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diff --git a/tutorial/tutorial.rst b/tutorial/tutorial.rst new file mode 100644 index 000000000..fab635030 --- /dev/null +++ b/tutorial/tutorial.rst @@ -0,0 +1,870 @@ +.. + Licensed under the Apache License, Version 2.0 (the "License"); you may + not use this file except in compliance with the License. You may obtain + a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, WITHOUT + WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the + License for the specific language governing permissions and limitations + under the License. + + Convention for heading levels in Open vSwitch documentation: + + ======= Heading 0 (reserved for the title in a document) + ------- Heading 1 + ~~~~~~~ Heading 2 + +++++++ Heading 3 + ''''''' Heading 4 + + Avoid deeper levels because they do not render well. + +======================================= +Open vSwitch Advanced Features Tutorial +======================================= + +Many tutorials cover the basics of OpenFlow. This is not such a tutorial. +Rather, a knowledge of the basics of OpenFlow is a prerequisite. If you do not +already understand how an OpenFlow flow table works, please go read a basic +tutorial and then continue reading here afterward. + +It is also important to understand the basics of Open vSwitch before you begin. +If you have never used ovs-vsctl or ovs-ofctl before, you should learn a little +about them before proceeding. + +Most of the features covered in this tutorial are Open vSwitch extensions to +OpenFlow. Also, most of the features in this tutorial are specific to the +software Open vSwitch implementation. If you are using an Open vSwitch port to +an ASIC-based hardware switch, this tutorial will not help you. + +This tutorial does not cover every aspect of the features that it mentions. +You can find the details elsewhere in the Open vSwitch documentation, +especially ``ovs-ofctl(8)`` and the comments in the +``include/openflow/nicira-ext.h`` and ``include/openvswitch/meta-flow.h`` +header files. + +Getting Started +--------------- + +This is a hands-on tutorial. To get the most out of it, you will need Open +vSwitch binaries. You do not, on the other hand, need any physical networking +hardware or even supervisor privilege on your system. Instead, we will use a +script called ``ovs-sandbox``, which accompanies the tutorial, that constructs +a software simulated network environment based on Open vSwitch. + +You can use ``ovs-sandbox`` three ways: + +* If you have already installed Open vSwitch on your system, then you should be + able to just run ``ovs-sandbox`` from this directory without any options. + +* If you have not installed Open vSwitch (and you do not want to install it), + then you can build Open vSwitch according to the instructions in the + `installation guide <INSTALL.rst>`__, without installing it. Then run + ``./ovs-sandbox -b DIRECTORY`` from this directory, substituting the Open + vSwitch build directory for ``DIRECTORY``. + +* As a slight variant on the latter, you can run ``make sandbox`` from an Open + vSwitch build directory. + +When you run ``ovs-sandbox``, it does the following: + +1. **CAUTION:** Deletes any subdirectory of the current directory named + "sandbox" and any files in that directory. + +2. Creates a new directory "sandbox" in the current directory. + +3. Sets up special environment variables that ensure that Open vSwitch programs + will look inside the "sandbox" directory instead of in the Open vSwitch + installation directory. + +4. If you are using a built but not installed Open vSwitch, installs the Open + vSwitch manpages in a subdirectory of "sandbox" and adjusts the ``MANPATH`` + environment variable to point to this directory. This means that you can + use, for example, ``man ovs-vsctl`` to see a manpage for the ``ovs-vsctl`` + program that you built. + +5. Creates an empty Open vSwitch configuration database under "sandbox". + +6. Starts ``ovsdb-server`` running under "sandbox". + +7. Starts ``ovs-vswitchd`` running under "sandbox", passing special options + that enable a special "dummy" mode for testing. + +8. Starts a nested interactive shell inside "sandbox". + +At this point, you can run all the usual Open vSwitch utilities from the nested +shell