Network Working Group M. Eubanks
Internet-Draft AmericaFree.TV LLC
Intended status: Standards Track P. Chimento
Expires: September 13, 2012 Johns Hopkins University Applied
Physics Laboratory
March 12, 2012
UDP Checksums for Tunneled Packets
draft-ietf-6man-udpchecksums-02
Abstract
This document provides an update of RFC 2460[RFC2460] in order to
improve the performance of IPv6 in an increasingly important use
case, the use of tunneling to carry new transport protocols. The
performance improvement is obtained by relaxing the IPv6 UDP checksum
requirement for suitable tunneling protocol where header information
is protected on the "inner" packet being carried. This relaxation
removes the overhead associated with the computation of UDP checksums
on tunneled IPv6 packets and thereby improves the efficiency of the
traversal of firewalls and other network middleware by such new
protocols. We describe how the IPv6 UDP checksum requirement can be
relaxed in the situation where the encapsulated packet itself
contains a checksum, the limitations and risks of this approach, and
provides restrictions on the use of this relaxation to mitigate these
risks.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on September 13, 2012.
Copyright Notice
Copyright (c) 2012 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Some Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. The Zero-Checksum Solution . . . . . . . . . . . . . . . . . . 7
6. Additional Observations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
This work constitutes the first upgrade of RFC 2460[RFC2460], in
order to improve the performance of IPv6 with transport layer
protocols carried encapsulated in tunnels. With the rapid growth of
the Internet, tunneling protocols have become increasingly important
to enable the deployment of new transport layer protocols. Tunneled
protocols can be deployed rapidly, while the time to upgrade and
deploy a critical mass of routers, switches and end hosts on the
global Internet for a new transport protocol is now measured in
decades. At the same time, the increasing use of firewalls and other
security related middleware means that truly new tunnel protocols,
with new protocol numbers, are also unlikely to be deployable in a
reasonable time frame, which has resulted in an increasing interest
in and use of UDP-based tunneling protocols. In such protocols,
there is an encapsulated "inner" packet, and the "outer" packet
carrying the tunneled inner packet is a UDP packet, which can pass
through firewalls and other middleware filtering that is a fact of
life on the current Internet.
As tunnel endpoints may be routers or middleware aggregating traffic
from large numbers of tunnel users, the computation of an additional
checksum on the outer UDP packet, when protected, is seen to be an
unwarranted burden on the nodes implementing lightweight tunneling
protocols, especially if the inner packet(s) are already protected by
a checksum. In IPv4, there is a checksum on the IP packet itself,
and the checksum on the outer UDP packet can be set to zero. However
in IPv6 there is not a checksum on the IP packet and RFC 2460
[RFC2460] explicitly states that IPv6 receivers MUST discard UDP
packets with a 0 checksum. So, while sending a UDP packet with a 0
checksum is permitted in IPv4 packets, it is explicitly forbidden in
IPv6 packets. In order to meet the needs of the deployers of IPv6
UDP tunnels, this document modifies RFC 2460 to allow for the
ignoring of UDP checksums under constrained situations (IPv6
tunneling where the inner packet exists and has a checksum), based on
the considerations set forth in [I-D.ietf-6man-udpzero].
While the origin of this I-D is the problem raised by the draft
titled "Automatic IP Multicast Without Explicit Tunnels", also known
as "AMT," [I-D.ietf-mboned-auto-multicast] we expect it to have wide
applicability, immediately to AMT and LISP [I-D.ietf-lisp], and in
the future to other tunneling protocols to come out of Softwires and
other IETF Working Groups.
Since the first version of this document, the need for an efficient,
lightweight UDP tunneling mechanism has increased. Indeed, other
workgroups, notably LISP [I-D.ietf-lisp] and Softwires [RFC5619] have
also expressed a need to have exceptions to the RFC 2460 prohibition.
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Other users of UDP as a tunneling protocol, for example, L2TP and
Softwires may benefit from a relaxation of the RFC 2460 restriction.
The third version of this document benefited from a close read by
Magnus Westerlund and Gorry Fairhurst.
