One document matched: draft-ietf-6man-udpzero-01.xml
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<rfc category="info" docName="draft-ietf-6man-udpzero-01" ipr="trust200902">
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<!-- ***** FRONT MATTER ***** -->
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
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<title abbrev="">IPv6 UDP Checksum Considerations</title>
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<author fullname="Godred Fairhurst" initials="G." surname="Fairhurst">
<organization>University of Aberdeen</organization>
<address>
<postal>
<street>School of Engineering</street>
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<country>Scotland, UK</country>
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<email>gorry@erg.abdn.ac.uk</email>
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</author>
<author fullname="Magnus Westerlund" initials="M" surname="Westerlund">
<organization>Ericsson Research</organization>
<address>
<postal>
<street>Torshamgatan 23</street>
<city>Stockholm</city>
<region></region>
<code>SE-164 80</code>
<country>Sweden</country>
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<facsimile></facsimile>
<email>magnus.westerlund@ericsson.com</email>
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</author>
<date day="8" month="August" year="2010" />
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<abstract>
<t>This document examines the role of the transport checksum when used
with IPv6, as defined in RFC2460. It presents a summary of the
trade-offs for evaluating the safety of updating RFC 2460 to permit an
IPv6 UDP endpoint to use a zero value in the checksum field to indicate
that no checksum is present. The document describes issues and design
principles that need to be considered when UDP is used with IPv6 to
support tunnel encapsulations and provides recommendations.</t>
<!--*********************************************************************************************************
********* Magnus - to do ******
-->
<!--*********************************************************************************************************
The text says: Allowing fragmentation would also open the receiver to a wide range of mis-behaviours.
- This text is somewhat vague on what the dangers actually are!
Mark Townsley maintains that fragmentation is essential to protocols like L2TP, and that if the tunnel
provides integrity checks itself this is safe.
Since IPv6 does not protect the reassembly buffer, the corruption of fragments is already something that
may be exected in normal IPv6 operation. -->
<!-- -->
</abstract>
</front>
<middle>
<section title="Introduction">
<t>The User Datagram Protocol (UDP) transport was defined by <xref
target="RFC0768">RFC768</xref> for IPv4 <xref
target="RFC0791">RFC791</xref> and is defined in <xref
target="RFC2460">RFC2460</xref> for IPv6 hosts and routers. A UDP
transport endpoint may be either a host or a router. The <xref
target="RFC5405">UDP Usage Guidelines</xref> provides overall guidance
for application designers, including the use of UDP to support
tunneling. These guidelines are applicable to this discussion.</t>
<t>This section provides a background to key issues, and introduces the
use of UDP as a tunnel transport protocol.</t>
<t>Section 2 describes a set of standards-track datagram transport
protocols that may be used to support tunnels.</t>
<t>Section 3 evaluates proposals to update the UDP transport behaviour
to allow for better support of tunnel protocols. It focuses on a
proposal to eliminate the checksum for this use-case with IPv6 and
assess the trade-offs that would arise.</t>
<t>Section 4 reviews the trade offs and provides recommendations.</t>
<section title="Background">
<t>An Internet transport endpoint should concern itself with the
following issues:</t>
<t><list style="symbols">
<t>Protection of the endpoint transport state from unnecessary
extra state (i.e. Invalid state from rogue packets).</t>
<t>Protection of the endpoint transport state from corruption of
internal state.</t>
<t>Pre-filtering by the endpoint of erroneous data, to protect the
transport from unnecessary processing and from corruption that it
can not itself reject.</t>
<t>Pre-filter of incorrectly addressed destination packets, before
responding to a source address.</t>
</list></t>
<t>UDP, as defined in <xref target="RFC0768"></xref>, supports two
checksum behaviours when used with IPv4. The normal behaviour is for
the sender to calculate a checksum over a block of data that includes
a pseudo header and the UDP datagram payload. The UDP header includes
a 16-bit one's complement checksum that provides a statistical
guarantee that the payload was not corrupted in transit. This also
allows a receiver to verify that the endpoint was the intended
destination of the datagram, because the transport pseudo header
covers the IP addresses, port numbers, transport payload length, and
Next Header/Protocol value corresponding to the UDP transport protocol
<xref target="RFC1071"></xref>. The length field verifies that the
datagram is not truncated or padded. The checksum therefore protects
an application against receiving corrupted payload data in place of,
or in addition to, the data that was sent. Although the IPv4 <xref
target="RFC0768">UDP</xref> checksum may be disabled, applications are
recommended to enable UDP checksums <xref
target="RFC5405"></xref>.</t>
<t>IPv4 UDP checksum control is often a kernel-wide configuration
control (e.g. In Linux and BSD), rather than a per socket call. There
are also Networking Interface Cards (NICs) that automatically
calculate <xref target="RFC0793">TCP </xref> and UDP checksums on
transmission when a checksum of zero is sent to the NIC, using a
method known as checksum offloading.</t>
<t>The network-layer fields that are validated by a transport checksum
are:</t>
<t><list style="symbols">
<t>Endpoint IP source address (always included in the pseudo
header of the checksum)</t>
<t>Endpoint IP destination address (always included in the pseudo
header of the checksum)</t>
<t>Upper layer payload type (always included in the pseudo header
of the checksum)</t>
<t>IP length of payload (always included in the pseudo header of
the checksum)</t>
<t>Length of the network layer extension headers (i.e. by correct
position of the checksum bytes)</t>
</list></t>
<t>The transport-layer fields that are validated by a transport
checksum are:<list style="symbols">
<t>Transport demultiplexing, i.e. ports (always included in the
checksum)</t>
<t>Transport payload size (always included in the checksum)</t>
</list></t>
<t>Transport endpoints also need to verify the correctness of
reassembly of any fragmented datagram (unless the application using
the payload is corruption tolerant, as indicated by UDP-Lite's
checksum coverage field). For UDP, this is normally provided as a part
of the integrity check. Disabling the IPv4 checksum prevents this
check. A lack of checksum can lead to issues in a translator or
middlebox (e.g. Many IPv4 Network Address Translators, NATs, rely on
port numbers to find the mappings, packet fragments do not carry port
numbers, so fragments get dropped). <xref
target="RFC2765">RFC2765</xref> provides some guidance on the
processing of fragmented IPv4 UDP datagrams that do not carry a UDP
checksum.</t>
<t>IPv6 does not provide a network-layer integrity check. The removal
of the header checksum from the IPv6 specification released routers
from a need to update a network-layer checksum for each router hop as
the IPv6 Hop Count is changed (in contrast to the checksum update
needed when an IPv4 router modifies the Time-To-Live (TTL)).</t>
<t>The IP header checksum calculation was seen as redundant for most
traffic (with UDP or TCP checksums enabled), and people wanted to
avoid this extra processing. However, there was concern that the
removal of the IP header checksum in IPv6 would lessen the protection
of the source/destination IP addresses and result in a significant (a
multiplier of ~32,000) increase in the number of times that a UDP
packet was accidentally delivered to the wrong destination address
and/or apparently sourced from the wrong source address when the UDP
checksum was set to zero. This would have had implications on the
detectability of mis-delivery of a packet to an incorrect
endpoint/socket, and the robustness of the Internet infrastructure.