environment. You can, for example, use ``ovs-vsctl`` to create a bridge: + + $ ovs-vsctl add-br br0 + +From Open vSwitch's perspective, the bridge that you create this way is as real +as any other. You can, for example, connect it to an OpenFlow controller or +use ``ovs-ofctl`` to examine and modify it and its OpenFlow flow table. On the +other hand, the bridge is not visible to the operating system's network stack, +so ``ifconfig`` or ``ip`` cannot see it or affect it, which means that +utilities like ``ping`` and ``tcpdump`` will not work either. (That has its +good side, too: you can't screw up your computer's network stack by +manipulating a sandboxed OVS.) + +When you're done using OVS from the sandbox, exit the nested shell (by entering +the "exit" shell command or pressing Control+D). This will kill the daemons +that ``ovs-sandbox`` started, but it leaves the "sandbox" directory and its +contents in place. + +The sandbox directory contains log files for the Open vSwitch dameons. You can +examine them while you're running in the sandboxed environment or after you +exit. + +Using GDB +--------- + +GDB support is not required to go through the tutorial. It is added in case +user wants to explore the internals of OVS programs. + +GDB can already be used to debug any running process, with the usual +``gdb <program> <process-id>`` command. + +``ovs-sandbox`` also has a ``-g`` option for launching ovs-vswitchd under GDB. +This option can be handy for setting break points before ovs-vswitchd runs, or +for catching early segfaults. Similarly, a ``-d`` option can be used to run +ovsdb-server under GDB. Both options can be specified at the same time. + +In addition, a ``-e`` option also launches ovs-vswitchd under GDB. However, +instead of displaying a ``gdb>`` prompt and waiting for user input, +ovs-vswitchd will start to execute immediately. ``-r`` option is the +corresponding option for running ovsdb-server under gdb with immediate +execution. + +To avoid GDB mangling with the sandbox sub shell terminal, ``ovs-sandbox`` +starts a new xterm to run each GDB session. For systems that do not support X +windows, GDB support is effectively disabled. + +When launching sandbox through the build tree's make file, the ``-g`` option +can be passed via the ``SANDBOXFLAGS`` environment variable. ``make sandbox +SANDBOXFLAGS=-g`` will start the sandbox with ovs-vswitchd running under GDB in +its own xterm if X is available. + +Motivation +---------- + +The goal of this tutorial is to demonstrate the power of Open vSwitch flow +tables. The tutorial works through the implementation of a MAC-learning switch +with VLAN trunk and access ports. Outside of the Open vSwitch features that we +will discuss, OpenFlow provides at least two ways to implement such a switch: + +1. An OpenFlow controller to implement MAC learning in a "reactive" fashion. + Whenever a new MAC appears on the switch, or a MAC moves from one switch + port to another, the controller adjusts the OpenFlow flow table to match. + +2. The "normal" action. OpenFlow defines this action to submit a packet to + "the traditional non-OpenFlow pipeline of the switch". That is, if a flow + uses this action, then the packets in the flow go through the switch in the + same way that they would if OpenFlow was not configured on the switch. + +Each of these approaches has unfortunate pitfalls. In the first approach, +using an OpenFlow controller to implement MAC learning, has a significant cost +in terms of network bandwidth and latency. It also makes the controller more +difficult to scale to large numbers of switches, which is especially important +in environments with thousands of hypervisors (each of which contains a virtual +OpenFlow switch). MAC learning at an OpenFlow controller also behaves poorly +if the OpenFlow controller fails, slows down, or becomes unavailable due to +network problems. + +The second approach, using the "normal" action, has different problems. First, +little about the "normal" action is standardized, so it behaves differently on +switches from different vendors, and the available features and how those +features are configured (usually not through OpenFlow) varies widely. Second, +"normal" does not work well with other OpenFlow actions. It is +"all-or-nothing", with