Network Working Group Youval Nachum Internet Draft Intended status: Experimental Linda Dunbar Expires: June 2015 Huawei Ilan Yerushalmi Tal Mizrahi Marvell December 15, 2014 Scaling the Address Resolution Protocol for Large Data Centers (SARP) draft-nachum-sarp-09.txt Abstract This document introduces SARP, an architecture that uses proxy gateways to scale large data center networks. SARP is based on fast proxies that significantly reduce switches' FDB table (MAC table) sizes and ARP/ND impact on network elements in an environment where hosts within one subnet (or VLAN) can spread over various locations. SARP is targeted for massive data centers with a significant number of VMs that can move across various physical locations. Status of this Memo This Internet-Draft is submitted to IETF 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Nachum, et al. Expires June 15, 2015 [Page 1] Internet-Draft SARP December 2014 This Internet-Draft will expire on June 15, 2015. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction ................................................. 3 1.1. SARP Motivation.......................................... 3 1.2. SARP Overview ........................................... 6 1.3. SARP Deployment Options ................................. 7 2. Terms and Abbreviations Used in this Document ................ 8 3. SARP - Theory of Operation ................................... 9 3.1. Control Plane: ARP/ND ................................... 9 3.1.1. ARP/NS Request for a Local VM ...................... 9 3.1.2. ARP/NS Request for a Remote VM .................... 10 3.1.3. Gratuitous ARP and Unsolicited Neighbor Advertisement (UNA) ...................................... 12 3.2. Data Plane: Packet Transmission ........................ 12 3.2.1. Local Packet Transmission ......................... 12 3.2.2. Packet Transmission Between Sites ................. 12 3.3. VM Migration ........................................... 13 3.3.1. VM Local Migration ................................ 13 3.3.2. VM Migration from One Site to Another ............. 13 3.3.2.1. Impact on IP<->MAC Mapping Cache Table of Migrated VMs .......................................... 15 3.4. Multicast and Broadcast ................................ 15 3.5. Non IP packet .......................................... 16 3.6. High availability and load balancing ................... 16 3.7. SARP Interaction with Overlay networks ................. 17 4. Security Considerations ..................................... 17 5. IANA Considerations ......................................... 18 6. References .................................................. 18 6.1. Normative References ................................... 18 Nachum, et al. Expires June 15, 2015 [Page 2] Internet-Draft SARP December 2014 6.2. Informative References ................................. 19 7. Acknowledgments ............................................. 19 1. Introduction This document describes a proxy gateway technique, called Scalable Address Resolution Protocol (SARP), which reduces switches' Filtering Data Base (FDB) size and ARP/Neighbor Discovery impact on network elements in an environment where hosts within one subnet (or VLAN) can spread over various access domains in data centers. The main idea of SARP is to represent all VMs (or hosts) under each access domain by their corresponding access (or aggregation) node's MAC address. For example (. Figure 1), when host A in the west site needs to communicate with host B, which is on the same VLAN but connected to a different access domain (east site), SARP requires A to use the MAC address of SARP proxy 2, rather than the address of host B. By doing so, switches in each domain do not need to maintain a list of MAC addresses for all the VMs (hosts) in different access domains; every switch only needs to be familiar with MAC addresses that reside in the current domain, and addresses of remote SARP proxy gateways. Therefore, the switches' FDB size is limited regardless of the number of access domains. +-------+ +-------+ _ __ +-------+ +-------+ | | | SARP | / \_/ \_ | SARP | | | |host A |<===>| proxy |<=>\_ \<==>| proxy |<===>|host B | | | | 1 | / _/ | 2 | | | +-------+ +-------+ \__ _/ +-------+ +-------+ \_/ <------west site------> <------east site------> Figure 1 SARP in a nutshell 1.1. SARP Motivation [ARMDProb] discusses the impacts and scaling issues that arise in data center networks when subnets span across multiple L2/L3 boundary routers. Unfortunately, when the combined number of VMs (or hosts) in all those subnets is large, this can lead to switches' MAC table size