Scaling the Address Resolution Protocol for Large Data Centers (SARP)
draft-nachum-sarp-05
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7586.
|
|
|---|---|---|---|
| Authors | Youval Nachum , Linda Dunbar , Ilan Yerushalmi , Tal Mizrahi | ||
| Last updated | 2013-07-11 | ||
| RFC stream | (None) | ||
| Formats | |||
| IETF conflict review | conflict-review-nachum-sarp, conflict-review-nachum-sarp, conflict-review-nachum-sarp, conflict-review-nachum-sarp, conflict-review-nachum-sarp, conflict-review-nachum-sarp, conflict-review-nachum-sarp, conflict-review-nachum-sarp | ||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | Became RFC 7586 (Experimental) | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-nachum-sarp-05
INTAREA Working Group Youval Nachum
Internet Draft Marvell
Intended status: Proposed Standard Linda Dunbar
Expires: January 2014 Huawei
Ilan Yerushalmi
Tal Mizrahi
Marvell
July 11, 2013
Scaling the Address Resolution Protocol for Large Data Centers
(SARP)
draft-nachum-sarp-05.txt
Abstract
This document introduces SARP, an architecture that uses proxy
gateways and allows to scale data center networks. SARP is based on
fast proxies that significantly reduce switches' FDB (MAC table)
sizes and ARP/ND impact on network elements in an environment that
hosts within one subnet (or VLAN) can spread over various locations.
SARP supports smooth and fast virtual machine (VM) mobility without
any modification to the VM, while keeping the connection up and
running efficiently. SARP is targeted for massive scaling 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.
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This Internet-Draft will expire on January 11, 2014.
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Copyright Notice
Copyright (c) 2013 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
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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 Description ............................................. 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 ..................... 9
3.1.3. Gratuitous ARP and Unsolicited Neighbor Advertisement
(UNA) .................................................... 10
3.2. Data Plane: Packet Transmission ........................ 10
3.2.1. Local Packet Transmission ......................... 10
3.2.2. Packet Transmission Between Sites ................. 10
3.3. VM Migration ........................................... 12
3.3.1. VM Local Migration ................................ 12
3.3.2. VM Migration from One Site to Another ............. 12
3.3.2.1. Impact to IP<->MAC Mapping Cache Table of VMs being
moved ................................................. 13
3.4. Multicast and Broadcast ................................ 14
3.5. Non IP packet .......................................... 14
3.6. IP<->MAC caching on SARP Proxy ......................... 14
3.7. High availability and load balancing ................... 15
3.8. SARP Interaction with Overlay networks ................. 16
4. Conclusions ................................................. 16
5. Security Considerations ..................................... 17
6. IANA Considerations ......................................... 17
7. References .................................................. 18
7.1. Normative References ................................... 18
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7.2. Informative References ................................. 19
8. Acknowledgments ............................................. 19
1. Introduction
1.1. SARP Motivation
SARP introduces an approach for scaling data center networks with a
large number of virtual Machines which can migrate from one location
to another without changing their IP/MAC addresses or allow serves in
one location to be instantiated with applications with IP addresses
in different subnets. [RFC6820] has documented various impacts and
scaling issues to data center networks when subnets span across
multiple L2/l3 boundary routers.
Note: The L2/L3 boundary routers in this draft are capable of
forwarding 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 [RFC4903]. They are different from
the "multi-link" subnets described in [Multi-Link] and [RFC4903]
which refer to a different physical media with the same prefix
connected to a router and the layer 2 frames cannot be natively
forwarded without header change.
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. 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 locations
(or Access Switches) and each Access Switch has multiple VLANs
enabled, the Access switches are exposed to all MAC addresses
among all the VLANs enabled.
For example, for an Access switch with 40 physical servers
attached, where each server has 100 VMs, there are 4000 hosts
under the Access Switch. 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.
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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) the 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-A initiates an ARP/ND
request to the L2/L3 boundary router if the router's MAC is not in
the host's cache; and the second hit is when the L2/L3 boundary
router initiates an ARP/ND request to the target in subnet-B if
the target is not in router's ARP/ND cache.
