Network Working Group M. Eubanks
Internet-Draft AmericaFree.TV LLC
Updates: 2460 (if approved) P. Chimento
Intended status: Standards Track Johns Hopkins University Applied
Expires: June 14, 2013 Physics Laboratory
M. Westerlund
Ericsson
December 11, 2012
IPv6 and UDP Checksums for Tunneled Packets
draft-ietf-6man-udpchecksums-06
Abstract
This document provides an update of the Internet Protocol version 6
(IPv6) specification (RFC2460) to improve the performance in the use
case when a tunnel protocol uses UDP with IPv6 to tunnel packets.
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 IPv6 packets used to carry tunnel protocols. The
specification describes how the IPv6 UDP checksum requirement can be
relaxed for the situation where the encapsulated packet itself
contains a checksum. The limitations and risks of this approach are
described, and restrictions specified on the use of the method.
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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on June 14, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Some Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Analysis of Corruption in Tunnel Context . . . . . . . . . 5
4.2. Limitation to Tunnel Protocols . . . . . . . . . . . . . . 7
4.3. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 8
5. The Zero-Checksum Update . . . . . . . . . . . . . . . . . . . 8
6. Additional Observations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This work constitutes an update of the Internet Protocol Version 6
(IPv6) Specification [RFC2460], in the use case when a tunnel
protocol uses UDP with IPv6 to tunnel packets. With the rapid growth
of the Internet, tunneling protocols have become increasingly
important to enable the deployment of new protocols. Tunneled
protocols can be deployed rapidly, while the time to upgrade and
deploy a critical mass of routers, middleboxes and hosts on the
global Internet for a new protocol is now measured in decades. At
the same time, the increasing use of firewalls and other security-
related middleboxes 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 middleboxes that perform filtering that is a fact
of life on the current Internet.
Tunnel endpoints may be routers or middleboxes aggregating traffic
from a number of tunnel users, therefore the computation of an
additional checksum on the outer UDP packet, may be seen as an
unwarranted burden on nodes that implement a tunneling protocol,
especially if the inner packet(s) are already protected by a
checksum. In IPv4, there is a checksum over the IP packet header,
and the checksum on the outer UDP packet may be set to zero. However
in IPv6 there is no checksum in the IP header and RFC 2460 [RFC2460]
explicitly states that IPv6 receivers MUST discard UDP packets with a
zero checksum. So, while sending a UDP datagram with a zero checksum
is permitted in IPv4 packets, it is explicitly forbidden in IPv6
packets. To improve support for IPv6 UDP tunnels, this document
updates RFC 2460 to allow endpoints to use a zero UDP checksum under
constrained situations (primarily IPv6 tunnel transports that carry
checksum-protected packets), following the applicability statements
and constraints in [I-D.ietf-6man-udpzero].
Unicast UDP Usage Guidelines for Application Designers [RFC5405]
should be consulted when reading this specification. It discusses
both UDP tunnels (Section 3.1.3) and the usage of checksums (Section
3.4).
While the origin of this specification 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. Since the first version of this document, the
need for an efficient UDP tunneling mechanism has increased. Other
IETF Working Groups, notably LISP [I-D.ietf-lisp] and Softwires
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[RFC5619] have expressed a need to update the UDP checksum processing
in RFC 2460. We therefore expect this update to be applicable in
future to other tunneling protocols specified by these and other IETF
Working Groups.
2. Some Terminology
This document discusses only IPv6, since this problem does not exist
for IPv4. Therefore all reference to 'IP' should be understood as a
reference to IPv6.
The document uses 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.
2.1. 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].
3. Problem Statement
When using tunnel protocols based on UDP, there can be both a benefit
and a cost to computing and checking the UDP checksum of the outer
(encapsulating) UDP transport header. In certain cases, where
reducing the forwarding cost is important, such as for nodes that
perform the checksum in software, where the cost may outweigh the
benefit. This document provides an update for usage of the UDP
checksum with IPv6. The update is specified for use by a tunnel
protocol that transports packets that are themselves protected by a
checksum.
4. Discussion
Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums [I-D.ietf-6man-udpzero] describes issues related to
allowing UDP over IPv6 to have a valid zero UDP checksum and is the
starting point for this discussion. Section 4 and 5 of
[I-D.ietf-6man-udpzero], respectively identify node implementation
and usage requirements for datagrams sent and received with a zero
UDP checksum. These introduce constraints on the usage of a zero
checksum for UDP over IPv6. The remainder of this section analyses
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the use of general tunnels and motivates why tunnel protocols are
being permitted to use the method described in this update. Issues
with middleboxes are also discussed.
