[agrawalkaushik]

Summary

Documents the Adaptive Path MTU feature introduced in SSR 7.2.0 (I95-60371), which lets the SSR correct an established flow's MTU in response to an underlay ICMP Fragmentation Needed message, without waiting for periodic Path MTU Discovery (PMTUD) or a session rebuild.

Context

Prior to SSR 7.2.0, a reduced underlay MTU was only discovered on the next periodic PMTUD cycle (every 10 minutes by default). Existing long-lived sessions (database replication, backups, tunnels) kept using a stale MTU until rebuilt, causing drops when a route change or added encapsulation reduced the available path MTU.

SSR 7.2.0 adds two complementary enhancements:

1. Underlay ICMP Reaction — the ingress SSR intercepts the ICMP Fragmentation Needed message, updates the affected flow, and relays a corrected message (accounting for SVR overhead) to the original sender.
2. Session Refresh — the affected flow is updated in place to the new MTU; the session is not rebuilt.

What's Covered

• Hub-and-spoke walkthrough: Client → Spoke SSR → Underlay Device → Hub SSR → Server
• Step-by-step trace of an underlay MTU drop (1500 → 1000), the ICMP response, in-place flow MTU update, and SVR-overhead-corrected client notification
• Scenario contrast: TCP sender-side MSS reduction vs. UDP/DF=0 fabric fragmentation fallback
• Asymmetric path / directional correction behavior
• Configuration reference for enforced-mss automatic and PMTUD interval tuning
• Verification via show peers, show stats icmp flow-mtu-updates, and packet capture guidance
• Troubleshooting section for common failure modes

Verification Done

• Reviewed full diff of docs/config_pmtu.md for formatting, Markdown validity, and Title Case headings
• Confirmed terminology matches RFC 1191 phrasing ("ICMP Destination Unreachable: Fragmentation Needed and DF Set (Type 3, Code 4)", "ICMP Fragmentation Needed") with no internal-only jargon
• Checked for trailing whitespace and tabs — none found
• Validated all 4 Mermaid sequence diagrams with mmdc; each compiles cleanly to SVG with no errors
• Confirmed "SVR" is an established public term used consistently across 90+ existing docs pages

[Chr1st0ph3rTurn3r]

I made some very minor text edits, mostly identifying the whole ICMP Destination Unreachable: message as a message and not just text (this was not initially clear to me).