Intel PCI User Manual

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The TCP/IP context transmit descriptor provides access to the enhanced checksum offload facility 
available in the Ethernet controller. This feature allows TCP and UDP packet types to be handled 

more efficiently by performing additional work in hardware, thus reducing the software overhead 

associated with preparing these packets for transmission. 

The TCP/IP context transmit descriptor does not point to packet data as a data descriptor does. 
Instead, this descriptor provides access to an on-chip context that supports the transmit checksum 

offloading feature of the controller. A “context” refers to a set of registers loaded or unloaded as a 

group to provide a particular function.

The context is explicit and directly accessible via the TCP/IP context transmit descriptor. The 
context is used to control the checksum offloading feature for normal packet transmission.

The Ethernet controller automatically selects the appropriate legacy or normal context to use based 

on the current packet transmission.

While the architecture supports arbitrary ordering rules for the various descriptors, there are 

restrictions including:

•

Context descriptors should not occur in the middle of a packet.

•

Data descriptors of different packet types (legacy or normal) should not be intermingled 

except at the packet level.

All contexts control calculation and insertion of up to two checksums. This portion of the context is 
referred to as the checksum context. 

In addition to checksum context, the segmentation context adds information specific to the 

segmentation capability. This additional information includes the total payload for the message 

(TDESC.PAYLEN), the total size

 

of the header (TDESC.HDRLEN), the amount of payload data 

that should be included in each packet (TDESC.MSS), and information about what type of protocol 

(TCP, IPv4, IPv6, etc.) is used. This information is specific to the segmentation capability and is 

therefore ignored for context descriptors that do not have the TSE bit set.

Because there are dedicated resources on-chip for the normal context, the context remains constant 
until it is modified by another context descriptor. This means that a context can be used for multiple 

packets (or multiple segmentation blocks) unless a new context is loaded prior to each new packet. 
Depending on the environment, it may be completely unnecessary to load a new context for each 

packet. For example, if most traffic generated from a given node is standard TCP frames, this 

context could be set up once and used for many frames. Only when some other frame type is 
required would a new context need to be loaded by software. After the “non-standard” frame is 

transmitted, the “standard” context would be setup once more by software. This method avoids the 

“extra descriptor per packet” penalty for most frames. The penalty can be eliminated altogether if 
software elects to use TCP/IP checksum offloading only for a single frame type, and thus performs 

those operations in software for other frame types.

This same logic can also be applied to the segmentation context, though the environment is a more 

restrictive one. In this scenario, the host is commonly asked to send a message of the same type, 
TCP/IP for instance, and these messages also have the same total length and same maximum 

segment size (MSS). In this instance, the same segmentation context could be used for multiple 

TCP messages that require hardware segmentation. The limitations of this scenario and the 
relatively small performance advantage make this approach unlikely; however, it is useful in 

understanding the underlying mechanism.