TSVWG                                                          J. Touch
Internet Draft                                                  USC/ISI
Intended status: Standards Track                           May 16, 2017
Intended updates: 768
Expires: November 2017

                         Transport Options for UDP

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   Transport protocols are extended through the use of transport header
   options. This document experimentally extends UDP by indicating the
   location, syntax, and semantics for UDP transport layer options.

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................3
   3. Background.....................................................3
   4. The UDP Option Area............................................4
   5. UDP Options....................................................7
      5.1. End of Options List (EOL).................................8
      5.2. No Operation (NOP)........................................8
      5.3. Option Checksum (OCS).....................................9
      5.4. Alternate Checksum (ACS).................................10
      5.5. Lite (LITE)..............................................10
      5.6. Maximum Segment Size (MSS)...............................12
      5.7. Timestamps (TIME)........................................13
      5.8. Fragmentation (FRAG).....................................13
         5.8.1. Coupling FRAG with LITE.............................16
      5.9. Authentication and Encryption (AE).......................16
      5.10. Experimental (EXP)......................................17
   6. UDP API Extensions............................................17
   7. Whose options are these?......................................18
   8. UDP options vs. UDP-Lite......................................18
   9. Interactions with Legacy Devices..............................19
   10. Options in a Stateless, Unreliable Transport Protocol........20
   11. UDP Option State Caching.....................................20
   12. Security Considerations......................................21
   13. IANA Considerations..........................................22
   14. References...................................................22
      14.1. Normative References....................................22
      14.2. Informative References..................................22
   15. Acknowledgments..............................................24
   Appendix A. Implementation Information...........................26

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 experimental extension to UDP that provides
   space for transport options including their generic syntax and

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   semantics for their use in UDP's stateless, unreliable message

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lowercase uses of these words are not to be
   interpreted as carrying significance described in RFC 2119.

   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. Background

   Many protocols include a default header and an area for header
   options. 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

   These options are used both in stateful (connection-oriented, e.g.,
   TCP [RFC793], SCTP [RFC4960], DCCP [RFC4340]) and stateless
   (connectionless, e.g., IPv4 [RFC791], IPv6 [RFC2460] 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 as well. One example of such uses is Substrate Protocol for
   User Datagrams (SPUD) [Tr15], and this document is intended to
   provide an out-of-band option area as an alternative to the in-band
   mechanism currently proposed [Hi15].

   UDP is one of the most popular protocols that lacks space for
   options [RFC768]. The UDP header was intended to be a minimal
   addition to IP, providing only ports and a data checksum for
   protection. This document experimentally extends UDP to provide a
   trailer area for options located after the UDP data payload.

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4. The UDP Option Area

   The UDP transport header includes demultiplexing and service
   identification (port numbers), a checksum, and a field that
   indicates the UDP datagram length (including UDP header). The UDP
   Length length field is typically redundant with the size of the
   maximum space available as a transport protocol payload (see also
   discussion in Section 9).

   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

   For IPv6, the IP Payload Length field indicates the datagram after
   the base IPv6 header, which includes the IPv6 extension headers and
   space available for the transport protocol, as shown in Figure 2
   [RFC2460]. 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 [RFC2460], so the typical (and
   largest valid) value for UDP Length is:

       UDP_Length = IPv6_Payload_Length - sum(extension header lengths)

   In both cases, the space available for the UDP transport protocol
   data unit is indicated by IP, either completely in the base header
   (for IPv4) or adding information in the extensions (for IPv6). In
   either case, this document will refer to this available space as the
   "IP transport payload".

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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 transport payload

      |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 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).

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                             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. It is
   important to note that this is not a requirement of UDP [RFC768]
   (discussed further in Section 9). UDP-Lite used the difference in
   these pointers to indicate the partial coverage of the UDP Checksum,
   such that the UDP user data, UDP header, and UDP pseudoheader (a
   subset of the IP header) are covered by the UDP checksum but
   additional user data in the surplus area is not covered [RFC3828].
   This document uses the surplus area for UDP transport options.

   The UDP option area is thus defined as the location between the end
   of the UDP payload and the end of the IP datagram as a trailing
   options area. This area can occur 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 offset".

   UDP options are defined using a TLV (type, length, and optional
   value) syntax similar to that of TCP [RFC793]. They are typically a
   minimum of two bytes in length as shown in Figure 4, excepting only
   the one byte options "No Operation" (NOP) and "End of Options List"
   (EOL) described below.

