More Accurately Naming IPv6 RA Router Lifetime

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Table of Contents

1. Introduction

According to [RFC4861], Router Advertisements (RAs) are used by IPv6 routers to¶

• [...] advertise their presence together with various link and Internet parameters either periodically, or in response to a Router Solicitation message. Router Advertisements contain prefixes that are used for determining whether another address shares the same link (on-link determination) and/or address configuration, a suggested hop limit value, etc.¶

The Router Lifetime field of an RA indicates whether or not the advertising router will act as a default router for the receiving IPv6 hosts [RFC4861], and if so, for how many seconds it will perform that role, unless it is refreshed by a subsequent RA from the router. The router is identified by its sending interface's Link-Local address [RFC4861].¶

If the RA Router Lifetime field value is zero, it means that the router is not or will no longer be acting as a default router [RFC4861].¶

The Router Lifetime field in IPv6 RAs is likely to be a carry over from the Lifetime field in Router Advertisements used in the IPv4 ICMP Router Discovery Protocol (IRDP) [RFC1256]. ICMP Router Discovery RAs only advertise default routers. IPv6 Router Advertisements have been generalised to carry other link and subnet parameter options, such as MTU and Prefix Information [RFC4861].¶

IPv6 Router Advertisements options are required to have their own lifetime fields if necessary [RFC4861]. They are not related to or in any way impacted by the Router Lifetime field value of the RA the options are carried within.¶

As the field name "Router Lifetime" is quite general, and is a permanent field in a Router Advertisement, the purpose of this field can be easily misunderstood. It could easily be misinterpreted as a maximum lifetime value applying to all of the parameters and options in the RA, acting as a constraint on all other lifetime values within the RA and its options. A Router Lifetime value of zero could seem to indicate that the RA is entirely invalid and that all of its options contents must be ignored or invalidated by the receivers of the RA.¶

This memo renames the Router Lifetime field to "Default Router Lifetime", more accurately and explicitly describing its purpose.¶

2. Updates to RFC4861

For all instances of the term "router lifetime", the term "default router lifetime" is to be used, independent of letter case.¶

When the term "lifetime" is used in the context of an IPv6 Router Advertisment, implying "router lifetime", it will now imply "default router lifetime."¶

3. Zero Value Default Router Lifetime Example Use Case

The use of a non-zero Default Router Lifetime is obvious; the router sending these RAs is willing to act as a default router for the receiving hosts.¶

An example of a router sending a zero value Default Router Lifetime is as follows.¶

On a link, there is a router acting as a default router for the attached hosts. This router is providing forwarding to default, unspecified destinations, such as the IPv6 Internet. The router's RAs have a non-zero value Default Router Lifetime, such as the [RFC4861] AdvDefaultLifetime default value of 1800 seconds (30 minutes).¶

Also attached to the link is another router that is sending RAs with a zero value Default Router Lifetime. This router is also attached to another physical or logical link to a specific destination, such as an IPsec VPN tunnel [RFC4301]. Within these RAs is a Route Information Option (RIO) [RFC4191], specifying an IPv6 prefix covering the IPv6 address space reachable over the other link.¶

Hosts receiving these zero value Default Router Lifetime RAs, that understand the Route Information Option, will send packets towards destinations within the prefix directly to the RIO advertising router, benefitting from a more optimal forwarding path, and also removing traffic forwarding load from the link's default router.¶

The router advertising itself as a default router will need to know about this other router and the address space reachable beyond it, so that it can forward traffic towards the the other link and path, for hosts that do not understand the Route Information Option and that will send all of their traffic to the non-zero Default Router Lifetime router.¶

4. Security Considerations

Security vulnerabilties can be created if the purpose or function of a mechanism is misunderstood. Renaming the IPv6 RA Router Lifetime field "Default Router Lifetime" will make it clearer what the purpose and function of this field in IPv6 RAs is.¶

5. Acknowledgements

Review and comments were provided by YOUR NAME HERE!¶

This memo was prepared using the xml2rfc tool.¶

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