The use of the UDP checksum is therefore required <xref
target="RFC2460"> </xref> when endpoint application s transmit UDP
datagrams over IPv6.</t>
</section>
<section title="Use of UDP Tunnels ">
<t>One increasingly popular use of UDP is as a tunneling protocol,
where a tunnel endpoint encapsulates the packets of another protocol
inside UDP datagrams and transmits them to another tunnel endpoint.
Using UDP as a tunneling protocol is attractive when the payload
protocol is not supported by the middleboxes that may exist along the
path, because many middleboxes support transmission using UDP. In this
use, the receiving endpoint decapsulates the UDP datagrams and
forwards the original packets contained in the payload <xref
target="RFC5405"></xref>. Tunnels establish virtual links that appear
to directly connect locations that are distant in the physical
Internet topology and can be used to create virtual (private)
networks.</t>
<section title="Motivation for new approaches">
<t>A number of tunnel encapsulations deployed over IPv4 have used
the UDP transport with a zero checksum. Users of these protocols
expect a similar solution for IPv6.</t>
<t>A number of tunnel protocols are currently being defined (e.g.
Automated Multicast Tunnels, <xref target="AMT">AMT</xref>, and the
Locator/Identifier Separation Protocol, <xref
target="LISP">LISP</xref>). These protocols have proposed an update
to IPv6 UDP checksum processing. These tunnel protocols could
benefit from simpler checksum processing for various reasons:<list
style="symbols">
<t>Reducing forwarding costs, motivated by redundancy present in
the encapsulated packet header, since in tunnel encapsulations,
payload integrity and length verification may be provided by
higher layer encapsulations (often using the IPv4, UDP,
UDP-Lite, or TCP checksums).</t>
<t>Eliminating a need to access the entire packet when
forwarding the packet by a tunnel endpoint.</t>
<t>Enhancing ability to traverse middleboxes, especially Network
Address Translators, NATs.</t>
<t>A desire to use the port number space to enable
load-sharing.</t>
</list></t>
</section>
<section title="Reducing forwarding cost">
<t>It is a common requirement to terminate a large number of tunnels
on a single router/host. Processing per tunnel concerns both state
(memory requirements) and per-packet processing costs.</t>
<t>Automatic IP Multicast Without Explicit Tunnels, known as <xref
target="AMT">AMT</xref> currently specifies UDP as the transport
protocol for packets carrying tunneled IP multicast packets. The
current specification for AMT requires that the UDP checksum in the
outer packet header should be 0 (see Section 6.6). It argues that
the computation of an additional checksum, when an inner packet is
already adequately protected, is an unwarranted burden on nodes
implementing lightweight tunneling protocols. The AMT protocol needs
to replicate a multicast packet to each gateway tunnel. In this
case, the outer IP addresses are different for each tunnel and
therefore require a different pseudo header to be built for each UDP
replicated encapsulation.</t>
<t>The argument concerning redundant processing costs is valid
regarding the integrity of a tunneled packet. In some architectures
(e.g. PC-based routers), other mechanisms may also significantly
reduce checksum processing costs: There are implementations that
have optimised checksum processing algorithms, including the use of
checksum-offloading. This processing is readily available for IPv4
packets at high line rates. Such processing may be anticipated for
IPv6 endpoints, allowing receivers to reject corrupted packets
without further processing. Relaxing RFC 2460 to minimise the
processing impact for existing hardware is a transition policy
decision, which seems undesirable if at the same time it yields a
solution that may reduce stability and functionality in future
network scenarios.</t>
</section>
<section title="Need to inspect the entire packet">
<t>The currently-deployed hardware in many routers uses a fast-path
processing that only provides the first n bytes of a packet to the
forwarding engine, where typically n < 128. This prevents fast
processing of a transport checksum over an entire (large) packet.