                        |  Kind  | Length |

                    Figure 4 UDP option default format

   >> UDP options MAY occur at any UDP length offset.

   >> The UDP length MUST be at least as large as the UDP header (8)
   and no larger than the IP transport payload. Values outside this
   range MUST be silently discarded as invalid and logged where rate-
   limiting permits.

   Others have considered using values of the UDP Length that is larger
   than the IP transport payload as an additional type of signal. Using

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   a value smaller than the IP transport payload is expected to be
   backward compatible with existing UDP implementations, i.e., to
   deliver the UDP Length of user data to the application and silently
   ignore the additional surplus area data. Using a value larger than
   the IP transport payload would either be considered malformed (and
   be silently dropped) or could cause buffer overruns, and so is not
   considered silently and safely backward compatible. Its use is thus
   out of scope for the extension described in this document.

   >> UDP options MUST be interpreted in the order in which they occur
   in the UDP option area.

5. UDP Options

   The following UDP options are currently defined:

             Kind    Length    Meaning
             0*      -         End of Options List (EOL)
             1*      -         No operation (NOP)
             2*      2         Option checksum (OCS)
             3       4         Alternate checksum (ACS)
             4       4         Lite (LITE)
             5       4         Maximum segment size (MSS)
             6       10        Timestamps (TIME)
             7       12        Fragmentation (FRAG)
             8       (varies)  Authentication and Encryption (AE)
             9-126   (varies)  UNASSIGNED (assignable by IANA)
             127-253           RESERVED
             254     N(>=4)    RFC 3692-style experiments (EXP)
             255               RESERVED

   These options are defined in the following subsections.

   >> An endpoint supporting UDP options MUST support those marked with
   a "*" above: EOL, NOP, and OCS.

   [QUESTION: Should we extend these, e.g., through #7?]

   >> All other options (without a "*") MAY be implemented, and their
   use SHOULD be determined either out-of-band or negotiated.

   >> Receivers MUST silently ignore unknown options. That includes
   options whose length does not indicate the specified value.

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   Receivers cannot treat unexpected option lengths as invalid, as this
   would unnecessarily limit future revision of options (e.g., defining
   a new ACS that is defined by having a different length).

   >> Option lengths MUST NOT exceed the IP length of the packet. If
   this occurs, the packet MUST be treated as malformed and dropped,
   and the event MAY be logged for diagnostics (logging SHOULD be rate

   >> Required options MUST come before other options. Each required
   option MUST NOT occur more than once (if they are repeated in a
   received segment, all except the first MUST be silently ignored).

   The requirement that required options come before others is intended
   to allow for endpoints to implement DOS protection, as discussed
   further in Section 12.

5.1. End of Options List (EOL)

   The End of Options List (EOL) option indicates that there are no
   more options. It is used to indicate the end of the list of options
   without needing to pad the options to fill all available option

                                 | Kind=0 |

                      Figure 5 UDP EOL option format

   >> When the UDP options do not consume the entire option area, the
   last non-NOP option SHOULD be EOL (vs. filling the entire option
   area with NOP values).

   >> All bytes after EOL MUST be ignored by UDP option processing. As
   a result, there can only ever be one EOL option (even if other bytes
   were zero, they are ignored).

5.2. No Operation (NOP)

   The No Operation (NOP) option is a one byte placeholder, intended to
   be used as padding, e.g., to align multi-byte options along 16-bit
   or 32-bit boundaries.

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                                 | Kind=1 |

                      Figure 6 UDP NOP option format

   >> If options longer than one byte are used, NOP options SHOULD be
   used at the beginning of the UDP options area to achieve alignment
   as would be more efficient for active (i.e., non-NOP) options.

   >> Segments SHOULD NOT use more than three consecutive NOPs. NOPs
   are intended to assig with alignment, not other padding or fill.

   [NOTE: Tom Herbert suggested we declare "more than 3 consecutive
   NOPs" a fatal error to reduce the potential of using NOPs as a DOS
   attack, but IMO there are other equivalent ways (e.g., using
   RESERVED or other UNASSIGNED values) and the "no more than 3"
   creates its own DOS vulnerability)

5.3. Option Checksum (OCS)

   The Option Checksum (OCS) is an 8-bit ones-complement sum (Ones8)
   that covers all of the UDP options. OCS is 8-bits to allow the
   entire option to occupy a total of 16 bits.