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 DC without VM mobility because each port
is only dedicated to one (or a few number of) VLANs. Thus, the
number of multicast addresses hitting each port is significantly
lower.
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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 probably an insignificant issue in today's data
center because the majority of data center servers are moving
towards 1G or 10G ports. The bandwidth taken by ARP/ND, even when
flooded to all physical links, becomes negligible compared to the
link bandwidth. In addition, the IGMP/MLD snooping [RFC4541] can
further reduce the ND multicast traffic to some physical link
segments.
Statistics done by Merit Network [ARMD-Statistics] has shown that the
major impact of a large number of mobile VMs in Data Centers is to
the L2/L3 boundary routers, i.e., issue 2 above. 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 those hosts do not communicate with external peers often
enough. The first hit is when the originating station in subnet-A
initiates an ARP/ND request to the L2/L3 boundary router if the
router's MAC is not in the host's cache; and the second hit is when
the L2/L3 boundary router initiates ARP/ND requests to the target in
subnet-B if the target is not in router's ARP/ND cache.
Overlay approaches, e.g. [NVo3-PROBLEM], can hide hosts (VMs)
addresses in the core but does 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.
In order to protect router CPUs from being overburdened by target
resolution requests, some routers rate limit the target MAC
resolution requests to CPU. When the rate limit is exceeded, the
incoming data frames are dropped.
In traditional Data Centers, it is less of an issue because 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 plus VMs, the number of hosts under one
router can grow 30 plus times. In addition, the traditional data
center has each subnet nicely placed in a limited number of server
racks, i.e., switches under router only need to deal with MAC
addresses of those limited subnets. With subnets being spread across
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many server racks, the switches are exposed to VLAN/MAC of many
subnets, greatly increasing the size of the FDB.
The solution proposed in this draft can eliminate or reduce the
likelihood of inter-subnet data frames being dropped and reduce the
host MAC addresses exposed to FDB on intermediate switches.
1.2. SARP Overview
SARP is a type of proxies that constrain the ARP/ND
broadcast/multicast messages to small segments regardless how wide
their corresponding Layer 2 domain spread.
Note: The Guidelines to proxy developers [RFC4389] have been
carefully considered for the SARP protocols. Section 3.3 has
demonstrated how SARP works when VMs are moved from one segment to
another.
In order to enable VMs to be moved across greater number of servers
while maintaining their MAC/IP addresses unchanged, the layer-2
network (e.g. VLAN) which interconnect those VMs may spread across
different server racks, different rows of server racks, or even
different data centers.
For ease of description, let's break the entire network which
interconnects all those VMs into two segments: interconnecting
segment and "access" segments. While the "Access" network is mostly
likely Layer 2, the "interconnecting" segment might be not.
The SARP proxies are located at the boundaries where the "Access"
segment connects to its "Interconnecting" segment. The boundary node
could 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 1 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 "interconnection" network. The ARP/ND
broadcast/multicast messages are bounded by the SARP proxies.
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*-------------------*
| |
+-------| Interconnect |-------+
| | | |
| *-------------------* |
| |
*-----------------* *----------------*
| SARP Proxies | | SARP Proxies |
*-----------------* *----------------*
| | | |
*-------* *-------* *-------* *-------*
| ACC | | ACC | | ACC | | ACC |
*-------* *-------* *-------* *-------*
|
*----------*
|Hypervisor|
*----------*
|
*--------*
|Virtual |
|Machine |
*--------*
(West Site) (East Site)
Figure 1 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 1
depicts), data center core, or data center aggregation. SARP can also
be implemented by the hypervisor (as Figure 2 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 1 depicts).
