# Packet library

## Introduction

Ryu packet library helps you to parse and build various protocol packets. dpkt is the popular library for the same purpose, however it is not designed to handle protocols that are interleaved; vlan, mpls, gre, etc. So we implemented our own packet library.

## Network Addresses

Unless otherwise specified, MAC/IPv4/IPv6 addresses are specified using human readable strings for this library. For example, '08:60:6e:7f:74:e7', '192.0.2.1', 'fe80::a60:6eff:fe7f:74e7'.

## Parsing Packet

First, let's look at how we can use the library to parse the received packets in a handler for OFPPacketIn messages.

    from ryu.lib.packet import packet

    @handler.set_ev_cls(ofp_event.EventOFPPacketIn, handler.MAIN_DISPATCHER)
    def packet_in_handler(self, ev):
        pkt = packet.Packet(array.array('B', ev.msg.data))
        for p in pkt.protocols:
            print p

You can create a Packet class instance with the received raw data. Then the packet library parses the data and creates protocol class instances included the data. The packet class 'protocols' has the protocol class instances.

If a TCP packet is received, something like the following is printed:

    <ryu.lib.packet.ethernet.ethernet object at 0x107a5d790>
    <ryu.lib.packet.vlan.vlan object at 0x107a5d7d0>
    <ryu.lib.packet.ipv4.ipv4 object at 0x107a5d810>
    <ryu.lib.packet.tcp.tcp object at 0x107a5d850>

If vlan is not used, you see something like:

    <ryu.lib.packet.ethernet.ethernet object at 0x107a5d790>
    <ryu.lib.packet.ipv4.ipv4 object at 0x107a5d810>
    <ryu.lib.packet.tcp.tcp object at 0x107a5d850>

You can access to a specific protocol class instance by using the packet class iterator. Let's try to check VLAN id if VLAN is used:

    from ryu.lib.packet import packet

    @handler.set_ev_cls(ofp_event.EventOFPPacketIn, handler.MAIN_DISPATCHER)
    def packet_in_handler(self, ev):
        pkt = packet.Packet(array.array('B', ev.msg.data))
        for p in pkt:
            print p.protocol_name, p
            if p.protocol_name == 'vlan':
                print 'vid = ', p.vid

You see something like:

    ethernet <ryu.lib.packet.ethernet.ethernet object at 0x107a5d790>
    vlan <ryu.lib.packet.vlan.vlan object at 0x107a5d7d0>
    vid = 10
    ipv4 <ryu.lib.packet.ipv4.ipv4 object at 0x107a5d810>
    tcp <ryu.lib.packet.tcp.tcp object at 0x107a5d850>

## Building Packet

You need to create protocol class instances that you want to send, add them to a packet class instance via add_protocol method, and then call serialize method. You have the raw data to send. The following example is building an arp packet.

    from ryu.ofproto import ether
    from ryu.lib.packet import ethernet, arp, packet

    e = ethernet.ethernet(dst='ff:ff:ff:ff:ff:ff',
                          src='08:60:6e:7f:74:e7',
                          ethertype=ether.ETH_TYPE_ARP)
    a = arp.arp(hwtype=1, proto=0x0800, hlen=6, plen=4, opcode=2,
                src_mac='08:60:6e:7f:74:e7', src_ip='192.0.2.1',
                dst_mac='00:00:00:00:00:00', dst_ip='192.0.2.2')
    p = packet.Packet()
    p.add_protocol(e)
    p.add_protocol(a)
    p.serialize()
    print repr(p.data)  # the on-wire packet