Packet-Loss Resiliency for Router Solicitations
updates
- rfc-4861 — Neighbor Discovery for IP version 6 (IPv6)
Extracted elements (15)
A known limitation is that hosts cannot differentiate between a network with no IPv6 routers and a link where an IPv6 router is temporarily unreachable; in the latter case the mechanism causes roughly 1 RS per hour to be sent indefinitely.
Exponential backoff rather than fixed-interval retransmission is used to achieve resiliency while keeping aggregate network traffic low when a large number of hosts are present.
On non-multicast links such as ISATAP, RSs must continue indefinitely even after receiving a valid RA because those links can only receive RAs in response to RSs, making continuous solicitation necessary for router reachability.
The initial retransmission time IRT was set to 4 seconds to align with RTR_SOLICITATION_INTERVAL from RFC 4861, so that short-term RS behavior is similar to what is experienced in current networks.
The maximum retransmission time MRT was set to 3600 seconds to align with the new SOL_MAX_RT from RFC 7083, so that longer-term persistent retransmission behavior trends towards being similar to that of DHCPv6.
On non-multicast links such as ISATAP (RFC 5214), RSs are unicast to specific routers rather than multicast; the stopping condition for multicast-capable links does not apply, and retransmission must continue indefinitely.
This document updates RFC 4861; hosts implementing RFC 4861 that give up after MAX_RTR_SOLICITATIONS (3) RSs will be without connectivity for up to 30 minutes (MaxRtrAdvInterval = 1800 s) if initial RSs are lost.
Hosts complying to this specification MUST use the exponential backoff algorithm for retransmits described in Section 14 of RFC 3315 in order to continuously retransmit Router Solicitations until a Router Advertisement is received.
Hosts SHOULD use the following variables as input to the retransmission algorithm: IRT=4 seconds, MRT=3600 seconds, MRC=0, MRD=0.
If a configuration option is provided to enable or disable RS retransmissions, it MUST enable RS retransmissions by default.
If an RA is received from a router but does not result in a default route (Router Lifetime is zero), the host MUST continue retransmitting the RSs.
On multicast-capable links, hosts SHOULD stop retransmitting RSs when Router Discovery is successful, i.e., an RA with a non-zero Router Lifetime that results in a default route is received.
On non-multicast links, hosts MUST continue retransmitting RSs even after receiving an RA that results in a default route, because on such links sending an RA can only be triggered by an RS.
The retransmission algorithm parameters are: IRT (Initial Retransmission Time) = 4 s, MRT (Maximum Retransmission Time) = 3600 s, MRC (Maximum Retransmission Count) = 0 (no limit), MRD (Maximum Retransmission Duration) = 0 (no limit).
This document does not present any additional security issues beyond those discussed in RFC 4861 and the RFCs that update it.