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*-------------------*
| |
+-------| TRANSPORT |-------+
| | | |
| *-------------------* |
| |
*-----------------* *----------------*
| Edge Device | | Edge Device |
*-----------------* *----------------*
| |
*-----------------* *----------------*
| Core | | Core |
*-----------------* *----------------*
| | | |
*-------* *-------* *-------* *-------*
| Agg | | Agg | | Agg | | Agg |
*-------* *-------* *-------* *-------*
|
*----------*
|Hypervisor|
*----------*
(West Site) (East Site)
Figure 2 SARP deployment options.
2. Terms and Abbreviations Used in this Document
ARP: Address Resolution Protocol
FIB: Forwarding Information Base
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. In this document, ND also
refers to Neighbor Solicitation, Neighbor Advertisement,
Unsolicited Neighbor Advertisement messages defined by RFC4861
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NS: IPv6 ND's Neighbor Solicitation
SARP Proxy: The components that participates in the SARP protocol.
UNA: IPv6 ND's Unsolicited Neighbor Advertisement
VM: Virtual Machine
3. SARP Description
3.1. Control Plane: ARP/ND
This section describes the ARP/ND procedure scenarios. In the first
scenario, VMs share the same Access Segment. In the second scenario,
the source VM is 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, the Address Resolution process is as described in [ARP] and
[ND]. When the VM sends an ARP request or IPv6's Neighbor
Solicitation (NS) to learn the IP to MAC mapping of another local VM,
it receives a reply from the other local VM with the IP-D to MAC-D
mapping.
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.
In our example, the source VM is located at the west Access Segment
and the destination VM is located at the east Access Segment.
When the source VM sends an ARP/NS request to find out the IP to MAC
mapping of a remote VM, if the local SARP proxy doesn't have the ARP
cache for the target IP address or the cache entry has expired, the
ARP/NS request is propagated to all Access Segments which might have
VMs in the same virtual network as the originating VM, including the
east Access Segment.
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The destination VM responds to the ARP/NS request and transmits an
ARP reply (IPv4) or Neighbor Advertisement (IPv6) having the IP-D to
MAC-D mapping.
The east SARP proxy functions as the proxy ARP of its Local VMs. The
east SARP proxy modifies the ARP reply or NA message's source MAC-D
to MAC-E and forwards the modified ARP reply or NA message to all the
SARP proxies.
The West SARP Proxy forwards the modified ARP reply message to the
source VM.
The west SARP proxy can also functions as an IP<->MAC cache of the
Remote VMs. By doing so, it significantly reduces the volume of the
ARP/ND transmission over the network.
When the west SARP proxy caches the IP<-> MAC mapping entries for
remote VMs, the timers for the entries to expire should be set
relatively small to prevent stale entries due to remote VMs being
moved or deleted. For environment where VMs move more frequently, it
is not recommended for SARP Proxy to cache the IP<-> MAC mapping
entries of remote VMs.
3.1.3. Gratuitous ARP and Unsolicited Neighbor Advertisement (UNA)
Hosts (or VMs) send out Gratuitous ARP (IPv4) and Unsolicited
Neighbor Advertisement - UNA (IPv6) for other nodes to refresh IP<-
>MAC entries in their cache.
The local SARP processes the Gratuitous ARP or UNA in the same way as
the ARP reply or IPv6 NA, i.e. replace the source MAC with its own
MAC.
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, there is no change in the data plane. The 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 traverse the SARP proxy of both
sites. In our example, all packets sent from the VM located at the
west site to the destination VM located at the east site traverse the
west SARP proxy and the east SARP proxy.
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The source VM follows its ARP table and sends packets to (IP-D, MAC-
E) destination addresses and with (IP-s, MAC-S) as the source
addresses.
The west SARP proxy can either 1) simply forward the data frame to
MAC-E, or 2)replace the packet source address to its own source
address (MAC-W), keeps the destination address to be (MAC-E), and
forwards the packet to the east proxy SARP.
It is recommended for west SARP proxy to replace Source Address with
its own if the "interconnecting segment" has address learning
enabled. Otherwise nodes in the "interconnecting segment" can't learn
the address of the switch on which west SARP proxy is running unless
the switch sends out frames periodically.
