Working Group: ARMD Himanshu Shah Intended Status: Proposed Standard Ciena Corp Internet Draft Anoop Ghanwani Expiration Date: May, 2011 Brocade Nabil Bitar Verizon October 25, 2010 ARP Broadcast Reduction for Large Data Centers draft-shah-armd-arp-reduction-01.txt Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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Abstract With the emergence server virtualization technologies, a host is able to support multiple Virtual Machines (VMs) in a single physical machine. Data centers can leverage these capabilities to instantiate on the order of 10s to 100s of VMs in a server. Each VM operates as an independent IP host with a set of Virtual Network Interface Cards (vNICs), each having its own MAC address and mapping to a physical Ethernet interface. These physical servers are typically installed in a rack with their Ethernet interfaces connected to a top-of-rack (ToR) switch. The ToR switches are interconnected through End-of- the-Row (EoR) or aggregation switches which are in turn connected to core switches. As discussed in [ARP-Problem] the host VMs use ARP broadcasts to find other host VMs and use periodic (broadcast) Gratuitous ARPs to refresh their IP to MAC address binding in other VM hosts. Such broadcasts in a large data center with potentially thousands of VM hosts in a Layer 2 based topology can overwhelm the network. This memo proposes mechanisms to reduce the number of broadcasts that are sent throughout the network. This is done by having the ToR switches intelligently process ARP packets, rather than simply broadcasting them throughout the broadcast domain. While this document specifically addresses ARP, the Neighbor Discovery mechanisms used by IPv6 hosts that make use of multicast rather than broadcast also pose similar issues for the data center. The solutions defined herein should be equally applicable to hosts running IPv6. The details will be specified in a subsequent revision. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC 2119]. Shah, et al. Expires May 2011 2 Internet Draft draft-shah-arp-reduction-01.txt Table of Contents Copyright Notice .................................................... 1 Abstract.............................................................. 2 1.0 Overview.......................................................... 3 1.1 Terminology ..................................................... 5 2.0 Configuration..................................................... 6 3.0 Building the ARP Tables........................................... 6 3.1 ARP Request ..................................................... 6 3.2 ARP Reply ....................................................... 7 3.3 Gratuitous ARP .................................................. 7 3.4 Uplink Versus Downlink Processing ............................... 8 3.5 Host Mobility ................................................... 8 4.0 Concluding Remarks................................................ 9 5.0 Security Considerations ......................................... 10 6.0 Acknowledgments ................................................. 10 7.0 References....................................................... 10 7.1 Normative References ........................................... 10 7.2 Informative References ......................................... 10 8.0 Author's Address................................................. 10 1.0 Overview The traditional topology in a data center consists of racks of servers connected to top-of-rack (ToR) switches, which connect to aggregation switches, which in turn connect to core switches. The network architecture is typically a combination Layer 2 and Layer 3 functionality. In some architectures, Layer 2 is terminated at the ToR, with Layer 3 being run in the aggregation and core devices. In other architectures, Layer 2 may be extended all the way to the aggregation switch. The primary concerns that have influenced network architectures in the data center have been keeping broadcast domains manageable and the spanning tree diameter contained. Moving forward, these traditional network architectures are being challenged due to emerging technologies such as server virtualization. Shah, et al. Expires May 2011 3 Internet Draft draft-shah-arp-reduction-01.txt The effect of server virtualization in the data center brings some challenges. Because of virtualization, the number of hosts seen by the network increases dramatically - 10 to 100 times the number of physical servers. These virtual hosts are referred to as Virtual machines (VMs). In addition, virtualized environments offer a feature referred to as "VM mobility" wherein a VM can be relocated to run on a different physical server. In order for the VM mobility to be non-disruptive to other hosts that have communication in progress with the VM being moved, the VM must retain its MAC address and IP address. Because of the requirement to retain the MAC and IP address, it is desirable to develop network architectures that would offer the least restrictions in terms of VM mobility. As an example, in a network architecture where TOR switches terminate the L2 domain, the range of VM mobility would be restricted to a single ToR switch. It would be more preferable to allow the flexibility of moving the VM anywhere within the data center, or perhaps even a different data center. Technologies such as TRILL [TRILL] overcome some of the issues of spanning trees that forced traditional Layer 2 topologies to be severely constrained. However, because of virtualization there are 2 specific problems that are introduced with respect to broadcast traffic. 