TSVWG J. Touch
Internet Draft Independent Consultant
Intended status: Standards Track C. Heard (Ed.)
Intended updates: 768 Unaffiliated
Expires: September 2025 March 16, 2025
Transport Options for UDP
draft-ietf-tsvwg-udp-options-45.txt
Abstract
Transport protocols are extended through the use of transport header
options. This document updates RFC 768 (UDP) by indicating the
location, syntax, and semantics for UDP transport layer options
within the surplus area after the end of the UDP user data but
before the end of the IP datagram.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 16, 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction ..................................................3
2. Conventions used in this document .............................3
3. Terminology ...................................................3
4. Background ....................................................5
5. UDP Option Intended Uses ......................................6
6. UDP Option Design Principles ..................................6
7. The UDP Option Area ...........................................8
8. The UDP Surplus Area Structure ...............................11
9. The Option Checksum (OCS) ....................................11
10. UDP Options .................................................13
11. SAFE UDP Options ............................................18
11.1. End of Options List (EOL) ..............................18
11.2. No Operation (NOP) .....................................19
11.3. Additional Payload Checksum (APC) ......................19
11.4. Fragmentation (FRAG) ...................................21
11.5. Maximum Datagram Size (MDS) ............................28
11.6. Maximum Reassembled Datagram Size (MRDS) ...............29
11.7. Echo request (REQ) and echo response (RES) .............30
11.8. Timestamps (TIME) ......................................31
11.9. Authentication (AUTH), RESERVED Only ...................33
11.10. Experimental (EXP) ....................................33
12. UNSAFE Options ..............................................34
12.1. UNSAFE Compression (UCMP) ..............................35
12.2. UNSAFE Encryption (UENC) ...............................35
12.3. UNSAFE Experimental (UEXP) .............................35
13. Rules for designing new options .............................35
14. Option inclusion and processing .............................37
15. UDP API Extensions ..........................................39
16. UDP Options are for Transport, Not Transit ..................41
17. UDP options vs. UDP-Lite ....................................42
18. Interactions with Legacy Devices ............................42
19. Options in a Stateless, Unreliable Transport Protocol .......43
20. UDP Option State Caching ....................................44
21. Updates to RFC 768 ..........................................44
22. Interactions with other RFCs (and drafts) ...................44
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23. Multicast and Broadcast Considerations ......................45
24. Network Management Considerations ...........................46
25. Security Considerations .....................................46
25.1. General Considerations Regarding the Use of Options ....46
25.2. Considerations Regarding On-Path Attacks ...............47
25.3. Considerations Regarding Option Processing .............47
25.4. Considerations for Fragmentation .......................48
25.5. Considerations for Providing UDP Security ..............48
25.6. Considerations Regarding Middleboxes ...................49
26. IANA Considerations .........................................49
27. References ..................................................51
27.1. Normative References ...................................51
27.2. Informative References .................................51
28. Acknowledgments .............................................55
Appendix A. Implementation Information ..........................57
1. Introduction
Transport protocols use options as a way to extend their
capabilities. TCP [RFC9293], SCTP [RFC9260], and DCCP [RFC4340]
include space for these options but UDP [RFC768] currently does not.
This document updates RFC 768 with an extension to UDP that provides
space for transport options including their generic syntax and
semantics for their use in UDP's stateless, unreliable message
protocol. The details of the impact on RFC 768 are provided in
Section 21. This extension does not apply to UDP-Lite, as discussed
further in Section 17.
2. Conventions used in this document
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.
In this document, the characters ">>" preceding an indented line(s)
indicates a statement using the key words listed above. This
convention aids reviewers in quickly identifying or finding the
portions of this RFC covered by these key words.
3. Terminology
The following terminology is used in this document:
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o IP datagram [RFC791][RFC8200] - an IP packet, composed of the IP
header (including any IPv4 options) and an IP payload area
(including any IPv6 extension headers or other shim headers)
o Must-support options - UDP options that all implementations are
required to support. Their use in individual UDP packets is
optional.
o SAFE options - UDP options that are designed to be safe to ignore
for a receiver that does not understand them. Such options do not
alter the UDP user data or signal a change in what its contents
represent.
o Socket pair - a pair of sockets defining a UDP exchange, defined
by a remote socket and a local socket, each composed of an IP
address and UDP port number (most widely known from TCP [RFC793])
o Surplus area - the area of an IP payload that follows a UDP
packet; this area is used for UDP options in this document
o UDP packet - the more contemporary term used herein to refer to a
user datagram [RFC768]
o UDP fragment - one or more components of a UDP packet and its UDP
options that enables transmission over multiple IP payloads,
larger than permitted by the maximum size of a single IP packet;
note that each UDP fragment is itself transmitted as a UDP packet
with its own options
o (UDP) User data - the user data field of a UDP packet [RFC768]
o UDP Length - the length field of a UDP header [RFC768]
o UNSAFE options - UDP options that are not designed to be safe for
a receiver that does not understand them to ignore. Such options
could alter the UDP user data or signal a change in what its
contents represent, but there are restrictions on how they can be
transmitted; these restrictions are noted in Sections 10 and 12.
o User - the upper layer application, protocol, or service that
produces and consumes content that UDP transfers
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o User datagram - a UDP packet, composed of a UDP header and UDP
payload; as discussed herein, that payload need not extend to the
end of the IP datagram. In this document, the original intent
that a UDP datagram corresponds to the user portion of a single
IP datagram is redefined, where a UDP datagram can span more than
one IP datagram through UDP fragmentation.
4. Background
Many protocols include a default, invariant header and an area for
header options that varies from packet to packet. These options
enable the protocol to be extended for use in particular
environments or in ways unforeseen by the original designers.
Examples include TCP's Maximum Segment Size, Window Scale,
Timestamp, and Authentication Options [RFC9293][RFC5925][RFC7323].
Header options are used both in stateful (connection-oriented, e.g.,
TCP [RFC9293], SCTP [RFC9260], DCCP [RFC4340]) and stateless
(connectionless, e.g., IPv4 [RFC791], IPv6 [RFC8200]) protocols. In
stateful protocols they can help extend the way in which state is
managed. In stateless protocols their effect is often limited to
individual packets, but they can have an aggregate effect on a
sequence of packets as well.
UDP is one of the most popular protocols that lacks space for header
options [RFC768]. The UDP header was intended to be a minimal
addition to IP, providing only port numbers and a checksum for error
detection. This document extends UDP to provide a trailer area for
such options, located after the UDP user data.
UDP options are possible because UDP includes its own length field,
separate from that of the IP header. Other transport protocols infer
transport payload length from the IP datagram length (TCP, DCCP,
SCTP). Internet historians have suggested a number of possible
reasons why the design of UDP includes this field, e.g., to support
multiple UDP packets within the same IP datagram or to indicate the
length of the UDP user data as distinct from zero padding required
for systems that require writes that are not byte-aligned. These
suggestions are not consistent with earlier versions of UDP or with
concurrent design of multi-segment multiplexing protocols, however,
so the real reason remains unknown. Regardless, this field presents
an opportunity to differentiate the UDP user data from the implied
transport payload length, which this document leverages to support a
trailer options field.
There are other ways to include additional header fields or options
in protocols that otherwise are not extensible. In particular, in-
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band encoding can be used to differentiate transport payload from
additional fields, such as was proposed in [Hi15]. This approach can
cause complications for interactions with legacy devices, and is
thus not considered further in this document.
IPv6 Teredo extensions [RFC4380][RFC6081] use a similar
inconsistency between UDP and IPv6 packet lengths to support
trailers, but in this case the values differ between the UDP header
and an IPv6 length contained as the payload of the UDP user data.
This allows IPv6 trailers in the UDP user data, but have no relation
to the surplus area discussed in this document. As a consequence,
Teredo extensions are compatible with UDP options.
5. UDP Option Intended Uses
UDP options can be used to provide a soft control plane to UDP. They
enable capabilities available in other transport protocols, such as
fragmentation and reassembly, that enable UDP frames larger than the
IP MTU to traverse devices that rely on transport ports, e.g., NATs,
without additional mechanisms or state. They add features that
could, in the future, protect transport integrity and validate
source identity (authentication), as well as those that could also
encrypt the user payload, while still protecting the UDP transport
header - unlike Datagram Transport Layer Security (DTLS) [RFC9147].
They also enable packetization-layer path MTU discovery (PLPMTUD)
over UDP, known as Datagram Packetization Layer Path Maximum
Transmission Unit Discovery DPLPMTUD [Fa25], providing a means for
probe packet validation without affecting the user data plane, as
well as providing explicit indication of the receiver transport
reassembly size.
UDP originally assumed that such capabilities would be provided by
the user or by a layer above UDP [RFC768]. However, enough protocols
have evolved to use UDP directly, so such an intermediate layer
would be difficult to deploy for legacy applications. UDP options
leverage the opportunity presented by the surplus area to enable
these extensions within the UDP transport layer itself. Among the
use cases where this approach could be of benefit are request-
response protocols such as DNS over UDP [He24].
6. UDP Option Design Principles
UDP options have been designed based on the following core
principles. Each is an observation about (preexisting) UDP [RFC768]
in the absence of these extensions that this document does not
intend to change or a lesson learned from other protocol designs.
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1. UDP is stateless; UDP options do not change that fact.
State required or maintained by the endpoints in intended to be
managed either by the application or a layer/library on behalf of
the application. Reassembly of fragments is the only limited
exception where this document introduces a notion of state to
UDP.
2. UDP is unidirectional; UDP options do not change that fact.
Responses to options are initiated by the application or a
layer/library on behalf of the application. A mechanism that
requires bidirectionality needs to be defined in a separate
document.
3. UDP options have no length limit separate from that of the UDP
packet itself.
Past experience with other protocols confirms that static length
limits will always need to be exceeded, e.g., as has been an
issue with TCP options and IPv4 addresses. Each implementation
can limit how long/many options there are, but a specification is
more robust when it does not introduce such a limit.
4. UDP options are not intended to replace or replicate other
protocols.
This includes NTP, ICMP (notably echo), etc. UDP options are
intended to introduce features useful for applications, not to
either replace these other protocols nor to instrument UDP to
replace the need for network testing devices.
5. UDP options are a framework, not a protocol.
Options can be defined in this initial document even when the
details are not sufficient to specify a complete protocol. Uses
of such options could then be described or supplemented in other
documents. Examples herein include REQ/RES and TIME; in both
cases, the option format is defined, but the protocol that uses
these is specified elsewhere (REQ/RES for DPLPMTUD [Fa25]) or
left undefined (TIME).
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6. The UDP option mechanism and UDP options themselves are intended
to default to the same behavior experienced by a legacy receiver.
By default, even when option checksums (OCS, APC),
authentication, or decryption fail, all received packets (with
the exception of UDP fragments) are passed (possibly with an
empty data payload) to the user application. Options that do not
modify user data are intended to (by default) result in the user
data also being passed, even if, e.g., option checksums or
authentication fails. It is always the user's or application's
obligation to override this default behavior explicitly.
These principles are intended to enable the design and use of UDP
options with minimal impact to legacy UDP endpoints, preferably
none. UDP is - and remains - a minimal transport protocol.
Additional capability is explicitly activated by user applications
or libraries acting on their behalf.
Finally, UDP options do not attempt to match the number of zero-
length UDP datagrams received by legacy and option-aware receivers
from a source using UDP fragmentation (see Section 11.4). Legacy
receivers interpret every UDP fragment as a zero-length packet
(because they do not perform reassembly), but option-aware receivers
would reassemble the packet as a non-zero-length packet. Zero-length
UDP packets have been used as "liveness" indicators see Section 5 of
[RFC8085]), but such use is dangerous because they lack unique
identifiers (the IPv6 base header has none, the IPv4 ID field is
deprecated for such use [RFC6994]).
7. The UDP Option Area
The UDP transport header includes demultiplexing and service
identification (port numbers), an error detection checksum, and a
field that indicates the UDP datagram length (including UDP header).