Hence the currently defined IPv6 UDP checksum is poorly suited to
use within a router that is unable to access the entire packet and
does not provide checksum-offloading.</t>
</section>
<section title="Interactions with middleboxes">
<t>In IPv4, UDP-encapsulation may be desirable for NAT traversal,
since UDP support is commonly provided.</t>
<t>IPv6 NAT traversal does not necessarily present the same protocol
issues as for IPv4. It is not clear that NATs will work the same way
for IPv6. Any change to RFC 2460 would also require rewriting (or
defining) IPv6 NAT behaviour to achieve consistent widescale
deployment.</t>
<t>The requirements for IPv6 firewall traversal are likely be to be
similar to those for IPv4. In addition, it can be reasonably
expected that a firewall conforming to RFC 2460 will not regard UDP
datagrams with a zero checksum as valid packets. If an updated IPv6
mode were to be defined for IPv6, this may also need firewalla to be
updated.</t>
<t>Key questions in this space include:</t>
<t><list style="symbols">
<t>What do IPv6 routers do today with zero-checksum UDP
packets?</t>
<t>What types of middleboxes does the tunnel protocol need to
cross (routers, NAT boxes, firewalls, etc.), and how will those
middleboxes deal with these packets?</t>
<t>What other IPv6 middleboxes exist today, and what would they
do?</t>
</list></t>
</section>
<section title="Support for load balancing">
<t>The UDP port number fields have been used as a basis to design
load-balancing solutions for IPv4. This approach could also be
leveraged for IPv6. However, support for extension headers would
increase the complexity of providing standards-compliant solutions
for IPv6.</t>
<t>An alternate method could utilise the IPv6 Flow Label to perform
load balancing. This would release IPv6 load-balancing devices from
the need to assume semantics for the use of the transport port
field. This use of the flow-label is consistent with the intended
use, although further clarity may be needed to ensure the field can
be consistently used for this purpose, (e.g. Equal-Cost Multi-Path
routing, ECMP <xref target="ECMP"></xref>). Router vendors could be
encouraged to start using the IPv6 Flow Label as a part of the flow
hash, providing support for ECMP without requiring use of UDP.</t>
</section>
</section>
</section>
<section title="Standards-Track Transports">
<t>The IETF has defined a set of IPv6 transports that at be used with
IPv6. These are described in the following sections, followed by a
description of standards tunnel encapsulations.</t>
<section title="UDP with Standard Checksum">
<t>UDP with standard checksum behaviour is defined in RFC 2460, and
should be the default choice. Guidelines are provided in <xref
target="RFC5405"></xref>.</t>
</section>
<section title="UDP-Lite">
<t>UDP-Lite <xref target="RFC3828"></xref> offers an alternate
transport to UDP, specified as a proposed standard, RFC 3828. A MIB is
defined in RFC 5097 and unicast usage guidelines in <xref
target="RFC5405"></xref>.</t>
<t>UDP-Lite provides a checksum with an optional partial coverage.
When using this option, a datagram is divided into a sensitive part
(covered by the checksum) and an insensitive part (not covered by the
checksum). Errors/corruption in the insensitive part will not cause
the datagram to be discarded by the transport layer at the receiving
endpoint. A minor side-effect of using UDP-Lite is that this was
specified for damage-tolerant payloads, and some link-layers may
employ different link encapsulations when forwarding UDP-Lite segments
(e.g. Over radio access bearers). When the checksum covers the entire
packet, which should be the default.</t>
<section title="Using UDP-Lite as a Tunnel Encapsulation">
<t>Tunnel encapsulations can use UDP-Lite (e.g. Control And
Provisioning of Wireless Access Points, CAPWAP), since UDP-Lite
provides a transport-layer checksum, including an IP pseudo header
checksum, in IPv6, without the need for a router/middelbox to
traverse the entire packet payload.</t>
<t>In the LISP case, the bytes that would need to be "checksummed"
for UDP-Lite would be the set of bytes that are added to the packet
by the LISP encapsulating router. When an IPv4/UDP header is
per-pended by a LISP router, the LISP ETR needs to calculate the IP
header checksum over 20 bytes (the IP header). If an IPv6/UDP-Lite
header were per-pended by a LISP router, the ETR would need to
calculate an IP header checksum over 48 bytes (the IP pseudo header
and the UDP header). This results in an increase in the number of
bytes to be the checksummed for IPv6 (48 bytes rather than 20), but
this is not thought to be a major additional processing overhead for
a well-optimized implementation where the pre-pended header bytes
are already in memory.</t>
</section>
</section>
<section title="IP in IPv6 Tunnel Encapsulations">
<t>The IETF has defined a set of tunneling protocols. These do not
include a checksum, since tunnel encapsulations are typically layered
directly over the Internet layer (identified by the upper layer type
field) and are also not used as endpoint transport protocols. That is,
there is little chance of confusing a tunnel-encapsulated packet with
other application data that could result in corruption of application
state or data.</t>
<t>From the end-to-end perspective, the principal difference is that
the network-layer Next Header field identifies a separate transport,
which reduces the probability that corruption could result in the
packet being delivered to the wrong endpoint or application.
Specifically, packets are only delivered to protocol modules that
process a specific next header value. The next header field therefore
provides a first-level check of correct demultiplexing. In contrast,
the UDP port space is shared by many diverse applications and
therefore UDP demultiplexing relies solely on the port numbers.</t>
</section>
</section>
<section anchor="Proposal"
title="Evaluation of proposal to update RFC 2460 to support zero checksum">
<t>This section evaluates a proposal to update IPv6 [RFC2460], to
provide the option that some nodes may suppress generation and checking
of the UDP transport checksum. The decision to omit an integrity check
at the IPv6 level means that the transport check is overloaded with many
functions including validating: <list style="symbols">
<t>the endpoint address was not corrupted within a router - i.e.
This packet was intended to be received by this destination and a
wrong header has not been spliced to a different payload;</t>
<t>that extension header processing is correctly delimited - i.e.
