Internet DRAFT - draft-ietf-tsvwg-sctp-zero-checksum
draft-ietf-tsvwg-sctp-zero-checksum
Network Working Group M. Tüxen
Internet-Draft Münster Univ. of Appl. Sciences
Intended status: Standards Track V. Boivie
Expires: 23 August 2024 F. Castelli
Google
R. Jesup
Mozilla
20 February 2024
Zero Checksum for the Stream Control Transmission Protocol
draft-ietf-tsvwg-sctp-zero-checksum-08
Abstract
The Stream Control Transmission Protocol (SCTP) uses a 32-bit
checksum in the common header of each packet to provide some level of
data integrity. If another method used by SCTP provides already the
same or a higher level of data integrity, computing this checksum
does not provide any additional protection, but does consume
computing resources.
This document provides a simple extension to SCTP allowing to save
these computing resources by using zero as the checksum in a
backwards compatible way. It also defines how this feature can be
used when SCTP packets are encapsulated in Datagram Transport Layer
Security (DTLS) packets.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 23 August 2024.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Alternate Error Detection Methods . . . . . . . . . . . . . . 3
4. A New Chunk Parameter . . . . . . . . . . . . . . . . . . . . 5
5. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Declaration of Feature Support . . . . . . . . . . . . . 6
5.2. Sender Side Considerations . . . . . . . . . . . . . . . 6
5.3. Receiver Side Considerations . . . . . . . . . . . . . . 7
6. Error Detection via SCTP over DTLS . . . . . . . . . . . . . 8
7. Socket API Considerations . . . . . . . . . . . . . . . . . . 8
7.1. Set Accepting a Zero Checksum
(SCTP_ACCEPT_ZERO_CHECKSUM) . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
SCTP as specified in [RFC9260] uses a CRC32c checksum to provide some
level of data integrity. When using, for example, Datagram Transport
Layer Security (DTLS) as the lower layer for SCTP as specified in
[RFC8261], using the CRC32c checksum does not provide any additional
protection over the one already provided by DTLS. However, computing
the CRC32c checksum at the sender and receiver side does consume
computational resources for no benefit. This is particularly
important for end points that are computational-limited and use SCTP
encapsulated in DTLS.
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The extension described in this document allows an SCTP end point to
declare that it accepts SCTP packets with a checksum of zero when
using a specific alternate error detection method. This declaration
happens during the setup of the SCTP association and allows end
points supporting this extension to be interoperable with end points
not supporting the extension described in this document. To provide
this backwards compatibility, end points using this extension still
need to implement the CRC32c checksum algorithm.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Alternate Error Detection Methods
SCTP uses a CRC32c checksum to provide some level of data integrity.
The CRC32c checksum is computed based on the SCTP common header and
the chunks contained in the packet. In particular, the computation
of the CRC32c checksum does not involve a pseudo header for IPv4 or
IPv6 like the computation of the TCP checksum, as specified in
[RFC9293], or the UDP checksum, as specified in [RFC0768].
Zero is a valid result of the CRC32c checksum algorithm. For
example, the following figure depicts an SCTP packet containing a
minimal INIT chunk with a correct CRC32c checksum of zero.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port Number = 5001 |Destination Port Number = 5001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Verification Tag = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 |Chunk Flags = 0| Chunk Length = 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag = 0xFCB75CCA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) = 1500 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Number of Outbound Streams = 1 | Number of Inbound Streams = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: SCTP Packet with a correct CRC32c checksum of zero
Using SCTP in combination with other mechanisms or protocol
extensions might provide an equal or better level of data integrity
protection than the one provided by using the CRC32c checksum
algorithm. When using such alternate error detection methods, the
SCTP common header containing the 32-bit checksum field might or
might not be visible to middleboxes on the paths between the two end
points.
Alternate error detection methods have two requirements:
1. An alternate error detection method MUST provide an equal or
better level of data integrity than the one provided by using the
CRC32c checksum algorithm. This MAY only apply to packets
satisfying some method specific constraints.
2. Using an alternate error detection method MUST NOT result in a
path failure for more than two retransmission timeouts (RTO) due
to middleboxes on the path expecting correct CRC32c checksums.
To fulfill the second requirement, alternate error detection methods
MAY use a heuristic to detect the existence of such middleboxes and
use correct CRC32c checksums on these affected paths.
One example fulfilling the first requirement is using DTLS as the
lower layer of SCTP as specified in [RFC8261]. Another example is
using SCTP Authentication as specified in [RFC4895]. Of course, this
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only applies to all SCTP packets having an AUTH chunk as its first
chunk. However, using SCTP Authentication without any heuristic does
not fulfill the second requirement. Since using DTLS as the lower
layer of SCTP as specified in [RFC8261] also fulfills the second
requirement, it can be used as an alternate error detection method
(see Section 6).
