Writing a Switchyard Program

A Switchyard program is simply a Python program that includes an explicit “startup” function. The startup function can either be named switchy_main, srpy_main, or simply main. This function must accept a single parameter, which is a reference to the Switchyard “network object” (described below). The network object is used to send and receive packets to and from network interfaces.

A Switchyard program isn’t executed directly with the Python interpreter. You will instead use the Switchyard program srpy.py to start up the Switchyard framework and tell srpy.py to load your code. Details on how to do this are given in the chapters on running a Switchyard in the “test” environment (Running in the test environment) and running Switchyard in a “live” environment (Running in a “live” environment).

A Switchyard program will typically also import other Switchyard modules such as modules for parsing and constructing packets, dealing with network addresses, and other functions. These modules are introduced below and described in detail in the API reference chapter (API Reference).

Introducing the “network object”

As mentioned above, a Switchyard program can simply have a main function that accepts a single argument. The parameter passed to main is called the “network object”. It is on this object that you can call methods for sending and receiving packets and getting information about ports on the device for which you’re implementing the logic.

Sending and receiving packets

Here is a program that receives one packet, prints it out, sends it back out the same interface, then quits.

from switchyard.lib.packet import *
from switchyard.lib.address import *
from switchyard.lib.common import *

def main(net):
    input_port,packet = net.recv_packet()
    print ("Received {} on {}".format(packet, input_port))
    net.send_packet(input_port, packet)

This program isn’t likely to be very useful — it is just meant as an illustration of two of the key methods on the network object:

recv_packet(timeout=None, timestamp=False)

Receive packets from any device on which one is available. Blocks until a packet is received, unless a timeout value >= 0 is supplied.

  • timeout (float) – The amount of time to wait to receive a packet, or None if the call should block until a packet is received (this is the default behavior)
  • timestamp (bool) – Indicate whether a timestamp associated with packet arrival is desired or not (default behavior is not to return a timestamp)

A tuple of length 2 or 3, depending whether the timestamp is desired. If no timestamp returns the device name (str) and the packet. If a timestamp, returns device name, timestamp, and the packet.

  • Shutdown – if the network device is shut down (i.e., by stopping the Switchyard program)
  • NoPackets – if no packets are received before the timeout expires.
send_packet(output_port, packet)

Send the Switchyard Packet object packet out the port named output_port.

  • output_port (str) – The name of the port on which to send the packet
  • packet (Packet) – A Switchyard packet object to send out the given interface


Raises SwitchyException:

if the output_port is invalid

Note that in the above call to recv_packet, no arguments are given so the call will block until a packet is received, and no timestamp will be returned (just the input port and the packet object). Importantly, note also that we aren’t handling any potential exceptions that could occur. In particular, we really should be handling at least the situation in which the framework is shut down (and we receive a Shutdown exception). Just for completeness, we should also handle the NoPackets exception, although if the code is designed to block indefinitely we shouldn’t receive that particular exception. (Note: these exceptions are defined in switchyard.lib.common.)

Let’s rewrite the code above, and now put everything in a while loop so that we keep reading and sending packets as long as we’re running. We will eventually turn this code into a working network hub implementation [1], but it’s currently broken because it still just sends a packet out the same port on which it arrived:

from switchyard.lib.packet import *
from switchyard.lib.address import *
from switchyard.lib.common import *

def main(net):
    while True:
            input_port,packet = net.recv_packet()
        except Shutdown:
            print ("Got shutdown signal; exiting")
        except NoPackets:
            print ("No packets were available.")

        # if we get here, we must have received a packet
        print ("Received {} on {}".format(packet, input_port))
        net.send_packet(input_port, packet)

Getting information about ports (interfaces) on the device

The only other methods available the network object relate to getting information about the ports/interfaces attached to the device on which the Switchyard code is running. The two basic methods are ports and interfaces:


Get a list of ports that are configured on the current network device. An alias method ports does exactly the same thing.

Returns:list of Interface objects

Each object returned from the interfaces or ports method is an instance of the class Interface and describes one interface/port on the device. The Interface class is defined in the module switchyard.lib.common:

class switchyard.lib.common.Interface

The name of the interface (e.g., eth0) as a string


The Ethernet address associated with the interface, as a switchyard.lib.address.EthAddr instance.


