RFC 6437

IPv6 Flow Label Specification

📅 published November 2011, from ietf

RFC 6437: IPv6 Flow Label Specification 🦋

Okay bestie, so this is RFC 6437, and it's called IPv6 Flow Label Specification 💖 It came out in November 2011 and it's on the Standards Track 👑 The authors are S. Amante from Level 3, B. Carpenter from the Univ. of Auckland, S. Jiang from Huawei, and J. Rajahalme from Nokia Siemens Networks 🫶 It's ISSN 2070-1721. It obsoletes RFC 3697 (so that one is retired now 💀) and it updates RFC 2205 and RFC 2460 🔥

Abstract

This document specifies the IPv6 Flow Label field and the minimum requirements for IPv6 nodes labeling flows, IPv6 nodes forwarding labeled packets, and flow state establishment methods. Even when mentioned as examples of possible uses of the flow labeling, more detailed requirements for specific use cases are out of the scope for this document.

The usage of the Flow Label field enables efficient IPv6 flow classification based only on IPv6 main header fields in fixed positions ✨

Status of This Memo

This is an Internet Standards Track document 🎀

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741 🌸

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6437 🧚

Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. 💅

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, because they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

Table of Contents

1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
2.  IPv6 Flow Label Specification  . . . . . . . . . . . . . . . .  4
3.  Flow Labeling Requirements in the Stateless Scenario . . . . .  5
4.  Flow State Establishment Requirements  . . . . . . . . . . . .  7
5.  Essential Correction to RFC 2205 . . . . . . . . . . . . . . .  7
6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
  6.1.  Covert Channel Risk  . . . . . . . . . . . . . . . . . . .  8
  6.2.  Theft and Denial of Service  . . . . . . . . . . . . . . .  8
  6.3.  IPsec and Tunneling Interactions . . . . . . . . . . . . . 10
  6.4.  Security Filtering Interactions  . . . . . . . . . . . . . 11
7.  Differences from RFC 3697  . . . . . . . . . . . . . . . . . . 11
8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
  9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
  9.2.  Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A.  Example 20-Bit Hash Function  . . . . . . . . . . . . 14

1. Introduction ✨

Okay so, from the network layer's point of view, a flow is a sequence of packets sent from a particular source to a particular unicast, anycast, or multicast destination that a node wants to label as a flow 🦋 From an upper-layer point of view, a flow could be all the packets going in one direction of a specific transport connection or media stream. However, a flow is not necessarily 1:1 mapped to a transport connection 💅

Traditionally, flow classifiers have been based on the 5-tuple of the source address, destination address, source port, destination port, and the transport protocol type. But some of these fields might be unavailable because of fragmentation or encryption, and finding them past a chain of IPv6 extension headers may be inefficient. Also, if classifiers only depend on IP-layer headers, then introducing alternative transport-layer protocols later will be easier 🌸

The usage of the 3-tuple of the Flow Label, Source Address, and Destination Address fields enables efficient IPv6 flow classification, where only IPv6 main header fields in fixed positions are used 🔥

The flow label could be used in both stateless and stateful scenarios. A stateless scenario is one where any node that processes the flow label in any way does not need to store any information about a flow before or after a packet has been processed. A stateful scenario is one where a node that processes the flow label value needs to store information about the flow, including the flow label value. A stateful scenario might also require a signaling mechanism to inform downstream nodes that the flow label is being used in a certain way and to establish flow state in the network. For example, RSVP [RFC2205] and General Internet Signaling Transport (GIST) [RFC5971] can signal flow label values 🧚

The flow label can be used most simply in stateless scenarios. This specification concentrates on the stateless model and how it can be used as a default mechanism. Details of stateful models, signaling, specific flow state establishment methods, and their related service models are out of scope for this specification. The basic requirement for stateful models is set forth in Section 4 🎀

