Comment 19 for bug 1692538
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| bugproxy (bugproxy) wrote Comment bridged from LTC Bugzilla | #19 |
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------- Comment From <email address hidden> 2017-11-21 02:33 EDT-------
Hi Daniel,
Yes. I meant TCP Segmentation Offload when I said large-send offload.
Your understanding of the current ibmveth code is right.
AIX Path MTU Discovery is enabled by default and so though the very first packet (larger payload) can leave the LPAR with higher MSS, AIX will discover that the path MTU size is lower than the MSS negotiated. This leads to AIX TCP recovering (based on path MTU values) and sending packets of right MTU size for further packets.
But if user disables Path MTU and configures an MTU higher than what physical NIC can support, then this will lead to packet drops (and AIX stack cannot rediscover MTU without path MTU). So, second scenario you mentioned in your note will lead to communication issues.
Also by default, AIX MTU settings are optimal ones i.e 1500 bytes (even when TSO is enabled). This helps in AIX LPAR sending larger payloads with MSS value of 1500. If an user knows that the physical NIC in VIOS or Linux IO server has Jumbo frames enabled, MTU can be set to 9000 (here again if TSO is enabled, larger payloads will go with MSS of 9000). So, the optimal values are the default values (1500/9000).
Regards,
Siva K.