The start of data has not been corrupted. In this case, reception of
a valid next header value provides some protection;</t>
<t>reassembly processing, when used;</t>
<t>the length of the payload;</t>
<t>the port values - i.e. The correct application receives the
payload (applications should also check the expected use of source
ports/addresses);</t>
<t>the payload integrity.</t>
</list></t>
<t>In IPv4, the first four checks are performed using the IPv4 header
checksum.</t>
<t>In IPv6, these checks occur within the endpoint stack using the UDP
checksum information. An IPv6 node also relies on the header information
to determine whether to send an ICMPv6 error message [RFC2463] and to
determine the node to which this is sent. Corrupted information may lead
to misdelivery to an unintended application socket on an unexpected
host.</t>
<section title="Alternatives to the Standard Checksum">
<t>There are several alternatives to the normal method for calculating
the UDP Checksum that do not require a tunnel endpoint to inspect the
entire packet when computing a checksum. These include (in decreasing
order of complexity):</t>
<t><list style="symbols">
<t>Delta computation of the checksum from an encapsulated checksum
field. Since the checksum is a cumulative sum (RFC 1624), an
encapsulating header checksum can be derived from the new pseudo
header, the inner checksum and the sum of the other network-layer
fields not included in the pseudo header of the encapsulated
packet, in a manner resembling incremental checksum update <xref
target="RFC1141"></xref>. This would not require access to the
whole packet, but does require fields to be collected across the
header, and arithmetic operations on each packet. The method would
only work for packets that contain a 2's complement transport
checksum (i.e. it would not be appropriate for SCTP or when IP
fragmentation is used). The process may be easier for IPv4 over
IPv6 encapsulation, where the encapsulated IPv4 header checksum
could be used as a basis.</t>
<t>UDP-Lite with the checksum coverage set to only the header
portion of a packet. This requires a pseudo header checksum
calculation only on the encapsulating packet header. The computed
checksum value may be cached (before adding the Length field) for
each flow/destination and subsequently combined with the Length of
each packet to minimise per-packet processing. This value is
combined with the UDP payload length for the pseudo header,
however this length is expected to be known when performing packet
forwarding.</t>
<t>The proposed UDP Tunnel Transport, UDPTT <xref
target="UDPTT"></xref> suggested a method where UDP would be
modified to derive the checksum only from the encapsulating packet
protocol header. This value does not change between packets in a
flow. The value may be cached per flow/destination to minimise
per-packet processing. This proposal is not discussed further in
this document, since function is nearly the same as for
UDP-Lite.</t>
<t>Use of a new IPv6 Extension Header that provides an end-to-end
validation check at the network layer. This would allow an
endpoint to verify delivery to an appropriate end point, but would
also require IPv6 nodes to correctly handle the additional
header.</t>
<t>UDP modified to disable checksum processing<xref target="UDPZ">
</xref> (if progressed). This requires no checksum calculation,
but would require constraints on appropriate usage.</t>
</list>These options are discussed further in the following
sections.</t>
</section>
<section title="Applicability of method">
<t>The expectation of the present proposal defined in <xref
target="UDPZ"></xref> is that this change would only apply to IPv6
router nodes that implement specific protocols which permit omission
of UDP checksums. However, the distinction between a router and a host
is not always clear, especially at the transport level. Systems (such
as unix-based operating systems) routinely provide both functions.
There is also no way to identify the role of a receiver from a
received packet.</t>
<t>Any new method would therefore need a specific applicability
statement indicating when the mechanism can (and can not) be used.
There are additional requirements, e.g. fragmentation must not be
performed, since correct reassembly can not be verified at the
receiver when there is no checksum. Allowing fragmentation would also
open the receiver to a wide range of mis-behaviours. Host-based
fragmentation must therefore be disabled. Policing this, and ensuring
correct interactions with the stack, implies much more than simply
disabling the checksum algorithm for specific packets at the transport
interface.</t>
<t>There are also proposals to simply ignore a specific received UDP
checksum value, however this also can result in problems (e.g. when
used with a NAT that always adjusts the checksum value).</t>
<t>The IETF should carefully consider constraints on sanctioning the
use of any new transport mode. If this is specified and widely
available, it may be expected to be used by applications that are
perceived to gain benefit. Any solution that uses an end-to-end
transport protocol, rather than an IP in IP encapsulation, also needs
to minimise the possibility that end-hosts could confuse a corrupted
or wrongly delivered packet with that of data addressed to an
application running on their endpoint.</t>
</section>
<section title="Effect of packet modification in the network">
<t>IP packets may be corrupted as they traverse an Internet path.
Evidence has been presented <xref target="Sigcomm2000"></xref> to show
that this was once an issue with IPv4 routers, and occasional
corruption could result from bad internal router processing in routers
or hosts. These errors are not detected by the strong frame checksums
employed at the link-layer (RFC 3819). There is no current evidence
that such cases are rare in the modern Internet, nor that they may not
be applicable to IPv6. It therefore seems prudent not to relax this
constraint. The emergence of low-end IPv6 routers and the proposed use
of NAT with IPv6 further motivate the need to protect from this type
of error.</t>
<t>Corruption in the network may result in: <list style="symbols">
<t>A datagram being mis-delivered to the wrong host/router or the
wrong transport entity within an endpoint. Such a datagram needs
to be discarded.</t>
<t>A datagram payload being corrupted, but still delivered to the
intended host/router transport entity. Such a datagram needs to be
either discarded or correctly processed by an application that
provides its own integrity checks.</t>
<t>A datagram payload being truncated by corruption of the length
field. Such a datagram needs to be discarded.</t>
</list></t>
<t>When a checksum is used with UDP over IPv6, this significantly
reduces the impact of errors, reducing the probability of undetected
corruption of state (and data) on both the host stack and the
applications using the transport service.</t>
<t>The following sections examine the impact of modifying each of
these header fields.</t>
<section title="Corruption of the destination IP address">
<t>An IP endpoint destination address could be modified in the
network (e.g. corrupted by an error). This is not a concern for
IPv4, because the IP header checksum will result in this packet
being discarded by the receiving IP stack. Such modification in the
network can not be detected when using IPv6.</t>
<t>There are two possible outcomes:</t>
<t><list style="symbols">
<t>Delivery to a destination address that is not in use (the
packet will not be delivered, but could result in an error
report).</t>
<t>Delivery to a different destination address. This
modification will normally be detected by the transport
checksum, resulting in silent discard. Without this checksum,
the packet would be passed to the endpoint port demultiplexing
function. If an application is bound to the associated ports,
the packet payload will be passed to the application (see the
subsequent section on port processing).</t>
</list></t>
</section>
<section title="Corruption of the source IP address">
<t>This section examines what happens when the source address is
corrupted in transit. (This is not a concern in IPv4, because the IP
header checksum will normally result in this packet being discarded
by the receiving IP stack).</t>
<t>Corruption of an IPv6 source address does not result in the IP
packet being delivered to a different endpoint protocol or
destination address. If only the source address is corrupted, the
datagram will likely be processed in the intended context, although
with erroneous origin information. The result will depend on the
application or protocol that processes the packet. Some examples
are:</t>
<t><list style="symbols">
<t>An application that requires a per-established context may
disregard the datagram as invalid, or could map this to another
context (if a context for the modified source address was
already activated).</t>
<t>A stateless application will process the datagram outside of
any context, a simple example is the ECHO server, which will
respond with a datagram directed to the modified source address.
