Scaling IPv6 Neighbor Discovery Ben Mack-Crane

Neighbor Solicitation (RFC4861) End-station 1 wants to resolve the L2 address of end-station 10; –End-station 1 sends Neighbor Solicitation packet using the solicited- node multicast address for end-station 10’s IPv6 address; The Neighbor Solicitation packet is flooded to all endpoints on the VLAN; –If the end-station 10 has configured its NIC to receive this multicast address, so no other end-stations must process the Neighbor Solicitation packet; –Note: there is a small probability that another end-station could register for the same solicited-node multicast address as end-station 10, but there are 2^24 addresses and so the probability of overlap is small and the impact is small as well (receiving unnecessary solicitations from a few end-stations) and therefore there would be no significant impact on end-station CPU cycles End-station 1 sends Neighbor Solicitation End-station 10 receives Neighbor Solicitation Other end-stations are not registered for multicast address

Problems with IPv6 self addressed hosts -What we learned on the way to BOF When Server is virtualized, –If the server’s MAC filter is smaller than the number of VMs supported, then effectively all the multicast messages will go into the server –impact end station CPU cycles. For user created subnet, the number of hosts in the subnet is up to the user. –IPv6 gives user more freedom to create a mega size subnet potentially –SLAAC & DAD could potentially blow up DHCP

Unsolicited Neighbor Advertisement End-station 1 wants to inform all end-stations of a change in L2 address; –End-station 1 sends an Unsolicited Neighbor Advertisement packet using the all-nodes multicast address; –The Unsolicited Neighbor Advertisement packet is flooded to all endpoints on the VLAN; –All end-stations in the VLAN process the Unsolicited Neighbor Advertisement; Note: this is expected to be a rare event (change of L2 address) and therefore, although all end-stations must process this packet, there would be no significant impact on end-station CPU cycles. Similar to Gratituous ARP Response End-station 1 sends Unsolicited Neighbor Advertisement All end-stations are registered for all-nodes multicast address

ND Scaling Gap Analysis – Performance nodes = routers + hosts; R = #routers; H = #hosts; P = #peers/node; s = small number Scalability looks very good for networks with a few routers and many hosts (each with a few peers) when servers are not virtualized. Who SendsHow OftenDAScaleHost Mobility Router Solicithostswhen new (seldom)all-routers mcastO(s) Router Advertroutersperiodic; when solicited all-nodes mcast; unicast O(R) Neighbor Solicitnodeswhen no/stale cache entry for Next Hop solicited-neighbor mcast O(P) Neighbor Advertnodeswhen solicitedunicastO(P) Unsolicited Neighbor Advert nodeswhen L2 address changes (seldom) all-nodes mcastO(s) Redirectrouterswhen needed ( Seldom in non-mobile environment, But frequent in Cloud Data Center unicastO(s)

ND Scaling Gap Analysis – Performance Additional features and considerations: Duplicate Address Detection –solicits all-nodes multicast Neighbor Advertisement if address is in use –this should be rare enough to be insignificant Anycast and Proxy address resolution –solicits multiple Neighbor Advertisements (from each node supporting the Anycast address) –increases the number of Neighbor Advertisements received by the requestor, randomized delay –may want to restrict this feature to a single site in a multi-site network Neighbor Unreachability Detection –is designed to take advantage of hints from higher layers, only send messages when connectivity is suspect (should be rare) –may not be suitable for core case since each router will have many peers and may not be able to take advantage of higher layer hints – may prefer alternate fault detection methods Redirect –rate limited, frequency depends on network design and management, impact should be limited –When VMs migration are used, the volume of re-direct could be huge.

ND Scaling Gap Analysis – Performance Additional features and considerations: Host-based Load Spreading (e.g. RFC 4311) –affects selection of Next Hop Router –does not increase ND traffic appreciably Router-based Load Spreading (i.e. use of NULL SA in Router Advertisement) –requires hosts to solicit Next Hop Router address –increases solicitations for router addresses –not significant if number of routers is small (may be inappropriate for core) Holding packet while address resolution occurs –in muiti-site networks or virtualized networks that may increase the edge-to-edge delay, hold time for packets awaiting address resolution may increase significantly IPv6 Subnet Model (RFC5942) –this RFC does not substantially change ND performance, it simply clarifies that there is no default subnet prefix size and makes small modifications for security