TSVWG J. Touch
Internet Draft Independent Consultant
Intended status: Standards Track March 26, 2022
Intended updates: 768
Expires: September 2022
Transport Options for UDP
draft-ietf-tsvwg-udp-options-18.txt
Abstract
Transport protocols are extended through the use of transport header
options. This document extends UDP by indicating the location,
syntax, and semantics for UDP transport layer options.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 26, 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................3
3. Terminology....................................................3
4. Background.....................................................4
5. The UDP Option Area............................................5
6. The UDP Surplus Area Structure.................................8
7. The Option Checksum (OCS)......................................8
8. UDP Options...................................................10
9. Safe UDP Options..............................................13
9.1. End of Options List (EOL)................................13
9.2. No Operation (NOP).......................................14
9.3. Alternate Payload Checksum (APC).........................14
9.4. Fragmentation (FRAG).....................................16
9.5. Maximum Datagram Size (MDS)..............................19
9.6. Maximum Reassembled Datagram Size (MRDS).................20
9.7. Echo request (REQ) and echo response (RES)...............21
9.8. Timestamps (TIME)........................................21
9.9. Authentication (AUTH)....................................22
9.10. Experimental (EXP)......................................23
10. UNSAFE Options...............................................24
10.1. UNSAFE Encryption (UENC)................................25
10.2. UNSAFE Experimental (UEXP)..............................25
11. Rules for designing new options..............................25
12. Option inclusion and processing..............................26
13. UDP API Extensions...........................................28
14. UDP Options are for Transport, Not Transit...................29
15. UDP options vs. UDP-Lite.....................................29
16. Interactions with Legacy Devices.............................30
17. Options in a Stateless, Unreliable Transport Protocol........30
18. UDP Option State Caching.....................................31
19. Updates to RFC 768...........................................31
20. Interactions with other RFCs (and drafts)....................32
21. Multicast Considerations.....................................33
22. Security Considerations......................................33
23. IANA Considerations..........................................34
24. References...................................................35
24.1. Normative References....................................35
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24.2. Informative References..................................35
25. Acknowledgments..............................................38
Appendix A. Implementation Information...........................39
1. Introduction
Transport protocols use options as a way to extend their
capabilities. TCP [RFC793], SCTP [RFC4960], and DCCP [RFC4340]
include space for these options but UDP [RFC768] currently does not.
This document defines 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.
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:
o IP datagram [RFC791][RFC8200] - an IP packet, composed of the IP
header and an IP payload area
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
o UDP packet - the more contemporary term used herein to refer to a
user datagram [RFC768]
o Surplus area - the area of an IP payload that follows a UDP
packet; this area is used for UDP options in this document
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o UDP fragment - one or more components of a UDP packet and its UDP
options that enables transmission as IP payloads larger than
permitted by IP datagram maximum sizes; 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 Must-support options - UDP options that all implementations are
required to support. Their use in individual UDP packets is
optional.
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 [RFC793][RFC5925][RFC7323].
Header options are used both in stateful (connection-oriented, e.g.,
TCP [RFC793], SCTP [RFC4960], 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 ports 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). There are a number of reasons why Internet historians suggest
that 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
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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-
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 [RFC6081] uses values of the UDP Length that are larger
than the IP payload as an additional type of signal, as noted in
Section 20. UDP options uses a value smaller than the IP payload to
enable backwards compatibility with existing UDP implementations,
i.e., to deliver the UDP Length of UDP user data to the application
and silently ignore the additional surplus area data. Using a value
larger than the IP payload could either be considered malformed (and
ought to be silently dropped by UDP processing) or could cause
buffer overruns, and so is not considered silently and safely
backward compatible.
5. 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 16). 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]. As a result, the
typical (and largest valid) value for UDP Length is:
UDP_Length = IPv4_Total_Length - IPv4_IHL * 4
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| 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) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 HDR 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 typical (and largest valid) value for UDP Length
is:
UDP_Length = IPv6_Payload_Length - sum(extension header lengths)
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Next Hdr | 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 (for IPv4) or by adding
information in the extensions (for IPv6). 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
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a requirement of UDP [RFC768] (discussed further in Section 16).