The UDP Length field is typically redundant with the size of the
maximum space available as a transport protocol payload, as
determined by the IP header (see detail in Section 18). The UDP
Option area is created when the UDP Length indicates a smaller
transport payload than implied by the IP header.
For IPv4, IP Total Length field indicates the total IP datagram
length (including IP header), and the size of the IP options is
indicated in the IP header (in 4-byte words) as the "Internet Header
Length" (IHL), as shown in Figure 1 [RFC791]. In exceptional cases,
the Protocol field in IPv4 might not indicate UDP (i.e., 17), e.g.,
when intervening shim headers are present such as IP Security
(IPsec) [RFC4301] or for IP Payload Compression (IPComp) [RFC3173].
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The upper bound for UDP Length when Protocol = 17 is given by:
UDP_Length <= IPv4_Total_Length - IPv4_IHL * 4
If shim headers are present, this upper bound must be reduced by the
sum of the lengths of shim headers that precede the UDP header.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL | DSCP |ECN| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Proto=17 (UDP)| Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... zero or more IP Options (using space as indicated by IHL) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... zero or more shim headers (each indicating size) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Source Port | UDP Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 IPv4 datagram with UDP header
For IPv6, the IP Payload Length field indicates the transport
payload after the base IPv6 header, which includes the IPv6
extension headers and space available for the transport protocol, as
shown in Figure 2 [RFC8200]. Note that the Next Header field in IPv6
might not indicate UDP (i.e., 17), e.g., when intervening IP
extension headers are present. For IPv6, the lengths of any
additional IP extensions are indicated within each extension
[RFC8200], so the upper bound for UDP Length is given by:
UDP_Length <= IPv6_Payload_Length - sum(extension header lengths)
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Next Header | Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
| Source Address (128 bits) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
| Destination Address (128 bits) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... zero or more IP Extension headers (each indicating size) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Source Port | UDP Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 IPv6 datagram with UDP header
In both cases, the space available for the UDP packet is indicated
by IP, either directly in the base header or by adding information
in the shim headers or extensions. In either case, this document
will refer to this available space as the "IP transport payload".
As a result of this redundancy, there is an opportunity to use the
UDP Length field as a way to break up the IP transport payload into
two areas - that intended as UDP user data and an additional
"surplus area" (as shown in Figure 3).
IP transport payload
<------------------------------------------------->
+--------+---------+----------------------+------------------+
| IP Hdr | UDP Hdr | UDP user data | surplus area |
+--------+---------+----------------------+------------------+
<------------------------------>
UDP Length
Figure 3 IP transport payload vs. UDP Length
In most cases, the IP transport payload and UDP Length point to the
same location, indicating that there is no surplus area. This is not
a requirement of UDP [RFC768] (discussed further in Section 18).
This document uses the surplus area for UDP options.
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The surplus area can commence at any valid byte offset, i.e., it
need not be 16-bit or 32-bit aligned. In effect, this document
redefines the UDP "Length" field as a "trailer options offset".
8. The UDP Surplus Area Structure
UDP options use the entire surplus area, i.e., the contents of the
IP payload after the last byte of the UDP payload. They commence
with a 2-byte Option Checksum (OCS) field aligned to the first 2-
byte boundary (relative to the start of the IP datagram) of that
area, adding zeroes before OCS as needed for alignment. The UDP
option area can be used with any UDP payload length (including zero,
i.e., a UDP Length of 8), as long as there remains enough space for
the aligned OCS and the options used.
>> UDP options MAY begin at any UDP length offset.
>> Option area bytes used for alignment before the OCS MUST be zero.
If this is not the case, all options MUST be ignored and the surplus
area silently discarded.
These alignment bytes, coupled with OCS as computed over the
remainder of the surplus area, ensure that the ones-complement sum
of the surplus area is zero. OCS is half-word (2-byte) aligned to
avoid the need for byte-swapping in its implementation.
The OCS contains an optional ones-complement sum that detects errors
in the surplus area, which is not otherwise covered by the UDP
checksum, as detailed in Section 9.
The remainder of the surplus area consists of options, all except
two of which are defined using a TLV (type, length, and optional
value) syntax similar to that of TCP [RFC9293], as detailed in
Section 10 (types NOP and EOL have an implicit length of one byte).
These options continue until the end of the surplus area or can end
earlier using the EOL (end of list) option, followed by zeroes
(discussed further in Section 10).
9. The Option Checksum (OCS)
The Option Checksum (OCS) option is a conventional Internet checksum
[RFC791] that detects errors in the surplus area. The OCS option
contains a 16-bit checksum that is aligned to the first 2-byte
boundary, preceded by zeroes for padding (if needed), as shown in
Figure 4.
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+--------+--------+--------+--------+
| UDP data | 0 |
+--------+--------+--------+--------+
| OCS | UDP options... |
+--------+--------+--------+--------+
Figure 4 UDP OCS format, here using one zero byte for alignment
The OCS consists of a 16-bit Internet checksum [RFC1071], computed
over the surplus area and including the length of the surplus area
as an unsigned 16-bit value. The OCS protects the surplus area from
errors in a similar way that the UDP checksum protects the UDP user
data (when not zero).
The primary purpose of the OCS is to detect existing non-standard
(i.e., non-option) uses of that area and accidental errors. It is
not intended to detect attacks, as discussed further in Section 25.
OCS is not intended to prevent future non-standard uses of the
surplus area, nor does it enable shared use with mechanisms that do
not comply with UDP options.
The design enables traversal of errant middleboxes that incorrectly
compute the UDP checksum over the entire IP payload [Fa18][Zu20],
rather than only the UDP header and UDP payload (as indicated by the
UDP header length). Because the OCS is computed over the surplus
area and its length and then inverted, OCS effectively negates the
effect that incorrectly including the surplus has on the UDP
checksum. As a result, when OCS is non-zero, the UDP checksum is the
same in either case.
>> The OCS MUST be non-zero when the UDP checksum is non-zero.
>> When the UDP checksum is zero, the OCS MAY be unused, and is then
indicated by a zero OCS value.
>> UDP option implementations MUST default to using OCS (i.e., as a
non-zero value); users overriding that default take the risk of not
detecting nonstandard uses of the option area (of which there are
none currently known).
Like the UDP checksum, the OCS is optional under certain
circumstances and contains zero when not used. UDP checksums can be
zero for IPv4 [RFC791] and for IPv6 [RFC8200] when the UDP payload
is already covered by another checksum, as might occur for tunnels
[RFC6935]. The same exceptions apply to the OCS when used to detect
bit errors; an additional exception occurs for its use in the UDP
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datagram prior to fragmentation or after reassembly (see Section
11.4).
The benefits are similar to allowing UDP checksums to be zero, but
the risks differ. OCS is additionally important to ensure packets
with UDP options can traverse errant middleboxes [Zu20]. When the
cost of computing an OCS is negligible, it is better to use OCS to
ensure such traversal. In cases where such traversal risks can
safely be ignored, such as controlled environments, over paths where
traversal is validated, or where upper layer protocols
(applications, libraries, etc.) can adapt (by enabling OCS when
packet exchange fails), and when bit errors at the UDP layer would
be detected by other layers (as with the UDP checksum) OCS can be
disabled, e.g., to conserve energy or processing resources or when
it can improve performance. This is why zeroing OCS is only safe
when UDP checksum is also zero, but why OCS might still be used in
that case.
The OCS covers the surplus area as formatted for transmission and is
processed immediately upon reception.
>> If the receiver validation of the OCS fails, all options MUST be
ignored and the surplus area silently discarded.
>> UDP user data that is validated by a correct UDP checksum MUST by
default be delivered to the application layer, even if the OCS
fails, unless the endpoints have negotiated otherwise for this UDP
packet's socket pair.
When not used (i.e., containing zero), the OCS is assumed to be
"correct" for the purpose of accepting UDP datagrams at a receiver
(see Section 14).
10. UDP Options
UDP options are a minimum of two bytes in length as shown in Figure
5, excepting only the one-byte options "No Operation" (NOP) and "End
of Options List" (EOL) described below.
+--------+--------+-------
| Kind | Length | (remainder of option...)
+--------+--------+-------
Figure 5 UDP option default format
The Kind field is always one byte, and is named after the
corresponding TCP field (though other protocols refer to this as
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Type). The Length field, which indicates the length in bytes of the
entire option, including Kind and Length, is one byte for all
lengths below 255 (including the Kind and Length bytes). A Length of
255 indicates use of the UDP option extended format shown in Figure
6. The Extended Length field is a 16-bit field in network standard
byte order. The length of the option refers to its Length field or
Extended Length field, whichever is applicable.
+--------+--------+--------+--------+
| Kind | 255 | Extended Length |
+--------+--------+--------+--------+
| (remainder of option...) |
+--------+--------+--------+--------+
Figure 6 UDP option extended format
>> The UDP length MUST be at least as large as the UDP header (8)
and no larger than the IP transport payload. Datagrams with length
values outside this range MUST be silently dropped as invalid and
logged.
>> All logging SHOULD be rate limited. Excess logging events can be
coalesced and reported as a count, or can be silently dropped if
needed to avoid resource overloading.
>> Option Lengths (or Extended Lengths, where applicable) smaller
than the minimum for the corresponding Kind MUST be treated as an
error. Such errors call into question the remainder of the surplus
area and thus MUST result in all UDP options being silently
discarded.
>> Any UDP option other than NOP or EOL whose length is 254 or less
MUST use the UDP option default format shown in Figure 5. NOP and
EOL never use either length format.
>> Any UDP option whose length is larger than 254 MUST use the UDP
option extended format shown in Figure 6.
>> For compactness, UDP options SHOULD use the smallest option
format possible.
>> UDP options MUST be interpreted in the order in which they occur
in the surplus area or, in the case of UDP fragments, in the order
in which they appear in the UDP fragment option area (see Section
11.4).
The following UDP options are currently defined:
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Kind Length Meaning
---------------------------------------------------------
0* - End of Options List (EOL)
1* - No operation (NOP)
2* 6 Additional payload checksum (APC)
3* 10/12 Fragmentation (FRAG)
4* 4 Maximum datagram size (MDS)
5* 5 Maximum reassembled datagram size (MRDS)
6* 6 Request (REQ)
7* 6 Response (RES)
8 10 Timestamps (TIME)
9 (varies) RESERVED for Authentication (AUTH)
10-126 (varies) UNASSIGNED (assignable by IANA)
127 (varies) RFC 3692-style experiments (EXP)
128-191 RESERVED
192 (varies) RESERVED for Compression (UCMP)
193 (varies) RESERVED for Encryption (UENC)
194-253 UNASSIGNED-UNSAFE (assignable by IANA)
254 (varies) RFC 3692-style experiments (UEXP)
255 RESERVED-UNSAFE
Options indicated by Kind values in the range 0..191 are known as
SAFE options because they do not interfere with use of that data by
legacy endpoints or when the option is unsupported. Options
indicated by Kind values in the range 192..255 are known as UNSAFE
options because might interfere with use by legacy receiving
endpoints (e.g., an option that alters the UDP data payload).
UNSAFE option nicknames are expected to begin with capital "U",
which needs to be avoided for SAFE option nicknames (see Section
26). RESERVED and RESERVED-UNSAFE are not assignable by IANA and not
otherwise defined at this time. The AUTH, UCMP, and UENC
reservations are intended for all future options supporting
authentication, compression, and encryption, respectively, and
remain reserved until assigned for those uses.
Although the FRAG option modifies the original UDP payload contents
(i.e., is UNSAFE with respect to the original UDP payload), it is
used only in subsequent fragments with zero-length UDP user data
payloads, thus is SAFE in actual use, as discussed further in
Section 11.4.
These options are defined in the following subsections. Options 0
and 1 use the same values as for TCP.
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>> An endpoint supporting UDP options MUST support those marked with
a "*" above: EOL, NOP, APC, FRAG, MDS, MRDS, REQ, and RES. This
includes both recognizing and being able to generate these options
if configured to do so. These are called "must-support" options.
The set of must-support options is defined herein. New options are
not eligible for this designation.