little potential to adjust its behavior slightly or to +compose it with other features. + +Scenario +-------- + +We will construct Open vSwitch flow tables for a VLAN-capable, +MAC-learning switch that has four ports: + +p1 + a trunk port that carries all VLANs, on OpenFlow port 1. + +p2 + an access port for VLAN 20, on OpenFlow port 2. + +p3, p4 + both access ports for VLAN 30, on OpenFlow ports 3 and 4, respectively. + +.. note:: + The ports' names are not significant. You could call them eth1 through eth4, + or any other names you like. + +.. note:: + An OpenFlow switch always has a "local" port as well. This scenario won't + use the local port. + +Our switch design will consist of five main flow tables, each of which +implements one stage in the switch pipeline: + +Table 0 + Admission control. + +Table 1 + VLAN input processing. + +Table 2 + Learn source MAC and VLAN for ingress port. + +Table 3 + Look up learned port for destination MAC and VLAN. + +Table 4 + Output processing. + +The section below describes how to set up the scenario, followed by a section +for each OpenFlow table. + +You can cut and paste the ``ovs-vsctl`` and ``ovs-ofctl`` commands in each of +the sections below into your ``ovs-sandbox`` shell. They are also available as +shell scripts in this directory, named ``t-setup``, ``t-stage0``, ``t-stage1``, +..., ``t-stage4``. The ``ovs-appctl`` test commands are intended for cutting +and pasting and are not supplied separately. + +Setup +----- + +To get started, start ``ovs-sandbox``. Inside the interactive shell that it +starts, run this command:: + + $ ovs-vsctl add-br br0 -- set Bridge br0 fail-mode=secure + +This command creates a new bridge "br0" and puts "br0" into so-called +"fail-secure" mode. For our purpose, this just means that the OpenFlow flow +table starts out empty. + +.. note:: + If we did not do this, then the flow table would start out with a single flow + that executes the "normal" action. We could use that feature to yield a + switch that behaves the same as the switch we are currently building, but + with the caveats described under "Motivation" above.) + +The new bridge has only one port on it so far, the "local port" br0. We need +to add ``p1``, ``p2``, ``p3``, and ``p4``. A shell ``for`` loop is one way to +do it:: + + for i in 1 2 3 4; do + ovs-vsctl add-port br0 p$i -- set Interface p$i ofport_request=$i + ovs-ofctl mod-port br0 p$i up + done + +In addition to adding a port, the ``ovs-vsctl`` command above sets its +``ofport_request`` column to ensure that port ``p1`` is assigned OpenFlow port +1, ``p2`` is assigned OpenFlow port 2, and so on. + +.. note:: + We could omit setting the ofport_request and let Open vSwitch choose port + numbers for us, but it's convenient for the purposes of this tutorial because + we can talk about OpenFlow port 1 and know that it corresponds to ``p1``. + +The ``ovs-ofctl`` command above brings up the simulated interfaces, which are +down initially, using an OpenFlow request. The effect is similar to ``ifconfig +up``, but the sandbox's interfaces are not visible to the operating system and +therefore ``ifconfig`` would not affect them. + +We have not configured anything related to VLANs or MAC learning. That's +because we're going to implement those features in the flow table. + +To see what we've done so far to set up the scenario, you can run a command +like ``ovs-vsctl show`` or ``ovs-ofctl show br0``. + +Implementing Table 0: Admission control +--------------------------------------- + +Table 0 is where packets enter the switch. We use this stage to discard +packets that for one reason or another are invalid. For example, packets with +a multicast source address are not valid, so we can add a flow to drop them at +ingress to the switch with:: + + $ ovs-ofctl add-flow br0 \ + "table=0, dl_src=01:00:00:00:00:00/01:00:00:00:00:00, actions=drop" + +A switch should also not forward IEEE 802.1D Spanning Tree Protocol (STP) +packets, so we can also add a flow to drop those and other packets with +reserved multicast protocols:: + + $ ovs-ofctl add-flow br0 \ + "table=0, dl_dst=01:80:c2:00:00:00/ff:ff:ff:ff:ff:f0, actions=drop" + +We could add flows to drop other protocols, but these demonstrate the pattern. + +We need one more flow, with a priority