2. Some Terminology
For the remainder of this document, we discuss only IPv6, since this
problem does not exist for IPv4. So any reference to 'IP' should be
understood as a reference to IPv6.
Although we will try to avoid them when possible, we may use the
terms "tunneling" and "tunneled" as adjectives when describing
packets. When we refer to 'tunneling packets' we refer to the outer
packet header that provides the tunneling function. When we refer to
'tunneled packets' we refer to the inner packet, i.e. the packet
being carried in the tunnel.
3. Problem Statement
The argument is that since in the case of AMT multicast packets
already have a UDP header with a checksum, there is no additional
benefit and indeed some cost to nodes to both compute and check the
UDP checksum of the outer (encapsulating) header. Consequently, IPv6
should make an exception to the rule that the UDP checksum MUST not
be 0, and allow tunneling protocols to set the checksum field of the
outer header only to 0 and skip both the sender and receiver
computation.
4. Discussion
[I-D.ietf-6man-udpzero] describes the issues related to allowing UDP
over IPv6 to have a valid checksum of zero and is not repeated here.
In Section 5.1 of [I-D.ietf-6man-udpzero], the authors propose nine
(9) constraints on the usage of a zero checksum for UDP over IPv6.
We agree with the restrictions proposed, and in fact proposed some of
those restrictions ourselves in the previous version of the current
draft. These restrictions are incorporated into the proposed changes
below.
As has been pointed out in [I-D.ietf-6man-udpzero] and in various
mailing list discussions, there is still the possibility of deep-
inspection firewall devices or other middleboxes actually checking
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the UDP checksum field of the outer packet and thereby discarding the
tunneling packets. This is would be an issue also for legacy systems
which have not implemented the change in the IPv6 specification. So
in any case, there may be packet loss of lightweight tunneling
packets because of mixed new-rule and old-rule nodes.
As an example, we discuss how can errors be detected and handled in a
lightweight UDP tunneling protocol when the checksum protection is
disabled. Note that other (non-tunneling) protocols may have
different approaches. We suggest that the following could be an
approach to this problem:
o Context (i.e. tunneling state) should be established via
application PDUs that are carried in checksummed UDP packets.
That is, any control packets flowing between the tunnel endpoints
should be protected by UDP checksums. The control packets can
also contain any negotiation that is necessary to set up the
endpoint/adapters to accept UDP packets with a zero checksum.
o Only UDP packets containing tunneled packets should have a UDP
checksum equal to zero.
o UDP keep-alive packets with checksum zero can be sent to validate
paths, given that paths between tunnel endpoints can change and so
middleboxes in the path may vary during the life of the
association. Paths with middleboxes that are intolerant of a UDP
checksum of zero will drop the keep-alives and the endpoints will
discover that. Note that this need only be done per tunnel
endpoint pair, not per tunnel context. Keep-alive traffic SHOULD
include both packets with tunnel checksums and packets with
checksums equal to zero to enable the remote end to distinguish
between path failures and the blockage of packets with checksum
equal to zero.
o Corruption of the encapsulating IPv6 source address, destination
address and/or the UDP source port, destination port fields : If
the 9 restrictions in [I-D.ietf-6man-udpzero] are followed, the
inner packets (tunneled packets) should be protected and run the
usual (presumably small) risk of having undetected corruption(s).
If lightweight tunneling protocol contexts contain (at a minimum)
source and destination IP addresses and source and destination
ports, there are 16 possible corruption outcomes. We note that
these outcomes not equally likely, as most require multiple bit
errors with errored bits in separate fields. The possible
corruption outcomes fall out this way:
* Half of the 16 possible corruption combinations have a
corrupted destination address. If the incorrect destination is
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reached and the node doesn't have an application for the
destination port, the packet will be dropped. If the
application at the incorrect destination is the same
lightweight tunneling protocol and if it has a matching context
(which can be assumed to be a very low probability event) the
inner packet will be decapsulated and forwarded. If it is some
other application, with very high probability, the application
will not recognize the contents of the packet.
