TSVWG J. Touch Internet Draft Intended status: Standards Track January 19, 2018 Intended updates: 768 Expires: July 2018 Transport Options for UDP draft-ietf-tsvwg-udp-options-02.txt Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. This document may not be modified, and derivative works of it may not be created, except to format it for publication as an RFC or to translate it into languages other than English. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on July 19, 2018. Copyright Notice Copyright (c) 2018 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 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with Touch Expires July 19, 2018 [Page 1] Internet-Draft Transport Options for UDP January 2018 respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Abstract 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...................................................3 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)........................................9 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......................................19 9. Interactions with Legacy Devices..............................19 10. Options in a Stateless, Unreliable Transport Protocol........20 11. UDP Option State Caching.....................................21 12. Multicast Considerations.....................................21 13. Security Considerations......................................21 14. IANA Considerations..........................................22 15. References...................................................22 15.1. Normative References....................................22 15.2. Informative References..................................23 16. Acknowledgments..............................................24 Appendix A. Implementation Information...........................26 Touch Expires July 19, 2018 [Page 2] Internet-Draft Transport Options for UDP January 2018 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 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", "MAY", and "OPTIONAL" in this 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 [RFC793][RFC5925][RFC7323]. 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 [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 as well. One example of such uses is Substrate Protocol for User Datagrams (SPUD) [Tr16], 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 Touch Expires July 19, 2018 [Page 3] Internet-Draft Transport Options for UDP January 2018 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. 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 [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) 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". Touch Expires July 19, 2018 [Page 4] Internet-Draft Transport Options for UDP January 2018 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |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). Touch Expires July 19, 2018 [Page 5] Internet-Draft Transport Options for UDP January 2018 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 Touch Expires July 19, 2018 [Page 6] Internet-Draft Transport Options for UDP January 2018 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. >> Only ACS and AE options depend on the contents of the option area. AE is always computed as if both the AE hash and ACS checksum Touch Expires July 19, 2018 [Page 7] Internet-Draft Transport Options for UDP January 2018 are zero; ACS is computed as if the ACS checksum is zero. Future options MUST NOT be defined as having a value independent of the contents of the option area. Otherwise, interactions between those values, ACS, and UDP-AE could be unpredictable. 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 limited). >> 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 13. 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 space. +--------+ | 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). Touch Expires July 19, 2018 [Page 8] Internet-Draft Transport Options for UDP January 2018 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. +--------+ | 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 assist 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. Touch Expires July 19, 2018 [Page 9] Internet-Draft Transport Options for UDP January 2018 OCS covers the entire UDP option, including the Lite option as formatted before swapping for transmission (or, equivalently, after the swap after reception). >> 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 When ACS is computed, its checksum (CRC) area is zeroed. No other options are zeroed before computing ACS. 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 Touch Expires July 19, 2018 [Page 10] Internet-Draft Transport Options for UDP January 2018 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. +--------+--------+--------+--------+ | 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 10). 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. Touch Expires July 19, 2018 [Page 11] Internet-Draft Transport Options for UDP January 2018 +---------+--------------+----------------+------------------+ | 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 following: 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]. Touch Expires July 19, 2018 [Page 12] Internet-Draft Transport Options for UDP January 2018 +--------+--------+--------+--------+ | Kind=5 | Len=4 | MSS size | +--------+--------+--------+--------+ Figure 13 UDP MSS option format The UDP MSS option MAY be used 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 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 [RFC8200]). 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 application. 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 Touch Expires July 19, 2018 [Page 13] Internet-Draft Transport Options for UDP January 2018 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 [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. +--------+--------+--------+--------+ | 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 [RFC8200]), 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 [RFC8200]. Touch Expires July 19, 2018 [Page 14] Internet-Draft Transport Options for UDP January 2018 >> 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 LITE. 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. Implementers are advised to limit the space available for UDP reassembly. >> 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 options. 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 fragment. Touch Expires July 19, 2018 [Page 15] Internet-Draft Transport Options for UDP January 2018 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 [To18ao]. For these reasons, the option is known as UDP-AE. Touch Expires July 19, 2018 [Page 16] Internet-Draft Transport Options for UDP January 2018 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. UDP-AE can be configured to either include or exclude TCP options, the same way as can TCP-AO. When UDP options are covered, the ACS option area checksum and UDP-AE hash areas are zeroed before computing the AE hash. It is important to consider that options not yet defined might yield unpredictable results if not confirmed as supported, e.g., if they contain other hashes or checksums that depend on the option area contents. Similar to TCP-AO-NAT, UDP-AE can be configured to support NAT traversal, excluding one or both of the UDP ports [RFC6978]. 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) [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 Touch Expires July 19, 2018 [Page 17] Internet-Draft Transport Options for UDP January 2018 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 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 [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 datagrams are not intended to be modified in-transit. However, the UDP option mechanism provides no specific protection against in-transit Touch Expires July 19, 2018 [Page 18] Internet-Draft Transport Options for UDP January 2018 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 interpret the UDP Length field as the prefix covered by the UDP checksum. UDP (protocol 17) already defines the UDP Length field as the limit of the UDP checksum, but by default also limits the data provided to the application as that which precedes the UDP Length. A goal of UDP-Lite is to deliver data beyond UDP Length as a default, which is why a separate transport protocol number was required. UDP options do not 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 Touch Expires July 19, 2018 [Page 19] Internet-Draft Transport Options for UDP January 2018 [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 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 Touch Expires July 19, 2018 [Page 20] Internet-Draft Transport Options for UDP January 2018 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 connections in sequence, known as TCP Sharing [RFC2140][To18cb]. 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 To18cb] 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. 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. 13. 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 Touch Expires July 19, 2018 [Page 21] Internet-Draft Transport Options for UDP January 2018 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 come first and at most once each (and all later duplicates silently ignored), this limits the DOS impact. 14. 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 [RFC8126]. 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 [RFC8126]. 15. References 15.1. 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 1980. [RFC791] Postel, J., "Internet Protocol," RFC 791, Sept. 1981. Touch Expires July 19, 2018 [Page 22] Internet-Draft Transport Options for UDP January 2018 15.2. 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. [RFC2140] Touch, J., "TCP Control Block Interdependence," RFC 2140, Apr. 1997. [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. [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. Touch Expires July 19, 2018 [Page 23] Internet-Draft Transport Options for UDP January 2018 [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. [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. [To18ao] Touch, J., "A TCP Authentication Option Extension for Payload Encryption", draft-touch-tcp-ao-encrypt, Jan. 2018. [To18cb] Touch, J., M. Welzl, S. Islam, J. You, "TCP Control Block Interdependence," draft-touch-tcpm-2140bis, Jan. 2018. [Tr16] 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. 16. 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. Touch Expires July 19, 2018 [Page 24] Internet-Draft Transport Options for UDP January 2018 Authors' Addresses Joe Touch Manhattan Beach, CA 90266 USA Phone: +1 (310) 560-0334 Email: touch@strayalpha.com Touch Expires July 19, 2018 [Page 25] Internet-Draft Transport Options for UDP January 2018 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 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 Touch Expires July 19, 2018 [Page 26] Internet-Draft Transport Options for UDP January 2018 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 Touch Expires July 19, 2018 [Page 27]