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L3 Mobility Implementation With WS5100 version 3.0

Friday, February 14, 2014 2:34 AM
L3 Mobility Implementation
With WS5100 version 3.0
·        Implementation Overview
·        Key Concept
·        Mobility Exchange Protocol : Peer Configuration
·        Mobility Exchange Protocol : MU Database
·        L3 Mobility Exchange Messages
·        L3 /L2 Roam operation and data forwarding
·        Test Network Diagram
·        Configuring L3 Mobility
·        Verify L3 Mobility
Implementation Overview
-         Driving Requirement:
In large campuses / enterprises with a highly segmented network, provide the ability for mobile units (MUs) to roam across Subnets (L3 boundaries) when associated with Symbol Wireless Switches and retain their IP Address.
Deliver QoS and Security as the devices move across Subnets
-         Layer 3 mobility is a mechanism which enables a Mobile Unit (MU) to maintain the same Layer 3 address while roaming throughout a multi-VLAN network.
This enables transparent routing of IP datagrams to MUs during their movement, so that data sessions can be initiated to them while they roam (in particular for Voice applications).
Layer 3 mobility also enables TCP/UDP sessions to be maintained in spite of roaming among different IP subnets.
-         Seamless roaming of MUs between wireless switches on different Layer 3 subnets, while retaining the same IP address.
-         Static configuration of “Mobility Peer” switches
-         Plans to automate the peer discovery process in a future release.
This solution does not require any changes to the MU. In comparison, other solutions to this problem such as Mobile IP [RFC 3344] require special functionality & software on the mobile unit.
This creates numerous inter-working problems with working with MUs from different vendors/ legacy devices which do not support this
Support for a maximum of 15 mobility peers, with each switch handling up to a maximum of 500 MUs.
A full mesh of GRE tunnels would be established between the mobility peers.
Each tunnel is between a pair of switches and would be capable of handling data traffic for all MUs (for all VLANs) associated directly or indirectly (if the switch is the HS and not the CS) with the MU.
Data traffic for roamed MUs is tunneled between and the home and current switches by encapsulating the entire L2 packet inside GRE with a proprietary code-point.
L2oGRE was chosen instead of IPoGRE so that VLAN headers can be carried across the tunnels to identify the VLAN of broadcast/multicast packets.
When MUs roam within the same VLAN, the current behavior is retained by re-homing the MU to the new switch so that extra hops are avoided while forwarding data traffic
Mobile units can be assigned IP addresses statically or dynamically.
The forward and reverse data paths for traffic originating from and destined to MUs that have roamed from one L3 subnet to another will be symmetric.
Key Concepts (Terminology)

•         Mobility Domain:
–        A Mobility Domain comprises of a network of Wireless Switches to which an MU can roam seamlessly without changing its IP address.
–        The initial implementation will support only a single mobility domain.
•         Home Switch (HS): 
–        As soon as a MU enters a mobility domain by associating with a switch, it is first assigned a “Home Switch”
–        The HS for a MU does not change for the remainder of the MU’s stay in the mobility domain.
–        All data packets transmitted/received by the MU including DHCP and ARP is tunneled through the HS.
–        The IP address for the MU is assigned from the RON subnet of the HS.
•         Current Switch (CS):
–        The CS for the MU is the switch in the mobility domain to which it is currently associated to and keeps changing as the MU continues to roam between the different switches.
–        The CS is also responsible for delivering data packets from the MU to its HS and vice-versa.
-         Mobility Exchange protocol : Peer Configuration
•         Peer Configuration and Establishment
–        All Wireless Switches that support the L3 mobility functionality peer with every other switch in the mobility domain to exchange mobility related control plane information.
•         This includes the IP address, MAC address, HS IP address, CS IP address and HS-VLAN-id of all the MUs in the mobility-domain.
•         A consistent peer configuration resulting in a full-mesh of peering sessions is required for L3 mobility to work correctly.
–        Peering sessions use TCP as the transport layer protocol to carry mobility update messages. Using TCP provides the following advantages:
•         TCP retransmits lost messages thereby providing reliable connectivity
•         TCP ensures in-order delivery of messages using sequence numbers.
•         TCP has a built-in keep-alive mechanism which helps detect loss of connectivity to the peer or peer failure.
–        In WS5100 v3.0, this feature will have support only for static configuration of mobility peers with plans to automate to automate the peer-discovery and establishment process in a future release.
•         Mobility Exchange Protocol : MU Database
•         MU-Database
–        The MU database is a complete set of all MUs currently associated with switches in the mobility domain.
–        Every switch needs to be aware of all the MUs and their mobility-related parameters to distinguish between new MUs entering the network and existing MUs roaming within the mobility domain.
–        The mobility related parameters include:
•         MU MAC address
•         MU IP-address
•         Home Switch IP address
•         Current Switch IP address
•         Home Switch VLAN identifier
–        As soon as a peering session is established between two switches, the initial data flow involves the exchange of the full MU database.
•         The protocol does not require periodic refresh of the entire MU database and only incremental updates are sent as the database changes.
–        The MU database within a switch consists of two distinct parts:
•         Home MU Database (HMDB): 
•         The set of MUs for which this switch is the HS.
•         Foreign MU Database (FMDB):
•         The set of MUs for which we are not the HS.
•         These MUs are learnt from other peers in the mobility domain via Mobility Exchange messages.
L3 Roam Operation and Data Forwarding
•         The MU first associates with a wireless switch in the mobility domain.
•         This switch becomes the “home switch” (HS) for the MU.
•         The HS sends a JOIN message with MU’s MAC-address, IP-address and HS-VLAN information to all its peers.
•         When the MU roams to a WS on a different L3 subnet, this switch becomes the CS for the MU and sends out a L3-ROAM message to the HS, which is then relayed out to all the peers.
•         The CS tunnels all data packets (including DHCP and ARP) transmitted by the MU out to the HS, which then decapsulates and forwards the packet as if the MU were local to the HS.
•         The MU thus continues to retain its IP address in the HS-VLAN.

•         All packets destined to the MU are tunneled back by the HS to the CS


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