Representation of Network topology in python

1
votes

This is the code for class Switches from https://github.com/osrg/ryu/blob/master/ryu/topology/switches.py#L429

The member variables of particular interest to me in the class Switches are the following.

    self.dps = {}                 # datapath_id => Datapath class
    self.port_state = {}          # datapath_id => ports
    self.ports = PortDataState()  # Port class -> PortData class
    self.links = LinkState()      # Link class -> timestamp
    self.is_active = True

These are the member variables that the RYU uses to cache the topology details.I am trying to figure out how a topology can be represented using the following variables.

1) dps is a dictionary which maps datapath_id to datapath class?

- Can someone explain to me what is a datapath_id and a datapath class?

2) port_state is a dictionary which maps datapath id to ports

- As per my understanding on a switch without a VLAN all the ports will belong to the           same datapath id? In case of a switch with VLAN ports on the switch can have multiple datapath id's. Is my understanding correct?

3) ports is again a dictionary which maps Port class to PortData class?

- what does this mean?

4) links is again a dictionary which maps Link class to timestamp

- again what does this mean?

I am trying to get my head around how RYU controller stores the topology information using the above structures.Any help in understanding or explanation would be greatly appreciated.

2
pythonnetworkingswitch-statementethernetopenflow

2 Answers

1
votes

Generically , Each OpenFlow switch connects to a controller on a specific port each such connection between a switch and a controller is seen as an individual 'datapath' and its generally referred to as DPID (DataPath ID).

a link is perceived as a connection between ports belonging to 2 DPIDs, hence DPID 1 (PORT1) <—> (PORT2)DPID2 is a link between SW1 P1 and SW2 P1.

I use POX controller, so not very much across Ryu, but I think you can now make sense of it.

0
votes

Here is what I think of the answers:

  1. save a datapath class for each datapth id. For example in line of 475 in switches.py: self.dps[dp.id] = dp This way with one datapath id the whole class is easily findable.

  2. PortState is defined like PortState(dict) at line 161 which means it's a dictionary. Where the dict has key of int port_no and a value of OFPPort port.
    So self.port_state saves the ports for each datapath id. For example for self.port_state[1] it would contain the ports with datapath id of 1.

  3. PortDataState is defined like PortDataState(dict) at line 206 which means it's a dictionary. It is some sort of doubly linked list in terms of dictionary. It seems this is more like a list of ports as a key and then their status as a value.

  4. LinkState keeps track of links. For a link between src and dst, it creates a Link object. And saved a mapping between dst and src (This is done using self._map structure). Also, for each created Link object it keeps a record of the time (I dont know why it does that).
    Have a look at line 317 of switches.py

Here is good tutorial for topo discovery in Ryu.