This document uses the surplus area for UDP options.
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".
6. 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, using zeroes for alignment. The UDP option area can be used
with any UDP payload length (including zero), 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.
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 7.
The remainder of the surplus area consists of options defined using
a TLV (type, length, and optional value) syntax similar to that of
TCP [RFC793], as detailed in Section 8. These options continue until
the end of the surplus area or can end earlier using the EOL (end of
list) option, followed by zeroes.
7. The Option Checksum (OCS)
The Option Checksum (OCS) option is 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.
+--------+--------+--------+--------+
| UDP data | 0 |
+--------+--------+--------+--------+
| OCS | UDP options... |
+--------+--------+--------+--------+
Figure 4 UDP OCS format, here using one zero for alignment
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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 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 22.
The design enables traversal of errant middleboxes that incorrectly
compute the UDP checksum over the entire IP payload [Fa18], 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.
>> 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.
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 UDP payload
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
datagram prior to fragmentation or after reassembly (see Section
9.4).
The OCS covers the surplus area as formatted for transmission and is
processed immediately upon reception.
>> If 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 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 12).
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8. UDP Options
UDP options are typically 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. The Length field 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.
+--------+--------+--------+--------+
| 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 where rate-limiting permits.
>> 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 EOL and NOP MAY use either the default
or extended option formats.
>> 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.
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>> UDP options MUST be interpreted in the order in which they occur
in the surplus area.
The following UDP options are currently defined:
Kind Length Meaning
----------------------------------------------
0* - End of Options List (EOL)
1* - No operation (NOP)
2* 6 Alternate payload checksum (APC)
3* 10/12 Fragmentation (FRAG)
4* 4 Maximum datagram size (MDS)
5* 4 Maximum reassembled datagram size (MRDS)
6* 6 Request (REQ)
7* 6 Response (RES)
8 10 Timestamps (TIME)
9 (varies) Authentication (AUTH)
10-126 (varies) UNASSIGNED (assignable by IANA)
127 (varies) RFC 3692-style experiments (EXP)
128-191 RESERVED
192 (varies) Encryption (UENC)
193-253 UNASSIGNED-UNSAFE (assignable by IANA)
254 (varies) RFC 3692-style experiments (UEXP)
255 RESERVED
Options indicated by Kind values in the range 0..127 are known as
SAFE options because they do not alter the UDP data payload and thus
do not interfere with use of that data by legacy endpoints. Options
indicated by Kind values in the range 192..254 are known as UNSAFE
options because they do alter the UDP data payload and thus would
interfere with legacy endpoints. UNSAFE option nicknames are
expected to begin with "U", which should be avoided for safe option
nicknames (see Section 23). Kind values 128-191 and 255 are RESERVED
and not otherwise defined at this time.
>> RESERVED Kind values MUST NOT be assumed to be either SAFE nor
UNSAFE until defined.
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 UDP payloads, thus is
SAFE in actual use, as discussed further in Section 9.4.
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.
>> An endpoint supporting UDP options MUST treat unsupported options
in the UNSAFE range as terminating all option processing.
>> 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
SAFE options (i.e., in the same way a legacy receiver would). That
includes options whose length does not indicate the specified
value(s), as long as the length is not inherently invalid (i.e.,
smaller than 2 for the default and 4 for the extended formats).
>> UNSAFE options are used only in with the FRAG option, in a manner
that prevents them from being silently ignored but passing the UDP
payload to the user when not supported. This ensures their safe use
in environments that might include legacy receivers (See Section
10).
>> Receivers supporting UDP options MUST silently drop all UDP
options in a datagram containing an UNSAFE option when any UNSAFE
option it contains is unknown. See Section 10 for further discussion
of UNSAFE options.
