| Internet-Draft | IOAM IPv6 Support | August 2026 |
| Song, et al. | Expires 4 February 2027 | [Page] |
IOAM pre-allocated trace option data fields can be encapsulated in the IPv6 Hop-by-Hop (HbH) Options header as described in RFC 9486. However, due to the potentially large size of the trace data and the location of the HbH Options header in the IPv6 packet, this scheme creates practical challenges for implementation, especially when other extension headers, such as a routing header, are also present and require on-path processing. In addition to IOAM Direct Export (DEX), this document proposes two alternative approaches to address this challenge: separating the IOAM incremental trace data from the IOAM instruction header, or applying the segment IOAM trace data export scheme, depending on the network scenario and application requirements. We discuss the pros and cons of each approach.¶
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 BCP 14 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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 4 February 2027.¶
Copyright (c) 2026 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.¶
In-situ OAM (IOAM) [RFC9197] defines two trace options, the pre-allocated trace option and the incremental trace option, which record hop-by-hop data along a packet's forwarding path. [RFC9486] describes a method to encapsulate IOAM pre-allocated trace option data fields in IPv6. Because the trace options require per-hop processing, such options can only be encapsulated in the IPv6 Hop-by-Hop (HbH) Options header.¶
[RFC8200] mandates that the HbH Options header, if present, must be the first extension header following the IPv6 header. However, the IOAM trace data can be large, amounting to tens or hundreds of bytes, which makes it difficult or even impossible for some routers to access the headers that follow it. There are practical limitations on how far forwarding hardware can reach into a packet. The IOAM trace option cannot be applied if it makes other extension headers inaccessible. Even if the other headers can be reached, the deeper they are, the higher the cost to access and process them, and the lower the forwarding performance. Note that [RFC9486] does not support the incremental trace option, because it would expand the HbH header at each hop and push back all the headers that follow it. The changing location of the later extension headers could further complicate the hardware implementation and degrade forwarding performance.¶
The issue becomes more severe when SRv6 and IOAM coexist. The Segment Routing Extension Header (SRH) [RFC8754] is encapsulated in a routing header, which follows the HbH Options header. The SRH itself can be large, and it requires read and write operations at each SRv6 segment endpoint node. If it is deeply embedded in a packet and its location keeps shifting, either it is beyond the reach of the hardware or the forwarding performance degrades.¶
We can avoid the problem by not using both at the same time, but this is not ideal, because IOAM is an important OAM tool and it is even more desirable when SRv6 introduces additional operational complexity into IPv6 networks.¶
A second recourse is to limit IOAM to SRv6 nodes only. That is, to consider SRv6 as an overlay tunnel over IPv6 and apply the IOAM pipe mode as discussed in [I-D.song-ippm-ioam-tunnel-mode], which only collects data at each SRv6 segment endpoint node. To realize this, [I-D.ali-spring-ioam-srv6] describes an approach that encapsulates the IOAM option data fields in an SRH TLV, and [RFC9259] describes another approach that enables postcard-based telemetry for SRv6 without needing IOAM option encapsulation. In either case, the SRH is close to the front of the packet and its location is fixed. While these approaches are useful for use cases that only need to monitor the segment endpoints, they fail to cover all the IPv6 nodes on the packet forwarding path in an IOAM domain.¶
The proposition of this draft is as follows: if we need to apply IOAM on all nodes in an SRv6 network, how can we amend the approach in [RFC9486] or use alternative approaches to circumvent the aforementioned issues? In this draft, we propose two viable approaches: (1) separating the IOAM trace data from the instruction header into a different extension header option placed after the routing header (if one exists), and (2) applying the segment IOAM trace export scheme. We discuss the pros and cons of each approach.¶
The IOAM trace-type data fields contain two parts: the instruction and the trace data. Although by convention the trace data part immediately follows the instruction part, there is no fundamental reason why these two parts must be kept together. This observation provides an optimization opportunity to amend the original proposal in [RFC9486].¶
We separate the IOAM trace-type data fields into the instruction part and the trace data part. We encapsulate only the instruction part in the HbH Options header, and encapsulate the trace data part in another extension header option placed after all the IPv6 extension headers that need to be examined and processed on the packet forwarding path (e.g., a routing header). This arrangement allows us to use the incremental trace option efficiently. Even if the trace data increases in size at each node, all IPv6 extension headers before it remain a fixed size, and new data is guaranteed to be inserted at a fixed location.¶
Figure 1 shows the HbH option format for the IOAM incremental trace type instruction. The field specification is identical to that in [RFC8200] and [RFC9197].¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Reserved | IOAM Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<--- | Namespace-ID |NodeLen | Flags | RemainingLen| IOAM +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Trace | IOAM-Trace-Type | Reserved | Type +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<Inst.
