Extensions to IOAM Trace Option for Carrying Fixed-Size Data

defines In situ Operations, Administration, and Maintenance (IOAM), which collects operational and telemetry information in the packet while the packet traverses a path between two points in the network. As specified in Section 4.1 of , IOAM tracing is defined as two separate options: Pre-allocated Trace Option and Incremental Trace Option. The two IOAM Trace Options share the same format of IOAM Trace-Option header and data. The IOAM Trace-Option data is composed of a set of node data lists; among them each node data list is populated by a node along the forwarding path of IOAM packet. The IOAM Trace-Option data defined in is a variable-length data, the length of this kind of data varies with the transited IOAM-capable nodes and the selection of data fields processed by each IOAM-capable node. This document extends the IOAM Trace Option to carry a fixed-size data, the length of this kind of data is fixed once the selection of data fields processed by each IOAM-capable node is determined, and doesn't vary with the transited IOAM-capable nodes.

ABW: Available Bandwidth BU: Buffer Utilization CSIG: Congestion Signaling HPCC++: Enhanced High Precision Congestion Control IOAM: In situ Operations, Administration, and Maintenance PD: Per-hop delay TTL: Time to Live

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 when, and only when, they appear in all capitals, as shown here.

As specified in Section 4.4 of , the IOAM Trace Option (including IOAM Pre-allocated Trace Option and IOAM Incremental Trace Option) has a format as shown in Figure 1.

The IOAM Trace Option defined in is used to stack up multiple metadata from multiple IOAM transit nodes. Specifically, along the forwarding path, each IOAM-capable node adds its own operational and telemetry information into the IOAM Trace Option carried by a data packet in a stacking manner. As described in , the IOAM Trace Option Data can be used as a congestion control signal for Enhanced High Precision Congestion Control (HPCC++) congestion control mechanism, and at the same time, Congestion Signaling (CSIG) Data can also be used as a congestion control signal for HPCC++. As specified in , a CSIG tag carries a fixed-size aggregate metric computed over the hop devices. Specifically, along the forwarding path, each CSIG-capable node optionally inputs its own congestion information into the CSIG tag carried by a data packet in a compare-and-replace manner. In different application scenarios, either the IOAM Trace Option defined in or the CSIG tag defined in is applicable, or even both of them can be used concurrently. In other words, they're complementary to each other. Then the question goes to that whether it's possible to integrate them together. That is also the problem this document intends to address.

In order to integrate the variable-size superimposed data and fixed-size aggregate data into IOAM Trace Option, this document defines the reserved Bit 3 of the Flags field of the IOAM Trace Option as shown in Figure 2.

Bit 0 "Overflow" (O-bit): When set, the Overflow flag indicates that there are not enough octets left to record the node data, as defined in Section 4.4.1 of . Bit 1 "Loopback" (L-bit): When set, the Loopback flag triggers the sending of a copy of a packet back towards the source, as defined in Section 3 of . Bit 2 "Active" (A-bit): When set, the Active flag indicates that a packet is an active measurement packet rather than a data packet, as defined in Section 3 of . Bit 3 "Fixed-size" (F-bit): When set, the Fixed-size flag indicates that a fixed-size aggregate data rather than a variable-size superimposed data is carried, as defined in Section 5 of this document. As defined in Section 4.4 of , there are two types of IOAM Trace Option, Pre-allocated Trace-Option and Incremental Trace-Option, the F-bit defined in this document can only be used for Pre-allocated Trace-Option. In other words, if the F-bit is set while the IOAM Trace Option-Type indicates it's an Incremental Trace-Option, then the F-bit MUST be ignored by the receiver.

When the IOAM Trace Option is used to carry fixed-size aggregate data, the format of the IOAM Trace Option is shown in Figure 3.

