# 📌 Understanding Packet Loss | Large Pings vs Fibre Uplink/Downlink Issues Packet loss is a common frustration in networking, whether you're troubleshooting **WiFi, wired Ethernet, or fibre optic links**. Two key scenarios often confuse engineers: 1. **Why do larger ping packets experience more packet loss than smaller ones?** 2. **Why might packet loss occur in only one direction on a fibre link?** In this article, we’ll break down **both issues**, explaining why they happen and how to diagnose them effectively. --- ## **📌 Why Larger Ping Packets Have More Packet Loss** When troubleshooting a network, **ICMP ping** is a go-to tool. But you may notice that small packets go through fine, while larger ones experience **higher loss rates**. ### 🔍 **1\. Fragmentation Issues** * If a ping packet **exceeds the Maximum Transmission Unit (MTU)**, it must be **fragmented** into smaller pieces. * If **any fragment is lost**, the entire packet is considered lost. * Firewalls or security policies may **drop fragmented packets** as a security measure. ### 🔍 **2\. Buffer Overflows** * Network devices (switches, routers, firewalls) have **limited buffer space**. * Larger packets fill up buffers **faster**, increasing the chance of dropped packets when congestion occurs. ### 🔍 **3\. Higher Chance of Corruption** * Larger packets spend more time **on the wire**, increasing exposure to **interference, bad cables, or faulty hardware**. * A single bit error causes the **entire packet to be discarded**. ### 🔍 **4\. Hardware Limitations** * Some older or overloaded devices **struggle with processing large packets**. * Firewalls and security appliances may **rate-limit large ICMP packets** to prevent DoS attacks. ### 🔍 **5\. Wireless-Specific Issues** * In **WiFi networks**, larger packets are more vulnerable to **signal interference** and retransmissions. * Some wireless setups have a **low fragmentation threshold**, causing higher loss rates for large packets. ### 🎯 **How to Test Large Packet Loss** You can check this behaviour using **ping** with different packet sizes: ```plaintext ping -s 56 8.8.8.8 # Small 64-byte packet (56 + 8-byte ICMP header) ping -s 1472 8.8.8.8 # Large 1500-byte packet (1472 + 8-byte ICMP header) ``` If large packets show **higher loss**, you should investigate **MTU mismatches, congestion, or hardware limitations**. --- ## **📌 Why Packet Loss Can Occur in One Direction on a Fibre Link** ### 🚦 **The Nature of Fibre Links** Fibre optic connections use **separate strands** for transmit and receive signals. This means: * **One strand handles traffic from A → B (Uplink).** * **The other strand handles traffic from B → A (Downlink).** If there’s packet loss **in only one direction**, it **usually means a problem with one of the fibre strands**. ### 🔍 **1\. A Faulty Fibre Strand Can Affect Only One Direction** If the **downlink strand is damaged or dirty**, traffic from **B → A** is impacted, while traffic from **A → B** works fine. 🔎 **Diagnosis:** * Use `ping` or `iperf` to test in both directions. * Swap the TX and RX fibre pairs. If the problem reverses, one fibre is faulty. ### 🔍 **2\. Uneven Optical Power Levels** Fibre optics rely on **precise optical signal levels**. If the **receive power** is too weak on one side: * The receiver struggles to interpret the signal, causing packet loss. * The sender might still transmit fine, making the issue **appear one-sided**. 🔎 **Diagnosis:** * Use an **optical power meter** to measure the signal at both ends. * Check that the **SFP or transceiver** is correctly seated and clean. ### 🔍 **3\. Unidirectional Congestion or Buffering Issues** Some **network devices handle traffic differently in each direction**. * A **firewall or router** might shape traffic more aggressively in one direction. * If one path has a congested **buffer**, large packets may get dropped **only on one side**. 🔎 **Diagnosis:** * Use `traceroute` and `mtr` to check for asymmetric routing. * Inspect firewall and QoS settings. --- ## **✅ How to Troubleshoot These Issues Effectively** ### **Step 1: Check for Large Packet Loss** * Use **different ping sizes** (`ping -s 56` vs `ping -s 1472`). * If large packets are dropping, check **MTU settings, buffer limits, or firewall rules**. ### **Step 2: Test Fibre Links in Both Directions** * Run **ping tests** between both IPs (`ping -s 56` both ways). * Swap the **TX/RX fibre strands** to see if the issue reverses. * Use an **optical power meter** to check signal levels. ### **Step 3: Identify Hardware or Congestion Issues** * Use `iperf` to test TCP/UDP throughput. * Check for **asymmetric routing or firewall shaping rules**. * Inspect for **buffer overruns or high CPU usage** on routers. --- ## **🔥 The Rant | Why Network Operators Always Say “No Fault Found”** You ever notice how network operators **never find a fault** unless their entire core network is on fire? It doesn’t matter **how much proof** you provide—ping logs, traceroutes, packet captures, live sacrifice to the networking gods—they **always** claim it’s fine **on their side**. ### **Their Favourite Move | Testing from the Core** * You report **packet loss** from your branch site. * Instead of testing **from your branch**, they test **from their core**—where, surprise surprise, everything is fine. * They tell you, **"Our network is clean. The issue must be on your side."** ### **How This Fails Reality** * If a link from **your branch to their core is bad**, testing from their core **does nothing**. * It’s like saying your house isn’t on fire because the fire station is fine. * They **never** test at the **problematic point**. ### **Why They Do This?** 1. **They Don’t Want to Admit Fault** – Because that would mean work. 2. **They Don’t Want SLA Penalties** – If they acknowledge an issue, they owe credits. 3. **They’re Too Lazy to Roll a Truck** – Dispatching someone costs money. ### **How to Beat the “No Fault Found” Excuse** ✔ **Make them test FROM the affected site.** ✔ **Provide proof** – timestamps, bidirectional ping tests, MTR logs. ✔ **Escalate and embarrass them** – If they refuse to test properly, escalate until someone with a brain gets involved. --- ## **🚀Wrap | Don’t Assume Packet Loss is Random** * **Large packet loss** is often caused by **MTU mismatches, buffer overflows, or hardware limitations**. * **Unidirectional packet loss** on a fibre link usually means **a faulty fibre strand, weak optical signal, or asymmetric traffic shaping**. * **Network operators will gaslight you** into thinking it’s not their problem, so force them to test **from the affected location**. By **understanding these principles**, you’ll troubleshoot **packet loss issues more effectively**, saving yourself **hours of frustration**—and maybe, just maybe, making your provider do their job. 🚀 %[https://bsky.app/profile/mastelek.bsky.social/post/3lqwup5gack27]