-padding or padding options
* Extension header too big
* Option too big
* Too many consecutive padding options
* Too many consecutive bytes of padding
* Extension header chain too long
* Aggregate header chain too long
* Too many extension headers
3.2. Host requirements
3.2.1. Sending extension headers
The requirements are:
* A host MUST NOT send more than 8 non-padding options in
Destination Options in a packet unless it has explicit knowledge
that the destination, and all intermediate destinations in the
case of Destination Options before the routing header, are able to
process a greater number of options.
* A host MUST NOT send more than 8 non-padding options in Hop-by-Hop
Herbert Expires 1 September 2023 [Page 12]
Internet-Draft Extension Header Limits February 2023
Options in a packet unless it has explicit knowledge that the
final destination host is able to process a greater number of
options.
* A host SHOULD NOT send more than 8 non-padding options in Hop-by-
Hop Options in a packet unless it has explicit knowledge that all
possible intermediate nodes are able to process a greater number
of options or will ignore options that exceeds their limit.
* A host MUST NOT send a packet with an extension header larger than
64 bytes unless it has explicit knowledge that all nodes that
might process the extension header are capable of processing a
larger header.
* A host MUST NOT send a packet with a Destination option or Hop-by-
Hop option with Data Length greater than 60 bytes unless it has
explicit knowledge that all nodes that might process the option
are capable of processing ones with a larger Data Length.
* A host node MUST NOT send a packet with an IPv6 header chain
larger than 104 bytes unless it has explicit knowledge that all
nodes in the path are capable of properly handling packets with
larger header chains. This requirement is equivalently stated as
a host MUST NOT send a packet with more than 64 bytes of aggregate
extension headers.
* A host MUST NOT set more than one consecutive pad option, either
PAD1 or PADN, in Destination options or Hop-by-Hop options.
* A host MUST NOT send a PadN option in Hop-by-Hop Options or
Destination Options with total length of more than seven bytes.
* A host node MUST NOT send more than 16 options (padding or non-
padding) Destination options in a packet unless it has explicit
knowledge that the destination, and all intermediate destinations
in the case of Destination Options before the routing header, are
able to process a greater number of options. Note that if the
above requirements on a host sending non-padding Destination
options and requirements on option padding are met, then this
requirement is implicitly satisfied.
* A host node MUST NOT send more than 16 options (padding or non-
padding) in Hop-by-Hop Options in a packet unless it has explicit
knowledge that the final destination host is able to process a
greater number of options. Note that if the above requirements on
a host sending non-padding Hop-by-Hop options and requirements on
padding are met, then this requirement is implicitly satisfied.
Herbert Expires 1 September 2023 [Page 13]
Internet-Draft Extension Header Limits February 2023
3.2.2. Receiving extension headers
Per [RFC8200], a host node that receives a packet with an extension
headers must process all the extension headers in the packet before
accepting the payload and processing the payload.
As described in [RFC8504] a host may establish limits on the
processing of extension headers. This specification reiterates and
updates those requirements to allow for a host to send an RFC8883
error if a limit has been exceeded.
* A host MAY set a limit on the maximum number of non-padding
options allowed in the Destination Options or Hop-by-Hop Options
extension headers. If this limit is supported then the maximum
number SHOULD be configurable, the limit MUST be greater than or
equal to 8, and the default value SHOULD be set to 8. The limits
for Destination options and Hop-by-Hop options MAY be separately
configurable. If a packet is received and the number of
Destination or Hop-by-Hop options exceeds the limit, then the
packet SHOULD be discarded and and an ICMP Parameter Problem with
code 9 MAY be sent to the packet's source address.
* A host MAY set a limit on the maximum number of options (padding
or non-padding) allowed in Destination Options or Hop-by-Hop
Options extension headers. If this limit is supported then the
maximum number SHOULD be configurable and the limit MUST be
greater than or equal to 16. The limits for Destination options
and Hop-by-Hop options MAY be separately configurable. If a
packet is received and the number of destination or Hop-by-Hop
options exceeds the limit, then the packet SHOULD be discarded and
and an ICMP Parameter Problem with code 9 MAY be sent to the
packet's source address
* A host node MAY set a limit on the length of an extension header.
If this limit is supported then the limit SHOULD be configurable
and the limit MUST be greater than or equal to 64 bytes. The
length limits for different extension headers MAY be separately
configurable.
* A host node MAY set a limit on the Data Length of a Hop-by-Hop or
Destination option. If this limit is supported then the limit
SHOULD be configurable, and the limit MUST be greater than or
equal to 60 bytes. The limits for Destination options and Hop-by-
Hop options MAY be separately configurable. If a packet is
received and a Hop-by-Hop or destination option has a length that
exceeds the limit, then the packet SHOULD be discarded and an ICMP
Parameter Problem with code 10 MAY be sent to the packet's source
address.