4.1. Analysis of Corruption in Tunnel Context
This section analyzes the impact of the different corruption modes in
the context of a tunnel protocol. It indicates what needs to be
considered by the designer and user of a tunnel protocol to be
robust. It also summarizes why use of a zero UDP checksum is thought
safe for deployment.
o Context (i.e. tunneling state) should be established by exchanging
application Protocol Data Units (PDUs) carried in checksummed UDP
datagrams or by other protocols with integrity protection against
corruption. These control packets should also carry any
negotiation required to enable the tunnel endpoint to accept UDP
datagrams with a zero checksum and identify the set of ports that
are used. It is important that the control traffic is robust
against corruption because undetected errors can lead to long-
lived and significant failures that affect not only the single
packet that was corrupted.
o Keep-alive datagrams with a zero UDP checksum should be sent to
validate the network path, because the path between tunnel
endpoints can change and therefore the set of middleboxes along
the path may change during the life of an association. Paths with
middleboxes that drop datagrams with a zero UDP checksum will drop
these keep-alives. To enable the tunnel endpoints to discover and
react to this behavior in a timely way, the keep-alive traffic
should include datagrams with both a non-zero checksum and ones
with a zero checksum.
o Corruption of the address information in an encapsulating packets,
i.e. IPv6 source address, destination address and/or the UDP
source port, and destination port fields. A robust tunnel
protocol should track tunnel context based on the 5-tuple, i.e.
the protocol and both the address and port for both the source and
destination. A corrupted datagram that arrives at a destination
may be filtered based on this check.
* If the datagram header matches the 5-tuple with a zero checksum
enabled, the payload is matched to the wrong context. The
tunneled packet will then be decapsulated and forwarded by the
tunnel egress.
* If a corrupted datagram matches a different 5-tuple with a zero
checksum enabled, the payload is matched to the wrong context,
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and may be processed by the wrong tunneling protocol, if it
passes the verification of that protocol.
* If a corrupted datagram matches a 5-tuple that does not have a
zero checksum enabled, it will be discarded.
When only the source information is corrupted, the datagram could
arrive at the intended applications/protocol which will process it
and try to match it against an existing tunnel context. If the
protocol restricts processing to only the source addresses with
established contexts the likelihood that a corrupted packet enters
a valid context is reduced. When both source and destination
fields are corrupted, this increases the likelihood of failing to
match a context, with the exception of errors replacing one packet
header with another one. In this case it is possible that both
are tunnels and thus the corrupted packet can match a previously
defined context.
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).
o Tunnel protocols using UDP have some advantages that reduce the
risk for a corrupted tunnel packet reaching a destination that
will receive it, compared to other applications. This results
from processing by the network of the inner (tunneled) packet
after being forwarded from the tunnel egress using a wrong
context:
* A tunneled packet may be forwarded to the wrong address domain,
for example a private address domain where the inner packet's
address is not routable, or may fail a source address check,
such as Unicast Reverse Path Forwarding [RFC2827], resulting in
the packet being dropped.
* The destination address of a tunneled packet may not at all be
reachable from the delivered domain. For example an Ethernet
packet where the destination MAC address is not present on the
LAN segment that was reached.
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* The type of the tunneled packet may prevent delivery for
example if an IP packet payload was attempted to be interpreted
as an Ethernet packet. This is likely to result in the packet
being dropped as invalid.
* The tunneled packet checksum or integrity mechanism may detect
corruption of the inner packet caused at the same time as
corruption to the outer packet header. The resulting packet
would likely be dropped as invalid.
These different examples each help to significantly reduce the
likelihood that a corrupted inner tunneled packet is finally
delivered to a protocol listener that can be affected by the packet.
While the methods do not guarantee correctness, they can reduce the
risk of relaxing the UDP checksum requirement for a tunnel
application using IPv6.
4.2. Limitation to Tunnel Protocols
This document describes the applicability of using a zero UDP
checksum to support tunnel protocols. There are good motivations
behind this and the arguments are provided here.
o Tunnels carry inner packets that have their own semantics that
makes any corruption less likely to reach the indicated
destination and be accepted as a valid packet. This is true for
IP packets with the addition of verification that can be made by
the tunnel protocol, the networks' processing of the inner packet
headers as discussed above, and verification of the inner packet
checksums. Also non-IP inner packets are likely to be subject to
similar effects that reduce the likelihood that an mis-delivered
packet are delivered.
o Protocols that directly consume the payload must have sufficient
robustness against mis-delivered packets from any context,
including the ones that are corrupted in tunnels and any other
usage of the zero checksum. This will require an integrity
mechanism. Using a standard UDP checksum reduces the
computational load in the receiver to verify this mechanism.
o Stateful protocols or protocols where corruption causes cascade
effects need to be extra careful. In tunnel usage each
encapsulating packet provides only a transport mechanism from
tunnel ingress to tunnel egress. A corruption will commonly only
effect the single packet, not established protocol state. One
common effect is that the inner packet flow will only see a
corruption and mis-delivery of the outer packet as a lost packet.
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o Some non-tunnel protocols operate with general servers that do not
know from where they will receive a packet. In such applications,
the usage of a zero UDP checksum is especially unsuitable because
there is a need to provide the first level of verification that
the packet was intended for the server. This verification
prevents the server from processing the datagram payload and spend
any significant amount of resources on it, including sending
replies or error messages.
