Scaling the Address Resolution Protocol for Large Data Centers (SARP)
draft-nachum-sarp-07
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 | 2014-01-12 | ||
| 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 | ||
| Additional resources | |||
| 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-07
Network Working Group Youval Nachum
Internet Draft
Intended status: Proposed Standard Linda Dunbar
Expires: July 2014 Huawei
Ilan Yerushalmi
Tal Mizrahi
Marvell
January 12, 2014
Scaling the Address Resolution Protocol for Large Data Centers
(SARP)
draft-nachum-sarp-07.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 (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.
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This Internet-Draft will expire on July 12, 2014.
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
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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 ................................. 8
2. Terms and Abbreviations Used in this Document ................ 9
3. SARP Description ............................................ 10
3.1. Control Plane: ARP/ND .................................. 10
3.1.1. ARP/NS Request for a Local VM ..................... 10
3.1.2. ARP/NS Request for a Remote VM .................... 10
3.1.3. Gratuitous ARP and Unsolicited Neighbor
Advertisement (UNA) ...................................... 11
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 to IP<->MAC Mapping Cache Table of
VMs being moved ....................................... 15
3.4. Multicast and Broadcast ................................ 16
3.5. Non IP packet .......................................... 16
3.6. IP<->MAC caching on SARP Proxy ......................... 16
3.7. High availability and load balancing ................... 17
3.8. SARP Interaction with Overlay networks ................. 18
4. Conclusions ................................................. 18
5. Security Considerations ..................................... 19
6. IANA Considerations ......................................... 19
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7. References .................................................. 19
7.1. Normative References ................................... 19
7.2. Informative References ................................. 20
8. Acknowledgments ............................................. 20
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 regardless whether the access
(or aggregation) node is the VMs (hosts)' gateway or not. For
example, when a host "a" under access domain "S" needs to
communicate with peers on the same VLAN but connected to
different access domains, SARP requires "a" to use remote
access node's MAC address rather than peers' MAC addresses. 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 in their FDBs. Therefore, the switches' FDB
size is limited regardless how VLAN is spread.
1.1. SARP Motivation
[ARMDProb] 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 [MultiLink].
They are different 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 and
the layer 2 frames cannot be natively forwarded without header
change.
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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 access
domains and each access domain has hosts belonging to
different VLANs, each access switch has to enable multiple
VLANs. Then, those access switches will be 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.
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-A
initiates an ARP/ND request to the L2/L3 boundary router if
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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.
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 [IGMPSnoop] can further
reduce the ND multicast traffic to some physical link
segments.
Statistics done by Merit Network [ARMDStats] 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
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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 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 proxy gateway technique to reduce switches' FDB (MAC
table) sizes and ARP/ND impact on network elements in an
environment where hosts within one subnet (or VLAN) can spread
over various access domains in data centers.
Note: The Guidelines to proxy developers [NDProxy] have been
carefully considered for the SARP protocols. Section 3.3 has
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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
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
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
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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
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.
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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.
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) [GratARP] 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.
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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.
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
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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.
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.
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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 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:
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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 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.
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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 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.
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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 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.
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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 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
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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.
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.
[GratARP] Cheshire, S., "IPv4 Address Conflict Detection",
RFC 5227, July 2008.
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[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.
7.2. 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-07 (work in progress), March 2013.
[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.
8. Acknowledgments
We want to thank Ted Lemon in providing many valuable comments
and suggestions to the draft.
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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
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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