The minimum level of IPv6 flow support consists of labeling the flows. A specific goal is to enable and encourage the use of the flow label for various forms of stateless load distribution, especially across Equal Cost Multi-Path (ECMP) and/or Link Aggregation Group (LAG) paths. ECMP and LAG are methods to bond together multiple physical links used to procure the required capacity necessary to carry an offered load greater than the bandwidth of an individual physical link. Further details are in a separate document [RFC6438]. IPv6 source nodes SHOULD be able to label known flows (e.g., TCP connections and application streams), even if the node itself does not require any flow-specific treatment. Node requirements for stateless flow labeling are given in Section 3 💖

This document replaces [RFC3697] and Section 6 and Appendix A of [RFC2460]. A rationale for the changes made is documented in [RFC6436]. The present document also includes a correction to [RFC2205] concerning the flow label 🫶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] 👑

2. IPv6 Flow Label Specification 🎀

The 20-bit Flow Label field in the IPv6 header [RFC2460] is used by a node to label packets of a flow. A Flow Label of zero is used to indicate packets that have not been labeled. Packet classifiers can use the triplet of Flow Label, Source Address, and Destination Address fields to identify the flow that a particular packet belongs to. Packets are processed in a flow-specific manner by nodes that are able to do so in a stateless manner or that have been set up with flow-specific state. The nature of the specific treatment and the methods for flow state establishment are out of scope for this specification 💅

Flow label values should be chosen so their bits show a high degree of variability, which makes them suitable as part of the input to a hash function used in a load distribution scheme. At the same time, third parties should be unlikely to be able to guess the next value that a source of flow labels will choose 🦋

In statistics, a discrete uniform distribution is defined as a probability distribution in which each value in a given range of equally spaced values (such as a sequence of integers) is equally likely to be chosen as the next value. The values in such a distribution show both variability and unguessability. So, as specified in Section 3, an approximation to a discrete uniform distribution is preferable as the source of flow label values. Intentionally, there are no precise mathematical requirements placed on the distribution or on the method used to achieve such a distribution 🌸

Once set to a non-zero value, the Flow Label is expected to be delivered unchanged to the destination node(s). A forwarding node MUST either leave a non-zero flow label value unchanged or change it only for compelling operational security reasons as described in Section 6.1. 🔥

There is no way to verify whether a flow label has been modified en route or whether it belongs to a uniform distribution. Therefore, no Internet-wide mechanism can depend mathematically on unmodified and uniformly distributed flow labels. They have a "best effort" quality. Implementers should be aware that the flow label is an unprotected field that could have been accidentally or intentionally changed en route (see Section 6). This leads to the following formal rule: 🧚

Best effort alert: the flow label is not verifiable, so nothing Internet-wide can lean on it being perfectly unmodified and uniform 🙈
  • Forwarding nodes such as routers and load distributors MUST NOT depend only on Flow Label values being uniformly distributed. In any usage such as a hash key for load distribution, the Flow Label bits MUST be combined at least with bits from other sources within the packet, so as to produce a constant hash value for each flow and a suitable distribution of hash values across flows. Typically, the other fields used will be some or all components of the usual 5-tuple. In this way, load distribution will still occur even if the Flow Label values are poorly distributed 💜

Although uniformly distributed flow label values are recommended below, and will always be helpful for load distribution, it is unsafe to assume their presence in the general case. The use case needs to work even if the flow label value is zero 🌈

As a general practice, packet flows should not be reordered, and the use of the Flow Label field does not affect this. In particular, a Flow label value of zero does not imply that reordering is acceptable 🍓

more coming in part 2, bestie 🧚💖

3. Flow Labeling Requirements in the Stateless Scenario ✨

okay so this section is the minimum rules for how to set the flow label value in the stateless scenario 🫶 it's giving "basic but make it important" 💅

To enable Flow-Label-based classification, source nodes SHOULD assign each unrelated transport connection and application data stream to a new flow 🦋 a typical flow for this is any set of packets carrying the same 5-tuple {dest addr, source addr, protocol, dest port, source port} ✨ and ngl, a source node always has easy, efficient access to that 5-tuple, which is NOT always true for the nodes that forward the packet later on 🙈