>> All other SAFE options (without a "*") MAY be implemented, and
their use SHOULD be determined either out-of-band or negotiated,
notably if needed to detect when options are silently ignored by
legacy receivers.
>> Receivers supporting UDP options MUST silently ignore unknown or
malformed SAFE options (i.e., in the same way a legacy receiver
would ignore all UDP options). An option is malformed when its
length does not indicate (one of) the value(s) stated in the
option's specification. A malformed FRAG option is an exception to
this rule; it SHALL be treated as an unsupported UNSAFE option.
>> Options with inherently invalid Length field values, i.e., those
that indicate underruns of the option itself or overruns of the
surplus area (pointing past the end of the IP payload), MUST be
treated as an indication of a malformed surplus area, and all
options MUST silently be discarded.
Receivers cannot generally treat unexpected option lengths as
invalid, as this would unnecessarily limit future revision of
options (e.g., defining a new APC that is defined by having a
different length).
>> When UNSAFE options are present, the UDP user data MUST be empty,
and any transport payload MUST be contained in a FRAG option (see
Section 11.4). Recall that such options may alter the transport
payload or signal a change in what its contents represent. This
restriction ensures their safe use in environments that might
include legacy receivers (see Section 12), because the transport
payload occurs inside the FRAG option area and is silently discarded
by legacy receivers.
>> Receivers supporting UDP options that receive unsupported options
in the UNSAFE range MUST terminate all option processing and MUST
silently drop all UDP options in that datagram. See Section 12 for
further discussion of UNSAFE options.
>> Other than FRAG, NOP, EXP, and UEXP, each option SHOULD NOT occur
more than once in a single UDP datagram. If an option other than
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these four occurs more than once, a receiver MUST interpret only the
first instance of that option and MUST ignore later instances.
Section 25 provides additional advice for Denial of Service (DoS)
issues that involve large numbers of options, whether valid,
unknown, or repeating.
>> NOP MAY occur multiple times, either in succession or between
other options, for option alignment. Additional repetition
constraints are indicated in Section 11.2.
>> If FRAG occurs more than once, the options area MUST be
considered malformed and MUST NOT be processed.
>> EXP and UEXP MAY occur more than once, but SHOULD NOT occur more
than once using the same ExID (see Sections 11.10 and 12.3).
>> Options other than OCS, AUTH, and UENC MUST NOT include fields
whose values depend on the contents of the surplus area.
AUTH and UENC are always computed as if their hash and the OCS are
zero; the OCS is always computed as if its contents are zero and
after the AUTH or UENC hash has been computed.
>> Future options MUST NOT be defined as having an option field
value dependent on the content or presence of other options or on
the remaining contents of the surplus area, i.e., the area after the
last option (presumably EOL).
If future options were to depend on the contents or presence of
other options, interactions between those values, the OCS, and the
AUTH and UENC options could be unpredictable. This does not prohibit
options that modify later options (in order of appearance within a
packet), such as would typically be the case for compression (UCMP).
Note that there is no need to reserve area after the last UDP option
for future uses, because any such use can be supported by defining a
new UDP option over that area instead. Using an option for this
purpose is safer than treating the region as an exception, because
its use can be verified based on option Kind and Length.
>> AUTH and UENC MUST NOT be used concurrently.
AUTH and UENC are never used together because UENC would serve both
purposes.
>> "Must-support" options other than NOP and EOL MUST be placed by
the transmitter before other SAFE UDP options. A receiver MAY drop
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all UDP options if this ordering is not honored. Such events MAY be
logged for diagnostic purposes.
The requirement that must-support options come before others is
intended to allow for endpoints to implement DoS protection, as
discussed further in Section 25.
11. SAFE UDP Options
SAFE UDP options can be silently ignored by legacy receivers without
affecting the meaning of the UDP user data. They stand in contrast
to UNSAFE options, which modify UDP user data in ways that render it
unusable by legacy receivers (Section 12). The following subsections
describe SAFE options defined in this document.
11.1. End of Options List (EOL)
The End of Options List (EOL, Kind=0) option indicates that there
are no more options. It is used to indicate the end of the list of
options without needing to use NOP options (see the following
section) as padding to fill all available option space.
+--------+
| Kind=0 |
+--------+
Figure 7 UDP EOL option format
>> When the UDP options do not consume the entire surplus area or
the options area of a UDP fragment, the last non-NOP option MUST be
EOL.
>> NOPs SHOULD NOT be used as padding before the EOL option. As a
one-byte option, EOL need not be otherwise aligned.
>> All bytes after EOL in the surplus area or the options area of a
UDP fragment MUST be set to zero on transmit.
>> Bytes after EOL in the surplus area or the options area of a UDP
fragment MAY be checked as being zero on receipt, but MUST NOT be
otherwise processed (except for OCS calculation, which zeros would
not affect) and MUST NOT be passed to the user.
>> If a receiver elects to check the bytes following EOL and finds
that they are not all set to zero, it MUST silently discard the
options area. In this case the UDP user data MUST be delivered to
the application layer, unless the socket has been explicitly
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configured to do otherwise, as decided by the upper layer or
negotiated with the other endpoint.
Requiring the post-option surplus area to be zero prevents side-
channel uses of this area, requiring instead that all use of the
surplus area be UDP options supported by both endpoints. It is
useful to allow this area to be used for zero padding to increase
the UDP datagram length without affecting the UDP user data length,
e.g., for UDP DPLPMTUD (Section 4.1 of [Fa25]).
11.2. No Operation (NOP)
The No Operation (NOP, Kind=1) option is a one-byte placeholder,
intended to be used as padding, e.g., to align multi-byte options
along 16-bit, 32-bit, or 64-bit boundaries.
+--------+
| Kind=1 |
+--------+
Figure 8 UDP NOP option format
>> UDP packets SHOULD NOT use more than seven consecutive NOPs,
i.e., to support alignment up to 8-byte boundaries. UDP packets
SHOULD NOT use NOPs at the end of the options area as a substitute
for EOL followed by zero-fill. NOPs are intended to assist with
alignment, not as other padding or fill.
>> Receivers persistently experiencing packets with more than seven
consecutive NOPs SHOULD log such events, at least occasionally, as a
potential DoS indicator.
NOPs are not reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4).
This issue is discussed further in Section 25.
11.3. Additional Payload Checksum (APC)
The Additional Payload Checksum (APC, Kind=2) option provides a
stronger supplement to the checksum in the UDP header, using a 32-
bit CRC of the conventional UDP user data payload only (excluding
the IP pseudoheader, UDP header, and surplus area). It is not an
alternative to the UDP checksum because it does not cover the IP
pseudoheader or UDP header, and it is not a supplement to the OCS
because the latter covers the surplus area only. Its purpose is to
detect user data errors that the UDP checksum might not detect.
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A CRC32c has been chosen because of its ubiquity and use in other
Internet protocols, including iSCSI [RFC3385] and SCTP. The option
contains the CRC32c in network standard byte order, as used for
iSCSI.
+--------+--------+--------+--------+
| Kind=2 | Len=6 | CRC32c... |
+--------+--------+--------+--------+
| CRC32c (cont.) |
+--------+--------+
Figure 9 UDP APC option format
When present, the APC always contains a valid CRC checksum. There
are no reserved values, including the value zero. A CRC value of
zero is a potentially valid checksum. As such, it does not indicate
that the APC is not used; instead, the option would simply not be
included if that were the desired effect.
APC does not protect the UDP pseudoheader; only the current UDP
checksum provides that protection (when used). APC cannot provide
that protection because it would need to be updated whenever the UDP
pseudoheader changed, e.g., during NAT address and port translation
(see [RFC1141]).
>> UDP packets with incorrect APC checksums SHOULD be passed to the
application with an indication of APC failure. This is the default
behavior for APC.
>> Like all SAFE UDP options, APC MUST be silently ignored when
failing, unless the receiver has been explicitly configured to do
otherwise.
Although all UDP option-aware endpoints support APC (being in the
required set), this silently-ignored behavior ensures that option-
aware receivers operate the same as legacy receivers unless
overridden. Another reason is because the APC check could fail even
where the user data has not been corrupted, such as when its
contents have been intentionally overwritten e.g. by a middlebox to
update embedded port numbers or IP addresses. Such overwrites could
be intentional and not widely known; defaulting to silent ignore
ensures that option-aware endpoints do not change how users or
applications operate unless explicitly directed to do otherwise.
>> UDP packets with unrecognized APC lengths MUST receive the same
treatment as UDP packets with incorrect APC checksums.
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Ensuring that unrecognized APC lengths are treated as incorrect
checksums enables future variants of APC to be treated as APC-like.
APC is reported to the user and useful only per-datagram, because
fragments have no UDP user data.
11.4. Fragmentation (FRAG)
The Fragmentation (FRAG, Kind=3) option supports UDP fragmentation
and reassembly, which can be used to transfer UDP messages larger
than allowed by the IP receive MTU (EMTU_R [RFC1122]). FRAG includes
a copy of the same UDP transport ports in each fragment, enabling
them to traverse stateless Network Address (and port) Translation
(NAT) devices, in contrast to the behavior of IP fragments
[RFC4787]. FRAG is typically used with the UDP MDS and MRDS options
to enable more efficient use of large messages, both at the UDP and
IP layers. The design of FRAG is similar to that of the IPv6
Fragmentation Header [RFC8200], except that the UDP variant uses a
16-bit Offset measured in bytes, rather than IPv6's 13-bit Fragment
Offset measured in 8-byte units. This UDP variant avoids creating
reserved fields.
The FRAG header also enables use of options that modify the contents
of the UDP payload, such as encryption (UENC, see Sec. 12.2). Like
fragmentation, such options would not be safely used on UDP payloads
because they would be misinterpreted by legacy receivers. FRAG
allows use of these options, either on fragments or on a whole,
unfragmented message (i.e., an "atomic" fragment at the UDP layer,
similar to atomic IP datagrams [RFC6864]). This is safe because FRAG
hides the payload from legacy receivers by placing it within the
surplus area.
>> When FRAG is present, it SHOULD come as early as possible in the
UDP options list.
When present, placing FRAG first can simplify some implementations,
notably those using hardware acceleration that assume a fixed
location for the FRAG option. However, there are cases where FRAG
cannot occur first, such as when combined with per-fragment UENC or
UCMP. In those cases, encryption or compression (or both) would
precede FRAG when they also encrypt or compress the fragment option
itself. Additional cases could include recoding, such as could be
used to support forward error correction (FEC) over a group of
fragments. FRAG not being first might result in software (so-called
"slow path") option processing, or might also be accommodated via a
small set of known cases.
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>> When FRAG is present, the UDP user data MUST be empty. If the
user data is not empty, all UDP options MUST be silently ignored and
the user data received sent to the user.
Legacy receivers interpret FRAG messages as zero-length user data
UDP packets (i.e., UDP Length field is 8, the length of just the UDP
header), which would not affect the receiver unless the presence of
the UDP packet itself were a signal (see Section 5 of [RFC8085]).
In this manner, the FRAG option also helps hide UNSAFE options so
they can be used more safely in the presence of legacy receivers.
The FRAG option has two formats; non-terminal fragments use the
shorter variant (Figure 10) and terminal fragments use the longer
(Figure 11). The latter includes stand-alone fragments, i.e., when
data is contained in the FRAG option but reassembly is not required.
+--------+--------+--------+--------+
| Kind=3 | Len=10 | Frag. Start |
+--------+--------+--------+--------+
| Identification |
+--------+--------+--------+--------+
| Frag. Offset |
+--------+--------+
Figure 10 UDP non-terminal FRAG option format
Most fields are common to both FRAG option formats. The option Len
field indicates whether there are more fragments (Len=10) or no more
fragments (Len=12).
Frag. Start indicates the location of the beginning of the fragment
data, measured from the beginning of the UDP header of the fragment.
The fragment data follows the remainder of the UDP options and
continues to the end of the IP datagram (i.e., the end of the
surplus area). Those options (i.e., any that precede or follow the
FRAG option) are applied to this UDP fragment.
