SD-WAN Router PMTU Detection to Prevent Packet Fragmentation
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Solution Overview
Problem
In Software Defined Wide Area Networks (SD-WAN), data packet fragmentation and loss occur due to unpredictable changes in Path Maximum Transfer Unit (PMTU) that are not promptly detected, leading to inefficient and costly fragmentation and de-fragmentation processes, which consume processing resources and cause network performance issues.
Innovation Solution
Implementing Bidirectional Forwarding Detection (BFD) data packets with modified headers to record and compare largest sent and received packet sizes, enabling immediate detection of PMTU reductions, thereby triggering a PMTU discovery process to adjust packet sizes and prevent fragmentation or loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If data packets are sent larger than the current PMTU without frequent discovery processes, then network throughput is improved, but data fragmentation and packet loss occur when PMTU changes
Solution Approach 1:
The system performs preliminary PMTU discovery actions by sending probe packets before actual data transmission when a path change is detected. This preliminary action determines the new PMTU value in advance, preventing data packet fragmentation before it occurs
Solution Approach 2:
The system implements feedback mechanisms where BFD packets carry PMTU information from remote endpoints back to the local router. This feedback loop continuously updates the local PMTU value, enabling the system to adapt to path changes and maintain reliable data transmission
2Reliability
If PMTU discovery processes are performed frequently, then data packet loss is reduced, but CPU usage and processing overhead increase
Solution Approach 1:
The system uses periodic BFD packets to carry PMTU information at controlled intervals rather than continuously discovering PMTU. This periodic action reduces CPU overhead while still maintaining adequate PMTU awareness for reliable transmission
Solution Approach 2:
The system leverages existing BFD control packets that are already being exchanged for forwarding purposes. By adding PMTU information to these self-service packets, the system obtains PMTU discovery functionality without requiring separate dedicated discovery packets, thus minimizing additional CPU overhead
3Adaptability or versatility
If packet fragmentation is enabled in the router, then data packets exceeding PMTU can be transmitted, but processing load and jitter increase due to fragmentation and reassembly operations
Solution Approach 1:
The system performs preliminary PMTU determination using BFD packets before actual data transmission. By knowing the PMTU in advance, the router can pre-adjust packet sizes to match the path capacity, eliminating the need for fragmentation operations during data transmission
Solution Approach 2:
The system takes preliminary anti-action by preventing packet fragmentation before it occurs. Through PMTU discovery using BFD packets, the system proactively configures packet sizes to avoid exceeding PMTU, thereby preventing the harmful fragmentation effect rather than dealing with it afterward
Data Source
AI summary
Techniques are described for detecting a change in Path Maximum Transfer Unit (PMTU) in a network and initiating a PMTU discovery process. A Bidirectional Forwarding Detection (BFD) data packet is generated having enhanced headers configured to record a largest packet sent value and a largest packet received value. The BFD data packet is sent from a first network device (such as a first router) to a second network device (such as a second router). A largest packet sent value and a largest packet received value are each recorded in the BFD data packet. If the largest data packet sent value is larger than the largest data packet received value, then a determination can be made that a path change has resulted in a reduction in PMTU which has resulted in either a data packet being fragmented, a data packet being dropped or both. A PMTU discovery can then be performed.