Flow label values should be uniformly distributed to help with load distribution 🌈 so it is RECOMMENDED that source hosts support the flow label by setting the flow label field for all packets of a given flow to the same value, picked from an approximation to a discrete uniform distribution 💜 both stateful and stateless methods of picking a value could be used, but this spec is NOT the place to mandate an algorithm 🧚 the algorithm SHOULD make sure the resulting flow label values are unique with high probability. however, if two simultaneous flows get the same flow label value by chance AND have the same source and destination addresses, they just get the same flow label treatment across the network. as long as that's a low-probability event, it won't really mess with load distribution 🍓

A possible stateless algorithm is to use a suitable 20-bit hash of values from the IP packet's 5-tuple 🎀 there's a simple example hash function in Appendix A.

An alternative is to use a pseudo-random number generator to assign a flow label value for a given transport session 🌸 this needs minimal local state at the source node, since you just record the flow label for each transport socket. iconic efficiency 👑

Viewed from the outside, either approach will produce values that look uniformly distributed and pseudo-random ✨

An implementation that assigns flow labels sequentially is NOT RECOMMENDED 🙅‍♀️ because it would be super easy for on-path observers to guess the next value 💀

A source node that does not otherwise set the flow label MUST set its value to zero 🍼

A node that forwards a flow whose flow label value in arriving packets is zero MAY change the flow label value. in that case, it is RECOMMENDED that the forwarding node sets the flow label field for a flow to a uniformly distributed value, the same way we just described for source nodes 🦋

  • The same considerations apply as for source hosts setting the flow label. the preferred case is that a flow is defined by the 5-tuple 💖 BUT there are cases where the complete 5-tuple isn't readily available to a forwarding node, especially for fragmented packets 😭 in those cases, a flow can be defined by fewer IPv6 header fields, typically just the 2-tuple {dest addr, source addr}. implementers could choose from some alternative approaches, like:
    • A forwarding node might use the 5-tuple to define a flow whenever possible, but use the 2-tuple when the complete 5-tuple isn't available. in this case, unfragmented and fragmented packets belonging to the same transport session would get different flow label values, which changes the effect of later load distribution based on the flow label 🌀
    • A forwarding node might use the 2-tuple to define a flow in all cases. in this case, load distribution would be based only on IP addresses 🫠
  • Setting the flow label in a forwarding node, if implemented, would presumably be valuable in first-hop or ingress routers 🛣️ it might put a considerable per-packet processing load on them, even if they used a stateless method of flow identification and label assignment 😵 that said, it will NOT interfere with host-to-router load sharing [RFC4311] ✨ it needs to be under the control of network managers, to avoid unwanted processing load and any other undesirable effects 🔐 for this reason, it MUST be a configurable option, disabled by default 🙅‍♀️

The rules above, taken together, allow a given network to include routers that set flow labels on behalf of hosts that don't do so 🌈 the complications described are exactly why the main recommendation is that source hosts should set the label 👑

4. Flow State Establishment Requirements 🧚

A node that sets the flow label MAY also take part in a flow state establishment method that assigns specific treatments to specific flows, possibly including signaling 💅 any such method MUST NOT disturb nodes taking part in the stateless scenario we just covered 🫶 so any node that sets flow label values according to a stateful scheme MUST choose labels that conform to Section 3 of this specification. further details aren't discussed in this document 🙈

5. Essential Correction to RFC 2205 🎀

[RFC2460] reduced the size of the flow label field from 24 to 20 bits 🍓 the references to a 24-bit flow label field in Section A.9 of [RFC2205] are updated accordingly ✨ a glow-up correction, honestly 💖

6. Security Considerations 🔥

okay this is the serious part 😭 this section covers security issues raised by the use of the Flow Label, including the potential for denial-of-service attacks and the related potential for theft of service by unauthorized traffic (Section 6.2). Section 6.3 covers the use of the Flow Label in the presence of IPsec, including its interaction with IPsec tunnel mode and other tunneling protocols 🛡️ we also note that inspecting unencrypted Flow Labels may allow some forms of traffic analysis by revealing some structure of the underlying communications 👀 even if the flow label was encrypted, its presence as a constant value in a fixed position might help traffic analysis and cryptoanalysis 💀