lower than the default, so that flows +that don't match either of the "drop" flows we added above go on to pipeline +stage 1 in OpenFlow table 1:: + + $ ovs-ofctl add-flow br0 "table=0, priority=0, actions=resubmit(,1)" + +.. note:: + The "resubmit" action is an Open vSwitch extension to OpenFlow. + +Testing Table 0 +--------------- + +If we were using Open vSwitch to set up a physical or a virtual switch, then we +would naturally test it by sending packets through it one way or another, +perhaps with common network testing tools like ``ping`` and ``tcpdump`` or more +specialized tools like Scapy. That's difficult with our simulated switch, +since it's not visible to the operating system. + +But our simulated switch has a few specialized testing tools. The most +powerful of these tools is ``ofproto/trace``. Given a switch and the +specification of a flow, ``ofproto/trace`` shows, step-by-step, how such a flow +would be treated as it goes through the switch. + +Example 1 +~~~~~~~~~ + +Try this command:: + + $ ovs-appctl ofproto/trace br0 in_port=1,dl_dst=01:80:c2:00:00:05 + +The output should look something like this:: + + Flow: metadata=0,in_port=1,vlan_tci=0x0000,dl_src=00:00:00:00:00:00,dl_dst=01:80:c2:00:00:05,dl_type=0x0000 + Rule: table=0 cookie=0 dl_dst=01:80:c2:00:00:00/ff:ff:ff:ff:ff:f0 + OpenFlow actions=drop + + Final flow: unchanged + Datapath actions: drop + +The first block of lines describes an OpenFlow table lookup. The first line +shows the fields used for the table lookup (which is mostly zeros because +that's the default if we don't specify everything). The second line gives the +OpenFlow flow that the fields matched (called a "rule" because that is the name +used inside Open vSwitch for an OpenFlow flow). In this case, we see that this +packet that has a reserved multicast destination address matches the rule that +drops those packets. The third line gives the rule's OpenFlow actions. + +The second block of lines summarizes the results, which are not very +interesting here. + +Example 2 +~~~~~~~~~ + +Try another command:: + + $ ovs-appctl ofproto/trace br0 in_port=1,dl_dst=01:80:c2:00:00:10 + +The output should be:: + + Flow: metadata=0,in_port=1,vlan_tci=0x0000,dl_src=00:00:00:00:00:00,dl_dst=01:80:c2:00:00:10,dl_type=0x0000 + Rule: table=0 cookie=0 priority=0 + OpenFlow actions=resubmit(,1) + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + No match + + Final flow: unchanged + Datapath actions: drop + +This time the flow we handed to ``ofproto/trace`` doesn't match any of our +"drop" rules, so it falls through to the low-priority "resubmit" rule, which we +see in the rule and the actions selected in the first block. The "resubmit" +causes a second lookup in OpenFlow table 1, described by the additional block +of indented text in the output. We haven't yet added any flows to OpenFlow +table 1, so no flow actually matches in the second lookup. Therefore, the +packet is still actually dropped, which means that the externally observable +results would be identical to our first example. + +Implementing Table 1: VLAN Input Processing +------------------------------------------- + +A packet that enters table 1 has already passed basic validation in table 0. +The purpose of table 1 is validate the packet's VLAN, based on the VLAN +configuration of the switch port through which the packet entered the switch. +We will also use it to attach a VLAN header to packets that arrive on an access +port, which allows later processing stages to rely on the packet's VLAN always +being part of the VLAN header, reducing special cases. + +Let's start by adding a low-priority flow that drops all packets, before we add +flows that pass through acceptable packets. You can think of this as a +"default drop" rule:: + + $ ovs-ofctl add-flow br0 "table=1, priority=0, actions=drop" + +Our trunk port ``p1``, on OpenFlow port 1, is an easy case. ``p1`` accepts any +packet regardless of whether it has a VLAN header or what the VLAN was, so we +can add a flow that resubmits everything on input port 1 to the next table:: + + $ ovs-ofctl add-flow br0 \ + "table=1, priority=99, in_port=1, actions=resubmit(,2)" + +On the access ports, we want to accept any packet that has no VLAN header, tag +it with the access port's VLAN number, and