The IPv4 address associated with the interface, if any. Returns an object of type IPv4Address. If there is no address assigned to the interface, the address is A limitation with the Interface implementation in Switchyard at present is that only one address can be associated with an interface, and it must be an IPv4 address.


The network mask associated with the IPv4 address assigned to the interface. The netmask defaults to (/32) if none is specified.

For example, to simply print out information regarding each interface defined on the current network device, you could use the following program:

def srpy_main(net):
    for intf in net.interfaces():
        print (intf.name, intf.ethaddr, intf.ipaddr, intf.netmask)

    # could also be:
    # for intf in net.ports():
    #    ...

Entirely depending on how the network device is configured, output from the above program might look like the following:

eth2 10:00:00:00:00:03
eth1 10:00:00:00:00:02
eth0 10:00:00:00:00:01

Note that there is no ordering to the list of interfaces returned.

There are a few convenience methods related to ports and interfaces, which can be used to look up a particular interface given a name, IPv4 address, or Ethernet (MAC) address:


This method returns an Interface object given a string name of a interface. An alias method port_by_name(name) also exists.

Parameters:name (str) – The name of the device, e.g., “eth0”
Returns:An Interface object or None if the name is invalid

This method returns an Interface object given an IP address configured on one of the interfaces. The IP address may be given as a string or as an IPv4Address object. An alias method port_by_ipaddr(devicename) also exists.

Parameters:ipaddr (IP address as a string or as an IPv4Address object) –
Returns:An Interface object or None if the IP address isn’t configured on one of the ports

This method returns an Interface object given an Ethernet (MAC) address configured on one of the interfaces. An alias method port_by_macaddr(devicename) also exists.

Parameters:ethaddr (Ethernet address as a string (e.g. “11:22:33:44:55:66”) or as an instance of EthAddr class) –
Returns:An Interface object or None if the MAC address isn’t configured on one of the ports

Other methods on the network object

The only other method available on the network object is shutdown:

  • shutdown() this signals to the Switchyard framework that your program is done and exiting. It should be the last thing you call in a Switchyard program.

A really complete implementation of our hub is now:

from switchyard.lib.packet import *
from switchyard.lib.address import *
from switchyard.lib.common import *

def main(net):
    # add some informational text about ports on this device
    print ("Hub is starting up with these ports:")
    for port in net.ports():
        print ("{}: ethernet address {}".format(port.name, port.ethaddr))

    while True:
            input_port,packet = net.recv_packet()
        except Shutdown:
            # got shutdown signal
        except NoPackets:
            # try again...

        # send the packet out all ports *except*
        # the one on which it arrived
        for port in net.ports():
            if port.name != input_port:
                net.send_packet(port.name, packet)

    # shutdown is the last thing we should do

Introduction to packet parsing and construction

This section provides an overview of packet construction and parsing in Switchyard. For full details on these capabilities, see Packet parsing and construction.

Switchyard’s packet construction/parsing library is found in switchyard.lib.packet. It’s design is based on a few other libraries out there, including POX’s library [2] and Ryu’s library [3].

There are a few key ideas to understand when using the packet library:

  • The Packet class acts as a container of headers (rather, header objects).
  • Headers within a packet can be accessed through methods on the Packet container object, and also by indexing. Headers are ordered starting with lowest layer protocols. For example, if a Packet has an Ethernet header (which is likely to be the lowest layer protocol), this header can be accessed with index 0 as in pktobj[0]. Indexes can be integers, and they can also be packet header class names (e.g., Ethernet, IPv4, etc.). For example, to access the Ethernet header of a packet, you can write pktobj[Ethernet].
  • Fields in header objects are accessed through standard Python properties. (The code to manipulate header fields thus looks like it is just accessing instance variables.)
  • A packet object can be constructed by either expliciting instantiating and object and adding headers, or it can be formed by “adding” (using the + operator) headers together, or by adding headers onto a packet (using + or +=).
  • The Switchyard framework generally automatically handles serializing and deserializing Packet objects to and from byte sequences (i.e., wire format packets), but you can also explicitly invoke those methods if you need to.

