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--- networking:mtu [2025/10/03 19:20] – aperez
+++ networking:mtu [2025/10/04 15:48] (current) – aperez
@@ Line 158: / Line 158: @@
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-===== PMTU and PMTUD =====
+> **Note: PMTU and PMTUD (Path MTU / Path MTU Discovery)**
-**Path MTU (PMTU)**
+The *Path Maximum Transmission Unit (PMTU)* is the **largest IP packet size that can travel end-to-end without fragmentation**.
-The *Path Maximum Transmission Unit* is the **largest IP packet size that can travel between two hosts along the entire path without being fragmented**.
+It is determined by the **smallest MTU along the entire path**.
-The PMTU is determined by the **smallest MTU of all the links** along the end-to-end path.
 Example:
@@ Line 168: / Line 167: @@
   * Link 2: MTU = 1500
   * Link 3: MTU = 1400
-  **PMTU = 1400 bytes**
+  → **PMTU = 1400 bytes**
 ---
-**Path MTU Discovery (PMTUD)**
+**Path MTU Discovery (PMTUD):**
-This is the **mechanism that automatically discovers the PMTU**.
+A dynamic mechanism to discover the PMTU using the **DF (Don’t Fragment)** bit.
-It uses the **DF (Don’t Fragment)** bit in IP packets:
-  - The source host sends a large packet with DF=1.
+Steps:
-  - If an intermediate router cannot forward it due to its MTU, it drops the packet and replies with an ICMP *Fragmentation Needed* (Type 3, Code 4).
+  - Source sends a large packet with DF=1.
-  - The source reduces the size and retries until it learns the **real PMTU**.
+  - 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 example on Linux**::
+**Verification on Linux**::
   ping -M do -s 1472 8.8.8.8
-    → If it replies: the path supports 1500 (1472+28 headers).
+    → If it replies: path supports 1500 (1472+28 headers).
-    → If it fails: the PMTU is smaller, reduce the size.
+    → If it fails: PMTU is smaller.
   tracepath -n 8.8.8.8
-    → Displays the estimated PMTU along the route.
+    → Displays estimated PMTU along the route.
 ---
-**Common issues**
+**Common pitfalls**
-  * ICMP blocked → breaks PMTUD (TCP sessions may hang).
+  * Blocking ICMP → breaks PMTUD (TCP sessions may hang).
-  * Tunnel/VPN overhead (e.g., IPSec reduces MTU from 1500 to ~1400).
+  * Tunnel/VPN overhead → reduces MTU (e.g., IPSec 1500 → ~1400).
-  * Inconsistent Jumbo Frame configuration → if any intermediate switch is at 1500, traffic will fail even if endpoints use 9000.
+  * Misconfigured Jumbo Frames → one 1500 hop breaks 9000 end-to-end.
 ---
-**Summary**
+**In summary:**
-  * **PMTU** = maximum packet size without fragmentation on the path.
+  * **PMTU** = max packet size without fragmentation.
-  * **PMTUD** = dynamic process that discovers that size using ICMP + DF.
+  * **PMTUD** = process to dynamically discover it using ICMP + DF.
@@ Line 314: / Line 312: @@
+{{ :aruba_networks:switch:6400:mtu_pmtu.pdf |}}
+{{pdfjs 46em >:aruba_networks:switch:6400:mtu_pmtu.pdf}}
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