then pass it along to the next +stage:: + + $ ovs-ofctl add-flows br0 - <<'EOF' + table=1, priority=99, in_port=2, vlan_tci=0, actions=mod_vlan_vid:20, resubmit(,2) + table=1, priority=99, in_port=3, vlan_tci=0, actions=mod_vlan_vid:30, resubmit(,2) + table=1, priority=99, in_port=4, vlan_tci=0, actions=mod_vlan_vid:30, resubmit(,2) + EOF + +We don't write any rules that match packets with 802.1Q that enter this stage +on any of the access ports, so the "default drop" rule we added earlier causes +them to be dropped, which is ordinarily what we want for access ports. + +.. note:: + Another variation of access ports allows ingress of packets tagged with VLAN + 0 (aka 802.1p priority tagged packets). To allow such packets, replace + ``vlan_tci=0`` by ``vlan_tci=0/0xfff`` above. + +Testing Table 1 +--------------- + +``ofproto/trace`` allows us to test the ingress VLAN rules that we added above. + +Example 1: Packet on Trunk Port +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Here's a test of a packet coming in on the trunk port:: + + $ ovs-appctl ofproto/trace br0 in_port=1,vlan_tci=5 + +The output shows the lookup in table 0, the resubmit to table 1, and the +resubmit to table 2 (which does nothing because we haven't put anything there +yet):: + + Flow: metadata=0,in_port=1,vlan_tci=0x0005,dl_src=00:00:00:00:00:00,dl_dst=00:00:00:00:00:00,dl_type=0x0000 + Rule: table=0 cookie=0 priority=0 + OpenFlow actions=resubmit(,1) + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + Rule: table=1 cookie=0 priority=99,in_port=1 + OpenFlow actions=resubmit(,2) + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + No match + + Final flow: unchanged + Datapath actions: drop + +Example 2: Valid Packet on Access Port +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Here's a test of a valid packet (a packet without an 802.1Q header) coming in +on access port ``p2``:: + + $ ovs-appctl ofproto/trace br0 in_port=2 + +The output is similar to that for the previous case, except that it +additionally tags the packet with ``p2``'s VLAN 20 before it passes it along to +table 2:: + + Flow: metadata=0,in_port=2,vlan_tci=0x0000,dl_src=00:00:00:00:00:00,dl_dst=00:00:00:00:00:00,dl_type=0x0000 + Rule: table=0 cookie=0 priority=0 + OpenFlow actions=resubmit(,1) + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + Rule: table=1 cookie=0 priority=99,in_port=2,vlan_tci=0x0000 + OpenFlow actions=mod_vlan_vid:20,resubmit(,2) + + Resubmitted flow: metadata=0,in_port=2,dl_vlan=20,dl_vlan_pcp=0,dl_src=00:00:00:00:00:00,dl_dst=00:00:00:00:00:00,dl_type=0x0000 + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + No match + + Final flow: unchanged + Datapath actions: drop + +Example 3: Invalid Packet on Access Port +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +This tests an invalid packet (one that includes an 802.1Q header) coming in on +access port ``p2``:: + + $ ovs-appctl ofproto/trace br0 in_port=2,vlan_tci=5 + +The output shows the packet matching the default drop rule:: + + Flow: metadata=0,in_port=2,vlan_tci=0x0005,dl_src=00:00:00:00:00:00,dl_dst=00:00:00:00:00:00,dl_type=0x0000 + Rule: table=0 cookie=0 priority=0 + OpenFlow actions=resubmit(,1) + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + Rule: table=1 cookie=0 priority=0 + OpenFlow actions=drop + + Final flow: unchanged + Datapath actions: drop + +Implementing Table 2: MAC+VLAN Learning for Ingress Port +-------------------------------------------------------- + +This table allows the switch we're implementing to learn that the packet's +source MAC is located on the packet's ingress port in the packet's VLAN. + +.. note:: + This table is a good example why table 1 added a VLAN tag to packets that + entered the switch through an access port. We want to associate a MAC+VLAN + with a port regardless of whether the VLAN in question was originally part of + the packet or whether it was an assumed VLAN associated with an access port. + +It only takes a single flow to do this. The following command adds it:: + + $ ovs-ofctl add-flow br0 \ + "table=2 actions=learn(table=10, NXM_OF_VLAN_TCI[0..11], \ + NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[], \ + load:NXM_OF_IN_PORT[]->NXM_NX_REG0[0..15]), \ + resubmit(,3)" + +The "learn" action (an Open vSwitch extension