Internet Engineering Task Force G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational M. Westerlund
Expires: November 9, 2010 Ericsson Research
May 8, 2010
IPv6 UDP Checksum Considerations
draft-ietf-6man-udpzero-00
Abstract
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 and provides
recommendations.
Status of this Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on November 9, 2010.
Copyright Notice
Copyright (c) 2010 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
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Use of UDP Tunnels . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Motivation for new approaches . . . . . . . . . . . . 5
1.2.2. Reducing forwarding cost . . . . . . . . . . . . . . . 6
1.2.3. Need to inspect the entire packet . . . . . . . . . . 7
1.2.4. Interactions with middleboxes . . . . . . . . . . . . 7
1.2.5. Support for load balancing . . . . . . . . . . . . . . 7
2. Standards-Track Transports . . . . . . . . . . . . . . . . . . 8
2.1. UDP with Standard Checksum . . . . . . . . . . . . . . . . 8
2.2. UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. Using UDP-Lite as a Tunnel Encapsulation . . . . . . . 8
2.3. IP in IPv6 Tunnel Encapsulations . . . . . . . . . . . . . 9
3. Evaluation of proposal to update to RFC 2460 to support
zero checksum . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Alternatives to the Standard Checksum . . . . . . . . . . 10
3.2. Applicability of method . . . . . . . . . . . . . . . . . 11
3.3. Effect of packet modification in the network . . . . . . . 12
3.3.1. Corruption of the destination IP address . . . . . . . 12
3.3.2. Corruption of the source IP address . . . . . . . . . 13
3.3.3. Delivery to an unexpected port . . . . . . . . . . . . 14
3.3.4. Validating the network path . . . . . . . . . . . . . 15
3.4. Comparision . . . . . . . . . . . . . . . . . . . . . . . 15
4. Requirements on the specification of transported protocols . . 16
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Document Change History . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The User Datagram Protocol (UDP) transport was defined by RFC768
[RFC0768] for IPv4 RFC791 [RFC0791] and is defined in RFC2460
[RFC2460] for IPv6 hosts and routers. A UDP transport endpoint may
be either a host or a router. The UDP Usage Guidelines [RFC5405]
provides overall guidance for application designers, including the
use of UDP to support tunneling. These guidelines are applicable to
this discussion.
This section provides a background to key issues, and introduces the
use of UDP as a tunnel transport protocol.
Section 2 describes a set of standards-track datagram transport
protocols that may be used to support tunnels.
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.
Section 4 reviews the trade offs and provides recommendations.
1.1. Background
An Internet transport endpoint should concern itself with the
following issues:
o Protection of the endpoint transport state from unnecessary extra
state (i.e. Invalid state from rogue packets).
o Protection of the endpoint transport state from corruption of
internal state.
o Pre-filtering by the endpoint of erroneous data, to protect the
transport from unnecessary processing and from corruption that it
can not itself reject.
o Pre-filter of incorrectly addressed destination packets, before
responding to a source address.
UDP, as defined in [RFC0768], 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
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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 [RFC1071].
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 UDP [RFC0768] checksum may be
disabled, applications are recommended to enable UDP checksums
[RFC5405].
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 Networking Interface Cards (NICs) that automatically
calculate TCP [RFC0793] and UDP checksums on transmission if a
checksum of zero is sent to the NIC, using a method known as checksum
offloading.
The network-layer fields that are validated by a transport checksum
are:
o Endpoint IP source address (always included in the pseudo header
of the checksum)
o Endpoint IP destination address (always included in the pseudo
header of the checksum)
o Upper Layer Payload type (always included in the pseudo header of
the checksum)
o IP length of payload (always included in the pseudo header of the
checksum)
o Length of the network layer extension headers (i.e. By correct
position of the hecksum bytes)
The transport-layer fields that are validated by a transport checksum
are:
o Transport demultiplexing, i.e. ports (always included in the
checksum)
o Transport payload size (always included in checksum)
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
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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). RFC2765 [RFC2765] provides some
guidance on the processing of fragmented IPv4 UDP datagrams that do
not carry a UDP checksum.
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 (comapraed to the checksum update
needed when an IPv4 router modifies the Time-To-Live (TTL)).
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[RFC2460] when applications
transmit UDP datagrams over IPv6.
1.2. Use of UDP Tunnels
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 [RFC5405].
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.
1.2.1. Motivation for new approaches
A number of tunnel protocols are currently being defined (e.g.
Automated Multicast Tunnels, AMT [AMT], and the Locator/Identifier
Separation Protocol, LISP [LISP]). These protocols have proposed an
update to IPv6 UDP checksum processing. These tunnel protocols could
benefit from simpler checksum processing for various reasons:
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o 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).
o Eliminating a need to access the entire packet when forwarding the
packet.
o Enhancing ability to traverse middleboxes, especially Network
Address Translators, NATs.
o A desire to use the port number space to enable load-sharing.
