GRE Tunnel Path MTU Determination via Feedback Packets
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Solution Overview
Problem
In network computing, determining the path maximum transmission unit (MTU) between endpoints of a Generic Routing Encapsulation (GRE) tunnel is challenging due to reliance on ICMP packets, which are not guaranteed to be delivered, especially when intermediary network devices filter them, leading to packet loss and unreliable data delivery.
Innovation Solution
A method involving the transmission of multiple outer GRE packets with varying payloads to determine the path MTU without relying on ICMP-based mechanisms, using GRE feedback packets to indicate packet receipt or loss, allowing the source network device to adjust the MTU accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ICMP packets are used for path MTU discovery, then the source network device can determine the path MTU, but the ICMP packets may be filtered by intermediary network devices leading to packet loss and unreliable determination
Solution Approach 1:
The patent introduces GRE feedback packets as an intermediary mechanism to relay path MTU information. Instead of relying directly on ICMP packets that may be filtered, the destination network device sends GRE feedback packets containing MTU information back to the source through the GRE tunnel, providing a more reliable communication channel that bypasses ICMP filtering issues.
Solution Approach 2:
The patent replaces the ICMP-based mechanical system with a GRE packet-based system. By embedding MTU discovery information within GRE protocol packets rather than using separate ICMP packets, the system leverages the existing GRE tunnel communication mechanism, which is already established and trusted, thereby improving reliability.
2Reliability
If packet fragmentation is disabled on intermediary devices, then packet delivery reliability improves, but the path MTU must be precisely known to avoid packet loss
Solution Approach 1:
The patent implements a feedback mechanism where the destination network device sends GRE feedback packets containing path MTU information back to the source network device. This feedback loop allows the source to learn the correct path MTU without complex probing mechanisms, simplifying the overall system while maintaining high reliability through accurate MTU knowledge.
Solution Approach 2:
The destination network device actively provides path MTU information to the source device through GRE feedback packets. Instead of requiring the source to perform complex active probing or rely on external ICMP mechanisms, the destination serves itself by supplying the necessary MTU information, reducing the complexity of the discovery mechanism.
3Reliability
If GRE feedback packets are used instead of ICMP packets, then path MTU determination reliability improves, but the mechanism becomes independent of upper-layer protocol mechanisms reducing standardization
Solution Approach 1:
The patent makes the GRE feedback mechanism independent of upper-layer protocols by embedding path MTU information directly within the GRE protocol layer. This universal approach allows the mechanism to function across different application layers without relying on TCP or other upper-layer protocols, providing both reliability and broad applicability through protocol independence.
Data Source
AI summary
A method of path MTU determination in Generic Routing Encapsulation (GRE) tunnel is presented. A source network device (ND) transmits, to a destination ND that is a second endpoint of the GRE tunnel, a first outer packet including a first inner packet, where the first inner packet includes a first inner header that is used to deliver the first inner packet to the source network device, a first inner GRE header, and a first payload. The source ND receives the first inner packet. The source ND transmits a second outer packet including a second inner packet that includes a second payload that has a size greater than a size of the first payload. The source ND determines that the second inner packet is not received and determines a path MTU between the source ND and the destination ND based on a size of the first and the second outer packets.