   OCS can be calculated by computing the 16-bit ones-complement sum
   and "folding over" the result (using carry wraparound). Note that
   OCS is direct, i.e., it is not negated or adjusted if zero (unlike
   the Internet checksum as used in IPv4, TCP, and UDP headers). OCS
   protects the option area from errors in a similar way that the UDP
   checksum protects the UDP user data.

                            | Kind=2 | Ones8  |

                      Figure 7 UDP OCS option format

   >> When present, the option checksum SHOULD occur as early as
   possible, preferably preceded by only NOP options for alignment and
   the LITE option if present.

   OCS covers the entire UDP option, including the Lite option as
   formatted before swapping for transmission (or, equivalently, after
   the swap after reception).

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   >> If the option checksum fails, all options MUST be ignored and any
   trailing surplus data (and Lite data, if used) silently discarded.

   >> UDP data that is validated by a correct UDP checksum MUST be
   delivered to the application layer, even if the UDP option checksum
   fails, unless the endpoints have negotiated otherwise for this
   segment's socket pair.

5.4. Alternate Checksum (ACS)

   The Alternate Checksum (ACS) is a 16-bit CRC of the UDP payload only
   (excluding the IP pseudoheader, UDP header, and UDP options). It
   does not include the IP pseudoheader or UDP header, and so need not
   be updated by NATs when IP addresses or UDP ports are rewritten. Its
   purpose is to detect errors that the UDP checksum might not detect.
   CRC-CCITT (polynomial x^16 + x^12 + x^5 + x or polynomial 0x1021)
   has been chosen because of its ubiquity and use in other packet
   protocols, such as X.25, HDLC, and Bluetooth.

                   | Kind=3 | Len=4  |     CRC16sum    |

                      Figure 8 UDP ACS option format

5.5. Lite (LITE)

   The Lite option (LITE) is intended to provide equivalent capability
   to the UDP Lite transport protocol [RFC3828]. UDP Lite allows the
   UDP checksum to cover only a prefix of the UDP data payload, to
   protect critical information (e.g., application headers) but allow
   potentially erroneous data to be passed to the user. This feature
   helps protect application headers but allows for application data
   errors. Some applications are impacted more by a lack of data than
   errors in data, e.g., voice and video.

   >> When LITE is active, it MUST come first in the UDP options list.

   LITE is intended to support the same API as for UDP Lite to allow
   applications to send and receive data that has a marker indicating
   the portion protected by the UDP checksum and the portion not
   protected by the UDP checksum.

   LITE includes a 2-byte offset that indicates the length of the
   portion of the UDP data that is not covered by the UDP checksum.

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                   | Kind=5 | Len=4  |      Offset     |

                      Figure 9 UDP LITE option format

   At the sender, the option is formed using the following steps:

   1. Create a LITE option, ordered as the first UDP option (Figure

   2. Calculate the location of the start of the options as an absolute
      offset from the start of the UDP header and place that length in
      the last two bytes of the LITE option.

   3. Swap all four bytes of the LITE option with the first 4 bytes of
      the LITE data area (Figure 11).

      | UDP Hdr |  user data   |  LITE data   |LITE| other opts  |
              UDP Length

            Figure 10   LITE option formation - LITE goes first

      | UDP Hdr |  user data   |  LITE data   |LITE| other opts  |
                                ^^^^           ^^^^
                                  |              |

    Figure 11   Before sending swap LITE option and front of LITE data

   The resulting packet has the format shown in Figure 12. Note that
   the UDP length now points to the LITE option, and the LITE option
   points to the start of the option area.

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      | UDP Hdr |  user data   |LITE| LITE data |Ldat| other opts  |
       <---------------------->    |             ^
              UDP Length           +-------------+

                      Figure 12   Lite option as sent

   A legacy endpoint receiving this packet will discard the LITE option
   and everything that follows, including the lite data and remainder
   of the UDP options. The UDP checksum will protect only the user
   data, not the LITE option or lite data.

   Receiving endpoints capable of processing UDP options will do the

   1. Process options as usual. This will start at the LITE option.

   2. When the LITE option is encountered, record its location as the
      start of the LITE data area and swap the four bytes there with
      the four bytes at the location indicated inside the LITE option,
      which indicates the start of all of the options, including the
      LITE one (one past the end of the lite data area). This restores
      the format of the option as per Figure 10.

   3. Continue processing the remainder of the options, which are now
      in the format shown in Figure 11.

   The purpose of this swap is to support the equivalent of UDP Lite
   operation together with other UDP options without requiring the
   entire LITE data area to be moved after the UDP option area.