If an alternate error detection method is available, the computation
of the CRC32c checksum consumes computational resources without
providing any benefit. To avoid this, an SCTP end point could be
willing to accept SCTP packets with an incorrect CRC32c checksum
value of zero in addition to SCTP packets with correct CRC32c
checksum values.
Because zero is a valid result of the CRC32c checksum algorithm, a
receiver of an SCTP packet containing a checksum value of zero cannot
determine whether the sender included an incorrect CRC32c checksum of
zero to reduce the CPU cost or the result of the CRC32c checksum
computation was actually zero. However, if the receiver is willing
to use an alternate error detection method, this ambiguity is
irrelevant, since the receiver is fine with not using the CRC32c
checksum to protect incoming packets.
4. A New Chunk Parameter
The Zero Checksum Acceptable Chunk Parameter is defined by the
following figure.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x8001 (suggested) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Detection Method Identifier (EDMID) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Zero Checksum Acceptable Chunk Parameter
Type: 16 bits (unsigned integer)
This field holds the IANA defined parameter type for the "Zero
Checksum Acceptable" chunk parameter. IANA is requested to assign
the value 32769 (0x8001) (suggested) for this parameter type.
Length: 16 bits (unsigned integer)
This field holds the length in bytes of the chunk parameter; the
value MUST be 8.
Error Detection Method Identifier (EDMID): 32 bits (unsigned
integer)
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An IANA registered value specifying the alternate error detection
method the sender of this parameter is willing to use for received
packets.
All transported integer numbers are in "network byte order" a.k.a.,
Big Endian.
The Zero Checksum Acceptable Chunk Parameter MAY appear in INIT and
INIT ACK chunks and MUST NOT appear in any other chunk. The
Parameter MUST NOT appear more than once in any chunk.
If an end point not supporting the extension described in this
document receives this parameter in an INIT or INIT ACK chunk, it is
REQUIRED to skip this parameter and continue to process further
parameters in the chunk. This behavior is specified by [RFC9260]
because the highest-order two bits of the Type are '10'.
5. Procedures
5.1. Declaration of Feature Support
An end point willing to accept SCTP packets with an incorrect
checksum of zero MUST include the Zero Checksum Acceptable Chunk
Parameter indicating the alternate error detection method it is
willing to use in the INIT or INIT ACK chunk it sends.
An SCTP implementation MAY also require the upper layer to indicate
that it is fine to use a specific alternate error detection method
for accepting SCTP packets with an incorrect CRC32c checksum value of
zero.
5.2. Sender Side Considerations
An SCTP end point cannot just use an incorrect CRC32c checksum value
of zero for all SCTP packets it sends. The following restrictions
apply:
1. If an end point has not received an INIT or INIT ACK chunk
containing a Zero Checksum Acceptable Chunk Parameter indicating
an alternate error detection method it supports from its peer
during the association setup, it MUST use a correct CRC32c
checksum. In particular, when an end point
a. sends a packet containing an INIT chunk, it MUST include a
correct CRC32c checksum in the packet containing the INIT
chunk.
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b. responds to an "Out of the Blue" (OOTB) SCTP packet, it MUST
include a correct CRC32c checksum in the response packet.
2. When an end point sends a packet containing a COOKIE ECHO chunk,
it MUST include a correct CRC32c checksum in the packet
containing the COOKIE ECHO chunk.
3. When an end point supports the dynamic address reconfiguration
specified in [RFC5061] and sends a packet containing an ASCONF
chunk, it MUST include a correct CRC32c checksum in the packet
containing the ASCONF chunk.
4. Alternate error detection methods might have some additional
conditions requiring that the sender MUST include a correct
CRC32c checksum in the packet.
The first restriction allows backwards compatibility, and the second
and third restrictions allow a simpler implementation of the
extension defined in this document. The last restriction covers
alternate error detection method specific constraints.
An SCTP end point MAY only send packets with an incorrect checksum of
zero, if the upper layer allowed the use of the alternate error
detection method that was announced by the peer.
If none of the above restrictions apply, an end point SHOULD use zero
as the checksum when sending an SCTP packet.
5.3. Receiver Side Considerations
If an end point has sent the Zero Checksum Acceptable Chunk Parameter
indicating the support of an alternate error detection method in an
INIT or INIT ACK chunk, it MUST accept SCTP packets fulfilling the
requirements of the announced alternate error detection method using
an incorrect checksum value of zero in addition to SCTP packets
containing the correct CRC32c checksum value for this association.
In addition to processing OOTB packets with a correct CRC32c checksum
as specified in [RFC9260], an SCTP implementation MAY also process
OOTB packets having an incorrect zero checksum. Doing so might
result in faster SCTP association failure detection.
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6. Error Detection via SCTP over DTLS
Using SCTP over DTLS as specified in [RFC8261] provides a stronger
error detection method than using the CRC32c checksum algorithm.