explosion and heavy impact on network elements. Nachum, et al. Expires June 15, 2015 [Page 3] Internet-Draft SARP December 2014 There are four major issues associated with subnets spanning across multiple L2/L3 boundary router ports: 1)Intermediate switches' MAC address table (FDB) explosion. When hosts in a VLAN (or subnet) span across multiple access domains and each access domain has hosts belonging to different VLANs, each access switch has to enable multiple VLANs. Thus, those access switches are exposed to all MAC addresses across all VLANs. For example, for an access switch with 40 attached physical servers, where each server has 100 VMs, the access switch has 4000 attached MAC addresses. If indeed hosts/VMs can be moved anywhere, the worst case for the Access Switch is when all those 4000 VMs belong to different VLANs, i.e. the access switch has 4000 VLANs enabled. If each VLAN has 200 hosts, this access switch's MAC table potentially has 200*4000 = 800,000 entries. It is important to note that the example above is relevant regardless of whether IPv4 or IPv6 are used. The example illustrates a scenario that is worse than what today's L2/3 Gateway has to face. In today's environment where each subnet is limited to a few access switches, the number of MAC addresses the gateway has to learn is of a significantly smaller scale. 2)ARP/ND processing load impact to the L2/L3 boundary routers. All VMs periodically send NDs to their corresponding gateway nodes to get gateway nodes' MAC addresses. When the combined number of VMs across all the VLANs is large, processing the responses to the ND requests from those VMs can easily exhaust the gateway's CPU utilization. A L2/L3 boundary router could be hit with ARP/ND twice when the originating and destination stations are in different subnets attached to the same router and when those hosts do not communicate with external peers very frequently. The first hit is when the originating station in subnet 1 initiates an ARP/ND request to the L2/L3 boundary router. The second hit is when the L2/L3 boundary router initiates an ARP/ND request to the target in subnet 2 if the target is not in router's ARP/ND cache. Nachum, et al. Expires June 15, 2015 [Page 4] Internet-Draft SARP December 2014 3)In IPv4, every end station in a subnet receives ARP broadcast messages from all other end stations in the subnet. IPv6 ND has eliminated this issue by using multicast. However, most devices support a limited number of multicast addresses, due to multicast filtering scaling. Once the number of multicast addresses exceeds the multicast filter limit, the multicast addresses have to be processed by devices' CPU (i.e. the slow path). It is less of an issue in data centers without VM mobility, since each port is only dedicated to one (or a small number of) VLANs. Thus, the number of multicast addresses hitting each port is significantly lower. 4)The ARP/ND messages are flooded to many physical link segments which can reduce the bandwidth utilization for user traffic. ARP/ND flooding is, in most cases, an insignificant issue in today's data center networks as the majority of data center servers are shifting towards 1G or 10G ports. The bandwidth used by ARP/ND, even when flooded to all physical links, becomes negligible compared to the link bandwidth. Furthermore, IGMP/MLD snooping [IGMPSnoop] can further reduce the ND multicast traffic to some physical link segments. Statistics gathered by Merit Network [ARMDStats] have shown that the major impact of a large number of mobile VMs in data centers is on the L2/L3 boundary routers, i.e., issue (2) above. An L2/L3 boundary router could be hit with ARP/ND twice when the originating and destination stations are in different subnets attached to the same router and those hosts do not communicate with external peers often enough. Overlay approaches, e.g. [NVo3-PROBLEM], can hide hosts (VMs) addresses in the core but do not prevent the MAC table explosion problem (issue (1)) unless the NVE is on a server. The scaling practices documented in [ARP-ND-PRACTICE] can only reduce some ARP impact to L2/L3 boundary routers in some scenarios, but not all. Nachum, et al. Expires June 15, 2015 [Page 5] Internet-Draft SARP December 2014 In order to protect router CPUs from being overburdened by target resolution requests, some routers rate limit the target MAC resolution requests to the router's CPU. When the rate limit is exceeded, the incoming data frames are dropped. In traditional data centers, this issue is less significant, since the number of hosts attached to one L2/L3 boundary router is limited by the number