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'''Network packet steering''' of incoming traffic for '''[[Multi-core_processor|multi-core architectures]]''' is needed in modern network computing environment, especially in [[Data_center|data centers]], where the high bandwidth and heavy loads would easily congestion a single core's queue.
[[File:Simple NIC and cores architecture.png|thumb|upright=1.6|Simple graph showing the path receiving packets need to travel to reach the cores' queues]]
For this reason many techniques, both in hardware and in software, are leveraged in order to distribute the incoming load of [[Network_packet|packets]] across the cores of the processor.
As shown by the figure beside, packets coming into the [[Network_interface_controller|network interface card (NIC)]] are processed and loaded to the receiving queues managed by the cores (which are usually implemented as [[Circular buffer|ring buffers]] within the [[User space and kernel space|kernel space]]).
The main objective is being able to leverage all the cores available within the CPU to process incoming packets, while also improving performances like [[Latency (engineering)|latency]] and [[Network throughput|throughput]].<ref name="RSS kernel linux docs">{{Cite web|title=RSS kernel linux docs|url=https://www.kernel.org/doc/html/v5.1/networking/scaling.html#rss-receive-side-scaling|access-date=2025-07-08|website=kernel.org|publisher=The Linux Kernel documentation|language=en-US}}</ref><ref name="RFS by redhat">{{Cite web|title=RFS by redhat|url=https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/performance_tuning_guide/network-rfs|access-date=2025-07-08|website=docs.redhat.com|publisher=Red Hat Documentation|language=en-US}}</ref><ref name="RFS by nvidea">{{Cite web|title=RFS by nvidea|url=https://docs.nvidia.com/networking/display/mlnxofedv23070512/flow+steering|access-date=2025-07-08|website=docs.nvidia.com|publisher=NVIDIA Documentation Hub|language=en-US}}</ref><ref name="RSS overview by microsoft">{{Cite web|title=RSS overview by microsoft|url=https://learn.microsoft.com/en-us/windows-hardware/drivers/network/introduction-to-receive-side-scaling|access-date=2025-07-08|website=learn.microsoft.com|language=en-US}}</ref><ref name="RSS++">{{Cite journal |last=Barbette |first=Tom |last2=Katsikas |first2=Georgios P. |last3=Maguire |first3=Gerald Q. |last4=Kostić |first4=Dejan |date=2019-12-03 |title=RSS++: load and state-aware receive side scaling |url=https://dl.acm.org/doi/10.1145/3359989.3365412 |journal=Proceedings of the 15th International Conference on Emerging Networking Experiments And Technologies |series=CoNEXT '19 |___location=New York, NY, USA |publisher=Association for Computing Machinery |doi=10.1145/3359989.3365412 |isbn=978-1-4503-6998-5}}</ref><ref name="General intro">{{Citation |last=Madden |first=Michael M. |title=Challenges Using the Linux Network Stack for Real-Time Communication |date=2019-01-06 |work=AIAA Scitech 2019 Forum |url=https://arc.aiaa.org/doi/10.2514/6.2019-0503 |access-date=2025-07-10 |series=AIAA SciTech Forum |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2019-0503 |pages=9-11}}</ref><ref>{{Cite web |last=Herbert |first=Tom |date=2025-02-24 |title=The alphabet soup of receive packet steering: RSS, RPS, RFS, and aRFS |url=https://medium.com/@tom_84912/the-alphabet-soup-of-receive-packet-steering-rss-rps-rfs-and-arfs-c84347156d68 |access-date=2025-07-10 |website=Medium |language=en}}</ref><ref>{{Cite journal |last=Wu |first=Wenji |last2=DeMar |first2=Phil |last3=Crawford |first3=Matt |date=2011-02-01 |title=Why Can Some Advanced Ethernet NICs Cause Packet Reordering? |url=https://ieeexplore.ieee.org/document/5673999/ |journal=IEEE Communications Letters |doi=10.1109/LCOMM.2011.122010.102022 |issn=1558-2558}}</ref>▼
▲<ref name="RSS kernel linux docs">{{Cite web|title=RSS kernel linux docs|url=https://www.kernel.org/doc/html/v5.1/networking/scaling.html#rss-receive-side-scaling|access-date=2025-07-08|website=kernel.org|publisher=The Linux Kernel documentation|language=en-US}}</ref><ref name="RFS by redhat">{{Cite web|title=RFS by redhat|url=https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/performance_tuning_guide/network-rfs|access-date=2025-07-08|website=docs.redhat.com|publisher=Red Hat Documentation|language=en-US}}</ref><ref name="RFS by nvidea">{{Cite web|title=RFS by nvidea|url=https://docs.nvidia.com/networking/display/mlnxofedv23070512/flow+steering|access-date=2025-07-08|website=docs.nvidia.com|publisher=NVIDIA Documentation Hub|language=en-US}}</ref><ref name="RSS overview by microsoft">{{Cite web|title=RSS overview by microsoft|url=https://learn.microsoft.com/en-us/windows-hardware/drivers/network/introduction-to-receive-side-scaling|access-date=2025-07-08|website=learn.microsoft.com|language=en-US}}</ref><ref name="RSS++">{{Cite journal |last=Barbette |first=Tom |last2=Katsikas |first2=Georgios P. |last3=Maguire |first3=Gerald Q. |last4=Kostić |first4=Dejan |date=2019-12-03 |title=RSS++: load and state-aware receive side scaling |url=https://dl.acm.org/doi/10.1145/3359989.3365412 |journal=Proceedings of the 15th International Conference on Emerging Networking Experiments And Technologies |series=CoNEXT '19 |___location=New York, NY, USA |publisher=Association for Computing Machinery |doi=10.1145/3359989.3365412 |isbn=978-1-4503-6998-5}}</ref><ref name="General intro">{{Citation |last=Madden |first=Michael M. |title=Challenges Using the Linux Network Stack for Real-Time Communication |date=2019-01-06 |work=AIAA Scitech 2019 Forum |url=https://arc.aiaa.org/doi/10.2514/6.2019-0503 |access-date=2025-07-10 |series=AIAA SciTech Forum |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2019-0503 |pages=9-11}}</ref><ref>{{Cite web |last=Herbert |first=Tom |date=2025-02-24 |title=The alphabet soup of receive packet steering: RSS, RPS, RFS, and aRFS |url=https://medium.com/@tom_84912/the-alphabet-soup-of-receive-packet-steering-rss-rps-rfs-and-arfs-c84347156d68 |access-date=2025-07-10 |website=Medium |language=en}}</ref><ref>{{Cite journal |last=Wu |first=Wenji |last2=DeMar |first2=Phil |last3=Crawford |first3=Matt |date=2011-02-01 |title=Why Can Some Advanced Ethernet NICs Cause Packet Reordering? |url=https://ieeexplore.ieee.org/document/5673999/ |journal=IEEE Communications Letters |doi=10.1109/LCOMM.2011.122010.102022 |issn=1558-2558}}</ref>
== Hardware techniques ==
Hardware accelerated techniques like RSS and aRFS are used to route and load balance incoming [[Network_packet|packets]] across the multiple cores' queues of a processor.<br>
Those hardware supported methods achieve extremely low latencies and reduce the load on the CPU, as compared to the software based ones. However they require a specialized hardware integrated within the [[Network_interface_controller|network interface controller]] (which could be for example a [[Data_processing_unit|SmartNIC]]).<ref name="RSS++" /><ref name="aRFS by redhat">{{Cite web|title=aRFS by redhat|url=https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/performance_tuning_guide/network-acc-rfs|access-date=2025-07-08|website=docs.redhat.com|publisher=Red Hat Documentation|language=en-US}}</ref><ref name="aRFS by nvidea">{{Cite web|title=aRFS by nvidea|url=https://docs.nvidia.com/networking/display/mlnxofedv23070512/flow+steering#src-2396583156_safe-id-Rmxvd1N0ZWVyaW5nLUFjY2VsZXJhdGVkUmVjZWl2ZUZsb3dTdGVlcmluZyhhUkZTKQ|access-date=2025-07-08|website=docs.nvidia.com|publisher=NVIDIA Documentation Hub|language=en-US}}</ref>
=== RSS ===
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Some notable example of header fields used are the [[Internet Protocol|layer 3 IP]] source and destination addresses, the protocol and the [[transport layer|layer 4]] source and destination port.
In this way, packets corresponding to the same flow will be directed to the same receiving queue, without loosing the original order causing an [[Out-of-order delivery|out-of-order delivery]]. Moreover all incoming flows will be [[Load balancing (computing)|load balanced]] across all the available cores thanks to the hash function. <br>
Another important feature introduced by the indirection table is the capability of changing the mapping of flows to the cores without having to change the hash function, but by simply updating the table entries.<ref>{{Cite web|title=RSS intel doc|url=https://www.intel.com/content/dam/support/us/en/documents/network/sb/318483001us2.pdf|access-date=2025-07-08|website=earn.microsoft.com|language=en-US}}</ref><ref name="RSS overview by microsoft" /><ref>{{Cite web|title=RSS by redhat|url=https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/performance_tuning_guide/network-rss|access-date=2025-07-08|website=docs.redhat.com|publisher=Red Hat Documentation|language=en-US}}</ref><ref name="RSS kernel linux docs" /><ref>{{Cite web|title=Receive-side scaling enhancements in windows server 2008|url=https://download.microsoft.com/download/a/d/f/adf1347d-08dc-41a4-9084-623b1194d4b2/rss_server2008.docx|access-date=2025-07-08|website=microsoft.com|publisher=Microsoft|language=en-US}}</ref><ref name="RSS++" />
=== aRFS ===
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Differently from RSS which is a fully hardware independent implementation, aRFS needs to interface with the software (the [[Kernel (operating system)|kernel]]) to properly function. <br>
RSS simply load balance incoming traffic across the cores; however if a packet flow is directed to the ''core i'' (as a result of the hash function) while the application needing the received packet is running on ''core j'', many cache misses could be avoided by simply forcing ''i=j'', so that packets are received exactly where they are needed. <br>
To do this aRFS doesn't forward packets directly from the result of the hash function, but using a configurable routing table (which can be filled and updated for instance by the [[Scheduling (computing)|scheduler]] through an [[API]]) packet flows can be steered to the specific consuming core.<ref name="aRFS by nvidea" /><ref name="aRFS by redhat" /><ref>{{Cite web|title=aRFS kernel linux docs|url=https://www.kernel.org/doc/html/v5.1/networking/scaling.html#accelerated-rfs|access-date=2025-07-08|website=kernel.org|publisher=The Linux Kernel documentation|language=en-US}}</ref>