* Half of the 8 possible corruption combinations with a correct
destination address have a corrupted source address. If the
tunnel contexts contain all elements of the address-port
4-tuple, then the likelihood is that this corruption will be
detected.
* Of the remaining 4 possibilities, with valid source and
destination IPv6 addresses, 1 has all 4 fields valid, the other
three have one or both ports corrupted. Again, if the
tunneling endpoint context contains sufficient information,
these error should be detected with high probability.
o Corruption of source-fragmented encapsulating packets: In this
case, a tunneling protocol may reassemble fragments associated
with the wrong context at the right tunnel endpoint, or it may
reassemble fragments associated with a context at the wrong tunnel
endpoint, or corrupted fragments may be reassembled at the right
context at the right tunnel endpoint. In each of these cases, the
IPv6 length of the encapsulating header may be checked (though
[I-D.ietf-6man-udpzero] points out the weakness in this check).
In addition, if the encapsulated packet is protected by a
transport (or other) checksum, these errors can be detected (with
some probability).
While this is not a perfect solution, it can reduce the risks of
relaxing the UDP checksum requirement for IPv6.
5. The Zero-Checksum Solution
The solution to the overhead associated with UDP packets carrying
encapsulated tunnel traffic is to allow a UDP checksum of zero on the
outer encapsulating packet of a lightweight tunneling protocol. UDP
endpoints that implement this solution MUST change their behavior and
not discard UDP packets received with a 0 checksum on the outer
packet of tunneling protocols. If this is done constraints in
Section 5.1 of [I-D.ietf-6man-udpzero] also MUST be adopted.
Specifically, the text in [RFC2460] Section 8.1, 4th bullet is
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amended. We refer to the following text:
"Unlike IPv4, when UDP packets are originated by an IPv6 node, the
UDP checksum is not optional. That is, whenever originating a UDP
packet, an IPv6 node must compute a UDP checksum over the packet and
the pseudo-header, and, if that computation yields a result of zero,
it must be changed to hex FFFF for placement in the UDP header. IPv6
receivers must discard UDP packets containing a zero checksum, and
should log the error."
This item should be taken out of the bullet list and should be
modified as follows:
Whenever originating a UDP packet, an IPv6 node SHOULD compute a
UDP checksum over the packet and the pseudo-header, and, if that
computation yields a result of zero, it must be changed to hex
FFFF for placement in the UDP header. IPv6 receivers SHOULD
discard UDP packets containing a zero checksum, and SHOULD log the
error. However, some protocols, such as lightweight tunneling
protocols that use UDP as a tunnel encapsulation, MAY omit
computing the UDP checksum of the encapsulating UDP header and set
it to zero, subject to the constraints described in
[I-D.ietf-6man-udpzero]. In cases where the encapsulating
protocol uses a zero checksum for UDP, the receiver of packets
sent to a port enabled to receive zero-checksum packets MUST NOT
discard packets solely for having a UDP checksum of zero. Note
that these constraints apply only to encapsulating protocols that
omit calculating the UDP checksum and set it to zero. An
encapsulating protocol can always choose to compute the UDP
checksum, in which case, its behavior should be as specified
originally.
1. IPv6 protocol stack implementations SHOULD NOT by default
allow the new method. The default node receiver behavior MUST
discard all IPv6 packets carrying UDP packets with a zero
checksum.
2. Implementations MUST provide a way to signal the set of ports
that will be enabled to receive UDP datagrams with a zero
checksum. An IPv6 node that enables reception of UDP packets
with a zero-checksum, MUST enable this only for a specific
port or port-range. This may be implemented via a socket API
call, or similar mechanism.
3. RFC 2460 specifies that IPv6 nodes should log UDP datagrams
with a zero-checksum. A port for which zero-checksum has been
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enabled MUST NOT log zero-checksum datagrams for that reason
(of course, there might be other reasons to log such packets).
4. 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.
5. UDP Tunnels that encapsulate IP may rely on the inner packet
integrity checks provided that the tunnel will not
significantly increase the rate of corruption of the inner IP
packet. If a significantly increased corruption rate can
occur, then the tunnel MUST provide an additional integrity
verification mechanism. An integrity mechanism is always
recommended at the tunnel layer to ensure that corruption
rates of the inner most packet are not increased.