Here are some examples using Ethernet, IPv4, and ICMP headers. First, let’s construct a packet object and add these headers to the packet:

>>> from switchyard.lib.packet import *
>>> p = Packet()   # construct a packet object
>>> e = Ethernet() # construct Ethernet header
>>> ip = IPv4()    # construct IPv4 header
>>> icmp = ICMP()  # construct ICMP header
>>> p += e         # add eth header to packet
>>> p += ip        # add ip header to packet
>>> p += icmp      # add icmp header to packet
>>> print (p)
Ethernet 00:00:00:00:00:00->00:00:00:00:00:00 IP | IPv4> ICMP | ICMP EchoRequest 0 0 (0 data bytes)

A shorthand for doing the above is:

>>> p = Ethernet() + IPv4() + ICMP()

The effect of “adding” headers together is to construct a packet, just as the first example. Note that with the above example, the default Ethertype for the Ethernet header is IPv4, and the default protocol number for IPv4 is ICMP. Thus, the above example is somewhat special in that we didn’t need to modify any of the packet header fields to create a (mostly) valid packet.

Switchyard does not ensure that a constructed Packet is sensible in any way. It is possible to put headers in the wrong order, to supply illogical values for header elements (e.g., a protocol number in the IPv4 header that doesn’t match the next header in the packet), and to do other invalid things. Switchyard gives you the tools for constructing packets, but doesn’t tell you how to do so.

The num_headers Packet method returns the number of headers in a packet, which returns the expected number for this example:

>>> p.num_headers()

Note that the len function on a packet returns the number of bytes that the Packet would consume if it was in wire (serialized) format. The size method returns the same value.

>>> len(p)
>>> p.size()

(Note: Ethernet header is 14 bytes + 20 bytes IP + 8 bytes ICMP = 42 bytes.)

Packet header objects can be accessed conveniently by indexing. Standard negative indexing also works. For example, to obtain a reference to the Ethernet header object and to inspect and modify the Ethernet header, we might do the following:

>>> p[0]
<switchyard.lib.packet.ethernet.Ethernet object at 0x104474248>
>>> p[0].src
>>> p[0].dst
>>> p[0].dst = "ab:cd:ef:00:11:22"
>>> str(p[0])
'Ethernet 00:00:00:00:00:00->ab:cd:ef:00:11:22 IP'
>>> p[0].dst = EthAddr("00:11:22:33:44:55")
>>> str(p[0])
'Ethernet 00:00:00:00:00:00->00:11:22:33:44:55 IP'
>>> p[0].ethertype
<EtherType.IP: 2048>
>>> p[0].ethertype = EtherType.ARP
>>> print (p)
Ethernet 00:00:00:00:00:00->00:00:00:00:00:00 ARP | IPv4> ICMP | ICMP EchoRequest 0 0 (0 data bytes)
>> p[0].ethertype = EtherType.IPv4 # set it back to sensible value

Note that all header field elements are accessed through properties. For Ethernet headers, there are three properties that can be inspected and modified, src, dst and ethertype, as shown above. Note again that Switchyard doesn’t prevent a user from setting header fields to illogical values, e.g., when we set the ethertype to ARP. All EtherType values are specified in switchyard.lib.packet.common, and imported when the module switchyard.lib.packet is imported.

Accessing header fields in other headers works similarly. Here are examples involving the IPv4 header:

>>> str(p[1])
'IPv4> ICMP'
>>> p[1].protocol
<IPProtocol.ICMP: 1>
>>> p[1].src
>>> p[1].dst
>>> p[1].dst = ''

IPv4 protocol values are specified in switchyard.lib.packet.common, just as with EtherType values. The full set of properties that can be manipulated in the IPv4 header as well as all other headers is described in the reference documentation for the packet library: Packet parsing and construction.

Lastly, an example with the ICMP header shows some now-familiar patterns. The main difference with ICMP is that the “data” portion of an ICMP packet changes, depending on the ICMP type. For example, if the type is 8 (ICMP echo request) the ICMP data becomes an object that allows the identifier and sequence values to be inspected and modified.

