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[[Network packet]] steering of transmitting and receiving 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 [[Queueing theory|queue]].<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>
[[File:Simple NIC and cores architecture.png|thumb|upright=1.7|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 packets across the cores of the [[Central processing unit|processor]].
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While for transmission XPS will be explained.<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><br>
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 [[Central processing unit|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="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
== 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.<ref name="RSS++" /><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 for example is usually available on more advanced cards, like the [[Data_processing_unit|SmartNIC]]).
=== RSS ===
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The hash function input is usually customizable and the header fields used can vary between use case and implementations.
Some notable examples of header fields chosen as keys for the hash are the [[Internet Protocol|layer 3 IP]] source and destination addresses, the protocol and the [[transport layer|layer 4]] source and destination ports.
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 properties.<ref name="RSS overview by microsoft" /><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
=== aRFS ===
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