Internet Engineering Task Force (IETF) M. Boucadair Request for Comments: 9870 Orange Category: Standards Track T. Reddy.K ISSN: 2070-1721 Nokia September 2025 Export of UDP Options Information in IP Flow Information Export (IPFIX) Abstract This document specifies new IP Flow Information Export (IPFIX) Information Elements for UDP options. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9870. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction 2. Conventions and Definitions 3. UDP Options at a Glance 4. New UDP IPFIX Information Elements 4.1. udpSafeOptions 4.2. udpUnsafeOptions 4.3. udpExID 4.4. udpSafeExIDList 4.5. udpUnsafeExIDList 5. Examples 5.1. Reduced-Size Encoding 5.2. SAFE Experimental Option 5.3. ExIDs and Reduced-Size Encoding 6. Security Considerations 7. IANA Considerations 7.1. IPFIX Information Elements 8. References 8.1. Normative References 8.2. Informative References Acknowledgments Authors' Addresses 1. Introduction IP Flow Information Export (IPFIX) [RFC7011] is a protocol that is widely deployed in networks for traffic management purposes (Section 2 of [RFC6632]). The protocol specifies the encoding of a set of basic data types and how the various Information Elements (IEs) are transmitted. In order to support the export of new Flow- related measurement data, new IEs can be defined and registered in a dedicated IANA registry [IANA-IPFIX] for interoperability. This document specifies new IPFIX Information Elements for UDP options (Section 4). A brief overview of UDP options is provided in Section 3. The IE specified in Section 4.1 uses the new abstract data type ("unsigned256") defined in [RFC9740]. Transport (including MTU) considerations are discussed in Section 10 of [RFC7011]. Examples to illustrate the use of the new IPFIX Information Elements are provided in Section 5. 2. Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. This document uses the IPFIX-specific terminology (e.g., Flow) defined in Section 2 of [RFC7011]. As in the base IPFIX specification [RFC7011], these IPFIX-specific terms have the first letter of a word capitalized. The document adheres to the naming conventions for Information Elements per Section 2.3 of [RFC7012]. Also, this document uses the terms defined in Section 3 of [RFC9868], especially "datagram" and "surplus area". 3. UDP Options at a Glance UDP [RFC0768] does not support an extension mechanism similar to the options supported by other transport protocols, such as TCP [RFC9293], Stream Control Transmission Protocol (SCTP) [RFC9260], or Datagram Congestion Control Protocol (DCCP) [RFC4340]. Such a mechanism can be useful for various applications, e.g., to discover a path MTU or share timestamps. To fill that void, [RFC9868] extends UDP with a mechanism to insert extensions in datagrams. To do so, and unlike the conventional approach that relies upon transport headers, [RFC9868] uses trailers. Concretely, UDP options are placed in the surplus area (that is, the area of an IP payload that follows a UDP packet). See Figure 1. An example of the use of UDP options for Datagram Packetization Layer Path MTU Discovery (DPLPMTUD) is described in [RFC9869]. IP transport payload <-------------------------------------------------> +--------+---------+----------------------+------------------+ | IP Hdr | UDP Hdr | UDP user data | surplus area | +--------+---------+----------------------+------------------+ <------------------------------> UDP Length Figure 1: Surplus Area Sections 4.1 and 4.2 introduce new IEs to export the observed UDP options. UDP options are unambiguously identified by means of a 1-byte field, called "Kind". Options indicated by Kind values in the range 0-191 are called SAFE options. Such options can be silently ignored by legacy receivers because they do not alter the UDP user data (Section 11 of [RFC9868]). SAFE options are exported using the IE defined in Section 4.1. Options indicated by Kind values in the range 192-255 are called UNSAFE options. Such options are not safe for legacy receivers to ignore because they alter the UDP user data (Section 12 of [RFC9868]). UNSAFE options are exported using the IE defined in Section 4.2. UDP options occur per-packet within a Flow and