IS-IS Database Fingerprinting

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Table of Contents

1. Introduction

In large IS-IS networks in stable state it is useful to observe whether all databases have synchronized properly and derive the delays involved in the propagation of information across the network.¶

This document defines a database fingerprinting mechanism that allows for quick validation of database synchronization across all nodes. A node implementing this specification computes a fingerprint over all fragments per level. Alongside the fingerprint itself, the node reports the time elapsed since the fingerprint value last changed, i.e. since the most recent recomputation that produced a value differing from the previous one. This facilitates measurement of flooding behavior across the network.¶

LSPs whose Remaining Lifetime is zero (including purged LSPs being retained for ZeroAgeLifetime [ISO10589]) MUST be excluded from the fingerprint calculation.¶

A reference checksum algorithm for a fragment is given in Appendix A. The overall fingerprint is the XOR of all fragment checksums.¶

2. Requirements Language

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. IS-IS Database Fingerprint TLV

To allow routers to rapidly detect link-state database synchronization differences directly with neighbors without waiting for or exchanging full CSNPs, this document defines an optional top-level IS-IS TLV: the Database Fingerprint TLV.¶

The Database Fingerprint TLV MAY be included in the following PDU types:¶

• Level 1 LAN IS-to-IS Hello PDU [ISO10589]¶
• Level 2 LAN IS-to-IS Hello PDU [ISO10589]¶
• Point-to-Point IS-to-IS Hello PDU [ISO10589]¶
• Complete Sequence Numbers PDU (CSNP) [ISO10589]¶

The format of the Database Fingerprint TLV is shown in Figure 1.¶

 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 (TBA)   | Length (10/14)|     Level     |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Database Fingerprint                      |
|                           (64 bits)                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Last Update (optional, 32 bits)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type:

TBA (1 octet), to be allocated by IANA from the "IS-IS Top-Level TLV Codepoints" registry.¶

Length:

10 or 14 octets. The Length field indicates whether the optional Last Update field is present: 10 octets indicates only the Level, Reserved, and Database Fingerprint fields are present; 14 octets indicates the optional 4-octet (32-bit) Last Update field is also present.¶

Level:

1 octet indicating the IS-IS level for the fingerprint: 1 for Level 1, or 2 for Level 2. Values other than 1 and 2 are reserved and MUST be ignored on receipt. In LAN Hello PDUs, the Level field MUST match the level of the LAN Hello PDU. In Point-to-Point Hello PDUs, a router MAY include up to two instances of this TLV (one for Level 1 and one for Level 2).¶

Reserved:

1 octet. MUST be set to 0 on transmission and ignored on receipt.¶

Database Fingerprint:

64-bit (8 octets) fingerprint for the specified level in network byte order, computed as defined in this document.¶

Last Update:

Optional 32-bit (4 octets) unsigned integer in network byte order indicating the time elapsed, in seconds, since the fingerprint value for this level's LSDB was last changed. This provides the in-band equivalent of the YANG last-update leaf. Its presence is indicated by the TLV Length being set to 14.¶

When a router receives an IS-IS Hello PDU containing a Database Fingerprint TLV for a given level, it MAY compare the received fingerprint value against its locally computed fingerprint for that level. If the fingerprints match, the link-state databases for that level are verified to be synchronized. If the fingerprints differ, normal IS-IS database synchronization procedures (e.g., CSNP and PSNP exchanges) continue as specified in [ISO10589].¶

5. Operational Considerations

The database fingerprint mechanism and the reported elapsed time (last-update) are intended to observe general database synchronization and propagation delays across the network in stable state. Measurement of flooding diffusion or synchronization events at sub-second granularity (under 1 second) is not intended by this specification.¶

The database fingerprint mechanism assumes that nodes within the same flooding domain synchronize identical sets of Link State PDUs (LSPs). If LSP flooding is filtered or constrained for any reason (for example, due to policy-based flooding filters, mesh groups, or topology reduction mechanisms), the link-state databases on nodes separated by such filtering will not contain the same set of LSPs.¶

Consequently, the computed fingerprint values will not be the same on both sides of the filtering boundary. Operators and network management systems must take any intentional flooding asymmetry or filtering into account when comparing database fingerprints between nodes, as a mismatch under these conditions does not indicate a protocol failure or synchronization defect.¶

6. IANA Considerations

   Value:     TBA
   Name:      Database Fingerprint
   IIH:       Y
   LSP:       N
   SNP:       Y
   Purge:     N
   Reference: RFC XXXX

   URI: urn:ietf:params:xml:ns:yang:ietf-isis-db-fingerprint
   Registrant Contact: The IESG.
   XML: N/A, the requested URI is an XML namespace.

   Name:      ietf-isis-db-fingerprint
   Namespace: urn:ietf:params:xml:ns:yang:ietf-isis-db-fingerprint
   Prefix:    isis-db-fp
   Reference: RFC XXXX

7. Security Considerations

The fingerprint mechanism defined in this document is purely informational and does not influence protocol behavior. It does not introduce new attack vectors beyond those inherent in IS-IS itself. The fingerprint value is derived from information already present in the LSDB and does not leak additional data.¶

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();
        // The LSP ID (system ID + pseudonode ID + fragment number)
        // MUST be folded in full. Omitting the fragment number byte
        // would make every fragment of a given system/pseudonode
        // contribute the same value here, leaving the checksum and
        // length fields below as the only differentiators between
        // fragments; two fragments whose checksum and length happen
        // to agree (or whose combined XOR happens to cancel across
        // the LSDB) would then be indistinguishable to the
        // fingerprint, silently defeating its purpose.
        for i in system_id
            .iter()
            .chain(&[self.lsp_id.pseudonode, self.lsp_id.fragment])
        {
            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>

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