The flow label is NOT protected in any way, even if IPsec authentication [RFC4302] is in use, so it can be forged by an on-path attacker 😱 implementers are advised that any en-route change to the flow label value is undetectable. on the other hand, a uniformly distributed pseudo-random flow label can't be readily guessed by an attacker 🔒 see [LABEL-SEC] for further discussion 📚 if a hash algorithm is used, as suggested in Section 3, it SHOULD include a step that makes the flow label value significantly difficult to predict [RFC4086], even with knowledge of the algorithm being used 🧠

6.1. Covert Channel Risk 🕵️‍♀️

the flow label could be used as a covert data channel, since apparently pseudo-random flow label values could, in fact, carry covert data [NSA] 👻 this could, for example, be done with a series of otherwise innocuous UDP packets whose flow label values form a covert message, or by co-opting a TCP session to carry a covert message in the flow labels of successive packets 🫣 both of these could be recognized as suspicious. the first, because isolated UDP packets wouldn't normally be expected to have non-zero flow labels, and the second, because the flow label values in a given TCP session should all be equal 🚩 however, other methods, like co-opting the flow labels of occasional packets, might be really hard to detect 🥷

in situations where the covert channel risk is considered significant, the only certain defense is for a firewall to rewrite non-zero flow labels 🧱 this would be an exceptional violation of the rule that the flow label, once set to a non-zero value, must not be changed 🙀 to preserve load distribution capability, such a firewall SHOULD rewrite labels by following the method described for a forwarding node (see Section 3), as if the incoming label value were zero, and it MUST NOT set non-zero flow labels to zero 🫶 this behavior is still undesirable, though, since (as discussed in Section 3) only source nodes have straightforward access to the complete 5-tuple 😤

6.2. Theft and Denial of Service 💀

okay so this one is about people trying to steal service or cause chaos 😭. The network maps traffic to special treatment based on the IP addresses and the Flow Label value in the IPv6 header, so an adversary could grab a class of service the network never meant to give them, either by modifying the IPv6 header or by injecting packets with fake addresses and/or labels. A concrete analysis is only possible for specific stateful methods of signaling and using the flow label, and those are out of scope for this document 🙈. Obviously a full analysis will be needed whenever one of those methods gets specified, but in general, networks SHOULD NOT make resource allocation decisions based on flow labels without some external means of assurance 🫶.

A denial-of-service attack [RFC4732] becomes possible in the stateless model when modified or injected traffic depletes the resources available to forward it and other traffic streams 💥. If someone launched a denial-of-service attack against a given Flow Label (or set of Flow Labels), then traffic carrying an affected Flow Label might end up with worse-than-best-effort performance 😬.

Here's a fun bit: the treatment of IP headers by nodes is typically unverified, so there's no guarantee that flow labels sent by a node are set according to the recommendations in this document 🧐. A man-in-the-middle or injected-traffic denial-of-service attack aimed specifically at flow label handling would involve setting unusual flow labels. For example, an attacker could set all flow labels reaching a given router to the same arbitrary non-zero value, or rapidly cycle flow label values so the packets of a given flow each have a different value 🌀. Either of these would make a stateless load distribution algorithm perform badly and would make a stateful classifier behave incorrectly. So stateless classifiers should not use the flow label alone to control load distribution, and stateful classifiers should include explicit methods to detect and ignore suspect flow label values 🔥.

Flows are identified by the 3-tuple of the Flow Label and the Source and Destination Address, so the denial-of-service risk from the Flow Label is closely tied to the risk of denial of service by address spoofing 👑. An adversary who can forge an address is also likely able to forge a label, and vice versa. It's giving matching energy 💅.

There are two issues with different properties here: spoofing of the Flow Label only, and spoofing of the whole 3-tuple, including Source and Destination Address 🦋.