The Frag. Offset field indicates the location of this fragment
relative to the original UDP datagram (prior to fragmentation or
after reassembly), measured from the start of the original UDP
datagram's header.
The Identification field is a 32-bit value that, when used in
combination with the IP source address, UDP source port, IP
destination address, and UDP destination port, uniquely identifies
the original UDP datagram.
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+--------+--------+--------+--------+
| Kind=3 | Len=12 | Frag. Start |
+--------+--------+--------+--------+
| Identification |
+--------+--------+--------+--------+
| Frag. Offset |Reass DgOpt Start|
+--------+--------+--------+--------+
Figure 11 UDP terminal FRAG option format
The terminal FRAG option format adds a Reassembled Datagram Option
Start (RDOS) pointer, measured from the start of the original UDP
datagram header, indicating the end of the reassembled data and the
start of the surplus area within the original UDP datagram. UDP
options that apply to the reassembled datagram are contained in the
reassembled surplus area, as indicated by RDOS. UDP options that
occur within the fragment are processed on the fragment itself. This
allows either pre-reassembly or post-reassembly UDP option effects,
such as using UENC on each fragment while also using TIME on the
reassembled datagram for round-trip latency measurements.
An example showing the relationship between UDP fragments and the
original UDP datagram is provided in Figure 12. In this example, the
trailer containing per-datagram options resides entirely within the
terminal fragment, but this need not always be the case.
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Constituent UDP Fragments Original UDP Datagram
+-------------+------------+
| Src Port | Dst Port |
+-------------+------------+
| UDP Len (8) | UDP Chksum |
+-------------+------------+
| OCS | K=3 L=10 | +-------------+------------+
+-------------+------------+ | Src Port | Dst Port |
,--| Frag. Start | Identifi- ~ +-------------+------------+
| +-------------+------------+ | UDP L.(RDOS)| UDP Chksum |
| ~ cation | Frag. Off. |----->+-------------+------------+
| +-------------+------------+ | Frag Data from 1st Frag. |
| ~ Per Fragment Options ~ | . |
'->+-------------+------------+ ~ . ~
~ Fragment Data ~ | . |
+-------------+------------+ ,-->+-------------+------------+
| | Frag Data from 2nd Frag. |
+-------------+------------+ | | . |
| Src Port | Dst Port | | ~ . ~
+-------------+------------+ | | . |
| UDP Len (8) | UDP Chksum | | ,>+-------------+------------+
+-------------+------------+ | | | OCS | UDP Options|
| OCS | K=3 L=12 | | | +-------------+ +
+-------------+------------+ | | ~ . ~
,--| Frag. Start | Identifi- ~ | | +-------------+------------+
| +-------------+------------+ | |
| ~ cation | Frag. Off. |--' |
| +-------------+------------+ |
| | RDOS | Frag.Opts. | |
'->+--|----------+------------+ |
~ | Fragment Data ~ |
+--|----------+------------+ |
| |
'----------------------------'
Figure 12 UDP fragments and Original UDP datagram
The FRAG option does not need a "more fragments" bit (as used by IP
fragmentation) because it provides the same indication by using the
longer, 12-byte variant, as shown in Figure 11.
>> The FRAG option MAY be used on a single fragment, in which case
the Frag. Offset would be zero and the option would have the 12-byte
format.
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>> Endpoints supporting UDP options MUST be capable of fragmenting
and reassembling at least two fragments, each of a size that will
fit within the standard Ethernet MTU of 1,500 bytes. For further
details, please see Section 11.6.
Use of the single fragment variant can be helpful in supporting use
of UNSAFE options without undesirable impact to receivers that do
not support either UDP options or the specific UNSAFE options.
During fragmentation, the UDP header checksum of each fragment
remains constant. It does not depend on the fragment data (which
appears in the surplus area) because all fragments have a zero-
length user data field.
>> The Identification field is a 32-bit value that MUST be unique
over the expected fragment reassembly timeout.
>> The Identification field SHOULD be generated in a manner similar
to that of the IPv6 Fragment ID [RFC8200].
>> UDP fragments MUST NOT overlap.
>> Similar to IPv6 reassembly [RFC8200], if any of the fragments
being reassembled overlap with any other fragments being reassembled
for the same UDP packet, reassembly of that UDP packet MUST be
abandoned and all the fragments that have been received for that UDP
packet MUST be discarded, and no ICMP error messages are to be sent
in this case (to avoid a potential DoS attack turning into an ICMP
storm in the reverse direction).
>> Note that fragments might be duplicated in the network. Instead
of treating these exact duplicate fragments as overlapping
fragments, an implementation MAY choose to detect this case and drop
exact duplicate fragments while keeping the other fragments
belonging to the same UDP packet.
UDP fragmentation relies on a fragment expiration timer, which can
be preset or could use a value computed using the UDP Timestamp
option.
>> The default UDP reassembly expiration timeout SHOULD be no more
than 2 minutes.
>> UDP reassembly expiration MUST NOT generate an ICMP error. Such
events are not an IP error and can be addressed by the
user/application layer if desired.
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>> UDP reassembly space SHOULD be limited to reduce the impact of
DoS attacks on resource use.
>> UDP reassembly space limits SHOULD NOT be computed as a shared
resource across multiple sockets, to avoid cross-socket pair DoS
attacks.
>> Individual UDP fragments MUST NOT be forwarded to the user. The
reassembled datagram is received only after complete reassembly,
checksum validation, and continued processing of the remaining UDP
options.
Per-fragment UDP options, if used in addition to FRAG, occur before
the fragment data. They typically occur after the FRAG option,
except where they modify the FRAG option itself (e.g., UENC or
UCMP). Per-fragment options are processed before the fragment is
included in the reassembled datagram. Such options can be useful to
protect the reassembly process itself, e.g., to prevent the
reassembly cache from being polluted (using AUTH or UENC).
>> Fragments of a single datagram MAY use different sets of options.
It is expected to be computationally expensive to validate
uniformity across all fragments and there could be legitimate
reasons for including options in a fragment but not all fragments
(e.g., MDS, MRDS).
If an option is used per-fragment but defined as not usable per-
fragment, it is treated the same as any other unknown option.
Per-datagram UDP options, if used, reside in the surplus area of the
original UDP datagram. Processing of those options occurs after
reassembly is complete. This enables the safe use of UNSAFE options,
which are required to result in discarding the entire UDP datagram
if they are unknown to the receiver or otherwise fail (see Section
12).
In general, UDP packets are fragmented as follows:
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1. Create a UDP packet with data and UDP options. This is the
original UDP datagram, which we will call "D". The UDP options
follow the UDP user data and occur in the surplus area, just as
in an unfragmented UDP datagram with UDP options.
>> UDP options for the original packet MUST be fully prepared
before the rest of the fragmentation steps that follow here.
>> The UDP checksum of the original packet SHOULD be set to zero
because it is never transmitted. Equivalent protection is
provided if each fragment has a non-zero OCS value, as will be
the case if each fragment's UDP checksum is non-zero. Similarly,
the OCS value of the original packet SHOULD be zero if each
fragment will have a non-zero OCS value, as will be the case if
each fragment's UDP checksum is non-zero.
2. Identify the desired fragment size, which we will call "S". This
value is calculated to take into account the path MTU (if known)
and to allow space for per-fragment options.
3. Fragment "D" into chunks of size no larger than "S"-12 each (10
for the non-terminal FRAG option and 2 for OCS), with one final
chunk no larger than "S"-14 (12 for the terminal FRAG option and
2 for OCS). Note that all the per-datagram options in step #1
need not be limited to the terminal fragment, i.e., the RDOS
pointer can indicate the start of the original surplus area
anywhere in the reassembled datagram.
4. For each chunk of "D" in step #3, create a UDP packet with no
user data (UDP Length=8) followed by the word-aligned OCS, the
FRAG option, and any additional per-fragment UDP options,
followed by the FRAG data chunk.
5. Complete the processing associated with creating these additional
per-fragment UDP options for each fragment.
Receivers reverse the above sequence. They process all received
options in each fragment. When the FRAG option is encountered, the
FRAG data is used in reassembly. After all fragments are received,
the entire UDP packet is processed with any trailing UDP options
applying to the reassembled user data.
>> Reassembly failures at the receiver result in silent discard of
any per-fragment options and fragment contents and such failures
SHOULD NOT generate zero-length frames to the user.
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>> Finally, because fragmentation processing can be expensive, the
FRAG option SHOULD be avoided unless the original datagram requires
fragmentation or it is needed for "safe" use of UNSAFE options.
>> The FRAG option MAY also be used to provide limited support for
UDP options in systems that have access to only the initial portion
of the data in incoming or outgoing packets, as such systems could
potentially access per-fragment options. Such packets would, of
course, be silently ignored by legacy receivers that do not support
UDP options.
The presence of the FRAG option is not reported to the user.
11.5. Maximum Datagram Size (MDS)
The Maximum Datagram Size (MDS, Kind=4) option is a 16-bit hint of
the largest UDP packet or UDP fragment that an endpoint believes can
be received without use of IP fragmentation. It helps UDP
applications limit the largest UDP packet that can be sent without
UDP fragmentation and helps UDP fragmentation determine the largest
UDP fragment to send - in both cases, to avoid IP fragmentation.
As with the TCP Maximum Segment Size (MSS) option [RFC9293], the
size indicated is the IP layer MTU decreased by the fixed IP and UDP
headers only [RFC9293]. The space needed for IP and UDP options
needs to be adjusted by the sender when using the value indicated.
The value transmitted is based on EMTU_R, the largest IP datagram
that can be received (i.e., reassembled at the receiver) [RFC1122].
However, as with TCP, this value is only a hint at what the receiver
believes, as when used with PLPMTUD at the UDP layer as discussed
later in this section.
>> MDS does not indicate a known path MTU and thus MUST NOT be used
to limit transmissions.
+--------+--------+--------+--------+
| Kind=4 | Len=4 | MDS size |
+--------+--------+--------+--------+
Figure 13 UDP MDS option format
>> The UDP MDS option MAY be used as a hint for path MTU discovery
[RFC1191][RFC8201], but this could be difficult because of known
issues with ICMP blocking [RFC2923] as well as UDP lacking automatic
retransmission.
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MDS is more likely to be useful when coupled with IP source
fragmentation or UDP fragmentation to limit the largest reassembled
UDP message as indicated by MRDS (see Section 11.6), e.g., when
EMTU_R is larger than the required minimums (576 for IPv4 [RFC791]
and 1500 for IPv6 [RFC8200]).
>> MDS can be used with DPLPMTUD [RFC8899] to provide a hint to the
packetization layer path MTU (PLPMTU) value, though it MUST NOT
prohibit transmission of larger UDP packets used as DPLPMTUD probes.
MDS is reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4). When used per-fragment, the
reported value is the minimum of the MDS values received per-
fragment.
11.6. Maximum Reassembled Datagram Size (MRDS)
The Maximum Reassembled Datagram Size (MRDS, Kind=5) option is a 16-
bit indicator of the largest reassembled UDP datagram that can be
received, including the UDP header and any per-datagram UDP options,
accompanied by an 8-bit indication of how many UDP fragments can be
reassembled. MRDS size is the UDP equivalent of IP's EMTU_R but the
two are not related [RFC1122]. Using the FRAG option (Section 11.4),
UDP packets can be transmitted as transport fragments, each in their
own (presumably not fragmented) IP datagram and be reassembled at
the UDP layer. MRDS segs is the number of UDP fragments that can be
reassembled.
+--------+--------+--------+--------+---------+
| Kind=5 | Len=5 | MRDS size |MRDS segs|
+--------+--------+--------+--------+---------+
Figure 14 UDP MRDS option format
>> Endpoints supporting UDP options MUST support a local MRDS size
of at least 2,926 bytes for IPv4 and 2,886 bytes for IPv6. Support
for larger values is encouraged.
>> Endpoints supporting UDP options MUST support a local MRDS segs
value of at least 2. Support for larger values is encouraged.