1. A larger number of hosts. A single physical server now hosts multiple VMs taking the scale factor to a different level. If each VM issues the same number of broadcasts as a physical server, the amount of broadcast traffic will increased 10 to greater than 100 times. 2. If the Layer 2 domains are extended to go across data centers, then broadcast traffic will now go across the backbone. If Layer 2 was terminated at the ToR switch, the increase in broadcast traffic would be been restricted to a single ToR switch, but as discussed earlier, this restriction is not desirable. Excessive broadcast traffic in Layer 2 networks results in wastage of network bandwidth, as well as in the wastage of CPU resources due to all of the VMs processing superfluous ARP broadcasts (IPv6 gets rid of the latter by running ND as a multicast service rather than a broadcast service). The solution presented here attempts to minimize the negative effects of ARP broadcast packets. The solution requires the first hop Ethernet switches, typically the ToR switch, to maintain an ARP table that is learned from the ARP packets received by the switch. The switch then selectively propagates the ARP packet to, or proxy- responds on behalf of, the remote peer. These types of ARP processing principles are well-known and are described in L2VPN Working Group documents such as [ARP-Mediation] and [IPLS]. Shah, et al. Expires May 2011 4 Internet Draft draft-shah-arp-reduction-01.txt The following sections describe the details of ARP snooping, the learning and maintenance of ARP tables, the use of learned information to limit broadcast propagation, and proxy (the response) on behalf of the remote peers. 1.1 Terminology ToR Top-of-Rack. An Ethernet switch present on top of a rack which provides network connectivity to the servers present on the rack. Downlink The Ethernet link between the ToR switch and a directly connected host (server in the rack). Uplink The network- facing Ethernet connection in the ToR switch. Typically, the uplinks from ToRs connect to end-of-row or aggregation switches. EoR End-of-Row. An Ethernet switch to which the ToR switches connect, also referred to as an aggregation switch. Uplinks from ToR switches connect to an EoR switch and uplinks from EoR switches connect to a core switch. Host/Server A host or server running the IP protocol. This could be a physical entity or a logical entity (such as a Virtual Machine) in a physical host. The term server refers to its role in the data center. Both terms are used interchangeably to refer to an IP host. Local hosts Used in the context of a ToR switch to denote the VM hosts connected to a ToR on the downlink, i.e. directly attached hosts. Remote hosts Used in the context of a ToR switch to denote the hosts that are accessible through uplink of the ToR. VM Virtual Machine. This is a logical instance of a host that operates independently in a physical host and has its own IP and MAC addresses. VMs allow efficient use of physical host resources (such as multiple CPU cores). Shah, et al. Expires May 2011 5 Internet Draft draft-shah-arp-reduction-01.txt 2.0 Configuration It is assumed that ARP reduction mechanisms that are defined in this document will be limited to ToR switches. The maximum benefit of restraining ARP broadcasts in the network is achieved by the first hop switches (the ones directly connected to the hosts) without placing additional burden on second or third tier switches. First, the ToR switches would need to be configured in order to enable the ARP reduction feature. Every Ethernet interface needs to be identified as either a downlink or uplink within the context of this feature. In addition the operator may optionally configure various ARP reduction related parameters such as: . ARP aging timer. . Size of the ARP table. . Static entries of IP to MAC address. 3.0 Building the ARP Tables When ARP reduction is enabled, the ToR switch will monitor all ARP traffic transiting the switch (regardless of uplink port or downlink port) and will process any ARP packets in the following manner: . ARP Request packets must be redirected to control plane CPU. . Gratuitous ARP packets (ARP Reply packet with a broadcast MAC DA) must be redirected to control plane CPU. . Other ARP Reply packets (ARP Reply packet with a unicast MAC DA) should be bi-casted; one copy sent to control plane CPU and other copy forwarded out normally. 