Spoofing the Flow Label only can be done inside a node that is using or transmitting the correct source address. Being able to spoof a Flow Label typically implies being in a position to forge an address too, but in lots of cases spoofing an address isn't very interesting to the spoofer, especially if their goal is theft of service rather than denial of service 🍓.

Spoofing the whole 3-tuple can be done by a host that isn't subject to ingress filtering [RFC2827] or by an intermediate router. Because of its properties, this is typically only useful for denial of service 💀. Without ingress filtering, almost any third party could instigate such an attack 😱.

With ingress filtering in place, forging a non-zero Flow Label on packets that originated with a zero label, or modifying or clearing a label, could only happen if an intermediate system such as a router was compromised, or through some other form of man-in-the-middle attack 🛡️.

6.3. IPsec and Tunneling Interactions 🌸

So IPsec, as defined in [RFC4301], [RFC4302], and [RFC4303], doesn't include the IPv6 header's Flow Label in any of its cryptographic calculations. (In tunnel mode, it's the outer IPv6 header's Flow Label that isn't included 🎀.) That means a network node changing the Flow Label has no effect on IPsec end-to-end security, because it can't cause any IPsec integrity check to fail. As a result, IPsec doesn't give any defense against an adversary modifying the Flow Label (aka a man-in-the-middle attack) 🙈.

IPsec tunnel mode does provide security for the encapsulated IP header's Flow Label ✨. A tunnel mode IPsec packet has two IP headers: an outer header supplied by the tunnel ingress node, and an encapsulated inner header supplied by the original source of the packet. When an IPsec tunnel is passing through nodes doing flow classification, those intermediate nodes operate on the Flow Label in the outer header 🦋. At the tunnel egress node, IPsec processing includes removing the outer header and forwarding the packet (if required) using the inner header. IPsec requires that the inner header's Flow Label not be changed by this decapsulation processing, so that modifications to the label can't be used to launch theft- or denial-of-service attacks across an IPsec tunnel endpoint 🔐. This document makes no change to that requirement; honestly, it forbids changes to the Flow Label 💅.

When IPsec tunnel egress decapsulation includes a sufficiently strong cryptographic integrity check of the encapsulated packet (where "sufficiently strong" is decided by local security policy), the tunnel egress node can safely assume the inner header's Flow Label has the same value it had at the tunnel ingress node 🫶.

This analysis and its implications apply to any tunneling protocol that performs integrity checks 🌈. Of course, any Flow Label set in an encapsulating IPv6 header is subject to the risks described in the previous section ⚠️.

6.4. Security Filtering Interactions 🧚

The Flow Label does nothing to get rid of the need for packet filtering based on headers past the IP header 🙅‍♀️. If that filtering is deemed necessary for security reasons on nodes like firewalls or filtering routers, it's still needed 🔥.

7. Differences from RFC 3697 🎀

okay so here's what changed from the predecessor [RFC3697] ✨. The main differences between this specification and its predecessor are:

  • This specification encourages non-zero flow label values to be used, and clearly defines how to set a non-zero value 💖.
  • This specification encourages a stateless model with uniformly distributed flow label values 🌸.
  • This specification does not specify any details of a stateful model 🙈.
  • This specification retains the rule that the flow label must not be changed en route, but allows routers to set the label on behalf of hosts that don't do so 🦋.
  • This specification discusses the covert channel risk and its consequences for firewalls 👑.

For further details, see [RFC6436] 💜.

8. Acknowledgements 🫶

Valuable comments and contributions were made by Jari Arkko, Ran Atkinson, Fred Baker, Richard Barnes, Steve Blake, Tassos Chatzithomaoglou, Remi Despres, Alan Ford, Fernando Gont, Brian Haberman, Tony Hain, Joel Halpern, Qinwen Hu, Chris Morrow, Thomas Narten, Mark Smith, Pascal Thubert, Iljitsch van Beijnum, and other participants in the 6man working group 🌟.

Cristian Calude suggested the von Neumann algorithm in Appendix A 🧠. David Malone and Donald Eastlake gave additional input about hash algorithms 🍓.

