Anycast & Multicast
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Anycast is a new address type in IPv6 that refers to one among many interfaces with the same address. It is used to identify sets of routers or servers. Anycast addresses are allocated from unicast space and packets sent to an anycast address are routed to the nearest interface. Multicast addresses use a class D range in IPv4 from 224.0.0.0 to 239.255.255.255 and have a specific format in IPv6 to identify multicast groups and allow delivery of packets to many destinations.
Anycast & Multicast
- 1. Anycast address overview A new type in IPv6. Not defined for IPv4 It is assigned to more than one interface Refers to one among many address A packet sent to an anycast address is routed to the nearest interface with that address. Unicast – one to one Multicast – one to many Broadcast – one to all Anycast – one to one of many
- 2. Anycast address & use Allocated from unicast address space Link local, site local, global It is an unicast address assigned to many interfaces Nodes with anycast address must be explicitly configured to receive anycast packets. Used to identify a set of routers of an ISP a set of routers in a subnet A set of routers providing entry to a routing domain
- 3. Anycast address restrictions Cannot be used as a source address Cannot be assigned to a host May be assigned to a router only
- 4. Multicasting One to many addressing Delivery of packets to many destinations Interactive conferencing Dissemination of mail News to multiple recepients Webcasts to multiple registered recipients Location of servers by clients
- 6. IPv4 multicast address Class D Range from 224.0.0.0 through 239.255.255.255 A set of hosts listening to a IP multicast address is called a host group A host group can span multiple networks Membership to a host group is dynamic No restriction on number of hosts to a group Non member can send a message to a group 1 1 1 0 28 bit multicast gp ID
- 7. Special multicast addresses 224.0.0.0 Reserved; not used 224.0.0.1 All devices on the subnet 224.0.0.2 All routers on the subnet 224.0.0.11 Mobile agents (for Mobile IP) 224.0.0.12 DHCP Server / Relay Agent
- 8. Ethernet Multicast address Host group to multicast source IP : group IP address MAC : next hop address Multicast source to Host group IP : group IP address MAC : Ethernet multicast address
- 9. IPv6 Multicast address Format prefix : FF Flags: 000T T= 0: well known permanent T=1: temporary Scope: 0 to 15 0: reserved 1: interface local 2: link local 3 : reserved 4 : admin local 5 : site local 8 : org local E : global F : reserved Multicast address structure
- 10. IPv6 Multicast address Group ID – Identifies the multicast group and is unique within the scope Two well known group IDs 1 : all nodes 2 : all routers Examples: FF01::1 (interface-local scope all-nodes multicast address) FF02::1 (link-local scope all-nodes multicast address) FF01::2 (interface-local scope all-routers multicast address) FF02::2 (link-local scope all-routers multicast address) FF05::2 (site-local scope all-routers multicast address)
- 11. Mapping IPv6 Multicast Addresses to Ethernet Addresses When sending IPv6 multicast packets on an Ethernet link, the corresponding destination MAC address is 33-33-mm-mm-mm-mm where mm-mm-mm-mm is a direct mapping of the last 32 bits of the IPv6 multicast address, as shown in Figure above
- 12. Solicited node multicast address Format: FF02:0:0:0:0:1:FFXX:XXXX It is formed by taking the lower order 24 bits of ( unicast or anycast ) When A node n1 which wants to find MAC address of node n2, it will send neighbor solicitation message with solicited multicast address to n2. Node n2 will reply with neighbor advertisement message
- 13. A Node’s multi cast addresses For example, a host with the Ethernet MAC address of 00-AA-00-3F-2A-1C (link-local address of FE80::2AA:FF:FE3F:2A1C) registers the following multicast MAC addresses with the Ethernet adapter: The address of 33-33-00-00-00-01, which corresponds to the link-local scope all-nodes multicast address of FF02::1. The address of 33-33-FF-3F-2A-1C, which corresponds to the solicited-node address of FF02::1:FF3F:2A1C. Remember that the solicited-node address is the prefix FF02::1:FF00:0/104 and the last 24-bits of the unicast IPv6 address.
- 14. Mapping of class D IP address into Ethernet Multicast Address IANA has allotted 01:00:5e:00:00:00 through 01:00:5e:7f:ff:ff for multicast Lower order 23 bits if gp id copied to ethernet address Eg1: 224.128.64.32 (hex e0.80.40.20) maps to hex 01:00:5e:00:40:20 Eg2 : 224.0.64.32 (hex e0.00.40.20) also maps to hex 01:00:5e:00:40:20 IEEE 802 address