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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.
31 2 74 65 8 9 10End-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
31 2 74 65 8 9 10End-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 Sends How Often DA Scale Host Mobility
Router Solicit hosts when new (seldom) all-routers mcast O(s)
Router Advert routers periodic;when solicited
all-nodes mcast;unicast
O(R)
Neighbor Solicit nodes when no/stale cache entry for Next Hop
solicited-neighbor mcast
O(P)
Neighbor Advert nodes when solicited unicast O(P)
UnsolicitedNeighbor Advert
nodes when L2 address changes (seldom)
all-nodes mcast O(s)
Redirect routers when needed (Seldom in non-mobile environment, But frequent in Cloud Data Center
unicast O(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