This would create unwanted additional processing load, and
generate traffic to the modified endpoint address.</t>
<t>Some datagram applications build state using the information
from packet headers. A previously unused source address would
result in receiver processing and the creation of unnecessary
transport-layer state at the receiver. For example, Real Time
Prottocol (RTP) flows commonly employ a source independent
receiver port. State is created for each received flow.
Reception of a datagram with a corrupted source address will
therefore result in accumulation of unnecessary state in the RTP
state machine, including collision detection and response (since
the same synchronization source, SSRC, value will appear to
arrive from multiple source IP addresses).</t>
</list></t>
<t>In general, the effect of corrupting the source address will
depend upon the protocol that processes the packet and its
robustness to this error. For the case where the packet is received
by a tunnel endpoint, the tunnel application is expected to
correctly handle a corrupted source address.</t>
<t>The impact of source address modification is more difficult to
quantify when the receiving application is not that originally
intended and several fields have been modified in transit.</t>
</section>
<section title="Delivery to an unexpected port">
<t>This section considers what happens if one or both of the UDP
port values are corrupted in transit. (This can also happen with
IPv4 in the zero checksum case, but not when UDP checksums are
enabled or with UDP-Lite). If the ports were corrupted in transit,
packets may be delivered to the wrong process (on the intended
machine) and/or responses or errors sent to the wrong application
process (on the intended machine).</t>
<t>There are several possible outcomes for a packet that passes and
does not use the UDP checksum validation:</t>
<t><list style="symbols">
<t>Delivery to a port that is not in use. The packet is
discarded, but could generate an ICMPv6 message (e.g. port
unreachable).</t>
<t>It could be delivered to a different node that implements the
same application, where the packet may be accepted, generating
side-effects or accumulated state.</t>
<t>It could be delivered to an application that does not
implement the tunnel protocol, where the packet may be
incorrectly parsed, and may be misinterpreted, generating
side-effects or accumulated state.</t>
</list></t>
<t>The probability of each outcome depends on the statistical
probability that the source address and the destination port of the
datagram (the source port is not always used in UDP) match those of
an existing connection. Unfortunately, such a match may be more
likely for UDP than for connection-oriented transports, because<list
style="numbers">
<t>There is no handshake prior to communication and no sequence
numbers (as in TCP, DCCP, or SCTP). Together, this makes it hard
to verify that an application is given only the data associated
with a transport session.</t>
<t>Applications writers often bind to wild-card values in
endpoint identifiers and do not always validate correctness of
datagrams they receive (guidance on this topic is provided in
<xref target="RFC5405"></xref>).</t>
</list>While these rules could in principle be revised to declare
naive applications as "Historic". This remedy is not realistic: the
transport owes it to the stack to do its best to reject bogus
datagrams.</t>
<t>If checksum coverage is suppressed, the application therefore
needs to provide a method to detect and discard the unwanted data.
The encapsulated tunnel protocol would need to perform its own
integrity checks on any control information and ensure an integrity
check is applied to the tunneled packet. It is not reasonable to
assume that it is safe for one application to use a zero checksum
value and that other applications will not. It is therefore
important to consider the possibility that a packet will be received
by a different node to that for which it was intended, or that it
will arrive at the correct tunnel destination with the wrong source
address in the external header.</t>
</section>
<section title="Validating the network path">
<t>IP transports designed for use in the general Internet should not
assume specific characteristics. Network protocols may reroute
packets and change the set of routers and middleboxes along a path.