can be inserted at any time in the Flow. [RFC9868] reserves two options for experiments: the Experimental (EXP, Kind=127) option for SAFE options and the UNSAFE Experimental option (UEXP, Kind=254). For both options, Experiment Identifiers (ExIDs) are used to differentiate concurrent use of these options. Known ExIDs are expected to be registered within IANA. Section 4.4 specifies a new IPFIX IE to export observed ExIDs in the EXP options. Also, Section 4.5 specifies a new IPFIX IE to export observed ExIDs in the UEXP options. Only 16-bit ExIDs are supported in [RFC9868]. This document does not intend to elaborate operational guidance/ implications of UDP options. The document focuses exclusively on exporting observed UDP options in datagrams. 4. New UDP IPFIX Information Elements Given the Kind structure of SAFE and UNSAFE UDP options, using one single IE that would multiplex both types of options will limit the benefits of reduced-size encoding in the presence of UNSAFE options. For example, at least 24 octets would be needed to report mandatory SAFE options that are observed in a Flow. In order to use less bits to report observed UDP options, distinct IEs are thus defined to report SAFE (Section 4.1) and UNSAFE (Section 4.2) UDP options. As further detailed in Section 5.1, only one octet is needed to report mandatory SAFE options. 4.1. udpSafeOptions Name: udpSafeOptions ElementID: 525 Description: Observed SAFE UDP options in a Flow. The information is encoded in a set of bit fields. Options are mapped to bits according to their option numbers. UDP option Kind 0 corresponds to the least significant bit in the udpSafeOptions IE, while Kind 191 corresponds to the 65th most significant bit of the IE. The bit is set to 1 if the corresponding SAFE UDP option is observed at least once in the Flow. The bit is set to 0 if the option is never observed in the Flow. The 64 most significant bits MUST be set to 0. The reduced-size encoding per Section 6.2 of [RFC7011] is followed whenever fewer octets are needed to report observed SAFE UDP options. For example, if only option Kinds <= 31 are observed, then the value of the udpSafeOptions IE can be encoded as unsigned32, or if only option Kinds <= 63 are observed, then the value of the udpSafeOptions IE can be encoded as unsigned64. The presence of udpSafeExIDList is an indication that the SAFE Experimental option is observed in a Flow. The presence of udpSafeExIDList takes precedence over setting the corresponding bit in the udpSafeOptions IE for the same Flow. In order to optimize the use of the reduced-size encoding in the presence of udpSafeExIDList IE, the Exporter MUST NOT set the EXP flag of the udpSafeOptions IE that is reported for the same Flow to 1. Abstract Data Type: unsigned256 Data Type Semantics: flags Additional Information: See the "UDP Option Kind Numbers" registry at [UDP_OPTIONS]. See [RFC9868] for more details about UDP options. Reference: RFC 9870 4.2. udpUnsafeOptions Name: udpUnsafeOptions ElementID: 526 Description: Observed UNSAFE UDP options in a Flow. The information is encoded in a set of bit fields. Options are mapped to bits according to their option numbers. UDP option Kind 192 corresponds to the least significant bit in the udpUnsafeOptions IE, while Kind 255 corresponds to the most significant bit of the IE. The bit is set to 1 if the corresponding UNSAFE UDP option is observed at least once in the Flow. The bit is set to 0 if the option is never observed in the Flow. The reduced-size encoding per Section 6.2 of [RFC7011] is followed whenever fewer octets are needed to report observed UNSAFE UDP options. The presence of udpUnsafeExIDList is an indication that the UNSAFE Experimental option is observed in a Flow. The presence of udpUnsafeExIDList takes precedence over setting the corresponding bit in the udpUnsafeOptions IE for the same Flow. In order to optimize the use of the reduced-size encoding in the presence of udpUnsafeExIDList IE, the Exporter MUST NOT set the UEXP flag of the udpUnsafeOptions IE that is reported for the same Flow to 1. Abstract Data Type: unsigned64 Data Type Semantics: flags Additional Information: See the "UDP Option Kind Numbers" registry at [UDP_OPTIONS]. See [RFC9868] for more details about UDP options. Reference: RFC 9870 4.3. udpExID Name: udpExID ElementID: 527 Description: Observed ExID in an Experimental (EXP, Kind=127) option or an UNSAFE Experimental (UEXP, Kind=254) option. A basicList of udpExID is used to report udpSafeExIDList and udpUnsafeExIDList values. Abstract Data Type: unsigned16 Data Type Semantics: identifier Additional Information: See the "TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)" registry at [UDP_ExIDs]. See [RFC9868] for more details about ExIDs. Reference: RFC 9870 4.4. udpSafeExIDList Name: udpSafeExIDList ElementID: 528 Description: Observed ExIDs in the Experimental (EXP, Kind=127) option. A basicList of udpExID Information Elements in which each udpExID Information Element carries the ExID observed in an EXP option. Abstract Data Type: basicList Data Type Semantics: list Additional Information: See the "TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)" registry at [UDP_ExIDs]. See [RFC9868] for more details about ExIDs. Reference: RFC 9870 4.5. udpUnsafeExIDList Name: udpUnsafeExIDList ElementID: 529 Description: Observed ExIDs in the UNSAFE Experimental (UEXP, Kind=254) option. A basicList of udpExID Information Elements in which each udpExID Information Element carries the ExID observed in an UEXP option. Abstract Data Type: basicList Data Type Semantics: list Additional Information: See the "TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)" registry at [UDP_ExIDs]. See [RFC9868] for more details about ExIDs. Reference: RFC 9870 5. Examples 5.1. Reduced-Size Encoding Given the UDP Kind allocation in Section 10 of [RFC9868] and the option mapping defined in Section 4.1 of this document, fewer octets are likely to be used for Flows with mandatory UDP options. Figure 2 shows an example of the Kind/bit mappings in the udpSafeOptions IE for a Flow in which End of Options List (EOL, Kind=0) and Additional Payload Checksum (APC, Kind=2) options are observed. Only the bits that corresponds to EOL and APC options are set to 1. MSB LSB 1 25 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 ... 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+-+-+-+-+ |0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0| |0|0|0|0|0|1|0|1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-++-++-+-+-+-+...+-+-+-+-+-+-+-+-+ Figure 2: An Example of udpSafeOptions IE with EOL and APC Options One octet is sufficient to report these observed options because the leading zeros are dropped per the reduced-size encoding guidance. Concretely, the reported udpSafeOptions IE will be set to 0x05 (Figure 3). MSB LSB 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |0|0|0|0|0|1|0|1| +-+-+-+-+-+-+-+-+ Figure 3: An Example of the Wire udpSafeOptions IE Value with EOL and APC Options 5.2. SAFE Experimental Option Let us now consider a UDP Flow in which SAFE Experimental options are observed. If a udpSafeOptions IE is exported for this Flow, then that IE will have the EXP bit set to 1 (Figure 4). This example does not make any assumption about the presence of other UDP options ("X" can be set to 0 or 1). MSB LSB 12 25 0 1 2 3 ... 7 8 9 0 1 2 3 4 5 6 7 8 9 ... 8 9 0 1 2 3 4 5 +-+-+-+-+...+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+-+-+-+ |X|X|X|X| |X|X|X|X|X|X|X|X|X|X|X|1|X|X| |X|X|X|X|X|X|X| +-+-+-+-+...+-+-+-+-+-+-+-+-++-++-+-+-+-+...+-+-+-+-+-+-+-+ Figure 4: An Example of udpSafeOptions with EXP Option 5.3. ExIDs and Reduced-Size Encoding Now assume that EOL, APC, EXP, and UEXP options are observed in a Flow. Let us also consider that the observed SAFE Experimental options have ExIDs set to 0x9858 and 0xE2D4 and UNSAFE Experimental options have ExIDs set to 0xC3D9 and 0x1234. Figure 5 shows an excerpt of the Data Set encoding with a focus on SAFE Experimental options that have ExIDs. The fields are defined in [RFC6313]. MSB LSB 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 : ... : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 255 | List Length = 9 |semantic=allof | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | udpExID = 527 | Field Length = 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAFE ExID = 0x9858 | SAFE ExID = 0xE2D4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 255 | List Length = 9 |semantic=allof | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | udpExID = 527 | Field Length = 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UNSAFE ExID = 0xC3D9 | UNSAFE ExID = 0x1234 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : ... : Figure 5: Example of UDP Experimental Option ExID IEs Following the guidance in Section 4.1, the reported udpSafeOptions IE will be set to 0x05 even in the presence of EXP options. 