Internet Engineering Task Force (IETF) M. Tüxen

Internet Engineering Task Force (IETF) M. Tüxen
Request for Comments: 9653 Münster Univ. of Appl. Sciences
Category: Standards Track V. Boivie
ISSN: 2070-1721 F. Castelli
Google
R. Jesup
Mozilla
September 2024

Zero Checksum for the Stream Control Transmission Protocol

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 already provides 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 allowing SCTP 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 is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9653.

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. Conventions
3. Alternate Error Detection Methods
4. A New Chunk Parameter
5. Procedures
5.1. Declaration of Feature Support
5.2. Sender-Side Considerations
5.3. Receiver-Side Considerations
6. Error Detection via SCTP over DTLS
7. Socket API Considerations
7.1. Set Accepting a Zero Checksum (SCTP_ACCEPT_ZERO_CHECKSUM)
8. IANA Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgments
Authors' Addresses

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 that already provided by DTLS. However, computing
the CRC32c checksum at the sender and receiver sides does consume
computational resources for no benefit. This is particularly
important for endpoints that are computationally limited and use SCTP
over DTLS.

The extension described in this document allows an SCTP endpoint 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 endpoints
that support this extension to be interoperable with endpoints that
don't. To provide this backwards compatibility, endpoints 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.

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 data integrity protection with an equal or
lower probability of false negatives 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 endpoints.

Alternate error detection methods have two requirements:

1. An alternate error detection method MUST provide an equal or
lower probability of false negatives 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 (RTOs) due
to middleboxes on the path expecting correct CRC32c checksums.

To fulfill the second requirement, alternate error detection methods
could use a heuristic to detect the existence of such middleboxes and
use correct CRC32c checksums on these affected paths.

Using DTLS as the lower layer of SCTP as specified in [RFC8261] is
one example that fulfills the first requirement. Another example is
using SCTP Authentication as specified in [RFC4895]. Of course, this
only applies to each SCTP packet 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 used, the computation of
the CRC32c checksum consumes computational resources without
providing any benefit. To avoid this, an SCTP endpoint 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 | 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 has assigned the value
32769 (0x8001) 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)
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 endpoint 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 endpoint 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
before including the corresponding Zero Checksum Acceptable Chunk
Parameter.

5.2. Sender-Side Considerations

An SCTP endpoint cannot just use an incorrect CRC32c checksum value
of zero for all SCTP packets it sends. The following restrictions
apply:

1. If an endpoint 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 endpoint

a. sends a packet containing an INIT chunk, it MUST include a
correct CRC32c checksum in the packet containing the INIT
chunk.

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 endpoint 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 endpoint 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. If an alternate error detection method has some method-specific
constraints, the sender MUST include a correct CRC32c checksum in
all packets that don't fulfill these method-specific constraints.

The first restriction allows backwards compatibility. The second and
third restrictions allow a simpler implementation of the extension
defined in this document, because looking up the association for SCTP
packets containing a COOKIE ECHO chunk or an ASCONF chunk might be
more complex than for other packets. Finally, the last restriction
covers constraints specific to the alternate error detection method.

An SCTP endpoint MAY require that the upper layer allow the use of
the alternate error detection method that was announced by the peer
before sending packets with an incorrect checksum of zero.

If none of the above restrictions apply, an endpoint SHOULD use zero
as the checksum when sending an SCTP packet.

5.3. Receiver-Side Considerations

If an endpoint has sent the Zero Checksum Acceptable Chunk Parameter
indicating the support of an alternate error detection method in an
INIT or INIT ACK chunk, in addition to SCTP packets containing the
correct CRC32c checksum value it MUST accept SCTP packets that have
an incorrect checksum value of zero and that fulfill the requirements
of the announced alternate error detection method used for this
association. Otherwise, the endpoint MUST drop all SCTP packets with
an incorrect CRC32c checksum.

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.

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 constraints (specific to the error detection
method) on packets when using DTLS encapsulation.

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 any alternate error detection
method. This means that all SCTP packets being received are only
accepted if they 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.

The default for this socket option is that the use of alternate error
detection methods is disabled.

8. IANA Considerations

A new chunk parameter type has been assigned by IANA in the "Chunk
Parameter Types" registry for SCTP:

+==========+===================================+===========+
| ID Value | Chunk Parameter Type | Reference |
+==========+===================================+===========+
| 32769 | Zero Checksum Acceptable (0x8001) | RFC 9653 |
+----------+-----------------------------------+-----------+

Table 1: New Entry in "Chunk Parameter Types" Registry

Furthermore, IANA has established 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 lower probability of false negatives than the one
provided by using the CRC32c checksum,

(c) any constraints (specific to the alternate error detection
method) that are 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
alternate error detection method uses a heuristic for
detecting such middleboxes, this heuristic needs to be
described.

The initial contents of the registry are as follows:

+================+========================+===========+
| ID Value | Error Detection Method | Reference |
+================+========================+===========+
| 0 | Reserved | RFC 9653 |
+----------------+------------------------+-----------+
| 1 | SCTP over DTLS | RFC 9653 |
+----------------+------------------------+-----------+
| 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 appropriately.

9. Security Considerations

This document does not change the considerations given in [RFC9260].

Due to the first requirement in Section 3, using an alternate error
detection method provides an equal or better level of data integrity
than the one provided by using the CRC32c checksum algorithm. The
second requirement in Section 3 ensures that the existence of
middleboxes expecting correct CRC32c checksums does not result in
permanent path failures.

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>.

[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 Bernard Aboba, Deb Cooley, Martin Duke,
Gorry Fairhurst, Mike Heard, Peter Lei, Nils Ohlmeier, Claudio
Porfiri, Greg Skinner, Timo Völker, Éric Vyncke, and Magnus
Westerlund for their invaluable comments.

Authors' Addresses

Michael Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: [email protected]

Victor Boivie
Google
Kungsbron 2
SE-11122 Stockholm
Sweden
Email: [email protected]

Florent Castelli
Google
Kungsbron 2
SE-11122 Stockholm
Sweden
Email: [email protected]

Randell Jesup
Mozilla Corporation
1835 Horse Shoe Trl
Malvern, PA 19355
United States of America
Email: [email protected]