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To make driver programming easier the TSO limits are changed to reflect the values used in the BUSDMA tag a network adapter driver is using. The TCP/IP network stack will subtract space for all linklevel and protocol level headers and ensure that the full mbuf chain passed to the network adapter fits within the given limits. Implementation notes: If a network adapter driver needs to fixup the first mbuf in order to support VLAN tag insertion, the size of the VLAN tag should be subtracted from the TSO limit. Else not. Network adapters which typically inline the complete header mbuf could technically transmit one more segment. This patch does not implement a mechanism to recover the last segment for data transmission. It is believed when sufficiently large mbuf clusters are used, the segment limit will not be reached and recovering the last segment will not have any effect. The current TSO algorithm tries to send MTU-sized packets, where the MTU typically is 1500 bytes, which gives 1448 bytes of TCP data payload per packet for IPv4. That means if the TSO length limitiation is set to 65536 bytes, there will be a data payload remainder of (65536 - 1500) mod 1448 bytes which is equal to 324 bytes. Trying to recover total TSO length due to inlining mbuf header data will not have any effect, because adding or removing the ETH/IP/TCP headers to or from 324 bytes will not cause more or less TCP payload to be TSO'ed. Existing network adapter limits will be updated separately. Differential Revision: https://reviews.freebsd.org/D3458 Reviewed by: rmacklem MFC after: 2 weeks
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