6. Security Considerations This document does not introduce new security considerations other than those already discussed in Section 11 of [RFC7011] and Section 8 of [RFC7012]. The reader may refer to Section 24 of [RFC9868] for the security considerations related to UDP options. 7. IANA Considerations 7.1. IPFIX Information Elements IANA has added the following new IEs to the "IPFIX Information Elements" registry under the "IP Flow Information Export (IPFIX) Entities" registry group [IANA-IPFIX]: +===========+===================+=========================+ | ElementID | Name | Reference | +===========+===================+=========================+ | 525 | udpSafeOptions | Section 4.1 of RFC 9870 | +-----------+-------------------+-------------------------+ | 526 | udpUnsafeOptions | Section 4.2 of RFC 9870 | +-----------+-------------------+-------------------------+ | 527 | udpExID | Section 4.3 of RFC 9870 | +-----------+-------------------+-------------------------+ | 528 | udpSafeExIDList | Section 4.4 of RFC 9870 | +-----------+-------------------+-------------------------+ | 529 | udpUnsafeExIDList | Section 4.5 of RFC 9870 | +-----------+-------------------+-------------------------+ Table 1: New IPFIX Information Elements udpSafeOptions uses the abstract data type ("unsigned256") defined in [RFC9740]. 8. References 8.1. Normative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC6313] Claise, B., Dhandapani, G., Aitken, P., and S. Yates, "Export of Structured Data in IP Flow Information Export (IPFIX)", RFC 6313, DOI 10.17487/RFC6313, July 2011, . [RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken, "Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of Flow Information", STD 77, RFC 7011, DOI 10.17487/RFC7011, September 2013, . [RFC7012] Claise, B., Ed. and B. Trammell, Ed., "Information Model for IP Flow Information Export (IPFIX)", RFC 7012, DOI 10.17487/RFC7012, September 2013, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9740] Boucadair, M. and B. Claise, "New IPFIX Information Elements for TCP Options and IPv6 Extension Headers", RFC 9740, DOI 10.17487/RFC9740, March 2025, . [RFC9868] Touch, J. and C. Heard, Ed., "Transport Options for UDP", RFC 9868, DOI 10.17487/RFC9868, September 2025, . 8.2. Informative References [IANA-IPFIX] IANA, "IP Flow Information Export (IPFIX) Entities", . [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, March 2006, . [RFC6632] Ersue, M., Ed. and B. Claise, "An Overview of the IETF Network Management Standards", RFC 6632, DOI 10.17487/RFC6632, June 2012, . [RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260, June 2022, . [RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)", STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022, . [RFC9869] Fairhurst, G. and T. Jones, "Datagram Packetization Layer Path MTU Discovery (DPLPMTUD) for UDP Options", RFC 9869, DOI 10.17487/RFC9869, September 2025, . [UDP_ExIDs] IANA, "TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)", . [UDP_OPTIONS] IANA, "UDP Option Kind Numbers", . Acknowledgments Thanks to Benoît Claise for the discussion on the ordering of IPFIX IEs. Thanks to Paul Aitken for the review and comments. Thanks to Tommy Pauly for the TSVART review, Joe Touch for the INTDIR review, Robert Sparks for the GENART review, Watson Ladd for the SECDIR review, and Jouni Korhonen for the OPSDIR review. Thanks to Thomas Graf for the shepherd review. Thanks to Mahesh Jethanandani for the AD review. Thanks to Éric Vyncke, Roman Danyliw, and Zahed Sarker for the IESG review. Authors' Addresses Mohamed Boucadair Orange 35000 Rennes France Email: mohamed.boucadair@orange.com Tirumaleswar Reddy.K Nokia India Email: kondtir@gmail.com