== Software techniques ==
Software techniques like RPS and RFS employ one of the CPU cores to steer incoming packets across the other cores of the processor. This comes at the cost of introducing additional [[Inter-processor interrupt|inter-processor interrupts (IPIs)]], however the number of hardware interrupts will not increase and thanks to [[Interrupt coalescing|interrupt aggregation]] it could even be reduced.<br>
The benefits of a software solutions is the ease in implementation, without having to change any component (like the [[Network_interface_controller|NIC]]) of the currently used architecture, but by simply deploying the proper [[Loadable kernel module|kernel module]]. This benefit can be crucial especially in cases where the server machine can't be customized or accessed (like in [[Cloud computing#Infrastructure as a service (IaaS)|cloud computing]] environment), even if the network performances could be reduced as compared the hardware supported ones.<ref name="RPS linux news (LWM)">{{Cite web|title=RPS linux news (LWM)|url=https://lwn.net/Articles/362339/|access-date=2025-07-08|website=lwn.net|publisher=Linux Weekly News|language=en-US}}</ref><ref name="RPS by redhat">{{Cite web|title=RPS by redhat|url=https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/performance_tuning_guide/network-rps|access-date=2025-07-08|website=docs.redhat.com|publisher=Red Hat Documentation|language=en-US}}</ref><ref name="RFS by nvidea" />
=== RPS ===
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Moreover thanks to the hash function, packets belonging to the same flow will always be steered to the same core.
This is usually done in the kernel, right after the NIC driver. Having handled the network interrupt and before it can be processed, the packet is sent to the receiving queue of a core, which is then notified thanks to an inter process interrupt. <br>
RPS can be used in conjunction with RSS, in case the number of hardware queues is lower than the number of cores. In this case after having distributed across the hardware queues the incoming packets, a pool of cores can be assigned to each queue and RPS will be used to spread again the incoming flows across the specified pool.<ref name="RPS linux news (LWM)" /><ref name="RPS by redhat" /><ref>{{Cite web|title=RPS kernel linux docs|url=https://www.kernel.org/doc/html/v5.1/networking/scaling.html#rps-receive-packet-steering|access-date=2025-07-08|website=kernel.org|publisher=The Linux Kernel documentation|language=en-US}}</ref>
=== RFS ===
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To do this, after having computed the hash of the header fields for the current packet, the result is used to index a lookup table.
This table is managed by the scheduler, which updates its entries when the application processes are moved between the cores.
The overall CPU load distribution is balanced as long as the applications in [[User space and kernel space|user-space]] are evenly distributed across the multiple cores.<ref name="RFS by redhat" /><ref name="RFS by nvidea" /><ref>{{Cite web|title=RFS kernel linux docs|url=https://www.kernel.org/doc/html/v5.1/networking/scaling.html#rfs-receive-flow-steering|access-date=2025-07-08|website=kernel.org|publisher=The Linux Kernel documentation|language=en-US}}</ref>
=== XPS (in transmission) ===
Transmit Packet Steering (XPS) differently from all the prevoius protocols exposed, is used in transmission. When packets need to be loaded on one of the transmission queues exposed by the NIC, there are again many possible optimization that could be done. <br>
For instance if multiple transmission queues are assigned to a single core, an hash function could be used to load balance outgoing packets across the queues (similarly to how RPS does in reception).
Moreover in order to improve cache locality and hit-rate (similarly to how RFS does), XPS ensures that applications producing the outgoing traffic and running in ''core i'' will favor the transmitting queues associated with the same ''core i''. This reduces the inter-core communication and cache coherency protocols overheads, resulting in better performances in heavy load environments.<ref>{{Cite web|title=XPS intel overview|url=https://www.intel.com/content/www/us/en/docs/programmable/683517/21-4/transmit-packet-steering-xps.html|access-date=2025-07-08|website=intel.com|publisher=Intel corp|language=en-US}}</ref><ref>{{Cite web|title=XPS linux news (LWM)|url=https://lwn.net/Articles/412062/|access-date=2025-07-08|website=lwn.net|publisher=Linux Weekly News|language=en-US}}</ref><ref>{{Cite web|title=XPS kernel linux docs|url=https://www.kernel.org/doc/html/v5.1/networking/scaling.html#xps-transmit-packet-steering|access-date=2025-07-08|website=kernel.org|publisher=The Linux Kernel documentation|language=en-US}}</ref><ref name="General intro" />
== See also ==
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