| Internet-Draft | tcp-ext-opt | May 2025 |
| Bonica & Li | Expires 1 December 2025 | [Page] |
The TCP header can accommodates 40 octets of TCP options. However, modern applications may require more than 40 octets of TCP Options. Therefore, this document describes an experiment that extends the TCP Options field. If this experiment is successful, it will demonstrate the extension procedures described herein are implementable and deployable. It will also demonstrate that they maintain backwards compatibility.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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."¶
This Internet-Draft will expire on 1 December 2025.¶
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.¶
Figure 1 depicts a TCP [RFC9293] segment.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ = | Source Port | Destination Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number | T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ C | Acknowledgment Number | P +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data | |C|E|U|A|P|R|S|F| | H | Offset| Rsrvd |W|C|R|C|S|S|Y|I| Window | E | | |R|E|G|K|H|T|N|N| | A +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ D | Checksum | Urgent Pointer | E +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R | [Options] | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ = | : : Data : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ =
Every TCP segment contains a header. Some TCP segments also contain data.¶
Each field in the TCP header, except for the last, has a fixed length. The fixed length fields in the TCP header occupy 20 octets. One of these fields is called the Data Offset field.¶
The last field in the TCP header is called the Options field. When present, the Options field contains a list of TCP options [TCPOPTS]. Its length varies from 0 to 40 octets.¶
The Data Offset field represents the offset of the Data field in the TCP segment, measured in 4-octet units. The Data Offset field also determines the length of the Options field. This is because the Options field consumes all of the space between the fixed length fields in the TCP header and the Data field.¶
The Data Offset field contains 4 bits. So, its value ranges nominally from 0 to 15. However, the value of the Data Offset field must be 5 or greater. This is because the Data field must follow the fixed length fields in the TCP header. As stated above, those fields occupy 20 octets.¶
Because the value of the Data Offset field cannot exceed 15, the offset of the Data field cannot exceed 60 and the length of the Options field cannot exceed 40 (i.e., 60 minus 20).¶
Modern applications may require more than 40 octets of TCP Options. Therefore, this document describes an experiment that extends the TCP Options field. If this experiment is successful, it will demonstrate the extension procedures described herein are implementable and deployable. It will also demonstrate that they maintain backwards compatibility.¶
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 BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The Data Offset field must have a value of 5 or greater¶
TCP Options can be present only when the Data Offset field has a value greater than 5.¶
This document allows TCP options to be present when the Data Offset field has a value of 0. When the Data Offset field has a value of 0, the format of the TCP Options field is as depicted in Figure 2.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length | : +-+-+-+-+-+-+-+-+ : | Individual Options : : : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length: 8-bit unsigned integer. Represents the length of the TCP Options field, including the length field, measured in 4-octet units.¶
When a TCP segment contains 40 bytes of TCP Options or fewer, it MUST be formatted exactly as described in [RFC9293]. When a TCP segment contains more than 40 bytes of TCP Options, the Data Offset field MUST be equal to 0 and the Options field MUST be formatted as described in this section.¶
As per [RFC9293], checksums are calculated over the entire Options field.¶
Legacy TCP implementations, including middleboxes, cannot parse segments in which the Data Offset is equal to 0. [RFC9293] is silent regarding the required behavior when a TCP implementation receives a segment with Data Offset equal to 0.¶
The following TCP implementations have been tested and are known to discard the segment:¶
Legacy middleboxes, including firewalls, cannot parse segments in which the Data Offset is equal to 0. They are likely to discard packets that they cannot parse.¶
This document does not request any IANA actions.¶
Parties participating in this experiment should publish experimental results within one year of the publication of this document Experimental results should address the following:¶
The authors wish to acknowledge Keshwan Hamlin, Jordan Head, and Joe Touch for their review and helpful comments.¶