The same as defined in . The same as defined in . MUST be set to 0. The same as defined in and . This document allocates a single flag as follows: "Fixed-size" (F-bit) (least significant bit). As defined in Section 4 of this document. When set, a fixed-size data space as shown in Figure 3 is carried in the IOAM Trace Option. The same as defined in . MUST be set to 0. The similar as the IOAM Trace-Type defined in Section 4.4.1 of . The only difference with the IOAM Trace-Type is that each bit indicates the presence of a fixed-size 8-octet data space, whose format is dependent on bit position. When Bit 0, 1, 2, or 3 is set, the data format of the Fixed-size Aggregate Data is the same, as shown in Figure 4.

The fields are defined as follows: Signal Value: A 20-bit field. Its meaning depends on bit position. Locator Metadata: A 16-bit field. It's the same as defined in Section 5.5.2.2 of , i.e., it contains Port ID, Device ID, and TTL (also called Hop Limit for IPv6). The 24-bit identifier of IOAM Fixed-Data Trace-Type and corresponding Signal Values are defined as follows: Most significant bit. When set, indicates the Signal Value in the data space is Minimum Available Bandwidth - min(ABW), the same as defined in Section 5.1 of . When set, indicates the Signal Value in the data space is Maximum link utilization - max(U/C) or min(ABW/C), the same as defined in Section 5.2 of . When set, indicates the Signal Value in the data space is Maximum Per-hop Delay - max(PD), the same as defined in Section 5.4 of . When set, indicates the Signal Value in the data space is Maximum Buffer Utilization - max(BU). This field value devided by 0xFFFFF equals the buffer utilization rate of the egress interface from where the packet is forwarded out. Each node along the forwarding path would compare its buffer utilization rate of the egress interface with the calculated buffer utilization rate by the received field value. If a node finds out its buffer utilization rate of the egress interface is bigger, then the node would populate this field with a value of 0xFFFFF multiplied by its buffer utilization rate; If a node finds out its buffer utilization rate of the egress interface is not bigger, then the node would not change this field. Undefined. These values are available for future assignment in the IOAM Fixed-Data Trace-Type Registry. An IOAM encapsulating node MUST set the value of each undefined bit to 0. If an IOAM transit node receives a packet with one or more of these bits set to 1, it MUST ignore these bits and SHOULD log and/or report an error. Reserved; MUST be set to zero upon transmission and be ignored upon receipt. This bit is reserved to allow for future extensions of the IOAM Fixed-Data Trace-Type bit field. Each Fixed-size Aggregate Data is a 8-octet data space. When only one bit within the bitmap of IOAM-Fixed-Data Trace-Type is set, the length of this field is eight octets. When more than one bit within the bitmap of IOAM Fixed-Data Trace-Type is set, the length of this field is equal to the number of set bits multiplied by eight octets.

As discussed in , IOAM is assumed to be deployed in a restricted administrative domain, thus limiting the scope of the threats above and their effect. However, even given this limited scope, security threats should still be considered and mitigated. Security issues discussed in apply to this document.

IANA is requested to allocate the following bit in the "IOAM Trace-Flags" registry as follows: "Fixed-size" (F-bit) This document is specified as the "Reference" in the registry for this bit. Note that bit 0 is the most significant bit in the "IOAM Trace-Flags" registry. This bit was allocated by as the 'Overflow' bit.

IANA is requested to create a new registry "IOAM Fixed-Data Trace-Type" within the defined registry group "In Situ OAM (IOAM)". This registry defines code points for each bit in the 24-bit IOAM Fixed-Data Trace-Type field for the Pre-allocated Trace Option-Type When the F-bit is set. Bits 0-2 are defined in Section 5 of this document: Minimum Available Bandwidth - min(ABW) and Locator Metadata Maximum link utilization - max(U/C) or min(ABW/C) and Locator Metadata Maximum Per-hop Delay - max(PD) and Locator Metadata Maximum Buffer Utilization - max(BU) and Locator Metadata reserved Bits 4-22 are available for assignment via the "IETF Review" process, as per . New registration requests MUST use the following template: desired bit to be allocated in the 24-bit IOAM Fixed-Data Trace-Type field for the Pre-allocated Trace Option-Type when the F-bit is set brief description of the newly registered bit reference to the document that defines the new bit

The authors would like to acknowledge Wei Duan and Jun Feng for their very helpful comments.