Herbert Expires 1 September 2023 [Page 14]
Internet-Draft Extension Header Limits February 2023
* A host MAY limit the number of consecutive PAD1 options in
destination options or Hop-by-Hop options to 7. In this case, if
there are more than 7 consecutive PAD1 options present, the packet
SHOULD be discarded and an ICMP Parameter Problem with code 10 MAY
be sent to the packet's source address
* A host MAY limit the number of bytes in a PADN option to be less
than 8. In such a case, if a PADN option is present that has a
length greater than 7, the packet SHOULD be discarded and an ICMP
Parameter Problem with code 10 MAY be sent to the packet's source
address.
* A host MAY set a limit on the maximum length of Destination
Options or Hop-by-Hop Options extension headers. This value
SHOULD be configurable, and if the limit is used then the limit
MUST be greater than or equal to 64 bytes. If a packet is
received and the length of the Destination or Hop-by-Hop Options
extension header exceeds the length limit, then the packet SHOULD
be discarded and an ICMP Parameter Problem with code 6 MAY be sent
to the packet's source address.
* A host node MAY set a limit on the maximum length of the IPv6
header chain, or equivalently a host MAY set a limit on the
aggregate length of extension headers in a packet. If the limit
is used then it MUST be greater than or equal to 104 bytes, or,
equivalently, the limit on aggregate header extension length MUST
be greater than or equal to 64 bytes. If a packet is received and
the aggregate length of the IPv6 header chain exceeds the limit
then the packet SHOULD be discarded and an ICMP Parameter Problem
with code 7 MAY be sent to the packet's source address.
* A host MAY disallow consecutive padding options, either PAD1 or
PADN, to be present in a packet. If consecutive padding options
are received and disallowed by the host, the then packet SHOULD be
discarded and an ICMP Parameter Problem with code 9 MAY be sent to
the packet's source address.
3.3. Intermediate node and intermediate destination requirements
The following common requirements are established for intermediate
nodes and intermediate destination nodes that receive and process
packets with extension headers.
* An intermediate node MUST be able to correctly forward packets
that contain an IPv6 header chain of 104 or fewer bytes, or
equivalently an intermediate node MUST be able to process a packet
with an aggregate length of extension headers less than or equal
to 64 bytes.
Herbert Expires 1 September 2023 [Page 15]
Internet-Draft Extension Header Limits February 2023
* Per [RFC8200] an intermediate node MAY be configured to not
process Hop-by-Hop Options. If a node is configured as such and a
packet with Hop-by-Hop options is received, the extension header
MUST be be skipped and the packet MUST otherwise be properly
processed and forwarded.
* An intermediate node MAY limit the number of non-padding Hop-by-
Hop options that it processes. If a limit is exceeded, that is a
packet contains more non-padding options than are configured to
process, the intermediate node SHOULD stop processing the Hop-by-
Hop Option and ignore any options in the chain beyond the limit.
It is NOT RECOMMENDED that an intermediate node discards the
packet because the limit is exceeded, however if it does so then
the intermediate node MAY send an ICMP Parameter Problem with code
10 to the packet's source address.
* An intermediate node MAY limit the number of Hop-by-Hop options
(padding or non-padding) that it processes. If a limit is
exceeded, that is a packet contains more non-padding options than
are configured to process, the intermediate node SHOULD stop
processing the Hop-by-Hop options and ignore any options in the
chain beyond the limit. It is NOT RECOMMENDED that the
intermediate node discards the packet because the limit is
exceeded, however if it does so then the intermediate node MAY
send an ICMP Parameter Problem with code 10 to the packet's source
address.
* If an intermediate node encounters an unknown Hop-by-Hop option
and the two high order bits are not 00 then the node SHOULD
immediately stop processing the option chain and ignore any
options in the chain beyond the unknown option. An intermediate
node MAY either elect to discard the packet and MAY send an ICMP
Parameter Problem per the requirements of [RFC8200] and
[I-D.ietf-6man-hbh-processing]; or the intermediate node MAY
forward the packet and effectively disregard the high order two
bits in the option type. The motivation for this requirement is
to simplify processing at intermediate nodes. Since intermediate
nodes are no longer required to process all options, ignoring an
unknown option is equivalent to had the node been configured to
process no options or a number of options up to the unknown option
with the high order two bits being non-zero. Note, that if the
high order two bits are non-zero for an option that is unknown to
the destination host then the packet will be discarded since the
destination host is required to process all Hop-by-Hop options in
a packet or to discard a packet if its limit for maximum number of
options to process is exceeded.
