ALG ICMP MTU Correction for SIP Packet Routing
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Solution Overview
Problem
The existing ALG application layer gateway protocol modifies SIP packets, causing them to exceed the maximum transmission size (MTU) of routers, leading to failed calls due to misinterpretation of ICMP error messages, especially in emergency situations where the sender does not understand the packet size adjustment made by the ALG.
Innovation Solution
Modify the MTU value in ICMP messages based on the size changes made by the ALG, calculating the actual processing size and adjusting the MTU value accordingly to ensure packets are resent within the router's maximum transmission size, allowing for successful transmission without fragmentation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the ALG application layer gateway protocol modifies SIP packets to enable translation between private and public networks, then network security and protocol compatibility are improved, but the packet size increases and may exceed the router's MTU, causing transmission failures
Solution Approach 1:
The invention performs preliminary action by calculating the size modification introduced by ALG processing in advance. The gateway determines the difference between original and modified packet sizes, and uses this information to adjust the MTU value in ICMP error messages before sending them back to the sender, preventing transmission failures
Solution Approach 2:
The gateway acts as an intermediary between the sender and the network path. It intercepts ICMP error messages from the network, modifies the MTU information within these messages based on its knowledge of ALG-induced size changes, and forwards the corrected messages to the sender, thereby mediating the information flow to ensure accurate MTU reporting
2Measurement precision
If the router sends ICMP error messages with the actual MTU value of the network path, then the sender can adjust packet sizes appropriately, but the sender misunderstands the error message because it does not account for the additional size introduced by ALG processing
Solution Approach 1:
The invention implements feedback by having the gateway observe the actual size modification caused by ALG processing and use this observed information to correct the MTU value in ICMP error messages. The gateway feeds back accurate MTU information that already accounts for ALG modifications, enabling the sender to make correct packet size adjustments without misunderstanding the error messages
3Stability of the object's composition
If the packet size exceeds the MTU and fragmentation is prevented by the DF flag, then packet integrity is maintained, but the call fails because the packet cannot be transmitted
Solution Approach 1:
The invention applies preliminary anti-action by preventing the transmission failure before it occurs. The gateway calculates the ALG-induced size modification in advance and uses this information to provide accurate MTU values in ICMP error messages, enabling senders to adjust packet sizes proactively before attempting transmission, thus avoiding the situation where packets would be discarded due to size violations
Data Source
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AI summary
A method for correcting a packet of an Internet control message protocol (ICMP), during routing of a packet (12, 12t) between a private network (LAN) and a public network (WAN) having been processed by an application level gateway (ALG), comprising the steps of: reading (PC1) a packet of an Internet control message protocol (ICMP) comprising a "need to fragment" message and the value of a maximum transmission unit (MTU) which defines the maximum size of data packets handled in a communication protocol used by a routing, calculating (PC2) a value (n) of a processing size by comparing a size (Lin) of a packet (12) not processed by the integrator of the application level gateway (ALG) and a size (Lout) of the same packet (12t) processed by the integrator of the application level gateway (ALG), modifying (PC3) the packet of an Internet control message protocol (ICMP) by overwriting the value of the maximum transmission unit (MTU) with an actual maximum packet size value which depends on the value of the processing size (n), sending (PC4), to a transmitter (1) having sent the non-processed packet (12t), the packet of an Internet control message protocol (ICMP).