Nt1310 Unit 8 Assignment 1

The problematic scenario is when the physical sequence has a loop. In this scenario, a switch which is attached to the middlebox literally cannot identify the middlebox segment based on the combination of the input interface and packet header fields. To address this scenario, SIMPLE introduce a ProcState tag that encodes the packet’s processing state inside the packet header fields using either VLAN tags, unused fields in the IP headers i.e., IP ToS fields or MPLS labels depending on the fields supported in the SDN-enabled switches. Therefore, SDN controller installs tag addition forwarding rules at the first switch of each middlebox segment, which is based on packet header fields and input ports of the switch. In addition, downstream switches consider these tags for the matching purpose and perform respective forwarding action. This process continues until the middlebox segment is finished.
Figure 15 shows how we can benefit from the tag addition forwarding rules at switch S2:{HTTP, from FW1} -> ProcState = FW; {HTTP, from Proxy1} -> ProcState = Proxy.
The forwarding rules at switch S5 are: {HTTP, ProcState = FW} -> forward to IDS1; and {HTTP, ProcState = Proxy} -> forward to destination. The key concept of this approach is that switch S5 can easily benefit from the ProcState tags to characterize between the first instance of the packet appears in the second segment then forward to IDS and the fourth segment of the packet appears then forward to the
destination.