Therefore transports such as TCP, SCTP and DCCP are designed to
negotiate protocol parameters, adapt to different network path
characteristics, and receive feedback that the current path is
suited to the intended application. Applications using UDP and
UDP-Lite need to provide their own mechanisms to confirm the
validity of the current network path.</t>
<t>Any application/tunnel that seeks to make use of zero checksum
must include functionality to both negotiate and verify that the
zero checksum support is provided by the path and validate that this
continues to work (e.g., in the case of re-routing events) between
the intended parties. This increases the complexity of using such a
solution.</t>
</section>
<section title="Requirements on the specification of transported protocols">
<t>If the IETF were to revise the standard for UDP using IPv6 for
specific use-cases there are a set of questions that would need to
be answered. These include:</t>
<t>Is there a reason why IP in IP is not a reasonable choice for
encapsulation?</t>
<t><list style="symbols">
<t>Examples of arguments for requiring an encapsulation beyond
IP-in-IP include the need for NAT traversal and/or firewall
traversal. However, the use of any new or non-standard transport
protocol or variant would additionally require specific support
in middleboxes.</t>
<t>Another example is a need to perform port-demultiplexing
(e.g. for load balancing or ECMP). This need could also be met
using UDP, UDP-Lite, or another supported transport, or by
utilising the IPv6 flow label.</t>
</list></t>
<t>Is there a reason why UDP-Lite is not a reasonable choice for
encapsulation?</t>
<t><list style="symbols">
<t>One argument against using UDP-Lite is that this transport is
not implemented on all endpoints. However, there is at least one
open source implementation as a part of the Linux kernel since
version 2.6.20.</t>
<t>Another argument against using UDP-Lite is that it uses a
different IPv6 Next Header, which is currently not widely
supported in middleboxes.</t>
<t>It has also been argued that UDP-Lite requires a checksum
computation. The UDP-Lite checksum, for instance includes the
length field, but need not include the UDP-Lite payload, and
therefore would not require access to the full datagram payload
by the tunnel endpoints.</t>
</list></t>
<t>If the IETF needs to revise the rationale for UDP checksums in
RFC 2460, should we remove the checksum or replace it with one
closer to UDP-Lite ?</t>
<t>Additional topics to be considered in making this decision:<list
style="symbols">
<t>In IPv6, a node selects the role of a router or host is
selected on a per interface basis. The role of a router and host
are therefore not fixed, and a consistent method must be
specified that can be used on all nodes. It can not be assumed
that a particular protocol (or transport mode) will only be used
on a specific type of network node (e.g. permitting the UDP
checksum to be disabled only on a router). It is important to
note that protocol changes intended for one specific use are
often re-used for different applications.</t>
<t>Behaviour of NAT/Middleboxes may need to be updated. This is
the case for a zero UDP checksum and also for use of an IPv6
Extension Header carrying a transport checksum.</t>
<t>The method needs to consider the impact of load balancing,
and whether this may be enabled for the chosen transport
protocol.</t>
</list></t>
<t></t>
<t>If a zero checksum approach were to be adopted by the IETF, the
specification should consider adding the following constraints on
usage:</t>
<t><list style="numbers">
<t>IPv6 protocol stack implementations should not by default
allow the new method. The default node behaviour must discard
all IPv6 packets carrying UDP packets with no checksum. RFC 2460
specifies that IPv6 nodes should log discarded packets.</t>
<t>A method must be specified to verify the integrity of the
inner (tunneled) packet for each tunnel application that uses a
zero-checksum. This method must be robust to the use of other
applications that also use a zero-checksum.</t>
<t>Non-IP inner (tunneled) packets must have a CRC or other
mechanism for checking packet integrity.</t>
<t>If a method proposes selective ignoring of the checksum on
reception, it needs to provide guidance that is appropriate for
all use-cases, including defining how currently standardised
nodes handle any new use.</t>
<t>The tunneling protocol must not allow fragmentation of the
inner packets being carried. We suggest the following
elaborations of the above restrictions, if a change in the IPv6
specification moves forward, the tunnel must not forward an
inner (tunneled) IPv4 packet that also has a UDP checksum equal
to 0. This includes not tunneling other tunneling protocols that
also use a UDP checksum equal to 0, even if more deeply
encapsulated packets have checksums or other integrity checking
mechanisms.</t>
<t>If a method proposes recursive tunnels, it needs to provide
guidance that is appropriate for all use-cases. Restrictions may
be needed to the use of a tunnel encapsulations and the use of
recursive tunnels (e.g. Necessary when the endpoint is not
verified).</t>
<t></t>
<t>The new method should remain restricted to endpoints that
explicitly enable this mode and adopt the above procedures.</t>
</list></t>
</section>
</section>
<section title="Comparision">
<t>This section compares different methods to support datagram
tunneling. This includes a proposal for updating the behaviour of UDP.
This is provided as an example, and does not seek to endorse any
specific method or suggest that these proposals are ready to be
standardised. The final column the expected functions if an additional
end-to-end IPv6 extension header were to be required in combination
with use of the zero checksum option.<!--
XXX Need to confirm checksum method for NH
--></t>
<figure>
<preamble>Comparison of functions for selected methods</preamble>
<artwork><![CDATA[ UDP UDPv4 UDPL IP IP UDPv6 UDPv6 UPv6
zero in in zero EH
IPv4 IPv6
Incremental cksum update? X - X N/A N/A X - ?
Verification of IP length? X X X X X X X X
Detect dest addr corruption? X X X X - X - X
Detect NH addr corruption? - - - X - - - X
Flow demux fields present? X X X - X X X -
Detect port corruption? X - X N/A N/A X - -
Detect illegal pay length? X X - N/A N/A X X X
Detect pay corruption? X - ? N/A N/A X - -
Static cksum per flow? - X - N/A N/A - X X
Partial/full midbox support? X * ? ? ? X ? ?
Restricted tunnel behaviour X * X X ? X - -
X = Provided/supported
- = Not provided/supported
N/A = Not applicable
? = Partial support
* = Supports a subset of functions (i.e. not all combinations)]]></artwork>
<postamble>Table 1</postamble>
</figure>
</section>
</section>
<section title="Requirements on the specification of transported protocols">
<t>If the IETF were to revise the standard for UDP using IPv6 for
specific use-cases there are a set of questions that would need to be
answered. These include:</t>
<t>Is there a reason why IP in IP is not a reasonable choice for
encapsulation?</t>
<t><list style="symbols">
<t>Examples of arguments for requiring an encapsulation beyond
IP-in-IP include the need for NAT traversal and/or firewall
traversal. However, the use of any new or non-standard transport
protocol or variant would additionally require specific support in
middleboxes.</t>
<t>Another example is a need to perform port-demultiplexing (e.g.