1.2.2. Reducing forwarding cost
It is a common requirement to terminate a large number of tunnels on
a single router/host. Processing costs per tunnel concern both state
(memory requirements) and processing costs.
Automatic IP Multicast Without Explicit Tunnels, known as AMT [AMT]
currently specifies UDP as the transport protocol for tunneled
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.
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 them 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.
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1.2.3. Need to inspect the entire packet
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.
1.2.4. Interactions with middleboxes
In IPv4, UDP-encapsulation may be desirable for NAT traversal, since
UDP support is commonly provided.
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 is going to require rewriting (or
defining) IPv6 NAT behaviour to achieve consistent widescale
deployment.
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, and if such a mode
were to be defined for IPv6, this may also need to be updated.
Key questions in this space include:
o What types of middleboxes does the protocol need to cross
(routers, NAT boxes, firewalls, etc.), and how will those
middleboxes deal with these packets?
o What do IPv6 routers do today with zero-checksum UDP packets?
o What other IPv6 middleboxes exist today, and what would they do?
1.2.5. Support for load balancing
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.
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,
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although further clarity may be needed to ensure the field can be
consistently used for this purpose, (e.g. ECMP [ECMP]). Router
vendors could be encouraged to start using the IPv6 Flow Label as a
part of the flow hash.
2. Standards-Track Transports
2.1. UDP with Standard Checksum
UDP with standard checksum behaviour is defined in RFC 2460, and
should be the default choice. Guidelines are provided in [RFC5405].
2.2. UDP-Lite
UDP-Lite [RFC3828] 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 [RFC5405]. UDP-Lite has been
implemented, e.g. as a part of the Linux kernel since version 2.6.20.
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
host. 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, UDP-Lite is
semantically identical to UDP and is specified for use with IPv4 and
IPv6. It uses an IP protocol type (or IPv6 next header) with a value
of 136 decimal. This value is different to that used by UDP.
2.2.1. Using UDP-Lite as a Tunnel Encapsulation
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.
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
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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.
2.3. IP in IPv6 Tunnel Encapsulations
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.
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 application
and therefore UDP demultiplexing relies solely on the port numbers.
3. Evaluation of proposal to update to RFC 2460 to support zero
checksum
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:
o 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.
o the extension header processing is correctly delimited - i.e. The
start of data has not been corrupted. The protocol type field
also provides some protection.
o reassembly processing, when used.
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o the length of the payload.
o the port values - i.e. The correct application gets the payload
(applications should also check the expecetd use of source ports/
addresses).
o the payload integrity.
In IPv4, the first 4 checks are performed using the IPv4 header
checksum.
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 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.
3.1. Alternatives to the Standard Checksum
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 complexity):
o 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 maaner resembling incremental checksum update
[RFC1141]. 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.
o UDP-Lite. Where the checksum coverage may be set to only the
header portion of a packet. This requires a pseudo header
checksum calculation only on the encapsulating packet header,
which includes extracting the UDP payload length for the pseudo
header, however this is expected to be also known when performing
packet forwarding. The value may be cached per flow/destination
and subsequently combined only with the Length field to minimise
per-packet processing.
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o The proposed UDP Tunnel Transport, UDPTT [UDPTT] proposed a method
where UDP is 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.
o 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 verfiy delivery to an appropriate end point, but would
also require IPv6 nodes to correctly handle the additional header.
o UDP modified to disable checksum processing[UDPZ] (if progressed).
This requires no checksum calculation.
These options are discussed further in later sections.
3.2. Applicability of method
The expectation of the present proposal to permit omission of UDP
checksums [UDPZ] is that this would apply only to IPv6 router nodes
that implement specific protocols. 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.
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 dsiabled. 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. 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).
The IETF should carefully consider constraints on sanctioning the use
of this 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
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delivered packet with that of data addressed to an application
running on their endpoint.
3.3. Effect of packet modification in the network
IP packets may be corrupted as they traverse an Internet path.
Evidence has been presented [Sigcomm2000] 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.
Corruption in the network may result in:
o 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.
o 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.
o A datagram payload being truncated by corruption of the length
field. Such a datagram needs to be discarded.
When a checksum is used with UDP/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.
3.3.1. Corruption of the destination IP address
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.
There are two possible outcomes:
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o Delivery to a destination address that is not in use (the packet
will not be delivered, but could result in an error report).
o 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).
3.3.2. Corruption of the source IP address
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 result in this packet being discarded by the
receiving IP stack).
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:
o An application that requires pre-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).
o 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.
o Some 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, 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).
In general, the effect of corrupting the source address will depend
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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.
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.
3.3.3. Delivery to an unexpected port
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).