These parameters plus the PMTU allow a sender to compute the size of
the largest pre-fragmentation UDP packet that a receiver will
guarantee to accept. Suppose that MMS_S is the PMTU less the size of
the IP header and the UDP header, i.e., the maximum UDP message size
that can be successfully sent in a single UDP datagram if there are
no IP options or extension headers and no UDP per-fragment options.
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Then size of the largest pre-fragmentation UDP packet that the
receiver will guarantee to accept is the smaller of the MRDS size
and
(MMS_S - 12) * (MRDS segs) - 2 - (Total Per-Frag IP/UDP Options) + 8
where Total Per-Frag IP/UDP Options includes the size of all IP
options and extension headers and all per-fragment UDP options
except for OCS and FRAG that are in the sequence of UDP fragments.
>> If no MRDS option has been received, a sender MUST assume that
MRDS size is 2,926 bytes for IPv4 and 2,886 bytes for IPv6 and that
MRDS segs is 2, i.e., the minimum values allowed.
MRDS is reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4). When used per-fragment, the
reported value is the minimum of the MRDS values received per-
fragment.
11.7. Echo request (REQ) and echo response (RES)
The echo request (REQ, Kind=6) and echo response (RES, Kind=7)
options provides UDP packet-level acknowledgments as a capability
for use by upper layer protocols, e.g., user applications,
libraries, operating systems, etc. Both the REQ and RES are under
the control of these upper layers, i.e., UDP option support
described in this document never automatically responds to a REQ
with a RES. Instead, the REQ is delivered to the upper layer, which
decides whether and when to issue a RES.
One such use is described as part of DPLPMTUD [Fa25]. This use case
is described as part of UDP options, but is logically considered to
be a capability of an upper layer that uses UDP options. The options
both have the format indicated in Figure 15, in which the token has
no internal structure or meaning.
+--------+--------+-----------------+
| Kind | Len=6 | token |
+--------+--------+-----------------+
1 byte 1 byte 4 bytes
Figure 15 UDP REQ and RES options format
>> As advice to upper layer protocol/library designers, when
supporting REQ/RES and responding with a RES, the upper layer SHOULD
respond with the most recently received REQ token.
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>> If the implementation includes a layer/library that produces and
consumes REQ/RES on behalf of the user/application, then that layer
MUST be disabled by default, in which case REQ/RES are simply sent
upon request by the user/application and passed to it when received,
as with most other UDP options.
For example, an application needs to explicitly enable the
generation of a RES response by DPLPMTUD when using UDP Options
[Fa25].
>> The token transmitted in a RES option MUST be a token received in
a REQ option by the transmitter. This ensures that the response is
to a received request.
REQ and RES option kinds appear at most once each in each UDP
packet, as with most other options. A single packet can include both
options, though they would be otherwise unrelated to each other.
Note also that the FRAG option is not used when sending DPLPMTUD
probes to determine a PLPMTU [Fa25].
REQ and RES are reported to the user, whether used per-datagram or
per-fragment (as defined in Section 11.4). When used per-fragment,
the reported value indicates the most recently received token.
11.8. Timestamps (TIME)
Timestamps are provided as a capability to be used by applications
and other upper layer protocols. They are based on a notion of time
as a monotonically non-decreasing unsigned integer, with wraparound.
They are defined the same way as TCP protection against wrapped
sequence numbers (PAWS), i.e., "without any connection to [real-
world, classical physics wall-clock] time" [RFC7323]. They are quite
similar to the behavior of relativistic time or the individual
scalars of Lamport clocks [La78]. However, if desired, they can
correspond to real-world time, e.g., as used for round-trip time
estimation. This option makes no assertions as to which is the case;
the decision is up to the application layer using this option.
The Timestamp (TIME, Kind=8) option exchanges two four-byte unsigned
timestamp fields. It serves a similar purpose to TCP's TS option
[RFC7323], enabling UDP to estimate the round-trip time (RTT)
between hosts. For UDP, this RTT can be useful for establishing UDP
fragment reassembly timeouts or transport-layer rate-limiting
[RFC8085].
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+--------+--------+------------------+------------------+
| Kind=8 | Len=10 | TSval | TSecr |
+--------+--------+------------------+------------------+
1 byte 1 byte 4 bytes 4 bytes
Figure 16 UDP TIME option format
TS Value (TSval) and TS Echo Reply (TSecr) are used in a similar
manner to the TCP TS option [RFC7323]. On transmitted UDP packets
using the option, TS Value is always set based on the local "time"
value. Received TSval and TSecr values are provided to the
application, which can pass the TSval value to be used as TSecr on
UDP messages sent in response (i.e., to echo the received TSval). A
received TSecr of zero indicates that the TSval was not echoed by
the transmitter, i.e., from a previously received UDP packet.
>> TIME MAY use an RTT estimate based on nonzero Timestamp values as
a hint for fragmentation reassembly, rate limiting, or other
mechanisms that benefit from such an estimate.
>> an application MAY use TIME to compute this RTT estimate for
further use by the user.
UDP timestamps are modeled after TCP timestamps and have similar
expectations. In particular, they are expected to follow these
guidelines:
o Values are monotonic and non-decreasing except for anticipated
number-space rollover events
o Values "increase" (allowing for rollover, i.e., modulo the field
size excepting zero) according to a typical 'tick' time
o A request is defined as TSval being non-zero and a reply is
defined as TSecr being non-zero.
o A receiver always responds to a request with the highest TSval
received (allowing for rollover), which is not necessarily the
most recently received.
Rollover can be handled as a special case or more completely using
sequence number extension [RFC9187], however zero values need to be
avoided explicitly.
>> TIME values MUST NOT use zeros as valid time values, because they
are used as indicators of requests and responses.
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TIME is reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4). When used per-fragment, the
reported value is the minimum and maximum of each of the timestamp
values received per-fragment.
>> Use of TIME per-fragment is NOT RECOMMENDED. Exceptions include
supporting diagnostics on the reassembly process itself, which could
be more appropriate to handle within the UDP option processing
implementation.
11.9. Authentication (AUTH), RESERVED Only
The Authentication (AUTH, Kind=9) option is reserved for all UDP
authentication mechanisms [To24]. AUTH is expected to cover the UDP
user data and UDP options, with possible additional coverage of the
IP pseudoheader and UDP header and potentially also support for NAT
traversal (i.e., by zeroing the remote socket - the source IP
address and UDP port - before computing the check), the latter in a
similar manner as per TCP-AO NAT traversal [RFC6978].
Like APC, AUTH is a SAFE option because it does not modify the UDP
user data. AUTH could fail even where the user data has not been
corrupted, such as when its contents have been overwritten. Such
overwrites could be intentional and not widely known; defaulting to
silent ignore ensures that option-aware endpoints do not change how
users or applications operate unless explicitly directed to do
otherwise. When a socket pair relies on AUTH, e.g., upon
configuration of a security policy, this default is expected to be
overridden, where incoming packets without AUTH or with a failed
AUTH check would be silently dropped, such that only authenticated
packets would be sent to the user. This approach enables security
checks for AUTH to occur above UDP, in a separate shim layer or
application library.
A specification for using AUTH is expected to define the
coordination of AUTH security parameters and configuration of the
socket pair when those parameters are installed. That specification
is expected to address rules for when AUTH is required upon
transmission and when the presence and correct validation of AUTH is
required on reception.
11.10. Experimental (EXP)
The Experimental option (EXP, Kind=127) is allocated for experiments
[RFC3692]. Only one such value is allocated because experiments are
expected to use an Experimental ID (ExIDs) to differentiate
concurrent use for different purposes, using UDP ExIDs registered
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with IANA according to the approach developed for TCP experimental
options [RFC6994].
+----------+----------+----------+----------+
| Kind=127 | Len | UDP ExID |
+----------+----------+----------+----------+
| (option contents, as defined)... |
+----------+----------+----------+----------+
Figure 17 UDP EXP option format
>> The length of the experimental option MUST be at least 4 to
account for the Kind, Length, and the 16-bit UDP ExID identifier
(similar to TCP ExIDs [RFC6994]).
The UDP EXP option uses only 16-bit ExIDs, unlike TCP ExIDs. In TCP,
the first 16 bits of the ExID is unique; the additional 16 bits,
where present, are used to decrease the chance of the entire ExID
occurring in legacy use of the TCP EXP option. This extended variant
provides no similar use for UDP EXP because ExIDs are required.
The UDP EXP option also includes an extended length format, where
the option LEN is 255 followed by two bytes of extended length.
+----------+----------+----------+----------+
| Kind=127 | 255 | Extended Length |
+----------+----------+----------+----------+
| UDP ExID |(option contents...) |
+----------+----------+----------+----------+
Figure 18 UDP EXP extended option format
Assigned UDP experimental IDs (ExIDs) assigned from a combined
TCP/UDP ExID registry managed by IANA (see Section 26). Assigned
ExIDs can be used in either the EXP or UEXP options (see Section
12.3 for the latter).
12. UNSAFE Options
UNSAFE options are not safe to ignore and can be used
unidirectionally or without soft-state confirmation of UDP option
capability. They are always used only when the user data occurs
inside a reassembled set of one or more UDP fragments, such that if
UDP fragmentation is not supported, the enclosed UDP user data would
be silently dropped anyway.
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>> Applications using UNSAFE options SHOULD NOT also use zero-length
UDP packets as signals, because they will arrive when UNSAFE options
fail. Those that choose to allow such packets MUST account for such
events.
>> UNSAFE options MUST be used only as part of UDP fragments, used
either per-fragment or after reassembly.
>> Receivers supporting UDP options MUST silently drop the UDP user
data of the reassembled datagram if any fragment or the entire
datagram includes an UNSAFE option whose UKind is not supported or
if an UNSAFE option appears outside the context of a fragment or
reassembled fragments.
12.1. UNSAFE Compression (UCMP)
The UNSAFE Compression (UCMP, Kind=192) option is reserved for all
UDP compression mechanisms. UCMP is expected to cover the UDP user
data and some (e.g., later, in sequence) UDP options.
12.2. UNSAFE Encryption (UENC)
The UNSAFE Encryption (UENC, Kind=193) option is reserved for all
UDP encryption mechanisms. UENC is expected to provide all of the
services of the AUTH option (Section 11.9) and in addition to
encrypt the UDP user data and some (e.g., later, in sequence) UDP
options, in a similar manner as TCP-AO-ENC [To18].
12.3. UNSAFE Experimental (UEXP)
The UNSAFE Experimental option (UEXP, Kind=254) is reserved for
experiments [RFC3692]. As with EXP, only one such UEXP value is
reserved because experiments are expected to use an Experimental ID
(ExIDs) to differentiate concurrent use for different purposes,
using UDP ExIDs registered with IANA according to the approach
developed for TCP experimental options [RFC6994].
Assigned ExIDs can be used with either the UEXP or EXP options.
13. Rules for designing new options
The UDP option Kind space allows for the definition of new options,
however the currently defined options (including AUTH, UENC, and
UCMP) do not allow for arbitrary new options. The following is a
summary of rules for new options and their rationales:
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>> New options MUST NOT be defined as "must-implement", i.e., they
are not eligible for the asterisk ("*") designation used in Section
10.
This document defines the minimum set of "must-implement" UDP
options. All new options are included at the discretion of a given
implementation.
>> New options MUST NOT modify the content of options that precede
them (in order of appearance and thus processing).
>> The fields of new options MUST NOT depend on the content of other
options.
UNSAFE options can both depend on and vary user data content because
they are contained only inside UDP fragments and thus are processed
only by UDP option capable receivers.
>> New options MUST NOT declare their order relative to other
options, whether new or old, even as a preference.
>> At the sender, new options MUST NOT modify UDP packet content
anywhere except within their option field, excepting only those
contained within the UNSAFE option; areas that need to remain
unmodified include the IP header, IP options, the UDP user data, and
the surplus area (i.e., other options).
>> Options MUST NOT be modified in transit. This includes those
already defined as well as new options.
