TCP offload's promises and limitations for enterprise networks

TCP offload products free up limited CPU resources in network-intensive data centers. But TCP offload is no panacea and can introduce more problems.

TCP offload was developed to improve data center network performance and reliability, but a confusing array of TCP offload techniques can do just the opposite.

In traditional networking, the CPU handles every major task involved with transmitting data: direct memory access to stage outgoing packet data, calculating the checksum for each packet, adding header information and moving packets to the network interface buffer. CPUs also handle network data reception -- essentially the reverse of transmission tasks.

Transmission Control Protocol/Internet Protocol (TCP/IP) network communication imposes overhead on the processor, which handles workload computing tasks. This processing overhead increases when data centers adopt high-bandwidth 1 Gigabit Ethernet or 10 Gigabit Ethernet networks. Some modern data center technologies, such as iSCSI and other network-based storage and virtualization, impose networking demands on the processor.

To free the CPU from overhead networking tasks, the network controller took over processing the entire TCP/IP stack on dedicated hardware. The controller handles all of the packet formation, checksum, buffering and other tasks, and exchanges blocks of data with the host.

TCP Chimney, TSO and TOE all refer to offload technology. TCP/IP offload engine (TOE) is the term coined by hardware-based network controller vendors. TCP segmentation offload (TSO) is used in some virtual environments, such as VMware. TCP Chimney Offload is Microsoft's software offload feature. Although features and functionality may vary based on the actual offload product, all three classifications serve essentially the same purpose.

TCP offload is no cure-all for network ailments

In spite of the promise of TCP offload technologies, adoption is limited. TCP offload controllers are more likely than other controllers to suffer resource shortages. Network performance problems occur if the controller is overtaxed. Proprietary TCP offload implementations can require extensive changes to the TCP/IP stack, degrading support and security, adding complexity and hurting Quality of Service, along with other network features.

Not every data center will reap benefits from TCP offload. Host systems are constantly getting faster; next-generation hosts without offload controllers handle networking better than older systems with offload controllers. If a server completes tasks faster than the TCP offload controller can acknowledge a transmission, it can cause communication errors.

Almost all functional or performance problems with TCP/IP offload products relate to software. When troubleshooting offload engine issues, always consider software updates or upgrades, usually firmware or driver updates. For example, if the offload engine is integrated onto the system's motherboard, consider a motherboard firmware update to address networking problems. If the offload engine is deployed as a PCI Express (PCIe) adapter, evaluate firmware upgrades for the adapter rather than the motherboard. Similarly, look for driver updates for the host and virtual machines' operating systems. As with any upgrade, test changes in a lab setting first.

If you must disable the network controller's offload capability for testing or troubleshooting, check for enable/disable controls in the network controller's BIOS. If the network controller is integrated onto the system's motherboard, you can access the system BIOS during a reboot cycle. If the offload adapter is in a PCIe slot, try accessing the BIOS through a dedicated utility provided with the adapter. Alternatively, try to disable offload features in the Advanced tab of the connection's Properties dialog under Windows. When the offload engine is software-based, such as Microsoft's TCP Chimney, you may need to access the system registry and disable the appropriate registry key. Always refer to vendor documentation for enabling or disabling offload features.

Alternatives to a network offload engine

Given the criticisms of full TCP offload products, some IT professionals choose another route.

One alternative is TCP checksum offload; the system CPU still shuttles data and assembles/disassembles header information but the network controller calculates checksum, inserts it into the packet and validates it during reception. Since checksum calculations and checks take time, offloading those tasks from the CPU will boost performance without a wholesale re-architecting of the operating system's TCP/IP stack.

Another popular alternative is the large segment offload (LSO) technique, or TSO. LSO/TSO offloads the networking tasks of all outgoing data. The host system simply shuttles outgoing data from memory to the network controller's buffer and the network controller segments the outgoing data into packets and facilitates their transmission. Network adapters commonly support LSO or TSO.

The counterpart to LSO/TSO is large receive offload (LRO), which offloads the networking tasks of all incoming data. The network controller strips incoming packet headers, verifies checksums, and assembles the incoming data into a buffer which is periodically passed over to host memory. LRO is less popular than LSO but is becoming more commonplace.

These techniques rely on the operating system's TCP/IP stack as a template, avoiding the problems that often accompany proprietary stack architectures.

Since server computing resources are advancing faster than TOE products, a modern server has plenty of CPU cores and memory to handle most networking tasks without imposing any proprietary TCP/IP stack changes. Unless your data center has extreme network demands, TOE adoption may not be the right path. Still, alternative offload technologies such as checksum, LSO and LRO have become widely deployed in data centers to optimize server resources for virtualized systems.

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