of physical ports of the switches/routers. When servers are virtualized to support 30+ VMs, the number of hosts under one router can grow by a factor of 30+. Furthermore, in traditional data center networks each subnet is neatly bound to a limited number of server racks, i.e., switches only need to be familiar with MAC addresses of hosts that reside in this small number of subnets. In contemporary data center networks, as subnets are spread across many server racks, switches are exposed to VLAN/MAC addresses of many subnets, greatly increasing the size of switches' FDB tables. The solution proposed in this document can eliminate or reduce the likelihood of inter-subnet data frames being dropped and reduce the number of host MAC addresses that intermediate switches are exposed to, thus reducing switches' FDB table sizes. 1.2. SARP Overview The SARP approach uses proxy gateways to address the problems discussed above. Note: The Guidelines to proxy developers [NDProxy] have been carefully considered for the SARP protocols. Section 3.3 discusses how SARP works when VMs are moved from one segment to another. In order to enable VMs to be moved across servers while maintaining their MAC/IP addresses unchanged, the Layer 2 network (e.g. VLAN) which interconnects those VMs may spread across different server racks, different rows of server racks, or even different data center sites. A multi-site data center network is comprised of two main building blocks: an interconnecting segment and an access segment. While the access network is, in most cases, a Layer 2 network, the interconnecting segment is not necessarily a Layer 2 network. Nachum, et al. Expires June 15, 2015 [Page 6] Internet-Draft SARP December 2014 The SARP proxies are located at the boundaries where the access segment connects to its interconnecting segment. The boundary node can be a hypervisor virtual switch, a top-of- rack switch, an aggregation switch (or end of row switch), or a data center core switch. Figure 2 depicts an example of two remote data centers that are managed as a single flat Layer 2 domain. SARP proxies are implemented at the edge devices connecting the data center to the transport network. SARP significantly reduces the ARP/ND transmissions over the interconnecting network. *-------------------* | | +-------| Interconnecting |-------+ | | network | | | *-------------------* | | | *-----------------* *----------------* | SARP Proxies | | SARP Proxies | *-----------------* *----------------* | | | | *-------* *-------* *-------* *-------* | ACC | | ACC | | ACC | | ACC | *-------* *-------* *-------* *-------* | *----------* |Hypervisor| *----------* | *--------* |Virtual | |Machine | *--------* (West Site) (East Site) Figure 2 SARP Networking Architecture Example. 1.3. SARP Deployment Options SARP deployment is tightly coupled with the data center architecture. SARP proxies are located at the point where the Layer 2 infrastructure connects to its Layer 2 cloud using overlay networks. SARP proxies can be located at the data center edge (as Figure 2 depicts), data center core, or data Nachum, et al. Expires June 15, 2015 [Page 7] Internet-Draft SARP December 2014 center aggregation. SARP can also be implemented by the hypervisor (as Figure 3 depicts). To simplify the description, we will focus on data centers that are managed as a single flat Layer 2 network, where SARP proxies are located at the boundary where the data center connects to the transport network (as Figure 2 depicts). *-------------------* | | +-------| TRANSPORT |-------+ | | | | | *-------------------* | | | *-----------------* *----------------* | Edge Device | | Edge Device | *-----------------* *----------------* | | *-----------------* *----------------* | Core | | Core | *-----------------* *----------------* | | | | *-------* *-------* *-------* *-------* | Agg | | Agg | | Agg | | Agg | *-------* *-------* *-------* *-------* | *----------* |Hypervisor| *----------* (West Site) (East Site) Figure 3 SARP deployment options. 