>> New options MUST NOT require or allow that any UDP options
(including themselves) or the remaining surplus area be modified in
transit.
>> All options MUST indicate whether they can be used per-fragment,
and, if so, MUST also indicate how their success or failure is
reported to the user. This document RECOMMENDS that options be
useful per-fragment and also RECOMMENDS that options used per-
fragment be reported to the user as a finite aggregate (e.g., a sum,
a flag, etc.) rather than individually.
Note that only certain of the initially defined options violate
these rules:
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o >> The FRAG option modifies UDP user data, splitting it across
multiple IP packets. UNSAFE options MAY modify the UDP user data,
e.g., by encryption, compression, or other transformations. All
other (SAFE) options MUST NOT modify the UDP user data.
14. Option inclusion and processing
The following rules apply to option inclusion by senders and
processing by receivers.
>> Senders MAY add any option, as configured by the API.
>> All "must-support" options MUST be processed by receivers, if
present (presuming UDP options are supported at that receiver).
>> Non-"must-support" options MAY be ignored by receivers, if
present, e.g., based on API settings.
>> All options MUST be processed by receivers in the order
encountered in the options area.
>> Unless configuration settings direct otherwise, all options
except UNSAFE options MUST result in the UDP user data being passed
to the upper layer protocol or application, regardless of whether
all options are processed, supported, or succeed.
The basic premise is that, for options-aware endpoints, the sender
decides what options to add and the receiver decides what options to
handle. Simply adding an option does not force work upon a receiver,
with the exception of the "must-support" options.
Upon receipt, the receiver checks various properties of the UDP
packet and its options to decide whether to accept or drop the UDP
packet and whether to accept or ignore some of its options as
follows (in order):
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if the UDP checksum fails then
silently drop the entire UDP packet (per RFC1122)
if the UDP checksum passes or is zero then
if (OCS != 0 and OCS fails) or
(OCS == 0 and UDP CS != 0) then
deliver the UDP user data but ignore other options
(this is required to emulate legacy behavior)
if (OCS != 0 and OCS passes) or
(OCS == 0 and UDP CS == 0) then
deliver the UDP user data after parsing
and processing the rest of the options,
regardless of whether each is supported or succeeds
(again, this is required to emulate legacy behavior)
The design of the UNSAFE options ensures that the resulting UDP data
will be silently dropped in both legacy and options-aware receivers
that do not recognize those options. Again, note that this still
results in the delivery of a zero-length UDP packet.
Options-aware receivers can drop UDP packets with option processing
errors via either an override of the default UDP processing or at
the application layer.
I.e., all options are treated the same, in that the transmitter can
add it as desired and the receiver has the option to require it or
not. Only if it is required (e.g., by API configuration) would the
receiver require it being present and correct.
I.e., for all options:
o if the option is not required by the receiver, then UDP packets
missing the option are accepted.
o if the option is required (e.g., by override of the default
behavior at the receiver) and missing or incorrectly formed,
silently drop the UDP packet.
o if the UDP packet is accepted (either because the option is not
required or because it was required and correct), then pass the
option with the UDP packet via the API. Note that FRAG, NOP, and
EOL are not passed to the user (see Section 15).
>> Any options whose length exceeds that of the UDP packet (i.e.,
intending to use data that would have been beyond the surplus area)
SHOULD be silently ignored (again to model legacy behavior).
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15. UDP API Extensions
UDP currently specifies an application programmer interface (API),
summarized as follows (with Unix-style command as an example)
[RFC768]:
o Method to create new receive ports
o E.g., bind(handle, recvaddr(optional), recvport)
o Receive, which returns data octets, source port, and source
address
o E.g., recvfrom(handle, srcaddr, srcport, data)
o Send, which specifies data, source and destination addresses, and
source and destination ports
o E.g., sendto(handle, destaddr, destport, data)
This API is extended to support options as follows:
o Extend the method to create receive ports to include per-packet
and per-fragment receive options that are required or omitted as
indicated by the application.
>> Datagrams not containing these required options MUST be
silently dropped and SHOULD be logged.
o Extend the method to create receive ports to have a means to
indicate that all packets containing UDP options that are
received on a particular socket pair are to be discarded.
>> The default value for the setting to drop all packets
containing UDP options MUST be to process packets containing UDP
options normally (i.e., not to discard them).
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o Extend the receive function to indicate the per-packet options
and their parameters as received with the corresponding received
datagram. Note that per-fragment options are handled within the
processing of each fragment.
>> Options and their processing status (success/fail) MUST be
available to the user (i.e., application layer or upper layer
protocol/service), both for the packet and for the fragment set,
except for FRAG, NOP, and EOL; those three options are handled
within UDP option processing only. As a reminder (from Section
14), all options except UNSAFE options MUST result in the UDP
user data being passed to the application layer (unless
overridden in the API), regardless of whether all options are
processed, supported, or succeed.
o For fragments, success for an option is reported only when all
fragments succeed for that option.
>> Per-fragment option status reporting SHOULD default as needed
(e.g., not computed and/or not passed up to the upper layers) to
minimize overhead unless actively requested (e.g., by the
user/application layer).
>> SAFE options associated with fragments are accumulated when
associated with the reassembled packet; values MAY be coalesced,
e.g., to indicate only that an AUTH failure of a fragment
occurred or not rather than indicating the AUTH status of each
fragment.
o Extend the send function to indicate the options to be added to
the corresponding sent datagram. This includes indicating which
options apply to individual fragments vs. which apply to the UDP
packet prior to fragmentation, if fragmentation is enabled. This
includes a minimum datagram length, such that the options list
ends in EOL and additional space is zero-filled as needed. It
also includes a maximum fragment size, e.g., as discovered by
DPLPMTUD, whether implemented at the application layer per
[RFC8899] or in conjunction with other UDP options [Fa25].
Examples of API instances for Linux and FreeBSD are provided in
Appendix A, to encourage uniform cross-platform implementations.
APIs are not intended to provide user control over option order,
especially on a per-packet basis, as this could create a covert
channel (see Section 25). Similarly, APIs are not intended to
provide user/application control over UDP fragment boundaries on a
per-packet basis, although they are expected to allow control over
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which options, including fragmentation, are enabled (or disabled) on
a per-packet basis. Such control over fragmentation is critical to
DPLPMTUD.
16. UDP Options are for Transport, Not Transit
UDP options are indicated in the surplus area of the IP payload that
is not used by UDP. That area is really part of the IP payload, not
the UDP payload, and as such, it might be tempting to consider
whether this is a generally useful approach to extending IP.
Unfortunately, the surplus area exists only for transports that
include their own transport layer payload length indicator. TCP and
SCTP include header length fields that already provide space for
transport options by indicating the total length of the header area,
such that the entire remaining area indicated in the network layer
(IP) is transport payload. UDP-Lite already uses the UDP Length
field to indicate the boundary between data covered by the transport
checksum and data not covered, and so there is no remaining area
where the length of the UDP-Lite payload as a whole can be indicated
[RFC3828].
UDP options are transport options. They are no more (or less)
appropriate to be modified in-transit than any other portion of the
transport datagram.
>> Generally, transport headers, options, and data are not intended
to be modified in-transit. UDP options are no exception and here are
specified as MUST NOT be altered in transit.
However, note that the UDP option mechanism provides no specific
protection against in-transit modification of the UDP header, UDP
payload, or surplus area, except as provided by the OCS or the
options selected (e.g., AUTH or UENC).
Unless protected by encryption (e.g., UENC or via other layers,
e.g., IPsec), UDP options remain visible to devices on the network
path. The decision to not require mandatory encryption for UDP
options to prevent such visibility was made because the key
distribution and management infrastructure necessary to support such
encryption does not exist in many of the deployment scenarios of
interest, notably those that use UDP directly as a stateless and
connectionless transport protocol (see, e.g., [He24]).
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17. UDP options vs. UDP-Lite
UDP-Lite provides partial checksum coverage, so that UDP packets
with errors in some locations can be delivered to the user
[RFC3828]. It uses a different transport protocol number (136) than
UDP (17) to interpret the UDP Length field as the prefix covered by
the UDP checksum.
UDP (protocol 17) already defines the UDP Length field as the limit
of the UDP checksum, but by default also limits the data provided to
the application as that which precedes the UDP Length. A goal of
UDP-Lite is to deliver data beyond UDP Length as a default, which is
why a separate transport protocol number was required.
UDP options do not use or need a separate transport protocol number
because the data beyond the UDP Length offset (surplus data) is not
provided to the application by default. That data is interpreted
exclusively within the UDP transport layer.
UDP-Lite cannot support UDP options, either as proposed here or in
any other form, because the entire payload of the UDP packet is
already defined as user data and there is no additional field in
which to indicate a surplus area for options. The UDP Length field
in UDP-Lite is already used to indicate the boundary between user
data covered by the checksum and user data not covered.
18. Interactions with Legacy Devices
It has always been permissible for the UDP Length to be inconsistent
with the IP transport payload length [RFC768]. Such inconsistency
has been utilized in UDP-Lite using a different transport number.
There are no known systems that use this inconsistency for UDP
[RFC3828]. It is possible that such use might interact with UDP
options, i.e., where legacy systems might generate UDP datagrams
that appear to have UDP options. The OCS provides protection against
such events and is stronger than a static "magic number".
UDP options have been tested as interoperable with Linux, macOS, and
Windows Cygwin, and worked through NAT devices. These systems
successfully delivered only the user data indicated by the UDP
Length field and silently discarded the surplus area.
One reported embedded device passes the entire IP datagram to the
UDP application layer. Although this feature could enable
application-layer UDP option processing, it would require that
conventional UDP user applications examine only the UDP user data.
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This feature is also inconsistent with the UDP application interface
[RFC768] [RFC1122].
It has been reported that Alcatel-Lucent's "Brick" Intrusion
Detection System has a default configuration that interprets
inconsistencies between UDP Length and IP Length as an attack to be
reported. Note that other firewall systems, e.g., CheckPoint, use a
default "relaxed UDP length verification" to avoid falsely
interpreting this inconsistency as an attack.
There are known uses of UDP exchanges of zero-length UDP user data
packets, notably in the TIME protocol [RFC868]. The need to support
such packets is also noted in the UDP usage guidelines [RFC8085].
Some of the mechanisms in this document can generate more zero-
length UDP packets for a UDP option aware endpoint than for a legacy
(non-aware) endpoint (e.g., based some error conditions) and some
can generate fewer (e.g., fragment reassembly). Because such packets
inherently carry no unique transport header or transport content,
endpoints are already expected to be tolerant of their (inadvertent)
replication or loss by the network, so such variations are not
expected to be problematic.
19. Options in a Stateless, Unreliable Transport Protocol
There are two ways to interpret options for a stateless, unreliable
protocol -- an option is either local to the message or intended to
affect a stream of messages in a soft-state manner. Either
interpretation is valid for defined UDP options.
It is impossible to know in advance whether an endpoint supports a
UDP option.
>> All UDP options other than UNSAFE ones MUST be ignored if not
supported or upon failure (e.g., APC).
>> All UDP options that fail MUST result in the UDP data still being
sent to the application layer by default, to ensure equivalence with
legacy devices.
UDP options that rely on soft-state exchange need allow for message
reordering and loss, in the same way as UDP applications [RFC8085].
The above requirements prevent using any option that cannot be
safely ignored unless it is hidden inside the FRAG area (i.e.,
UNSAFE options). Legacy systems also always need to be able to
interpret the transport fragments as individual UDP packets.
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20. UDP Option State Caching
Some TCP connection parameters, stored in the TCP Control Block, can
be usefully shared either among concurrent connections or between
connections in sequence, known as TCP Sharing [RFC9040]. Although
UDP is stateless, some of the options proposed herein could have
similar benefit in being shared or cached. We call this UCB Sharing,
or UDP Control Block Sharing, by analogy. Just as TCB sharing is not
a standard because it is consistent with existing TCP
specifications, UCB sharing would be consistent with existing UDP
specifications, including this one. Both are implementation issues
that are outside the scope of their respective specifications, and
so UCB sharing is outside the scope of this document.
