Bug 65 - passt/pasta MTU occasionally still requires IP fragmentation
Summary: passt/pasta MTU occasionally still requires IP fragmentation
Status: CONFIRMED
Alias: None
Product: passt
Classification: Unclassified
Component: IPv4 (show other bugs)
Version: unspecified
Hardware: All Linux
: Normal normal
Assignee: David Gibson
URL:
Depends on: 66
Blocks: 55
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Reported: 2023-07-04 03:42 UTC by David Gibson
Modified: 2023-07-04 06:04 UTC (History)
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Description David Gibson 2023-07-04 03:42:47 UTC 
The guest facing link of passt/pasta has an MTU of 65520, just slightly smaller than the maximum allowed IPv4 packet size of 65535 bytes.  This means that if the guest tries to send very large (but legal) ICMP or UDP packets, they will be fragmented, and those packets will then not be propagated properly by passt because it doesn't handle IP defragmentation.

It should be possible to increase the paast/pasta MTU to 65535.  However, this will require increasing the size of a number of internal buffers over another page boundary.  We want to be careful this doesn't have too much of a performance impact.
Comment 1 David Gibson 2023-07-04 06:04:40 UTC 
Drat.  Unfortunately, it isn't going to be possible to do this, at least for pasta:  the kernel appears to limit the mtu for tap devices to 65521 or less bytes.
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