>>> p[2]
<switchyard.lib.packet.icmp.ICMP object at 0x104449c78>
>>> p[2].icmptype
<ICMPType.EchoRequest: 8>
>>> p[2].icmpcode
<EchoRequest.EchoRequest: 0>
>>> p[2].icmpdata
<switchyard.lib.packet.icmp.ICMPEchoRequest object at 0x1044742c8>
>>> icmp.icmpdata.sequence
>>> icmp.icmpdata.identifier
>>> icmp.icmpdata.identifier = 42
>>> icmp.icmpdata.sequence = 13
>>> print (p)
Ethernet 00:00:00:00:00:00->00:11:22:33:44:55 IP | IPv4> ICMP | ICMP EchoRequest 42 13 (0 data bytes)

By default, no “payload” data are included in with an ICMP header, but we can change that using the data property on the icmpdata part of the header:

>>> icmp.icmpdata.data = "hello, world"
>>> print (p)
Ethernet 00:00:00:00:00:00->00:11:22:33:44:55 IP | IPv4> ICMP | ICMP EchoRequest 42 13 (12 data bytes)

To serialize the packet into a wire format sequence of bytes, we can use the to_bytes() method:

>>> p.to_bytes()
b'\x00\x11"3DU\x00\x00\x00\x00\x00\x00\x08\x00E\x00\x00(\x00\x00\x00\x00\x00\x01\xba\xd6\x00\x00\x00\x00\x00\x00\x00\x00\x08\x00\xb7|\x00*\x00\rhello, world'

Other header classes that are available in Switchyard include Arp, UDP, TCP, IPv6, and ICMPv6. Again, see Packet parsing and construction for details on these header classes, and full documentation for all classes.

Utility functions (e.g., logging)

There are a few additional utility functions that are useful when developing a Switchyard program related to logging and debugging. These functions are all included by importing the module switchyard.lib.common.

Logging functions

Switchyard uses the standard Python logging facilities, but provides four convenience functions. Each of these functions takes a string as a parameter and prints it to the console as a logging message. The only difference with the functions relates to the logging level (see https://docs.python.org/3.4/library/logging.html#levels), and whether the output is colored to visually highlight a problem. The default logging level is INFO within Switchyard. If you wish to include debugging messages, you can use the -d flag for the various invocation programs (e.g., srpy), as described in Running in the test environment and Running in a “live” environment.


Write a debugging message to the log using the log level DEBUG.


Write a debugging message to the log using the log level INFO.


Write a debugging message to the log using the log level WARNING. Output is colored magenta.


Write a debugging message to the log using the log level CRITICAL. Output is colored red.

Alternatively, you can simply use the print statement to write to the console, but writing to the log provides a much more structured way of writing information to the screen.

Invoking the debugger

Although a longer discussion of debugging is included in a later section (debugging), it is worth mentioning that there is a built-in function named debugger that can be used anywhere in Switchyard code to immediately invoke the standard Python pdb debugger.

For example, if we add a call to debugger() in the example code above just after the try/except block, then run the code in a test environment (for details on how to do this, see Running in the test environment), the program pauses immediately after the call to debugger and the pdb prompt is shown:

# after hub code is started in test environment,
# some output is shown, followed by this:

> /Users/jsommers/Dropbox/src/switchyard/xhub.py(29)main()
-> for port in net.ports():
(Pdb) list
 25                 debugger()
 27                 # send the packet out all ports *except*
 28                 # the one on which it arrived
 29  ->             for port in net.ports():
 30                     if port.name != input_port:
 31                         packet[0].src = 'ab:cd:ef:ff:ff:ff'
 32                         net.send_packet(port.name, packet)

As you can see, the program is paused on the next executable line following the call to debugger(). At this point, any valid pdb commands can be given to inspect program state. Once again, see later sections for details on running Switchyard code (Running in the test environment, Running in a “live” environment) and on other debugging capabilities (debugging).

[1]A hub is a network device with multiple physical ports. Any packet to arrive on a port is sent back out all ports except for the one on which it arrived.