* An intermediate node MAY set a limit on the maximum length of Hop-
Herbert Expires 1 September 2023 [Page 16]
Internet-Draft Extension Header Limits February 2023
by-Hop Options extension headers. This value SHOULD be
configurable. If this limit is exceeded, that is a packet has an
extension header larger then the limit, then the intermediate node
SHOULD stop processing the Hop-by-Hop Option and ignore any
options in the chain beyond the limit. It is NOT RECOMMENDED that
the intermediate node discards the packet because the limit is
exceeded, however if it does so then the intermediate node MAY
send an ICMP Parameter Problem with code 10 to the packet's source
address.
3.4. Intermediate destination requirements
The following are requirements specific to intermediate destinations
pertaining to the processing of Destination Options before the
Routing header. For processing Hop-by-Hop options at an intermediate
destination, the requirements for processing them at an intermediate
node are assumed.
* An intermediate destination MAY limit the maximum length of
Destination Options extension header before the Routing header.
This value SHOULD be configurable, and the default is to accept
options of any length. If a limit is defined is MUST be at least
64 bytes. If the limit is exceeded then the intermediate
destination SHOULD discard the packet and MAY send an ICMP
Parameter Problem with code 6 to the packet's source address.
* An intermediate destination node MAY limit the number of non-
padding options in Destination Options before the Routing header.
If this limit is supported then the maximum number SHOULD be
configurable and the limit MUST be greater than or equal to 8. If
a limit is exceeded, that is a packet contains more non-padding
options than are configured to process, the intermediate
destination node SHOULD discard the packet and MAY send an ICMP
Parameter Problem with code 10 to the packet's source address.
* An intermediate destination node MAY limit the number of options
(padding or non-padding) in Destination Options before the Routing
header. If this limit is supported then the maximum number SHOULD
be configurable and the limit MUST be greater than or equal to 16.
If a limit is exceeded, that is a packet contains more options
than are configured to process, the intermediate destination node
SHOULD discard the packet and MAY send an ICMP Parameter Problem
with code 10 to the packet's source address.
* An intermediate destination MAY limit the total number bytes in
consecutive PAD1 options in Destination Options before the Routing
Header 7. If the limit is exceeded, that is there are more than
seven bytes in consecutive PAD1 or PADN options present, the
Herbert Expires 1 September 2023 [Page 17]
Internet-Draft Extension Header Limits February 2023
intermediate destination node SHOULD discard the packet and MAY
send an ICMP Parameter Problem with code 10 to the packet's source
address.
* An intermediate destination MAY limit the number of bytes in a
PADN option in Destination Options before the Routing header to be
less than 8. In such a case, if a PADN option is present that has
a length greater than 7, the packet SHOULD be discarded and the
intermediate destination node SHOULD discard the packet and MAY
send an ICMP Parameter Problem with code 10 to the packet's source
address.
* An intermediate MAY limit the maximum length of Destination
Options extension header before the Routing header. If this limit
is supported then the limit SHOULD be configurable and the limit
MUST be greater than or equal to 64 bytes. If a packet is
received and the length of the Destination or Hop-by-Hop Options
extension header exceeds the length limit, the intermediate
destination node SHOULD discard the packet and MAY send an ICMP
Parameter Problem with code 10 to the packet's source address.
4. References
4.1. Normative References
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8504] Chown, T., Loughney, J., and T. Winters, "IPv6 Node
Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
January 2019, <https://www.rfc-editor.org/info/rfc8504>.
[RFC8883] Herbert, T., "ICMPv6 Errors for Discarding Packets Due to
Processing Limits", RFC 8883, DOI 10.17487/RFC8883,
September 2020, <https://www.rfc-editor.org/info/rfc8883>.
4.2. Informative References
[I-D.ietf-6man-hbh-processing]
Hinden, R. M. and G. Fairhurst, "IPv6 Hop-by-Hop Options
Processing Procedures", Work in Progress, Internet-Draft,
draft-ietf-6man-hbh-processing-05, 23 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
hbh-processing-05>.
Herbert Expires 1 September 2023 [Page 18]
Internet-Draft Extension Header Limits February 2023
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", RFC 7872,
DOI 10.17487/RFC7872, June 2016,
<https://www.rfc-editor.org/info/rfc7872>.
[RFC9098] Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston,
G., and W. Liu, "Operational Implications of IPv6 Packets
with Extension Headers", RFC 9098, DOI 10.17487/RFC9098,
September 2021, <https://www.rfc-editor.org/info/rfc9098>.
Author's Address
Tom Herbert
SiPanda
Santa Clara, CA,
United States of America
Email: tom@herbertland.com
Herbert Expires 1 September 2023 [Page 19]