>> Except for NOP, EXP, and UEXP, each option SHOULD NOT occur more
than once in a single UDP datagram. If an option other than these
occurs more than once, a receiver MUST interpret only the first
instance of that option and MUST ignore all others.
>> EXP and UEXP MAY occur more than once, but SHOULD NOT occur more
than once using the same ExID (see Sections 9.10 and 10.2).
>> Only the OCS and the AUTH and UENC options depend on the contents
of the surplus area. AUTH and UENC are never used together, as UENC
would serve both purposes. 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 a value
dependent on the contents of the surplus area. Otherwise,
interactions between those values, the OCS, and the AUTH and UENC
options could be unpredictable.
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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). The exception is only for lengths that imply a
physical impossibility, e.g., smaller than two for conventional
options and four for extended length options. Impossible lengths
should indicate a malformed surplus area and all options silently
discarded. Lengths other than those expected should result in safe
options being ignored and skipped over, as with any other unknown
safe option.
>> Option lengths MUST NOT exceed the IP length of the overall IP
datagram. If this occurs, the options MUST be treated as malformed
and all options dropped, and the event MAY be logged for diagnostics
(logging SHOULD be rate limited).
>> "Must-support" options other than NOP and EOL MUST come before
other options.
The requirement that must-support options come before others is
intended to allow for endpoints to implement DOS protection, as
discussed further in Section 22.
9. 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 10). The following subsections
describe safe options defined in this document.
9.1. 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, the
last non-NOP option MUST be EOL.
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>> NOPs SHOULD NOT be used as padding before the EOL option. As a
one byte option, it need not be otherwise aligned.
>> All bytes in the surplus area after EOL MUST be set to zero on
transmit.
>> Bytes after EOL in the surplus area MAY be checked as being zero
on receipt but MUST be treated as zero regardless of their content
and are not passed to the user (e.g., as part of the surplus area).
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 [Fa22]).
9.2. 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.
This issue is discussed further in Section 22.
9.3. Alternate Payload Checksum (APC)
The Alternate Payload Checksum (APC, Kind=2) option provides a
stronger alternative 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 an
"alternate" to the UDP checksum that covers the user data - not to
the OCS (the latter covers the surplus area only). Unlike the UDP
checksum, APC does not include the IP pseudoheader or UDP header,
thus it does not need to be updated by NATs when IP addresses or UDP
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ports are rewritten. Its purpose is to detect user data errors that
the UDP checksum, when used, might not detect.
A CRC32c has been chosen because of its ubiquity and use in other
Internet protocols, including iSCSI and SCTP. The option contains
the CRC32c in network standard byte order, as described in
[RFC3385].
+--------+--------+--------+--------+
| 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 of zero. If the CRC is
zero, this must indicate a valid checksum (i.e., 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;
because this is not the case, APC does not cover the pseudoheader.
>> UDP packets with incorrect APC checksums MUST be passed to the
application by default, e.g., with a flag indicating APC failure.
Like all safe UDP options, APC needs to be silently ignored when
failing by default, unless the receiver has been 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.
>> UDP packets with unrecognized APC lengths MUST be receive the
same treatment as UDP packets with incorrect APC checksums.
Ensuring that unrecognized APC lengths are treated as incorrect
checksums enables future variants of APC to be treated as APC-like.
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9.4. Fragmentation (FRAG)
The Fragmentation (FRAG, Kind=3) option supports UDP fragmentation
and reassembly, which can be used to transfer UDP messages larger
than limited 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 Network Address (and port) Translation (NAT)
devices, in contrast to the behavior of IP fragments. 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. FRAG
is designed similar to 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.
>> When FRAG is present, it SHOULD come as early as possible in the
UDP options list.
>> 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
In the non-terminal FRAG option format, 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
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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
are applied to this UDP fragment. Non-terminal fragments never have
options after the fragment.
The Frag. Offset field indicates the location of this fragment
relative to the original UDP datagram (prior to fragmentation),
measured from the start of the original UDP datagram's UDP header.
The FRAG option does not need a "more fragments" bit 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.