5.6. Maximum Segment Size (MSS)

   The Maximum Segment Size (MSS, Kind = 3) is a 16-bit indicator of
   the largest UDP segment that can be received. As with the TCP 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].

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                   | Kind=5 | Len=4  |    MSS size     |

                     Figure 13   UDP MSS option format

   The UDP MSS option MAY be used for path MTU discovery
   [RFC1191][RFC1981], 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 to limit the largest reassembled UDP message,
   e.g., when EMTU_R is larger than the required minimums (576 for IPv4
   [RFC791] and 1500 for IPv6 [RFC2460]).

5.7. Timestamps (TIME)

   The UDP Timestamp option (TIME) exchanges two four-byte 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=6 | Len=10 |     TS Value     |   TS Echo Reply  |
          1 byte   1 byte       4 bytes            4 bytes

                    Figure 14   UDP TIME option format

   TS Value (TSval) and TS Echo (TSecr) are used in a similar manner to
   the TCP TS option [RFC7323]. A host using the Timestamp option sets
   TS Value on all UDP segments issued. Received TSval values are
   provided to the application, which passes this value as TSecr on UDP
   messages sent in response to such a message.

   >> UDP 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.

   >> UDP SHOULD make this RTT estimate available to the user

5.8. Fragmentation (FRAG)

   The Fragmentation option (FRAG) supports UDP fragmentation and
   reassembly, which can be used to transfer UDP messages larger than
   limited by the IP receive MTU (EMTU_R [RFC1122]). It is typically

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   used with the UDP MSS option to enable more efficient use of large
   messages, both at the UDP and IP layers. FRAG is designed similar to
   the IPv6 Fragmentation Header [RFC2460], 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.

                   | Kind=8 | Len=8  |  Frag. Offset   |
                   |          Identification           |

              Figure 15   UDP non-terminal FRAG option format

   The FRAG option also lacks a "more" bit, zeroed for the terminal
   fragment of a set. This is possible because the terminal FRAG option
   is indicated as a longer, 12-byte variant, which includes an
   Internet checksum over the reassembled payload (omitting the IP
   pseudoheader and UDP header, as well as UDP options), as shown in
   Figure 16.

   >> The reassembly checksum SHOULD be used, but MAY be unused in the
   same situations when the UDP checksum is unused (e.g., for transit
   tunnels or applications that have their own integrity checks
   [RFC2460]), and by the same mechanism (set the field to 0x0000).

                   | Kind=8 | Len=12 |  Frag. Offset   |
                   |          Identification           |
                   |    Checksum     |

                Figure 16   UDP terminal FRAG option format

   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 Len field indicates whether there are more
   fragments (Len=8) 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 [RFC2460].

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   >> UDP fragments MUST NOT overlap.

   FRAG needs to be used with extreme care because it will present
   incorrect datagram boundaries to a legacy receiver, unless encoded
   as LITE data (see Section 5.8.1).

   >> A host SHOULD indicate FRAG support by transmitting an
   unfragmented datagram using the Fragmentation option (e.g., with
   Offset zero and length 12, i.e., including the checksum area),
   except when encoded as LITE.

   >> A host MUST NOT transmit a UDP fragment before receiving recent
   confirmation from the remote host, except when FRAG is encoded as

   UDP fragmentation relies on a fragment expiration timer, which can
   be preset or could use a value computed using the UDP Timestamp

   >> The default UDP reassembly SHOULD be no more than 2 minutes.

   Implementers are advised to limit the space available for UDP

   >> UDP reassembly space SHOULD be limited to reduce the impact of
   DOS attacks on resource use.

   >> UDP reassembly space limits SHOULD NOT be implemented as an
   aggregate, 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

   Any additional UDP options would follow the FRAG option in the final
   fragment, and would be included in the reassembled packet.
   Processing of those options would commence after reassembly.

   >> UDP options MUST NOT follow the FRAG header in non-terminal
   fragments. Any data following the FRAG header in non-terminal
   fragments MUST be silently dropped. All other options that apply to
   a reassembled packet MUST follow the FRAG header in the terminal

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5.8.1. Coupling FRAG with LITE

   FRAG can be coupled with LITE to avoid impacting legacy receivers.
   Each fragment is sent as LITE un-checksummed data, where each UDP
   packet contains no legacy-compatible data. Legacy receivers
   interpret these as zero-payload packets, which would not affect the
   receiver unless the presence of the packet itself were a signal. The
   header of such a packet would appear as shown in Figure 17 and
   Figure 18.

                 | UDP Hdr |   LiteFrag   |LITE|FRAG|
                  <-------> ^^^^           ^^^^
            Zero UDP Length  |              |

                 Figure 17   Preparing  FRAG as Lite data

                 | UDP Hdr |LITE|LiteFrag |FRAG|
                  <------->  |             ^
            Zero UDP Length  |             |

                Figure 18   Lite option before transmission

   When a packet is reassembled, it appears as a complete LITE data
   region. The UDP header of the reassembled packet is adjusted
   accordingly, so that the reassembled region now appears as
   conventional UDP user data, and processing of the UDP options
   continues, as with the non-LITE FRAG variant.