2. Terms and Abbreviations Used in this Document ARP: Address Resolution Protocol [ARP] FDB: Filtering Data Base, which is used for Layer-2 switches (IEEE802.1Q). Layer 2 switches flood data frames when DA is not in FDB, whereas routers drop data frames when the DA is not in the Forwarding Information Base (FIB). That is why Filtering Data Base (FDB) is used for Layer 2 switches. FIB: Forwarding Information Base Nachum, et al. Expires June 15, 2015 [Page 8] Internet-Draft SARP December 2014 IP-D: IP address of the destination virtual machine IP-S: IP address of the source virtual machine MAC-D: MAC address of the destination virtual machine MAC-E: MAC address of the East Proxy SARP Device MAC-S: MAC address of the source virtual machine NA: IPv6 ND's Neighbor Advertisement ND: IPv6 Neighbor Discovery Protocol [ND]. In this document, ND also refers to Neighbor Solicitation, Neighbor Advertisement, Unsolicited Neighbor Advertisement messages defined by RFC4861 NS: IPv6 ND's Neighbor Solicitation SARP Proxy: The components that participates in the SARP protocol. UNA: IPv6 ND's Unsolicited Neighbor Advertisement [ND] VM: Virtual Machine 3. SARP - Theory of Operation 3.1. Control Plane: ARP/ND This section describes the ARP/ND procedure scenarios. The first scenario addresses a case where both the source and destination VMs reside in the same access segment. In the second scenario, the source VM is in the local access segment and the destination VM is located at the remote access segment. In all scenarios, the VMs (source and destination) share the same L2 broadcast domain. 3.1.1. ARP/NS Request for a Local VM When source and destination VMs are located at the same access segment (Figure 4), the address resolution process is as Nachum, et al. Expires June 15, 2015 [Page 9] Internet-Draft SARP December 2014 described in [ARP] and [ND]; host A sends an ARP request or an IPv6 Neighbor Solicitation (NS) to learn the IP-to-MAC mapping of host B, and receives a reply from host B with the IP-D to MAC-D mapping. +-------+ _ __ +-------+ _ __ |host A | / \_/ \_ | SARP | / \_/ \_ | IP-S |<--->\_access \<==>| proxy |<===>\_interc.\ | MAC-S | /network_/ | 1 | /network_/ +-------+ +->\__ _/ +-------+ \__ _/ | \_/ \_/ +-------+ | |host B |<-+ | IP-D | | MAC-D | +-------+ <--------------west site------------> Figure 4 SARP: two hosts in the same access segment 3.1.2. ARP/NS Request for a Remote VM When the source and destination VMs are located at different access segments, the address resolution process is as follows. +-------+ +-------+ _ __ +-------+ +-------+ |host A | | SARP | / \_/ \_ | SARP | |host B | | IP-S |<===>|proxy 1|<=>\_ \<==>|proxy 2|<===>| IP-D | | MAC-S | | MAC-W | / _/ | MAC-E | | MAC-D | +-------+ +-------+ \__ _/ +-------+ +-------+ \_/ <------west site------> <------east site------> Figure 5 SARP: two hosts that reside at different segments In the example illustrated in Figure 5, the source VM is located at the west access segment and the destination VM is located at the east access segment. When host A sends an ARP/NS request to find out the IP-to-MAC mapping of host B: 1. If SARP proxy 1 does not have IP-D in its ARP cache, the ARP/NS request is propagated to all access segments which Nachum, et al. Expires June 15, 2015 [Page 10] Internet-Draft SARP December 2014 might have VMs in the same virtual network as the originating VM, including the east access segment. 2. As SARP proxy 1 forwards the ARP/NS message, it replaces the source MAC address, MAC-S, with its own MAC address, MAC-W. Thus, all switches that reside in the interconnecting segment are not exposed to MAC-S. 3. The ARP/NS request reaches SARP proxy 2. 4. If SARP proxy 2 does not have IP-D in its ARP cache, the ARP/NS request is forwarded to the east access network. Host B responds with an ARP reply (IPv4) or a Neighbor Advertisement (IPv6) to the request with MAC-D. 5. When the response message reaches SARP proxy 2, it replaces MAC-D with MAC-E, and thus the response reaches SARP proxy 1 with MAC-E. 6. As SARP proxy 1 forwards the response to host A, it replaces the destination address from MAC-W to MAC-S. SARP Proxy ARP/ND Cache SARP proxies maintain a cache of the IP<->MAC mapping. This cache is based on ARP/ND messages that are sent by hosts and traverse the SARP proxies. In step 1 and step 4 . above, if the SARP proxy has IP-D in its ARP cache, it responds with MAC-E, without forwarding the ARP/NS request. This caching approach significantly reduces the volume of the ARP/ND transmission over the network, and reduces the round trip time of ARP/ND requests. When the west SARP proxy caches the IP<-> MAC mapping entries for remote VMs, the expiration timers should be set to relatively low value to prevent stale entries due to remote VMs being moved or deleted. In environments where VMs move more frequently, it is not recommended for SARP proxies to cache the IP<-> MAC mapping entries of remote VMs. Nachum, et al. Expires June 15, 2015 [Page 11] Internet-Draft SARP December 2014 3.1.3. Gratuitous ARP and Unsolicited Neighbor Advertisement (UNA) Hosts (or VMs) send out Gratuitous ARP (IPv4) [GratARP] and Unsolicited Neighbor Advertisement - UNA (IPv6) to allow other nodes to refresh IP<->MAC entries in their caches. The local SARP proxy processes the Gratuitous ARP or UNA in the same way as the ARP reply or IPv6 NA, i.e. replaces the MAC addresses in the same manner. 