Network working group X. Xu
Internet Draft Huawei Technologies
Category: Informational Kai Lee
China Telecom
Expires: January 2013 July 9, 2012
Path Optimization for LAN Extension
draft-xu-nvo3-lan-extension-path-optimization-00
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Internet-Draft Path Optimization for LAN Extension January 2013
Abstract
This document describes path optimization issues caused by LAN
extension across geographically dispersed data centers. In addition,
this document also describes requirements for possible solutions to
these issues.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
Table of Contents
1. Problem Statement ........................................... 3
1.1. Suboptimal Routing for Incoming Traffic ................ 4
1.2. Suboptimal Routing for Outgoing Traffic ................ 4
2. Terminology ................................................. 5
3. Solution Requirements ....................................... 5
3.1. Path Optimization for Incoming Traffic ................. 5
3.2. Path Optimization for Outgoing Traffic ................. 5
4. Security Considerations ..................................... 5
5. IANA Considerations ......................................... 6
6. Acknowledgements ............................................ 6
7. References .................................................. 6
7.1. Normative References ................................... 6
7.2. Informative References ................................. 6
Authors' Addresses ............................................. 6
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Internet-Draft Path Optimization for LAN Extension January 2013
1. Problem Statement
Virtual Machine (VM) migration and geo-clustering across data
centers usually require a LAN to be extended across these data
centers. Figure 1 depicts a generic data center interconnect
architecture where multiple data centers are interconnected with a
given LAN extension solution and remote VPN sites (e.g., cloud user
sites) are connected to these data centers with L3VPN solution
[RFC4364].
---------------
/ \
| Cloud User Site |
\ /
-------+-------
|
+---+---+
| PE-3 |
+---+---+
|
-------+------
/ \
/ \
| MPLS/IP Backbone |
\ /
/\ /\
/ ---------------- \
/ \
+-------/----+ +----\-------+
| GW-1(PE-1) | | GW-2(PE-2) |
+-----+------+ +------+-----+
| |
+---------+----------------------------+----------+
| LAN Extension |
+---------+----------------------------+----------+
| |
----+----- ----+-----
/ \ / \
| DC West | | DC East |
\ / \ /
---------- ----------
Figure 1: A Generic Data Center Interconnect Architecture
Since the LAN has been extended across multiple data center
locations, the IP subnet associated with this LAN is also extended
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Internet-Draft Path Optimization for LAN Extension January 2013
across these locations. As such, the traffic to/from the extended
subnet (e.g., the traffic between cloud user sites and data centers)
would encounter suboptimal routing issues as described in the
following sub-sections. Such suboptimal routing not only
unnecessarily consumes the bandwidth intended for data center
interconnect, but also decreases the cloud users' experiences due to
increased path latency. Note that here the traffic to/from the
extended subnet refers to L3VPN traffic between a remote L3VPN site
(e.g., a cloud user site) and data centers, rather than Internet
traffic. How to optimize the path for Internet traffic to/from the
extended subnet would be explored in the future.
1.1. Suboptimal Routing for Incoming Traffic
Since an IP subnet has been extended across multiple locations, the
subnet no longer retains its location semantics. As a result, the
incoming traffic towards a given server within the extended subnet
could travel through suboptimal paths if the traffic is forwarded
based on the corresponding subnet route. For example, assume a
server is physically located at data center East of an extended
subnet, the incoming traffic towards that server would possibly
travel through the default gateway router at data center West when
entering that subnet.
1.2. Suboptimal Routing for Outgoing Traffic
Let's assume the existing VPLS solution [RFC4761, RFC4762] is used
to achieve LAN extension across multiple data center locations. In
this case, VRRP would usually be enabled on default gateway routers
of different locations and only one of them would be selected as the
VRRP Master for the subnet associated with the extended LAN, which
is available for forwarding outgoing traffic of the subnet. In
addition, although multiple default gateway routers of different
locations could be selected as VRRP masters by filtering VRRP
messages among them, since the existing VPLS solution however
perform MAC learning as a traditional bridge, the route (e.g., MAC
forwarding entry) for a given MAC address would be determined
without taking the network distance into account. As a result, if
the forwarding path to the VRRP virtual MAC is currently pointed to
a default gateway router at data center East, for those servers
located at data center West, their outgoing traffic would have to
traverse the data center interconnection path so as to reach that
default gateway router at data center East, which in turn forwards
the traffic out of that subnet.
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2. Terminology
This memo makes use of the terms defined in [RFC4364] and [RFC2338].
3. Solution Requirements
3.1. Path Optimization for Incoming Traffic
The basic idea is to allow each default gateway router acting as a
L3VPN PE router to propagate host routes for local servers within
the extended subnet to remote PE routers. More specifically, a
default gateway router at a given data center is allowed to
advertise hosts routes only for servers located in that data center,
rather than those ones located in other data centers. In this way,
remote PE routers would be able to forward traffic destined for a
given server within the extended subnet according to the
corresponding host route for that server, rather than the subnet
route for that extended subnet.
The challenge here is how to make default gateway routers be able to
tell which servers within the extended subnet are their local ones.
Hence the possible solution for this path optimization issue SHOULD
ensure default gateway routers to be able to obtain enough information
so as to distinguish local servers from remote ones.
3.2. Path Optimization for Outgoing Traffic
To realize the purposes of default gateway redundancy and VM live
mobility across data centers, default gateway routers of a given
extended subnet at different locations SHOULD be configured with an
identical virtual IP/MAC address pair (i.e., virtual router). As
such, servers within the extended subnet could use that virtual
router's IP address as their default gateway. To ensure the outgoing
traffic with destination MAC address being the virtual router's MAC
address to be forwarded to a local default gateway router, rather
than any remote default gateway router, just like the anycast manner
in IP networks, the LAN extension solution SHOULD be able to select
the best route for a given MAC address (e.g., the virtual router's
MAC address) among multiple possible routes, e.g., by taking network
distance as one factor in the decision-making process of best-route
selection.
4. Security Considerations
TBD.
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5. IANA Considerations
There is no requirement for IANA.
6. Acknowledgements
TBD.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[RFC2338] Knight, S., et al., "Virtual Router Redundancy Protocol",
RFC 2338, April 1998.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling", RFC
4761, January 2007.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
Authors' Addresses
Xiaohu Xu
Huawei Technologies,
Beijing, China.
Phone: +86 10 60610041
Email: xuxiaohu@huawei.com
Kai Lee
China Telecom,
Beijing, China.
Leekai@ctbri.com.cn
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