21. Updates to RFC 768
This document updates RFC 768 as follows:
o This document defines the meaning of the IP payload area beyond
the UDP length but within the IP length as the surplus area used
herein for UDP options.
o This document extends the UDP API to support the use of UDP
options.
22. Interactions with other RFCs (and drafts)
This document clarifies the interaction between UDP Length and IP
length that is not explicitly constrained in either UDP or the host
requirements [RFC768] [RFC1122].
Teredo extensions (TE) define use of a similar difference between
these lengths for trailers [RFC4380][RFC6081]. TE defines the length
of an IPv6 payload inside UDP as pointing to less than the end of
the UDP payload, enabling trailing options for that IPv6 packet:
"..the IPv6 packet length (i.e., the Payload Length value in
the IPv6 header plus the IPv6 header size) is less than or
equal to the UDP payload length (i.e., the Length value in
the UDP header minus the UDP header size)"
UDP options are not affected by the difference between the UDP user
payload end and the payload IPv6 end; both would end at the UDP user
payload, which could end before the enclosing IPv4 or IPv6 header
indicates - allowing UDP options in addition to the trailer options
of the IPv6 payload. The result, if UDP options were used, is shown
in Figure 19.
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Outer IP Length
<---------------------------------------------------------->
+--------+---------+------------------------------+----------+
| IP Hdr | UDP Hdr | IPv6 packet/len | TE trailer | surplus |
+--------+---------+------------------------------+----------+
<--------------->
Inner IPv6 Length
<-------------------------------------->
UDP Length
Figure 19 TE trailers and UDP options used concurrently
UDP options cannot be supported when a UDP packet has no independent
UDP Length. One such case is when UDP Length==0 in IPv6, intended
for (but not limited to) IPv6 Jumbograms [RFC2675]. Note that
although this technique is "Standard", the specification did not
"update" UDP [RFC768]. Another such case arises when UDP is proxied
via HTTP [RFC 9298], as the UDP header is omitted and only the UDP
user data is transported.
This document is consistent the UDP profile for Robust Header
Compression (ROHC)[RFC3095], noted here:
"The Length field of the UDP header MUST match the Length
field(s) of the preceding subheaders, i.e., there must not
be any padding after the UDP payload that is covered by the
IP Length."
ROHC compresses UDP headers only when this match succeeds. It does
not prohibit UDP headers where the match fails; in those cases, ROHC
default rules (Section 5.10) would cause the UDP header to remain
uncompressed. Upon receipt of a compressed UDP header, Section A.1.3
of that document indicates that the UDP length is "INFERRED"; in
uncompressed packets, it would simply be explicitly provided.
This issue of handling UDP header compression is more explicitly
described in more recent specifications, e.g., Sec. 10.10 of Static
Context Header Compression [RFC8724].
23. Multicast and Broadcast Considerations
UDP options are primarily intended for unicast use. Using these
options over multicast or broadcast IP requires careful
consideration, e.g., to ensure that the options used are safe for
different endpoints to interpret differently (e.g., either to
support or silently ignore) or to ensure that all receivers of a
multicast or broadcast group confirm support for the options in use.
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24. Network Management Considerations
UDP options use and configuration may be useful to track and manage
remotely. IP Flow Information Export (IPFIX [RFC7011]) Information
Elements for UDP options have been defined in [Bo24]. Similar to
what has been done for TCP [RFC9648], a YANG model [RFC7950] for use
by network management protocols (e.g., NETCONF [RFC6241] or RESTCONF
[RFC8040]), may be developed. Development of these models is outside
the scope of this document.
25. Security Considerations
There are a number of security issues raised by the introduction of
options to UDP. Some are specific to this variant, but others are
associated with any packet processing mechanism; all are discussed
further in this section.
25.1. General Considerations Regarding the Use of Options
Note that any user application that considers UDP options to
adversely affect security need not enable them. However, their use
does not impact security in a way substantially different than TCP
options; both enable the use of a control channel that has the
potential for abuse. Similar to TCP, there are many options that, if
unprotected, could be used by an attacker to interfere with
communication.
UDP options are not covered by DTLS [RFC9147]. Neither TLS [RFC8446]
(transport layer security, for TCP) nor DTLS (TLS for UDP) protect
the transport layer; both operate as a shim layer solely on the user
data of transport packets, protecting only their contents.
Just as TLS does not protect the TCP header or its options, DTLS
does not protect the UDP header or the new options introduced by
this document. Transport security is provided in TCP by the TCP
Authentication Option (TCP-AO [RFC5925]) and (when defined) in UDP
by the Authentication (AUTH) option (Section 11.9) and (when
defined) the UNSAFE Encryption (UENC) option (Section 12). Transport
headers are also protected as payload when using IP security (IPsec)
[RFC4301].
Some UDP options are never passed to the receiving application,
notably FRAG, NOP, and EOL. They are not intended to convey
information, either by their presence (FRAG, EOL) or number (NOP).
It could also be useful to provide the options received in a
reference order (e.g., sorted by option number), to avoid the order
of options being used as a covert channel.
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All logging is rate-limited to avoid logging itself becoming a
resource vulnerability.
25.2. Considerations Regarding On-Path Attacks
UDP options, like any options, have the potential to expose option
information to on-path attackers, unless the options themselves are
encrypted (as might be the case with some configurations of UENC,
when defined). Application protocol designers are expected to ensure
that information in UDP options is not used with the assumption of
privacy unless UENC provides that capability. Application protocol
designers using secure payload contents (e.g., via DTLS) are
expected to be aware that UDP options add information that is not
inside the UDP payload and thus not protected by the same mechanism,
and that alternate mechanisms (again, as might be the case with some
configurations of UENC) could be additionally required to protect
against information disclosure.
>> Implementations concerned with the potential use of UDP options
as a covert channel MAY consider limiting use of some or all
options. Such implementations SHOULD return options in an order not
related to their sequence in the received packet.
UDP options create new potential opportunities for distributed DoS
(DDos) attacks, notably through the use of fragmentation. When
enabled, UDP options cause additional work at the receiver, however,
of the "must-support" options, only REQ (e.g., when used with
DPLPMTUD [Fa25]) will cause the upper layer to initiate a UDP
response in the absence of user transmission.
>> Implementations concerned with the potential for DoS attacks
involving large numbers of UDP options, either implemented or
unknown, or excessive sequences of valid repeating options (e.g.,
NOPs) SHOULD detect excessive numbers of such occurrences and limit
resources they use, e.g., through silent packet drops. Such
responses SHOULD be logged. Specific thresholds for such limits will
vary based on implementation and are thus not included here.
25.3. Considerations Regarding Option Processing
UDP options use the TLV syntax similar to that of TCP. This syntax
is known to require serial processing and could pose a DoS risk,
e.g., if an attacker adds large numbers of unknown options that need
to be parsed in their entirety, as is the case for IPv6 [RFC8504].
The use of UDP packets with inconsistent IP and UDP Length fields
has the potential to trigger a buffer overflow error if not properly
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handled, e.g., if space is allocated based on the smaller field and
copying is based on the larger. However, there have been no reports
of such vulnerability, and it would rely on inconsistent use of the
two fields for memory allocation and copying.
Because required options come first and at most once each (with the
exception of NOPs, which never need to come in sequences of more
than seven in a row), their DOS impact is limited. Note that TLV
formats for options do require serial processing, but any format
that allows future options, whether ignored or not, could introduce
a similar DoS vulnerability.
>> Implementations concerned with the potential for UDP options
introducing a vulnerability MAY implement only the required UDP
options and SHOULD also limit processing of TLVs, either in number
of non-padding options or total length, or both. The number of non-
zero TLVs allowed in such cases MUST be at least as many as the
number of concurrent options supported with an additional few to
account for unexpected unknown options, but SHOULD also consider
being adaptive and based on the implementation, to avoid locking in
that limit globally.
E.g., if a system supports 10 different option types that could
concurrently be used, it is expected to allow up to around 13-14
different options in the same packet. This document avoids
specifying a fixed minimum, but recognizes that a given system might
not expect to receive more than a few unknown option types per
packet.
25.4. Considerations for Fragmentation
UDP fragmentation introduces its own set of security concerns, which
can be handled in a manner similar to IP reassembly or TCP segment
reordering [CERT18]. In particular, the number of UDP packets
pending reassembly and effort used for reassembly is typically
limited. In addition, it could be useful to assume a reasonable
minimum fragment size, e.g., that non-terminal fragments are never
be smaller than 500 bytes.
>> Implementations concerned with the potential for UDP
fragmentation introducing a vulnerability SHOULD implement limits on
the number of pending fragments.
25.5. Considerations for Providing UDP Security
UDP security is not intended to rely solely on transport layer
processing of options. UNSAFE options are the only type that share
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fate with the UDP data, because of the way that data is hidden in
the surplus area until after those options are processed. All other
options default to being silently ignored at the transport layer but
could be dropped either if that default is overridden (e.g., by
configuration) or discarded at the application layer (e.g., using
information about the options processed that are passed along with
the UDP packet).
Options providing UDP security, e.g., AUTH and UENC, require
endpoint key and security parameter coordination, which UDP options
(being stateless) do not facilitate. These parameters include
whether and when to override the defaults described herein,
especially at the transmitter as to when emitted packets need to
include AUTH and at the receiver as to whether (and when) packets
with failed AUTH and/or without AUTH (or that fail the AUTH checks)
are not be forwarded to the user/application.
25.6. Considerations Regarding Middleboxes
Some middleboxes operate as UDP relays, forwarding data between a
UDP socket and another transport socket by modifying the IP and/or
UDP headers without properly acting as a protocol endpoint (i.e., an
application layer proxy). In such cases, a sender might add UDP
options that could be stripped by the middlebox before the packet is
forwarded to the second socket. A remote application will not
receive the options (for SAFE options the payload data will be
received, for UNSAFE options the payload data will not be received).
In such cases the application will function as it would if
communicating with a remote endpoint that does not support UDP
options.
Additionally, [Zu20] reported that packets containing UDP options do
not traverse certain Internet paths, most likely those options were
stripped (e.g., by resetting the IP length to correspond to the UDP
length, truncating the surplus area) or packets with options
dropped. UDP options do not function over such paths.
26. IANA Considerations
Upon publication, IANA is hereby requested to create a new registry
group for UDP Options, consisting of UDP Option Kind numbers and to
rename the TCP experimental IDs (ExIDs) registry as a unified
registry for TCP/UDP ExIDs, with a link from the UDP protocol
parameters area to this unified TCP/UDP ExID registry. IANA is also
hereby requested to update the unified TCP/UDP ExID registry with
the direction that "16-bit ExIDs can be used with either TCP or UDP;
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32-bit ExIDs can be used with TCP or their first 16 bits can be used
with UDP", and with further detail provided below.
Initial values of the UDP Option Kind registry are as listed in
Section 10, including those both assigned and reserved. Additional
values in this registry are to be assigned from the UNASSIGNED
values in Section 10 by IESG Approval or Standards Action [RFC8126].
Those assignments are subject to the conditions set forth in this
document, particularly (but not limited to) those in Section 13.
>> Although option nicknames are not used in-band, within new UNSAFE
option names MUST commence with the capital letter "U" and new SAFE
options MUST NOT commence with either uppercase or lowercase "U".
IANA is also requested to add a note to that list regarding which
entries are mandatory to implement when UDP options are supported,
as follows. No new options may be created that are mandatory to
implement in all UDP options implementations.
"Code points 0-7 MUST be supported an any implementation
supporting UDP options. All others are supported at the
discretion of each implementation."
UDP Experimental Option Experiment Identifiers (UDP ExIDs) are
intended for use in a similar manner as TCP ExIDs [RFC6994]. Both
TCP and UDP ExIDs are managed as a single, unified registry because
such options could be used for both transport protocols and because
the option space is large enough that there is no clear need to
maintain them separately. This new TCP/UDP ExID registry has entries
for both transports, although each codepoint needs to be explicitly
defined for each transport protocol in which it is used, i.e.,
defining a codepoint in TCP does not imply it has a similar use in
UDP. IANA is requested to add a field called "Protocol" and update
the current TCP ExIDs to be indicated as defined for TCP, and to
proceed with new assignments that indicate the transport for which
it is defined.