3.1 ARP Request The ToR examines the source IP and the source hardware address (MAC address) in the ARP Request . The source IP and MAC address association is learned, or is updated/refreshed if already learned. The destination IP address is searched in the ARP table. If an entry exists, the associated MAC address from the table is used to prepare a unicast ARP Reply packet. The same MAC address is used as the source MAC address in the MAC header, as well as for the target hardware address, in the unicast ARP Reply packet. If the destination IP address in the ARP Request is not present in the ARP table, then the original ARP Request packet is broadcast to all the switch ports that are members of the same VLAN except the source port that the ARP Request was received from. However, if the requested (destination) IP address is present in the ARP table, a unicast ARP Reply packet is prepared as described above and sent to the switch port from which the ARP Request was received and original ARP Request packet is dropped. Shah, et al. Expires May 2011 6 Internet Draft draft-shah-arp-reduction-01.txt The intent is to prevent propagation of ARP Request broadcasts as much as possible using the information present in the ARP table. The following observations can be made from such behavior. . Most of the ARP Request packets from the local hosts of a ToR switch for the local hosts of that ToR switch can be prevented from being broadcast on uplinks or downlinks. . Most of the ARP Request packets from remote hosts of a ToR switch for local hosts of that ToR switch can be prevented from being broadcast on downlinks or other uplinks of the ToR switch. . Many of the ARP Request packets from local hosts of a ToR switch for remote hosts of that ToR switch can be prevented from being forwarded on uplinks if the remote host IP to MAC association is known to the ToR switch. 3.2 ARP Reply The unicast ARP Reply is examined to learn/update the ARP table for source and destination IP/MAC address association, but is also forwarded out as a normal frame. 3.3 Gratuitous ARP Gratuitous ARP is a broadcast ARP Reply packet with the destination IP address set to the IP address of the sender and target hardware address set to the MAC address of the sender. It is typically used by IP hosts (including VMs) to keep its IP-to-MAC address association fresh in its peers' ARP cache. The ToR switch should process Gratuitous ARP in the following manner. . Learn/update/refresh the ARP table entry. . If the IP address is new, or exists but with a different hardware address, then the Gratuitous ARP packet is forwarded out; otherwise the packet is discarded. The goal for handling of Gratuitous ARP packets received from the downlinks (i.e. local hosts) is to avoid propagating it into the 'network' (i.e. to the uplinks), unless there is a new association. By suppressing the propagation of Gratuitous ARP packets, the peer IP hosts will end up aging out the corresponding ARP table entries. This will result in generation of the broadcast ARP Requests by those IP hosts if they need to continue to communicate with the IP host whose Gratuitous ARPs were obstructed. The handling of the ARP Request by the first-hop ToR switch, as described above, will be able to respond to this request based on the ARP cache maintained in the ToR switch. In essence, the presence of large ARP tables with longer aging times compensates for the smaller ARP table present in Shah, et al. Expires May 2011 7 Internet Draft draft-shah-arp-reduction-01.txt the IP hosts and eliminates the need for periodic use of Gratuitous ARPs in order to refresh the ARP table in the IP hosts. 3.4 Uplink Versus Downlink Processing With respect to processing of the ARP packets as described above, the behavior is different depending on whether the packet was received from an uplink or downlink in the following ways. . The aging timer will typically be higher for entries learned from an uplink versus those learned from a downlink. The reason for this is to avoid flooding ARP broadcast packets on uplinks since they have a much larger negative impact. . If ARP table fills up, then entries learned from downlinks (i.e. directly attached hosts) will take precedence over those learned from an uplink (i.e. remote hosts). This will trade off sending broadcasts on host links versus sending them into the core of the network. The reason for this is that access links are typically lower bandwidth, and also this will conserve CPU resources involved in processing unnecessary ARP traffic. 