Network Service Header (NSH) Encapsulation for In-situ OAM (IOAM) Data

In-situ OAM (IOAM), as defined in , records OAM information within the packet while the packet traverses a particular network domain. The term "in-situ" refers to the fact that the OAM data is added to the data packets rather than is being sent within packets specifically dedicated to OAM. This document defines how IOAM data fields are transported as part of the Network Service Header (NSH) encapsulation for the Service Function Chaining (SFC) . The IOAM-Data-Fields are defined in . An implementation of IOAM which leverages NSH to carry the IOAM data is available from the FD.io open source software project .

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. Abbreviations used in this document: In-situ Operations, Administration, and Maintenance Network Service Header Operations, Administration, and Maintenance Service Function Chaining Type, Length, Value

The NSH is defined in . IOAM-Data-Fields are carried in NSH using a next protocol header which follows the NSH MD context headers. An IOAM header is added containing the different IOAM-Data-Fields. The IOAM-Data-Fields MUST follow the definitions in . If "proof-of-transit" is used in conjunction with NSH, the implementation of proof of transit MUST follow . In an administrative domain where IOAM is used, insertion of the IOAM header in NSH is enabled at the NSH tunnel endpoints, which also serve as IOAM encapsulating/decapsulating nodes by means of configuration.

The NSH header and fields are defined in . The "NSH Next Protocol" value (referred to as "NP" in the diagram above) is TBD_IOAM. The IOAM related fields in NSH are defined as follows: 8-bit field defining the IOAM-Option-Type, as defined in the IOAM Option-Type Registry (see Section 7.2 of ). 8 bit Length field contains the length of the IOAM header in 4-octet units. Reserved bits are present for future use. The reserved bits MUST be set to 0x0 upon transmission and ignored upon receipt. 8-bit unsigned integer that determines the type of header following IOAM. The semantics of this field are identical to the Next Protocol field in . IOAM-Option-Type and IOAM-Data-Field as specified by the IOAM-Type field are present (see Section 4 of ). Multiple IOAM-Option-Types MAY be included within the NSH encapsulation. For example, if a NSH encapsulation contains two IOAM-Option-Types before a data payload, the Next Protocol field of the first IOAM option will contain the value of TBD_IOAM, while the Next Protocol field of the second IOAM-Option-Type will contain the "NSH Next Protocol" number indicating the type of the data payload.

This section summarizes a set of considerations on the overall approach taken for IOAM data encapsulation in NSH, as well as deployment considerations.

This section discusses several approaches for encapsulating IOAM-Data-Fields in NSH and presents the rationale for the approach chosen in this document. An encapsulation of IOAM-Data-Fields in NSH should be friendly to an implementation in both hardware as well as software forwarders and support a wide range of deployment cases, including large networks that desire to leverage multiple IOAM-Data-Fields at the same time. Hardware and software friendly implementation: Hardware forwarders benefit from an encapsulation that minimizes iterative look-ups of fields within the packet: Any operation which looks up the value of a field within the packet, based on which another lookup is performed, consumes additional gates and time in an implementation - both of which are desired to be kept to a minimum. This means that flat TLV structures are to be preferred over nested TLV structures. IOAM-Data-Fields are grouped into several categories, including trace, proof-of-transit, and edge-to-edge. Each of these options defines a TLV structure. A hardware-friendly encapsulation approach avoids grouping these three option categories into yet another TLV structure, but would rather carry the options as a serial sequence. Total length of the IOAM-Data-Fields: The total length of IOAM-Data-Fields can grow quite large in case multiple different IOAM-Data-Fields are used and large path-lengths need to be considered. If for example an operator would consider using the IOAM Trace Option-Type and capture node-id, app_data, egress/ingress interface-id, timestamp seconds, timestamps nanoseconds at every hop, then a total of 20 octets would be added to the packet at every hop. In case this particular deployment would have a maximum path length of 15 hops in the IOAM domain, then a maximum of 300 octets were to be encapsulated in the packet. Different approaches for encapsulating IOAM-Data-Fields in NSH could be considered: Encapsulation of IOAM-Data-Fields as "NSH MD Type 2" (see , Section 2.5). Each IOAM-Option-Type (e.g. trace, proof-of-transit, and edge-to-edge) would be specified by a type, with the different IOAM-Data-Fields being TLVs within this the particular option type. NSH MD Type 2 offers support for variable length meta-data. The length field is 6-bits, resulting in a maximum of 256 (2^6 x 4) octets. Encapsulation of IOAM-Data-Fields using the "Next Protocol" field. Each IOAM-Option-Type (e.g trace, proof-of-transit, and edge-to-edge) would be specified by its own "next protocol". Encapsulation of IOAM-Data-Fields using the "Next Protocol" field. A single NSH protocol type code point would be allocated for IOAM. A "sub-type" field would then specify what IOAM options type (trace, proof-of-transit, edge-to-edge) is carried. The third option has been chosen here. This option avoids the additional layer of TLV nesting that the use of NSH MD Type 2 would result in. In addition, this option does not constrain IOAM data to a maximum of 256 octets, thus allowing support for very large deployments.

defines an "O bit" for OAM packets. Per the O bit must be set for OAM packets and must not be set for non-OAM packets. Packets with IOAM data included MUST follow this definition, i.e. the O bit MUST NOT be set for regular customer traffic which also carries IOAM data and the O bit MUST be set for OAM packets which carry only IOAM data without any regular data payload.

IANA is requested to allocate protocol numbers for the following "NSH Next Protocol" related to IOAM:

IOAM is considered a "per domain" feature, where one or several operators decide on leveraging and configuring IOAM according to their needs. Still, operators need to properly secure the IOAM domain to avoid malicious configuration and use, which could include injecting malicious IOAM packets into a domain. For additional IOAM related security considerations, see Section 8 in . For proof of transit related security considerations, see Section 7 in .

The authors would like to thank Eric Vyncke, Nalini Elkins, Srihari Raghavan, Ranganathan T S, Karthik Babu Harichandra Babu, Akshaya Nadahalli, Stefano Previdi, Hemant Singh, Erik Nordmark, LJ Wobker, and Andrew Yourtchenko for the comments and advice.

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