TCP/UDP ExIDs can be used in either (or both) the UDP EXP (Section
11.10) or UEXP (Section 12.3) options. TCP/UDP ExID entries for use
in UDP consist of a 16-bit ExID (in network-standard order), and (as
with the original TCP ExIDs) will preferentially also include a
short description and acronym for use in documentation. TCP/UDP
ExIDs used for UDP are always 16 bits because their use in EXP and
UEXP options is required and thus do not need a larger codepoint
value to decrease the probability of accidental occurrence with non-
ExID uses of the experimental options, as is the case with TCP ExIDs
(e.g., when using 32-bit ExIDs). ExIDs defined solely for TCP
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options could be either 16 or 32 bits and all ExIDs (including now
UDP) need to be unique in their first 16 bits, as originally
described for TCP [RFC6994].
Values in the TCP/UDP ExID registry are to be assigned by IANA using
first-come, first-served (FCFS) rules applied to both the ExID value
and the acronym [RFC8126]. UDP options using these ExIDs are subject
to the same conditions as new UDP options, i.e., they too are
subject to the conditions set forth in this document, particularly
(but not limited to) those in Section 13.
27. References
27.1. Normative References
[Fa25] Fairhurst, G., T. Jones, "Datagram PLPMTUD for UDP
Options," draft-ietf-tsvwg-udp-options-dplpmtud, Feb.
2025.
[RFC768] Postel, J., "User Datagram Protocol," RFC 768, August
1980.
[RFC791] Postel, J., "Internet Protocol," RFC 791, Sept. 1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts --
Communication Layers," RFC 1122, Oct. 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC 2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words," BCP 14, RFC 8174, May 2017.
27.2. Informative References
[Bo24] Boucadair, M., and T. Reddy.K, "Export of UDP Options
Information in IP Flow Information Export (IPFIX)", draft-
ietf-opsawg-tsvwg-udp-ipfix, Jul. 2024.
[CERT18] CERT Coordination Center, "TCP implementations vulnerable
to Denial of Service,", Vulnerability Note VU 962459,
Software Engineering Institute, CMU, 2018,
https://www.kb.cert.org/vuls/id/962459.
[Fa18] Fairhurst, G., T. Jones, R. Zullo, "Checksum Compensation
Options for UDP Options", draft-fairhurst-udp-options-cco,
Oct. 2018.
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[He24] Heard, C., "Use of UDP Options for Transmission of Large
DNS Responses," draft-heard-dnsop-udp-opt-large-dns-
responses, Apr. 2024.
[Hi15] Hildebrand, J., B. Trammel, "Substrate Protocol for User
Datagrams (SPUD) Prototype," draft-hildebrand-spud-
prototype, Mar. 2015.
[La78] Leslie Lamport. 1978. Time, clocks, and the ordering of
events in a distributed system. Commun. ACM 21, 7 (July
1978), 558-565. https://doi.org/10.1145/359545.359563
[RFC793] Postel, J. (Ed.), "Transmission Control Protocol," RFC
793, September 1981.
[RFC868] Postel, J., K. Harrenstien, "Time Protocol," RFC 868, May
1983.
[RFC1071] Braden, R., D. Borman, C. Partridge, "Computing the
Internet Checksum," RFC 1071, Sept. 1988.
[RFC1141] Mallory, T., A. Kullberg, "Incremental Updating of the
Internet Checksum," RFC 1141, January 1990.
[RFC1191] Mogul, J., S. Deering, "Path MTU discovery," RFC 1191,
November 1990.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery," RFC
2923, September 2000.
[RFC3095] Bormann, C. (Ed), et al., "RObust Header Compression
(ROHC): Framework and four profiles: RTP, UDP, ESP, and
uncompressed," RFC 3095, July 2001.
[RFC3173] Shacham, A., Monsour, B., Pereira, R., and M. Thomas, "IP
Payload Compression Protocol (IPComp)", RFC 3173,
September 2001.
[RFC3385] Sheinwald, D., J. Satran, P. Thaler, V. Cavanna, "Internet
Protocol Small Computer System Interface (iSCSI) Cyclic
Redundancy Check (CRC)/Checksum Considerations," RFC 3385,
Sep. 2002.
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[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful," RFC 3692, Jan. 2004.
[RFC3828] Larzon, L-A., M. Degermark, S. Pink, L-E. Jonsson (Ed.),
G. Fairhurst (Ed.), "The Lightweight User Datagram
Protocol (UDP-Lite)," RFC 3828, July 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, Dec. 2005.
[RFC4340] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)," RFC 4380, Feb. 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP," RFC 4787,
Jan. 2007.
[RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
Option," RFC 5925, June 2010.
[RFC6081] Thaler, D., "Teredo Extensions," RFC 6081, Jan 2011.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, June 2011.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field,"
RFC 6864, Feb. 2013.
[RFC6935] Eubanks, M., P. Chimento, M. Westerlund, "IPv6 and UDP
Checksums for Tunneled Packets," RFC 6935, April 2013.
[RFC6978] Touch, J., "A TCP Authentication Option Extension for NAT
Traversal", RFC 6978, July 2013.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options," RFC
6994, Aug. 2013.
[RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
"Specification of the IP Flow Information Export (IPFIX)
Protocol for the Exchange of Flow Information", STD 77,
RFC 7011, September 2013.
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[RFC7323] Borman, D., R. Braden, V. Jacobson, R. Scheffenegger
(Ed.), "TCP Extensions for High Performance," RFC 7323,
Sep. 2014.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, August 2016.
[RFC8085] Eggert, L., G. Fairhurst, G. Shepherd, "UDP Usage
Guidelines," RFC 8085, Feb. 2017.
[RFC8126] Cotton, M., B. Leiba, T. Narten, "Guidelines for Writing
an IANA Considerations Section in RFCs," RFC 8126, June
2017.
[RFC8200] Deering, S., R. Hinden, "Internet Protocol Version 6
(IPv6) Specification," RFC 8200, Jul. 2017.
[RFC8201] McCann, J., S. Deering, J. Mogul, R. Hinden (Ed.), "Path
MTU Discovery for IP version 6," RFC 8201, Jul. 2017.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3," RFC 8446, Aug. 2018.
[RFC8504] Chown, T., J. Loughney, T. Winters, "IPv6 Node
Requirements," RFC 8504, Jan. 2019.
[RFC8724] Minaburo, A., L. Toutain, C. Gomez, D. Barthel, JC.,
"SCHC: Generic Framework for Static Context Header
Compression and Fragmentation," RFC 8724, Apr. 2020.
[RFC8899] Fairhurst, G., T. Jones, M. Tuxen, I. Rungeler, T. Volker,
"Packetization Layer Path MTU Discovery for Datagram
Transports," RFC 8899, Sep. 2020.
[RFC9040] Touch, J., M. Welzl, S. Islam, "TCP Control Block
Interdependence," RFC 9040, Jul. 2021.
[RFC9147] Rescorla, E., H. Tschofenig, N. Modadugu, "Datagram
Transport Layer Security Version 1.3," RFC 9147, Apr.
2022.
[RFC9187] Touch, J., "Sequence Number Extension for Windowed
Protocols," RFC 9187, Jan. 2022.
[RFC9260] Stewart, R., M. Tuxen, K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, June 2022.
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[RFC9293] Eddy, W. (Ed.), "Transmission Control Protocol," STD 7,
RFC 9293, Aug. 2022.
[RFC9298] Schinazi, D., "Proxying UDP in HTTP", RFC 9298, August
2022.
[RFC9648] Scharf, M., Jethanandani, M., and V. Murgai, "YANG Data
Model for TCP", RFC 9648, October 2024.
[To18] Touch, J., "A TCP Authentication Option Extension for
Payload Encryption," draft-touch-tcp-ao-encrypt, Jul.
2018.
[To24] Touch, J., "The UDP Authentication Option," draft-touch-
tsvwg-udp-auth-opt, Mar. 2024.
[Zu20] Zullo, R., T. Jones, and G. Fairhurst, "Overcoming the
Sorrows of the Young UDP Options," 2020 Network Traffic
Measurement and Analysis Conference (TMA), IEEE, 2020.
28. Acknowledgments
This work benefitted from feedback from Erik Auerswald, Bob Briscoe,
Ken Calvert, Ted Faber, Gorry Fairhurst (including OCS for errant
middlebox traversal), C. M. Heard (editor of this doc, including
combining previous FRAG and LITE options into the new FRAG, as well
as Figure 12), Tom Herbert, Tom Jones, Mark Smith, Carl Williams,
and Raffaele Zullo, as well as discussions on the IETF TSVWG and
SPUD email lists.
This work was partly supported by USC/ISI's Postel Center.
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Joe Touch
Manhattan Beach, CA 90266 USA
Phone: +1 (310) 560-0334
Email: touch@strayalpha.com
C. M. (Mike) Heard (Ed.)
PO Box 2667
Redwood City, CA 94064-2667 USA
Phone: +1 (408) 499-7257
Email: heard@pobox.com
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Appendix A.Implementation Information
The following information is provided to encourage consistent naming
for API implementations.
System-level variables (sysctl):
Name default meaning
----------------------------------------------------
net.ipv4.udp_opt 0 UDP options available
net.ipv4.udp_opt_ocs 1 Use OCS
net.ipv4.udp_opt_apc 0 Include APC
net.ipv4.udp_opt_frag 0 Fragment
net.ipv4.udp_opt_mds 0 Include MDS
net.ipv4.udp_opt_mrds 0 Include MRDS
net.ipv4.udp_opt_req 0 Include REQ
net.ipv4.udp_opt_resp 0 Include RES
net.ipv4.udp_opt_time 0 Include TIME
net.ipv4.udp_opt_auth 0 Include AUTH
net.ipv4.udp_opt_exp 0 Include EXP
net.ipv4.udp_opt_ucmp 0 Include UCMP
net.ipv4.udp_opt_uenc 0 Include UENC
net.ipv4.udp_opt_uexp 0 Include UEXP
Socket options (sockopt), cached for outgoing datagrams:
Name meaning
----------------------------------------------------
UDP_OPT Enable UDP options (at all)
UDP_OPT_OCS Use UDP OCS
UDP_OPT_APC Enable UDP APC option
UDP_OPT_FRAG Enable UDP fragmentation
UDP OPT MDS Enable UDP MDS option
UDP OPT MRDS Enable UDP MRDS option
UDP OPT REQ Enable UDP REQ option
UDP OPT RES Enable UDP RES option
UDP_OPT_TIME Enable UDP TIME option
UDP OPT AUTH Enable UDP AUTH option
UDP OPT EXP Enable UDP EXP option
UDP_OPT_UCMP Enable UDP UCMP option
UDP_OPT_UENC Enable UDP UENC option
UDP OPT UEXP Enable UDP UEXP option
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Send/sendto parameters:
(Same as sysctl, with different prefixes)
Connection parameters (per-socket pair cached state, part UCB):
Name Initial value
----------------------------------------------------
opts_enabled net.ipv4.udp_opt
ocs_enabled net.ipv4.udp_opt_ocs
NB: The JUNK option is included for debugging purposes, and is not
intended to be enabled otherwise.
System variables
net.ipv4.udp_opt_junk 0
System-level variables (sysctl):
Name default meaning
----------------------------------------------------
net.ipv4.udp_opt_junk 0 Default use of junk
Socket options (sockopt):
Name params meaning
------------------------------------------------------
UDP_JUNK - Enable UDP junk option
UDP_JUNK_VAL fillval Value to use as junk fill
UDP_JUNK_LEN length Length of junk payload in bytes
Connection parameters (per-socket pair cached state, part UCB):
Name Initial value
----------------------------------------------------
junk_enabled net.ipv4.udp_opt_junk
junk_value 0xABCD
junk_len 4
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