+In-Band Control
+===============
+
+In-band control allows a single network to be used for OpenFlow traffic and
+other data traffic. See ovs-vswitchd.conf.db(5) for a description of
+configuring in-band control.
+
+This comment is an attempt to describe how in-band control works at a
+wire- and implementation-level. Correctly implementing in-band
+control has proven difficult due to its many subtleties, and has thus
+gone through many iterations. Please read through and understand the
+reasoning behind the chosen rules before making modifications.
+
+In Open vSwitch, in-band control is implemented as "hidden" flows (in that
+they are not visible through OpenFlow) and at a higher priority than
+wildcarded flows can be set up by through OpenFlow. This is done so that
+the OpenFlow controller cannot interfere with them and possibly break
+connectivity with its switches. It is possible to see all flows, including
+in-band ones, with the ovs-appctl "bridge/dump-flows" command.
+
+The Open vSwitch implementation of in-band control can hide traffic to
+arbitrary "remotes", where each remote is one TCP port on one IP address.
+Currently the remotes are automatically configured as the in-band OpenFlow
+controllers plus the OVSDB managers, if any. (The latter is a requirement
+because OVSDB managers are responsible for configuring OpenFlow controllers,
+so if the manager cannot be reached then OpenFlow cannot be reconfigured.)
+
+The following rules (with the OFPP_NORMAL action) are set up on any bridge
+that has any remotes:
+
+ (a) DHCP requests sent from the local port.
+ (b) ARP replies to the local port's MAC address.
+ (c) ARP requests from the local port's MAC address.
+
+In-band also sets up the following rules for each unique next-hop MAC
+address for the remotes' IPs (the "next hop" is either the remote
+itself, if it is on a local subnet, or the gateway to reach the remote):
+
+ (d) ARP replies to the next hop's MAC address.
+ (e) ARP requests from the next hop's MAC address.
+
+In-band also sets up the following rules for each unique remote IP address:
+
+ (f) ARP replies containing the remote's IP address as a target.
+ (g) ARP requests containing the remote's IP address as a source.
+
+In-band also sets up the following rules for each unique remote (IP,port)
+pair:
+
+ (h) TCP traffic to the remote's IP and port.
+ (i) TCP traffic from the remote's IP and port.
+
+The goal of these rules is to be as narrow as possible to allow a
+switch to join a network and be able to communicate with the
+remotes. As mentioned earlier, these rules have higher priority
+than the controller's rules, so if they are too broad, they may
+prevent the controller from implementing its policy. As such,
+in-band actively monitors some aspects of flow and packet processing
+so that the rules can be made more precise.
+
+In-band control monitors attempts to add flows into the datapath that
+could interfere with its duties. The datapath only allows exact
+match entries, so in-band control is able to be very precise about
+the flows it prevents. Flows that miss in the datapath are sent to
+userspace to be processed, so preventing these flows from being
+cached in the "fast path" does not affect correctness. The only type
+of flow that is currently prevented is one that would prevent DHCP
+replies from being seen by the local port. For example, a rule that
+forwarded all DHCP traffic to the controller would not be allowed,
+but one that forwarded to all ports (including the local port) would.
+
+As mentioned earlier, packets that miss in the datapath are sent to
+the userspace for processing. The userspace has its own flow table,
+the "classifier", so in-band checks whether any special processing
+is needed before the classifier is consulted. If a packet is a DHCP
+response to a request from the local port, the packet is forwarded to
+the local port, regardless of the flow table. Note that this requires
+L7 processing of DHCP replies to determine whether the 'chaddr' field
+matches the MAC address of the local port.
+
+It is interesting to note that for an L3-based in-band control
+mechanism, the majority of rules are devoted to ARP traffic. At first
+glance, some of these rules appear redundant. However, each serves an
+important role. First, in order to determine the MAC address of the
+remote side (controller or gateway) for other ARP rules, we must allow
+ARP traffic for our local port with rules (b) and (c). If we are
+between a switch and its connection to the remote, we have to
+allow the other switch's ARP traffic to through. This is done with
+rules (d) and (e), since we do not know the addresses of the other
+switches a priori, but do know the remote's or gateway's. Finally,
+if the remote is running in a local guest VM that is not reached
+through the local port, the switch that is connected to the VM must
+allow ARP traffic based on the remote's IP address, since it will
+not know the MAC address of the local port that is sending the traffic
+or the MAC address of the remote in the guest VM.
+
+With a few notable exceptions below, in-band should work in most
+network setups. The following are considered "supported' in the
+current implementation:
+
+ - Locally Connected. The switch and remote are on the same
+ subnet. This uses rules (a), (b), (c), (h), and (i).
+
+ - Reached through Gateway. The switch and remote are on
+ different subnets and must go through a gateway. This uses
+ rules (a), (b), (c), (h), and (i).
+
+ - Between Switch and Remote. This switch is between another
+ switch and the remote, and we want to allow the other
+ switch's traffic through. This uses rules (d), (e), (h), and
+ (i). It uses (b) and (c) indirectly in order to know the MAC
+ address for rules (d) and (e). Note that DHCP for the other
+ switch will not work unless an OpenFlow controller explicitly lets this
+ switch pass the traffic.
+
+ - Between Switch and Gateway. This switch is between another
+ switch and the gateway, and we want to allow the other switch's
+ traffic through. This uses the same rules and logic as the
+ "Between Switch and Remote" configuration described earlier.
+
+ - Remote on Local VM. The remote is a guest VM on the
+ system running in-band control. This uses rules (a), (b), (c),
+ (h), and (i).
+
+ - Remote on Local VM with Different Networks. The remote
+ is a guest VM on the system running in-band control, but the
+ local port is not used to connect to the remote. For
+ example, an IP address is configured on eth0 of the switch. The
+ remote's VM is connected through eth1 of the switch, but an
+ IP address has not been configured for that port on the switch.
+ As such, the switch will use eth0 to connect to the remote,
+ and eth1's rules about the local port will not work. In the
+ example, the switch attached to eth0 would use rules (a), (b),
+ (c), (h), and (i) on eth0. The switch attached to eth1 would use
+ rules (f), (g), (h), and (i).
+
+The following are explicitly *not* supported by in-band control:
+
+ - Specify Remote by Name. Currently, the remote must be
+ identified by IP address. A naive approach would be to permit
+ all DNS traffic. Unfortunately, this would prevent the
+ controller from defining any policy over DNS. Since switches
+ that are located behind us need to connect to the remote,
+ in-band cannot simply add a rule that allows DNS traffic from
+ the local port. The "correct" way to support this is to parse
+ DNS requests to allow all traffic related to a request for the
+ remote's name through. Due to the potential security
+ problems and amount of processing, we decided to hold off for
+ the time-being.
+
+ - Differing Remotes for Switches. All switches must know
+ the L3 addresses for all the remotes that other switches
+ may use, since rules need to be set up to allow traffic related
+ to those remotes through. See rules (f), (g), (h), and (i).
+
+ - Differing Routes for Switches. In order for the switch to
+ allow other switches to connect to a remote through a
+ gateway, it allows the gateway's traffic through with rules (d)
+ and (e). If the routes to the remote differ for the two
+ switches, we will not know the MAC address of the alternate
+ gateway.
+
+
git://git.onelab.eu / sliver-openvswitch.git / blobdiff