for load balancing or ECMP). This need could also be met using UDP,
UDP-Lite, or another supported transport, or by utilising the IPv6
flow label.</t>
</list></t>
<t>Is there a reason why UDP-Lite is not a reasonable choice for
encapsulation?</t>
<t><list style="symbols">
<t>One argument against using UDP-Lite is that this transport is not
implemented on all endpoints. However, there is at least one open
source implementation as a part of the Linux kernel since version
2.6.20.</t>
<t>Another argument against using UDP-Lite is that it uses a
different IPv6 Next Header, which is currently not widely supported
in middleboxes.</t>
<t>It has also been argued that UDP-Lite requires a checksum
computation. The UDP-Lite checksum, for instance includes the length
field, but need not include the UDP-Lite payload, and therefore
would not require access to the full datagram payload by the tunnel
endpoints.</t>
</list></t>
<t>If the IETF needs to revise the rationale for UDP checksums in RFC
2460, should we remove the checksum or replace it with one closer to
UDP-Lite ?</t>
<t>Additional topics to be considered in making this decision:<list
style="symbols">
<t>In IPv6, a node selects the role of a router or host is selected
on a per interface basis. The role of a router and host are
therefore not fixed, and a consistent method must be specified that
can be used on all nodes. It can not be assumed that a particular
protocol (or transport mode) will only be used on a specific type of
network node (e.g. permitting the UDP checksum to be disabled only
on a router). It is important to note that protocol changes intended
for one specific use are often re-used for different
applications.</t>
<t>Behaviour of NAT/Middleboxes may need to be updated. This is the
case for a zero UDP checksum and also for use of an IPv6 Extension
Header carrying a transport checksum.</t>
<t>The method needs to consider the impact of load balancing, and
whether this may be enabled for the chosen transport protocol.</t>
<t>If a method proposes selective ignoring of the checksum on
reception, it needs to provide guidance that is appropriate for all
use-cases, including defining how currently standardised nodes
handle any new use.</t>
</list></t>
<section title="Constraints required oin usage of a zero checksum">
<t>If a zero checksum approach were to be adopted by the IETF, the
specification should consider adding the following constraints on
usage:</t>
<t><list style="numbers">
<t>IPv6 protocol stack implementations should not by default allow
the new method. The default node receiver behaviour must discard
all IPv6 packets carrying UDP packets with no checksum.</t>
<t>Implementations must provide a way to signal which ports will
be enabled to receive UDP datagrams with a zero checksum. An IPv6
node that enables reception of must enable this only for a
specific port or port-range. This may be implemented via a socket
API call, or similar mechanism.</t>
<t>RFC 2460 specifies that IPv6 nodes should log UDP datagrams
with a zero checksum. This should remain the case for any datagram
received on a port that does not explicitly enable zero-checksum
processing. A port for which zero-checksum has been enabled must
not log the datagram.</t>
<t>(that pass the checksum). A stack may separately identify UDP
datagrams that are discarded with a zero checksum. It should not
add these to the standard log, since the endpoint has not been
verified.</t>
<t>A method must be specified to verify the integrity of the inner
(tunneled) packet for each tunnel application that uses a
zero-checksum. This method must be robust to the use of other
applications that also use a zero-checksum.</t>
<t>Non-IP inner (tunneled) packets must have a CRC or other
mechanism for checking packet integrity.</t>
<t>UDP applications that support use of a zero-checksum, should
not rely upon correct reception of the IP and UDP protocol
information when decoding and processing the packet payload. In
particular, the application must be designed so that corruption of
this information does not result in accumulated state or incorrect
processing of a tunneled payload.</t>
<t>The tunnel must not forward an inner (tunneled) IPv4 packet
that also has a UDP checksum equal to 0. This includes not
tunneling other tunneling protocols that also use a UDP checksum
equal to 0, even if more deeply encapsulated packets have
checksums or other integrity checking mechanisms.</t>
<t>The tunneling protocol must not allow fragmentation of the
inner packets being carried.</t>
<t>If a method proposes recursive tunnels, it needs to provide
guidance that is appropriate for all use-cases. Restrictions may
be needed to the use of a tunnel encapsulations and the use of
recursive tunnels (e.g. Necessary when the endpoint is not
verified).</t>
<t>IPv6 nodes that receive ICMPv6 messages that relate to packets
with a zero UDP checksum must provide appropriate checks
concerning the consistency of the reported packet was actually
originated by the node, before acting upon the information (e.g.
validating the address and port numbers in the ICMPv6 message
body).</t>
</list>Deployment of the new method should remain restricted to
endpoints that explicitly enable this mode and adopt the above
procedures</t>
</section>
</section>
<section title="Summary">
<t>This document examines the role of the transport checksum when used
with IPv6, as defined in RFC2460.</t>
<t>It presents a summary of the trade-offs for evaluating the safety of
updating RFC 2460 to permit an IPv6 UDP endpoint to use a zero value in
the checksum field to indicate that no checksum is present. A decision
not to include a UDP checksum in received IPv6 datagrams could impact a
tunnel application that receives these packets. However, a well-designed
tunnel application should include consistency checks to validate any
header information encapsulated with a packet and ensure that an
integrity check is included for each tunneled packet. When correctly
implemented, such a tunnel endpoint will not be negatively impacted by
omission of the transport-layer checksum. However, other applications at
the intended destination node or another IPv6 node can be impacted if
they are allowed to receive datagrams without a transport-layer
checksum.</t>
<t>In particular, it is important that already deployed applications are
not impacted by any change at the transport layer. If these applications
execute on nodes that implement RFC 2460, they will reject all datagrams
without a UDP checksum.</t>
<t>The implications on firewalls, NATs and other middleboxes need to be
considered. It should not be expected that NATs handle IPv6 UDP
datagrams in the same way as they handle IPv4 UDP datagrams. Firewalls
are intended to be configured, and therefore may need to be explicitly
updated to allow new services or protocols.</t>
<t>In general, UDP-based applications need to employ a mechanism that
allows a large percentage of the corrupted packets to be removed before
they reach an application, both to protect the applications data stream
and the control plane of higher layer protocols. These checks are
currently performed by the UDP checksum for IPv6, or the reduced
checksum for UDP-Lite when used with IPv6.</t>
<t>Although the use of UDP over IPv6 with no checksum may have merits as
a tunnel encapsulation and is widely used in IPv4, there are dangers for
IPv6 nodes (hosts and routers). If the use of UDP transport without a
checksum were to become prevalent for IPv6 (e.g. tunnel and host
applications using this are widely deployed), there would also be a
significant danger of the Internet carrying an increased volume of
packets without a transport checksum for other applications, potentially
including applications that have traditionally used IPv4 UDP transport
without a checksum. This result is highly undesirable. Other solutions
need to be found, such as the use of IPV6 with the minimal checksum
coverage for UDP-Lite. This requires that the IPv4 and IPv6 solutions to
differ, since there are different deployed infrastructures.</t>
<t>Guidance has also been provided to help evaluate the case for
disabling the checksum for specific applications</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>Brian Haberman, Brian Carpenter, Magaret Wasserman, Lars Eggert,
Magnus Westerlund, others in the TSV directorate.</t>
<t>Thanks also to: Rémi Denis-Courmont, Pekka Savola and many
others who contributed comments and ideas via the 6man, behave, lisp and
mboned lists.</t>
</section>
<!-- Possibly a 'Contributors' section ... -->
<section anchor="IANA" title="IANA Considerations">
<t>This document does not require IANA considerations.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>Transport checksums provide the first stage of protection for the
stack, although they can not be considered authentication mechanisms.