6. Tunnels that encapsulate Non-IP packets MUST have a CRC or
other mechanism for checking packet integrity, unless the
Non-IP packet specifically is designed for transmission over
lower layers that do not provide any packet integrity
guarantee. 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.
7. UDP applications that support use of a zero-checksum, SHOULD
NOT rely upon correct reception of the IP and UDP protocol
information (including the length of the packet) 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.
8. If a method proposes recursive tunnels, it MUST 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).
9. IPv6 nodes that receive ICMPv6 messages that refer to packets
with a zero UDP checksum MUST provide appropriate checks
concerning the consistency of the reported packet to verify
that the reported packet actually originated from the node,
before acting upon the information (e.g. validating the
address and port numbers in the ICMPv6 message body).
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Middleboxes MUST allow IPv6 packets with UDP checksum equal to
zero to pass. Implementations of middleboxes MAY allow
configuration of specific port ranges for which a zero UDP
checksum is valid and may drop IPv6 UDP packets outside those
ranges.
6. Additional Observations
The persistence of this issue among a significant number of protocols
being developed in the IETF requires a definitive policy. The
authors would like to make the following observations:
o An empirically-based analysis of the probabilities of packet
corruptions (with or without checksums) has not (to our knowledge)
been conducted since about 2000. It is now 2011. We strongly
suggest that an empirical study is in order, along with an
extensive analysis of IPv6 header corruption probabilities.
o A key cause of this issue generally is the lack of protocol
support in middleboxes. Specifically, new protocols, such as LISP
[I-D.ietf-lisp], are being forced to use UDP tunnels just to
traverse an end-to-end path successfully and avoid having their
packets dropped by middleboxes. If this were not the case, the
use of UDP-lite [RFC3828] might become more viable for some (but
not necessarily all) lightweight tunneling protocols.
o Another cause of this issue is that the UDP checksum is overloaded
with the task of protecting the IPv6 header for UDP flows (as it
the TCP checksum for TCP flows). Protocols that do not use a
pseudo-header approach to computing a checksum or CRC have
essentially no protection from mis-delivered packets.
7. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
8. Security Considerations
It is of course less work to generate zero-checksum attack packets
than ones with full UDP checksums. However, this does not lead to
any significant new vulnerabilities as checksums are not a security
measure and can be easily generated by any attacker, as properly
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configured tunnels should check the validity of the inner packet and
perform any needed security checks, regardless of the checksum
status, and finally as most attacks are generated from compromised
hosts which automatically create checksummed packets (in other words,
it would generally be more, not less, effort for most attackers to
generate zero UDP checksums on the host).
9. Acknowledgements
We would like to thank Brian Haberman, Magnus Westerlund and Gorry
Fairhurst for discussions and reviews.
10. References
10.1. Normative References
[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.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
[RFC5619] Yamamoto, S., Williams, C., Yokota, H., and F. Parent,
"Softwire Security Analysis and Requirements", RFC 5619,
August 2009.
10.2. Informative References
[I-D.ietf-6man-udpzero]
Fairhurst, G. and M. Westerlund, "IPv6 UDP Checksum
Considerations", draft-ietf-6man-udpzero-05 (work in
progress), December 2011.
[I-D.ietf-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-22 (work in progress), February 2012.
[I-D.ietf-mboned-auto-multicast]
Bumgardner, G. and T. Morin, "Automatic Multicast
Tunneling", draft-ietf-mboned-auto-multicast-12 (work in
progress), February 2012.
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Authors' Addresses
Marshall Eubanks
AmericaFree.TV LLC
P.O. Box 141
Clifton, Virginia 20124
USA
Phone: +1-703-501-4376
Fax:
Email: marshall.eubanks@gmail.com
P.F. Chimento
Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Road
Laurel, MD 20723
USA
Phone: +1-443-778-1743
Fax:
Email: Philip.Chimento@jhuapl.edu
URI:
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