Updating the IP Headers for Coalesced Segments

When finalizing a single coalescing unit (SCU), a receive segment coalescing (RSC)-capable miniport driver updates the fields in the IP headers as described in the following tables.

• Updating IPv4 header fields for coalesced segments
• Updating IPv6 header fields for coalesced segments

Updating IPv4 header fields for coalesced segments

Field	Description
Version	The value of this field must be the same for all coalesced segments.
Header Length	The length of a basic IPv4 header without any IP options.
Differentiated Services	The value of this field must be the same for all coalesced segments.
ECN bits	See Exception 8 in Exception Conditions that Terminate Coalescing. Datagrams should be coalesced if they all have the same values for the ECN bits.
Total Length	The value of this field must be recomputed every time a new segment with non-zero TCP payload length is coalesced into an existing SCU. See Exception Conditions that Terminate Coalescing for special cases that arise from the value in this field.
Identification	Must be set to the IP ID of the first coalesced segment.
Flags	Datagrams may be coalesced as long as they have the same value for the DF (Don’t Fragment) bit: either all set or all clear. Segments with the MF (More Fragments) bit set must not be coalesced.
Fragment Offset	Not applicable. Fragmented IP datagrams are not coalesced.
Time To Live	Must be set to the minimum time to live (TTL) value of the coalesced segments.
Protocol	Always set to 6, for TCP.
Header Checksum	The value of this field must be recomputed by the miniport driver.
Source Address	The value of this field must be the same for all coalesced segments.
Destination Address	The value of this field must be the same for all coalesced segments.

Updating IPv6 header fields for coalesced segments

Field	Description
Version	The value of this field must be the same for all coalesced segments.
Traffic Class	The value of this field must be the same for all coalesced segments.
Flow Label	The value of this field must be the same for all coalesced segments.
Payload Length	The value of this field must be recomputed whenever a new segment with nonzero TCP payload length is coalesced into an existing segment.
Next Header	Always set to 6, for TCP.
Hop Limit	Must be set to the minimum Hop Limit value of the coalesced segments.
Source Address	The value of this field must be the same for all coalesced segments.
Destination Address	The value of this field must be the same for all coalesced segments.