These checks are also desirable to ensure packet counters correctly log
actual activity, and can be used to detect unusual behaviours.</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
<!-- -->
<references title="Normative References">
<!--?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?-->
&RFC2119;
<?rfc include='reference.RFC.0791'?>
<?rfc include='reference.RFC.0793'?>
<?rfc include='reference.RFC.1071'?>
<?rfc include='reference.RFC.2460'?>
</references>
<references title="Informative References">
<!-- Here we use entities that we defined at the beginning. -->
<reference anchor="AMT">
<front>
<title>Automatic IP Multicast Without Explicit Tunnels (AMT)</title>
<author fullname="D. Thaler, et al" surname="">
<organization>Internet draft,
draft-ietf-mboned-auto-multicast-10</organization>
</author>
<date day="07" month="March" year="2010" />
</front>
</reference>
<reference anchor="UDPTT">
<front>
<title>The UDP Tunnel Transport mode</title>
<author fullname="G Fairhurst" surname="">
<organization></organization>
</author>
<date day="20" month="Feb" year="2010" />
</front>
</reference>
<reference anchor="UDPZ">
<front>
<title>UDP Checksums for Tunneled Packets</title>
<author fullname="M. Eubanks and P. Chimento" surname="">
<organization></organization>
</author>
<date day="01" month="(Oct" year="2009" />
</front>
</reference>
<reference anchor="LISP">
<front>
<title>Locator/ID Separation Protocol (LISP)</title>
<author fullname="D. Farinacci et al" surname="">
<organization>Internet draft,
draft-farinacci-lisp-12.txt</organization>
</author>
<date day="02" month="March" year="2009" />
</front>
</reference>
<reference anchor="Sigcomm2000">
<front>
<title>When the CRC and TCP Checksum Disagree</title>
<author fullname="Jonathan Stone and Craig Partridge " surname="">
<organization>http://conferences.sigcomm.org/sigcomm/2000/conf/abstract/9-1.htm</organization>
</author>
<date year="2000" />
</front>
</reference>
<reference anchor="ECMP">
<front>
<title>Using the IPv6 flow label for equal cost multipath routing in
tunnels (draft-carpenter-flow-ecmp)</title>
<author fullname="B. Carpenter">
<organization></organization>
</author>
<date />
</front>
</reference>
<?rfc include='reference.RFC.0768'?>
<?rfc include='reference.RFC.1141'?>
<?rfc include='reference.RFC.2463'?>
<?rfc ?>
<?rfc include='reference.RFC.2765'?>
<?rfc include='reference.RFC.4302'?>
<?rfc include='reference.RFC.4303'?>
<?rfc include='reference.RFC.3828'?>
<?rfc include='reference.RFC.5405'?>
<!-- A reference written by by an organization not a person.
Discusses IPv4 cksum=0, and alludes to IPv6 case:
UDP Encapsulation of IPsec ESP Packets (draft-ietf-ipsec-udp-encaps-09)
-->
</references>
<section title="Document Change History">
<t>{RFC EDITOR NOTE: This section must be deleted prior to
publication}</t>
<t><list style="hanging">
<t hangText="Individual Draft 00 ">This is the first DRAFT of this
document - It contains a compilation of various discussions and
contributions from a variety of IETF WGs, including: mboned, tsv,
6man, lisp, and behave. This includes contributions from Magnus with
text on RTP, and various updates.</t>
<t hangText="Individual Draft 01"><list style="symbols">
<t>This version corrects some typos and editorial NiTs and adds
discussion of the need to negotiate and verify operation of a
new mechanism (3.3.4).</t>
</list></t>
<t hangText="Individual Draft 02"><list style="symbols">
<t>Version -02 corrects some typos and editorial NiTs.</t>
<t>Added reference to ECMP for tunnels.</t>
<t>Clarifies the recommendations at the end of the document.</t>
</list></t>
<t hangText="Working Group Draft 00"><list style="symbols">
<t>Working Group Version -00 corrects some typos and removes
much of rationale for UDPTT. It also adds some discussion of
IPv6 extension header.</t>
</list></t>
<t hangText="Working Group Draft 01"><list style="symbols">
<t>Working Group Version -01 updates the rules and incorporates
off-list feedback. This version is intended for wider review
within the 6man working group.</t>
</list></t>
<t hangText="**TO BE DONE **"><list style="symbols">
<t>This version requires review from proponents and opponents to
the UDP zero checksum proposal.</t>
<t>Work still to be done includes:<list style="numbers">
<t>Text on issues with fragmentation needs to be updated to
provide more clarity on issues.</t>
<t>Need a recommendation on whether to permit a multicast
destination address with a zero UDP checksum.</t>
<t>Is it OK to send ICMPv6 messages in response to
non-delivered UDP datagrams with a zero checksum?</t>
<t>The final section may need to be reworked if this
document recommends a specific change to RFC 2460.</t>
</list></t>
</list></t>
</list></t>
</section>
<!-- Change Log
-->
</back>
</rfc>
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