There are several possible outcomes for a packet that passes and does
not use the UDP checksum validation:
o Delivery to a port that is not in use. The packet is discarded,
but could generate an ICMPv6 message (e.g. port unreachable).
o 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.
o 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.
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
1. 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 session.
2. Applications writers often bind to wild-card values in endpoint
identifiers and do not always validate correctness of datagrams
they receive.
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While these ruled could 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.
If checksum coverage is suppressed, the application 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.
3.3.4. Validating the network path
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 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.
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.
3.4. Comparision
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.
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Comparison of functions for selected methods
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)
Table 1
4. Requirements on the specification of transported protocols
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:
Is there a reason why IP in IP is not a reasonable choice for
encapsulation?
o 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 require specific support in middleboxes.
o Another example is a need to perform port-demultiplexing (e.g. for
load balancing). This need could also be met using UDP, UDP-Lite,
or another supported transport, or by utilising the IPv6 flow
label.
Is there a reason why UDP-Lite is not a reasonable choice for
encapsulation?
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o 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.
o Another argument against using UDP-Lite is that it uses a
different IPv6 Next Header, which is currently not widely
supported in middleboxes (see previous).
o 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.
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 ?
Additional topics to be considered in making this decision:
o The role of a router and host are not fixed, and a consistent
method must be specified that can be used on all nodes. In IPv6,
a node selects the role of a router or host on a per interface
basis. 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.
o Behaviour of NAT/Middleboxes may need to be updated. This is the
case for UDP cksum==0 and also for use of an IPv6 Extension Header
carrying a transport checksum.
o The method needs to consider the impact of load balancing, and
whether this may be enabled for the chosen transport protocol.
If a zero checksum approach were to be adopted by the IETF, the
specification should consider adding the following constraints on
usage:
1. A method must be specified to verify the integrity of the inner
(tunneled) packet.
2. Non-IP inner (tunneled) packets must have a CRC or other
mechanism for checking packet integrity.
3. If a method proposes selective ignoring of the checksum on
reception, it needs to provide guidance that is appropriate for
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all use-cases, including defining how currently standardised
nodes handle any new use.
4. 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: That is a 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.
5. 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).
6. General protocol stack implementations should not by default
allow the new method. The new method should remain restricted to
endpoints that explicitly enable this mode and adopt the above
procedures.
5. Summary
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. 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 a 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.
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.
The implications on firewalls, NATs and other middleboxes need to be
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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.
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.
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.
Guidance has also been provided to help evaluate the case for
disabling the checksum for specific applications
6. Acknowledgements
Brian Haberman, Brian Carpenter, Magaret Wasserman, Lars Eggert,
Magnus Westerlund, others in the TSV directorate.
Thanks also to: Remi Denis-Courmont, Pekka Savola and many others who
contributed comments and ideas via the 6man, behave, lisp and mboned
lists.
7. IANA Considerations
This document does not require IANA considerations.
8. Security Considerations
Transport checksums provide the first stage of protection for the
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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.
9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer,
"Computing the Internet checksum", RFC 1071,
September 1988.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
9.2. Informative References
[AMT] Internet draft, draft-ietf-mboned-auto-multicast-10,
"Automatic IP Multicast Without Explicit Tunnels (AMT)",
March 2010.
[ECMP] "Using the IPv6 flow label for equal cost multipath
routing in tunnels (draft-carpenter-flow-ecmp)".
[LISP] Internet draft, draft-farinacci-lisp-12.txt, "Locator/ID
Separation Protocol (LISP)", March 2009.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC1141] Mallory, T. and A. Kullberg, "Incremental updating of the
Internet checksum", RFC 1141, January 1990.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
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(UDP-Lite)", RFC 3828, July 2004.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[Sigcomm2000]
http://conferences.sigcomm.org/sigcomm/2000/conf/abstract/
9-1.htm, "When the CRC and TCP Checksum Disagree", 2000.
[UDPTT] "The UDP Tunnel Transport mode", Feb 2010.
[UDPZ] "UDP Checksums for Tunneled Packets", (Oct 2009.
Appendix A. Document Change History
{RFC EDITOR NOTE: This section must be deleted prior to publication}
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.
Individual Draft 01
* 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).
Individual Draft 02
* Version -02 corrects some typos and editorial NiTs.
* Added reference to ECMP for tunnels.
* Clarifies the recommendations at the end of the document.
*
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Working Group Draft 00
* Working Group Version -00 corrects some typos and removes much
of rationale for UDPTT. It also adds some discussion of IPv6
extension header
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Aberdeen, AB24 3UE,
Scotland, UK
Phone:
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/users/gorry
Magnus Westerlund
Ericsson Research
Torshamgatan 23
Stockholm, SE-164 80
Sweden
Phone:
Fax:
Email: magnus.westerlund@ericsson.com
URI:
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