3.2. Data Plane: Packet Transmission 3.2.1. Local Packet Transmission When a VM transmits packets to a destination VM that is located at the same site (Figure 4), the data plane is unaffected by SARP; packets are sent from (IP-S, MAC-S) to (IP-D, MAC-D). 3.2.2. Packet Transmission Between Sites Packets that are sent between sites (. Figure 5) traverse the SARP proxy of both sites. A packet sent from host A to host B undergoes the following procedure: 1. Host A sends a packet to IP-D, and based on its ARP table it uses the MAC addresses {MAC-E, MAC-S}. 2. SARP proxy 1 receives the packet and replaces the source MAC address, such that the packet includes {MAC-E, MAC-W}. 3. SARP proxy 2 receives the packet and replaces the destination MAC address, and the packet is sent to host B with {MAC-D, MAC-W}. SARP proxy 1 replaces the source MAC address with its own since switches in the interconnecting segment are only familiar with SARP proxy MAC addresses, and are not familiar with host addresses. Note: it is a common security practice in data center networks to use access lists, allowing each VM to communicate only with a list of authorized peer VMs. In most cases, such access Nachum, et al. Expires June 15, 2015 [Page 12] Internet-Draft SARP December 2014 control lists are based on IP addresses, and hence are not affected by the MAC address replacement in SARP. 3.3. VM Migration 3.3.1. VM Local Migration When a VM migrates locally within its access segment, the SARP protocol does not require any special behavior. VM migration is resolved entirely by the Layer 2 mechanisms. 3.3.2. VM Migration from One Site to Another This section focuses on a scenario where a VM migrates from the west site to the east site while maintaining its MAC and IP addresses. VM migration might affect networking elements based on their respective location: - Origin site (west site) - Destination site (east site) - Other sites +-------+ +-------+ _ __ +-------+ + - - - + |host A | | SARP | / \_/ \_ | SARP | host A | IP-D |<===>|proxy 1|<=>\_ \<==>|proxy 2|<===>| IP-D | | MAC-D | | MAC-W | / _/ | MAC-E | MAC-D +-------+ +-------+ \__ _/ +-------+ + - - - + \_/ <------west site------> <------east site------> Origin site Destination site Figure 6 SARP: host A migrates from west site to east site Origin site The Origin site is the site where the VM resides before the migration (west site). Before the VM (IP=IP-D, MAC=MAC-D) is moved, all VMs at the west site that have an ARP entry of IP-D in their ARP table have the IP-D -> MAC-D mapping. VMs on other access segments Nachum, et al. Expires June 15, 2015 [Page 13] Internet-Draft SARP December 2014 have an ARP entry of IP-D -> MAC-W mapping where MAC-W is the MAC address of the SARP proxy on the west access segment. After the VM (IP-D) in the west site moves to the east site, if a Gratuitous ARP (IPv4) or an Unsolicited Neighbor Advertisement (IPv6) is sent out by the destination hypervisor on behalf of the VM (IP-D), then the IP<->MAC mapping cache of the VMs in all access segments is updated by IP-D -> MAC-E where MAC-E is the MAC address of the SARP proxy on the east site. If no Gratuitous ARP or Unsolicited Neighbor Advertisement is sent out by the destination hypervisor, the IP<->MAC cache on the VMs in the west site (and other sites) is eventually aged out. Until the IP<->MAC mapping cache tables are updated, the source VMs from the west site continue sending packets locally to MAC-D, and switches at the west site are still configured with the old location of MAC-D. This transient condition can be resolved by having the VM manager send out a fake Gratuitous ARP or Unsolicited Neighbor Advertisement on behalf of the destination Hypervisor. Another alternative is to have a shorter aging timer configured for IP<->MAC cache table. Destination Site The destination site is the site to which the VM migrated, i.e., the east site in Figure 6. Before any Gratuitous ARP or Unsolicited Neighbor Advertisement messages are sent out by the destination hypervisor, all VMs at the east site (and all other sites) might have IP-D -> MAC-W mapping in their IP<->MAC mapping cache. The IP<->MAC mapping cache is updated by aging or by a Gratuitous ARP or UNA message sent by the destination hypervisor. Until the IP<->MAC mapping caches are updated, VMs from the east site continue to send packets to MAC-W. This can be resolved by having the VM manager sending out a fake Gratuitous ARP/UNA immediately after the VM