Revision of the tcpControlBits IP Flow Information Export (IPFIX) Information Element

Octets 12 and 13 (counting from zero) of the TCP header encode the data offset (header length) in 4 bits, as well as 12 bits of flags. The least significant 6 bits of these were defined in as URG, ACK, PSH, RST, SYN, and FIN for TCP control. Subsequently, defined the CWR and ECE flags for Explicit Congestion Notification (ECN) negotiation and signaling; additionally defined the NS flag for the ECN Nonce Sum. As defined in the IANA IPFIX Information Element Registry, taken from , the tcpControlBits Information Element for IPFIX only covers the original 6 bits from . To allow IPFIX to be used to measure the use of ECN, and to bring the IPFIX Information Element definition in line with the current definition of the TCP Flags header field, it is necessary to revise this definition. The revised definition of the Information Element in was developed and approved through the IE-DOCTORS process in August 2013. Section 5.1 of states "This process should not in any way be construed as allowing the IE-DOCTORS to overrule IETF consensus. Specifically, Information Elements in the IANA Information Element registry that were added with IETF consensus require IETF consensus for revision or deprecation". Since the tcpControlBits Information Element was originally defined in , an IETF Proposed Standard, any revision of this Information Element definition requires IETF Consensus. The publication of this document fulfills that requirement. defines the revised tcpControlBits Information Element as in Section 9.1 of .

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 .

6 unsigned16 flags TCP control bits observed for the packets of this Flow. This information is encoded as a bit field; for each TCP control bit, there is a bit in this set. The bit is set to 1 if any observed packet of this Flow has the corresponding TCP control bit set to 1. The bit is cleared to 0 otherwise. The values of each bit are shown below, per the definition of the bits in the TCP header :

MSb LSb 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | | N | C | E | U | A | P | R | S | F | | Zero | Future | S | W | C | R | C | S | S | Y | I | | (Data Offset) | Use | | R | E | G | K | H | T | N | N | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ bit flag value name description ------+-----+------------------------------------- 0x8000 Zero (see tcpHeaderLength) 0x4000 Zero (see tcpHeaderLength) 0x2000 Zero (see tcpHeaderLength) 0x1000 Zero (see tcpHeaderLength) 0x0800 Future Use 0x0400 Future Use 0x0200 Future Use 0x0100 NS ECN Nonce Sum 0x0080 CWR Congestion Window Reduced 0x0040 ECE ECN Echo 0x0020 URG Urgent Pointer field significant 0x0010 ACK Acknowledgment field significant 0x0008 PSH Push Function 0x0004 RST Reset the connection 0x0002 SYN Synchronize sequence numbers 0x0001 FIN No more data from sender As the most significant 4 bits of octets 12 and 13 (counting from zero) of the TCP header are used to encode the TCP data offset (header length), the corresponding bits in this Information Element MUST be exported as zero and MUST be ignored by the collector. Use the tcpHeaderLength Information Element to encode this value. Each of the 3 bits (0x800, 0x400, and 0x200), which are reserved for future use in , SHOULD be exported as observed in the TCP headers of the packets of this Flow. If exported as a single octet with reduced-size encoding, this Information Element covers the low-order octet of this field (i.e, bits 0x80 to 0x01), omitting the ECN Nonce Sum and the three Future Use bits. A collector receiving this Information Element with reduced-size encoding must not assume anything about the content of these four bits. Exporting Processes exporting this Information Element on behalf of a Metering Process that is not capable of observing any of the ECN Nonce Sum or Future Use bits SHOULD use reduced-size encoding, and only export the least significant 8 bits of this Information Element. Note that previous revisions of this Information Element's definition specified that the CWR and ECE bits must be exported as zero, even if observed. Collectors should therefore not assume that a value of zero for these bits in this Information Element indicates the bits were never set in the observed traffic, especially if these bits are zero in every Flow Record sent by a given exporter. 1

IANA has updated the definition of the tcpControlBits Information Element in the IANA IPFIX Information Element Registry to reflect the changes in above, setting the revision to 1 as noted, and the revision date to the date of publication of this document.

This document changes the data type (and therefore the native size) of the tcpControlBits Information Element, from unsigned8 (1 octet) to unsigned16 (2 octets). As Exporting and Collecting Processes use the Information Element Length field in Templates, Options Templates, and specifications for reduced-size encoding where appropriate, as opposed to abstract data type sizes, for encoding and decoding Data Records, it is not expected that this will have any impact on buffer sizing during encoding and decoding. Otherwise, note that the security considerations for IPFIX apply.

Thanks to Andrew Feren, Lothar Braun, Michael Scharf, and Simon Josefsson for comments on the revised definition. This work is partially supported by the European Commission under grant agreement FP7-ICT-318627 mPlane; this does not imply endorsement by the Commission.