Tunnel protocols encapsulating IP this will generally be safe, since
all IPv4 and IPv6 packets include at least one checksum at either the
network or transport layer and the network delivery of the inner
packet will further reduce the effects of corruption. Tunnel
protocols carrying non-IP packets may provide equivalent protection
due to the non-IP networks reducing the risk of delivery to
applications. However, there is need for further analysis to
understand the implications of mis-delievery of corrupted packets for
that each non-IP protocol. The analysis above suggests that non-
tunnel protocols can be expected to have significantly more cases
where a zero checksum would result in mis-delivery or negative side-
effects.
One unfortunate side-effect of increased use of a zero-checksum is
that it also increases the likelihood of acceptance when a datagram
with a zero UDP checksum is mis-delivered. This requires all tunnel
protocols using this method to be designed to be robust to mis-
delivery.
4.3. Middleboxes
Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums [I-D.ietf-6man-udpzero] notes that middlebox devices that
conform to RFC 2460 will discard datagrams with a zero UDP checksum
and should log this as an error. Thus tunnel protocols intending to
use a zero UDP checksum needs to ensure that they have defined a
method for handling cases when a middlebox prevents the path between
the tunnel ingress and egress from supporting transmission of
datagrams with a zero UDP checksum.
5. The Zero-Checksum Update
This specification updates IPv6 to allow a zero UDP checksum in the
outer encapsulating datagram of a tunneling protocol. UDP endpoints
that implement this update MUST follow the node requirements
"Applicability Statement for the use of IPv6 UDP Datagrams with Zero
Checksums" [I-D.ietf-6man-udpzero].
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The following text in [RFC2460] Section 8.1, 4th bullet should be
deleted:
"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 text should be replaced by:
Whenever originating a UDP packet in the default mode, 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 by default discard UDP packets containing a zero
checksum, and SHOULD log the error. As an alternative usage for
some protocols, such as protocols that use UDP as a tunnel
encapsulation, MAY enable the zero-checksum mode for specific sets
of ports. Any node implementing the zero-checksum mode MUST
follow the node requirements specified in Section 4 of
Applicability Statement for the use of IPv6 UDP Datagrams with
Zero Checksums [I-D.ietf-6man-udpzero].
Any protocol using the zero-checksum mode MUST follow the usage
requirements specified in Section 5 of Applicability Statement for
the use of IPv6 UDP Datagrams with Zero Checksums
[I-D.ietf-6man-udpzero].
Middleboxes supporting IPv6 MUST follow the requirements 9, 10 and
11 of the usage requirements specified in Section 5 of
Applicability Statement for the use of IPv6 UDP Datagrams with
Zero Checksums [I-D.ietf-6man-udpzero].
6. Additional Observations
This update was motivated by the existence of a number of protocols
being developed in the IETF that are expected to benefit from the
change. The following observations are made:
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. At the time of publication, it
is now 2012. We strongly suggest a new empirical study, along
with an extensive analysis of the corruption probabilities of the
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IPv6 header.
o A key motivation for the increase in use of UDP in tunneling is a
lack of protocol support in middleboxes. Specifically, new
protocols, such as LISP [I-D.ietf-lisp], may prefer to use UDP
tunnels to traverse an end-to-end path successfully and avoid
having their packets dropped by middleboxes. If middleboxes were
updated to support UDP-Lite [RFC3828], this would provide better
protection than offered by this update. This may be suited to a
variety of applications and would be expected to be preferred over
this method for many tunnel protocols.
o Another issue is that the UDP checksum is overloaded with the task
of protecting the IPv6 header for UDP flows (as is 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
Less work is required required to generate an attack using a zero UDP
checksum than one using a standard full UDP checksum. However, this
does not lead to significant new vulnerabilities because checksums
are not a security measure and can be easily generated by any
attacker. Properly configured tunnels should check the validity of
the inner packet and perform security checks.
9. Acknowledgements
We would like to thank Brian Haberman, Dan Wing, Joel Halpern and the
IESG of 2012 for discussions and reviews. Gorry Fairhurst has been
very diligent in reviewing and help ensuring alignment between this
document and [I-D.ietf-6man-udpzero].
10. References
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10.1. Normative References
[I-D.ietf-6man-udpzero]
Fairhurst, G. and M. Westerlund, "Applicability Statement
for the use of IPv6 UDP Datagrams with Zero Checksums",
draft-ietf-6man-udpzero-07 (work in progress),
October 2012.
[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.
10.2. Informative References
[I-D.ietf-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-24 (work in progress), November 2012.
[I-D.ietf-mboned-auto-multicast]
Bumgardner, G., "Automatic Multicast Tunneling",
draft-ietf-mboned-auto-multicast-14 (work in progress),
June 2012.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[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.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[RFC5619] Yamamoto, S., Williams, C., Yokota, H., and F. Parent,
"Softwire Security Analysis and Requirements", RFC 5619,
August 2009.
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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
Email: Philip.Chimento@jhuapl.edu
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
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