migration, or redirecting the packets from the SARP proxy of the east site back to the migrated VM by updating the destination MAC of the packets to MAC-D. Other Sites All VMs at the other sites that have an ARP entry of IP-D in their ARP table have the IP-D -> MAC-W mapping. The ARP mapping is updated by aging or by a Gratuitous ARP message Nachum, et al. Expires June 15, 2015 [Page 14] Internet-Draft SARP December 2014 sent by the destination hypervisor of the migrated VM and modified by the SARP proxy of the east site to an IP-D -> MAC- E mapping. Until ARP tables are updated, VMs from other sites continue sending packets to MAC-W. 3.3.2.1. Impact on IP<->MAC Mapping Cache Table of Migrated VMs When a VM (IP-D) is moved from one site to another, its IP<- >MAC mapping entries for VMs located at other sites (i.e., neither the east site nor the west site) are still valid, even though most guest OSs (or VMs) will refresh their IP<->MAC cache after migration. The migrated VM's IP<->MAC mapping entries for VMs located at the east site, if not refreshed after migration, can be kept with no change until the ARP aging time since they are mapped to MAC-E. All traffic originated from the migrated VM in its new location to VMs located at the east site traverses the SARP proxy of the east site, which can redirect the traffic back to the corresponding destinations on the east site. Furthermore, an ARP/UNA sent by the SARP proxy of the east site or by the VMs on the east site can refresh the corresponding entries in the migrated VM's IP<->MAC cache. The migrated VM's ARP entries for VMs located at the west site remain unchanged until either the ARP entries age out or new data frames are received from the remote sites. Since all MAC addresses of the VMs located at the west site are unknown at the east site, all unknown traffic from the VM is intercepted by the SARP proxy of the east site and forwarded to the SARP proxy of the west site (during the transient period before the ARP entries age out). This transient behavior is avoided if the SARP proxy has the destination IP address in its ARP cache, and upon receiving a packet with an unknown destination MAC address it can send a Gratuitous ARP/UNA to the migrated VM. Note that overlay networks providing Layer 2 network virtualization services configure their edge device MAC aging timers to be greater than the ARP request interval. 3.4. Multicast and Broadcast Multicast and broadcast traffic is forwarded by SARP proxies as follows: Nachum, et al. Expires June 15, 2015 [Page 15] Internet-Draft SARP December 2014 o SARP proxies modify the source MAC address of multicast and broadcast packets as described in Section 3.2. o SARP proxies do not modify the destination MAC address of multicast and broadcast packets. 3.5. Non IP packet The L2/L3 boundary routers in the current document are capable of forwarding non-IP IEEE802.1 Ethernet frames (Layer 2) without MAC header change. When subnets span across multiple ports of those routers, they are still under the category of a single link, or a multi-access link model recommended by [MultiLink]. They differ from the "multi-link" subnets described in [MultLinkSub] and [MultiLink], which refer to a different physical media with the same prefix connected to a router, where the Layer 2 frames cannot be natively forwarded without header change. 3.6. High availability and load balancing The SARP proxy is located at the boundary where the local Layer 2 infrastructure connects to the interconnecting network. All traffic from the local site to the remote sites traverses the SARP proxy. The SARP proxy is subject to high availability and bandwidth requirements. The SARP architecture supports multiple SARP proxies connecting a single site to the transport network. In the SARP architecture all proxies can be active and can backup one another. The SARP architecture is robust and allows network administrators to allocate proxies according to bandwidth and high availability requirements. Traffic is segregated between SARP proxies by using VLANs. An SARP proxy is the Master-SARP proxy of a set of VLANs and the Backup-SARP proxy of another set of VLANs. For example, assume the SARP proxies of the west site are SARP proxy 1 and SARP proxy 2. The west site supports VLAN 1 and VLAN 2 while SARP proxy 1 is the Master SARP proxy of VLAN 1 and the Backup proxy of VLAN 2 and SARP proxy 2 is the Master SARP proxy of VLAN 2 and the Backup SARP proxy of VLAN 1. Both proxies are members of VLAN 1 and VLAN 2. Nachum, et al. Expires June 15, 2015 [Page 16] Internet-Draft SARP December 2014 The Master SARP proxy updates its Backup proxy with all the ARP reply messages. The Backup SARP proxy maintains a backup database to all the VLANs that it is the Backup SARP proxy of. The Master and the Backup SARP proxies maintain a keepalive mechanism. In case of a failure the Backup proxy becomes the Master SARP proxy. The failure decision is per VLAN. When the Master and the Backup proxies switch-over, the backup SARP proxy can use the MAC address of the Master SARP proxy. The backup SARP proxy sends locally a Gratuitous ARP message with the MAC address of the Master SARP proxy to update the forwarding tables on the local switches. The backup SARP proxy also updates the remote SARP proxies on the change. 