Figure 2 shows the TLV option format for the IOAM trace type data. The IOAM trace type data format is compliant with [RFC9197].¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IOAM Type | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ IOAM Trace Type Data ~ ~ ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There are basically two methods to encapsulate the IOAM incremental trace data. First, we can define a new IPv6 extension header dedicated to metadata. Once standardized, this extension header could also be used to host potential metadata from other applications, such as NSH for SFC [RFC8300]. Second, this option can be carried as a TLV option in an existing extension header, such as the Destination Options header (DOH). The only requirement is that this extension header be the last one in the extension header chain. The first method is cleaner, but it requires standardizing a new extension header type; the second method is simpler, but it needs to overcome the access constraints imposed by [RFC8200].¶
If the overhead of the IOAM trace-type data fields is under control, we may still manage to encapsulate both the instruction and the data in the HbH Options header as described in [RFC9486]. To this end, we introduce two sub-approaches.¶
[I-D.song-ippm-segment-ioam] proposes an enhancement to the IOAM trace type that can configure the allowable overhead of the IOAM trace-type data fields. Once the trace data size reaches the limit at a network node (i.e., a segment, or a fixed number of network nodes, has been traversed), the trace data is stripped and exported so that room is made to accommodate new trace data from nodes in the next segment of the forwarding path.¶
This approach requires some moderate updates to the IOAM trace-type data fields, as described in [I-D.song-ippm-segment-ioam]. Figure 3 shows the format of the HbH Option header containing segment IOAM trace-type data fields. A flag bit (#23) in the Flags field is used to indicate that the current header is a segment IOAM header. In this context, the last octet in the IOAM header is partitioned into two 4-bit nibbles. The first nibble (SSize) is used to save the segment size, and the second nibble (RHop) is used to save the remaining hops. This limits the maximum segment size to 15.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Reserved | IOAM Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<------+ | Namespace-ID |NodeLen|Flags|1| SSize | RHop | IOAM +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Segment | IOAM-Trace-Type | Reserved | Trace +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type | | Data | Node Data List [] | Fields | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<------+
At the beginning of each segment, the segment size (SSize) and the remaining hops (RHop) are initialized: RHop is set equal to SSize. At each hop, if RHop is not zero, the node data is added to the node data list and then RHop is decremented by 1. If RHop is equal to 0 when the packet is received, the node needs to remove (in the incremental trace option) or clear (in the pre-allocated trace option) the IOAM node data list and reset RHop to SSize.¶
In this case, if we use the IOAM pre-allocated trace type, the size and location of each IPv6 extension header are fixed and predictable, and the hardware capability and performance can be guaranteed.¶
Whenever a packet with the IOAM option reaches an SRv6 segment endpoint node that needs to access the SRH, we can configure the node to immediately export the IOAM trace data accumulated so far. In this case, at each SRv6 segment endpoint node, after the trace data export, the HbH header size is fixed and the header contains an IOAM option with only the instruction part. After the SRH processing, this node can add local IOAM trace data to the HbH option header before forwarding the packet.¶
The incremental trace type is more appropriate than the pre-allocated trace type in this approach, due to the uncertainty in the number of hops between two segment endpoints. In an extreme case where every node is also an SRv6 node, this approach regresses to a per-hop postcard-based telemetry approach such as IOAM DEX, as described in [RFC9326].¶
It is worth noting that, instead of using the IOAM trace options, the IOAM Direct Export (DEX) option type [RFC9326] can be used for fixed and small packet overhead, since it only needs to encapsulate a fixed-size instruction header in the HbH Options header. This scheme is covered in [RFC9486].¶
The following table compares the existing approach (RFC 9486) with the alternative approaches discussed in this draft.¶
+--------------+-------------------------+--------------------------+ | Approach | Pros | Cons | | | | | +--------------+-------------------------+--------------------------+ |IOAM Trace |Comply w/ IOAM Data Spec |Variable, long HbH | |Option in | |header impedes access of | |HbH (RFC9486) | |other extension headers | +--------------+-------------------------+--------------------------+ |IOAM Trace |Fix-size and short HbH |Need extra extension | |Data Separate |header, good for |header option to hold | |and Postpose |accessing other extension|trace data | |(Sec. 2) |headers | | +--------------+-------------------------+--------------------------+ |Segment IOAM |Fix-size and controllable|Need to update IOAM trace | |Data Export |HbH header size |type data field spec. | |(Sec. 3.1) | | | +--------------+-------------------------+--------------------------+ |Trace Export |Can be done through |Specific to SRv6; | |at SRv6 nodes |configuration |No better than IOAM | |(Sec. 3.2) | |DEX in the worst case | +--------------+-------------------------+--------------------------+ |IOAM Direct |Comply w/ IOAM DEX Spec; |Need export data | |Export in HbH |Fix-size and short HbH |correlation, and other | |(RFC9486) | |issues of DEX (RFC9326) | +--------------+-------------------------+--------------------------+
The IOAM trace option is easy to implement and extensible in its data types. Complementary to [RFC9486], the scalable solutions for using it in IPv6 networks discussed in this document can fully realize the benefits of the IOAM trace option while avoiding the potential issues associated with variable and high header overhead.¶
No new security issue is identified other than those for the IOAM trace option [RFC9197], IOAM DEX [RFC9326], and IPv6 extension headers [RFC8200].¶
This document requires no IANA actions.¶