>> Endpoints supporting UDP options MUST be capable of fragmenting
and reassembling at least 2 fragments, for a total of at least 3,000
bytes (see MRDS in Section 9.6).
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.
+--------+--------+--------+--------+
| Kind=3 | Len=12 | Frag. Start |
+--------+--------+--------+--------+
| Identification |
+--------+--------+--------+--------+
| Frag. Offset | Dgram Opt Start |
+--------+--------+--------+--------+
Figure 11 UDP terminal FRAG option format
The terminal FRAG option format adds a Datagram Option Start
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 after the original UDP datagram. In this variant,
UDP options that apply to the reassembled datagram may occur after
the terminal fragment data. UDP options that occur before the FRAG
data are processed on the fragment; UDP options after the FRAG data
are processed after reassembly, such that the reassembled data
represents the original UDP user data. 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.
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>> During fragmentation, the UDP header checksum of each fragment
remains constant and does not depend on the fragment data (which
appears in the surplus area), because all fragments have a zero-
length user data field.
The Fragment Offset is 16 bits and indicates the location of the UDP
payload fragment in bytes from the beginning of the original
unfragmented payload. The option Len field indicates whether there
are more fragments (Len=10) or no more fragments (Len=12).
>> 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 should be sent.
It should be noted that fragments may 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 SHOULD be no more than 2 minutes.
>> 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-socketpair 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.
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Any per-datagram UDP options, if used, follow the FRAG option in the
final fragment and would be included in the reassembled UDP packet.
Processing of those options would commence after reassembly. This is
especially important for UNSAFE options, which are interpreted only
after FRAG.
In general, UDP packets are fragmented as follows:
1. Create a UDP packet with data and UDP options, which we will call
"D". Note that the UDP options treat the data area as UDP user
data and thus must follow that data.
Process these UDP options before the rest of the fragmentation
steps below. Note that 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 the UDP checksum is non-zero).
2. Identify the desired fragment size, which we will call "S". This
value should take into account the path MTU (if known) and allow
space for per-fragment options.
3. Fragment "D" into chunks of size no larger than "S"-10 each, with
one final chunk no larger than "S"-12. Note that all the non-FRAG
options in step #1 need not be limited to the terminal fragment,
i.e., the Dgram Opt. Start pointer can indicate the start of the
original surplus area anywhere in the reassembled data.
4. For each chunk of "D" in step #3, create a zero-data UDP packet
followed by the word-aligned OCS, the FRAG option, and any
additional UDP options, followed by the FRAG data chunk.
The last chunk includes the non-FRAG options noted in step #1
after the end of the FRAG data. These UDP options apply to the
reassembled user data as a whole when received.
5. Process the pre-reassembly UDP options of 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.
9.5. Maximum Datagram Size (MDS)
The Maximum Datagram Size (MDS, Kind=4) option is a 16-bit hint of
the largest unfragmented UDP packet that an endpoint believes can be
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received. As with the TCP Maximum Segment Size (MSS) option
[RFC793], the size indicated is the IP layer MTU decreased by the
fixed IP and UDP headers only [RFC6691]. The space needed for IP and
UDP options need 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; it does not indicate a known path MTU and
thus MUST NOT be used to limit transmissions.
+--------+--------+--------+--------+
| Kind=4 | Len=4 | MDS size |
+--------+--------+--------+--------+
Figure 12 UDP MDS option format
The UDP MDS option MAY be used as a hint for path MTU discovery
[RFC1191][RFC8201], but this may be difficult because of known
issues with ICMP blocking [RFC2923] as well as UDP lacking automatic
retransmission. It 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 9.6),
e.g., when EMTU_R is larger than the required minimums (576 for IPv4
[RFC791] and 1500 for IPv6 [RFC8200]). It can also be used with
DPLPMTUD [RFC8899] to provide a hint to maximum DPLPMTU, though it
MUST NOT prohibit transmission of larger UDP packets (or fragments)
used as DPLPMTU probes.