3.5 Host Mobility As mentioned earlier, server virtualization technology allows mobility of VMs to different physical servers. The flexibility to move VMs is one of the key benefits of server virtualization. VM mobility could be manual (operator initiated) or may be done automatically in reaction to demands placed by the application users. The important point is that in either case, VM movement is not transparent and is made known to the network. There is ongoing work in IEEE 802.1 standards organization (IEEE 802.1Qbg) to coordinate/communicate the presence and capabilities of the VMs to the directly connected network switch. VMs typically retain their MAC and IP address across a VM mobility event, and as such, there would be little impact to the ARP table maintained by the ARP reduction mechanism described herein. However, the ARP reduction mechanism would benefit from knowing if a VM is completely decommissioned so that the ToR switch can remove the ARP entry that it has for that VM in a timely fashion, rather than waiting for it to age out. 3.6 Scaling Considerations Depending on the number of hosts in the network, the ARP table in a ToR switch needed for the ARP reduction mechanisms described above can be quite large. Although it is possible to implement some of the mechanisms for ARP reduction in hardware in the forwarding plane, Shah, et al. Expires May 2011 8 Internet Draft draft-shah-arp-reduction-01.txt the number of ARP entries favors maintaining the ARP table in the control plane memory. 3.7 Miscellaneous Issues Because of the distributed nature of the mechanisms described herein, there are a few additional issues that warrant consideration from the network operator. Earlier in the document, we had mentioned the configuration of a aging timer for ARP entries. A longer timer for holding onto ARP entries helps with reduction of broadcasts. However, having a "too large timer" can lead to problems in certain situations. Consider the following scenario. Host A is attached to ToR switch #1, and host B is attached to ToR switch #2. If host B issues an ARP Request for host A, and if the entry is available at switch #2, then switch #2 would send the ARP Reply on behalf of host A. It is possible that host A is no longer available, but there is no way for switch #2 to know this, and it would continue to respond on behalf of host A, until its entry for host A has aged out. In this case, it is easy to see that a smaller aging timer would be beneficial. Additionally, since host B has an ARP aging timer, it means that host B would find out about host A's unavailability only after its entry has aged out, which would be some time after it the entry has aged out of switch #2. Another issue that can be somewhat problematic could be the inconsistency of tables in switches. Once again, consider a scenario similar to the one described above with two hosts each connected to its respect ToR switch. Let the ARP entries at both A and B be learned by both switches. Now assume that the IP address on host A changes. This change is signaled to switch #1 which in turn broadcasts the message on its uplink. Now, if this message is discarded due to network congestion or signal integrity issues, then switch #2 will not learn about the change and will continue to respond to host B's ARP Requests for host A's old IP address with stale information. This lasts until the ARP entry for A ages out at Switch #2. 4.0 Concluding Remarks Based on the procedures described in this document, it is possible for ToR switches in the data center to contain ARP broadcasts significantly. The solution is based on well known, non-intrusive procedures and strives to curtail ARP broadcasts that are increasingly becoming a cause for concern in the data centers. In essence, ToR switches offload some of the ARP table management from the IP hosts to themselves. The ARP table aging timer can be tuned higher by the operator based on the available switch resources and network traffic behavior. The larger capacity of the ARP table Shah, et al. Expires May 2011 9 Internet Draft draft-shah-arp-reduction-01.txt coupled with a long aging time for entries in the table directly translates to more effective subduing of the ARP broadcasts. 5.0 Security Considerations Security aspects will be addressed in a subsequent revision. 6.0 Acknowledgments This document resulted from discussions with Linda Dunbar (Huawei), Sue Hares (Huawei), and T Sridhar (Force10). We would like to acknowledge their contribution to this work. 7.0 References 7.1 Normative References [ARP] D. Plummer, "An Ethernet Address Resolution Protocol: Or Converting Network Protocol Addresses to 48.bit Ethernet Addresses for Transmission on Ethernet Hardware, " RFC 826 (also STD 37), November 1982. [ARP-Problem] L.Dunbar et al., "Scalable Address Resolution for Large Data Center Problem Statements," , July 2010. 7.2 Informative References [ARP-Mediation] H. Shah et al., "ARP Mediation for IP interworking in Layer 2 VPN," , July 2010. [IPLS] H.Shah et al., "IP-only LAN service," , February 2010. [PROXY-ARP] J. Postel, "Multi-LAN Address Resolution," RFC 925, October 1984. [TRILL] R. Perlman et al., "RBridges: Base Protocol Specification", , March 2010. 8.0 Author's Address Himanshu Shah Ciena Corp Email: hshah@ciena.com Anoop Ghanwani Shah, et al. Expires May 2011 10 Internet Draft draft-shah-arp-reduction-01.txt Brocade Email: anoop@brocade.com Nabil Bitar Verizon Email: nabil.n.bitar@verizon.com Shah, et al. Expires May 2011 11