Since middleboxes will not observe the unencrypted SCTP packet, there
is no risk in interfering with using zero as an incorrect checksum.
There are no additional error detection method specific constraints
on packets when using DTLS encapsulation.
IANA is requested to assign the Error Detection Method Identifier of
1 for this method.
7. Socket API Considerations
This section describes how the socket API defined in [RFC6458] needs
to be extended to provide a way for the application to control the
acceptance of a zero checksum.
A 'Socket API Considerations' section is contained in all SCTP
related specifications published after [RFC6458] describing an
extension for which implementations using the socket API as specified
in [RFC6458] would require some extension of the socket API. Please
note that this section is informational only.
A socket API implementation based on [RFC6458] is extended by
supporting one new write-only IPPROTO_SCTP-level socket option.
7.1. Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM)
This IPPROTO_SCTP-level socket option with the name
SCTP_ACCEPT_ZERO_CHECKSUM can be used to control the acceptance of a
zero checksum. It is a write-only socket option and applies only to
future SCTP associations on the socket.
This option expects an unsigned integer. Possible values include:
SCTP_EDMID_NONE: Disable the use of alternate error detection
method. This means that all SCTP packets being sent have a
correct CRC32c checksum value.
SCTP_EDMID_LOWER_LAYER_DTLS: Use the alternate error detection
method described in Section 6.
An implementation might only send packets with an incorrect checksum
of zero, if the alternate error detection method announced by the
peer is also enabled locally via this socket option.
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The default for this socket option is that the use of alternate error
detection methods is disabled.
8. IANA Considerations
[NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number
you assign this document.]
[NOTE to RFC-Editor: The suggested value for the parameter type is
tentative and to be confirmed by IANA.]
This document (RFCXXXX) is the reference for the registration
described in this section.
A new chunk parameter type has to be assigned by IANA. This requires
an additional line in the "Chunk Parameter Types" registry for SCTP:
+===================+==========================+===========+
| ID Value | Chunk Parameter Type | Reference |
+===================+==========================+===========+
| 32769 (suggested) | Zero Checksum Acceptable | [RFCXXXX] |
| | (0x8001 (suggested)) | |
+-------------------+--------------------------+-----------+
Table 1: New entry in "Chunk Parameter Types" registry
Furthermore, IANA is requested to establish a new "Error Detection
Method" registry for SCTP. The assignment of new error detection
methods is done through the Specification Required policy as defined
in [RFC8126]. Documentation for a new error detection method MUST
contain the following information:
1. A name of an alternate error detection method.
2. A reference to a specification describing:
(a) the alternate error detection method,
(b) why the alternate error detection method provides an equal
or better level of data integrity protection than the one
provided by using the CRC32c checksum,
(c) any alternate error detection method specific constraints
referred to in the fourth exception in Section 5.2, and
(d) why using the alternate error detection method does not
result in path failures due to middleboxes expecting correct
CRC32c checksums for more than two RTOs. In case the
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alternate error detection method uses a heuristic for
detecting such middleboxes, this heuristic needs to be
described.
IANA is requested to use the following as the initial contents of the
registry:
+================+========================+===========+
| ID Value | Error Detection Method | Reference |
+================+========================+===========+
| 0 | Reserved | [RFCXXXX] |
+----------------+------------------------+-----------+
| 1 | SCTP over DTLS | [RFCXXXX] |
+----------------+------------------------+-----------+
| 2 - 4294967295 | Unassigned | |
+----------------+------------------------+-----------+
Table 2: Initial Contents of the "Error Detection
Method" registry
A Designated Expert (DE) is expected to ascertain the existence of
suitable documentation (a specification) as described in [RFC8126]
and to verify that the document is permanently and publicly
available. Furthermore, the DE is expected to ensure that the above
four points have been addressed in an appropriate way.
9. Security Considerations
This document does not change the considerations given in [RFC9260].
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<https://www.rfc-editor.org/info/rfc5061>.
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[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
"Datagram Transport Layer Security (DTLS) Encapsulation of
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.
[RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
June 2022, <https://www.rfc-editor.org/info/rfc9260>.
10.2. Informative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <https://www.rfc-editor.org/info/rfc4895>.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<https://www.rfc-editor.org/info/rfc6458>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
Acknowledgments
The authors wish to thank Gorry Fairhurst, Mike Heard, Peter Lei,
Nils Ohlmeier, Claudio Porfiri, Greg Skinner, Timo Völker, and Magnus
Westerlund for their invaluable comments.
Authors' Addresses
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Michael Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
Victor Boivie
Google
Kungsbron 2
SE-11122 Stockholm
Sweden
Email: boivie@google.com
Florent Castelli
Google
Kungsbron 2
SE-11122 Stockholm
Sweden
Email: orphis@google.com
Randell Jesup
Mozilla Corporation
1835 Horse Shoe Trl
Malvern, PA 19355
United States of America
Email: randell-ietf@jesup.org
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