9.6. Maximum Reassembled Datagram Size (MRDS)
The Maximum Reassembled Segment Size (MRDS, Kind=5) option is a 16-
bit indicator of the largest reassembled UDP segment that can be
received. MRDS is the UDP equivalent of IP's EMTU_R but the two are
not related [RFC1122]. Using the FRAG option (Section 9.4), UDP
packets can be transmitted as transport fragments, each in their own
(presumably not fragmented) IP datagram and be reassembled at the
UDP layer.
+--------+--------+--------+--------+
| Kind=5 | Len=4 | MRDS size |
+--------+--------+--------+--------+
Figure 13 UDP MRDS option format
>> Endpoints supporting UDP options MUST support a local MRDS of at
least 3,000 bytes.
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9.7. Echo request (REQ) and echo response (RES)
The echo request (REQ, Kind=6) and echo response (RES, Kind=7)
options provide a means for UDP options to be used to provide UDP
packet-level acknowledgements. One such use is described as part of
the UDP options variant of packetization layer path MTU discovery
(PLPMTUD) [Fa22]. The options both have the format indicated in
Figure 14, in which the token has no internal structure or meaning.
+--------+--------+------------------+
| Kind | Len=6 | token |
+--------+--------+------------------+
1 byte 1 byte 4 bytes
Figure 14 UDP REQ and RES options format
Each of these option kinds appears at most once in each UDP packet,
as with other options. Note also that the FRAG option is not used
when sending DPLPMTUD probes to determine a PLPMTU [Fa22].
9.8. Timestamps (TIME)
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].
+--------+--------+------------------+------------------+
| Kind=8 | Len=10 | TSval | TSecr |
+--------+--------+------------------+------------------+
1 byte 1 byte 4 bytes 4 bytes
Figure 15 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.
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>> 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 be:
o Values are monotonic and non-decreasing except for anticipated
number-space rollover events
o Values should "increase" (allowing for rollover) 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 should always respond 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.
9.9. Authentication (AUTH)
The Authentication (AUTH, Kind=9) option is intended to allow UDP to
provide a similar type of authentication as the TCP Authentication
Option (TCP-AO) [RFC5925]. AUTH covers the UDP user data. AUTH
supports NAT traversal in a similar manner as TCP-AO [RFC6978].
Figure 16 shows the UDP AUTH format, whose contents are identical to
that of the TCP-AO option.
+--------+--------+--------+--------+
| Kind=9 | Len | TCP-AO fields...|
+--------+--------+--------+--------+
| TCP-AO fields (con't)... |
+--------+--------+--------+--------+
Figure 16 UDP AUTH option format
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Like TCP-AO, AUTH is not negotiated in-band. Its use assumes both
endpoints have populated Master Key Tuples (MKTs), used to exclude
non-protected traffic.
TCP-AO generates unique traffic keys from a hash of TCP connection
parameters. UDP lacks a three-way handshake to coordinate
connection-specific values, such as TCP's Initial Sequence Numbers
(ISNs) [RFC793], thus AUTH's Key Derivation Function (KDF) uses
zeroes as the value for both ISNs. This means that the AUTH reuses
keys when socket pairs are reused, unlike TCP-AO.
>> UDP packets with incorrect AUTH HMACs MUST be passed to the
application by default, e.g., with a flag indicating AUTH failure.
Like all non-UNSAFE UDP options, AUTH needs to be silently ignored
when failing. This silently-ignored behavior ensures that option-
aware receivers operate the same as legacy receivers unless
overridden.
In addition to the UDP user data (which is always included), AUTH
can be configured to either include or exclude the surplus area, in
a similar way as can TCP-AO can optionally exclude TCP options. When
UDP options are covered, the OCS value and AUTH (and later, UENC)
hash areas are zeroed before computing the AUTH hash. It is
important to consider that options not yet defined might yield
unpredictable results if not confirmed as supported, e.g., if they
were to contain other hashes or checksums that depend on the surplus
area contents. This is why such dependencies are not permitted
except as defined for the OCS and the AUTH (and later, UENC) option.
