DESIGN: Move in-band control design discussion here.

It seems more likely that interested users and administrators will be able
to find it here.

1 files changed, 163 insertions, 0 deletions

diff --git a/DESIGN b/DESIGN
index 6e25f01b..2e3fced8 100644
--- a/DESIGN
+++ b/DESIGN
@@ -71,6 +71,169 @@ nodes that do not connect to link with such large MTUs.  Currently, Open
 vSwitch doesn't process jumbograms.
 
 
+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.
+
+
 Suggestions
 ===========