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DESIGN
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Design Decisions In Open vSwitch
================================
This document describes design decisions that went into implementing
Open vSwitch. While we believe these to be reasonable decisions, it is
impossible to predict how Open vSwitch will be used in all environments.
Understanding assumptions made by Open vSwitch is critical to a
successful deployment. The end of this document contains contact
information that can be used to let us know how we can make Open vSwitch
more generally useful.
IPv6
====
Open vSwitch supports stateless handling of IPv6 packets. Flows can be
written to support matching TCP, UDP, and ICMPv6 headers within an IPv6
packet. Deeper matching of some Neighbor Discovery messages is also
supported.
IPv6 was not designed to interact well with middle-boxes. This,
combined with Open vSwitch's stateless nature, have affected the
processing of IPv6 traffic, which is detailed below.
Extension Headers
-----------------
The base IPv6 header is incredibly simple with the intention of only
containing information relevant for routing packets between two
endpoints. IPv6 relies heavily on the use of extension headers to
provide any other functionality. Unfortunately, the extension headers
were designed in such a way that it is impossible to move to the next
header (including the layer-4 payload) unless the current header is
understood.
Open vSwitch will process the following extension headers and continue
to the next header:
* Fragment (see the next section)
* AH (Authentication Header)
* Hop-by-Hop Options
* Routing
* Destination Options
When a header is encountered that is not in that list, it is considered
"terminal". A terminal header's IPv6 protocol value is stored in
"nw_proto" for matching purposes. If a terminal header is TCP, UDP, or
ICMPv6, the packet will be further processed in an attempt to extract
layer-4 information.
Fragments
---------
IPv6 requires that every link in the internet have an MTU of 1280 octets
or greater (RFC 2460). As such, a terminal header (as described above in
"Extension Headers") in the first fragment should generally be
reachable. In this case, the terminal header's IPv6 protocol type is
stored in the "nw_proto" field for matching purposes. If a terminal
header cannot be found in the first fragment (one with a fragment offset
of zero), the "nw_proto" field is set to 0. Subsequent fragments (those
with a non-zero fragment offset) have the "nw_proto" field set to the
IPv6 protocol type for fragments (44).
Jumbograms
----------
An IPv6 jumbogram (RFC 2675) is a packet containing a payload longer
than 65,535 octets. A jumbogram is only relevant in subnets with a link
MTU greater than 65,575 octets, and are not required to be supported on
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
===========
Suggestions to improve Open vSwitch are welcome at [email protected].