Similar to TCP-AO-NAT, AUTH (and later, UENC) can be configured to
support NAT traversal, excluding (by zeroing out) one or both of the
UDP ports and corresponding IP addresses [RFC6978].
9.10. Experimental (EXP)
The Experimental option (EXP, Kind=127) is reserved for experiments
[RFC3692]. Only one such 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].
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+----------+----------+----------+----------+
| 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 minimum 16-bit UDP ExID
identifier (similar to TCP ExIDs [RFC6994]).
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 option format
Assigned UDP experimental IDs (ExIDs) assigned from a single
registry managed by IANA (see Section 23). Assigned ExIDs can be
used in either the EXP or UEXP options (see Section 10.2 for the
latter).
10. 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.
>> 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
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datagram includes an UNSAFE option whose UKind is not supported.
Note that this still results in the receipt of a zero-length UDP
datagram.
10.1. UNSAFE Encryption (UENC)
UNSAFE encryption (UENC, Kind=192) has the same format as AUTH
(Section 9.9), except that it encrypts (modifies) the user data. It
provides a similar encryption capability as TCP-AO-ENC, in a similar
manner [To18]. Its fields, coverage, and processing are the same as
for AUTH, except that UENC encrypts only the user data, although it
can (optionally) depend on the surplus area (with certain fields
zeroed, as per AUTH, e.g., providing authentication over the surplus
area). Like AUTH, UENC can be configured to be compatible with NAT
traversal.
10.2. 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.
11. Rules for designing new options
The UDP option Kind space allows for the definition of new options,
however the currently defined options do not allow for arbitrary new
options. The following is a summary of rules for new options and
their rationales:
>> New options MUST NOT modify other option content.
>> 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.
>> At the sender, new options MUST NOT modify UDP packet content
anywhere except within their option field, excepting only those
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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 intend optionally for
modification of any UDP options, including their new areas, in
transit.
Note that only certain of the initially defined options violate
these rules:
o >> Only FRAG and UNSAFE options are permitted to modify the UDP
body.
The following recommendation helps enable efficient zero-copy
processing:
o >> FRAG SHOULD be the first option, when present.
12. 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.
>> All options except UNSAFE options MUST result in the UDP user
data being passed to the application layer, 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.
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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 its options as follows
(in order):
if the UDP checksum fails then
silently drop the entire UDP packet (per RFC1122)
if the UDP checksum passes then
if OCS != 0 and fails or is zero when UDP CS != 0 then
deliver the UDP user data but ignore other options
(this is required to emulate legacy behavior)
if OCS is nonzero and passes or is zero 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 as used only inside the FRAG area
ensures that the resulting UDP data will be silently dropped in both
legacy and options-aware receivers. 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) should 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.
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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).
13. 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 as indicated
by the application. Datagrams not containing these required
options MUST be silently dropped and MAY be logged. This includes
a minimum datagram length, such that the options list ends in EOL
and additional space is zero-filled as needed.
o WG QUESTION: DO WE ALSO WANT A MIN FRAG SIZE? OR MAX?
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.
o WG QUESTION: SHOULD WE ACCUMULATE THOSE OPTIONS? OR DISCARD THEM?
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.
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Examples of API instances for Linux and FreeBSD are provided in
Appendix A, to encourage uniform cross-platform implementations.
14. 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 intended for use only by the transport endpoints.
They are no more (or less) appropriate to be modified in-transit
than any other portion of the transport datagram.
UDP options are transport options. Generally, transport headers,
options, and data re not intended to be modified in-transit. UDP
options are no exception and here are specified as "MUST NOT" be
altered in transit. However, 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).
15. 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.
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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.
16. 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.
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.
17. 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
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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 MUST allow for
message reordering and loss.
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.
18. 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 may 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.
19. 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.