to OpenFlow) modifies a flow +table based on the content of the flow currently being processed. Here's how +you can interpret each part of the "learn" action above: + +``table=10`` + Modify flow table 10. This will be the MAC learning table. + +``NXM_OF_VLAN_TCI[0..11]`` + Make the flow that we add to flow table 10 match the same VLAN ID that the + packet we're currently processing contains. This effectively scopes the + MAC learning entry to a single VLAN, which is the ordinary behavior for a + VLAN-aware switch. + +``NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[]`` + Make the flow that we add to flow table 10 match, as Ethernet destination, + the Ethernet source address of the packet we're currently processing. + +``load:NXM_OF_IN_PORT[]->NXM_NX_REG0[0..15]`` + Whereas the preceding parts specify fields for the new flow to match, this + specifies an action for the flow to take when it matches. The action is + for the flow to load the ingress port number of the current packet into + register 0 (a special field that is an Open vSwitch extension to OpenFlow). + +.. note:: + A real use of "learn" for MAC learning would probably involve two additional + elements. First, the "learn" action would specify a hard_timeout for the new + flow, to enable a learned MAC to eventually expire if no new packets were + seen from a given source within a reasonable interval. Second, one would + usually want to limit resource consumption by using the Flow_Table table in + the Open vSwitch configuration database to specify a maximum number of flows + in table 10. + +This definitely calls for examples. + +Testing Table 2 +--------------- + +Example 1 +~~~~~~~~~ + +Try the following test command:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=1,vlan_tci=20,dl_src=50:00:00:00:00:01 -generate + +The output shows that "learn" was executed, but it isn't otherwise informative, +so we won't include it here. + +The ``-generate`` keyword is new. Ordinarily, ``ofproto/trace`` has no side +effects: "output" actions do not actually output packets, "learn" actions do +not actually modify the flow table, and so on. With ``-generate``, though, +``ofproto/trace`` does execute "learn" actions. That's important now, because +we want to see the effect of the "learn" action on table 10. You can see that +by running:: + + $ ovs-ofctl dump-flows br0 table=10 + +which (omitting the ``duration`` and ``idle_age`` fields, which will vary based +on how soon you ran this command after the previous one, as well as some other +uninteresting fields) prints something like:: + + NXST_FLOW reply (xid=0x4): + table=10, vlan_tci=0x0014/0x0fff,dl_dst=50:00:00:00:00:01 actions=load:0x1->NXM_NX_REG0[0..15] + +You can see that the packet coming in on VLAN ``20`` with source MAC +``50:00:00:00:00:01`` became a flow that matches VLAN ``20`` (written in +hexadecimal) and destination MAC ``50:00:00:00:00:01``. The flow loads port +number ``1``, the input port for the flow we tested, into register 0. + +Example 2 +~~~~~~~~~ + +Here's a second test command:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=2,dl_src=50:00:00:00:00:01 -generate + +The flow that this command tests has the same source MAC and VLAN as example 1, +although the VLAN comes from an access port VLAN rather than an 802.1Q header. +If we again dump the flows for table 10 with:: + + $ ovs-ofctl dump-flows br0 table=10 + +then we see that the flow we saw previously has changed to indicate that the +learned port is port 2, as we would expect:: + + NXST_FLOW reply (xid=0x4): + table=10, vlan_tci=0x0014/0x0fff,dl_dst=50:00:00:00:00:01 actions=load:0x2->NXM_NX_REG0[0..15] + +Implementing Table 3: Look Up Destination Port +---------------------------------------------- + +This table figures out what port we should send the packet to based on the +destination MAC and VLAN. That is, if we've learned the location of the +destination (from table 2 processing some previous packet with that destination +as its source), then we want to send the packet there. + +We need only one flow to do the lookup:: + + $ ovs-ofctl add-flow br0 \ + "table=3 priority=50 actions=resubmit(,10), resubmit(,4)" + +The flow's first action resubmits to table 10, the table that the "learn" +action modifies. As you saw previously, the