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 discuss@openvswitch.org.