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20. 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 [RFC6081]. TE defines the UDP length
pointing beyond (larger) than the location indicated by the IP
length rather than shorter (as used herein):
"..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)"
As a result, UDP options are not compatible with TE, but that is
also why this document does not update TE. Additionally, it is not
at all clear how TE operates, as it requires network processing of
the UDP length field to understand the total message including TE
trailers.
TE updates Teredo NAT traversal [RFC4380]. The NAT traversal
document defined "consistency" of UDP length and IP length as:
"An IPv6 packet is deemed valid if it conforms to [RFC2460]:
the protocol identifier should indicate an IPv6 packet and
the payload length should be consistent with the length of
the UDP datagram in which the packet is encapsulated."
IPv6 is clear on the meaning of this consistency, in which the
pseudoheader used for UDP checksums is based on the UDP length, not
inferred from the IP length, using the same text in the current
specification [RFC8200]:
"The Upper-Layer Packet Length in the pseudo-header is the
length of the upper-layer header and data (e.g., TCP header
plus TCP data). Some upper-layer protocols carry their own
length information (e.g., the Length field in the UDP header);
for such protocols, that is the length used in the pseudo-
header."
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
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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].
21. Multicast Considerations
UDP options are primarily intended for unicast use. Using these
options over multicast 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 group confirm support
for the options in use.
22. 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
in this section further.
The use of UDP packets with inconsistent IP and UDP Length fields
has the potential to trigger a buffer overflow error if not properly
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.
UDP options are not covered by DTLS (datagram transport-layer
security). Despite the name, neither TLS [RFC8446] (transport layer
security, for TCP) nor DTLS [RFC6347] (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]) or in UDP by the
Authentication (AUTH) option (Section 9.9) and UNSAFE Encryption
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(UENC) option (Section 10). Transport headers are also protected as
payload when using IP security (IPsec) [RFC4301].
UDP options use the TLV syntax similar to that of TCP. This syntax
is known to require serial processing and may pose a DOS risk, e.g.,
if an attacker adds large numbers of unknown options that must be
parsed in their entirety, as is the case for IPv6 [RFC8504].
>> Implementations concerned with the potential for this
vulnerability MAY implement only the required UDP options and MAY
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 8.
Because required options come first and at most once each (with the
exception of NOPs, which should never need to come in sequences of
more than seven in a row), this limits their DOS impact. Note that
TLV formats for options does require serial processing, but any
format that allows future options, whether ignored or not, could
introduce a similar DOS vulnerability.
UDP security should never rely solely on transport layer processing
of options. UNSAFE options are the only type that share 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 may 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).
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 may be useful to assume a reasonable
minimum fragment size, e.g., that non-terminal fragments should
never be smaller than 500 bytes.
23. IANA Considerations
Upon publication, IANA is hereby requested to create a new registry
for UDP Option Kind numbers, similar to that for TCP Option Kinds.
Initial values of this registry are as listed in Section 8.
Additional values in this registry are to be assigned from the
UNASSIGNED values in Section 8 by IESG Approval or Standards Action
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[RFC8126]. Those assignments are subject to the conditions set forth
in this document, particularly (but not limited to) those in Section
11.
Although option nicknames are not used in-band, IANA should require
UNSAFE safe option values to commence with the letter "U" and avoid
that letter as commencing safe options.
Upon publication, IANA is hereby requested to create a new registry
for UDP Experimental Option Experiment Identifiers (UDP ExIDs) for
use in a similar manner as TCP ExIDs [RFC6994]. UDP ExIDs can be
used in either (or both) the EXP or UEXP options. This registry is
initially empty. Values in this registry are to be assigned by IANA
using first-come, first-served (FCFS) rules [RFC8126]. Options using
these ExIDs are subject to the same conditions as new options, i.e.,
they too are subject to the conditions set forth in this document,
particularly (but not limited to) those in Section 11.
24. References
24.1. Normative References
[Fa22] Fairhurst, G., T. Jones, "Datagram PLPMTUD for UDP
Options," draft-ietf-tsvwg-udp-options-dplpmtud, Feb.
2022.
