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Implementing IPv6 Segment Routing in the Linux kernel


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IPv6 Segment Routing is a major IPv6 extension that provides a modern version of source routing that is currently being developed within the Internet Engineering Task Force (IETF). We propose the first open-source implementation of IPv6 Segment Routing in the Linux kernel. We first describe it in details and explain how it can be used on both endhosts and routers. We then evaluate and compare its performance with plain IPv6 packet forwarding in a lab environment. Our measurements indicate that the performance penalty of inserting IPv6 Segment Routing Headers or encapsulat- ing packets is limited to less than 15%. On the other hand, the optional HMAC security feature of IPv6 Segment Routing is costly in a pure software implementation. Since our implementation has been included in the official Linux 4.10 kernel, we expect that it will be extended by other researchers for new use cases.

Presented at ANRW'17 on behalf of David Lebrun

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Implementing IPv6 Segment Routing in the Linux kernel

  1. 1. Implementing IPv6 Segment Routing in the Linux Kernel David Lebrun, Olivier Bonaventure ICTEAM, UCLouvain, Belgium Work supported by ARC grant 12/18-054 (ARC-SDN) and a Cisco grant
  2. 2. Agenda • IPv6 Segment Routing • Implementation in the Linux kernel • Performance evaluation
  3. 3. Packet along shortest path to R5 What is Segment Routing ? • The return of Source Routing – Each packet contains a loose route to encode any path inside the network R1 R2 R3 R5 R7 R6 R5->R2->R6 Packet along shortest path to R2 R5->R2->R6 Normal IPv6 forwarding Normal IPv6 forwarding R2->R6 ->R6 Packet along shortest path to R6 ->R6
  4. 4. IPv6 Segment Routing • Basic principles – IGP distributes IPv6 prefixes and router loopback addresses – Loose source route encoded inside IPv6 extension header containing a list of segments – Main types of segments • Node segment (router's loopback address) • Adjacency segment (router outgoing interface) • Virtual function (operator defined function)
  5. 5. The IPv6 Segment Routing Header Remaining segments Each segment is one IPv6 address Index of last segment Extensibility
  6. 6. IPv6 Segment Routing use cases • Paths controlled by the endhosts R1 R2 R3 R5 R7 R6 R5->R2->R6->D R5->R2->R6->D R5->R2->R6->D R5->R5->R6->D R5->R2->R6->D R5->R2->R6->D R5->R2->R6->D Source adds SRH to all packets Destination removes SRH from all packets
  7. 7. Network Function Virtualisation • Force packets to pass through NFV R1 R2 R3 R5 R7 R6 R5->FCT->R6 R5->FCT->D R5->FCT->R6 R5->FCT->D FCT performed on R5 R5->FCT->D FCT
  8. 8. Encap and decap • Routers can also tunnel SRH packets R1 R2 R3 R5 R7 R6 ->D R1->R5->R7->R6 [->D] ->D Ingress router encaps to R6 with SRH R1->R5->R7->R6 [->D] Egress router decaps and removes SRH R1->R5->R7->R6 [->D] R1->R5->R7->R6 [->D] R1->R5->R7->R6 [->D]
  9. 9. Security: Learning from the past • How to avoid past failures of source routing ?
  10. 10. The IPv6 SRH HMAC TLV Different keys and different hash functions can be used
  11. 11. Utilisation of the HMAC TLV • All routers are configured with an HMAC key • Clients receive SRH with HMAC key – E.g. from SDN controlled • Trusted servers configured with HMAC key R1 R2 R3 R5 R7 R6
  12. 12. Agenda • IPv6 Segment Routing • Implementation in the Linux kernel • Performance evaluation
  13. 13. Basics of Linux packet processing Packet recvd for local process Packet sent by local process Forwarded packet
  14. 14. Packet forwarding with IPv6 SR • Router is one of the segments in the list SRH updated, Packet forwarded to next segment
  15. 15. Packet forwarding with IPv6 SR • Egress router receiving encapsulated packet SRH processed encapsulation removed
  16. 16. How to configure IPv6 SR ? • IPv6 SR implementation extends iproute2 – Commands passed through rtnetlink – Example ip −6 route add fc42::/64 encap seg6 mode encap segs fc00::1,2001:db8::1,fc10::7 dev eth0 Destination match SRv6 encap Segments added in the encapsulated packet
  17. 17. SRH usage by applications • Endhosts can control the SRH on a per flow basis through the socket API
  18. 18. HMAC processing • Three modes of operations can be configured – Ignore • All packets are forwarded independently of the HMACs – Verify • Packets containing an HMAC are processed if HMAC is valid • Packets without HMAC are processed – Enforce • Packets containing an HMAC are processed if HMAC is valid • Packets without HMAC are discarded
  19. 19. Agenda • IPv6 Segment Routing • Implementation in the Linux kernel • Performance evaluation
  20. 20. Lab measurements • Lab setup – Intel Xeon X3440 processors (4 cores 8 threads at 2.53 GHz – 16 GB of RAM – two Intel 82599 10 Gbps Ethernet • One queue per CPU, one IRQ per queue – Linux kernel 4.11-rc3, TSO and GRO disabled • Traffic generator – Pktgen, in-kernel module sending UDP packets 10 Gbps Ethernet 10 Gbps Ethernet
  21. 21. First measurements with one CPU Why this gap ? Baseline Plain IPv6 forwarding No difference between SRH forwarding and encap+forwarding
  22. 22. Performance limitations of the first implementation • Route lookup – Destination cache was implemented for locally generated packets but not forwarded ones • Fixed with a dest cache • Issue with memory allocation – Forced free to take a slow path involving spinlocks in case packet was processed by different CPU than NIC IRQ • Fixed with a better utilisation of the skb
  23. 23. Improved performance on one CPU IPv6 SRH forwarding and encap are now close to plain IPv6 packet fowarding performance
  24. 24. Does packet size affect performance ? 1000 bytes 1000 bytes 64 64
  25. 25. Cost of HMAC Pure C code Special intel instructions
  26. 26. Leveraging multiple cores Performance scales well with the number of physical CPUS
  27. 27. Conclusion • IPv6 Segment Routing has matured – Stable specification – Various use cases • Implementation in the Linux kernel 4.11+ – Endhost functions for clients and servers – Router functions • Performance evaluation – Good forwarding and encap/decap performance – Unsurprisingly HMAC TLV affects performance