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diff --git a/tutorial/Tutorial.md b/tutorial/Tutorial.md new file mode 100644 index 000000000..0cf52fb15 --- /dev/null +++ b/tutorial/Tutorial.md @@ -0,0 +1,830 @@ +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 header file. + +>>> In this tutorial, paragraphs set off like this designate notes + with additional information that readers may wish to skip on a + first read. + + +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 INSTALL, 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. + + +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 and p4, both access ports for VLAN 30, on OpenFlow ports 3 + and 4, respectively. + +>>> The ports' names are not significant. You could call them eth1 + through eth4, or any other names you like. + +>>> 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. + +>>> 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. + +>>> 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)" + +(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. + +>>> 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. + +>>> 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). + +>>> 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)" + +>>> 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] + +>>> 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 + +>>> 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 + +>>> 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/ |