Brocade Communications Systems ServerIron ADX 12.4.00 Service Manual download pdf (Page 61)

ServerIron ADX Advanced Server Load Balancing Guide 49

53-1002435-01

Other TCS options

• The BAR does not send an ICMP redirect in this case, as you might expect. A router sends

ICMP redirects only if the following conditions are met:

• The interface on which the packet comes into the router is the same as the interface on which

the packet gets routed out.

• The route being used for the outgoing packet must not have been created or modified by an

ICMP redirect, and must not be the router's default route.

• The sub-net of the source IP address is the same as the sub-net of the next-hop IP address of

the routed packet.

• The packet must not be source routed.

• The router must be configured to send redirects.

The third rule is violated here because caches put the web server’s address in the source address

field rather than the cache’s address. Thus in this scenario, the packet is retransmitted to the best

next hop router (the RAS) but no ICMP redirect is sent.

The ServerIron ADX Solution

The CRO feature of the ServerIron ADX is a Layer 2 mechanism that solves the problem described

above. When the cache server responds to a client, the first packet is forwarded to the BAR as

discussed above. The BAR then retransmits the packet with the RAS as the destination MAC

address and the BAR as the source MAC. The ServerIron ADX examines the packet at Layer 4. The

ServerIron ADX finds a session table entry for this packet and knows it came from the cache server.

The ServerIron ADX knows the packet has been re-transmitted because the packet’s source MAC

address isn’t the cache server’s MAC address and the input port isn’t the cache server’s port. The

ServerIron ADX also recognizes that for this particular TCP session, it has seen the same packet

with two different destination MAC addresses and knows that the second MAC address is the more

appropriate one to use.

The ServerIron ADX contains a state table that includes a field for a MAC address. Initially this field

is blank. If the ServerIron ADX sees that a packet has been re-transmitted, the ServerIron ADX

places the new destination MAC address (the RAS MAC address) in the state table. When

subsequent packets are sent from the cache server, the ServerIron ADX sees that there is a MAC

address in the state table and replaces the destination MAC address with this address and

forwards the packet.

How Cache Route Optimization Works

Each TCP connection between the cache and a client is tracked by the ServerIron ADX in a state

table. The state table uses a key made up of the Layer 4 header: Source IP address, Source TCP

port, Destination IP address, and Destination TCP port. The state table also has a field for a MAC

address. This field is initially set to null (empty). When the cache server sends a packet a client,

the ServerIron ADX examines its Layer 4 header and checks to see whether it matches an entry in

the state table. The ServerIron ADX also examines the source MAC address to verify that the cache

sent the packet. If the MAC address field in the state table is null, and it will be for the first packet,

the ServerIron ADX simply forwards the packet at Layer 2 to the cache’s default gateway, the BAR.

When the packet is re-transmitted by the BAR, the ServerIron ADX examines the Layer 4 header

again, and sees that it matches an existing connection. The ServerIron ADX also examines the

source MAC address to be sure the cache server sent the packet. In this case, the source MAC

address is the BAR’s MAC, not the cache server’s. The ServerIron ADX concludes that this packet

has been retransmitted and places the destination MAC address of the packet, the RAS’s MAC, into

the state table’s MAC address field for this connection. Then the packet is forwarded to the RAS at

Layer 2.

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Brocade Communications Systems ServerIron ADX 12.4.00 Service Manual Page 61

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