Steve Deering and Alex Conta were co-authors of RFC 3697, on which this document is based 💖.

Contributors to the original development of RFC 3697 included Ran Atkinson, Steve Blake, Jim Bound, Francis Dupont, Robert Elz, Tony Hain, Robert Hancock, Bob Hinden, Christian Huitema, Frank Kastenholz, Thomas Narten, Charles Perkins, Pekka Savola, Hesham Soliman, Michael Thomas, Margaret Wasserman, and Alex Zinin 🌈.

9. References 📚

9.1. Normative References

  • [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
  • [RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, September 1997.
  • [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
  • [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.

9.2. Informative References

  • [LABEL-SEC] Gont, F., "Security Assessment of the IPv6 Flow Label", Work in Progress, November 2010.
  • [NSA] Potyraj, C., "Firewall Design Considerations for IPv6", National Security Agency I733-041R-2007, 2007, http://www.nsa.gov/ia/_files/ipv6/I733-041R-2007.pdf.
  • [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000.
  • [RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6 Flow Label Specification", RFC 3697, March 2004.
  • [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005.
  • [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 2005.
  • [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005.
  • [RFC4311] Hinden, R. and D. Thaler, "IPv6 Host-to-Router Load Sharing", RFC 4311, November 2005.
  • [RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-Service Considerations", RFC 4732, December 2006.
  • [RFC5971] Schulzrinne, H. and R. Hancock, "GIST: General Internet Signalling Transport", RFC 5971, October 2010.
  • [RFC6436] Amante, S., Carpenter, B., and S. Jiang, "Rationale for Update to the IPv6 Flow Label Specification", RFC 6436, November 2011.
  • [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels", RFC 6438, November 2011.
  • [vonNeumann] von Neumann, J., "Various techniques used in connection with random digits", National Bureau of Standards Applied Math Series 12, 36-38, 1951.

Appendix A. Example 20-Bit Hash Function ✨

okay so remember Section 3? 🧚 A stateless hash function can be used to generate a flow label value from an IPv6 packet's 5-tuple. Picking a good hash function is not trivial, and it's expected that you'll need a lot of real-world experience to find the best choices 💅. This example is based on an algorithm by von Neumann that's known to give an approximately uniform distribution [vonNeumann]. For each packet that needs a flow label, follow these steps:

  1. Split the destination and source addresses into two 64-bit values each, which turns the 5-tuple into a 7-tuple 🎀.
  2. Add these five components together using unsigned 64-bit arithmetic, throwing away any carry bits: both parts of the source address, both parts of the destination address, and the protocol number.
  3. Apply the von Neumann algorithm to the resulting string of 64 bits 🦋:
    1. Starting at the least significant end, select two bits.
    2. If the two bits are 00 or 11, discard them.
    3. If the two bits are 01, output a 0 bit.
    4. If the two bits are 10, output a 1 bit.
    5. Repeat with the next two bits in the input 64-bit string.
    6. Stop when 16 bits have been output (or when the 64-bit string is exhausted).
  4. Add the two port numbers to the resulting 16-bit number.
  5. Shift the resulting value 4 bits left, and mask with 0xfffff.
  6. In the highly unlikely event that the result is exactly zero, set the flow label arbitrarily to the value 1 🌈.
Heads up, bestie: this simple example does NOT include a step to prevent predictability, even though Section 6 recommends one. Don't skip that part just because this example does 😭

Authors' Addresses

Here are the authors' details, exactly as they were written 🫶:

Shane Amante
Level 3 Communications, LLC
1025 Eldorado Blvd
Broomfield, CO  80021
USA

EMail: shane@level3.net


Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland  1142
New Zealand

EMail: brian.e.carpenter@gmail.com


Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing  100095
P.R. China

EMail: jiangsheng@huawei.com


Jarno Rajahalme
Nokia Siemens Networks
Linnoitustie 6
02600  Espoo
Finland

EMail: jarno.rajahalme@nsn.com
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