5.9. Authentication and Encryption (AE)

   The Authentication and Encryption option (AE) is intended to allow
   UDP to provide a similar type of authentication as the TCP
   Authentication Option (TCP-AO) [RFC5925]. It uses the same format as
   specified for TCP-AO, except that it uses a Kind of 8. UDP-AO
   supports NAT traversal in a similar manner as TCP-AO [RFC6978]. UDP-
   AO can also be extended to provide a similar encryption capability
   as TCP-AO-ENC, in a similar manner [To17ao]. For these reasons, the
   option is known as UDP-AE.

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   Like TCP-AO, UDP-AE 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 UDP-AE's Key Derivation Function (KDF) uses
   zeroes as the value for both ISNs. This means that the UDP-AE reuses
   keys when socket pairs are reused, unlike TCP-AO.

5.10. Experimental (EXP)

   The Experimental option (EXP) 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].

   >> 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]).

6. UDP API Extensions

   UDP currently specifies an application programmer interface (API),
   summarized as follows (with Unix-style command as an example)

   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

       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:

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   o  Extend the method to create receive ports to include receive
      options that are required. Datagrams not containing these
      required options MUST be silently dropped and MAY be logged.

   o  Extend the receive function to indicate the options and their
      parameters as received with the corresponding received datagram.

   o  Extend the send function to indicate the options to be added to
      the corresponding sent datagram.

   Examples of API instances for Linux and FreeBSD are provided in
   Appendix A, to encourage uniform cross-platform implementations.

7. Whose options are these?

   UDP options are indicated in an 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

   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 datagrams
   are not intended to be modified in-transit. However, the UDP option
   mechanism provides no specific protection against in-transit
   modification of the UDP header, UDP payload, or UDP option area,
   except as provided by the options selected (e.g., OCS, ACS, or AE).

8. UDP options vs. UDP-Lite

   UDP-Lite provides partial checksum coverage, so that 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

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   interpret the UDP Length field as the prefix covered by the UDP

   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 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 options support a similar service to UDP-Lite by terminating the
   UDP options with an EOL option. The additional data not covered by
   the UDP checksum follows that EOL option, and is passed to the user
   separately. The difference is that UDP-Lite provides the un-
   checksummed user data to the application by default, whereas UDP
   options can provide the same capability only for endpoints that are
   negotiated in advance (i.e., by default, UDP options would silently
   discard this non-checksummed data). Additionally, in UDP-Lite the
   checksummed and non-checksummed payload components are adjacent,
   whereas in UDP options they are separated by the option area -
   which, minimally, must consist of at least one EOL option.

   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 separate 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.

9. 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 UDP OCS provides protection
   against such events and is stronger than a static "magic number".

   UDP options have been tested as interoperable with Linux, Max OS-X,
   and Windows Cygwin, and worked through NAT devices. These systems

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   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 payload.
   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.

   (TBD: test with UDP checksum offload and UDP fragmentation offload)

10. 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.

   >> UDP options MUST allow for silent failure on first receipt.

   >> UDP options that rely on soft-state exchange MUST allow for
   message reordering and loss.

   >> A UDP option MUST be silently optional until confirmed by
   exchange with an endpoint.

   The above requirements prevent using any option that cannot be
   safely ignored unless that capability has been negotiated with an
   endpoint in advance for a socket pair. Legacy systems would need to
   be able to interpret the transport payload fragments as individual
   transport datagrams.

11. UDP Option State Caching

   Some TCP connection parameters, stored in the TCP Control Block, can
   be usefully shared either among concurrent connections or between

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   connections in sequence, known as TCP Sharing [RFC2140][To17cb].
   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.

   [TBD: extend this section to indicate which options MAY vs. MUST NOT
   be shared and how, e.g., along the lines of To17cb]

   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.

   o  This document extends the UDP API to support the use of options.