When the east proxy SARP receives the packet, it replaces the
destination MAC address to be (MAC-D) based on the packet destination
IP (i.e., IP-D), but it does not change the source MAC addresses.
When the destination VM receives the packet, the Source Address field
would be the MAC address of the VM on the west side or the MAC
address of the west side SARP proxy,
Noted: it is common for data center network to have security policies
to enforce some VMs can communicate with each other, and some VMs
can't. Most likely, those policies are configured by VM's IP
addresses. Even though the originating VM's MAC address might be lost
when the packet arrives at the destination VM, the originating VM's
IP address is still present in the data packets for security policy
to be enforced.
Noted: for the option which doesn't need west SARP to change source
MAC of the data frames, the originating VM's MAC will be present when
the data frames arrive at the destination VMs. Therefore, this option
is valuable when hosts/VMs need to validate source VMs MAC addresses
to comply any policies imposed.
Noted: Most hosts/VMs refresh its IP<->MAC mapping cache, with the
Source MAC and Source IP of a received data frame. For the option
which west SARP changes data frame's source MAC with its own MAC
address, the destination VM's IP<->MAC cache can be refreshed with
the valid mapping of the Source-VM-IP <->West-SARP-MAC. For the
option of West SARP not changing source MAC, the destination VM has
to turn off the learning of IP<->MAC mapping from the received data
frames.
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3.3. VM Migration
3.3.1. VM Local Migration
When a VM migrates locally within its Access segment, the SARP
protocol is not required to perform any action. VM migration is
resolved entirely by the Layer 2 mechanisms.
3.3.2. VM Migration from One Site to Another
In our example, the 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
Origin site:
The Origin site is the site where the VM is before migration. It is
the west site in our example.
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 to
MAC-D) mapping. VMs on any other "Access Segments" will have ARP
entry of (IP-D to 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 East Site, if there is
gratuitous ARP (IPv4) or Unsolicited Neighbor Advertisement (IPv6)
sent out by the destination hypervisor for the VM (IP-D), then the
IP<->MAC mapping cache of VMs on all Access Segments will be updated
by (IP-D to MAC-E) where MAC-E is the MAC address of the SARP proxy
on the East Site. If there isn't any gratuitous ARP or Unsolicited
Neighbor Advertisement sent out by the destination hypervisor, the
IP<->MAC cache on the VMs in west site (and other sites) will
eventually aged out.
Until IP<->MAC mapping cache tables are updated, the source VMs from
the west site continue sending packets to MAC-D. Switches at the west
site are still configured with the old location of MAC-D. This can be
resolved by VM manager sending out a fake gratuitous ARP or
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Unsolicited Neighbor Advertisement on behalf of destination
Hypervisor, shorter aging timer configured for IP<->MAC cache table,
or by redirecting the packets to the proxy SARP of the west site.
Destination Site:
The destination site is the site to which the VM migrated, the east
site in our example.
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 to MAC-W) mapping in
their IP<->MAC mapping cache. IP<->MAC mapping cache is updated by
aging or by a gratuitous ARP or UNA message sent by the destination
hypervisor. Until IP<->MAC mapping caches are updated, the source VMs
from the east site continue to send packets to MAC-W. This can be
resolved by 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 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 to MAC-W) mapping. ARP mapping is updated by
aging or by a gratuitous ARP message sent by the destination
hypervisor of the migrated VM and modified by the SARP proxy of the
east site (IP-D to MAC-E) mapping. Until ARP tables are updated, the
source VMs from the west site continue sending packets to MAC-W. This
can be resolved by redirecting the packets from the SARP proxy of the
west site to the SARP proxy of the east site by updating the
destination MAC of the packets to MAC-E.
3.3.2.1. Impact to IP<->MAC Mapping Cache Table of VMs being moved
When a VM (IP-D) is moved from one site to another site, its IP<->MAC
mapping entries for VMs located at the other sites (i.e. neither east
site nor west site) are still valid, even though most Guest OSs (or
VMs) will refresh their IP<->MAC cache after migration.
