Skip to content

Commit

Permalink
DESIGN: Move in-band control design discussion here.
Browse files Browse the repository at this point in the history 
It seems more likely that interested users and administrators will be able
to find it here.
  • Loading branch information
@blp
blp committed May 4, 2011
1 parent 2ba7924 commit 946350d
Show file tree
Hide file tree
Showing 2 changed files with 163 additions and 160 deletions.
163 changes: 163 additions & 0 deletions DESIGN
View file
Expand Up @@ -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
===========

Expand Down
160 changes: 0 additions & 160 deletions ofproto/in-band.c
View file
Expand Up @@ -40,166 +40,6 @@

VLOG_DEFINE_THIS_MODULE(in_band);

/* 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.
*/

/* Priorities used in classifier for in-band rules. These values are higher
* than any that may be set with OpenFlow, and "18" kind of looks like "IB".
* The ordering of priorities is not important because all of the rules set up
Expand Down

0 comments on commit 946350d

Please sign in to comment.