RFC 4311

IPv6 Host-to-Router Load Sharing 🎀

okay bestie, RFC 4311 is about how a host picks which router to send traffic through when more than one router would work 🦋 It's a Standards Track document that updates RFC 2461. It doesn't list anything it obsoletes. The authors are Robert Hinden (Nokia) and Dave Thaler (Microsoft), and it's dated November 2005. The title is "IPv6 Host-to-Router Load Sharing." ✨

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Network Working Group · Request for Comments: 4311 · Updates: 2461 · Category: Standards Track · November 2005

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. 💌 Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. 🌸

Copyright Notice

Copyright (C) The Internet Society (2005). 🌈

Abstract

The original IPv6 conceptual sending algorithm does not do load sharing among equivalent IPv6 routers, and suggests schemes that can be problematic in practice. 🙈 This document updates the conceptual sending algorithm in RFC 2461 so that traffic to different destinations can be distributed among routers in an efficient fashion. 💅

1. Introduction ✨

okay so, the conceptual sending algorithm in [ND] and the optional extension in [ROUTERSEL] both pick a next hop when there's no destination cache entry for an off-link destination, or when talking through an existing router is failing. 🫶 Usually a router gets picked the first time traffic goes to a particular destination IP address. After that, the same destination keeps using the same router, unless something gives it a reason to switch, like a redirect message showing up or the router turning out to be unreachable. 🔥

Also, as [ADDRSEL] explains, picking the next hop can affect which source address gets picked. That means it can indirectly (and to a smaller degree) affect which router handles inbound traffic too. 👑

In both the base sending algorithm and the optional extension, a host sometimes has several equivalent routers to choose from for a destination. 🧚 All other factors are equal, so the host has to break the tie using some implementation-specific method. 💜

It's often a good idea, when there's more than one equivalent router, for hosts to spread their outgoing traffic across those routers. ✨ That shares the load and gives the host's traffic better performance. 🍓

BUT, load sharing isn't always the move 💀 If one router has plenty of capacity, sending everything through that single router can make traffic patterns more predictable. It also makes it easier to diagnose connectivity problems beyond the first-hop routers. 🔍

[ND] doesn't require any particular behavior here. 🤷‍♀️ It says an implementation may always choose the same router (for example, the first one in the list), or it may cycle through the routers in a round-robin manner. Both of those suggestions are problematic. 😬

Always choosing the same router obviously doesn't do load sharing. 🙃 The problems with load sharing using naive tie-breaking techniques, like round-robin and random, are discussed in [MULTIPATH]. The destination cache gives some stability, since the choice isn't made per packet. But cache evictions or timeouts can still lead to unstable or unpredictable paths over time, which lowers performance and makes problems harder to diagnose. 😤 Round-robin selection can also cause synchronization issues among hosts. In the worst case, the load ends up concentrated on one router at a time. 💔

In the rest of this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119]. 📚 These are the official key words, so pay attention to them 🫡

2. Load Sharing 🦋

When a host chooses from multiple equivalent routers, it SHOULD support choosing with a method that spreads load across the equivalent routers for different destinations, instead of always picking the same one (for example, the first in the list). ✨ This memo takes no stance on whether load sharing support should be turned on or off by default. 🤷‍♀️ Also, a host that does attempt to distribute load among routers SHOULD use a hash-based scheme that takes (at least) the destination IP address into account, like the ones described in [MULTIPATH], to choose a router. 🔮

Here's the tea: traffic to a given destination address keeps using the same router for as long as its Destination Cache Entry isn't deleted. 🍵 With a hash-based scheme, traffic to a given destination address keeps using the same router over time even if the Destination Cache Entry is deleted, as long as the list of equivalent routers stays the same. 💖

3. Security Considerations 💀

Security is serious, so let's be real 🚨 As [MULTIPATH] points out, when next-hop selection is predictable, an application can craft traffic that all hashes to the same value. That makes it possible to launch a denial-of-service attack against the load-sharing algorithm and overload one particular router. 😱 This can even be done by a remote application that can make a host respond to a given destination address. One special case is when the same single next hop is always selected, like the algorithm [ND] allows. Adding hashing can make that kind of attack harder. The less predictable the hash, the harder it gets to launch a denial-of-service attack against any single router. 🛡️

BUT, a malicious local application can bypass the algorithm for its own traffic by using mechanisms like raw sockets, and remote attackers can still overload the routers directly. 🙅‍♀️ So the mechanisms discussed here have no significant incremental impact on Internet infrastructure security. 🫠

Heads up, bestie: hashing makes attacks harder, but it does NOT stop a local app using raw sockets, and remote attackers can still hit the routers directly. 💀

4. Acknowledgements 🫶

The authors would like to thank Erik Nordmark, Brian Haberman, Steve Deering, Aron Silverton, Christian Huitema, and Pekka Savola. 💜 Thank you, legends! 👑

5. Normative References 📚

• [ND] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998.
• [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
• [ADDRSEL] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, February 2003.
• [ROUTERSEL] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005.

6. Informative References 🌸

• [MULTIPATH] Thaler, D. and C. Hopps, "Multipath Issues in Unicast and Multicast Next-Hop Selection", RFC 2991, November 2000.

Authors' Addresses 💌

Robert Hinden
Nokia
313 Fairchild Drive
Mountain View, CA  94043

Phone: +1 650 625-2004
EMail: bob.hinden@nokia.com


Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA  98052

Phone: +1 425 703 8835
EMail: dthaler@microsoft.com

Full Copyright Statement 🌈

Copyright (C) The Internet Society (2005).

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