learned flows in this table write +the learned port into register 0. If the destination for our packet hasn't +been learned, then there will be no matching flow, and so the "resubmit" turns +into a no-op. Because registers are initialized to 0, we can use a register 0 +value of 0 in our next pipeline stage as a signal to flood the packet. + +The second action resubmits to table 4, continuing to the next pipeline stage. + +We can add another flow to skip the learning table lookup for multicast and +broadcast packets, since those should always be flooded:: + + $ ovs-ofctl add-flow br0 \ + "table=3 priority=99 dl_dst=01:00:00:00:00:00/01:00:00:00:00:00 \ + actions=resubmit(,4)" + +.. note:: + We don't strictly need to add this flow, because multicast addresses will + never show up in our learning table. (In turn, that's because we put a flow + into table 0 to drop packets that have a multicast source address.) + +Testing Table 3 +--------------- + +Example +~~~~~~~ + +Here's a command that should cause OVS to learn that ``f0:00:00:00:00:01`` is +on ``p1`` in VLAN ``20``:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_vlan=20,dl_src=f0:00:00:00:00:01,dl_dst=90:00:00:00:00:01 \ + -generate + +Here's an excerpt from the output that shows (from the "no match" looking up +the resubmit to table 10) that the flow's destination was unknown:: + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + Rule: table=3 cookie=0 priority=50 + OpenFlow actions=resubmit(,10),resubmit(,4) + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + No match + +You can verify that the packet's source was learned two ways. The most direct +way is to dump the learning table with:: + + $ ovs-ofctl dump-flows br0 table=10 + +which ought to show roughly the following, with extraneous details removed:: + + table=10, vlan_tci=0x0014/0x0fff,dl_dst=f0:00:00:00:00:01 actions=load:0x1->NXM_NX_REG0[0..15] + +.. note:: + If you tried the examples for the previous step, or if you did some of your + own experiments, then you might see additional flows there. These + additional flows are harmless. If they bother you, then you can remove + them with `ovs-ofctl del-flows br0 table=10`. + +The other way is to inject a packet to take advantage of the learning entry. +For example, we can inject a packet on p2 whose destination is the MAC address +that we just learned on p1: + + $ ovs-appctl ofproto/trace br0 \ + in_port=2,dl_src=90:00:00:00:00:01,dl_dst=f0:00:00:00:00:01 -generate + +Here's an interesting excerpt from that command's output. This group of lines +traces the ``resubmit(,10)``, showing that the packet matched the learned flow +for the first MAC we used, loading the OpenFlow port number for the learned +port ``p1`` into register ``0``:: + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + Rule: table=10 cookie=0 vlan_tci=0x0014/0x0fff,dl_dst=f0:00:00:00:00:01 + OpenFlow actions=load:0x1->NXM_NX_REG0[0..15] + +If you read the commands above carefully, then you might have noticed that they +simply have the Ethernet source and destination addresses exchanged. That +means that if we now rerun the first ``ovs-appctl`` command above, e.g.: + + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_vlan=20,dl_src=f0:00:00:00:00:01,dl_dst=90:00:00:00:00:01 \ + -generate + +then we see in the output that the destination has now been learned:: + + Resubmitted flow: unchanged + Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0 + Resubmitted odp: drop + Rule: table=10 cookie=0 vlan_tci=0x0014/0x0fff,dl_dst=90:00:00:00:00:01 + OpenFlow actions=load:0x2->NXM_NX_REG0[0..15] + + +Implementing Table 4: Output Processing +--------------------------------------- + +At entry to stage 4, we know that register 0 contains either the desired output +port or is zero if the packet should be flooded. We also know that the +packet's VLAN is in its 802.1Q header, even if the VLAN was implicit because +the packet came in on an access port. + +The job of the final pipeline stage is to actually output packets. The job is +trivial for output to our trunk port ``p1``:: + + $ ovs-ofctl add-flow br0 "table=4 reg0=1 actions=1" + +For output to the access ports, we just have to strip the VLAN header before +outputting the packet:: + + $ ovs-ofctl add-flows br0 - <<'EOF' + table=4 