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--- networking:mtu [2025/10/03 18:48] – aperez
+++ networking:mtu [2025/10/04 15:48] (current) – aperez
@@ Line 156: / Line 156: @@
+----
+> **Note: PMTU and PMTUD (Path MTU / Path MTU Discovery)**
+The *Path Maximum Transmission Unit (PMTU)* is the **largest IP packet size that can travel end-to-end without fragmentation**.
+It is determined by the **smallest MTU along the entire path**.
+Example:
+  * Link 1: MTU = 9000
+  * Link 2: MTU = 1500
+  * Link 3: MTU = 1400
+  → **PMTU = 1400 bytes**
+---
+**Path MTU Discovery (PMTUD):**
+A dynamic mechanism to discover the PMTU using the **DF (Don’t Fragment)** bit.
+Steps:
+  - Source sends a large packet with DF=1.
+  - If a router cannot forward due to its MTU, it discards the packet and replies with ICMP *Fragmentation Needed* (Type 3, Code 4).
+  - The source reduces the size until it learns the **real PMTU**.
+---
+**Verification on Linux**::
+  ping -M do -s 1472 8.8.8.8
+    → If it replies: path supports 1500 (1472+28 headers).
+    → If it fails: PMTU is smaller.
+  tracepath -n 8.8.8.8
+    → Displays estimated PMTU along the route.
+---
+**Common pitfalls**
+  * Blocking ICMP → breaks PMTUD (TCP sessions may hang).
+  * Tunnel/VPN overhead → reduces MTU (e.g., IPSec 1500 → ~1400).
+  * Misconfigured Jumbo Frames → one 1500 hop breaks 9000 end-to-end.
+---
+**In summary:**
+  * **PMTU** = max packet size without fragmentation.
+  * **PMTUD** = process to dynamically discover it using ICMP + DF.
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+{{ :aruba_networks:switch:6400:mtu_pmtu.pdf |}}
+{{pdfjs 46em >:aruba_networks:switch:6400:mtu_pmtu.pdf}}
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