12. Security Considerations

   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 [RFC5246] (transport layer
   security, for TCP) nor DTLS [RFC6347] (TLS for UDP) protect the
   transport layer. Both operate as a shim layer solely on the payload
   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 Extension option (Section 5.9). 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. Implementations concerned with the
   potential for this vulnerability MAY implement only the required
   options and MAY also limit NOPs (e.g., no more than three
   consecutive NOPs or some total number that might occur between the
   required options, if all are present). Because the required options

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   come first and at most once each (and all later duplicates silently
   ignored), this limits the DOS impact.

13. 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 5.
   Additional values in this registry are to be assigned by IESG
   Approval or Standards Action [RFC5226].

   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]. This registry is
   initially empty. Values in this registry are to be assigned by IANA
   using first-come, first-served (FCFS) rules [RFC5226].

14. References

14.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC768]  Postel, J., "User Datagram Protocol", RFC 768, August

   [RFC791]  Postel, J., "Internet Protocol," RFC 791, Sept. 1981.

14.2. Informative References

   [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.

   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts --
             Communication Layers," RFC 1122, Oct. 1989.

   [RFC1191] Mogul, J., S. Deering, "Path MTU discovery," RFC 1191,
             November 1990.

   [RFC1981] McCann, J., S. Deering, J. Mogul, "Path MTU Discovery for
             IP version 6," RFC 1981, Aug. 1996.

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   [RFC2140] Touch, J., "TCP Control Block Interdependence," RFC 2140,
             Apr. 1997.

   [RFC2460] Deering, S., R. Hinden, "Internet Protocol Version 6
             (IPv6) Specification," RFC 2460, Dec. 1998.

   [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery," RFC
             2923, September 2000.

   [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.

   [RFC4960] Stewart, R. (Ed.), "Stream Control Transmission Protocol",
             RFC 4960, September 2007.

   [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.

   [RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA
             Considerations Section in RFCs," RFC 5226, May 2008.

   [RFC5246] Dierks, T., E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2," RFC 5246, Aug. 2008.

   [RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
             Option," RFC 5925, June 2010.

   [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.

   [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.

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   [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.

   [To17ao]  Touch, J., "A TCP Authentication Option Extension for
             Payload Encryption", draft-touch-tcp-ao-encrypt, Apr.

   [To17cb]  Touch, J., M. Welzl, S. Islam, J. You, "TCP Control Block
             Interdependence," draft-touch-tcpm-2140bis, Jan. 2017.

   [Tr15]    Trammel, B. (Ed.), M. Kuelewind (Ed.), "Requirements for
             the design of a Substrate Protocol for User Datagrams
             (SPUD)," draft-trammell-spud-req-04, May 2016.

15. Acknowledgments

   This work benefitted from feedback from Bob Briscoe, Ken Calvert,
   Ted Faber, Gorry Fairhurst, C. M. Heard (including the FRAG/LITE
   combination), Tom Herbert, and Mark Smith, as well as discussions on
   the IETF TSVWG and SPUD email lists.

   This work is 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
   4676 Admiralty Way
   Marina del Rey, CA 90292 USA

   Phone: +1 (310) 448-9151
   Email: touch@isi.edu

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Appendix A.                 Implementation Information

   The following information is provided to encourage interoperable API

   System-level variables (sysctl):

           Name                   default   meaning
           net.ipv4.udp_opt       0         UDP options available
           net.ipv4.udp_opt_ocs   1         Default include OCS
           net.ipv4.udp_opt_acs   0         Default include ACS
           net.ipv4.udp_opt_lite  0         Default include LITE
           net.ipv4.udp_opt_mss   0         Default include MSS
           net.ipv4.udp_opt_time  0         Default include TIME
           net.ipv4.udp_opt_frag  0         Default include FRAG
           net.ipv4.udp_opt_ae    0         Default include AE

   Socket options (sockopt), cached for outgoing datagrams:

           Name           meaning
           UDP_OPT        Enable UDP options (at all)
           UDP_OPT_OCS    Enable UDP OCS option
           UDP_OPT_ACS    Enable UDP ACS option
           UDP_OPT_LITE   Enable UDP LITE option
           UDP_OPT_MSS    Enable UDP MSS option
           UDP_OPT_TIME   Enable UDP TIME option
           UDP_OPT_FRAG   Enable UDP FRAG option
           UDP_OPT_AE     Enable UDP AE option

   Send/sendto parameters:

   (TBD - currently using cached parameters)

   Connection parameters (per-socketpair cached state, part UCB):

           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

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   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-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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