reg0=2 actions=strip_vlan,2 + table=4 reg0=3 actions=strip_vlan,3 + table=4 reg0=4 actions=strip_vlan,4 + EOF + +The only slightly tricky part is flooding multicast and broadcast packets and +unicast packets with unlearned destinations. For those, we need to make sure +that we only output the packets to the ports that carry our packet's VLAN, and +that we include the 802.1Q header in the copy output to the trunk port but not +in copies output to access ports:: + + $ ovs-ofctl add-flows br0 - <<'EOF' + table=4 reg0=0 priority=99 dl_vlan=20 actions=1,strip_vlan,2 + table=4 reg0=0 priority=99 dl_vlan=30 actions=1,strip_vlan,3,4 + table=4 reg0=0 priority=50 actions=1 + EOF + +.. note:: + Our rules rely on the standard OpenFlow behavior that an output action will + not forward a packet back out the port it came in on. That is, if a packet + comes in on p1, and we've learned that the packet's destination MAC is also + on p1, so that we end up with ``actions=1`` as our actions, the switch will + not forward the packet back out its input port. The + multicast/broadcast/unknown destination cases above also rely on this + behavior. + +Testing Table 4 +--------------- + +Example 1: Broadcast, Multicast, and Unknown Destination +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Try tracing a broadcast packet arriving on ``p1`` in VLAN ``30``:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_dst=ff:ff:ff:ff:ff:ff,dl_vlan=30 + +The interesting part of the output is the final line, which shows that the +switch would remove the 802.1Q header and then output the packet to ``p3`` +and ``p4``, which are access ports for VLAN ``30``:: + + Datapath actions: pop_vlan,3,4 + +Similarly, if we trace a broadcast packet arriving on ``p3``:: + + $ ovs-appctl ofproto/trace br0 in_port=3,dl_dst=ff:ff:ff:ff:ff:ff + +then we see that it is output to ``p1`` with an 802.1Q tag and then to ``p4`` +without one:: + + Datapath actions: push_vlan(vid=30,pcp=0),1,pop_vlan,4 + +.. note:: + Open vSwitch could simplify the datapath actions here to just + ``4,push_vlan(vid=30,pcp=0),1`` but it is not smart enough to do so. + +The following are also broadcasts, but the result is to drop the packets +because the VLAN only belongs to the input port:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_dst=ff:ff:ff:ff:ff:ff + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_dst=ff:ff:ff:ff:ff:ff,dl_vlan=55 + +Try some other broadcast cases on your own:: + + $ ovs-appctl ofproto/trace br0 + in_port=1,dl_dst=ff:ff:ff:ff:ff:ff,dl_vlan=20 + $ ovs-appctl ofproto/trace br0 + in_port=2,dl_dst=ff:ff:ff:ff:ff:ff + $ ovs-appctl ofproto/trace br0 + in_port=4,dl_dst=ff:ff:ff:ff:ff:ff + +You can see the same behavior with multicast packets and with unicast +packets whose destination has not been learned, e.g.:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=4,dl_dst=01:00:00:00:00:00 + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_dst=90:12:34:56:78:90,dl_vlan=20 + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_dst=90:12:34:56:78:90,dl_vlan=30 + +Example 2: MAC Learning +~~~~~~~~~~~~~~~~~~~~~~~ + +Let's follow the same pattern as we did for table 3. First learn a MAC on port +``p1`` in VLAN ``30``:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_vlan=30,dl_src=10:00:00:00:00:01,dl_dst=20:00:00:00:00:01 \ + -generate + +You can see from the last line of output that the packet's destination is +unknown, so it gets flooded to both ``p3`` and ``p4``, the other ports in VLAN +``30``:: + + Datapath actions: pop_vlan,3,4 + +Then reverse the MACs and learn the first flow's destination on port ``p4``:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=4,dl_src=20:00:00:00:00:01,dl_dst=10:00:00:00:00:01 -generate + +The last line of output shows that the this packet's destination is known to be +``p1``, as learned from our previous command:: + + Datapath actions: push_vlan(vid=30,pcp=0),1 + +Now, if we rerun our first command:: + + $ ovs-appctl ofproto/trace br0 \ + in_port=1,dl_vlan=30,dl_src=10:00:00:00:00:01,dl_dst=20:00:00:00:00:01 \ + -generate + +...we can see that the result is no longer a flood but to the specified learned +destination port ``p4``: + + Datapath actions: pop_vlan,4 + +Contact +======= + +bugs@openvswitch.org +http://openvswitch.org/ |