The VM (IP-D)'s IP<->MAC mapping entries for VMs located at 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 VM (IP-D) in its new location to VMs located at
the east site traverses the SARP proxy of the east Site. The ARP/UNA
sent by the SARP proxy of the east site or by the VMs on east side
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can always refresh the corresponding entries in the VM (IP-D)'s IP<-
>MAC cache .
The VM (IP-D)'s ARP entries (i.e. IP to MAC mapping) for VMs located
at west sites will not be changed either until 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 (just for ARP aging time). This can be resolved by the east SARP
proxy having mapping entries for VMs in the west side. Upon receiving
unknown packets, it can update the migrating VM with the new IP to
MAC mapping by sending a modified gratuitous ARP with (IP-D to MAC-W)
mapping.
Note that overlay networks providing the 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
To be added in a future version of this document
3.5. Non IP packet
To be added in a future version of this document
3.6. IP<->MAC caching on SARP Proxy
ARP/NS Requests for a VM located at a remote site require flooding
messages over the interconnecting network to all sites which have
enabled the virtual network on which the VM belongs to. This
scenario is described in details at 3.1.2. In such cases, SARP
caching can reduce the number of ARP/ND transmissions over
interconnecting networks.
In the example presented at section 3.1.2. the source VM is located
at the west site and the destination VM is located at the east site.
When the source VM sends an ARP or Neighbor Solicitation request to
discover the IP to MAC mapping of the remote VM, the request can be
intercepted by the west SARP proxy.
The west SARP proxy learns or refreshes the source IP to source MAC
mapping and looks up the IP to MAC translation of the destination IP.
If the destination IP entry is found and is valid, the west SARP
proxy replies with an ARP reply or Neighbor Advertisement without
propagating the packet to other sites. Otherwise, the packet is
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propagated to all sites which have the virtual network enabled
including the east site.
The propagated ARP/NS request is intercepted again by the east SARP
proxy. It learns or refreshes the source IP to source MAC mapping and
looks up the destination IP to MAC translation. If the destination IP
entry is found and is valid the SARP proxy replies with an ARP reply
or NA without propagating the ARP request to the east site.
Otherwise, the ARP/NS request is broadcasted within the east site.
The destination VM responds to the ARP/NS request and transmits an
ARP reply or NA having the IP-D to MAC-D mapping.
The east side SARP proxy intercepts the ARP reply or NA and learns or
refreshes the Destination IP to Destination MAC mapping, replace the
source MAC with its own MAC before sending the ARP reply or NA to the
west SARP proxy (so that requesting VM can learn the IP-D to MAC-E
mapping).
The West SARP Proxy intercepts the ARP reply or NA and learns or
refreshes the Destination IP to Destination MAC mapping and
propagates the ARP reply to the source VM.
The SARP proxies maintain an ARP caching table of IP to MAC mapping
for all recent ARP/NS requests and replies. This table allows the
SARP proxy to respond with low latency to the ARP/NS requests sent
locally and avoid the broadcast transmissions of such requests over
the transport network and all over the broadcast domains at the
remote sites.
3.7. 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 SARP architecture all
proxies can be active and can backup one another. The SARP
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architecture is robust and allows the network administrator to
allocate proxies according to the 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 the SARP proxies of the west site (as Figure 1 depicts)
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.
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.
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 switchover, 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.
3.8. SARP Interaction with Overlay networks
SARP interaction with overlay networks providing L2 network
virtualization (such as IP, VPLS, Trill, OTV, NVGRE and VxLAN) is
efficient and scalable.
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. Conclusions
SARP distributes the Layer 2 Forwarding Information Base (FIB) from
the edge devices (functioning as SARP proxies) to the VMs. By doing
so, it significantly reduces table sizes on the edge devices. The
source VM maintains the mapping of its destination VMs to the
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destination site/cloud in the ARP table. The destination VM IP is
translated to the destination MAC address of the SARP proxy at the
destination site. The SARP proxies only maintain Layer 2 FIB of local
VMs and remote edge devices.
