Optional IS-IS Fragment Timestamping and Database Fingerprint

Status of This Memo

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 23 July 2026.¶

Copyright Notice

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.¶

Table of Contents

1. Introduction

Many applications in today’s networks rely on reliable and timely flooding of link-state information, such as, but not limited to Traffic Engineered networks and advanced telemetry solutions. If such information is delayed during flooding it can be difficult for those applications to adequately fulfill their intended purpose. This document describes extensions to ISIS allowing it to carry the origination time on each fragment. The origination time can be used to aid troubleshooting of large domains and/or by the applications themselves to improve their behavior.¶

As an example, in the case of Traffic Engineered Networks synchronization of the Traffic Engineering Database (TED) enables the compute nodes to adapt to changes in the network state and/or react to network events in a timely manner. If link state information is delayed during the flooding process this can result in an unsynchronized TED and easily lead to service degradation due to substandard re-optimization of network load. More specifically, in RSVP-TE networks, a TE path computed using a specific snapshot of the TED may be rejected during signaling by a transit node because of bandwidth unavailability on a specific link (link bandwidth information in the snapshot of TED used during computation may not be “current”). When the ingress is subsequently notified of this “error” via RSVP signaling, the link in question is avoided in the subsequent path computation and an alternate path is sought. An implementation may use a configurable “hold time” to determine how long this link needs to be avoided. The awareness of the distribution delay statistics can be used by implementations to dynamically adapt an appropriate “hold time” for a given TE link (instead of using a blanket topology-wide configuration). Therefore, the origination time proposed in this document is meant to be used by a compute node(s) or by an operator of Traffic Engineered Network to measure any delays incurred in TED synchronization. The awareness of delays in the distribution of information can be incorporated further into algorithms and network tooling to improve the responsiveness and quality of decisions taken.¶

In a similar vein it is important in large networks when they are in a stable state to measure whether all databases have synchronized properly and derive the delays involved in the propagation of information across the network. This document proposes an extension to the yang schema that allows easy access to such information.¶

2. Timestamp TLV

This section defines a new, optional TLV that can be present in any fragment. In case of multiple instances of the TLV in a fragment only the first occurrence MUST be used. The semantics of the TLV is the point in time the fragment with the current sequence number has been generated. Its absence signifies that such information is not available due to host of possible issues, one of them lack of clock with synchronization precise enough.¶

For practical purposes, although desirable, timestamping the moment a fragment is flooded would be preferable but beside practical implementation problems this could generate on different interfaces the same fragment with different content which breaks one of the fundamental tenants of link-state protocols. However, an implementation is free to choose to use, e.g. the moment the fragment is queued for flooding first time rather than the time the version is generated.¶

To save space the timestamp is following semantically NTP seconds epoch [RFC5905] with the exception of an extra bit in the seconds field to extend the wrap around and carrying only 2^-8 of a second as maximum resolution of the timestamp since this is considered sufficient for link-state purposes. The specification follows further guidelines of [RFC8877] as far as possible.¶

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Type      |     Length    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         Seconds                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |H|     Frac      |    Prec     |
  +-+-+-+-+-+-+-+-+---------------+

• Type: TBD1¶
• Length: ...¶
• Seconds: 4 bytes of number of seconds since the NTP [RFC5905] epoch.¶
• H(-Bit): 1 bit. Extra high order bit is used to prevent wrap-around in 2036 and pushes it out to 2242. The offset can be constructed in network order `HB` shifted to left without overflow by 32 bits and the `Seconds` field OR'ed into the according value.¶
• Fraction: 8-bits of fraction of the second in units of 2^-8 which is equivalent to 1/256 of a second or roughly 4 msecs resolution.¶
• Precision: 7 bits indicating the maximum possible slip (either in future or past) of the clock used to generate the timestamp (depending on the synchronization protocol) as 2^Precision where at minimum of the range signifies 2 msec or better precision and the maximum of the range amounts to 256 msec precision or less. A node that cannot achieve this minimum precision required SHOULD NOT advertise the fragment timestamp.¶

Appendix A. Checksum Reference Algorithm

<CODE BEGINS>
    pub(crate) fn lsdb_fingerprint_component(&self) -> u64 {
        let mut result: u64 = 0;
        let system_id: &[u8] = self.lsp_id.system_id.as_ref();
        for i in system_id.iter().chain(&[self.lsp_id.pseudonode]) {
            result <<= 8;
            result ^= *i as u64;
        }
        // fragment checksum
        result ^= (self.cksum as u64) << 48;
        // This is equal to the PDU length advertised by the fragment
        result ^= (self.raw.len() as u64) << 32;
        result
    }
<CODE ENDS>

Authors' Addresses