[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," RFC 2119, May 2017.
24.2. Informative References
[Fa18] Fairhurst, G., T. Jones, R. Zullo, "Checksum Compensation
Options for UDP Options", draft-fairhurst-udp-options-cco,
Oct. 2018.
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[Hi15] Hildebrand, J., B. Trammel, "Substrate Protocol for User
Datagrams (SPUD) Prototype," draft-hildebrand-spud-
prototype-03, Mar. 2015.
[RFC793] Postel, J., "Transmission Control Protocol" RFC 793,
September 1981.
[RFC1071] Braden, R., D. Borman, C. Partridge, "Computing the
Internet Checksum," RFC 1071, Sept. 1988.
[RFC1191] Mogul, J., S. Deering, "Path MTU discovery," RFC 1191,
November 1990.
[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.
[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.
[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.
[RFC4960] Stewart, R. (Ed.), "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
Option," RFC 5925, June 2010.
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[RFC6081] Thaler, D., "Teredo Extensions," RFC 6081, Jan 2011.
[RFC6347] Rescorla, E., N. Modadugu, "Datagram Transport Layer
Security Version 1.2," RFC 6347, Jan. 2012.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS),"
RFC 6691, July 2012.
[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.
[RFC7323] Borman, D., R. Braden, V. Jacobson, R. Scheffenegger
(Ed.), "TCP Extensions for High Performance," RFC 7323,
Sep. 2014.
[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.
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[RFC9040] Touch, J., M. Welzl, S. Islam, "TCP Control Block
Interdependence," RFC 9040, Jul. 2021.
[RFC9187] Touch, J., "Sequence Number Extension for Windowed
Protocols," RFC 9187, Jan. 2022.
[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.
[To18] Touch, J., "A TCP Authentication Option Extension for
Payload Encryption," draft-touch-tcp-ao-encrypt, Jul.
2018.
25. Acknowledgments
This work benefitted from feedback from Erik Auerswald, Bob Briscoe,
Ken Calvert, Ted Faber, Gorry Fairhurst (including OCS for
misbehaving middlebox traversal), C. M. Heard (including combining
previous FRAG and LITE options into the new FRAG), Tom Herbert, Mark
Smith, 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.
Authors' Addresses
Joe Touch
Manhattan Beach, CA 90266 USA
Phone: +1 (310) 560-0334
Email: touch@strayalpha.com
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Appendix A. Implementation Information
The following information is provided to encourage interoperable API
implementations.
System-level variables (sysctl):
Name default meaning
----------------------------------------------------
net.ipv4.udp_opt 0 UDP options available
net.ipv4.udp_opt_ocs 1 Default use OCS
net.ipv4.udp_opt_apc 0 Default include APC
net.ipv4.udp_opt_frag 0 Default fragment
net.ipv4.udp_opt_mds 0 Default include MDS
net.ipv4.udp_opt_mrds 0 Default include MRDS
net.ipv4.udp_opt_req 0 Default include REQ
net.ipv4.udp_opt_resp 0 Default include RES
net.ipv4.udp_opt_time 0 Default include TIME
net.ipv4.udp_opt_auth 0 Default include AUTH
net.ipv4.udp_opt_exp 0 Default include EXP
net.ipv4.udp_opt_uenc 0 Default include UENC
net.ipv4.udp_opt_uexp 0 Default 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_UENC Enable UDP UENC option
UDP OPT UEXP Enable UDP UEXP option
Send/sendto parameters:
Connection parameters (per-socketpair cached state, part UCB):
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Name Initial value
----------------------------------------------------
opts_enabled net.ipv4.udp_opt
ocs_enabled net.ipv4.udp_opt_ocs
The following option is included for debugging purposes, and MUST
NOT be enabled otherwise.
System variables
net.ipv4.udp_opt_junk 0
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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-socketpair cached state, part UCB):
Name Initial value
----------------------------------------------------
junk_enabled net.ipv4.udp_opt_junk
junk_value 0xABCD
junk_len 4
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