3.7. SARP Interaction with Overlay networks SARP can be used over overlay networks, providing L2 network virtualization (such as IP, VPLS, TRILL, OTV, NVGRE and VXLAN). The mapping of SARP to overlay networks is straightforward; the VM does the destination IP to SARP proxy MAC mapping. The mapping of the proxy MAC to its correct tunnel is done by the overlay networks. SARP significantly scales down the complexity of the overlay networks and transport networks by reducing the mapping tables to the number of SARP proxies. 4. Security Considerations SARP proxies are located at the boundaries of access networks, where the local Layer 2 infrastructure connects to its Layer 2 cloud. SARP proxies interoperate with overlay network protocols that extend the Layer 2 subnet across data centers or between different systems within a data center. The SARP protocol does not expose the network to additional security threats that do not exist in the absence of SARP. SARP proxies may be exposed to Denial of Service (DoS) attacks by means of ARP/ND message flooding. Thus, SARP proxies must have sufficient resources to support the SARP control plane without making the network more vulnerable to DoS than without SARP proxies. SARP adds security to the data plane in terms of network reconnaissance, by hiding all the local Layer 2 MAC addresses from potential attackers located at the interconnecting Nachum, et al. Expires June 15, 2015 [Page 17] Internet-Draft SARP December 2014 network, and significantly limiting the number of addresses exposed to an attacker at a remote site. 5. IANA Considerations There are no IANA actions required by this document. RFC Editor: please delete this section before publication. 6. References 6.1. Normative References [ARP] Plummer, D., "An Ethernet Address Resolution Protocol", RFC 826, November 1982. [ND] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. [GratARP] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227, July 2008. [ProxyARP] Carl-Mitchell, S., Quarterman, J., "Using ARP to Implement Transparent Subnet Gateways", RFC 1027, October 1987. [NDProxy] Thaler, D., Talwar, M., Patel, C., "Neighbor Discovery Proxies (ND Proxy)", RFC 4389, April 2006. [IGMPSnoop] Christensen, M., Kimball, K., Solensky, F., "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", RFC 4541, May 2006. [MultiLink] Thaler, D., "Multilink Subnet Issues", RFC 4903, June 2007. [ARMDProb] Narten, T., Karir , M., Foo, I., "Address Resolution Problems in Large Data Center Networks", RFC 6820, Jan 2013. Nachum, et al. Expires June 15, 2015 [Page 18] Internet-Draft SARP December 2014 6.2. Informative References [ARMDStats] Karir, M., Rees, J., "Address Resolution Statistics", draft-karir-armd-statistics-01 (expired), July 2011. [ARPPractice] Dunbar, L., Kumari, W., Gashinsky, I., "Practices for scaling ARP and ND for large data centers", draft-dunbar-armd-arp-nd-scaling- practices-08 (work in progress), May 2014. [NVO3Prob] Narten, T., Gray, E., Black, D., Fang, L., Kreeger, L., Napierala, M., "Problem Statement: Overlays for Network Virtualization", draft- ietf-nvo3-overlay-problem-statement (work in progress), July 2013. [MultLinkSub] Thaler, D., Huitema, C., "Multi-link Subnet Support in IPv6", draft-ietf-ipv6-multi-link- subnets-00 (expired), June 2002. 7. Acknowledgments We want to thank Ted Lemon in providing many valuable comments and suggestions to the draft. This document was prepared using 2-Word-v2.0.template.dot. Authors' Addresses Youval Nachum Email: youval.nachum@gmail.com Linda Dunbar Huawei Technologies 5430 Legacy Drive, Suite #175 Plano, TX 75024, USA Phone: (469) 277 5840 Email: ldunbar@huawei.com Nachum, et al. Expires June 15, 2015 [Page 19] Internet-Draft SARP December 2014 Ilan Yerushalmi Marvell 6 Hamada St. Yokneam, 20692 Israel Email: yilan@marvell.com Tal Mizrahi Marvell 6 Hamada St. Yokneam, 20692 Israel Email: talmi@marvell.com Nachum, et al. Expires June 15, 2015 [Page 20]