SARP proxies can support FAST VM migration and provide minimum
transition phase. When SARP proxy indicates or is informed of VM
migration, it can update all its peers and trigger a fast update.
SARP seamlessly supports Layer 2 network virtualization services over
the overlay network and significantly reduces their complexity in
terms of table size and performance. The overlay networks are only
required to map MAC addresses of the SARP proxies to the correct
tunnel.
5. Security Considerations
The SARP proxies are located at the boundaries where the local Layer
2 infrastructure connects to its Layer 2 cloud. The SARP proxies
interoperate with overlay network protocols that extend the Layer-2
subnet across data centers or between different systems within a data
center.
SARP control plane and data plane are traversed by the overlay
network hence SARP does not expose the network to additional security
threats.
SARP proxies may be exposed to Denial of Service (DoS) attacks by
means of ARP/ND message flooding. Thus, the 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 by hiding all the local layer 2
MAC addresses from potential attacker located at the remote clouds.
The only MAC addresses that are exposed at remote sites are the MAC
addresses of the SARP proxies.
6. IANA Considerations
There are no IANA actions required by this document.
RFC Editor: please delete this section before publication.
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7. References
7.1. 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.
[GratuitousARP] S. Cheshire, "IPv4 Address Conflict Detection", RFC
5227, July 2008.
[IGMP-MLD-tracking] H. Aseda, and N. Leymann, "IGMP/MLD-Based
Explicit Membership Tracking Function for Multicast
Routers" (http://tools.ietf.org/html/draft-ietf-pim-
explicit-tracking-02), Oct, 2012.
[RFC826] D.C. Plummer, "An Ethernet address resolution protocol."
RFC826, Nov 1982.
[RFC1027] Mitchell, et al, "Using ARP to Implement Transparent Subnet
Gateways" (http://datatracker.ietf.org/doc/rfc1027/)
[RFC4389] Thaler, et al, "Neighbor Discovery Proxies (ND Proxy)",
RFC4389, April 2006.
[RFC4541] Christensen, et al, "Considerations for Internet Group
Management Protocol (IGMP) and Multicast Listener Discovery
(MLD) Snooping Switches", RFC 4541, May 2006
[RFC4861] Narten, et al, "Neighbor Discovery for IP version 6
(IPv6)", RFC4861, Sept 2007
[RFC4903] Thaler, "Multilink Subnet Issues", RFC4903, July 2007.
[RFC6820] Narten, et al, "Address Resolution Problems in Large Data
Center Networks", RFC6820, Jan 2013.
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7.2. Informative References
[Impatient-NUD] E. Nordmark, I. Gashinsky, "draft-ietf-6man-
impatient-nud"
[ARMD-Statistics] M. Karir, J. Rees, "Address Resolution Statistics",
draft-karir-armd-statistics-01.txt (expired), July 2011.
https://datatracker.ietf.org/doc/draft-karir-armd-
statistics/
[ARP_Reduction] Shah, et al, "ARP Broadcast Reduction for Large Data
Centers", draft-shah-armd-arp-reduction-02.txt (expired),
Oct 2011. https://datatracker.ietf.org/doc/draft-shah-armd-
arp-reduction/
[ARP-ND-PRACTICE] Dunbar, Kumari, Gashinsky, "Practices for scaling
ARP and ND for large data centers", draft-dunbar-armd-arp-
nd-scaling-practices-06, Feb 2013
[NVo3-PROBLEM] 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, May 2013.
[Multi-Link] Thaler, et al, "Multi-link Subnet Support in IPv6",
draft-ietf-ipv6-multi-link-subnets-00.txt (expired), Dec
2002. https://datatracker.ietf.org/doc/draft-ietf-ipv6-
multilink-subnets/
8. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Youval Nachum
Email: youval.nachum@gmail.com
Linda Dunbar
Huawei Technologies
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5430 Legacy Drive, Suite #175
Plano, TX 75024, USA
Phone: (469) 277 5840
Email: ldunbar@huawei.com
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
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